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rotate
06-04-2009, 12:33 AM
I'm trying to find out what the average pressure is on top and bottom of the an commercial jet plane wing (e.g. 747) during take off and cruising.

I'm guessing that the top will be slightly lower than 1 atm, while the bottom would have several atmosphere of pressure....or am I completely off the mark? I'm trying to settle a bet.

interrupted_cut
06-04-2009, 12:54 AM
There's less than 1 PSI differential between the top and bottom of the wing in level flight or normal climbing.
The wing area of a 747-400 is ~5900 ft^2 and its max takeoff weight is ~400K lbs. That gives ~68 lb/ft^2 which is less than .5 psi. It would average about 1 psid in a sustained 60 (2 G) banked turn. Commercial planes rarely exceed 30 bank angle.

Evan
06-04-2009, 03:48 AM
That can't be determined by deriving the "answer" by doing the math based on surface area and weight. The Bernoulli principle supplies only a relatively small proportion of the lift that an aircraft wing produces.

The majority of the lift is produced simply by accelerating the air to flow downward. This holds for every type of airfoil but is especially true for fully symmetrical shapes and those that come close to approximating a flat plate such as the F-101. All aircraft wings while in a normal flight attitude produce a downwash from the trailing edge of the wing. It is this downwash and the accelerated mass of the air it contains that produces the equal and opposite reaction called lift.

barts
06-04-2009, 03:56 AM
That can't be determined by deriving the "answer" by doing the math based on surface area and weight. The Bernoulli principle supplies only a relatively small proportion of the lift that an aircraft wing produces.

The majority of the lift is produced simply by accelerating the air to flow downward. This holds for every type of airfoil but is especially true for fully symmetrical shapes and those that come close to approximating a flat plate such as the F-101. All aircraft wings while in a normal flight attitude produce a downwash from the trailing edge of the wing. It is this downwash and the accelerated mass of the air it contains that produces the equal and opposite reaction called lift.

Whether Bernoulli or momentum transfer to downward air is responsible doesn't matter - in either case, the lift is seen as a pressure difference between the top and bottom of the wing.

- Bart

Circlip
06-04-2009, 04:57 AM
My indoctrination into the theory of flight seems to have followed the same principles as Barts, ie. pressure differential between upper and lower surfaces. Symetrical and knife sections tending to rely more on raw power to drag/push them around. The other factor when these two sections are used is that when a control surface is incorporated into them, air deflection takes place to change the flight attitude rather than the increase in lift generated from the increased profile section in a "Conventional" Airfoil.

Regards Ian.

aostling
06-04-2009, 05:37 AM
Whether Bernoulli or momentum transfer to downward air is responsible doesn't matter - in either case, the lift is seen as a pressure difference between the top and bottom of the wing.
- Bart

The lift is the net upward force obtained by integrating all the pressure forces on the body of the aircraft. For an airplane in level flight this is (as you say) the excess of the pressure on the bottom of the wing compared to the pressure on the top of the wing. In vertical flight the unbalanced upward pressure forces are confined to the inner surfaces of the engine or rocket motor, or to the propeller blades.

rantbot
06-04-2009, 06:50 AM
[delete - changed my mind - it would just add to general obfuscation]

Greg Q
06-04-2009, 06:56 AM
A 747-400 max TOW is >400,000 Kg. Thats of course around 880,000 lb.

For those of you who are adherents to the air deflection theory, how do you explain the obvious and visible change on a wings droop as the aircraft accelerates down the runway. A 747 wingtip will flex upward about 12 ft prior to rotation.

I can assure you that it is possible to fly at close to zero theta, given sufficient speed. At lower speeds a higher theta (and hence alpha) is required to generate sufficient airflow velocity differential over the wing.

WRT to absolute values, the leading edge of a jet airliner is seeing approximately 30 degrees temperature rise due to compression. It follows that dynamic effects would raise the underwing local pressure to >1 bar.

At the centre of pressure above the wing the texts* say the pressure drop s equal to about 150mm water, or about 0.015bar drop= surprisingly little.

*text in this case is "The Mechanics of Flight" A.C. Kermode
the example cited speaks of "normal wings at normal speeds", whatever the hell that means.


Greg. (who operates somewhere between 70 and 460 kts normally.)

Evan
06-04-2009, 07:22 AM
For those of you who are adherents to the air deflection theory, how do you explain the obvious and visible change on a wings droop as the aircraft accelerates down the runway. A 747 wingtip will flex upward about 12 ft prior to rotation.



??? Every part of the wing is responsible for accelerating the air as the wing passes through it. Action=Reaction. There is no inconsistency. Of course the wing flexes when doing work, it isn't rigid.

The Bernoulli effect presupposes that there must be a velocity difference between the top and the bottom airflows over the wing. Why should this be the case? It isn't like the molecules that take one or the other route have some sort of appointment with each other.

Invoking the Bernoulli effect as the sole contributor to lift fails miserably to explain the observed movment of air as a wing passes through it. It also fails to explain what happens to the airflow and the loss of lift that takes place when even a small portion detaches from the boundary layer and becomes turbulent flow instead of laminar flow.

As well, helicopters (rotary WING aircraft) do not fly by sucking themselves into the air.

Note, all of this is as determined by NASA.



Lift occurs when a moving flow of gas is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction. Because air is a gas and the molecules are free to move about, any solid surface can deflect a flow. For an aircraft wing, both the upper and lower surfaces contribute to the flow turning. Neglecting the upper surface's part in turning the flow leads to an incorrect theory of lift.

http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html

Greg Q
06-04-2009, 08:02 AM
Evan, I perhaps was not as accurate in my post as I should have been. While ascribing all lift effects to pure Bernoulli venturi effect is incorrect, the pressure differential does in fact account for 80% of the lift in most cases*

Downward acceleration and Newton indeed play a role, but it would be wrong to ascribe a disproportionate role to that effect.

As you say you can get a flat plate or delta wing to fly, but even they are developing lift based on the conventional lift model to some extent. The angle of attack will convert even a flat plate into an airfoil when you draw the flow lines.

* most cases+ see the aforementioned text. see also "High Speed Aerodynamics for Naval Aviators" and "Fly the Wing"

Greg

On edit: Thanks very much for the link. It has been many years since I revisited lift theory. When I was taught 40 years ago the Bernoulli theory was given as the sole cause of lift. I have heard the rather naive suggestion that the wing acts purely as a deflector before, and knew that to be untrue.

The Nasa link has a nifty 12 page explanaton of the current model and demonstrates that the total is much more complex than any one facile explanation. There are a series of equations, the Euler and the Navier-Stokes which integrate the total effects of a body moving in a fluid.

I shall have to be careful in how I throw around terms like Bernoulli, venturi, Newton.

I am glad that I no longer have little kids in the cockpit asking how the wings work. "Its like this kid...oh, do you do calculus?"

dockterj
06-04-2009, 09:02 AM
this helped me understand a lot of things that are glossed over in flight training:
http://www.av8n.com/how/htm/airfoils.html

Any comments? Is it accurate?

andy_b
06-04-2009, 09:19 AM
has physical science changed since i was in high school? i was always under the impression "lift" in an airplane was caused by the pressure differential on the top and underside of the wing. the NASA link Evan posted seems to suggest that it is caused by something different, more like how a boat propeller works. what am i missing?

oh, how about that Air France crash? that must have been one heck of a thunderstorm to just break the plane apart in mid-air.

andy b.

Greg Q
06-04-2009, 09:35 AM
I don't think that the thunderstorm activity will be proven to have directly caused an inflight break-up, although it has certainly happened before.

The aircraft had an automatic data link to base which transmitted a few data bursts including news of a complete electrical generation failure. When the Airbus runs out of amps it departs controlled flight. There is no direct connection between the flight crew and the control surfaces- it is all filtered through the flight control computers.

The lack of a distress call of any kind indicates something catastrophic (and sudden) though. The Acars data link would need pretty much a complete airplane in order to transmit.

Greg

departs controlled flight=lawn dart

Evan
06-04-2009, 09:55 AM
The Airbus can be flown manually using the trim tabs and differential engine thrust. The elevator and rudder trim have mechanical controls. Of course in turbulence that may not be sufficient.

The question is what caused a catastrophic failure of such magnitude?

One potential cause is something that hasn't been mentioned. Lightning over the southern oceans has been seen to occur in what have been dubbed "super bolts". These were first observed when the US orbited the VELA satellites to detect atomic tests. The satellites were detecting pulses of energy that had the signature of an atomic explosion over the open ocean. It was eventually determined that sometimes thunderstorms may produce lightning with an energy level that exceeds "ordinary" lightning by orders of magnitude. This places the energy contained in such a bolt in another realm entirely. It's the difference between a regular breaker rolling into the beach and a Tsunami.

If an aircraft were to be struck by such a bolt the ElectroMagnetic Pulse would probably be sufficient to instantly fry all electrical systems that had any significant amount of extended wiring.

Evan
06-04-2009, 10:00 AM
this helped me understand a lot of things that are glossed over in flight training:


That's a long read but from what I did read it seems pretty accurate.

The old explanation that relies on the Bernoulli theory as the sole contributor to lift is simply wrong, as are many, if not most text books and other reference sources. It's one of those "facts" that refuses to die.

J Tiers
06-04-2009, 10:02 AM
Evan and others.......

IT MAKES NO DIFFERENCE whether you use one theory or another...... in this case.

It is perfectly OBVIOUS and incontrovertible that SOMETHING holds the aircraft up.

Can we agree on that?

OK, so, the ENTIRE MASS of the aircraft is supported.

The only available support is some effect of the aircraft on the air.

The fuselage and wings DO share the support job, but we can consider that virtually all the support in a commercial aircraft is due to wing area, to a first approximation.

The ONLY way for this support to occur is due to some sort of pressure difference on top vs bottom of the wing. Only in that way can the wing deflect the airflow.*

The deflection of airflow is the key, since a force due to a fluid comes from accelerating a mass of that fluid in the direction opposite to the force.

The acceleration MUST result in a local pressure difference, or else there won't be a flow. Flow is a response to a pressure difference.

Therefore, there must be a pressure difference of some sort, and since the wings do the majority of the "support" of the aircraft, the average pressure across the underside wing area must be that which will support the aircraft's mass.

And, that PRESSURE DIFFERENCE supports the aircraft....... Your various arguments are to a certain extent simply arguing over whether there is MORE pressure on the bottom, or LESS pressure on the top of the wing.

In that sense, a helicopter DOES fly by "sucking itself into the air", but it cheats, by creating it's own high pressure area UNDER the blades.

*Note that whenever the airflow is re-directed, some acceleration is applied to do it, and there is an equal and opposite force on the deflecting device. An airfoil, or a flat plate, same-same. The plate restrains airflow in one direction, so it must flow away from high pressure in the opposite direction.

That force is from a pressure difference between the side with the force on it, and the opposite side.

Jim Caudill
06-04-2009, 10:24 AM
I flew C-130's as a USAF pilot between 1979 and 1981. I lost some friends and acquaintances from my squadron when their C-130 exploded after being struck by lightning in Turkey. Around that same period there was another USAF C-130 that also exploded on approach to Charleston AFB due to lightning and a Kuwait owned C-130 that similarly exploded in France. As a direct result of our C-130 loss, the USAF installed fire-supressant foam in the fuel tanks. The issue was induced currents in the fuel probes. I have no idea how the standards for shielding the Airbus "fly-by-wire" system were developed and tested, but doubt if lab simulations could take into account either the myriad of conductivity patterns or the intensity of worldwide lightning strikes. 2 things that I have a healthy fear of: ice and thunderstorms. I've been in a lot of both, and is one aspect of flying that I don't miss.

I have had a number of Aerodynamics classes over the years, I don't understand how the shuttle flies; and apparently don't understand how lift is truly developed, but I can fly a wing using AOA, airspeed, or control feel.

Greg Q
06-04-2009, 10:24 AM
The Airbus can be flown manually using the trim tabs and differential engine thrust. The elevator and rudder trim have mechanical controls. Of course in turbulence that may not be sufficient.

The question is what caused a catastrophic failure of such magnitude?

.

No. The trim is electrically operated. There is zero mechanical connection between the thrust levers and the engines. Further, where are you going to derive your attitude information over a dark ocean at night? (All in the context of total electrical failure)

Additionally, the notion that you can fly a jet transport aircraft at altitude using trim is fanciful. Having attempted to do so in a simulator with extreme fidelity I can report that it is barely possible, and I am no dummy on stick n' rudder. (30,000 hrs airline, air shows and some test work-not to brag but to qualify my remarks)

I fly with highly trained people, most of whom are dandy pilots. You have to appreciate that airline pilot flying training includes no basic pilot skills, nor does it focus on the one-in-a-billion event. Even dead sticking an aircraft to a visible runway in front of the aircraft is not a no-brainer. Flight on trim is has been done once...the Sioux City DC-10 crash, which ended in a fireball precisely due to the lack of control authority. And they at least had trim to work with, and direct thrust control.

Greg

Glenn Wegman
06-04-2009, 10:30 AM
The Airbus can be flown manually using the trim tabs and differential engine thrust.
.

That is if you have any control over the engines, as I was under the impression that they used FADEC or a similar form of "control by wire" computer controlled system.

Glenn

Greg Q
06-04-2009, 10:35 AM
I flew C-130's as a USAF pilot between 1979 and 1981. I lost some friends and acquaintances from my squadron when their C-130 exploded after being struck by lightning in Turkey. Around that same period there was another USAF C-130 that also exploded on approach to Charleston AFB due to lightning and a Kuwait owned C-130 that similarly exploded in France. As a direct result of our C-130 loss, the USAF installed fire-supressant foam in the fuel tanks. The issue was induced currents in the fuel probes. I have no idea how the standards for shielding the Airbus "fly-by-wire" system were developed and tested, but doubt if lab simulations could take into account either the myriad of conductivity patterns or the intensity of worldwide lightning strikes. 2 things that I have a healthy fear of: ice and thunderstorms. I've been in a lot of both, and is one aspect of flying that I don't miss.

Jim, the C-130 was known for that. I think the Douglass C-133 Vanisher was another plane with the same unfortunate characteristics. Maybe JP-4 fuel played a role? More modern metal aircraft* have better bonding and lightning strike resistance.

The aircraft that I mostly fly now is a lightning rod. It has so much plastic in it that we can get smoked from a cell 6 or 7 miles away. Every plane in the fleet has been plugged a dozen times or more despite being operated by prudent, conservative pilots. (All southern hemisphere lightning, although mostly not in the ITCZ). Most of us now aim for a 15 mile margin around cells. The older version of this plane was the opposite: all metal, you had to fly right through a cell to get lit up.

The A-330 has lots of composite structures and fairings. It too has a magical lighting rod quality despite state of the art static wicks.

My flight bag carries the sticker "If its not Boeing I'm not going". For a reason.

Greg

A.K. Boomer
06-04-2009, 10:40 AM
I'm trying to find out what the average pressure is on top and bottom of the an commercial jet plane wing (e.g. 747) during take off and cruising.

I'm guessing that the top will be slightly lower than 1 atm, while the bottom would have several atmosphere of pressure....or am I completely off the mark? I'm trying to settle a bet.



Rotate - You asked so im not picking on you - just want to help you understand but you do have one thing right and that's that your completely off the mark;)

To understand lift we need to leave the bottom part of the wing alone as neutral with the atmosphere, in normal flight it is after all just a straight vector in line with the air flow and in working theory changes very little -- What this equates to is that the bottom part of the wing does not "lift" the plane - it does not provide several atmospheres - its simply neutral in its medium --- All the effective lift actually comes from the top of the wing and its of course not due to "pressure" but actually lack there of ---- This is what keeps the big birds in the air.

In order to understand this phenomenon properly I use a different tactic than most textbooks, I use a time frame analogy to describe the cause and effect;
The shortest distance between two points is a line correct?
this is exactly what the bottom part of the wing is doing -- there is really no change - there is really no "interruption".
The "magic" of lift occurs in the upper section of the wings profile for it does two very important things -- it creates a longer distance that the air flow has to travel across the surface between separation of the leading edge to where its pieced back together in the rear and it does this all while keeping obstructions of flow to a minimum.
What this in effect does is create a time frame delay at about the last 2/3rds of the top part of the wings surface as the air has to be accelerated to blend back into the mix of the "normal" air flowing from the bottom side,
This is called the Bernoulli effect, when this occurs the air has to be "stretched out" in effect its like putting it into a slight vacuum chamber --- This is whats keeping the plane in the air - its like that last 2/3rds of the airplanes wing on top is using the atmosphere like a giant vacuum cleaner --- next time you get a window seat on a plane keep an eye on that section and you will see intermittent condensation build up in this area (like fog)

I know you probably never heard it put this way but whats keeping that bird up in the sky is the air on top of the last 2/3rds of the wing is actually playing "catch-up" almost as if it were "late for its flight":)

Next time your on a big bird and you have a window seat try this - while the plane is still on the ground pick a shiny spot on the top of the wing @ that last 2/3rds section around a bunch of rivets and such and memorize the slight deviation patterns (they all have them) Then when the craft gets in the air look at it again - the pucker factor changes the entire aluminum skin as its being sucked away from the wings mainframe.

Evan
06-04-2009, 10:42 AM
No. The trim is electrically operated. There is zero mechanical connection between the thrust levers and the engines. Further, where are you going to derive your attitude information over a dark ocean at night? (All in the context of total electrical failure)

I am under the impression that that it is still a requirement to have needle, ball and airspeed instruments on the panel that are independent of the electrical system. Also, perhaps you need to review the operators manual. The airbus trims are hydraulic according to the computers system flight law degradation sequence. The APU should also be independent of electrical systems as well as the RAT.


In case of a complete loss of electrical flight control signals, the aircraft can be temporarily controlled by mechanical mode.
Pitch control is achieved through the horizontal stabilizer by using the manual trim wheel.
Lateral control is accomplished using the rudder pedals.
Both controls require hydraulic power.
A red MAN PITCH TRIM ONLY warning appears on the PFD.


http://www.airbusdriver.net/airbus_fltlaws.htm

Evan
06-04-2009, 10:45 AM
To understand lift we need to leave the bottom part of the wing alone as neutral with the atmosphere, in normal flight it is after all just a straight vector in line with the air flow and in working theory changes very little --

That is incorrect. Turn it all upside down and it still works just fine.

dockterj
06-04-2009, 10:50 AM
Evan is right. Take a look at this:
http://www.av8n.com/how/htm/airfoils.html#fig-3pv
(and iirc "Stick and Rudder" has actual pictures of wind tunnel experiments of this). There is nothing that forces the air on top to rejoin at the same spot. In fact, it was this fallacy that made me not understand wingtip vortices or ground effect (not that I remember the details now :) )!


That is incorrect. Turn it all upside down and it still works just fine.

Jim Caudill
06-04-2009, 10:50 AM
For Greg: Thanks for the stimulus to research the old Cargomaster. From www.johnweeks.com:
"If there was a flaw with the C-133, it was their tendency to fall out of the sky for unexplained reasons. 10 of the 50 were lost in crashes. Many of these crashes were over water and left little or no debris. As a result, some of these crashes remain unexplained. "

Hence the moniker, "vanisher"

Also, I like Boeing's approach to flight computers and pilots as opposed to Airbus.

A.K. Boomer
06-04-2009, 11:00 AM
Turn it all upside down and it still works just fine.



Of course it does -- changing its position in its medium does not change a thing as the medium is the same in all directions --- it works perfectly in all directions be it sideways/upside down whatever,
But keep in mind that whatever its attached to will either fly correctly or be driven directly into the terra firma...

Evan
06-04-2009, 11:03 AM
Of course it does -- changing its position in its medium does not change a thing as the medium is the same in all directions --- it works perfectly in all directions be it sideways/upside down whatever,
But keep in mind that whatever its attached to will either fly correctly or be driven directly into the terra firma...

Wrong again.

A.K. Boomer
06-04-2009, 11:05 AM
Wrong again.

Don't be so rough on yourself:p

Greg Q
06-04-2009, 11:18 AM
I am under the impression that that it is still a requirement to have needle, ball and airspeed instruments on the panel that are independent of the electrical system. Also, perhaps you need to review the operators manual. The airbus trims are hydraulic according to the computers system flight law degradation sequence. The APU should also be independent of electrical systems as well as the RAT.


http://www.airbusdriver.net/airbus_fltlaws.htm

Evan, to be clear, I do not fly Airbus aircraft, although I have many friends who do. Reviewing the FCOM is not an option for me as a result, but thank you for the helpful suggestion. We are speaking of a total permanent loss of electrics in this scenario, hence the eventual loss of all control of primary and secondary flight controls. Hence the lawn dart scenario. Needle ball and airspeed? No. Its all electronic. We have no direct reading pressure instruments any more. I have not seen a turn needle in twenty years since the last time I flew a CV-580. And to reiterate, you are not going to be flying a 480 kt jet with instruments designed as back-ups for 120 kt DC-3s

As to hydraulic trim systems: Maybe, probably not. In any event all hydraulic secondary systems are hydraulically operated BUT electrically actuated. In other words, we don't have a bank of hydraulic valves in the flight deck.

The APU would be running for a twin engine oceanic flight anyway...ETOPS rules. The ACARS still signaled total electrics loss. RATS*...don't think the A-330 has one. I'll check.

Greg

*RATS= ram air turbine system, a drop-down prop-driven (from the airflow) generator/pump set first installed on the 767 back in 1981 ish. A condition for the regulatory authorities to allow the 767 to fly more than 60 minutes from an airport, the first EROPS/ETOPS authorities issued. We now fly up to 217 minutes from an airport with some twins, 180 with most. I think modern engines and APU's give sufficient redundancy without the RAT.

Circlip
06-04-2009, 11:31 AM
This was always a point of difference in the rules pertaining to one of the toy R/C glider classifications over here A.K.

The rules stated that only "primary control surfaces" were to be controlled (Rudder and Elevator) and controls to lift generating surfaces ie Ailerons were banned.

So we have the case of a tailplane with an aerofoil section. On downward movement of the elevator, how does it know it's not a "Lift controlling device"?? Likewise the Rudder/fin assembly? surely the same thing but on its side??

Funny stuff the theory of flight.

Just as an aside, a test flight of an Airbus on one occasion resulted in the aircraft to be totally uncontrollable from human "interfearence" for a rather long time untill the "Brain" switched back and allowed the flight crew to again assume control.

Tother thing, on the edges of thunder storms you can get into a serious up/down draft situation, "air pockets" as we are told, but the air currents travel at extremely high velocity which I would imagine we are not capable of doing emulating on a ground test rig.

Fly safe now,:D Regards Ian.

Circlip
06-04-2009, 11:38 AM
Cos Airbus is a joint Anglo-French system, RATS has been designed out and the much Superior PLB units were fitted to all types. Due to the indeterminate position in the air that an Aircraft can adopt, the Aux Genny pops UP into the cab and two arms automatically deploy for the Co-pilot to put his feet on and then Pedal Like Buggery. Simple ain't it??

Regards Ian.

Greg Q
06-04-2009, 11:49 AM
We can emulate the effects of severe vertical wind**. But it doesn't matter because the wing and structure are only certified to a finite amount. It is possible to find non-survivable winds and up/down drafts that would break an F-15. Tornadoes come to mind.

If it helps to confuse matters at all, a local A-330 had an uncommanded* elevator runaway some months ago, causing huge and injurious altitude excursions. If you do that a few more times the airplane breaks.

The A-300 that broke over NY in 2001 only had two full rudder cycles-enough to snap it in half.

*this incident echoed an earlier similar thing in a 777. These aircraft share the same model flight control computer. Implicated is the fact that they both occurred in the vicinity of a naval submarine coms stations which uses extremely high power VLF signals. EMP is the current suspicion.

**aircraft certification includes the destruction of two "iron birds", the aircraft bare structure that lives in a hydraulic jig. One is a fatigue tester which gets many thousand cycles, the other is an ultimate strength test dummy-it gets loaded to failure. Interestingly, if a structure is much stronger than the minimum allowed it is also considered a failure-it is obviously too heavy, and heavy costs lots of fuel over the life of the airframe.

Greg

Evan
06-04-2009, 12:11 PM
My wife was on an Airbus 330 (I think, maybe 320) on a flight from Calgary to Vancouver last year. About 1 hour into the flight the ship hit the fan, all the lights went out, the air conditioning etc. Captain comes on and says they have had a computer failure and he is returning to Calgary rather than proceeding to Vancouver because he doesn't want to fly over water. Said he is flying the aircraft manually. They landed without flaps deployed and used the entire runway to stop, apparently without reverse thrust.

According to the flight law document I posted that appears to correspond to a near total failure of the computer system as the very last mode before total loss of the fly by wire is "Direct Control" where all stick movement is directly transmitted to the actuators with no conditioning. My wife was unimpressed as was the helicopter pilot she was sitting next to.

On a flight from Europe to Vancouver on an Airbus we had to stop in Iceland for fuel due to extremely adverse headwinds. On the decent the flight profile was decidely uncomfortable for the majority of occupants as the craft decended in very pronounced steps. At each step it initiated a very discernable pitch over with a corresponding gee reduction of perhaps 30% or more. I refuse to fly Airbus after these events.

Also, see here for a pronounced tendency for the rudder to fall off various Airbus aircraft.

http://www.iasa.com.au/folders/Safety_Issues/others/rudder-sep.htm

Greg Q
06-04-2009, 12:37 PM
On a flight from Europe to Vancouver on an Airbus we had to stop in Iceland for fuel due to extremely adverse headwinds. On the decent the flight profile was decidely uncomfortable for the majority of occupants as the craft decended in very pronounced steps. At each step it initiated a very discernable pitch over with a corresponding gee reduction of perhaps 30% or more. I refuse to fly Airbus after these events.


I don't know what it is about them that causes that, but I've noticed it too, especially when flying into an airport surrounded by mountains. It's like the airplane is trying to fly down a set of stairs. Hardly confidence building. The airplane that I fly has modes that allow you to use the autopilot yet still imitate a skillful hand flown approach. Did I mention my Boeing sticker?

It's been a while since I last flew into Iceland-and that was Reykjavik. Did you go into Keflavik? While they are only about 30 miles apart I recall that Kef was more wide open-farther from the volcanic cones etc. Greenland's another story altogether...a frozen nightmare.

rotate
06-04-2009, 12:46 PM
Thanks for all the answers. I wasn't asking so much about theory of flight, but rather a simple empirical data relating to the measured pressure.

Would it be fair to say that during cruising the pressure above the wings is just less than 1 atm and the pressure below the wings is just above 1 atm?

Funny because in school, they make it sound like the difference has to be substantial.

Circlip
06-04-2009, 12:46 PM
Awwwww, lets not get into a finger pointing exercise cos I won't fly on Boyng 737's :D Cargo doors and rudder things. Yer can't beat a good old Dragon.

The difference IS substantial Rotate, if it were tother way round it wouldn't take orft.

Regards Ian.

Evan
06-04-2009, 12:51 PM
We flew into Keflavik. Very nice facility. They even have an old sticks and rags restored aircraft hanging from the ceiling in the main lobby. I didn't recognize it though.

Probably the most disconcerting experience I have had ( I have flown commercial more times than I can count and on just about everything) was on an L1011 from New York to LA. I was sitting in the very front of the business class for the legroom and began to smell that very unmistakeable aroma of fried wiring. I called over a stewardess and quietly mentioned this to here and suggested she inform the flight deck. She just shrugged and told me that it was "normal". :eek:

That was the last time I flew on an L1011. I also prefer Boeing.

Evan
06-04-2009, 12:53 PM
Would it be fair to say that during cruising the pressure above the wings is just less than 1 atm and the pressure below the wings is just above 1 atm?


Only if sitting at a full stop on the ground on a book standard day. As the top of the wing is at a slightly higher altitude it will have slightly less pressure on it. :D

Once off the ground both surfaces will be below 1 atmosphere.

Greg Q
06-04-2009, 01:01 PM
Awwwww, lets not get into a finger pointing exercise cos I won't fly on Boyng 737's :D Cargo doors and rudder things. Yer can't beat a good old Dragon.

The difference IS substantial Rotate, if it were tother way round it wouldn't take orft.

Regards Ian.
The 737 has never had a cargo door issue, and the rudder theory has been addressed with a re-design. While the 737 is not the last word in airplane design, it is a robust and reliable device.

The Dragon? as in Dragon Rapide? Now there's a funky aerothang. I think I passed one the other day when I was giving my Bleriot a whirl. ;) I always wanted to have a go at one of those. That and the Brabazon. And the Saunders-Roe Princess.

dp
06-04-2009, 01:01 PM
There are pressure waves on the wing and in front of the wing. The whole idea of a wing is that air will pass around it with the least amount of drag. A wing is a propeller blade except linear, not spinning. They get the plane off the ground because of downward thrust, same as propeller. This was described in Wolfgang Langewiesche's book, "Stick and Rudder" eons ago. It's still a good read.

philbur
06-04-2009, 01:07 PM
How else can airflow over a wing provide lift other than by a difference in air pressure. It simple Newtonian physic. The up force under the wing provided by the air pressure has to be great than the down force on top of the wing provided by the air pressure. The difference gives you the rate of acceleration. How else does any fluid transfer a force to a plain surface other than by pressure. It doesn't matter what principles you use to calculate it, the end result has to be a difference in air pressure. So using the aircraft weight and the wing area gives you the average differential pressure needed to lift the aircraft. Simple.

Phil

rotate
06-04-2009, 01:12 PM
Yes, I understand that it's the difference in pressures that gives rise to lift, but I want to know what the absolute pressure is on top and on the bottom. I can't find any reference on the web that provide that measurement.

Greg Q
06-04-2009, 01:26 PM
Well, its not. Read the Nasa link in full. Now here's some more numbers for you:

A 747 can reach say 35,000 ft after take-off, it'll weigh about 390,000 kg. It can go Mach 0.86. The air over the wing is doing about 0.94. The ambient pressure is 250 mb. approximately; the temperature is -56 C. The angle of attack is approximately 4 degrees. Solve for dynamic pressure above and below the wing.

Langanobob
06-04-2009, 01:54 PM
Hope I don't get permanently banned for dredging this one up:D

A plane is standing on a runway that can move (some sort of conveyor belt). The plane moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the plane speed and adjusts the speed of the runway to be exactly the same (but in the opposite direction). Can the plane take off?

Greg Q
06-04-2009, 02:00 PM
Yes. The planes airspeed is a result of the Newtonian reaction of the airframe to the rearward thrust produced by the powerplant(s). The only difference between the hypothetical moving runway and a conventional runway is that the tire speed will be twice as fast on your (theoretical) conveyor belt. The aircraft does not rely on a relationship between tire speed and the runway to accelerate. In fact on a perfect zero friction surface you can take off with the brakes on. A Chevy Impala with wings on the other hand would not reach flying speed.

philbur
06-04-2009, 02:12 PM
Yes it says it's due to newton third law of motion. The force on the walls of a gas cylinder is also due to Newtons third law, it's a result of the impact of air molecules on the wall, we call it air pressure. The Nasa link even uses the phrase "center of pressure". Look at the assosiated link for "center of pressure" the diagram there even shows the PRESSURE Variation profile across the wing. Nasa quote:

"As an object moves through a fluid, the velocity of the fluid varies around the surface of the object. The variation of velocity produces a variation of pressure on the surface of the object as shown by the the thin red lines on the figure. Integrating the pressure times the surface area around the body determines the aerodynamic force on the object."

So lift is calculated by an integration of the pressure over the surface. Simple.

Phil


Well, its not. Read the Nasa link in full.

fixxit
06-04-2009, 02:20 PM
The Myth Busters tested the myth of the plane on the conveyor belt.
It took off.... myth busted !

philbur
06-04-2009, 02:30 PM
Interrupted cut gave you the order of magnitude answer in post #2. If the differential is of the order of 1 psi. then under the wing can't ever be more than 1 psi over atmospheric and over the wing can't be lgreater than 1 psi under atmospheric. So split it 50/50 for fairness. If you want a more accurate answer you will need to define you conditions much more fully and then obtain a PhD in aerodynamics.;)

Phil:)


Yes, I understand that it's the difference in pressures that gives rise to lift, but I want to know what the absolute pressure is on top and on the bottom. I can't find any reference on the web that provide that measurement.

dockterj
06-04-2009, 02:43 PM
rotate - read just this section I think it illustrates what you are looking for:
http://www.av8n.com/how/htm/airfoils.html#sec-pressure
They say for GA planes (small planes) Q is on the order of 1/2 PSI. So for large planes maybe 1psi is in the ballpark. The most interesting thing is that there is actually suction (low pressure) on both the top and bottom of the wing, just stronger suction on the top.

rotate
06-04-2009, 03:03 PM
rotate - read just this section I think it illustrates what you are looking for:
http://www.av8n.com/how/htm/airfoils.html#sec-pressure
They say for GA planes (small planes) Q is on the order of 1/2 PSI. So for large planes maybe 1psi is in the ballpark. The most interesting thing is that there is actually suction (low pressure) on both the top and bottom of the wing, just stronger suction on the top.

Thanks.

I'm actually quite surprised by two things.

- when cruising (i.e. when angle of attack is low), the lift is generated mostly by the suctioning action (i.e. pressure lower than ambient) on the top of the wing rather than the pressure at the bottom pushing the wing upwards.

- half the lift is generated by the first 1/4 of the wing.


Unfortunately, this means that I lose my bet with a friend, however I'm glad that I learned something from this bet. :) I just love it when truth proves intuition wrong. Of course, one could instead say bad intuition can prove how obvious truth can be.

Circlip
06-04-2009, 03:18 PM
But the lower pressure above the wing always displays suction. This was a basic demo of pressure reduction and suction effect, two sheets of paper with a one finger space between them held between both hands, blow into the parallel gap between them and observe the results.

Regards Ian.

rotate
06-04-2009, 03:21 PM
Yes. The planes airspeed is a result of the Newtonian reaction of the airframe to the rearward thrust produced by the powerplant(s). The only difference between the hypothetical moving runway and a conventional runway is that the tire speed will be twice as fast on your (theoretical) conveyor belt. The aircraft does not rely on a relationship between tire speed and the runway to accelerate. In fact on a perfect zero friction surface you can take off with the brakes on. A Chevy Impala with wings on the other hand would not reach flying speed.

I don't quite understand this.

If the conveyor belt is moving so as to keep the ground speed of the plane to zero, then there would be no air moving around the wings which would prevent it from taking off.

I can see how if the plane is in flight, then the ground speed can be zero (with wind blowing in the opposite direction), but for a plane to take off from the ground (provided that there's little or no wind blowing) it must have ground speed. No?

Circlip
06-04-2009, 03:32 PM
Errr if lift is generated by the forward movement of the airframe with air going OVER the wing surface how does engine thrust affect a stationary airframe?? Did Mythbusters prove this with a TOY aircraft??

fasto
06-04-2009, 04:11 PM
but for a plane to take off from the ground (provided that there's little or no wind blowing) it must have ground speed. No?
No. It must have AIRSPEED.

Evan
06-04-2009, 04:56 PM
How else can airflow over a wing provide lift other than by a difference in air pressure

How do you think the Harrier is able to hover? Or a helicopter? A fixed wing aircraft flies the same way. It isn't the suction of the air above the wing that somehow holds it stuck to the sky ( which is the precise corollary of air pressure below the wing). As I originally wrote and as NASA also describes, it is the action of accelerating the mass of air that provides the reaction called lift. The force that keeps an aircraft suspended in the air is a result of continually making a large mass of air move downward. Air has non-negligble mass. It is about 1 kilogram per cubic meter.

As an aircraft passes through the air the wings cause that air to be put into motion and blasted downward. Simple Newtonian physics gives the result.

There is absolutely no requirement that there be a pressure differential between the top and bottom of the wing. All that is needed is for the air to be accelerated downward. The pressure as measured at the surface of a wing represents air velocity differences over the surface of the wing, not a force that is keeping the wing aloft.

The situation is greatly complicated by various factors. It is possible for a wing to operate almost entirely by simply deflecting the air downward from the bottom surface. It is extremely inefficient as the wing will exhibit enormous induced drag as it will be fully stalled, but as the old saying goes, if you put enough power on it you can make a brick fly. How do you think a missile such as a sidewinder flys in level flight? It has no wings.

A well designed wing is simply an efficient shape to transfer the forward momentum of the aircraft to downward momentum of the air.

A.K. Boomer
06-04-2009, 05:08 PM
No. It must have AIRSPEED.



Not exactly, it must have air resistance, big difference.

dockterj
06-04-2009, 05:11 PM
quote:
This conveyor has a control system that tracks the plane speed and adjusts the speed of the runway to be exactly the same (but in the opposite direction).

The question as stated is meaningless. Assume that "tracks the plane speed" means groundspeed. Lets further assume that this means the runway moves in a speed and direction to reduce the planes groundspeed to zero. I'm pretty sure that you will soon exceed the speed rating for the wheel bearings.

Assume that "tracks the plane speed" means air speed. Again, the runway is quickly going to be moving faster than the speed rating of the wheel bearings.

Let's assume that the wheel bearings are perfect and can sustain an infinite speed. In either interpretation the plane takes of as easily as if the runway were stationary.

A.K. Boomer
06-04-2009, 05:12 PM
There is absolutely no requirement that there be a pressure differential between the top and bottom of the wing. All that is needed is for the air to be accelerated downward.



Evan as usual your a contradiction in terms ------ if air is getting accelerated downward then there is a pressure differential between top and bottom, this is what makes a plane fly.

rotate
06-04-2009, 05:33 PM
No. It must have AIRSPEED.

Yes, that's true, however when the plane is initially stationary on the ground it must gain airspeed by moving relative to the ground. If the plane is not moving relative to the ground (i.e. while it's stationary on the ground), it cannot gain airspeed.

fasto
06-04-2009, 05:38 PM
Yes, that's true, however when the plane is initially stationary on the ground it must gain airspeed by moving relative to the ground. If the plane is not moving relative to the ground (i.e. while it's stationary on the ground), it cannot gain airspeed.

This is NOT true. The airplane has to gain AIRSPEED. AIRSPEED is not related to GROUNDSPEED. The airplane does not have to have move relative to the ground to gain AIRSPEED, although it usually does. The airplane DOES NOT push against the GROUND to gain airspeed. It pushes against the AIR. The airplane uses its propellers or turbofan engine's fans pushing against the AIR. The ground is totally irrevelant. The groundspeed is totally irrevelant. The airplane does not use its TIRES to gain airspeed.

rotate
06-04-2009, 05:54 PM
This is NOT true. The airplane has to gain AIRSPEED. AIRSPEED is not related to GROUNDSPEED. The airplane does not have to have move relative to the ground to gain AIRSPEED, although it usually does. The airplane DOES NOT push against the GROUND to gain airspeed. It pushes against the AIR. The airplane uses its propellers or turbofan engine's fans pushing against the AIR. The ground is totally irrevelant. The groundspeed is totally irrevelant. The airplane does not use its TIRES to gain airspeed.

I agree with you that airspeed and ground speed need not be related. In fact, as I noted earlier, the plane's ground speed can be zero while air speed can be 200km/h. This is the case because the air is moving relative to the ground (aka wind).

If there's no wind and the plane is flying at a fixed altitude, then airspeed must be very close to ground speed.

Having said, this I have yet to see a winged airplane (not helicopters and vertical take off Jet planes) take off from a runway without having actually moved relative to the ground.

hitnmiss
06-04-2009, 06:16 PM
Having said, this I have yet to see a winged airplane (not helicopters and vertical take off Jet planes) take off from a runway without having actually moved relative to the ground.


I have an electric r/c plane (slo-stick) that routinely does exactly that. Takes off with no forward movement. All it needs is about a 8+mph breeze.

I can take off and land it with zero ground speed. At about 14mph it can take off backing up!

rotate
06-04-2009, 06:25 PM
I have an electric r/c plane (slo-stick) that routinely does exactly that. Takes off with no forward movement. All it needs is about a 8+mph breeze.

I can take off and land it with zero ground speed. At about 14mph it can take off backing up!

Yes, that's because there's wind. The wind provides the airspeed. For that matter, an airplane can even take off without any engine if it's tethered by a rope to the ground (like a kite) and there's sufficient head wind.

My point is that when there's no wind, ground speed = airspeed, when the altitude is fixed.

Evan
06-04-2009, 06:42 PM
if air is getting accelerated downward then there is a pressure differential between top and bottom, this is what makes a plane fly.


No. Tell me, which part of this statement by NASA do you not understand?

"Lift occurs when a moving flow of gas is turned by a solid object."

rantbot
06-04-2009, 06:58 PM
If the conveyor belt is moving so as to keep the ground speed of the plane to zero, then there would be no air moving around the wings which would prevent it from taking off.
That was not how the problem was posed.

The control system moves the conveyor at the same speed as the aircraft, but in the opposite direction. This is not the same as saying that the aircraft is held stationary. The aircraft's speed is unaffected; only the conveyor's speed is being controlled. The faster the aircraft moves, the faster the conveyor moves. But so what? The aircraft, its wings, and the airflow over the wings are all unaffected by the conveyor's antics, and the aircraft takes off normally.

The only unusual thing going on is that at any particular ground speed (and while the aircraft is still on the ground), the wheels are turning twice as fast as they would if the conveyor was not moving.

NickH
06-04-2009, 06:58 PM
That is incorrect. Turn it all upside down and it still works just fine.

You can fly a plane upside down but what does that do to the fuel consumption?

Evan
06-04-2009, 06:59 PM
As for the very lame "what if the runway has a conveyor....." question...

What if the plane has widely spaced wheels that straddle the conveyor belt so that no part of the aircraft is actually touching the belt. For extra points explain how this differs from having the wheels on the belt. For double credit explain how it differs from projecting an image of a moving belt on the runway. And, for an automatic passing grade explain how the plane can take off if we paint the words STOP on the runway.

Evan
06-04-2009, 07:03 PM
You can fly a plane upside down but what does that do to the fuel consumption?


On most aircraft the fuel consumption falls almost immediately to zero. It has no bearing on the question at all.

A better question is "how does a glider maintain level flight in a rising air mass.?"

Greg Q
06-04-2009, 08:10 PM
As far as the conveyor belt question, the problem many people are having is that their own experience of vehicles consists of cars. Cars need friction for the motor to drive the tires. Airplanes do not have driven wheels. The conveyor can merrily spin a thousand mph the other way, doesn't matter.

Since the whole hypothetical question is unreal my explanation also requires tires with a very rapid speed rating, bullet proof bearings etc.

Greg

Barrington
06-04-2009, 08:52 PM
if air is getting accelerated downward then there is a pressure differential between top and bottom, this is what makes a plane fly.No. Tell me, which part of this statement by NASA do you not understand?

"Lift occurs when a moving flow of gas is turned by a solid object."
What exactly is the conceptual difficulty here ? How do you imagine the turning of the flow produces a force on the solid body other than by creating a pressure differential ????

.

Evan
06-04-2009, 09:54 PM
How do you imagine the turning of the flow produces a force on the solid body other than by creating a pressure differential ????



You can build a device that flies by accelerating air using an electrical field. By ionizing the air molecules they may be attracted to a charged grid and energy of motion is imparted to the air by electrostatic attraction. The air does not interact with any surface and there is no pressure differential because of that.

Where does the lift come from?


A biplane takes advantage of something called the "slot effect". This is an increase in lift that is present when the air flows between the upper surface of one wing and the lower surface of the wing above. The pair of wings provide more than 2 times the lift that one would expect. The velocity of flow between the wings increases as the slot acts as a restriction to the flow of air. While this produces a decrease in pressure above the lower wing it produces a corresponding decrease under the upper wing as well. How does this improve the lift of the upper wing?

The answer is that it doesn't. What it improves is the velocity of the air mass as it leaves the slot having been directed downward by the slot. This accelerated mass provides greater reaction than it would otherwise because of the increased velocity even though it reduces the pressure differential on the top wing and should by your explanation reduce the overall lift instead of increasing it.

Rustybolt
06-04-2009, 10:35 PM
What if we use a sailboat sail instead of an airplane wing? Are the prinicples the same for a schooner or a cat rigged sail, close hauled against the wind?

Most sailboats have their best and fastest sailing position with the wind just a few points off dead ahead. IE sailing into the wind.

andy_b
06-04-2009, 10:37 PM
I don't quite understand this.

If the conveyor belt is moving so as to keep the ground speed of the plane to zero, then there would be no air moving around the wings which would prevent it from taking off.

I can see how if the plane is in flight, then the ground speed can be zero (with wind blowing in the opposite direction), but for a plane to take off from the ground (provided that there's little or no wind blowing) it must have ground speed. No?


not this again. an airplane can move forward or reverse as fast as it wants regardless of the RPM of its wheels. all the wheels do is keep the landing gear from scratching the pavement, the exhaust gas from the engines provides the thrust for mobility.

andy b.

doctor demo
06-04-2009, 11:20 PM
not this again. an airplane can move forward or reverse as fast as it wants regardless of the RPM of its wheels.
andy b.
OK Andy.... but how do I get the clock on My VCR to stop flashing 12:00 ?

Steve:D

dp
06-04-2009, 11:20 PM
On most aircraft the fuel consumption falls almost immediately to zero. It has no bearing on the question at all.

A better question is "how does a glider maintain level flight in a rising air mass.?"

The angle of attack and the angle of the lift combine algebraically to provide level flight with level attitude. It's close to being an optical illusion but only because one cannot the the direction of air movement relative to the point of observation.

You're pretty damned aware of it in the cockpit of the glider, though :)

J Tiers
06-04-2009, 11:47 PM
You can build a device that flies by accelerating air using an electrical field. By ionizing the air molecules they may be attracted to a charged grid and energy of motion is imparted to the air by electrostatic attraction. The air does not interact with any surface and there is no pressure differential because of that.

Where does the lift come from?



From the pressure differential.

Glenn Wegman
06-04-2009, 11:52 PM
How does a Frisbee fly?

J Tiers
06-05-2009, 12:05 AM
Posting in a second post to allow the maximum shock value from the statement in the first post..... despite the fact that it isn't really exactly true..... it was too good to leave alone......

In the case of the electrostatic gizmo, the air is , say, attracted to the grid...... OK, it moves towards the grid, LEAVING AN AREA OF LOWER PRESSURE ABOVE IT, into which air moves......

Of course this is also not really the issue, because the movement of a mass of air is an acceleration of that mass, which produces a reaction on whatever does the accelerating.

However, the acceleration itself is usually due to, or associated with, a difference of pressure.... The wing accelerates air downward, due to its angle of attack, because it creates an area of higher pressure below it, forcing the air to move away, in a certain direction because it cannot pass through the solid wing.

The AIRFOIL shape does not matter a lot..... many wings for aerobatic model and otehr aircraft are symmetrical, knocking a hole in the idea that the shape matters in creating lift.

Hidden inside the streamlined wing is a flat plate equivalent, which will produce lift as well.. I think Langwiesch has an illustration of that, actually.

But for sure the angle of attack produces a higher pressure below than above the wing..... Anyone doubting that is free to explain the whorls left in the air at the wingtips some other way........ if they can.

The consequence of the pressure is that the air moves away from the wing, i.e. it is accelerated downwards, producing an opposite reaction which lifts the wing.

Evan's "grid" is EXACTLY THE SAME. And it would produce wingtip whorls also......

The electrostatic attraction produces a 'virtual hard surface", which is somewhat impermeable to the charged air, and the air is again forced by an externally imposed condition to flow "away from" this "virtual hard surface", i.e. it is accelerated downwards, producing an opposite reaction which lifts the "wing". Even if the grid is stationary, the act of accelerating the first bit of air produces a higher pressure below that moving air.. The electrostatic attraction is acting the same as the pressure differential does, and in fact it produces a pressure differential as the air is accelerated down..*

It would be the same if he postulated a "repulsion" from a similar grid, but in that case the "virtual hard surface" would be very much more directly visible.

With some modification, the electrostatic example becomes merely a rocket.......

*the pressure differential entrains air that isn't charged, and so is not directly affected by the grid. That air is also moved, and so it becomes part of the lifting action by virtue of the pressure which accelerates it.

Jim Hubbell
06-05-2009, 12:07 AM
When you put your hand out of the window of an auto moving at a decent rate and suppose the hand to be an airfoil, rotate your arm to vary the angle of attack. Now tell me the pressure is the same on my palm as on the back of my hand.
Respectfully,

dp
06-05-2009, 12:08 AM
How does a Frisbee fly?


It's all in the wrist.

dp
06-05-2009, 12:12 AM
When you put your hand out of the window of an auto moving at a decent rate and suppose the hand to be an airfoil, rotate your arm to vary the angle of attack. Now tell me the pressure is the same on my palm as on the back of my hand.
Respectfully,

At angles of attack that are compatible with flight with a well designed wing the pressure differentials are not very high. It isn't pressure that holds a wing in the air - it is thrust. Same thing that makes a propeller work. What you describe with your hand? That's what holds a kite in the air and keeps a parachute from collapsing, but does not apply to a wing.

J Tiers
06-05-2009, 12:14 AM
At angles of attack that are compatible with flight with a well designed wing the pressure differentials are not very high. It isn't pressure that holds a wing in the air - it is thrust. Same thing that makes a propeller work. What you describe with your hand? That's what holds a kite in the air and keeps a parachute from collapsing, but does not apply to a wing.

Egg and chicken?

Now explain what makes the thrust.........

Greg Q
06-05-2009, 12:21 AM
At angles of attack that are compatible with flight with a well designed wing the pressure differentials are not very high. It isn't pressure that holds a wing in the air - it is thrust. Same thing that makes a propeller work. What you describe with your hand? That's what holds a kite in the air and keeps a parachute from collapsing, but does not apply to a wing.

Well why have the wing at all then if it's thrust which sustains flight.

Evan's Harrier and helicopter examples are examples of downward thrust being> than the mass of the aircraft, but that's not what it going on in sustained flight. The gliders, both sailplane and Hudson river demonstrate that it is in fact lift which keeps you in the air. Thrust just helps you stay on schedule.

Jim Hubbell
06-05-2009, 12:30 AM
If the kite string is equated to thrust then it works for me. The pressure on the kite transmits as tension on the string. Substitute a thruster in place of the string and behold a flying machine! Sort of like an ultralite aircraft.

Evan
06-05-2009, 12:32 AM
The electrostatic attraction produces a 'virtual hard surface", which is somewhat impermeable to the charged air, and the air is again forced by an externally imposed condition to flow "away from" this "virtual hard surface", i.e. it is accelerated downwards, producing an opposite reaction which lifts the "wing". Even if the grid is stationary, the act of accelerating the first bit of air produces a higher pressure below that moving air.. The electrostatic attraction is acting the same as the pressure differential does, and in fact it produces a pressure differential as the air is accelerated down..*



Uh no.


Pressure has nothing to do with it. The grid creates charged particles by ionizing the air molecules. Once ionized they can be attracted and repelled by electrostatic fields. This will happen even if the molecules are so sparse that they never encounter one another and therefore pressure doesn't exist.

The grid is equipped with an array of sharply pointed wires that act to create an directional electronic charge gradient strong enough to strip electrons from the atoms in the molecules of air. That requires a positive charge on the grid. Once deprived of an electron the molecule is repelled from the field of the grid and is accelerated by the electromagnetic force acting between the molecules and the field gradient of the charged wires. Pressure changes in the air will exist due to the movement of the air but have nothing to do with the transfer of momentum forces between the molecules and the grid. Forces are transmitted via the electromagnetic fields which in no way depend on the presence of other air molecules.

J Tiers
06-05-2009, 12:33 AM
Well why have the wing at all then if it's thrust which sustains flight.

Evan's Harrier and helicopter examples are examples of downward thrust being> than the mass of the aircraft, but that's not what it going on in sustained flight. The gliders, both sailplane and Hudson river demonstrate that it is in fact lift which keeps you in the air. Thrust just helps you stay on schedule.

He refers to downward thrust..........the accelerating of air in a downward direction by the wing, exactly like the acceleration in a (usually) rearward direction by the propeller, which has several little wings on it which work the same way as the wings on the airframe.

Bruce Griffing
06-05-2009, 12:35 AM
Evan-
While it is true that a wing "throws air downward" creating a reaction force, there is no mechanism for force transfer to the wing other than a pressure differential. If you believe there is another mechanism - what is it?

dp
06-05-2009, 12:42 AM
Well why have the wing at all then if it's thrust which sustains flight.

Evan's Harrier and helicopter examples are examples of downward thrust being> than the mass of the aircraft, but that's not what it going on in sustained flight. The gliders, both sailplane and Hudson river demonstrate that it is in fact lift which keeps you in the air. Thrust just helps you stay on schedule.

Lift provided by the legendary skyhook, no doubt. :)

Thrust from the propeller or jet engine is not the same as thrust from the wing. By that I mean the direction of the thrust. A Harrier in hover is standing on columns of pure thrust, of course - they are reaction thrusters.

The mechanism by which a propeller generates thrust is identical to how a wing generates thrust. They have the same basic profile, in fact. A propeller is twisted because it is rotating about a center and the angle of attack along its length is made consistent with the twist. Without the twist the propeller would stall somewhere toward the tip, and a stalled wing/propeller is incredibly inefficient.

In the case of a bird's wing it is producing thrust down and to the rear which is how it can both remain airborne and get where it wants to go. The vertical component of thrust is what holds the bird in the air, and the rear component of thrust is what makes it go places.

J Tiers
06-05-2009, 12:50 AM
Uh no.


Pressure has nothing to do with it. The grid creates charged particles by ionizing the air molecules. Once ionized they can be attracted and repelled by electrostatic fields. This will happen even if the molecules are so sparse that they never encounter one another and therefore pressure doesn't exist.

The grid is equipped with an array of sharply pointed wires that act to create an directional electronic charge gradient strong enough to strip electrons from the atoms in the molecules of air. That requires a positive charge on the grid. Once deprived of an electron the molecule is repelled from the field of the grid and is accelerated by the electromagnetic force acting between the molecules and the field gradient of the charged wires. Pressure changes in the air will exist due to the movement of the air but have nothing to do with the transfer of forces between the molecules and the grid. Forces are transmitted via the electromagnetic fields which in no way depend on the presence of other air molecules.

Uh, YES.......

You are describing a rocket engine, merely accelerating a mass of material and profiting by the reaction.

it is similar to the Harrier hovering, which is not "flight", in a macro sense. At least not "winged flight".

That acceleration of a mass can be done with a physical pressure, inside an actual rocket engine/jet engine, OR AN ARTIFICIAL PRESSURE. Your acceleration derives from an artificial electrostatic "pressure" created by your device.

The ultimate effect of flight in an aircraft with wings, distinct from your 'rocket", can be said to be acceleration, or pressure, almost as you wish, since the two are so related..... one creates the other.

Fluid flows result from pressure differences.

An area of high pressure is potential energy, which dissipates as the high pressure areas have fluid flow out into the lower pressure areas.

Please explain the wingtip vortex if there is no pressure difference.

The wing is re-directing air down, yet some flows UP.......... Rather difficult to explain without allowing pressure differences.......

Ditto with upward flex of the wings....... But then the flow itself is difficult to explain without pressure differences being associated.

Nope, you can explain it away anyhow you want, but there ARE pressure differences. They result from the downward acceleration of the air...... in fact that is rather the point of doing the acceleration in the first place...... The upward reaction is the pressure....... and it ain't on the top of the wing.

I think you are tied up in knots about this due to what is basically a semantic issue......

dp
06-05-2009, 12:56 AM
Well why have the wing at all then if it's thrust which sustains flight.

Evan's Harrier and helicopter examples are examples of downward thrust being> than the mass of the aircraft, but that's not what it going on in sustained flight. The gliders, both sailplane and Hudson river demonstrate that it is in fact lift which keeps you in the air. Thrust just helps you stay on schedule.

Wings are efficient at what they do. The fuel consumption to generate pure thrust with reaction engines is not so efficient. Let's see if this makes sense.

A Mars lander that uses pure thrust to brake and land uses a huge amount of fuel. It also requires a powerful engine to kill off the speed. It has to continue burning and providing thrust in excess of the weight of the lander right to touchdown.

A Mars lander with wings doesn't need any fuel at all. It enters the atmosphere at high speed and maximum drag to bleed off energy. When enough energy is dissipated, the wings deploy and it uses the energy in it's altitude to maintain flight speed, while maintaining a reasonable rate of descent. In fact, the best rate of descent is a very high angle of attack that produces very little lift but is still stable in pitch, roll, and yaw - very near a stall, in other words. Simple spoilers can provide this. You get a high rate of descent with very little increase in forward speed. Diving the plane will increase the rate of descent but also increase the speed over the ground and push it toward a realm of instability. Or it will porpoise - alternately diving and swooping up kind of like balsam airplane toys do with they're out of trim.

It's very quiet as opposed to the rocket-assisted landing. When the last of the energy is used up it is at the surface and it can optionally deploy a 'chute for the last mile, or skid to a stop. It is probably a bad idea to skid to a stop on a surface as battered and rocky as that of Mars, so the 'chute will probably be used.

aostling
06-05-2009, 01:01 AM
I think it's time for NASA to weigh in on this argument. From http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html


The proponents of the arguments usually fall into two camps: (1) those who support the "Bernoulli" position that lift is generated by a pressure difference across the wing, and (2) those who support the "Newton" position that lift is the reaction force on a body caused by deflecting a flow of gas.


After explaining these two positions further, it is stated that


So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force.

Both positions are right, unless you deny the validity of the opposing explanation.

J Tiers
06-05-2009, 01:05 AM
I think it's time for NASA to weigh in on this argument. From http://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html


The proponents of the arguments usually fall into two camps: (1) those who support the "Bernoulli" position that lift is generated by a pressure difference across the wing, and (2) those who support the "Newton" position that lift is the reaction force on a body caused by deflecting a flow of gas.


After explaining these two positions further, it is stated that


So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force.

Both positions are right, unless you deny the validity of the opposing explanation.

Exactly....... you can view it either way you want. Just what I have been saying.

A.K. Boomer
06-05-2009, 01:38 AM
What exactly is the conceptual difficulty here ? How do you imagine the turning of the flow produces a force on the solid body other than by creating a pressure differential ????

.


Let it go Barrington -- he'll never get it, someone asks a simple question and the next thing you know he's replying to your above question by voiding out the typical airplane wing and stating;

"You can build a device that flies by accelerating air using an electrical field. By ionizing the air molecules they may be attracted to a charged grid and energy of motion is imparted to the air by electrostatic attraction. The air does not interact with any surface and there is no pressure differential because of that."

Such is the nature of "squirming" (Evan at his best):rolleyes:

And by the Way Ev -- if your somehow creating an opposing effect (downforce of air) without having to have the effect to set it off with in the first place (the "wing") Youv not only discovered a brand spanking new perpetual motion machine -- your also in violation of one of the old classic Newtonian laws of physics - Where in the hell do you think the downforce of air is coming from? (duh) do you think its coming from a pressure neutral situation??? :o --- bottom line pal, If all pressures remain identical on both sides of the wing your going absolutely nowhere (except for taxying) -- now what part of that do you not understand? Now stop the madness and get some kind of a clue.... Really.

dp
06-05-2009, 01:40 AM
There's definitely a pressure difference across the two surfaces of a wing - it's just nowhere near what many people think it is. What is very different is the velocity of air above and below the wing, and what happens to those molecules when they reach the rear edge of the wing.

There is an explanation that shows two molecules parting at the leading edge of the wing. In wing theory they meet again at the rear of the wing, but the one that goes over the top, having farther to go, has to go faster in order to meet it's partner at the trailing edge. And it is a fact that air over the top surface is moving faster than air moving past the lower surface. And we're clever enough to isolate eddy and laminar flow - we're talking about free molecules here.

Anyway, the higher velocity air on the top also has a direction it's going (its vector) - to the rear, and downward. That is what provides the downward thrust in the case of a wing, and the rearward thrust in the case of a propeller.

Bernoulie's principle provides the speed increase over the top of the wing (which is a cross section of a portion of a Venturi tube). Venturi comes in to play as well though it is a bit more abstract because all of Venturi's dealings were within cylinders of various profiles and deals with air pressure gradients along a curved surface entirely within the structure. There is no outer surface in the Venturi/Bernouli equation as there is with a wing/propeller.

Bruce Griffing
06-05-2009, 01:45 AM
No matter who's principle you believe in, reaction force or not - the outcome is pressure!! The reaction force of a bunch of gas molecules on any surface is called pressure. There can be no force of reaction without some connection between the gas streams and the wing surface. That connection is called pressure.

Evan
06-05-2009, 01:47 AM
While it is true that a wing "throws air downward" creating a reaction force, there is no mechanism for force transfer to the wing other than a pressure differential. If you believe there is another mechanism - what is it?


It is via momentum transfer from the molecules that are being accelerated in a particular direction consistently. That is also consistent with the definition of pressure. However, it makes no difference whether there is a differential between the top and the bottom of the wing since the air over the top is also being accelerated downward to an even greater degree than the air below the wing. It is not the difference in pressure that provides the lift, it is the acceleration of the air and the momentum that is exchanged as a result.

The air that flows over the top of the wing follows the surface of the wing because of the downward pressure of the air above it. It is the air that flows over the top of the wing that provides the most lift because it leaves the wing trailing edge with a greater downward velocity vector than the air that follows the bottom of the wing. The transfer of momentum on the top occurs because of reduced pressure on the top side which is a consequence of the increase in velocity. This is independent of what happens on the bottom.

Pressure is responsible for causing the air to change direction but it isn't a differential in pressure between the top and bottom that causes lift. Each side of the wing produces lift on it's own regardless of what happens on the other side.

To claim that a differential in pressure is responsible for the lift is equivalent to saying that the top of the wing makes no contribution to lift.

Bruce Griffing
06-05-2009, 01:54 AM
Evan-
Just how does that momentum transfer happen?? Are you suggesting that moving air transfers momentum to the wing without contact?

Evan
06-05-2009, 02:14 AM
I just explained that.

dp
06-05-2009, 02:19 AM
To claim that a differential in pressure is responsible for the lift is equivalent to saying that the top of the wing makes no contribution to lift.

It also requires a skyhook to attach the low pressure to and that skyhook exists only in our imagination. It is as we've described.

It is also useful to recall that the air molecules had no important velocity at all relative to their molecule buddies until that damn wing came along and nudged them. In other words they were stable - having no energy differential relative to one another. Once displaced by the wing they immediately went about restoring their energy state relative to their molecule buddies to zero. They had no energy to contribute to the process of holding up that wing. The energy that went into displacing and accelerating them is what we call drag.

Bruce Griffing
06-05-2009, 02:29 AM
Evan-
You did not explain how force is transmitted to the wing from air. My position is simple. Force is transmitted to the wing from air by pressure. What exactly is your position on that point?

OldRedFord
06-05-2009, 02:51 AM
As for the very lame "what if the runway has a conveyor....." question...

What if the plane has widely spaced wheels that straddle the conveyor belt so that no part of the aircraft is actually touching the belt. For extra points explain how this differs from having the wheels on the belt. For double credit explain how it differs from projecting an image of a moving belt on the runway. And, for an automatic passing grade explain how the plane can take off if we paint the words STOP on the runway.


Its still going to fly.

So you think the whole belt thing was faked?

philbur
06-05-2009, 04:11 AM
Differential pressure by it's definition is dictated by both the top and bottom pressures.

Pbottom - Ptop = Pdifferential

The lower the pressure on the top of the wing the greater the differential. Your statement seems to show a very simple and fundimental missunderstanding.

Phil:)


To claim that a differential in pressure is responsible for the lift is equivalent to saying that the top of the wing makes no contribution to lift.

boslab
06-05-2009, 05:08 AM
That can't be determined by deriving the "answer" by doing the math based on surface area and weight. The Bernoulli principle supplies only a relatively small proportion of the lift that an aircraft wing produces.

The majority of the lift is produced simply by accelerating the air to flow downward. This holds for every type of airfoil but is especially true for fully symmetrical shapes and those that come close to approximating a flat plate such as the F-101. All aircraft wings while in a normal flight attitude produce a downwash from the trailing edge of the wing. It is this downwash and the accelerated mass of the air it contains that produces the equal and opposite reaction called lift.
if only some of the lift comes from Bernaulis eqn, where does the rest come from? are the laws of physics that have a different name? i'm confused some explanation would help
mark

philbur
06-05-2009, 06:23 AM
I think you may find that helicopter blades, jet engines and rocket engines all ultimately rely on the differential pressure of a gas for their motive force. How you calculated that motive force may not require you to get directly involved in calculation pressures but the bottom line is still difference in pressure. How else does a gas apply a force to a solid body.

Phil:)

Circlip
06-05-2009, 06:54 AM
Many years ago, as a kid I went on a gliding course at the RAF No 2 Gliding centre.(Kirton Lindsey at that time Norm) The last day the ground based towing equipment dragged us up to 1000Ft and release was effected by the auto mech. only, over the TOP of the winch. Our "Flights" lasted a short time and consisted of keeping the glider facing the wind, straight and level, indicated airspeed was 45Kts. and we were being pushed BACKWARDS to the edge of the airfield and at the point of being on the boundary, landing straight forward. Although the indicated airspeed was less than that required to overcome the sink ratio, it wouldn't have taken much to attain positive lift by adjusting the attitude of the aircraft.

Hmmm, still milking the downward deflection argument?? That TOTALLY buggers the reflex sections used on tailless concoctions then.

Jump jets?? Prototype was nicknamed the flying bedstead and consisted of two damn great Rolls Royce jet engines bolted into a tubular steel frame with some poor sod sat on top in the middle of the nightmare. NO wings, just pure down thrust, directional control like the Harriers, snifter jets.

Regards Ian.

Had to edit the station, sorry Norm, All zimmers (Again)

Barrington
06-05-2009, 07:41 AM
The air pressure at every point on the object exerts a force perpendicular to the surface. The vector sum of all these forces is the aerodynamic force on the object.

A discussion on the relative contribution of 'Bernouli' and 'Newtonian' mechanisms to these pressure differentials does not alter this simple fact.

In an aircraft with no 'electrostatic' lift device :rolleyes: these pressure differentials produce the only relevant lift mechanism.


http://i564.photobucket.com/albums/ss82/MrBarrington/Image1.jpg

The air mass is turned by forces acting upon it. Momentum is transferred by forces acting upon it. These forces result in pressure changes in the air mass both below and above the wing. The aerodynamic force is the result of these pressure changes.

Evan - when you're in a hole - stop digging !
.

Rustybolt
06-05-2009, 08:10 AM
The air pressure at every point on the object exerts a force perpendicular to the surface. The vector sum of all these forces is the aerodynamic force on the object.

A discussion on the relative contribution of 'Bernouli' and 'Newtonian' mechanisms to these pressure differentials does not alter this simple fact.

In an aircraft with no 'electrostatic' lift device :rolleyes: these pressure differentials produce the only relevant lift mechanism.


http://i564.photobucket.com/albums/ss82/MrBarrington/Image1.jpg

The air mass is turned by forces acting upon it. Momentum is transferred by forces acting upon it. These forces result in pressure changes in the air mass both below and above the wing. The aerodynamic force is the result of these pressure changes.

Evan - when you're in a hole - stop digging !
.



Same with a sailboat, right? I asked that because, the way I see it, there is really nothing for the wind to deflect against in order to create forward motion. So It has to be differential pressure, right?

OldRedFord
06-05-2009, 08:21 AM
Same with a sailboat, right? I asked that because, the way I see it, there is really nothing for the wind to deflect against in order to create forward motion. So It has to be differential pressure, right?

I think the wind is deflecting on the fabric of the sail. Hold a piece of paper in front of a fan with both hands.

Evan
06-05-2009, 08:52 AM
Evan - when you're in a hole - stop digging !
.


I already explained that pressure is the means by which force is transmitted to the wing by the air and vice versa. It is not the pressure differential between the top and the bottom of the wing that is responsible for lift. Both the top and the bottom of the wing create lift independently of each other. You can subtract each from the actual operation of a wing by spoiling the airflow on either side and see the difference. Invoking a difference in pressure as the source of lift is incorrect. NASA points out that the Bernoulli equations are a convenient approximation to the truth but are not the correct explanation for the actual lift.

The air pressures on each side of the wing are different but it is not the difference that causes the lift. Each side of the wing accelerates air and it is that transfer of momentum that produces an equal and opposite reaction called lift.

I think there is a fundamental misunderstanding of the term "accelerate". In physics accelerate does not mean to go faster. It means to change momentum of mass. That can be the same as braking, or any change in direction and velocity of any mass. Any time a mass is accelerated there is an exchange of momentum. Both the top and the bottom of the wing accelerate the air as it passes over each side. Each side independently of the other exchanges momentum with the air by the action of the air pressure on that side. The sign of the pressure is opposite because sides are opposite. The sign of the momentum transfers are the same and are additive, not differential.

Evan
06-05-2009, 09:00 AM
The sail acts in exactly the same way as a wing as does the leeboard, centre board, rudder or keel as well as the hull. All are moving through a fluid and all change the momentum of that fluid which transfers that momentum to the boat. A sail does not operate by itself. It cannot propel a boat upwind without the action of parts of the boat in the water such as a keel. The sail provides a force that that points in the approximate direction of the centre of the bulge of the fabric. The keel or equivalent then acts to exchange momentum with the water to exchange that force in the opposite direction. The actual course made good is the vector sum of both forces.

I will be off line today most of the day as I have a backhoe to pick up and a ditch to dig.

philbur
06-05-2009, 09:07 AM
Evan - when you're in a hole - stop digging !
.

He's off to pick up a backhoe so I guess his plan is to keep on digging.

Phil

Evan
06-05-2009, 09:10 AM
The ditch is to help drain incoming BS from near the house.

J Tiers
06-05-2009, 09:43 AM
The ditch is to help drain incoming BS from near the house.

Or to capture escaping BS and keep it localised?

Your position on the pressure issue is logically inconsistent. Even your pet explanation at NASA has let you down.

I would advise you to do as you usually do whenever you find yourself in defense of the indefensible......... simply stop posting to the thread.

Evan
06-05-2009, 09:53 AM
I will stop posting as I must leave the computer at this very moment. That may be interpreted as an admission that I am correct and have nothing else to add. I also have work to do.

mooney1el
06-05-2009, 11:07 AM
As a pilot (hence "mooney1el"), I read these posts with great interest. It seems that most people just cannot understand the latest discoveries on the principles of flight from the 2004 published papers in the following link. All of the Bernouli, differential pressure and angle of attack theories have been put aside. Lift demons and thrust pixies are responsible for all flight :)

http://www.messybeast.com/dragonqueen/liftdemon.htm

dp
06-05-2009, 11:21 AM
As a pilot (hence "mooney1el"), I read these posts with great interest. It seems that most people just cannot understand the latest discoveries on the principles of flight from the 2004 published papers in the following link. All of the Bernouli, differential pressure and angle of attack theories have been put aside. Lift demons and thrust pixies are responsible for all flight :)

http://www.messybeast.com/dragonqueen/liftdemon.htm

Well it's back to the books for me. One never gets used to sudden and irrevocable shifts in principles of physics. Hell, I remember when Pluto was a planet... :)

Glenn Wegman
06-05-2009, 12:12 PM
Now I'm afraid to go flying after all of the above confusion over what will hold me up there!

kf2qd
06-05-2009, 12:25 PM
Lift is just a composite of vertical forces acting on an airplane of helicopter. As was mentioned very early on in the discussion here something creates lower pressure above the lifting surface and higher pressure below the lifting surface. The result is lift. using Bernoulli as an explanation worked fine - until others proposed more complex systems that they can't explain very well. If you work with the Bernoulli's ideas you can come up with a fairly simple, well behaved model that works for most of the non-extreme cases. Start playing around with supersonic and you need a new set of rules.

It is just like Newtonian Physics. Even though we now have Quantum Physics - for much of the discussion Newtonian works just fine. When you get to the special cases then Quantum Physics has to be used because Newtonian Physics has its limitations. (So does Quantum Physics...)

Much of this discussion showed how little the presenters understood of the newer ideas. For instance - how can you even have an area of high pressure under a helicopter rotor if you don't have a corresponding area of low pressure above? Which supplies the lift? (just a hint - ITS BOTH)

Some of these discussions seem to follow Einstien's adage - Genius has it limits. Ignorance doesn't.

A.K. Boomer
06-05-2009, 12:43 PM
Well it's back to the books for me. One never gets used to sudden and irrevocable shifts in principles of physics. Hell, I remember when Pluto was a planet... :)


Don't knock it the lady was from NASA -- it explains so much -

"The reason that planes stall at high alpha is that the leading edge demons get scared and let go when they can't see the ground anymore."

Even though I think the word "alpha" got by their spell check and it should have been "Altitude" I went over most of this article with a fine tooth tooth brush and for the most part it sounds legit,
Put yourself in one of the demons shoe's - what would you do? How would you handle the situation? Keep in mind its easy to answer the question in the comfort of your home instead of trying to cling to a freezing surface at some kind of mach speed. It almost sounds like these little guys are being tortured...

Bruce Griffing
06-05-2009, 03:29 PM
kf2qd-
While it is true that there are many new ideas and better understanding of the origins of lift, none of these new ideas get away from the basic idea that force is transmitted from air to the wing by pressure. The pressure balance around the wing is a basic and fundamental idea that is central to any explanation lift. The pressure balance idea does not, by itself, imply any particular explanation for origin of pressure differences.

boslab
06-05-2009, 07:33 PM
out of curiosity, what of the temperature of the upper and lower wing surface, would there not be a cooling effect [more on one surface] as one gets when a fluid rapidly expands say in a venturi [mr B's formula applies with the addition of G and Z to either side of the eqn where G=9.81 and Z=height above a datum, but they cancell out for areofoils]
I still think that the lift comes from Bernaullis and am not entirly convinced about the other Newtonian descriptions, I'm not saying that they are wrong but i am not a great fan of Mr Newton, i tend to think that he was one of the greatest [i,m going to get burned] plagerisers on record, well Mr Hookes record anyway.
an interesting thread

toastydeath
06-05-2009, 08:08 PM
Just because I can't resist getting involved in a clusterf*** thread, I have a question for all the "lift isn't generated by an acceleration of mass" people.

How do you measure the pressure of one individual particle?

philbur
06-05-2009, 09:00 PM
You measure the force applied to the surface when the particle strikes it and then you divide by your chosen units of area.

Phil:)


Just because I can't resist getting involved in a clusterf*** thread, I have a question for all the "lift isn't generated by an acceleration of mass" people.

How do you measure the pressure of one individual particle?

toastydeath
06-05-2009, 09:07 PM
You measure the force applied to the surface when the particle strikes it and then you divide by your chosen units of area.

Phil:)

What surface? Where'd you get the surface from in my question?

Edit:

Just to clarify, I'm asking about just one, lone, solitary particle. If you could look at any of the properties that particle (say, a hydrogen atom for simplicity), what would you look at to tell me what pressure it had?

philbur
06-05-2009, 09:22 PM
A simple definition of pressure:

"Pressure (symbol: p or sometimes P) is the force per unit area applied to an object in a direction perpendicular to the surface."

I think that without an object for the particle to strike the concept of pressure has no meaning.

Phil:)


What surface? Where'd you get the surface from in my question?

Edit:

Just to clarify, I'm asking about just one, lone, solitary particle. If you could look at any of the properties that particle (say, a hydrogen atom for simplicity), what would you look at to tell me what pressure it had?

toastydeath
06-05-2009, 09:45 PM
Exactly. Whereas you CAN talk about force and acceleration with a single particle, in isolation. Well, not entirely; you have to assume some other reference frame. But even then, you can discuss changes in energy and maintain the same reference frame as the particle.

To be fair, both Newton and Bernoulli deal with classical mechanics, which are obviously a simplification, an abstraction, and a statistical measure of the fundamental principles all at once.

For funsies, talking about a particle hitting a surface (which, I assume, has to be made of particles). Particles are pointlike (let's ignore wavelike things for now, since they're not terribly pertinent); to the best of our knowledge, massive particles are made up of smaller, more fundamental (or purely fundamental) particles with zero size. These things of zero size interact with each other through fields; they never come into contact. So when you have one particle "hitting" a wall of particles, they are never touching. The influence they have on each other increases with the inverse of distance by virtue of the field each projects, until the interaction becomes so great that the particles repel (assuming electrodynamics, and not say, gravity).

So there's no "area," even on a surface. There are no solid objects, just arrested particles in fields whose effects become so strong at small distances that we perceive them as solids.

So Bernoulli's principle isn't WRONG, but it ain't right either - and to say that Newton's laws aren't in effect is equally ridiculous. They're both wrong, and they're both right, in hillariously similar ways.

The only way to generate a force in the open atmosphere - where you can't maintain a static density increase - is to accelerate particles.

The aerofoil imparts energy to the particles in front of it to push them down, and that energy transfer is seen in the form of aerodynamic drag (plus whatever parasitic drag the shape has).

From there, you can look at it two ways in classical mechanics, and both produce the exact damn same answer so why is everyone arguing about it:

You can look at it as pressure. The air below the wing has a slightly higher density than the air above it because you just moved a some of the air that would have been on the top of the wing below it. So you have pressure; higher density on the bottom of the wing, lower density on top as compared to the unperturbed atmosphere.

http://www.grc.nasa.gov/WWW/K-12/airplane/presar.html

Or you can look at it as accelerated mass. The air you knocked down has accelerated, but now there's a giant column of air ABOVE the wing that is trying to accelerate downwards to fill the gap. If you take the whole mass of air that is now moving downwards, you get the same result as the pressure method. To my knowledge, classical mechanics doesn't deal with how that huge column of air above the wing took energy FROM the wing; that's why it looks a bit ridiculous to people. But then, that's why classical mechanics is wrong.

http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html

And both of these are wrong because they're classical mechanics and not quantum field theory. But they're accurate enough to use on the scale we care about, so why not? Bonus, we have computers fast enough to calculate the PDEs for classical mechanics, so we can get useful answers. We're some time away from being able to calculate the pressure on a wing by using QFT.

philbur
06-05-2009, 09:58 PM
Seems like you had a hidden agenda when you asked a simple question. If you want state of the art fundamental principles then you possibly need to throw Superstring theory in there somewhere. Newton and Bernoulli just gave us mechanics some rules of thumb to muddle along with.

Phil:)


Exactly. Whereas you CAN talk about force and acceleration with a single particle, in isolation. Well, not entirely; you have to assume some other reference frame. But even then, you can discuss changes in energy and maintain the same reference frame as the particle.

To be fair, both Newton and Bernoulli deal with classical mechanics, which are obviously a simplification, an abstraction, and a statistical measure of the fundamental principles all at once.

For funsies, talking about a particle hitting a surface (which, I assume, has to be made of particles). Particles are pointlike (let's ignore wavelike things for now, since they're not terribly pertinent); to the best of our knowledge, massive particles are made up of smaller, more fundamental (or purely fundamental) particles with zero size. These things of zero size interact with each other through fields; they never come into contact. So when you have one particle "hitting" a wall of particles, they are never touching. The influence they have on each other increases with the inverse of distance by virtue of the field each projects, until the interaction becomes so great that the particles repel (assuming electrodynamics, and not say, gravity).

So there's no "area," even on a surface. There are no solid objects, just arrested particles in fields whose effects become so strong at small distances that we perceive them as solids.

So Bernoulli's principle isn't WRONG, but it ain't right either - and to say that Newton's laws aren't in effect is equally ridiculous. They're both wrong, and they're both right, in hillariously similar ways.

The only way to generate a force in the open atmosphere - where you can't maintain a static density increase - is to accelerate particles.

The aerofoil imparts energy to the particles in front of it to push them down, and that energy transfer is seen in the form of aerodynamic drag (plus whatever parasitic drag the shape has).

From there, you can look at it two ways in classical mechanics, and both produce the exact damn same answer so why is everyone arguing about it:

You can look at it as pressure. The air below the wing has a slightly higher density than the air above it because you just moved a some of the air that would have been on the top of the wing below it. So you have pressure; higher density on the bottom of the wing, lower density on top as compared to the unperturbed atmosphere.

http://www.grc.nasa.gov/WWW/K-12/airplane/presar.html

Or you can look at it as accelerated mass. The air you knocked down has accelerated, but now there's a giant column of air ABOVE the wing that is trying to accelerate downwards to fill the gap. If you take the whole mass of air that is now moving downwards, you get the same result as the pressure method. To my knowledge, classical mechanics doesn't deal with how that huge column of air above the wing took energy FROM the wing; that's why it looks a bit ridiculous to people. But then, that's why classical mechanics is wrong.

http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html

And both of these are wrong because they're classical mechanics and not quantum field theory. But they're accurate enough to use on the scale we care about, so why not? Bonus, we have computers fast enough to calculate the PDEs for classical mechanics, so we can get useful answers. We're some time away from being able to calculate the pressure on a wing by using QFT.

toastydeath
06-05-2009, 10:05 PM
I apologize for misleading you; I thought the tone of the question was sufficient to indicate my stance. If I mislead, that was not my intent.

Additionally, no, M-theory (the popular branch of string theory) does not need to be included as nobody has preformed any experiments to validate the predictions of that situation. It provides an easier mathematical framework to solve certain quantum interactions, but nobody has made any novel predictions using M-theory.

barts
06-05-2009, 10:14 PM
The air pressures on each side of the wing are different but it is not the difference that causes the lift. Each side of the wing accelerates air and it is that transfer of momentum that produces an equal and opposite reaction called lift.


Draw a free-body diagram of the airplane in level flight. The only force acting on the plane are air pressure, gravity and engine thrust. If you integrate over the surface of the plane, you'll get the net aerodynamic loads on the plane; the horizontal drag component is offset by the engine thrust, gravity is offset by the lift generated by the wings - which is responsible for the pressure difference between the top of the wing and the bottom.

Air cannot act on the plane except via a pressure differential across the surface, since if the pressure is the same, the surface integral will always sum to zero, regardless of the complexities of the shape of the aircraft - otherwise objects that were aircraft shaped would start moving w/o any engine or gravitational effects.

- Bart

Evan
06-05-2009, 10:23 PM
From there, you can look at it two ways in classical mechanics, and both produce the exact damn same answer so why is everyone arguing about it:


They don't produce the same answer. Bernoulli's equations are a very useful approximation but they are an approximation. One of the major flaws limits their use in the standard form to velocities below about 300 knots. That is because they treat the air as an incompressible fluid which exhibits NO density changes with changes in flow. The differences are fairly minor at low velocities but they aren't all that small in total. It's the reason that we still use wind tunnels to quantify the performance of air foils. Empirical testing is still the only way to fully qualify the calculated performance of a wing.

Incidentally, the pressure distribution on a standard Clark Y airfoil should produce a significant thrust in the direction of motion. There is a huge low pressure area in front of the leading edge according to the flow velocity equations. It's a good thing that induced drag cancels that result or airplanes wouldn't need engines.

Evan
06-05-2009, 10:25 PM
Draw a free-body diagram of the airplane in level flight

The standard diagram does not consider pressure but lift instead. Lift, gravity, thrust and drag are the factors.

barts
06-05-2009, 11:20 PM
The standard diagram does not consider pressure but lift instead. Lift, gravity, thrust and drag are the factors.

Lift is the vertical component of the surface integral of pressure, and drag is the horizontal component.

- Bart

Evan
06-05-2009, 11:30 PM
Lift is the vertical component of the surface integral of pressure...

That is one way of calculating an approximation.

dp
06-05-2009, 11:32 PM
Why is it most people can easily understand how a propeller works but can't fathom how a wing works? It's the exact same process. The wing and all attached parts are lifted because air is directed downward. Classic Newtonian reaction.

Airplanes cannot be held up in the sky by low pressure above the wing because that low pressure hasn't anything to hold it up. Airplanes are not buoyant. We know that air is deflected downward by a wing because that same air moves very quickly back where it was.

http://www.johnnybaas.nl/zeilvliegen/theory/images/Image17.gif

http://www.aerospaceweb.org/question/aerodynamics/vortex/trailing-vortex.jpg

J Tiers
06-05-2009, 11:37 PM
From there, you can look at it two ways in classical mechanics, and both produce the exact damn same answer so why is everyone arguing about it:


Which I have said at least twice. A NASA explanation has also been quoted which apparently says the same thing.

BTW... "pushing particles down", or in any other direction, involves "pressure".

Earlier Evan was in the same boat as you, trying to say that one particle, or a very small number of particles, has no pressure.......

The evident answer is that it has a *very low* pressure..... might as well be zero. But an enclosed space with one free gas particle in it has a pressure.

Suppose we said it did not........ then when do you actually GET a "pressure"? Does it take 10 particles? 100? one mole?

Once you agree that a cubic meter of air at sea level conditions has a definable pressure (a fact I doubt you would deny), then you are stuck with agreeing that one single particle in a huge space ALSo has a pressure...... Otherwise you have a logical inconsistency.


Airplanes cannot be held up in the sky by low pressure above the wing because that low pressure hasn't anything to hold it up. Airplanes are not buoyant. We know that air is deflected downward by a wing because that same air moves very quickly back where it was.

Then you go ahead and explain how the air in the tip vortex flows UP unless there is a low pressure above the wing...............it should be flowing downward only.

Gas flows from high pressure to low..... The reason you CAN deflect air downwards is that you artificially create a higher pressure with the effective angle of attack, and the gas flows away from it.

The clear and obvious answer is further above, I said it, Toasty said it, others said it, and even NASA agrees....... choose the important variable, and then you get your answer, if you like velocity, that will explain it. Choose pressure, that will also.

Just don't choose one and then DENY the other..... they are completely interconnected.

BTW, "that same air" does not "move quickly back where it was"........ it is deflected down, and OTHER air "particles" replace it......It isn't as if there is a vacuum created, which is then re-filled by the very same air that was driven away previously.

dp
06-06-2009, 12:01 AM
Air pressure is the measurable consequence of molecules banging against an immovable object.

Place a sensitive sensor on the lid of a jar. At sea level, wind the lid onto the jar. Measure the pressure in the jar. Because it is the same as the pressure outside the jar it has a net pressure of zero. Now put that jar in a vacuum chamber and remove all the air. It now has a net positive pressure of one atmosphere relative to a vacuum.

Now lets measure the jar pressure at sea level again. Zero relative to the ambient air. Put the jar in a pot of boiling water. The air heats up, the molecules move faster, and they hit the sensor harder. Still the same number of molecules but they're at a higher energy level, hence, a higher pressure relative to sea level.

I've already said this but it needs saying again, I can tell. The air was stationary before being parted by the wing. The air and wing can't be in the same place at the same time, something has to give and it is the air. The wing, having parted the molecules, moves on. The displaced molecules need desperately to get back where they were and at the same energy level they were at.

The shape of the wing forces the molecules on the top of the wing to arch up and over the thickest part of the wing and then allows them to move quickly back to the point they started from.

Molecules on the bottom are not so advantaged and they are stuck at the offset position until the rear edge of the wing has freed them. But by then the molecules on the top of the wing are accelerating back to their starting point at ever higher speed and when the trailing edge of the wing goes by they keep going because of inertia.

But the disturbing force is gone so the air, initially in a laminar downward flow, breaks up and rattles around against other air until it finally comes back to a rest. The energy put into the air by the wing, just like stretching a rubber band, has created a downward force, and like a rubber band that is stretched and released, the energy level will quickly return to zero and all will be stable again. In the process it has created a reactive force that when analyzed looks just like the lift vector diagrams seen in this thread.

toastydeath
06-06-2009, 12:05 AM
J Tiers, I agree with a lot of what you've said. I haven't read the whole thread (it's friggin' huge and there's a lot of really bad arguments), so I apologize for any repetition, omission, or lack of acknowledgment of prior, better arguments.

The one point Evan makes I think I agree with is the whole thing that Bernoulli breaks down once the flow compression becomes substantial. Naver-Stokes/whatever other PDE you want to use that handles compressible flow might be a better thing to discuss. As I don't know much about it, I'm not going to guess which is appropriate. The issue I agree with is that if you have a macroscopic force, and you have a macroscopic area, you are going to be able to represent the phenomenon as a pressure. If Bernoulli breaks down, then go to another, more advanced pressure model that can handle higher mach numbers.

Pressure:

The one thing I disagree with is that pressure does not exist, period. Not "A little pressure" or "nonzero pressure." Or if it does, it exists in the same way "capitalism" or "communism" exist: as a useful concept or framework for thought, but never actually translated into the real world.

Pressure is statistical physics - just like temperature. Individual particles do not have a pressure. We use the concept of pressure because the average action of billions and billions of interactions between particles can be very well represented by statistical methods.

In your example, the cubic meter of air, I deny it actually has a pressure. It has an enormous number of particles, each affecting the electrodynamic field around it, while being affected by the same. The number of them is so large that we cannot hope to model the interactions anytime in the near future, so we have to resort to classical mechanics with such barbaric notions as pressure to hope to guess what will happen to our mechanical systems in that environment.

I think that our volume of air has an average particle density, and an average particle energy. Each particle is going to be different, but when we look at the averages, we can get the useful but fictitious measure we call pressure.

Evan
06-06-2009, 12:06 AM
Once you agree that a cubic meter of air at sea level conditions has a definable pressure (a fact I doubt you would deny), then you are stuck with agreeing that one single particle in a huge space ALSo has a pressure...... Otherwise you have a logical inconsistency.


No, what you get is quantum mechanics instead of Newtonian mechanics. Quantum mechanics is chock full of counter intuitive "logical inconsistencies". "Pressure" is the force exhibited by an aggregate of a large number of atoms or molecules interacting with an equally large number of the same. If you try to take it down to the level of individual particles the values become absurd and the concept of pressure become entirely invalid.

Quantum mechanics does not allow for averages when dealing with individual particles. You cannot know both the position and the velocity of a particle and so you cannot calculate a value of force that it would have in interacting directly in a transfer of kinetic energy. You can only give a range of possible values based on the probabilities of an interaction. That range includes both negative and positive values. You do not and cannot know what it will be.

J Tiers
06-06-2009, 12:41 AM
For a massive particle like a gas molecule, you can come damn close....... because the interaction necessary to see it has not the same effect it would on a very light "particle".

And with a bus, your knowledge is only "theoretically" incomplete. As a practical matter you know where it is and where it's going .

But that isn't the point

The point, without complications, is that a single particle with some velocity, will impact the sides of the vessel from taime to time, and produce the same effect that we call "pressure" when a lot of them do it.

We'll use the common scientific and engineering meanings, please, they are sufficient for our purposes here.

So I ask again..... how many particles must we put in before you will agree to call it a "pressure"? If you don't like one, how about ten thousand? perhaps one mole?

If that one mole is lacking one particle of teh full count, does that push it to the point there is no longer a "pressure" in the common scientific and engineering meaning of the word?

I am confident that you see the point. You must deny the existence of "pressure" altogether, or admit it for any quantity of particles per unit volume.

dp
06-06-2009, 01:02 AM
I am confident that you see the point. You must deny the existence of "pressure" altogether, or admit it for any quantity of particles per unit volume.

This clarifies the line between theoretical and practical science. A single atom of oxygen in an infinite container has no important characteristics applicable in a discussion of airfoils at normal flight altitudes on the third stone from the sun.

But let us continue this discussion of wings and air. Suppose you have created a horizontal vortex 50 meters in diameter and 10 miles long. You did this with your Acme Vortex Generator, a prominent advertiser on the PM bbs. Now you fly a large airplane along this vortex with one wing dipped into the vortex at the bottom of the diameter so that it has 95% of the low-pressure vortex above the wing and spans the length of it. It is a very low pressure core in the vortex because Acme sells nothing but the best. Does that low pressure acting on the top of the wing generate lift (and that is a lot of low pressure), sucking the wing up, or does the lift go to hell causing the wing to fall out the bottom of it? No peeking.

andy_b
06-06-2009, 01:14 AM
OK Andy.... but how do I get the clock on My VCR to stop flashing 12:00 ?

Steve:D

unplug it. :)

andy b.

A.K. Boomer
06-06-2009, 01:35 AM
Why is it most people can easily understand how a propeller works but can't fathom how a wing works? It's the exact same process. The wing and all attached parts are lifted because air is directed downward. Classic Newtonian reaction.

Airplanes cannot be held up in the sky by low pressure above the wing because that low pressure hasn't anything to hold it up. Airplanes are not buoyant. We know that air is deflected downward by a wing because that same air moves very quickly back where it was.

http://www.johnnybaas.nl/zeilvliegen/theory/images/Image17.gif

http://www.aerospaceweb.org/question/aerodynamics/vortex/trailing-vortex.jpg

DP, its more complex than that - esp. when you use a perfectly trimmed wing as an example - by perfectly trimmed I mean that the bottom of the wing is directly aligned with the air flow -- the rest is up to the top of the wing and Yes it does indeed hold the plane in the sky (if design and speed allows) by means of a low pressure area in the last 2/3rds of the top wing structure,
You say the low pressure does not have anything to "hold against" but in fact it does in the form of resistance as it takes energies to separate it from the ambient -- keep in mind its a time frame thing - it has to get it done in a set time or there will be no effect --- after the air has been utilized for this purpose and then meets the underlying ambient air there is an abrupt filling of the void from the top air that "was" thin --- This has mass - what this does is give you your final Newtonian reaction that pushes air downward -- but the bottom of the wing has nothing to do with it...

Its classic newton "cause and effect" Times 2.

dp
06-06-2009, 01:59 AM
Time to introduce sails on boats. Do you know that your pressure view of things is demonstrated with stark clarity at any regatta?

Points of sail:

In irons - nose into the wind, sails luffing, the boat is actually moving backward.

Tight reach - boat is advancing against the wind at an angle. Relative wind speed is the vector sum of the boat speed and wind speed. There is a best angle of reach against the wind. This is not the best speed the hull makes through the water but is the best speed the boat makes through the air. You can actually go faster than the wind.

Broad reach - appx 90' to the wind. The best speed through the water is in this region. Again, you can outpace the wind.

Before the wind - the wind is behind the boat, at an angle. Not a good point of sail, there are no airfoil generated efficiencies. You are a hole in the water with large bags catching air. Your passengers are begging to head to shore and many are going to lose their lunch. The wind is passing you by.

Downwind - your sails are configured wing and wing and if you have a spinnaker it is out and full. You have trailing seas and can feel the boat surge as waves come up from behind, lift the stern, and move on. It is a roller coaster ride. With a stiff breeze the boat will have the bone in her teeth while running down the swells and then she will nearly stop, petulantly, and then surge on again. You have no prayer of keeping up with the wind.

You are now being propelled entirely by pressure differential - there are no useful aerodynamics involved in this point of sail. This is not how airplane wings work.

J Tiers
06-06-2009, 02:11 AM
This clarifies the line between theoretical and practical science. A single atom of oxygen in an infinite container has no important characteristics applicable in a discussion of airfoils at normal flight altitudes on the third stone from the sun.

But let us continue this discussion of wings and air. Suppose you have created a horizontal vortex 50 meters in diameter and 10 miles long. You did this with your Acme Vortex Generator, a prominent advertiser on the PM bbs. Now you fly a large airplane along this vortex with one wing dipped into the vortex at the bottom of the diameter so that it has 95% of the low-pressure vortex above the wing and spans the length of it. It is a very low pressure core in the vortex because Acme sells nothing but the best. Does that low pressure acting on the top of the wing generate lift (and that is a lot of low pressure), sucking the wing up, or does the lift go to hell causing the wing to fall out the bottom of it? No peeking.

Yep. theoretical, sort-of, not very rigorous......

but a vortex made by the acme machine is now moving gas, with energy content. if you put an object in it, the kinetic energy will be transferred to the object in a way we would describe as 'higher PRESSURE" on one side of it, and it will be moved away from that pressure....... the effect depends on how the aircraft hits the vortex....... but it could be "interesting"......

One factor folks seem to be forgetting is that "pressure" is a difference of pressure, so LESS on one side is equal to MORE on the other..... same-same.

Expanding on what someone pointed out, if there is no higher pressure on one side of the wing vs the other, we can remove the wings and the aircraft will still fly, because the gas (air) is not affecting the wing at all. I would not recommend trying that in mid-air...... it has been tried many thousands of times, and it has yet to work;) .

Please see and understand that the downward velocity imparted to the air is imparted via pressure, and that pressure IS the opposite and equal reaction on the aircraft demanded by classical physics.

dp
06-06-2009, 02:34 AM
Expanding on what someone pointed out, if there is no higher pressure on one side of the wing vs the other, we can remove the wings and the aircraft will still fly, because the gas (air) is not affecting the wing at all. I would not recommend trying that in mid-air...... it has been tried many thousands of times, and it has yet to work;) .

Please see and understand that the downward velocity imparted to the air is imparted via pressure, and that pressure IS the opposite and equal reaction on the aircraft demanded by classical physics.

Who ever said that - it's wrong vis a vis removing the wings. The very act of stuffing a wing through the air is what imparts energy into the air that is returned as thrust. Removing the wings removes the mechanism by which energy is imparted to the air. Seriously, I'm trying hard to not laugh.

The idea of pressure in your second statement is coming closer to the reality. Energy is imparted to the air around the wing and at the first opportunity it rebounds as free particles will, and that rebound is asymmetrical where wings are involved because wings are carefully asymmetrical by design. The rebounding air molecules are heavily involved in the thrust.

Evan
06-06-2009, 06:55 AM
For a massive particle like a gas molecule, you can come damn close....... because the interaction necessary to see it has not the same effect it would on a very light "particle".

Atoms are quite small enough to be subject to the laws of quantum mechanics along with the particles from which they are made. Atoms exhibit the same diffraction effects in the classic two slit experiment that photons exhibit. The concept of pressure has no meaning at the level of an individual atom.

Circlip
06-06-2009, 07:00 AM
Why is it most people can easily understand how a propeller works but can't fathom how a wing works?

Think I need a refresher on this one please.

Sorry, the propeller bit.

Regards Ian

John Stevenson
06-06-2009, 09:04 AM
Think I need a refresher on this one please.

Sorry, the propeller bit.

Regards Ian

Usually fits on the front blunt bit of the wing, clear enough ?

Greg Q
06-06-2009, 09:18 AM
The part that I like about all of this is the old texts all had the Bernoulli explanation as the sole reason. Every thing printed in the fifties-seventies was the same, and they used those explanations for both sub, trans and super-sonic flight.

Here's the kicker: Every plane I've ever flown was designed in that period or earlier. The 747 is still the world's best high sub sonic civil jet ever made. Beats me how those things can still show their face at the airport knowing there wings were built on the wrong math.

Also, as a trivial aside about wing alpha: In cruise the wing is not at zero pitch. Don't forget that most planes have some positive angle of incidence. ie, the wing is mounted slightly nose-up. With level pitch there is still some positive pitch, and alpha (angle of attack) on the wing.

In a 737 for example, a typical cruise nose-up attitude might be 3 degrees + a 2 degree angle of incidence giving a five degree angle of attack. More at slower speeds typical of a holding pattern.

Aside number two: wind tunnels are not used in wing design to nearly the degree that they used to be. Now computational fluid dynamics has provided the profiles for the A-380 and the 787. It appears that this is not an entirely mature technology however, as the predicted performance has not been fully met on the A-380 (for example)

Aside number three: Someone asked about upper wing temperatures: At jet cruise speeds there is a temperature rise at the leading edge of 30 C or so because of the dynamic pressure rise. The lowest pressure aft of the wing is sen at approx 1/4 span, and here the temperature drop is most pronounced. We have all seen images of fighters in high-g situations where the water vapour is condensing above the wing due to the pressure/temp drop. The same is visible on landing airliners on humid days. I'm guessing that the magnitude is 3 or 4 degrees C drop below ambient because you typically see the condensation in conditions when the temperature and dew point are within a couple or three degrees.

Greg

Circlip
06-06-2009, 09:24 AM
Very droll Sir John, Very droll. :rolleyes:

J Tiers
06-06-2009, 10:55 AM
Who ever said that - it's wrong vis a vis removing the wings. The very act of stuffing a wing through the air is what imparts energy into the air that is returned as thrust. Removing the wings removes the mechanism by which energy is imparted to the air. Seriously, I'm trying hard to not laugh.

The idea of pressure in your second statement is coming closer to the reality. Energy is imparted to the air around the wing and at the first opportunity it rebounds as free particles will, and that rebound is asymmetrical where wings are involved because wings are carefully asymmetrical by design. The rebounding air molecules are heavily involved in the thrust.

Of course it is wrong...... it is a reductio ad absurdum argument.... :rolleyes: the wings are obviously essential, except of course in the case of a "lifting body", which IS the wing......

BTW, a wing need not be asymmetrical. They will produce lift even if perfectly symmetrical. or flat. That will limit the useful angle of attack possible.

The asymmetry has much more to do with avoiding a 'stall" or the formation of large volume turbulence over the top of the wing, which kills the lift and drastically increases the drag.

The point is that if there WERE no pressure difference, there would be no lifting force transmitted from the gas to the wing, and they would be non-essential., and could be removed, which is, as you seem to have noticed, naturally ridiculous.

In actual fact, the forcing of the wing through the gas produces a pressure under the wing, which pushes the wing UP, and the gas DOWN.

In the case of an updraft, as your vortex, the kinetic energy in the flow which hits the wing is converted to pressure under the wing and forces it up, as any airline passenger has felt.

Downdrafts same in reverse.

Remarkably similar to normal wing action........

Pressure below, redirection of gas molecules, both arguments are the same.

Evan
06-06-2009, 11:11 AM
Here's the kicker: Every plane I've ever flown was designed in that period or earlier. The 747 is still the world's best high sub sonic civil jet ever made. Beats me how those things can still show their face at the airport knowing there wings were built on the wrong math.


It's why they build wind tunnels large enough to hold entire aircraft. It's also why there are so many dead test pilots. It's also why there are aircraft that STILL have bits that fall off.

Greg Q
06-06-2009, 11:18 AM
Oh Evan really. They never had a 747 sized wind tunnel. Geez, it would have taken 200,000 hp to drive the fan. My point is that all of these approximations are approximately representative of the real world. The wing can't do math after all-yet they fly mostly as predicted by the old formulae.

There haven't been too many fatalities in civil transport testing since the last Canadair accident back when the Challenger was doing its stall testing-even that was an unfortunate parachute snagging the tail and the poor guy rode it down.

As far as things falling off...almost always bad or incomplete maintenance. Apart from DC-10 cargo doors I can't think of anything which falls off with regularity. Oh, blue ice of course.

Evan
06-06-2009, 11:24 AM
Oh Evan really. They never had a 747 sized wind tunnel

I didn't say that, did I? They use scale models if the real article won't fit. The math isn't good enough to predict what will actually happen. It hasn't been possible up until very recently to accurately model the airflow over a wing and airframe. Even now the models are still subject to errors and require actual physical testing. Back when the 747 was designed they were losing test pilots at Edwards and Dryden at the rate of 2 per week, frequently because the aircraft would not fly.



Apart from DC-10 cargo doors I can't think of anything which falls off with regularity

The rudders on Airbus aircraft. NOT a maintenance issue either.

Greg Q
06-06-2009, 11:50 AM
It's why they build wind tunnels large enough to hold entire aircraft. .

Maybe its just me, but we were speaking within the context of the 747. You can see my confusion.

As to Edwards test pilots back in the sixties: Yes, it was a bloody time. But lots of that was failed weapons testing, just plane goofy **** like the VTOL aircraft, the X series planes in the 50's and 60's, the late century series aircraft (F-105 and on. Remember the 107? There was a high speed coffin. How about the XB-70? That one killed three guys in one go. (collision with F-104 chase plane). But all of that was envelope-pushing expediency at the height of the cold war. The civilian stuff was a walk in the park in comparison.

Airbus rudders....hmm, forgot about that one. The JFK accident though was attributed to the F/O inputing several rudder full deflections and reversals, a condition akin to cracking the whip-not survivable in anything. He flew Pitts Specials a lot, an airplane that wants rudder hard and often. Now that I think of it, Boeing had a few problems with pylon fuse pins too some time ago. Rudders notwithstanding, most of it was still bad maintenance or corrosion*

*Aloha airlines 737 cabriolet and many others

jkilroy
06-06-2009, 12:20 PM
"Aloha airlines 737 cabriolet"

Just about launched my breakfast with that, ROTHLMAO! :D

Evan
06-06-2009, 02:34 PM
Aside from the JFK accident there have been several incidents of disappearing rudders on Airbus aircraft.

I believe the first was a flight from Cuba to Canada or some such. The rudder was found to be missing when they landed. The flight control system simply adjusted the flight rules to make do.

http://ixian.ca/pics6/rudder.jpg


Airbus has a poor safety record. They have lost 1400 passengers in 12 years. The main problem seem to be that the engineers don't have a complete understanding of the life cycle and failure modes of composite materials. Composite materials have a nasty habit of failing catastrophically and without visible warning. There is also speculation that the problem is aerodynamic and that flutter is to blame.

Greg Q
06-06-2009, 09:12 PM
Somehow that escaped my attention, living as I do in the Boeing world. At least the airline pictured adheres to truth-in-advertising laws...their Airbus has a see-through fin logo!

That does not look good, does it?

In the old days control surface separation was often triggered by dynamic unporting and flutter.

One other comment about that airplane pictured: The Airbus aircraft do not have rudder input except in an assymetric thrust situation. It's entirely possible that the autopilot never missed the rudder, although the yaw damper actuators must have ben going crazy trying to offset the minor fish-tailing (to obviously no avail)

Thruthefence
06-06-2009, 10:19 PM
At 9:50 AM we have:
"Airbus rudders....hmm, forgot about that one. The JFK accident though was attributed to the F/O inputing several rudder full deflections and reversals"

Followed by, at 7:12 PM:
"The Airbus aircraft do not have rudder input except in an assymetric thrust situation."

Am I missing something?

or were the 'rudder full deflections' referred to in the 1st post a response to asymmetric thrust conditions?

Anyway, the AA flight in New York lost the whole damn vertical stabilizer, not just the rudder.

Greg Q
06-06-2009, 11:09 PM
Sorry, I should have been more clear...In the JFK AA accident the aircraft was an earlier A-300 series with more conventional controls. The rudder input was made in response to a wake turbulence encounter (which can manifest as sudden roll. To the accomplished aerobatic pilot who was flying, a rudder input would have been second nature-not for most airline pilots) . Because sudden control reversal can so overload the airframe something breaks-in this case the entire fin/rudder assembly. Since then accident airline pilots are drilled in jet upset and to use the rudder very gingerly if at all. The rudder has to be so powerful to counter the loss of directional control with an engine failure, but its obviously a double-edged sword.

The second comment I made was in reference to the Autopilot, (which I did not specify). Specifically then: the Airbus auto flight system does not make gross rudder inputs unless called for due to a thrust loss. Human pilots rarely do either as there is no call for it. The yaw damper on the other hand is a device that senses minute changes in yaw and compensates with a miniscule but timely rudder application. This is necessary because of the swept wings-you have to control yaw to prevent the onset of dutch roll. The yaw damper is a full time device, always on.

edited spelling

Thruthefence
06-06-2009, 11:43 PM
thanks, Greg

KenL
06-07-2009, 12:23 AM
Wow, that was quite a discussion! I am sure that the answer to the original post lies within somewhere but it was lost on me just where:( .

Suffice it to say for the layman that the pressures in question are fairly low, the loads are huge and the distribution of those loads is a variable according to flight regime and density. There is no simple answer to the original question, because it depends on many factors.

There were quite a few lost souls in designing and testing aircraft, some from things we did not know, some from poor judgement and more than a few from attempting to use emperical data where it did not apply. XB70 was caused by the chase pilot in the 104 getting too close, he goofed!

You folks did a good job of chasing the old lift argument around the barrel! And managed to keep it civil too; at least in the estimation of this old aerospace engineer!!

KenL in Ottawa