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View Full Version : The Tailgate question revisited: An investigation using CFD



Evan
02-25-2013, 05:23 PM
Heh, this will be interesting.

I found some very excellent software at Autodesk. It is a beta test of Computational Fluid Dynamics (CFD) software. It is a wind tunnel simulation. This is a stand alone program so you don't need anything else. Very unfortunately the beta test runs out on March 15 next month. Fortunately there are still almost three weeks to play with it. This is very easy to use software with no math required at all. You may insert models in 2D as simple JPEG or PNG shapes or as full 3D models from just about any software. I use Sketchup and export the 3D model as an STL file. There is a free plugin for that. It works perfectly.

The program is Falcon and is available here: http://labs.autodesk.com/utilities/falcon

I decided to test it using the old question: Is it more efficient to drive a pickup truck with the tailgate present and up or with it removed? There are numerous people that claim it is obviously more efficient with it gone since it won't be producing drag. Airflow is not quite so intuitive. What may seem obvious can be far from reality. However, the way air behaves at less than about 1/2 mach one is exactly the same as a thin fluid. It is essentially incompressible in a free flow situation. All that varies is the rate of flow and the pressure, not the volume. This makes it easy to calculate using well known physical laws. There are no difficulties with non-linear events such as shock waves.

What looks like should happen is frequently not the case. That is very evident in this simulation. This question was addressed here some years ago and I and my son did some actual wind tunnel testing with a physical model. The apparent result was that having the tailgate in place was more efficient. I have run much more accurate simulations in Falcon using an accurate 3D model of my short box F250. It very clearly verifies that having the tailgate in place is much more efficient than removing it and it shows why.

Examples: The air flow is shown by the line models. They indicate the turbulence patterns and vortices. As can be seen, the major effect of the tailgate is to trap a vortex or sometimes two inside the box. A vortex always produces a low pressure region. When that region is trapped in the box the low pressure is acting as drag (or suction) against all surfaces with which it interacts. That includes the back of the cab, the floor, sides and especially the tailgate. Having the tailgate in place not only traps the vortex in the box, it also counteracts the "suction" on an equal area of the cab ahead of it. Those forces then cancel. The magnitude of the forces is large.

This is modeled at 29.2 metres per second which is about 65 mph. That is close to the actual speed limit here (100km) and the speed that I drive on the hiway.

http://ixian.ca/pics10/cfdwithgate.jpg

On the other hand, removing the tailgate using the same parameters shows a dramatic difference. The vortex now spills out the back of the box. It enlarges and produces a significant low pressure area that engulfs the entire back end of the truck. There is no compensating "pull" on the missing tailgate and the entire back of the truck is now under a much larger amount of drag. As a WAG this is probably equal to at least a couple of mpg reduction.

http://ixian.ca/pics10/cfdnogate.jpg

This example also reveals during the simulation that this is not a static situation. The vortex may blow away and reform very quickly but the illustrations above closely resemble the average condition. Also, the actual formation and extent of the vortices is very velocity dependent. I have seen people claim that their vehicle actually may get better mileage at a particular higher speed than at a lower one. There is evidence for this in the simulation. At certain speeds the vortex doesn't form or is much smaller. At others it is greater. I selected 65 because it is the speed at which I actually drive.

The real value of a program such as this is that it can operate in close to real time. A static calculation can badly mislead since it does not give any indication of the dynamic variability that will always be present.

Unfortunately for some, this software requires a lot of computer horsepower. I am using the equivalent of about 20 ghz of 64 bit processing power with 16 gig of ram to run it. Even then on the pressure plots it runs very slowly. The velocity plots are close to real time. In full 3D mode (not shown) it is very slow at decent model resolution. This is the type of software can can bring the most powerful machines to their knees.

rohart
02-25-2013, 05:41 PM
You remind me of some work I did at Imperial College looking at a 2-D model of airflow of a city street. We were interested in the process of dilution of vehicle exhaust fumes.

We used what we called a water table. Water about an inch or so deep, flowing slowly from a feed bath at one end to a sump at the other end, modelling the profile of the building with plywood and ally strips. As a marker we used, if I remember, a mixture of nitrobenzene and glycerine with a density of one. We would inject globules of the mixture into the feed bath and use long exposures to show the movement of the water and the cloud of injected globules.

What we found was that once the buildings were high enough the vortex would double up, so the airflow at ground level across the street would reverse. No ground-breaking (pun intended) but the work was fun.

gcude
02-25-2013, 05:41 PM
Evan, I seem to remember an episode of Mythbusters where they tested the theory about drafting behind a tractor-trailer giving better gas milage. They found that indeed milage improved the closer you drafted behind the trailer. Taking your simulation and applying it to Mythbusters results, it may be that their vehicle was closing (creating) the box for creating the vortex and low pressure effect. Could be more there than the tailgate effect though.

Dr Stan
02-25-2013, 05:57 PM
Mythbusters also did an episode to test the tailgate/no tailgate theory. They ran three tests one with the tail gate up, one with the tail gate removed, and one with a nylon web replacement gate. If I remember correctly there was little to not difference and possibility a little better fuel efficiency with the tail gate in place.

The Artful Bodger
02-25-2013, 06:00 PM
Evan, very interesting, could you do one with a tonneau cover on the pickup bed, please?

I expect that would be more efficient and well worth the few minutes it takes to button the cover down on an empty bed.

[Thinks, is it usual for US pickups to be supplied with a tonneau?]

Jon Heron
02-25-2013, 06:11 PM
Mythbusters also did an episode to test the tailgate/no tailgate theory. They ran three tests one with the tail gate up, one with the tail gate removed, and one with a nylon web replacement gate. If I remember correctly there was little to not difference and possibility a little better fuel efficiency with the tail gate in place.
Yep, I saw the same one, you get better mileage with the tailgate up. http://dsc.discovery.com/tv-shows/mythbusters/mythbusters-database/driving-tailgate-fuel-consumption.htm
Who woulda thunk it eh?
Cheers,
Jon

Evan
02-25-2013, 06:12 PM
"could you do one with a tonneau cover on the pickup bed, please?"


Sure, that's easy.

bollie7
02-25-2013, 08:36 PM
This is interesting. I have a medium (for australia anyway) sized box trailer. The vehicle I had before the one I have now was a station wagon and when towing the trailer with the rear gate on I could hardly tell I had the trailer on.
The car I have now is a sedan and the difference is remarkable. With the rear gate on (trailer empty) its like driving with the handbrake on 2 or 3 notches. Gate off and again can hardly notice the trailer is there. Whilst I have not done accurate fuel figures there is a noticable difference in fuel consumption as well.
So now when I'm towing it empty I take the rear gate off and tie it onto the drawbar in front of the front gate.
Thanks Even for posting this info.

regards
bollie7

legendboy
02-25-2013, 08:55 PM
that software looks badass i'll download a copy

i wonder how much it will cost when released? cfdesigns software is 25k if i remember

legendboy
02-25-2013, 08:57 PM
i just looked and autodesk bought cfdesign

falcon beta is open to aug 30 for inventor users

Evan
02-25-2013, 09:25 PM
I don't expect it will be affordable. I don't run pirate software so I will be doing some testing while it still works.

The testing of the tonneau cover was a big surprise. I expected it to be more efficient but it isn't. It seems that the key is the normal configuration tends to trap the vortex in the box. With a tonneau it slides right off and forms a much larger vortex quickly. I changed the parameters slightly by raising the height to avoid possible edge effects similar to the ground effect with aircraft. That made a slight difference to the standard tailgate as it seemed to allow the vortex to leave the box periodically. However, the average was still highly favorable whereas the tonneau produced a huge low pressure area, possibly even worse than the "no tailgate" option. I ran it several times with slightly different parameters and the result is the same each time.

http://ixian.ca/pics10/cfdwithton.jpg

http://ixian.ca/pics10/cfdwithtonp.jpg

http://ixian.ca/pics10/cfdnotonline.jpg

http://ixian.ca/pics10/cfdnoton.jpg

aostling
02-26-2013, 12:01 AM
Evan,

Welcome back. I was wondering what you were doing during your absence. I didn't guess that you had become a CFD modeler and an adept at Autodesk. You've made a good choice in your test case -- the results of your simulation have immediate practical application to anybody with a truck.

CFD is all about external flows over objects. I wonder if Falcon can also handle internal flows, in manifolds and heat exchangers for example, where the flow is one-dimensional but with gradients normal to the walls (i.e. shear, causing fluid friction). I suspect not, but that doesn't make it any less valuable for its intended purpose. The visualizations are far beyond what I was seeing ten years ago.

The Artful Bodger
02-26-2013, 12:21 AM
Thanks Evan, I am surprised that the tonneau is less efficient that the open cargo box but I am not surprised by the flow patterns you show as I know half a canvas tonneau bulges up and half bulges down (I cant recall if it is the front half or the rear half that bulges up).

My current ute (i.e. pickup) has a solid lid so I don't really have an option. My vehicle is a bit more streamlined than the model you used and has a quite pronounced lip at the rear of the cabin. Maybe I should tape some ribbons to the lid and see what the patterns are.

Evan
02-26-2013, 12:27 AM
Hi Allan,

I will send you some items for your consideration soon. I have been very sidetracked on other pursuits lately. Among other things I am now working on getting the attention of the Minister directly. If that doesn't seem to work then it will be the Prime Minister. There is much more to what is happening than meets the public eye and I have discovered some of it. This is not the place to discuss it in any detail so I shall not.

As for the CFD software, it is proving very useful in verifying my design for a shuttle replacement. It is a design exercise that I started about a year ago and it goes far beyond just a shuttle. It is basically what the entire space program should have been by now and could be if the will existed.

A test for rudder blanking at high angles of attack. No problems found. I will be posting more about this some time in the next while. It goes far beyond this model. I am not spending much time here as I have other interests since I cannot spend much time machining. I sure wish I could.

http://ixian.ca/pics10/rudderblank.jpg

J Tiers
02-26-2013, 12:47 AM
The tonneau cover isn't really that surprising.... and yet it is in some ways....

I saw a version of a "cap", such as lots of folks have, that was alleged by tests at some engineering dept to be more efficient than any other. It was, of course, not useful at all as far as holding "stuff"......

It was the front 1/4 of a standard "cap", and then tapered down to the tailgate. Obviously, it filled the entire volume where the circulation currents normally are, so it SHOULD work.

I would expect a standard cap to be very slightly, if any, better than a tonneau cover..... it moves the vortex to the rear, but adds more height to the surface causing it.

Caps with a slight side tilt-in going up might be slightly better, by reducing the flat surface at the back

One item that could be of very significant help is a "slot" (like aircraft) at the top of the cab, directing air down over the bed, aimed generally at the top of the tailgate. The strong directed flow from the slot might block the vortex circulation off from the airflow pattern over the vehicle, and act somewhat like the "efficient cap" mentioned above.

With any luck, it can be done without adding too much direct drag, so that the net is a positive overall reduction in drag.

The Artful Bodger
02-26-2013, 12:57 AM
A pickup cap is the same basic shape as a station wagon? The lid I have is a solid tonneau.

Evan
02-26-2013, 01:07 AM
What is very apparent is that the results will vary with the shape of the vehicle. While the general effect is probably applicable to all similar vehicles, the speed at which certain effects appear is likely to be different. I strongly suspect that at least in the US the vehicles are optimized for the exact highway fuel mileage test conditions mandated by the government.

Paul Alciatore
02-26-2013, 01:49 AM
This is most interesting. I would love to see one with a full cap on the back. It might also be interesting to compare long bed vs. short.

J Tiers
02-26-2013, 08:45 AM
In ships, length gives speed, but that is to a large extent due to the effect of the bow wave and how it interacts with the ship... where the following trough is.

length would generally give better results than width, for similar volumes..... I have no idea if it would be of use in a truck-sized length compared to cab length only, or at truck speeds.

Clearly actual streamlining does work at auto sizes and speed, look at the fact that nearly every car looks alike anymore.... physics takes no account of styling trends, and one shape is going to be best.

Weston Bye
02-26-2013, 09:13 AM
And indeed, as automakers are mandated with ever increasing MPG requirements I'm sure that they have considered the tailgate question as low hanging fruit, considering some of the efforts they have gone to in other less obvious areas of the vehicle. Some that I have been dragged into involve electronic engine fan clutches, radiator shutters, cam phasing and transmission clutches.

vincemulhollon
02-26-2013, 10:04 AM
With a tonneau it slides right off and forms a much larger vortex quickly.

Try a cargo net and see if it "traps" little vortexes. I've seen a couple trucks around town with those cargo strap nets lets say 2 inch wide belts in a 50% or so coverage mesh.

I have occasionally wondered if an air duct from the bottom of a vehicle to the back of a vehicle would lower air resistance appreciably while also gluing it to the road at high speed.

I also think it would be hilarious to model cars moving in reverse because I know an ex-truck driver who pulled a car hauler and he swore stereotypical cars gave him much better mileage on his truck when the cars were all pointed backwards and from what I know about real aerodynamics vs the demands of fake marketing aerodynamics he's probably correct, you'd likely get higher MPG in reverse than forward for most cars... maybe even pickup trucks.

JohnAlex141r
02-26-2013, 10:35 AM
I also think it would be hilarious to model cars moving in reverse because I know an ex-truck driver who pulled a car hauler and he swore stereotypical cars gave him much better mileage on his truck when the cars were all pointed backwards and from what I know about real aerodynamics vs the demands of fake marketing aerodynamics he's probably correct, you'd likely get higher MPG in reverse than forward for most cars... maybe even pickup trucks.

Think of a rain drop. Not being an expert in this, but it is my understanding that a raindrop (round "front" long tail) is the ultimate subsonic form; so putting the cars backwards (especially hatchbacks) would make sense to me.

Another JohnS.

Evan
02-26-2013, 11:25 AM
I will give the cap a try. I already have a model of that.

garagemark
02-26-2013, 12:14 PM
This may seem a dumb question, but... if the standard tailgate adds efficiency as opposed to no gate, then how "high" could the tail wall be before drag catches up? I guess I am saying, if I added a two inch strip across the top of my tailgate, would I be even more efficient? How about six inches?

What is the correlation to wall height verses drag equals maximum efficiency?

Mark

digger_doug
02-26-2013, 12:43 PM
Very nicely done Evan.

How about some vortex generators on the roof, just ahead of the back of the cab ?

Or, open the cab door windows, and the rear sliding window (which generally results
in all the dirt from the bed coming in thru the back window on my truck.....)

aostling
02-26-2013, 03:33 PM
I mentioned this thread to a friend who worked at the GM Proving Grounds in East Mesa, as a test engineer. He said that GM did CFD modeling and coast-down tests and found that the drag is less with the tailgate up, as shown by Evan's model.

However, they also found that a tonneau cover helps. You can read about it, and get a pdf of the article here: http://www.motor.com/article.asp?article_ID=784. This claims that a full tonneau can save 1.35 mpg, on the particular truck they were testing.

Evan
02-26-2013, 03:45 PM
I'm afraid that this program doesn't have the necessary resolution to easily answer the questions that follow with small modifications of a few inches here and there. It looks like the resolution for a truck model is about a minimum cell size of maybe around 4x4 inches or so. It isn't specified so that is a guess. It is also very time consuming to calculate at that resolution. I would need a machine with multiple cores on multiple CPUs to run that in a reasonable time. Great if you are GM but I am not. The coast down test would provide a good average across the entire velocity regime. As I previously indicated, the results are very velocity sensitive. Strangely, at very high velocities the vortices generally do not form. However, the frontal drag increases as the square of velocity and the power required to overcome it increases as the cube.

garagemark
02-26-2013, 05:01 PM
Well Evan, you have piqued my interest in any event. It seems to be universally accepted (except maybe to Bubba) that tailgate up efficiency has some validity. And since my questions cannot easily be modeled without a Cray sized processor, I suppose the next logical step might be to experiment with various heights and geometries of rear walls (tailgates). I drive a certain route to and from work every day, about 53 miles each direction, and my fuel mileage is extremely consistent over the course. I may just alter the tailgate a bit and see what happens over a few weeks.

Glad to see you back.

aostling
02-26-2013, 05:07 PM
Great if you are GM but I am not.

On the other hand, GM is impressed with your model. From my friend Mike, the former test engineer:


A few notes. The Motor.com article shows what appears to be a "generic" truck model while Evan's is an accurate (as far as I can tell) Ford F250. So maybe there are some differences there. It would be nice to find the SAE paper which is mentioned in the Motor article.

On the tonneau cover. When I was at GM, they offered an electric S10 pickup using the EV1 powertrain (here: http://en.wikipedia.org/wiki/Chevrolet_S-10_EV).

GM did some aerodynamic clean-up which included a half-length tonneau cover over the rear of the pickup bed. So apparently this is better than no cover.

Weston Bye
02-26-2013, 05:31 PM
Years ago I has a Chevy S10 that I used for work. After a while I got a gang box that spent most of it's time right behind the cab. It was low enough that I could still see over it through the rear view. I believe that it's presence improved my fuel economy at highway speed, even though it and the junk inside added extra weight.

J Tiers
02-26-2013, 08:16 PM
I can absolutely vouch for the airflow predictions of the program. before we put the cap on the truck (a 2000 S10), one could see that the airflow patterns would move sand and small gravel up against the back of the bed next to the cab. Obviously there was forward moving air there, and equally obviously, the airflow was reasonably powerful.

Anything that 'fills up" the space where the circulation occurs will tend to stop it. The effect of better "streamlining" is to fill up ALL the space where the vortexes occur, so that there are none, and flow is smooth.

I would expect that a toolbox might do something along those lines. It's a question whether it is better or worse....

Any vortex at all represents turbulent lossy flow, requiring energy to supply that which is put into the vortex. If the vortices are shed, that is probably MORE lossy, even if they are smaller, because new ones must be created, with added input of energy.

Now, I was told that the cap would improve efficiency, but have not noticed much.

Tyro 001
02-26-2013, 08:48 PM
Several years ago, the Indianapolis Sunday Star included a nationally syndicated car repair column. One week the writer asked the same question and published the answers the following week. Several automotive engineers responded, as did a former CEO of one of the car companies. They all agreed leaving the tailgate on was the most aerodynamic and the most structurally sound. They didn't think the cargo nets, etc. in place of the tail gate was worth while. They all mentioned a "bubble" of air which circulates in the back of the truck. You can see the bubble in Evan's diagrams.

I don't remember them saying anything about the effects of a cap. FWIW, I have a '99 F150 extended cab pick-up truck, with an 8 foot bed and a cap on it. The engine is a 4.6L and the truck has a 5-speed manual transmission. When I bought my mill, I made two 600 mile round trips to bring it home. On the way to it, I drove 75-85mph. On the way back I was either carrying 1200lbs of mill heads or towing the base on a trailer. My round trip MPG for the 1200 miles was 17mpg.

John Stevenson
02-26-2013, 08:50 PM
Evan,
In the UK they have brought out aerodynamic trailers that go behind rigs.

http://static.commercialmotor.com/big-lorry-blog/teardrop.JPG

Instead of the usual box type, would be interesting to know if it works or it's just a gimmick ?

Also interesting variation

http://www.logisticsmediauk.co.uk/articles/tnt---6-fuel-savings-with-aerodynamic-trailer-tail

Evan
02-26-2013, 10:27 PM
There sure are a lot of things I would like to model. Three weeks isn't enough but will have to do for now. I have looked for easy to use CFD software for many years and this is the first I have found. I have even written some myself but very basic stuff with no dynamics at all.

Here is the test with a cap. It seems surprisingly good although the one I modeled I designed with the best possible forward streamlining. I should mention the truck model is not my work. I found it on the 3D warehouse. The credits show several people's components put together. Also, the program doesn't just provide a qualitative result. It will also give apparently accurate quantitative measures of drag given the appropriate scaling info. Autodesk claims it has been tested and verified for accuracy and has a document detailing those claims. The drag calculations require it be run in full 3D mode and are exceedingly slow to calculate.

http://ixian.ca/pics10/cfd1.jpg

Just for the heck of it I also ran a test of an extreme front air dam. Air dams you see on all sorts of vehicles but the applicability to ordinary street machines is very doubtful. An air dam (ground effects) is intended to produce negative pressure under the vehicle. This is just what you need in road racing where ample power is available and mpg isn't very important. It improves handling and braking because of the extra download. It isn't necessarily a benefit in street driving. It does seem to reduce the trailing vortices and negative pressure but it seems to be at the direct cost of increased frontal pressure and area. I doubt there is any net gain.

http://ixian.ca/pics10/cfd2.jpg

Just to show the actual model: (no point in reinventing the wheel when you can get one for free)

http://ixian.ca/pics10/cfd3.jpg

The Artful Bodger
02-26-2013, 10:34 PM
Evan, it is my observation that utes with a front air dam throw up less dust on a gravel road than those without.

Evan
02-26-2013, 10:48 PM
That is likely true as verified by the airflow when seen dynamically. Too bad they seem to have disabled the video output on this version. It goes through the motions but doesn't write the file. Here it probably doesn't make much difference. It's so dry that the unpaved roads are more like powdered dirt rather than graveled. You need to follow about a kilometre back on a still day.

Bob Pekny
02-26-2013, 11:14 PM
It would be very cool to see how Buckminster Fullers Dymaxion car would fare, I imagine it would be very good up to about 80 mph or so. That man was so very far ahead of his time.

J Tiers
02-26-2013, 11:17 PM
Hmmm....

The results with cap look considerably worse than with tailgate up.... more like the no tailgate case... Am I missing something?

The Artful Bodger
02-26-2013, 11:31 PM
Evan, the difference with an air dam on a gravel road is that less dust is blown out the sides of the vehicle and the sides of the vehicle tend to stay cleaner. The tail gate seems to get dirty no matter air dam or not, tonneau cover or not. I never drive with the gate down as stones do too much damage.

Evan
02-27-2013, 12:13 AM
Jerry,

It is hard to pick the right moment to show the average condition. Also, the program does not seem to have a way to save settings. That means I have to try and set it up the same each time and that isn't easy. There are bound to be some uncontrolled variables between tests. It helps a lot to watch it in action so you can integrate it in your head. The cap looks good, certainly at least as good as just the tailgate so it isn't costing anything to leave it on. I also should have made the min-max pressures readable in the earlier images.

I should also mention that the vortices are not necessarily as low pressure when in contact with the vehicle as when they have shed. They seem to tighten up after shedding which deepens the pressure drop (in smaller volume) but that has no impact on the vehicle. They are already bought and paid for. It's only the pressure zone in which the vehicle is immersed that matters.

J Tiers
02-27-2013, 01:07 AM
OK, that ,makes sense..... it's a demo version, too, so they probably won't make it easy to do any real work with it.... like saved setups, etc

Looks pretty cool, I might try to use it.... I have some stuff that I could make into an STL file ..... I better get a move on if I want to though!

Thanks.

Paul Alciatore
02-27-2013, 01:14 AM
That's about what I expected with the cap.

That is one neat toy. You said this is a evaluation copy: perhaps if you send in a nice critique of it they will provide a copy of the final version for free or at least at a reduced price.

Wish I had time to play with it.

Evan
02-27-2013, 03:47 AM
It's actually in beta test rather than demo. It isn't yet near ready for prime time. It's pretty stable but it has crashed twice for me so far and I have written them up in full detail with all the system information and error logs. I can think of a dozen small but necessary features that are not yet present but almost certainly will be when it releases. You aren't required to participate in the beta release forum to try it.

I don't know if Autodesk gives away software to beta testers, especially when the beta test is wide open. I have officially beta tested for other companies under NDA and they usually hand out a free copy in payment but that is a different matter. I am currently beta testing for two European companies but am not permitted to say who or what I am testing.

A.K. Boomer
02-27-2013, 11:49 AM
There are numerous people that claim it is obviously more efficient with it gone since it won't be producing drag. Airflow is not quite so intuitive. What may seem obvious can be far from reality. However, the way air behaves at less than about 1/2 mach one is exactly the same as a thin fluid. It is essentially incompressible in a free flow situation. All that varies is the rate of flow and the pressure, not the volume. This makes it easy to calculate using well known physical laws.

http://ixian.ca/pics10/cfdwithgate.jpg


What a great experiment, I agree there is so much that is counter intuitive when it comes to flow dynamics be it liquid or air, although I was always taught that the air is more elastic even at lower speeds and in fact that's the principle behind the common airfoil, we separate the incoming air with the leading edge and make the top part have to travel further yet have to reconnect and meet back up on the trailing edge with the lower part in the same given time frame - the results are lower pressure on the top part of the foil due to thinner air (less pressure)...

To me your example puts things together quite nicely - to understand where the true efficiency savings are one needs to look at the effects of the air just past the cab, what is happening is that you are basically creating one giant air powered "pneumatic roller bearing"
The added efficiencies are the results of powering this "air bearing" with what would most other wise be waste turbulence...

Want to make it even better? put a slight rounded scoop edge that's actually facing towards the direction that the truck is moving and put it at the top of the gait (no higher than the gait itself) but inside so it pushes the air down, now round off the bottom of the gate where it meets the truck bed,

Now - remember what JT said about sand collecting right behind the cab on the bed? round that off too, and then put a rounded off foot long extension trailing the top of the trucks cab and you've increased the internal velocity of this entire "air bearing" and you did it at less drag expense, it's a beautiful thing - it's what you call making a vortex work for you - if you cannot achieve an aerodynamic longtail that puts the air back together all nice and non-turbulent the next best thing is to create a controlled departure,,, and since the tailgates lower than the cab itself it will help to put your next big worry back together in time for it to reap some benefits (at certain speeds) As some of the air in the "bearing" escapes it jets air slightly downward past the gate and creates yet another semi-controlled departure...

controlled eddies on blunt shorter structures can actually be more efficient than if you tried to re-assemble the air with solid polished sculpted structure.
Sometimes with only so much length VS height and width you just have to cut your losses and go with plan B...


What's funny is if that was a picture of a truck in the river with water flowing over it the currents for the most part would be reversed - the water behind the cab and on top would actually be moving forward - whilst the water closer to the bed would be jetting downstream, it would form a dreaded keeper hydraulic - but that's not making any kind of statement against what Evan said about the similarities between fluid and air dynamics - it's more the results of what you get when you MIX the two together...

Great to see you posting again Evan!

If your program has the ability to use the honda CRX as an example id love to see the visual results - the car has a very low CD yet it's somewhat blunt in the back - im thinking there has to be one nice little curler coming off the back of that pup...

id also like to see the honda del-sol --- but that's more to find out why it's such a wind pig...
(I have my theories- lol)

Evan
02-27-2013, 12:42 PM
although I was always taught that the air is more elastic even at lower speeds

That is one of the very counterintuitive aspects of airflow at low velocities. Below about 1/2 mach 1 the compressibility is so small that it is only a fraction of a percent and universally ignored in aerodynamics. That only applies to situations where the air is in a free flow condition.

I will be posting occasionally and may go for periods of time when I do not post. My wife has orders to post if my inoperable brain aneurysm decides to blow.

A.K. Boomer
02-27-2013, 12:48 PM
Is compressibility and separability one and the same?

Take as good a care as you can Evan - were all hoping the best for you...

Evan
02-27-2013, 12:53 PM
we separate the incoming air with the leading edge and make the top part have to travel further yet have to reconnect and meet back up on the trailing edge with the lower part in the same given time frame -

That is not true in reality. The molecules moving over the top do not have an appointment with the ones on the bottom. They move at rates governed by the flow mechanics and may end up never coming close to any previous others again, such as when the flow over the top detaches. The primary component of lift is caused by the simple redirection of mass to a different velocity vector. That occurs on the bottom and the top of the wing and explains how a fully symmetrical airfoil works, including flat plates which work well below six degrees angle of attack. A lesser contribution is made by the Bernoulli Effect on asymmetrical airfoils but it is never the major part of lift except when used in the Coanda Effect.

Note that a symmetrical airfoil at zero degrees angle of attack does not produce lift.

A.K. Boomer
02-27-2013, 01:22 PM
It's not an appointment per say - it's more like a dinner date,
goes kinda something like this;

her; Ok babe, i'll see you at 7:15 then

him; alright - but I got some stuff to do first, if it goes well should be good but if not i'll give you a quick one and tell you what's up...

her; ok


{a few milliseconds later;}



her; what happened to you? you almost hit me in the parking lot?

him; yeah sorry about that - was speeding to keep the appointment.

her; well why didn't you call then?

him; I could not get a signal - it's like there was a wall between us,

her; you look like hell.

him; you would too if you had to work for a living

her; why I never ----

him; yeah - I know - and never will with me either toots...

her; why you bastard

him; why you little biatch...


her; you still want to get something to eat?

him; sure....

DICKEYBIRD
02-27-2013, 03:47 PM
The primary component of lift is caused the simple redirection of mass to a different velocity vector.THANK YOU! I can't tell you how much yelling & screaming there
used to be on the model airplane forums about what caused a wing (or whatever) to create lift.

(By the way it's so great to have you back!):)

The Artful Bodger
02-27-2013, 05:55 PM
So then, just how does a wing create lift?;)

A.K. Boomer
02-27-2013, 07:07 PM
So then, just how does a wing create lift?;)


More importantly "how it know" how to go up or down?

This is a timely post as I just finished my new plane yesterday and been flying it,
actually had it done awhile ago but it had a real radical swept wing and was a PITA for stability as it would go into the dreaded "sabre dance" at lower speeds.

it's just a simple foamie with carbon fiber leading edges and packing tape over the foam - it's about indestructible, the fuselage is made out of a carbon fiber arrow shaft...

http://i146.photobucket.com/albums/r249/AK_Boomer/DSC03051_zps88ba1eb3.jpg

No matter how durable you make the body though your still up against the laws of physics and kinetic energy when it comes to the engine ripping itself out of its "tri-spoke" mounts, the solution?

Jettison the 1 1/2 oz engine (the entire plane weighs only 7 oz) along with the ducted fan unit upon impact... they not only survive the crash they make it easier on the rest of the plane...

http://i146.photobucket.com/albums/r249/AK_Boomer/DSC03053_zps81bfa746.jpg

http://i146.photobucket.com/albums/r249/AK_Boomer/DSC03055_zpsd3250391.jpg

A.K. Boomer
02-27-2013, 07:13 PM
when trying to round up parts semi-locally i went to two different hobby shops looking for ducted fan parts and the guys up front laughed at me and then mentioned it to the "guru's" in the back and they stated they don't sell ducted fan kits at this altitude because they will never get off the ground,

I told them I did all the math about the reduced wing lift and also the lower fan purchase (and I hate to do math) and came out with a comfortable performance margin, ( I got 4 to 4.5 oz of thrust in a 7 oz plane) - they said "good luck with that" and smirked lol

today I put this bird almost out of sight multiple times with a 7.4volt 360mah lipo... maybe they weren't counting on some guy with a milling machine building up his own velocity intake ring for the ducted fan unit, it's about .025" thick and made from black plumbing pipe - the results were an extra ounce of thrust... - it's a great plane and lots of fun to fly - yes very simple but still fun...

the pic below is a dramatization - still intimidating when your made aware that a hunting tip can screw directly into the arrow shaft if you wanted ( like if someone was shooting at you or something )
and this little fokker's got enough balls to pull it off...

it's one bad little mamajamma

http://i146.photobucket.com/albums/r249/AK_Boomer/DSC03058_zpsc345033b.jpg

Astronowanabe
02-27-2013, 07:43 PM
It would be very cool to see how Buckminster Fullers Dymaxion car would fare, I imagine it would be very good up to about 80 mph or so. That man was so very far ahead of his time.

except for the rear wheel steering part... in practice, having to steer into what you need to get away from does not work well for panic mode

Evan
02-27-2013, 10:01 PM
I like to say that helicopters, aka rotary wing machines, do not suck themselves into the sky. They quite simply accelerate large quantities of air downward. Newton's laws do the rest. Air has considerable mass, nearly exactly one kilogram per cubic metre at STP. A fixed wing operates on the same principle. As it runs into large amounts of air per second that air is accelerated downwards. With the average low speed airfoil the lift produced by the Bernoulli Effect is maybe 20% of the total lift. The rest is Newton. Nasa has been trying to correct the science text books for about 30 years but it is nearly impossible to do.

It falls in the same category as the "bumblebees can't fly" story. The fact is that nobody ever said they couldn't. What a couple of science types worked out many years ago is that they cannot glide, which they can't, unless you consider a brick to have a glide ratio..


BTW, you will all have to put up with me missing out the occasional word and especially the letter s at the end of many words. Also some other mixups in typing that aren't ordinary typos. Since my stroke where I couldn't read at all for about 24 hours I still have some slight side effects in seeing words in context with others. My brain seems to fill in the word in my head even if it isn't visible. Strange effect.

tdmidget
02-27-2013, 10:34 PM
" when trying to round up parts semi-locally i went to two different hobby shops looking for ducted fan parts and the guys up front laughed at me and then mentioned it to the "guru's" in the back and they stated they don't sell ducted fan kits at this altitude because they will never get off the ground,"

If you had a location in your profile we might have some idea what "this altitude" means.

TGTool
02-27-2013, 10:57 PM
" when trying to round up parts semi-locally i went to two different hobby shops looking for ducted fan parts and the guys up front laughed at me and then mentioned it to the "guru's" in the back and they stated they don't sell ducted fan kits at this altitude because they will never get off the ground,"

If you had a location in your profile we might have some idea what "this altitude" means.

Actually it's heartening to know that this board isn't the only place that has a bunch of guys with a bad altitude. :)

A.K. Boomer
02-27-2013, 11:28 PM
I like to say that helicopters, aka rotary wing machines, do not suck themselves into the sky. They quite simply accelerate large quantities of air downward. Newton's laws do the rest.

That is actually a great example - and yes id have to agree that Newtonian laws are by far the majority of the workhorse esp. when you throw in an increased angle of attack.

Im kinda surprised at just your 20% credit to the Bernoulli effect on a fixed wing though as I would think an airfoil that's being used in a trimmed out low drag mode would be much greater - and your right - part of that's due to being taught back in the day that it was responsible for ALL lift but the other part is thinking that if the bottom part of the wing is directly level going through the air it will not "accelerate large quantities of air downward" Yet - the wing will still produce substantial lift.

So when a wing is in this mode what would you guess the majority of the workhorse is...? still only 20%?

where's the rest of the force being generated and in what way? remember there's no real angle of attack here - just an airfoil in full trim...


I do believe there are varying degree's of different operating principles depending on the degree's that are varied...

A.K. Boomer
02-27-2013, 11:33 PM
" when trying to round up parts semi-locally i went to two different hobby shops looking for ducted fan parts and the guys up front laughed at me and then mentioned it to the "guru's" in the back and they stated they don't sell ducted fan kits at this altitude because they will never get off the ground,"

If you had a location in your profile we might have some idea what "this altitude" means.

sorry - mile high colorado springs area is where the shops were - I had to get the parts on-line,

one good thing about going electric is at least along with the lower prop bite and sour-arss wing lift your also not dealing with a de-tuned engine :p

A.K. Boomer
02-28-2013, 12:42 AM
I like to say that helicopters, aka rotary wing machines, do not suck themselves into the sky. They quite simply accelerate large quantities of air downward. Newton's laws do the rest.

That's also a great example on the flip side, your taking things to extreme to prove a point and I understand that - but instead of a horsepower robbing Newtonian craft lets look at something far more efficient with its lift principles - Do you not think a glider would operate more towards the Bernoulli principle? or Coanda? either one of these operating principles are very slippery in comparison to placing a flat sheet at a 6 degree angle and applying force to it, (not that that's a typical helicopter blade - just bringing up what you stated earlier)

Im trying to keep the topic somewhat lined up with your original post, it had to do with efficiency - and typical "Newtonian" lift does not,,,,
the goal here is to utilize this "fabric" that we call "air" and then assemble it back together in the cleanest way possible whilst still achieving our lift results...
you can never make it perfect - if you could you would have no results...

but there are far better operating principles when it comes to efficiency than just grabbing hold of the air and shoving it down to the ground...

Gliders have to rely on the most efficient means possible, they do not have steep angles of attack on their wings to "pump" the air at a high rate of speed downward and create all kinds of turbulence in the process - they allow the air to flow smoothly past - and milk just enough to create negative pressure on the last 2/3rds of the trailing top of the wing.

That's how you hang up in the sky for over 15 hours without any fuel ( of course riding thermals helps - but how long you think a chopper would last :p)

J Tiers
02-28-2013, 08:26 AM
Gliders have to rely on the most efficient means possible, they do not have steep angles of attack on their wings to "pump" the air at a high rate of speed downward and create all kinds of turbulence in the process - they allow the air to flow smoothly past - and milk just enough to create negative pressure on the last 2/3rds of the trailing top of the wing.



A glider takes it's angle of attack out in angle of descent .... since it NEVER goes straight forward, and is always descending. So the angle of attack has less of a fixed relationship to the structure, centerline, etc. A powered aircraft can actually go straight forward, a free glider cannot in the absence of a thermal, etc, equalling the rate of descent.

The glider STILL has to accelerate mass downward in order to stay up. In fact even the traditional airfoil explanation eventually comes down to accelerating a mass of air downward, the means is just a bit more roundabout. There isn't really any way to escape that, unless you rely on buoyancy like a balloon. Energy is extracted from the aircraft, and added to the air.
.
.
.

Not sure how this side matter of lift relates to the flow over a truck, which is the opposite of an efficient shape, and doesn't want lift, but......

With a truck/car/etc the idea is not about creating lift, but avoiding drag, which means as much as possible AVOIDING the extraction of energy from the vehicle, AVOIDING adding it to the air. No energy extraction, no drag. Zero is impossible, but closer is better.

in the case of an aircraft, the idea is to avoid any energy extraction which does not end up as a downward acceleration of air.... because only that creates the desired lift....

Everything else is "wasted", although an aircraft that didn't move from place-to-place would not be regarded as an unmitigated success...... so some frictional energy loss in "drag" is inevitable due to the need to actually GO somewhere.

THAT part of the matter is common to both aircraft and ground vehicles.

DICKEYBIRD
02-28-2013, 10:43 AM
I like to say that helicopters, aka rotary wing machines, do not suck themselves into the sky.Couldn't resist posting a pic of a gyrocopter I designed & built 10 yrs. ago for the 1/2A Design Contest on the RCU forum. It definitely sucked on the 1st test flights!

It’s powered by an exhaust-throttled Cox TeeDee .049 and has 4 channel control: rotor tilt left/right, elevator, rudder and engine throttle.

Most of my peers thought it couldn’t be done with Cox power but the darned thing flew well once I got the weight down as much as possible (17.4 oz.) and changed the rotor blade airfoil from a thinned Clark-Y to a Selig 6042 The blades have a couple degrees negative incidence to generate autorotation. I usually hand launched it since I flew mostly on grass fields and the little engine & 5” prop had trouble generating enough thrust to get it moving on grass. Once airborne, it’d handle touch & goes on grass if it was mowed short.

It has a “flapper” rotor hub (each blade is hinged at the root with thin/flexible polypropylene sheet which allows the advancing blade to rise & dump unneeded lift. This prevents the advancing/retreating blade asymmetrical lift problem. Initially it had rotor fore-aft tilt control as well but test flights proved that the elevator control by itself was crisp & powerful enough that I dumped the rotor fore/aft servo & linkage to save weight.

http://i57.photobucket.com/albums/g227/DBAviation/flyby2sm.jpg

http://i57.photobucket.com/albums/g227/DBAviation/launch800.jpg

A.K. Boomer
02-28-2013, 11:01 AM
Very cool - reminds me of the benson gyrocopter, can you still land it fairly well if you lost power?

that's one unique machine... The fact that you can hand launch it is amazing to me as I would have thought it took a little travel time to get the main rotors up to speed so they could start supplying lift.

DICKEYBIRD
02-28-2013, 11:46 AM
Thanks. Yup, I ran it outta fuel several times. You just slide in full down elevator trim which gets you a pretty steep descent, make a gentle turn toward the runway (if you have enough altitude in the bank) wait until she's very low & ease back on the stick to flare. If you do it right, she'll hover a couple inches off the deck for a split-second at zero ground speed & settle gently down on all three. An unfortunate side effect of the light rotor is there's not much energy to be stored in the rotor during power off descents.

Full scale gyros need a rotor pre-spin system. My rotor is light and spins up by itself at (I'm guessing) 5 to 7 mph. I had to tie the rotor up between flights to keep it from spinning up just sitting on the ground between flights. The picture of me launching it was on a fairly calm day and I had to trot faster than normal. If the wind was 8-10 mph, I could face the wind with it cocked back, take a few steps, tilt it forward to a more normal angle & gently push it forward.

I did only a few "normal" take-offs on paved runways as I felt disconnected from the spin-up process and worried that I'd screw up & stuff it in. The main gear is fairly far forward and it was squirrelly during the run up to take-off speed. Too much weight on the tail skid = ground loop city!

Evan
02-28-2013, 12:43 PM
So when a wing is in this mode what would you guess the majority of the workhorse is...? still only 20%?


First, that isn't my number, its NASA's. And the answer is still yes, if the wing is something like a standard Clark Y airfoil. While the air over the bottom is not forced to change direction the air over the top IS as long as it isn't stalled. That sort of airfoil always produces a downwash even at slightly negative angles of attack. The attached airflow is following the contour of the upper surface which is changing the direction of the airflow as it passes over the wing. Newton still wins.

Evan
02-28-2013, 12:49 PM
Not sure how this side matter of lift relates to the flow over a truck, which is the opposite of an efficient shape, and doesn't want lift, but......

With a truck/car/etc the idea is not about creating lift, but avoiding drag, which means as much as possible AVOIDING the extraction of energy from the vehicle, AVOIDING adding it to the air. No energy extraction, no drag. Zero is impossible, but closer is better.

in the case of an aircraft, the idea is to avoid any energy extraction which does not end up as a downward acceleration of air.... because only that creates the desired lift....


It is instructive to look at the drag figures for aircraft vs autos. There is no comparison even with the very best cars. Aircraft have numbers that are a very small fraction of cars.

A.K. Boomer
02-28-2013, 02:58 PM
First, that isn't my number, its NASA's. And the answer is still yes, if the wing is something like a standard Clark Y airfoil. While the air over the bottom is not forced to change direction the air over the top IS as long as it isn't stalled. That sort of airfoil always produces a downwash even at slightly negative angles of attack. The attached airflow is following the contour of the upper surface which is changing the direction of the airflow as it passes over the wing. Newton still wins.


Thanks Evan - I did not know the proper name for it but the clark Y foil is the exact example im talking about ---

If I see one more example of an airfoil pitched positive at 15 degree's to demonstrate how a wing produces lift im going to puke, im not talking about that - that is pure blunt force Newtonian at its best - im talking about why we have the basic shape of the wing to begin with, and the clark Y as you stated will even produce downwash at slightly negative angles of attack,,,

the great thing about using the clark y as an example is the lower platform is directly flat, and you can fly it with the lower platform in direct correlation to the oncoming air - so there is no added pressures to the lower part of the wing to produce lift,

Now people are saying "AK's crazy -------- he thinks that you can get a reaction of lifting a plane without the plane having the opposite reaction on the air itself, not so --- the wing produces downwash AFTER it disconnects itself from the two conflicting forces that it was separating - These two sides were going through different pressure phases - the lower remained very close to constant (atmospheric) - not only that - it's path is virtually unaltered as compared to the uppers and due to air having mass it wants to try and remain that way, the upper portion of most of the trailing wing is however going through something completely different - it's in a low pressure mode - this in turn does a series of things - due to it unloading the atmospheric pressure that was normally on top of the wing it creates a huge imbalance between the top and the bottom - and even though the bottom has remained neutral and is at the same pressure of the atmosphere the top is not - so the bottom part of the wing is lifting the plane with no direct physical interference angle - and because the bottom air is traveling in a virtually straight line and the top air is trying to follow the curvature of the wing (choose a theory here) and is coming back into this "void vector" and is not only accelerated but also accelerated downward, the results are that when the separation median of the wing leaves these two conflicting sides there is a massive "whoosh" of thrust directed downward,,, and whilst not perfect the wing puts the air back together and is gone without much drag,
far better than if a flat blade was held at an angle to achieve the same lift results... it's not because the flat blade is extremely inefficient on the lower side that's getting a direct purchase on the air itself - it has more to do with all the turbulence that is being created on the upper part - much of which the archaic design is having to drag along with it instead of piecing it back together and letting it go....

This does not mean im not in agreement with you on it still being a "Newtonian effect" but it's a secondary Newtonian effect if it is - it's not as direct and simple as pushing off the air as many people think --- at least not in the form im talking about, one of the reasons there is so much controversy is that the rules of the game change from design to design - and angle of attack and speed of craft and on and on,

But when a wing is looked at in this pure form it's hard for many people to grasp the concept that the downward thrust that is being induced is actually coming from above the wings cordline...

there is no added pressure to the lower part of the wing to lift the craft, or accelerate air downward for that matter, as far as all the theories go as to what the initial working principles are that create this effect to begin with to me your description earlier about the vector change at the top of the wing is about as basic and real as it gets, and the rest may be as simple as trying to fill a void...

One thing for sure is controversy abounds, :)

" It is amazing that today, almost 100 years after the first flight of the Wright Flyer, groups of engineers, scientists, pilots, and others can gather together and have a spirited debate on how an airplane wing generates lift. Various explanations are put forth, and the debate centers on which explanation is the most fundamental.

— John D. Anderson, Curator of Aerodynamics at the National Air and Space Museum"




Im really glad your back and talking about planes and stuff Evan...

hitnmiss
02-28-2013, 03:09 PM
A clark Y at 0 deg looks a lot like a symmetrical airfoil at 10 deg or so angle of attack.

I can believe the 20% number because I've flown many RC planes with Clark Y flat bottom airfoils inverted.

Evan
02-28-2013, 03:28 PM
Here is a simulator from NASA specifically intended to test airfoils. It requires JAVA be installed (but not in the browsers AFAIK).

http://www.grc.nasa.gov/WWW/K-12/FoilSim/index.html

A.K. Boomer
02-28-2013, 03:40 PM
A clark Y at 0 deg looks a lot like a symmetrical airfoil at 10 deg or so angle of attack.


Yes but you really can't call that an angle of attack if the bottom part is level - it's just the upper part that's tapered back down that makes it look that way,

You can make a simple symmetrical one by simply putting half of a sphere up front and connecting the tails together - then you just mount it with the bottom flat and it will lift - but the typical ones iv seen in diagrams look like a piece of quarter round up front with the round part leading top, then connect the tails together...

the entire point is - is they produce lift without any extra purchase angle on the bottom, this can only mean one thing.

DICKEYBIRD
02-28-2013, 04:33 PM
The Clark Y as plotted from the original coordinates is only flat on the bottom from about the 30% (of chord) point aft. From the the 30% point forward it has a gentle curve upwards.

Come to think about it, I think what we perceive as flat actually has a very slight undercamber when plotted from the original (ancient) coordinates. I'd have to fire up Eric Sanders' (AiR_GuNNeR here on the forum) Compufoil program to check & be sure.

Come to think about it #2, haven't seen much on here from Eric lately. Hope he's OK and just obsessing over that nice Emco lathe he drug home.:)

A.K. Boomer
02-28-2013, 04:55 PM
Take the experiment to the most basic level - a triangle with a 1" blunt vertical wall up front (your "leading edge" lol)
and a 90 degree from that bottom base that's 6" long, then connect the top of the vert. to the trailing end of the bottom part,

now pass air directly in line with the bottom and directly confronting the vert. leading edge, and walla - you still have lift...

J Tiers
02-28-2013, 09:35 PM
It is instructive to look at the drag figures for aircraft vs autos. There is no comparison even with the very best cars. Aircraft have numbers that are a very small fraction of cars.

Yep.....

But aircraft don't have to have all that crap on the bottom of them, and the users can be TOLD how they will get in and out, what they can put in, etc...... With cars, the landing gear is always deployed, the shape is smaller, and still has to fit the usual shape of payload.

And the payload has opinions about what it will and will not get into, what else will get in with it (and need space).......... ;)

A.K. Boomer
02-28-2013, 11:33 PM
You can always combine the two and check out the draft signature on them,

just have Evan run his program on an indy or formula 1 race car, it's like driving a car and an upside down airplane both at the same time...

Evan
03-01-2013, 12:55 PM
The program doesn't have sufficient resolution to test something with lots of little projections and skinny suspension members etc. If it did it would need a much more powerful computer. CFD is exceedingly heavy on number crunching resources. It is also very well suited to parallel processing which is why they are building machines with large clusters of CPUs including the Nvidia graphics cards with huge numbers of onboard graphics CPUs. It's a shame the Falcon program doesn't take advantage of the graphics card. I have some fractal programs that run using PhysX on my Nvidia GTX430 which is rather dated now and it still runs a hundred times fasted than CPU based software.

Black_Moons
03-01-2013, 01:35 PM
gpu speed really is amazing, I moved a simple fluid/erosion simulation from CPU to GPU.
512x512 x 5000 loops on cpu: 30 minutes
1920x1080 x 5000 loops on gpu: 15 seconds

Evan
03-02-2013, 12:27 PM
For a top of the head WAG my 100 times was pretty close. Your speedup was 120 times.

MrSleepy
03-04-2013, 03:50 PM
The Force India Sahara Formula 1 team have a video that shows their windtunnel and shots of their CFD screens

https://www.youtube.com/watch?feature=player_embedded&v=aX33YQ1tpKI

11mins = windtunnel
12.50 = CFD.

Rob

The cfd screenshots have been captured here (http://www.f1technical.net/forum/viewtopic.php?f=6&t=285&start=510)

DICKEYBIRD
04-29-2013, 02:44 PM
I saw one of these "Trailer Tails" on the Interstate this weekend and thought about this thread. I 'spect there's more than a little fuel savings to be had using one and am puzzled why you don't see more of them.

http://www.atdynamics.com/fleets.htm

The Artful Bodger
04-29-2013, 04:32 PM
I don't know why 'trailer tails' etc must be rigid as I expect a simple fabric bag of the right shape could be fashioned and inflated when on the road. Soft landing for tail-gaters too!

dalee100
04-29-2013, 10:14 PM
I saw one of these "Trailer Tails" on the Interstate this weekend and thought about this thread. I 'spect there's more than a little fuel savings to be had using one and am puzzled why you don't see more of them.

http://www.atdynamics.com/fleets.htm

Hi,

I suspect trailer lengths. Added to a trailer would possibly make them illegal to use because they would be over length.

dalee

Willy
04-29-2013, 10:59 PM
The trailer tails do have a very positive effect on fuel consumption figures, and yes in certain archaic jurisdictions they do contravene overall length guidelines. It will take a few years before we see a North America wide agreement about implementing these types of aerodynamic aids and their effect on length dimensions.
The tails in conjunction with skirts and under trailer air control devices that reconfigure airflow around the trailers axles are all being actively researched and increasingly put into service.
Class eight truck manufactures have been very busy themselves as well over the last ten years or so in order to increase fuel mileage as well. Aerodynamics improvements have already made huge impacts on fuel efficiency. Drive-train efficiency has also helped considerably as well. It is now normal to run an 18 wheeler at 1200 rpm or even less at 60 mph. Doing this has meant redesigning the entire power train from front to back.

Fuel is the biggest cost to those that transport just about every item in our lives. Whether you operate one truck or have a fleet of five thousand, cost reduction and efficiency is going to sell trucks and trailers. The industry stands to save billions in reduced fuel consumption. Efficiency whether mechanical or aerodynamic is not going to go away, it's becoming the biggest sales tool in the transportation industry as fuel prices continue to climb.