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O.T. Helicopter physics test

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  • O.T. Helicopter physics test

    Watch the vid - stop it when you know what's being asked of you to speculate on - think about it for awhile then decide/commit and take the test...

    Do not feel bad about your results, I got the first one wrong and the modified second two right... But - I believe I actually might have got the first one right despite the claims of the vid... most of all - have fun with it, should be fun to discuss...

    https://youtu.be/q-_7y0WUnW4

  • #2
    Originally posted by A.K. Boomer View Post
    Watch the vid - stop it when you know what's being asked of you to speculate on - think about it for awhile then decide/commit and take the test...

    Do not feel bad about your results, I got the first one wrong and the modified second two right... But - I believe I actually might have got the first one right despite the claims of the vid... most of all - have fun with it, should be fun to discuss...

    https://youtu.be/q-_7y0WUnW4
    Ok Boomer. Ill have to Preface before I watch whatever in the heck you are getting me to do.

    I built a Bell 47 RC helicopter. Ground up. She was about 4 feet in blade sweep. The top part? The swash( you know what I mean, I know what I mean, heck. I had to fine tune it), very nice. Oh, and I had to try and fly it? Waa, waa..

    So I will go look. Thanks AK. JR

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    • #3
      Dang it. Hahaaa. I watched that entire waist of time. I thought it was going to be something fun ")

      I flew is a Sea Hawk, its the Navy's variant of the Black Hawk. Thay thing is a beast and they would never lower an unweighted line out board.

      That was a... I dont know. Informative lets say JR

      P,S, You wanna know what the Sea Hawk is good at? Popping sono boeys over Russian Subs in the Bearing Sea. And if need be she will stuff a very fast torp up the offending Subs back side. Yes. JR
      Last edited by JRouche; 11-07-2021, 12:46 AM.

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      • #4
        I do think the test is somewhat flawed due to propwash,,, so it's really not how a rope reacts to oncoming air it's how a rope reacts when being confronted with two different air currents....

        and in bringing up the swash --- think of all the air being distributed at the tail end of the craft to make it move foreword in the first place - you know - the area where the rope is trailing without the protection of the crafts body/mainframe...

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        • #5
          they say later that wash has a negligible effect much past the aircraft itself so does not influence the shape of rope. Unless the test was about one's helicopter rotor wash knowledge, it's a bit of poor show not mentioning that in the question
          in Toronto Ontario - where are you?

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          • #6
            I know -- that's what "they" say - but where's the test on that?


            fly a chopper over water at the same level that the bottom of the rope hangs and then tell me there's insignificant propwash as your watching the whitecaps it creates...
            Last edited by A.K. Boomer; 11-07-2021, 10:39 AM.

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            • #7
              The rotorwash probably is really and actually not significant. The forward motion of the helicopter means that it, and anything attached to it , is continuously moving away from the rotorwash. If you could see it, the rotorwash would be going down at an angle behind the helicopter.

              You are probably making the mistake of looking at the rotorwash when hovering, and extrapolating that to the effect when moving. There is plenty of disturbance when hovering.

              To stay in the air, the helicopter has to accelerate a certain mass of air downwards per second. When hovering, all that air moves down in a certain spot. But when flying forward, the moving air is distributed along the path of the helicopter. At any given point along the path, a lot less air volume is being moved than when hovering. So the rotorwash is much less intense at any single location than when hovering. How much less depends on speed of movement.

              Think of a fan blowing at you. When it is held pointing at you, you feel a definite wind over your body. Now, imagine that fan zipping past you instead of staying steady. It's entirely possible that you would not even notice any wind from the fan, depending on how far away you are from it's path. And what you do notice, if anything, happens after the fan has already passed you and is off to the side.

              It takes time for the moving air to reach you or the rope below the helicopter. By the time it reaches that level, the helicopter has moved on, away from that disturbed air.

              Also, in order to produce forward motion, there must be a thrust vector toward the rear of the helicopter. So the helicopter and rotor must tilt down at the nose That points the disturbed air behind the helicopter, away from anything hanging below it.

              The rope should be in undisturbed air, regardless of what may be the case behind the helicopter and rope.

              Now for the shape of the rope below the helicopter.

              Think of the old time radio antennas on cars. What shape to they take at highway speed? Yes, they take a curved shape similar to choice C. But, what does the antenna have that the rope does not? Stiffness. The antenna has stiffness, which tends to keep it straight up (or at whatever angle it was mounted). The rope has no stiffness keeping it sticking straight down.

              If you consider the rope to be a chain, with definite identical links, each link must support the links below it. So it hangs straight down with no wind. Any net force in a rope or chain must be along the chain at every point, since it cannot support any bending.

              In a wind, the last link at the bottom is pushed back by the wind. But the top end is held by the link above it. There is a moment around that attachment point, since the force back is displaced from the point of attachment. The ,link will be forced to the angle where the gravitational moment due to mass of the link at that angle equals the moment created by the wind, and will then be stable.

              The next link upward has the same thing going on. Without the link below, it would clearly be stable at the same angle.

              So what about when there is another link? Well, we already established that the moment due to wind was balanced by the moment due to gravity. Those are canceled out on the lower link. What is left is the tension in the chain, which has to be along the axis of the chain, so the two have no angle between them. If those were not canceled, the lower link would have to move until they were.

              For there to be an angle between them with the lower one pointing further back, the force of the wind would have to be larger in the lower link, so that the stable balance of the moments was at a different angle. (Or gravity must be weaker on that link😏🤔).

              As with those two, the same argument goes right up the chain. A rope is just a chain with vanishingly short links.

              It HAS TO take a form which has the force in it as tension axially oriented (along the rope or chain) at that point. Any force that is not tension along the rope will cause the rope to move until the force IS axial.
              Last edited by J Tiers; 11-07-2021, 12:12 PM.
              CNC machines only go through the motions

              Comment


              • #8
                Originally posted by J Tiers View Post
                The rotorwash probably is really and actually not significant. The forward motion of the helicopter means that it, and anything attached to it , is continuously moving away from the rotorwash. If you could see it, the rotorwash would be going down at an angle behind the helicopter.

                You are probably making the mistake of looking at the rotorwash when hovering, and extrapolating that to the effect when moving. There is plenty of disturbance when hovering.
                don't think im the one making the mistake with that, the rotor wash is not directed "downward" --- it's actually directed down AND at a fair angle backwards,,, that's what propels the craft forward (remember we are NOT talking about hovering)

                the swash plate acts upon the rotors blades in the rear of the craft more so than the front - this creates an imbalance of the CG in the craft tilting it in the process, this result creates "propulsion"

                so besides the rope just seeing "oncoming air" it's also seeing a blend of oncoming/downward mix from the rotor blades... it's the blending of the two that actually keep the craft in the sky whilst giving it propulsion...


                Since rotor wash could add to erroneous test results - they would have been far better off to hang a rope from a slow moving airplane,,, in fact better yet - hang one from a glider...
                Last edited by A.K. Boomer; 11-07-2021, 12:49 PM.

                Comment


                • #9
                  Originally posted by A.K. Boomer View Post
                  .............................................

                  so besides the rope just seeing "oncoming air" it's also seeing a blend of oncoming/downward mix from the rotor blades... it's the blending of the two that actually keep the craft in the sky whilst giving it propulsion...
                  Is the ROPE seeing that? If the forward movement is enough to blow the rope back, there is a fair bit of movement, the volume and mass of disturbed air is spread out along a "track". Because it takes time for the disturbed air to get down to the rope level, there is a good chance that the rope sees NONE of the disturbed air, especially down at the end of it, even though it is stretched out behind.

                  Time is key. How far has the helicopter moved forward by the time the moving air has reached the rope? At just 20 mph, it will have moved ahead about 30 feet in one second. And it is likely moving faster than that.

                  Remember the moving fan example.
                  CNC machines only go through the motions

                  Comment


                  • #10
                    Originally posted by J Tiers View Post

                    Is the ROPE seeing that? If the forward movement is enough to blow the rope back, there is a fair bit of movement, the volume and mass of disturbed air is spread out along a "track". Because it takes time for the disturbed air to get down to the rope level, there is a good chance that the rope sees NONE of the disturbed air, especially down at the end of it, even though it is stretched out behind.

                    Time is key. How far has the helicopter moved forward by the time the moving air has reached the rope? At just 20 mph, it will have moved ahead about 30 feet in one second. And it is likely moving faster than that.

                    Remember the moving fan example.
                    How fast would you say the air is coming off the rotors blades just to keep a bird like that in the sky - let alone keep it in the sky whilst also propelling it forward --- ?

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                    • #11
                      What is the frequency Kenneth?

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                      • #12
                        I think we can all agree that helicopters are badass. I still find it hard to believe that they are any kind of reliable.
                        Saw this show almost 40 years ago (Horizon - The Chopper):
                        https://www.youtube.com/watch?v=5VOttDBZDWI
                        There is a shocking view of the rotor blade in action at 15:57 - yikes!
                        Location: North Central Texas

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                        • #13
                          Wow --- yeah even if those blades were made out of flexible rubber id still be worried about fatigue,,, iv always been amazed at the swash plate linkage and how rinky dink it all looks yet I guess it must be pretty dependable...

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                          • #14
                            Originally posted by A.K. Boomer View Post

                            How fast would you say the air is coming off the rotors blades just to keep a bird like that in the sky - let alone keep it in the sky whilst also propelling it forward --- ?
                            We have no way of knowing for sure without more info, although the rearward air flow needed to move the helicopter is probably far less than that required to lift it. Plus in order to HAVE a rearward flow, the downwash has to start out pointed at an angle backward. Clearly then, when moving, the airflow starts out being tilted away from anything below the helicopter.

                            BUT...... the area swept by the rotor is a LOT larger than that swept by a propeller on an aircraft. The principle depends on both the mass of air and the acceleration imparted to it. So a large swept area means considerably less velocity is needed to get the same lift required.

                            Then also, when it IS moving forward, the situation is a LOT different from hovering. Even when hovering, the actual air velocity is not super high. a person can duck and run up to the helicopter without being blown head over heels away, as could be the case with a propeller.

                            Again.... think about the moving fan example.

                            Perhaps the best indicator that the downwash is not very important is the fact that the rope did hang in an angled straight line, as one would expect from an analysis of the situation with no effect of downwash included.

                            Also, in the video, at some times the end of the rope was actually curved up a bit, not what would be expected if it is being blown downward.
                            CNC machines only go through the motions

                            Comment


                            • #15
                              Originally posted by A.K. Boomer View Post
                              I do think the test is somewhat flawed due to propwash,,, so it's really not how a rope reacts to oncoming air it's how a rope reacts when being confronted with two different air currents....

                              and in bringing up the swash --- think of all the air being distributed at the tail end of the craft to make it move foreword in the first place - you know - the area where the rope is trailing without the protection of the crafts body/mainframe...
                              Originally posted by Mcgyver View Post
                              they say later that wash has a negligible effect much past the aircraft itself so does not influence the shape of rope. Unless the test was about one's helicopter rotor wash knowledge, it's a bit of poor show not mentioning that in the question
                              As a comparison, it would be interesting to repeat the tests using another helicopter that has a substantially higher gross weight, and therefore more rotor downwash. Should yield the same results, but with a different shape to the curves.

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