sieanr said: omgwtfbbq said: Entroper said: blaydcor said: they're using an "Ultralight flying machine", not a jet, if anybody would take the time to read the OP. A lot of you are arguing using pseudo-physics, anyway. Personally I'ld be amazed if they got the plane to take off. An ultralight flying machine still uses a propeller, does it not? It's just a really small prop plane. What "pseudo-physics" are you referring to? yes the concept is the same. Probably even more so since on an ultra light plane the wheels are not connected to anything and are just there so the plane can move on the ground, so there will be even less friction than on a place with motors on the wheels. Not going to make any difference unless the treadmill starts moving before the plane, in which case a small plane may struggle to start moving. pseudo physics either means "physics I don't understand" or "no calculations". Anyway, prepare to be amazed, blaydcor. There's no way that plane isn't taking off.   Uhh, planes dont have motors on the wheels, unless you got confused with brakes.

some do, I thought. However, I may be wrong. Whether they do or not doesn't change my point, in fact it just makes it more likely for the plane to take off

 

Entroper said: elendar said: The planes engine produces thrust. Planes taxi ect. So they can move using just wheels. The treadmill allows the engines to use more thrust. Further, there are jets that take off vertical. So we know by channeling the thrust down you will be able to take off. It will just be a very very fast treadmill. Brain... melting... from... absurdity... overload...

Oh? Whats so absurd about it? I reread it. Perhaps you didn't understand what i meant by move using just wheels. A better way to say that would be move with the weight being supported by the wheels instead of using wings for lift. In my example I was using a jet. Would it be possible to put a jet on a treadmill instead of an ultra light prop plane? Possible yeah.. End result they should spin the theoretical treadmill.

Uhm ok...wait...

Is the plane actually moving at takeoff speeds relative to the air or only relative to the treadmill? If the plane is only moving at takeoff speed relative to the treadmill its not going anywhere.

If the treadmill is going at 200mph + takeoff speed it will takeoff.

The actual situation needs to be properly defined.

omgwtfbbq said: epsilon72 said: They're doing this myth? I would've thought it would be a no-brainer... Maybe one of their short 5 minute ones or something...   edit: wow, some people here think it might be possible? No forward velocity--->no lift--->no takeoff. Now, if they were to blow a huge amount of air at a high velocity at the conveyor-belt-plane, then it would possibly work (and I can see mythbusters attempting this on a small scale, possibly) it's clearly not a no-brainer since you got it wrong =) no forward velocity won't happen, no treadmill's going to stop a plane taking off

It's really a tricky situation. A specially built propeller plane would actually be able to take off (or a model plane on a real life conveyor belt) - and probably result in a spectacular crash due to uncontrollable sudden forward movement. A jet engine plane will not take off.

Now we all agree that planes fly because the forward movement of the plane creates an airflow on the wings that pushes air down and momentum conservation requires something else and of equal magnitude pushing up (yes I know that the demo in the Smithsonian museum tells you something different..)

Now if the plane rolls forward and the conveyor belt spins at the exact opposite speed, the plane has a net forward speed of zero (we can actually enter and leave the plane if we dare) - and there is now airflow over the wings since the plane is not moving in reference to us or the air around us. Imagine these tests they do with your car in a garage where the conveyor belt is two rolls and the wheels of your axis sit on the wheels. You can accelerate to any speed and still be on the same spot.

Now the tricky part: Let's make a propeller plane that blows the propelled air entirely over its wings (jet engines don't blow air over their wings). This plane is still stationary on the belt rolling in the opposite direction. Now the situation is different, we actually have airflow over the wings and therefore generate momentum. If we accelerate the plane and the conveyor belt to takeoff speed, we actually take off and hover shortly above the belt - and then rapidly accelerate forward after loosing contact with the belt which eould make this test rather uncomfortable to the pilot.

(Greeting from Dieter, who has taught physics to students for some years, so I hope I am not too wrong or none of my former students reads this...)

Sqrl said: Uhm ok...wait... Is the plane actually moving at takeoff speeds relative to the air or only relative to the treadmill? If the plane is only moving at takeoff speed relative to the treadmill its not going anywhere. If the treadmill is going <----- at 200 mph and the plan is going ----> at 200mph + takeoff speed it will takeoff. The actual situation needs to be properly defined.

 See that's the problem it's impossible to test because there is no way the treadmill will hold the plane stationary.

elendar said:   Oh? Whats so absurd about it? I reread it. Perhaps you didn't understand what i meant by move using just wheels. A better way to say that would be move with the weight being supported by the wheels instead of using wings for lift. In my example I was using a jet. Would it be possible to put a jet on a treadmill instead of an ultra light prop plane? Possible yeah.. End result they should spin the theoretical treadmill.

I did misunderstand if that's what you meant about the wheels, I thought you meant the wheels were motorized.  This is still absurd though: "The treadmill allows the engines to use more thrust."  How exactly does that happen?

 

@drkohler, the plane will move forward, not just roll forward.  Remember, the plane's engines, whether they're jet or prop, are thrusting against the air, not the treadmill.  Your comparison with the car is invalid, because the car's method of propulsion is by its wheels pushing against the treadmill.

Entroper said: elendar said: @drkohler, the plane will move forward, not just roll forward. Remember, the plane's engines, whether they're jet or prop, are thrusting against the air, not the treadmill. Your comparison with the car is invalid, because the car's method of propulsion is by its wheels pushing against the treadmill.

Well the problem is worded in a way that assumes the plane does _not_ move forward.... 

Maybe the car thing was more confusing the matter than resolving it. We must take care of the reference frame we look at (and let's say our reference is somebody standing still at the conveyor belt) where the plane is initially:

Assume the plane engines do not work but the conveyor belt runs backward at speed x. What happens? Of course, the plane will be moving backwards at speed -x of the conveyor belt (assuming friction holds the plane on the belt). Its wings are also moving backwards with speed -x in relation to us and the still air surrounding us, therefore creating downward momentum (and increased friction) since the plane moves backwards so the wings 'point in the wrong direction'.

Now assume the plane's engines run and it rolls forward exactly with the speed +x of the conveyor belt, on the moving belt. Its relative velocity to an observer standing next to the belt is now -x (from the belt) + +x (from the plane), so -x +x = 0, the plane is stationary to an observer - and therefore stationary in relation to the air surrounding the observer.

However, the working engines themselves now move air backwards in reference to the wings at a velocity z (z is high enough to achieve a forward roll movement with speed +x). It now depends on where the air comes from and where it goes to. If we somehow glue an engine to the wheels, the air sucked into the engine and expelled behind it 'never sees the wings' - therefore it does not create any vertical momentum (assuming the engine is glued horizontally onto the wheels, otherwise we have built a rocket, not a plane). If the air flow from the engine is along the wings (as is the case with a propeller engine), it creates upward momentum on the wings and the plane will, in fact, lift off the belt when it surpasses lift-off speed.

That's my take and I stick to it

You guys seem to all be assuming zero friction between tire and wheel. There is friction. If you were able to somehow feather the throttle up slowly while increasing the treadmill speed, the friction of the wheels of the plane on the treadmill will exert a rearward force which could easily be countered by an increase in throttle. I've flown a plane and it doesn't take much throttle, once rolling, to keep plane moving down the runway at a decent speed and that is with wind resistance. Without wind resistance, throttle percent won't be nearly as high because the only friction to overcome is the treadmill.

All that said, planes work on one simple principle: lift generated by wind moving over the wing's airfoil shape. Without that lift, they plane is going nowhere. It doesn't matter if the treadmill reaches 300 miles per hour and the plane is running throttle at 30% to counter the friction. There is no lift without AIRSPEED.

This seems like a particularly silly myth to try to prove/disprove...

you're all getting it wrong! you're all saying "if the plane has no airspeed it won't fly" which is of course true, but you're missing the point that they're trying to slow down a plane with a treadmill. It simply won't work! The plane is pushing against the air. If it were a car it would be stationary. Imagine this scenario then:

You have a car driving at 100 MPH. It's a front wheel drive. The two back wheels are on a treadmill (ok, you'll have to imagine a mobile treadmill here so that the back wheels are on it and the front wheels aren't) and this treadmill is going backwards at 100 MPH. Will the car stay stationary? Of course not! The front wheels are providing the force, the back wheels are simply there for the ride. It doesn't matter how fast those back wheels are spinning, the front wheels will keep the car moving at 100 MPH.

This is the exact same scenario. The plane will move forward because no treadmill in the world can stop a plane from moving.

omgwtfbbq said: you're all getting it wrong! you're all saying "if the plane has no airspeed it won't fly" which is of course true, but you're missing the point that they're trying to slow down a plane with a treadmill. It simply won't work! The plane is pushing against the air. If it were a car it would be stationary. Imagine this scenario then: You have a car driving at 100 MPH. It's a front wheel drive. The two back wheels are on a treadmill (ok, you'll have to imagine a mobile treadmill here so that the back wheels are on it and the front wheels aren't) and this treadmill is going backwards at 100 MPH. Will the car stay stationary? Of course not! The front wheels are providing the force, the back wheels are simply there for the ride. It doesn't matter how fast those back wheels are spinning, the front wheels will keep the car moving at 100 MPH. This is the exact same scenario. The plane will move forward because no treadmill in the world can stop a plane from moving.

Oh boy... nobody is trying to slow down anything in that experiment. Now in your car case, put the belt where it belongs, under the FRONT wheels that DEFINE the momentary axis of rotation (it does not make any logic sense to place the belt under the wheels of the free running axle).

Of course the plane can and will stay stationary on the conveyor belt if its forward speed (generated by whatever means) exactly matches that of the belt. Now you can increase both speeds slowly to 'lift-off' speed, If the pilot can balance the plane long enough. At some (probably very high) speed, a propeller plane will lift off although being stationary to an observer standing at the belt. However, in the actual experiment a normal propeller engine might not be able to reach that speed (which is higher than normal 'lift-off' speeds) but the plane lifts off anways because it is probably not running horizontally but with engines slightly inclined giving the accelerated air a downward momentum.

 

Ever watched those fitness freaks running on a treadmill? They do exactly the same, they run just as fast forward as the belt runs backwards to stay in place...