Mythbusters to take on: PLANE ON A TREADMILL!!

[a_m]

You may have PROPWASH over the wing roots.

Propwash is not enough to fly in most aircraft.

Try it with a jet.. No airflow = no bernoullis = no lift = no fly.

http://www.airwarriors.com/forum/showthread.php?t=137919

(Is is possible that there is a thread you haven't posted in)

and also

http://www.straightdope.com/columns/060203.html

 

[JIMC5499]

Wrong, try again.

The speed of the aircraft has NOTHING to do with the speed of the wheels. All propulsion is from the engine, not the wheels. The wheels will end up spinning twice as fast as normal, but the aircraft will still take off.

So your saying that the propellor is pulling the airframe through the air which will give the airframe speed relative to the air not the conveyor? You might be right. Be interesting to find out. I have the R/C plane, I just don't have a big enough belt sander or I'd try it myself.

 

[MasterBates]

matt, you got your reference frames AFU..

The tire speed vs wheel speed will be whatever it is..

The plane still needs to accelerate relative to the air to liftoff speed..

Now if you had a really big treadmill, and pointed it into the wind, and drove it so the relative wind was above Vlof, they yah, it could work, but you could also fly it off the spot without a treadmill then.

 

[a_m]

So your saying that the propellor is pulling the airframe through the air which will give the airframe speed relative to the air not the conveyor? You might be right. Be interesting to find out. I have the R/C plane, I just don't have a big enough belt sander or I'd try it myself.

Ding! We have a winner. Also why the wheels are rotating twice as fast on liftoff.

 

[a_m]

matt, you got your reference frames AFU..

The tire speed vs wheel speed will be whatever it is..

The plane still needs to accelerate relative to the air to liftoff speed..

Now if you had a really big treadmill, and pointed it into the wind, and drove it so the relative wind was above Vlof, they yah, it could work, but you could also fly it off the spot without a treadmill then.

The tire/wheel speed is immaterial to the speed of the aircraft. Remember this is a frictionless wheel and conveyor with no wind.


If you read the 2nd link I just posted, it's explained pretty easily.

 

[MasterBates]

I just "built" a T-45 with frictionless wheels. And 1000kt tire speed.

And NASK put in a 8000'x 200' conveyor belt on RW 31L.

I taxi out. I arm the conveyor. I lock my brakes. Pust throttle to MRT. Wipe it out. Brake release.

Conveyor starts spinning as fast as the wheels.. since they are frictionless, and have no limit speed, they gain speed as fast as the conveyor can accelerate, up to some limit. The T-45 accelerates a little better than a normal one, because it has those speical frictionless bearings that have NO drag. I still accelerate the ClownJet to 120 kt, in about 1200', and rotate, and take off 300' later.

My tires are going some recockulous speed, and the plane flies away..

But I still burned up 1500' of conveyor using that whole F=ma thing, having to accelerate my 13,500lb jet to 120kt with 5500lb of thrust.

You will not stand still and "jump off the runway".. You will go down the runway about the same distance (same if frictionless wheels).

You may gain a little airspeed by conveyor pulling some air along with it, but nowhere near enough airflow to generate sufficient lift for takeoff.

 

[a_m]

I just "built" a T-45 with frictionless wheels. And 1000kt tire speed.

And NASK put in a 8000'x 200' conveyor belt on RW 31L.

I taxi out. I arm the conveyor. I lock my brakes. Pust throttle to MRT. Wipe it out. Brake release.

Conveyor starts spinning as fast as the wheels.. since they are frictionless, and have no limit speed, they gain speed as fast as the conveyor can accelerate, up to some limit. The T-45 accelerates a little better than a normal one, because it has those speical frictionless bearings that have NO drag. I still accelerate the ClownJet to 120 kt, in about 1200', and rotate, and take off 300' later.

My tires are going some recockulous speed, and the plane flies away..

But I still burned up 1500' of conveyor using that whole F=ma thing, having to accelerate my 13,500lb jet to 120kt with 5500lb of thrust.

You will not stand still and "jump off the runway".. You will go down the runway about the same distance (same if frictionless wheels).

You may gain a little airspeed by conveyor pulling some air along with it, but nowhere near enough airflow to generate sufficient lift for takeoff.

You just stated that it will move down the runway and accelerate. Why don't you believe it will take off if it can accelerate and gain speed? Your post has you both taking off and not taking off.

 

[MasterBates]

You guys have been talking about the conveyor as if it will allow the plane to just "accelerate in place" and pop into the air.

It will do nothing but wear out your tires. Now, if it tries to match the tires speed regardless (tires rotating for 30kt, treadmill goes 30, go 31, treadmill accels to match) you will do nothing but drive the system to its limit.

I saw that in a conveyor at SEMASS power plant in Rochester, MA, where it took the speed from a drive wheel, and senses speed with hall effect sensors embeded in the belt.. The thing ran away as soon as they turned it on, due to a slight mismatch in the sensor calibration.. (and it did not need to be that complex, perfect example of "overengineering)

 

[a_m]

You guys have been talking about the conveyor as if it will allow the plane to just "accelerate in place" and pop into the air.

Not in this scenario. Of course it won't jump right into the air.

 

[MasterBates]

Here is my professional opinion on the two:

Whacko theory land: (the land of perfect collisions, frictionless bearings and perfect conductors)
GIVEN A TREADMILL AS LONG AS THE NORMAL RUNWAY:
Plane accelerates, takes off in normal length.
Wheels accelerating at exponential rate approaching infinity, if the treadmill is matched to TIRE SPEED.
Wheels accelerating at twice the normal rate to twice takeoff speed if the treadmill is matched to AIRPLANE SPEED.

Real world, T-45, 176kt tire speed, non-frictionless bearings.
Some dumbass builds me an 8000x200' conveyor. I let Joboy fly this one, as I dont like blown tires and how the 45 handles with them.
Plane MAY take off, if tires don't blow/bearings melt. Increased takeoff run due to increased drag.

Belt matched to TIRE SPEED:
Belt accelerates to whatever its mechanical limit is.. Most likely blowing tires.

Belt matched to AIRPLANE SPEED:
Belt accelerates to 120kt, wheel speed 240kt at rotation. 264 at takeoff.
REALLY risking blowing a tire.. I'm not sure how much over speeding they can take.

 

[a_m]

Yes, in the real world, I doubt any of this is really possible, given tire limits and whatnot.

It seems our discussion was more over semantics than theory.

 

[MasterBates]

Maybe I misunderstood, but you guys were sounding like you thought it would allow the aircraft to take off in some really short distance. It won't.

 

[a_m]

Maybe I misunderstood, but you guys were sounding like you thought it would allow the aircraft to take off in some really short distance. It won't.

That is not what I was talking about. I was referring to the normal scenario given.

 

[Raptor2216]

Indeed it won't. Now I just can't wait to see them actually try and fail.

Maybe I misunderstood, but you guys were sounding like you thought it would allow the aircraft to take off in some really short distance. It won't.

 

[phrogpilot73]