Conveyer Belt Takeoff- Mythbusters jumps in!!!

[Guest jboweruk]

The point is though that the aircraft will have to reach forward velocity. If it doesn't attain enough forward momentum it won't takeoff, which I believe was the whole point of the excercise. To prove whether a plane, on a conveyer going in the opposite direction enough to keep the plane still, will or won't lift.

[LAdamson 240]

>The point is though that the aircraft will have to reach>forward velocity. If it doesn't attain enough forward momentum>it won't takeoff, which I believe was the whole point of the>excercise. To prove whether a plane, on a conveyer going in>the opposite direction enough to keep the plane still, will or>won't lift.A year or so ago, I tried an experiment, when this conveyer belt question came up. The test consisted of a model airplane with fairly smooth turning wheels, sitting on a treadmill with rubber bands tied to a post.The idea, was to see how friction would stretch the rubber bands, as the treadmill was speeded up. I originally believed, that the aircraft would not take off either.As the treadmill belt speed was increased, the rubber band just didn't seem to stretch. At least not to a real noticeable degree.I then, changed my mind. The airplane WILL takeoff! :-hah L.Adamson

[bob.bernstein 0]

John...imagine this....you and your friend stand on either side of a treadmill. You can set the treadmill to run at any speed you wish.You and your friend are holding an axel with a free spinning wheel that rests on the treadmill. Do you think you will be unable to walk forward?Now increase the speed of the treadmill...you think you will be unable to walk forward now?The fact is, that walking forward is independant of the treadmill, as your propulsion to walk forward is all about your legs, friction between your shoes and the ground. Has nothing to do with the treadmill.Just like the airplane...its propulsion is propellor and air...has nothing to do with the treadmill.When a treadmill is not part of a propulsion force system, its presence is irrelevant....its speed is irrelevant...If you doubt this, try my experiment at a gym...if you cant find a wheel, try a barbell and hold it loosley letting the bar rotate in your hands.BobPs...while we can't know how credible we are on the internet, please understand that my writing is not based on opinion...its simple dynamic equilibrium analysis...taught in 2nd year engineering school...I'm a BS in Mechanical Engineering (Penn State, '77). (Go Nittany Lions!)

[michal 44]

>The point is though that the aircraft will have to reach>forward velocity. If it doesn't attain enough forward momentum>it won't takeoff, which I believe was the whole point of the>excercise. Not sure what your 'point' is. Yes, the aircraft WILL reach 'forward velocity' and WILL 'attain forward momentum' (using your vocabulary), this is exactly what I have been writing about. Also read excellent and very simple arguments by Larry and Bob.Michael J.http://img142.imageshack.us/img142/9320/apollo17vf7.jpg

[Guest jboweruk]

Guess we'll just have to wait and see, this could go back and forth like a tennis match forever at this rate.

[kerosene31 349]

I love this question as it really shows how the human mind works and how we can be so easily trapped by our own assumptions. When I first heard this question, I got it totally wrong like so many others. I was sure that the treadmill would hold that airplane in place! As I visualized a plane on a treadmill, I only imagined the treadmill to be slightly bigger than the plane, so I didn't even give it a chance to take off! I made a false assumption and couldn't get past it. It all comes down to assumptions. When you tell someone to imagine a hammer, what pops into your head? A normal size hammer that you can get in any hardware store most likely? One that probably every one of us has lying around the house. But what about a sledge hammer? Or a hammer on a big drilling machine? This is what happens with this problem and why it is so tricky. When you tell someone "treadmill", what do you envision? Something keeping something else in place, because that's what a treadmill does! However, the truth is that a powered airplane cannot be stopped by a treadmill in the same way that a runner is kept in place. The plane most definitely will fly, assuming:-the plane's wheels are free-spinning and can take the extra friction-it also depends on exactly what speed the treadmill matches: airspeed, wheel speed, ground speed? etc? Assuming ground speed, it should take off no problem.

[bob.bernstein 0]

Craig, I was with you right til the end...why do you think it depends on treadmill speed?Bob

[bob.bernstein 0]

Perhaps you can't accept analysis as a valid form of decision making. There is no tennis match...there are only people who haven't studied dynamics and people who have. Perhaps some of you haven't learned what rigorous analysis even is yet. Therefore you treat the analysis offered as an opposing opinion. Its not an opinion, its rigorous analysis.Have you made an attempt to imagine the gym experiment? Do you really think that you and your friend cannot walk forward beside the treadmill holding the freespinning barbell between you (resting on the treadmill)?Somehow that treadmill is going to nullify your ability to walk across the gym floor with a bar freely rotating in your hands?LOLI guess some folks think the world is flat in spite of the orbital imagry shown to them. Perhaps this is the same thing.

[kerosene31 349]

>Craig, I was with you right til the end...why do you think it>depends on treadmill speed?>>BobWell, basically some people try to justify the "it will not take off" answer by saying that the treadmill speed will match the actual wheel speed of the plane, which would technically keep accelerating until either the plane took off or the wheels exploded. However, that goes way beyond the original problem IMHO. People work backwards from their wrong answer and try to find a way to make it right :)Depending on how the problem is worded, it is usually not specific on speed, and some people get hung up on that. Also a few argue that the wheels will need to spin faster than originally designed and will fail, which I suppose is plausible. This goes way beyond the original problem though. Making reasonable assumptions, the plane will easily take off :)

[michal 44]

>there are only people who haven't studied dynamics and people who have... >Its not an opinion, its rigorous analysis.Bob, thanks for underlying this fact. And actually not more than high school level of Newtonian physics is needed here. We are not talking discussing here Einstein's space warping/dragging experiments in relativistic physics. *:-*Michael J.http://img142.imageshack.us/img142/9320/apollo17vf7.jpg

[Guest jboweruk]

Ah but the people with the barbell have something solid to push against, the aircraft has only air, which is not solid. Also if the friction on the barbell increases beyond the push of the two people holding it they will fall over and drop it, You ever seen somebody lose it on a treadmill?I still think that as long as the belt is sped up along with the airspeed then the plane is going to sit stationary, and if it's stationary no air flow, and no airflow means no fly.An aircraft does not have to move forward to takeoff, if the opposite wind is equal to the takeoff speed then a cessna sitting on a runway not moving will still generate lift, however if the airflow is not equal to takeoff speed then it doesn't matter how fast the aircraft is going it won't lift, for instance if the wind is behind you on takeoff and equal to your takeoff airspeed then you will need to be going at double normal speed before you get any lift, though your airspeed will be the same.I may be wrong, if I'm proven so then great, and the plane takes off, if I'm right, it won't, because it won't get enough forward speed to get the airflow.

[Guest RonB49]

"Ah but the people with the barbell have something solid to push against, the aircraft has only air, which is not solid."But John, the aircraft is pushing against the same "nonsolid" air whether 'tis on the treadmill or off. So if it moves forward on a runway, it should do the same on the treadmill; I think. :)"I may be wrong, if I'm proven so then great, and the plane takes off, if I'm right, it won't, because it won't get enough forward speed to get the airflow."I have no doubt that the airplane will take off on the Mythbusters show and that the myth will be declared BUSTED because I don't believe their treadmill will be fast enough to prove anything meaningful. My only question involves theory. If we could move the belt at an infinite speed, and if we could keep the tires from exploding, and if we could prevent a bearing from seizing, and if, and if, and if ... would the airplane still take off? IMO, probably yes but they won't prove that.R-

[michal 44]

John,I am afraid what you need is a solid course in basic physics and not more TV. Watching the program may not help you - there may be technical glitches, the belt may be too slow to make a difference, there could be countless reasons why a casual observer would be left unconvinced (my main interest in this program is exactly seeing if they can overcome significant technical obstacles). If you really knew basic physics the whole experiment would be superfluous. If your physics teacher gave you this problem then the answer "if it takes off then it takes off if it doesn't then it doesn't" I am afraid would not cut it.Michael J.http://img142.imageshack.us/img142/9320/apollo17vf7.jpg

[bob.bernstein 0]

John, Enjoy your TV show.Bob

[btacon 1,831]

Remove the analogy of a treadmill and a wheeled aircraft. In its place put a float plane and a rushing river. Prevailing winds make it necessary to depart "upstream". Will the float plane take off? Yes. Why? Because the propulsion system of the aircraft is completely independant of the ground surface (unlike a car). In this example the river may have more effect on the A/C, as there is much more friction involved, but in the end the plane will have no trouble reaching V1 (80 - 110 kts) in spite of the river.I know this has been already shown ad nauseum, but I thought the floatplane might add a new light.No need for me to watch the show, I already know the answer.Cheers,bt