Physics question/riddle. This is a good one, read it.

[sisukid1975]

Originally Posted by White Shark

Low air pressure due to air flow over the greater surface area on top of the wing generates lift. No air flow over the wings = no lift. A prop plane will provide forward motion, but if the ground moves and negates forward motion, then the props are generating nothing but prop wash and sufficient lift will not occur to get the plane off the ground.

The question states that the conveyor matches the forward motion. That would case a stasis to occur where the plane is at full throttle and is still maintaining the original position. Thus, no air flow over the wings other than prop wash, and the jet turbines throw thrust behind the wings.

That's what I thought at first too, and I went with that reaction. But, the planes's engines don't act on the GROUND, they act on the AIR, which isn't moving.

So the plane's propulsion is not dependant on the ground like it is with a car. THink of it this way:

Take a toy plane to the airport where they have a moving sidewalk. put the plane on the moving sidewalk. tie a string to it. Walk beside the plane, but not on the moving sidewalk. You are replaicating the way the plane gets it's thrust: By moving the air which is static. Newtons third law, for every action there is a an equal, opposite reaction. The jet engines (or prop) pumping massive cubic meters of air per second is where the reaction that generates speed comes from, not the wheels relative to the ground.

Fred

[Arya Ebrahimi]

Originally Posted by LoveTractor

It's like walking up the wrong way of an escalator. You are stationary. No wind is magically produced on your face. If it were moving at 200 knots and I could miraculously keep up and then grew wings, I wouldn't go anywhere because I'm stationary. There is no wind to push on the front of my wings to produce lift.


I've got a headache.

No, it's not like walking the wrong way up an escalator, because you depend on your feet to propel you forward against the ground(which in this case is moving). In the planes case, it is propeling itself with the air. The ground just happens to be moving the opposite direction.

Imagine this. Someone is walking up the stairs next to the escalator at 2mph and holding you over the escalator. You are walking up the escalator and the escalator starts moving the other direction at 2mph. Now, the person holding/carrying you is STILL moving 2mph forward, but YOU have to move your legs at 4mph to keep in contact with the ground.

Think about it.

It's all about relativity.

[rusty_tlc]

Originally Posted by Arya Ebrahimi

White Shark, I see you as a man of logic and reasoning, so I am perplexed. How does the ground negate forward motion? The wheels are free spinning, simply holding the plane off the ground. The plane has to move forward in order for the conveyor to start turning. Now, the conveyor is spinning, but the plane is still moving. The only affect the turning belt has on the plane is the wheels are spinning twice as fast as the plane's actual speed.

Draw a vector diagram.

[baxter650]

Originally Posted by rusty_tlc

If the plane is exerting forward thrust equivelent to 200 knots and the belt is moving backward at 200 knots the net air speed will be zero.

There is no air in the windmills of your mind.

[NorCalDoug]

I'm in the "it's not going to fly" camp. And I'm firmly rooted here. As already mentioned, flight is a result of thrust and lift -- lift is generated by air speeding over and under the wing.

If the forward thrust is negated by the backward movement of the conveyor, there is no net thrust -- therefore no air movement under the wings.

Think about running on a treadmill. When you're running on a treadmill...do you feel the wind in your face? Nope. No wind...no lift...no fly.

[Arya Ebrahimi]

To completely throwout the thought that a plane's wheels are driven, I will state that they are not. If they were, the plane would fall down as soon as it lost contact with the runway.

[baxter650]

Originally Posted by LoveTractor

In order for flight to occur, enough wind must hit the front of the wing so that lift occurs under the wing and therefore causing flight. If the plane is on a conveyor belt that is moving backwards and only the wheels are moving, there is no wind on the front of the wing at all to cause lift. Therefore no flight.

What did I win?

Crap I'm a slow typer.

There is no air in the windmills of your mind.

[IdahoDoug]

A plane's engines produce thrust against the air. This thrust will move the plane forward as if it were in still air in this example. The belt will only produce a small amount of friction caused by the wheel bearings and tire. If the plane's normal takeoff speed is 200 knots, it will take off here at 200 knots, but be using a tiny fraction more power because the freewheeling tires are spinning at twice their normal speed. The belt moving the opposite way inhibits the plane from gaining forward momentum in the same way the ground passing under it does on a normal takeoff - hardly at all. Just a tad more to account for the increased wheel/tire rotation.

DougM

[cruiseroutfit]

Not on this board too...

The plane takes off, the wheels of the plane spin at twice the forward speed of the plane relative to the outside world.

Draw a free body diagram...

[Arya Ebrahimi]

Originally Posted by IdahoDoug

A plane's engines produce thrust against the air. This thrust will move the plane forward as if it were in still air in this example. The belt will only produce a small amount of friction caused by the wheel bearings and tire. If the plane's normal takeoff speed is 200 knots, it will take off here at 200 knots, but be using a tiny fraction more power because the freewheeling tires are spinning at twice their normal speed. The belt moving the opposite way inhibits the plane from gaining forward momentum in the same way the ground passing under it does on a normal takeoff - hardly at all. Just a tad more to account for the increased wheel/tire rotation.

DougM

Wooohooo! Doug's on my side, maybe now people will believe me!

[Hawke]

" who gives a rats a$$, I'm on the second step of 6-4-6, leave me alone!"

There's steps? Any instruction manual?

[rusty_tlc]

Originally Posted by Arya Ebrahimi

To completely throwout the thought that a plane's wheels are driven, I will state that they are not. If they were, the plane would fall down as soon as it lost contact with the runway.

By this logic we wouldn't need runways because planes would lift straight off the ground.

[subzali]

sisukid was on the right track. Draw a Free-body diagram and examine the forces. Plane's engines create forward thrust (force). Now assuming the wheels are frictionless on their bearings (which they are pretty close to being), then the ONLY forces on the wheels are the weight of the plane pointing down, the normal forces from the ground, pointing exactly upwards to counteract the weight of the plane, and a frictional force on the wheel, acting at the surface of the wheel and creating a moment. The frictional forces from the conveyor on the wheels ONLY causes them to rotate about their (close to frictionless) axle. Since this axle is pert near frictionless, there is NO latent force from the conveyor to counteract the thrust from the engines. Voila there is a net force in the x direction (if that's the axis you choose, relative to a stationary object, like the earth), and according to F=ma the plane accelerates regardless of the conveyor. Once its speed relative to the stationary ground is high enough, enough air will be passing over the wings to create a pressure differential (lift), and the plane will take off, its tires on fire because they're spinning so fast.

Never argue with a School of Mines student

[subzali]

Doug, the reason you don't move on a treadmill is because you're using your feet to propel each thrust forward...a plane doesn't work the same way. Now imagine your holding a fan on a treadmill...it'll pull you off the treadmill. Okay bad example.

[Arya Ebrahimi]

Originally Posted by rusty_tlc

By this logic we wouldn't need runways because planes would lift straight off the ground.

How do you figure? The planes ENGINES are generating and applying to the air the force to move it forward, not the wheels. The wheels simply eliminate the friction that would be inherint if the plane's belly were resting on the runway.

Ary

[LoveTractor]

I'm going to bed.

[NorCalDoug]

so...by this logic a plane needs no forward movement to take off?




(exlcuding VTO aircraft)

[import silvia]

No forces were given in the problem. Newton's first law wins.

[wesintl]

my logic says no which i'm sure is flawed.. That's why i'm a computer dork. When I first looked I at I knew it had to be a trick question and the answer would be yes in the long run. I don't understand how the plane is actually moving if it's on rollers it's stationary with no air over the wing creating no lift. To me it's like a car on a dyno there's no wind drag on it when it's doing 70. I can see how some thrust over the wing from a prop could create lift though. sadly my only physics class was in 11th grade almost 20 years ago....

Maybe I should have went and 6-4-6'd it.


ok.. matt made it make sense for us mere tards...

[Cube Dweller]

I agree with LoveTractor and Whiteshark. Ary wrote "a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction."

Since a PLANE requires the relative velocity of AIR to be greater on the top of the wing than on the bottom to generate lift, since the plane is stationary, there is no velocity differential and thus no pressure differential to lift the plane.

Rockets are different, they fly based on thrust generated by the gases created during combustion of propellant.

Draw a freebody diagram

[e9999]

dang, somebody else beat me to the greater friction force on the wheel...

but wait!

you guys forgot that the conveyor belt will actually drag air with it by friction in a boundary layer whose thickness depend on the speed. So, contrary to some of the statements above, the wing would actually see airflow even if the plane is at the same speed as the conveyor... (Effect on lift very small, though, likely)

[Cube Dweller]

Ary, you're still in school, your hw is to take this to one of your past physics profs or an ME prof and have them prove I'm right.

[rusty_tlc]

Originally Posted by Arya Ebrahimi

How do you figure? The planes ENGINES are generating and applying to the air the force to move it forward, not the wheels. The wheels simply eliminate the friction that would be inherint if the plane's belly were resting on the runway.

Ary

Exactly except that they don't eliminate it they reduce it.

[baxter650]

Originally Posted by White Shark

Low air pressure due to air flow over the greater surface area on top of the wing generates lift. No air flow over the wings = no lift. A prop plane will provide forward motion, but if the ground moves and negates forward motion, then the props are generating nothing but prop wash and sufficient lift will not occur to get the plane off the ground.

The question states that the conveyor matches the forward motion. That would case a stasis to occur where the plane is at full throttle and is still maintaining the original position. Thus, no air flow over the wings other than prop wash, and the jet turbines throw thrust behind the wings.

Matches FORWARD motion. In order to MATCH forward motion you have to HAVE FORWARD MOTION. There is no airflow in the windmill of your mind.

[Arya Ebrahimi]

Cube Dweller, how does the plane move in the first place? If the plane stayed stationary the belt would never begin to move. In order for the belt to begin moving, the plane has to start moving. In order to start moving the engines generate thrust to move the plane. At that point the plane is moving forward. The conveyor, now sensing the plane's motion, starts moving in the other direction at the same speed. This in turn accelerates the planes wheels to a new speed, which will in turn accelerate the conveyor's speeds. Eventually the conveyor will reach its mechanical limitis and breakdown. Then the plane will take off.

Either way, the plane is taking off.

Ary