Topic: Can this airplane lift off?

[sangyo soichiro]

You folks seem mechanically-minded...

There is an airplane on a runway. The runway is like a conveyor belt, and can detect the speed of the wheels of the airplane. When the runway senses the wheels moving, the runway will match the speed of the airplane's wheels, but in the opposite direction.

Can this airplane take off?

Discuss.

[Duke McDukiedook]

The runway will keep the plane in one static spot so the plane will not be able to create lift unless the plane is one of those fancy jump/harrier jets.

[Gordon]

Yes, the plane will take off just like with a stationary runway.  The conveyor belt runway may be able to keep up with the speed that the wheels are spinning, but it won't affect the speed that the plane itself is traveling through the air. 

The only difference to the plane will be that its wheels will be spinning much faster before it lifts off than they would be on a normal runway. 

[kghost]

dukiedook has it right Gordon....no forward motion = no airspeed

However if the airplane was say Jet powered.....you'd need one hell of a conveyor belt.

For example a Boeing 727-200 generates about 45,000 lbs of thrust.

You'd spin the conveyor belt up to about 500 mph very quickly to keep it stationary

[Joel]

The Mythbusters tried this with a propeller driven plane.  The wheels of the plane aren't propelling it so it doesn't matter if the belt is matching the wheel speed.  If the propeller or whatever other power plant is throttled up, the plane will move forward and take off.

[Gordon]

You guys are thinking of this like it's a car.  The plane is being pushed through the air by its propellers or jets.  It doesn't matter what the ground under its wheels is doing, the plane will still be moved forward through the air by its propellers.  

[Duke McDukiedook]

No, I'm not thinking of this like a car. The plane has to have forward motion to create lift against the wing. If the runway was keeping the plane in one spot the plane is not moving forward, hence no lift.

[kghost]

Point is the airplane has to move.

If the conveyor keeps it stationary by rotating in the opposite direction....how does it generate lift?

You can run 30 MPH on a treadmill....great.... your airspeed is Zero.

Thrust still has to overcome weight.

At some point you could overspeed the belt...unless it were a hell of a belt and then you would in fact fly.

You'd have a hard time maintaining zero wheel rotation with a belt.

[Damfino]

If the feezlebobber struts on the main jetison pak suspension are not seperating enough, then the only place this plane will go is on a time-space warp continuim where it will continually circle Uranus in search of Klingons...........

Just my .02 cents worth

[Gordon]

The point is that the conveyor belt can't keep the plane from moving forward because what's moving it forward has nothing to do with the wheels.  The wheels are just there to minimize the friction with the ground, nothing else.  

The faster the conveyor belt is moving backwards, the faster the wheels will spin in the opposite direction, but it will have no bearing on how fast the plane is moving forward through the air, and air moving over the wings is what causes the plane to lift off.  

Think of it this way.  Instead of a normal airplane, you have an unpowered glider.  The glider is being towed along this conveyor belt by a vehicle that is off to the side on the stationary part of the runway.  The glider's wheels will be spinning furiously in the opposite direction of what the conveyor belt is moving, but the glider will still move forward and achieve lift.  

A regular airplane's propeller is the same as the vehicle towing the glider.  The propeller is towing the airplane through the air.  It doesn't matter what the wheels are doing or the ground that those wheels are rolling along is doing.      

[my78k]

http://www.youtube.com/results?search_type=&search_query=mythbusters+plane+treadmill&aq=0&oq=mythbusters+plane

Sorry guys but the plane WILL (and does) take off....

Dennis

[alltherightpills]

Yep, Gordon's got it.  The wheels on a plane aren't powered, therefore they could be rolling backwards and the thrust generated by the engines (prop or jet) would still cause the plane to move forward.

[HavocTurbo]

The point is that the conveyor belt can't keep the plane from moving forward because what's moving it forward has nothing to do with the wheels.  The wheels are just there to minimize the friction with the ground, nothing else.  

The faster the conveyor belt is moving backwards, the faster the wheels will spin in the opposite direction, but it will have no bearing on how fast the plane is moving forward through the air, and air moving over the wings is what causes the plane to lift off.  

Think of it this way.  Instead of a normal airplane, you have an unpowered glider.  The glider is being towed along this conveyor belt by a vehicle that is off to the side on the stationary part of the runway.  The glider's wheels will be spinning furiously in the opposite direction of what the conveyor belt is moving, but the glider will still move forward and achieve lift.  

A regular airplane's propeller is the same as the vehicle towing the glider.  The propeller is towing the airplane through the air.  It doesn't matter what the wheels are doing or the ground that those wheels are rolling along is doing.      

What you are talking about is a static position of the tow vehicle. Then the tow vehicle moving forward. That's different than if the tow vehicle was also on the belt.

[Inigo Montoya]

Yes up it will go. A plane does not gain speed using its wheels. It bites the air and uses that friction to move so the motion of the runway does not matter.

[Gordon]

What you are talking about is a static position of the tow vehicle. Then the tow vehicle moving forward. That's different than if the tow vehicle was also on the belt.

You're correct, it is different.  The tow vehicle is an analogy for the propeller.  Both the tow vehicle and the propeller will pull the plane forward through the air regardless of how fast the conveyor belt is moving backwards, because neither are affected by the conveyor belt in any way.    

The ONLY part of the plane that is affected by the backwards motion of the runway is its wheels.  Therefore, the only effect that the backwards-moving conveyor belt will have on the plane is to cause its wheels to spin faster in the opposite direction.  This will not stop the propeller from moving the plane forward through the air, and the plane will take off just like it always does.  

[mattcb350f]

You folks seem mechanically-minded...

There is an airplane on a runway. The runway is like a conveyor belt, and can detect the speed of the wheels of the airplane. When the runway senses the wheels moving, the runway will match the speed of the airplane's wheels, but in the opposite direction.

Can this airplane take off?

Yes.

Regardless of the speed or direction of the conveyor belt, as long as the aircraft is allowed to accelerate through the air it will take off.

However, I have a hard time understanding which way the conveyor is moving. If the conveyor matches the speed of the wheels to keep them stationary...it will take off no problem.

If the conveyor moves in the opposite direction to spin the wheels at twice the speed then they would likely fail prior to rotation (depending on the Vr speed and the speed rating of the tires) ...but it would still take off, just with blown tires.

 Matt.

[Borkunit]

Gordon's right.  The difference is that cars or people running on a treadmill are using the thrust against the treadmill's surface to generate the forward motion.  In the case of the airplane, it's using a propeller against the air to generate that forward thrust.  Completely different and unconnected medium from the treadmill.  

What ever the treadmill is doing is completely irrelevant.  The propeller will act on the air and pull the plane forward.  The treadmill has little affect on the plane, save for some trace friction in the wheel bearings.  It certainly has no ability to hold the plane in place, assuming the wheels spin freely.

[SD750F]

The free spinning wheels will freely rotate up to 2 times the speed required for the plane to lift off, then at that time the plane will be airborne. If the plane requires 58 MPH to product enough lift to fly, and the tires are 10 inches in diameter, then the tires will have to rotate 1,949 RPM.

This is a typical calculus problem... Where you can calculate the wheel rotational speed at any given moment in time.

[Gordon]

Okay, how about this:

Instead of traditional wheels on the landing gear you have one giant ball bearing where each wheel would normally be.  These ball bearings can rotate in any direction.  

With this type of landing gear the runway could even be moving sideways, but the plane would still be pulled forward by its propeller, and it would take off normally.  

On a plane with wheeled landing gear, the motion of the runway, as long as it's perpendicular to the wheel axles, will have no bearing on the direction or airspeed that the plane is capable of achieving.  

[HavocTurbo]

What you are talking about is a static position of the tow vehicle. Then the tow vehicle moving forward. That's different than if the tow vehicle was also on the belt.

You're correct, it is different.  The tow vehicle is an analogy for the propeller.  Both the tow vehicle and the propeller will pull the plane forward through the air regardless of how fast the conveyor belt is moving backwards, because neither are affected by the conveyor belt in any way.    

The ONLY part of the plane that is affected by the backwards motion of the runway is its wheels.  Therefore, the only effect that the backwards-moving conveyor belt will have on the plane is to cause its wheels to spin faster in the opposite direction.  This will not stop the propeller from moving the plane forward through the air, and the plane will take off just like it always does.  

OK!!!

Now I understand what you are talking about. Nevermind. 

[Gordon]

Wow!  It IS actually possible for forum members to disagree with each other without resorting to insults and name-calling!  Whoodathunkit? 



















ya bastard!  

[HavocTurbo]

Don't make me call Mikey and Tony over to make you an offer you can't refuse....

Agree with me or else. 

 

[Duke McDukiedook]

Holy crap, I need to sign up for whatever physics class you guys took in college.
What class was that, magical physics?

Yes up it will go. A plane does not gain speed using its wheels. It bites the air and uses that friction to move so the motion of the runway does not matter.

Bite the air, you guys are hilarious! Was that a joke or are you serious?
 

[Gordon]

We took physics classes that pertain to THIS universe.

[bunghole]

This one always throws me off at first, but then I remember that the plane DOES move through the air even though the runway is moving opposite the direction of travel of the plane.  Unless there was a lot of friction in the wheel bearings, the movement of the runway wouldn't have a significant effect on the movement of the plane relative to the ground (ie airspeed).