Will it Fly??

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Chet18013

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Mar 5, 2005
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Full time in RV. Home is where we are parked
Here's one for you to think about.

Let's say you have a single engine airplane sitting on a runway that is 3000 feet long, and through some great engineering the runway is actually a giant treadmill. It's engineered so that when the plane starts to move forward the runway moves in the opposite direction at exactly the same speed., i.e. as the airplane's wheels pick up speed the runway matches it.
Will the plane get airborne?

What do you think?

Chet18013
 
Chet,
No, it won't. To fly, the planes wings must pass through the air at a speed high enough to provide the necessary pressure differential between the upper wing surface and the lower wing surface to cause 'lift'. Being on a treadmill doesn't cause any relative wind speed, so you'll just sit there. Interestingly though, if you are landing into a headwind (as you should), and the wind is blowing straight down the runway at or above the stall speed of the plane, you can land on the runway with absolutely no forward motion - land on a dime and stay there! I've done it.
 
An airplane doesn't depend on the runway surface to provide forward motion.  The propeller pulls it through the air.  Therefore to prevent forward motion, the treadmill would have to instantly reach whatever speed necessary for the wheel friction to counteract the propeller thrust.  If the engine has enough power to overcome the wheel friction, the airplane will move forward and take off even if the treadmill has infinite speed.
Art
 
That would sure wear out the tires in a hurry :)
 
Art is correct, The plane interacts with the air, the treadmill would fly apart before it kept the plane grounded. there is no power to the wheels on a plane.
 
Chet18013 said:
It's engineered so that when the plane starts to move forward the runway moves in the opposite direction at exactly the same speed., i.e. as the airplane's wheels pick up speed the runway matches it.
Will the plane get airborne?

If the moving runway keeps up exactly with the speed of the airplane, the airplane never moves relative to the world around it.  There wouldn't be enough air moving over/under the wings to create lift to take off.  The thrust of the propeller doesn't create enough wind over/under the wings for lift.  Let's say the airplane's stall speed is 50 mph.  Then the the airplane has to be moving through the air or the air moving past the airplane at at least 50+ miles per hour to fly.  If the airpne never moves relative to the wind around it, it stays down.
 
It would even be worse trying to land the dang thing on the treadie.  Bottoms up!
 
Art has it exactly right.

    Keep an eye on the upcoming show on MythBusters, called Plane on a treadmill.

    Will be shown soon on the Discovery channel.

carson FL
 
Sorry; I'm not buying it. Those large, long wings aren't there just to look pretty, they have to be that large and long to provide enough 'lift' (actually atmospheric pressure on the underside of the wing) to overcome the force of gravity. On a regular, single engine aircraft like a Cessna 150, the air flow from the propeller covers only a small portion of the wing - certainly not enough to actually lift the plane and cause it to fly. Yes, a really large engine and propeller might get it off the ground and moving in a forward direction, but that's not flying; that's a missile kept in the air only by thrust. No wing aerodynamics involved.

Maybe this was a trick question? One plane could do it. A helicopter! :D
 
Karl, this is fun...

Take this scenario:
    The plane is sitting on the belt. Engine off, belt not running.
    Tie a rope on the front of the plane in order to hold it from moving backwards when the belt starts. Activate the belt slowly. The plane will only try move backwards due to wheel/tire friction. One person will be able to hold it from moving backwards. (assuming a small plane).

Next- Start the engine, belt moving slowly. Will the plane move forward?

Next- Increase engine power. What happens?

Next - Speed up the belt. Wheels are turning faster now. Will the plane stop?

Next - Full power, belt at high speed, the wheels will be turning much faster. The plane will move forward the same as on land, except for the slowdown of the friction factor of the spinning wheels, which would be very minor compared to the pulling force of the prop.

    These are not trick questions and it is not a helicopter.

What do you think?

    Tell me I am wrong. I want to learn.

carson FL


 
Carson:

The plane's engine will not be at full power; it will only be generating enough thrust to offset the friction of the wheels on the belt.

IF you applied full power the plane would no longer be on the belt but piled into whatever was just in front of the belt. ;D
 
It's the airplane's forward motion relative to the air mass that creates lift and lets it fly.  It doesn't matter how fast the wheels are moving relative to the runway.  A glider doesn't have any thrust but its forward motion relative to the airmass creates the lift that allows it to fly. 

Lets try this another way.  Lets assume the airplane is setting on the deck of an aircraft carrier and the nose is tied to a point forward on the deck with the aircraft engine off.  If the aircraft carrier can accelerate past the airplanes stalling speed, the airplane could lift off the deck and the wheels of the airplane have never turned.

I love this type of disscussion.
 
Jeff, my point.

    The belt is the runway - 3000feet. Somewhere along the belt the plane will lift and gain altitude.

Lowell, I don't see your point. The belt is not moving the plane, the propeller is. The belt is going backwards as in a mill in the workout gym.

carson Fl
 
Carson:

Get on a treadmill holding a kite. and start running. Tell me how much wind you feel in your face at full speed?

The wind is what makes the kite (airplane) fly, not the treadmill.
 
Let's put another way, unless the plne gets enough air speed to reach takeoff air speed, it will not fly.  Now if it can do that in less than the 3000 feet with a moving runway, great. Otherwise it will just sit there!!  On the other hand if it can, why use the treadmill!!  VBG
 
Jeff Cousins said:
Carson:

Get on a treadmill holding a kite. and start running. Tell me how much wind you feel in your face at full speed?

The wind is what makes the kite (airplane) fly, not the treadmill.

You forget though when you are flying a kite the force that moves the kite through the air is your feet in contact with the ground, There is "power to the ground"

An air plane has no power to ground, none, zip, nada, there is no drive to the wheels, it moves itself by pulling air over past the prop or through jet engines, action and reaction, not friction, is what moves the plane.  The propeller on a prop plane is a rotary wing (Well Wings) that pull air

In another language an airplane is called an air-screw because the prop 'Screws" through the air like a wood screw through wood

The wheels are just there to hold it up when parked, not to move it so the treadmill  won't keep it down.

The kite analogy is ... Not applicable
 
Well, I'm rethinking this.  The propeller should be moving the airplane forward.  In the problem statement, it says the runway moves keeping up with the planes wheels.  So now I'm thinking that the runway doesn't move at all as it's backward motion keeps up with the wheels turning, and the airplane does take off.  ::)
 
Gosh what a bunch of thinkers.  Ask a lady.  They think with both sides of their brain.
 
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