can a plane takeoff on a conveyor belt? (mythbusters)

[Matt Taylor]

From a future mythbusters (wed, December 12th)

"Finally, Adam and Jamie carefully navigate their way through a myth that has baffled everyone from web bloggers to pilots. If a plane is traveling at takeoff speed on a conveyor belt, and the belt is matching that speed in the opposite direction, can the plane take off? Extensive small-scale testing with a super treadmill and a nearly uncontrollable model airplane don't completely resolve the myth, so our flight cadets supersize the myth with help from a willing pilot and his Ultralight flying machine. "

On another forum I visit, the idiots are arguing vehemently about this. I know there are some pilots on here; what are your thoughts?

 

[jim-00-4.6]

I'm not a pilot, but I saw one on TV, so I'm qualified to answer.
Let's say "takeoff speed" is 165 mph.
The plane is "travelling" at 165 on a treadmill means the tires are turning, but the plane is not moving.
Therefore, there is no airflow over the lift surfaces, and no lift.

Now that I've typed this, I suspect you were just joking, right?

 

[David Despain]

its all about realitivity (thank you einstien). speed of the plane doesnt matter. speed of the treadmill doesnt matter. the only thing that matters is the speed of the air passing over the airfoil shape of the wing. if the plane cant move itself through the air fast enough it wont fly.
edit: damn i didnt hit post fast enough

 

[noee]

Relativity? General Theory or Special Theory?

There's a little thing called Bernoulli's Principle that comes into play with any "wing" in a medium. There are also issues of Thrust, Lift, Gravity and Drag.

May I suggest "Stick and Rudder" by Langeweische.

 

[TurdFerguson]

HOLY crap! It actually made it to TV?????? I suggested this WAYY back...check it out...

Post #52

http://www.landroversonly.com/forums/showthread.php?t=6296&page=3&highlight=plane+treadmill

 

[az_max]

It's all about lift. You need the airfoil traveling at VR to generate lift.

Haven't seen the myth yet, but it could mean a plane is rolling on a treadmill that is going 165kts, or it could mean a plane is sitting on a treadmill that is going 165kts. The latter will achieve takeoff speed, but will soon stall if there is nothing else propelling it forward.

(that's a big plane to need 165kts to take off. I'm used to Cubs that can take off near 35kts.)

 

[Matt Taylor]

Well, I would agree with you guys.

But there are pilots and others stating that since the plane doesn't rely on friction against the earth to move, that the thrust of the engines will easily overcome the treadmill. They site bush planes in Alaska fitted with skis that take of from frozen lakes, etc.

The question isn't whether the plane will take off if it's stationary; of course it won't.

The question is whether or not the treadmill will make the plane stationary.

 

[TurdFerguson]

Again...the conveyor belt is MATCHING the speed of the freespinning wheels of an airplane...like i said in the LRO thread...if the plane moves a foot foward, the belt move a foot backward at the same speed...so yes, the Treadmill will make the plane stationary and therefore, will nto lift off.

 

[David Despain]

or another way to look at it. the conveyor is pushing the plane one way at 165 mph, in order to stay static the plane has to push on the conveyor at 165 mph. but the thing is the planes engines dont drive the wheels, they push on the air, which is not moving at 165 mph. so the plane accelerates and takes off.

 

[az_max]

Well, I would agree with you guys.

But there are pilots and others stating that since the plane doesn't rely on friction against the earth to move, that the thrust of the engines will easily overcome the treadmill. They site bush planes in Alaska fitted with skis that take of from frozen lakes, etc.

The question isn't whether the plane will take off if it's stationary; of course it won't.

The question is whether or not the treadmill will make the plane stationary.

Engine thrust is another matter alltogether. I'm assuming there's no other thrust involved other than the thrust needed to turn the treadmill.

Think of your hand as a treadmill moving forward. Put a paper plane in your hand. if you throw hard enough, the plane will fly when you release it. since it has no onboard thrust it will glide (crash) to the ground.

 

[TurdFerguson]

or another way to look at it. the conveyor is pushing the plane one way at 165 mph, in order to stay static the plane has to push on the conveyor at 165 mph. but the thing is the planes engines dont drive the wheels, they push on the air, which is not moving at 165 mph. so the plane accelerates and takes off.

AH here we go...there's where teh confusion is...

The theory states...that the Conveyor Belt is MATCHING the speed of the airplne" so that only leads me to believe that there is some sort of sensor on it and the belt moves under its own power

 

[az_max]

or another way to look at it. the conveyor is pushing the plane one way at 165 mph, in order to stay static the plane has to push on the conveyor at 165 mph. but the thing is the planes engines dont drive the wheels, they push on the air, which is not moving at 165 mph. so the plane accelerates and takes off.

In your case, the plane's wheels would be doing 330mph (not that it matters). the plane is doing 165mph ground speed, 330mph versus the treadmill. It will take off, and since it's been supplying thrust, it will continue to fly. but I'd check those wheel bearings on your next landing

Turn that around now. The treadmill is going the same direction as the plane. Parking brake is set on the plane so it doesn't roll, the plane is now doing 165mph vs the true ground, 0mph vs the treadmill. It will take off, but will need additional thrust to stay airborne.

 

[Matt Taylor]

From another forum:

"But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly."

 

[noee]

It's a weird thing to conceptualize. I know that we love a nice headwind (2000 ft field), because, of course, your airspeed is instantly greater than your ground speed. When the wind is pushing at you, it's a good thing. But when the ground is pushing at you....well....that's hard to imagine.

But really, it's a question of how much thrust is needed to produce enough groundspeed to overcome the fact that the "ground" is moving you the opposite direction.

The point at which the thrust generated produces the groundspeed necessary to equal the moving ground is where the plane would become stationary.

After that, the point at which the thrust can overcome the moving ground and provide the airspeed needed for rotation is when the takeoff could occur.

 

[David Despain]

In your case, the plane's wheels would be doing 330mph (not that it matters). the plane is doing 165mph ground speed, 330mph versus the treadmill. It will take off, and since it's been supplying thrust, it will continue to fly. but I'd check those wheel bearings on your next landing

Turn that around now. The treadmill is going the same direction as the plane. Parking brake is set on the plane so it doesn't roll, the plane is now doing 165mph vs the true ground, 0mph vs the treadmill. It will take off, but will need additional thrust to stay airborne.

yes exactly.

 

[David Despain]

But really, it's a question of how much thrust is needed to produce enough groundspeed to overcome the fact that the "ground" is moving you the opposite direction.

The point at which the thrust generated produces the groundspeed necessary to equal the moving ground is where the plane would become stationary.

After that, the point at which the thrust can overcome the moving ground and provide the airspeed needed for rotation is when the takeoff could occur.

yes exactly, and i propose in this imaginary airplane that amount of thrust would not be very high at all. it really doesnt take all that much power to continue to roll on the gound once you are moving. some planes require the use beta range or thrust atenuators so you dont cook the brakes.

 

[p m]

or another way to look at it. the conveyor is pushing the plane one way at 165 mph, in order to stay static the plane has to push on the conveyor at 165 mph. but the thing is the planes engines dont drive the wheels, they push on the air, which is not moving at 165 mph. so the plane accelerates and takes off.

god damn....
That's some funny shit.

 

[I HATE PONIES]

So thats why they have the long motorized walkways in airports. Its actualy a backup runway. You learn something everyday.

 

[disco_fever]

the plane's engines are pushing the air for thrust, if the wheels drove the plane it would be a different story. thus the conveyor belt will just turn the wheels twice as fast as the plane takes off.(assuming low friction and the wheels don't reach their limits)

test this yourself, take a toy car, put in on a treadmill. push with your hand, it requires almost no more force to push it forward on the treadmill as it does on the ground. the wheels just spin twice as fast.

 

[I HATE PONIES]

The planes engines are pushing the air to keep the plane stationary. There is no air speed however. Forward motion is required to provide lift. The only other way to provide lift is to have headwind. Wind or air is the key here. A plane wont fly in a vacum. It also will not fly if there is no air passing over the wings. From the original statement there will be no air movement for the wings. Rearward thrust does not exist if the plane is merely equalizing the front rear movement. Thrust is measured by forward movement vs rearward power. If they are equal you have no thrust.