This Should Leave Your Brain In A Knot- Part 2!

[FlyingLow]

. .

Think of it like this. . . You're standing on a treadmill wearing some roller skates and you're holding onto a rope that another person is holding onto in front of the treadmill. Turn the treadmill on and the belt starts to move. Do you actually move? No. . .

Now turn the treadmill on full speed . . Have the other person reel in the rope, do you move forward? Yes. . . Does that other person have to pull harder on the rope to get you to move forward due to the treadmill rotating at a high rate of speed? I highly doubt it. . .

This idea does not work. Yes you would move forward. But with the airplane problem you don't have an external source pulling the rope. The person on the treadmill is the plane. The rope is the thrust (engines). Just holding onto the rope allows the person to over come the movement of the treadmill. That is thrust and drag being equal. Now if you have a friend pull the rope then the person would move forward. But what does the pulling of the rope equate too? Nothing, it is an external factor.

The wheel speed has NOTHING to do with this equation. The wheels are free spinning.

They kind of do. The wheel speed might be 1000 knots, but if the airplane speed is zero, due to thrust being equal to drag then there is no lift being generated. I might get up to 100 mph on a dyno, but if a cop shot me with his radar then what would it show? Zero because the car is not actually moving, only the wheels. The airplane is not moving, only the wheels. The airplane can only move if the thrust from the engines can propel the airplane at a speed faster then the moving runway. Even then the thrust has to be enough to overcome the moving runway and get the airplane moving fast enough to get wind over the wind foils to create lift.

If you are running on a treadmill while holding a paper airplane and you just release the airplane, will it fly? No, unless there is a fan generating wind for lift or you push the plane forward with your arm as you release it. But then you added extra thrust (your arm). The engine (your legs) equaled the drag (treadmill) which generated zero lift.

 

[Roz]

But the engines are acting on the air, which is stationary, so the engines by newtons theory will still produce forward thrust, moving the plane forward relative to the air.

The conveyor belt speed is set to cancel the speed of the wheels turning, not the forward movement of the aircraft. The wheels are not connected to the engine(s) mechanically. The thrust of the engines will push the plane forward, and the conveyor belt will rotate the wheels in the opposite direction. The turning of the wheels will not excert any force on the aircraft, so it will continue to move forward.

 

[Roz]

Imagine this too. . . It's kind of opposite of this scenerio.

Stationary runway.
Stationary wheels. (locked)

With the engines at full thrust, the engines would DRAG/skid the wheels across the tarmac. Wheel speed is completely irrelevant to the engines creating thrust. Period.

 

[FlyingLow]

But the engines are acting on the air, which is stationary, so the engines by newtons theory will still produce forward thrust, moving the plane forward relative to the air.

The conveyor belt speed is set to cancel the speed of the wheels turning, not the forward movement of the aircraft. The wheels are not connected to the engine(s) mechanically. The thrust of the engines will push the plane forward, and the conveyor belt will rotate the wheels in the opposite direction. The turning of the wheels will not excert any force on the aircraft, so it will continue to move forward.

The engines are pushing the air. That air is causing thrust to move the plane forward to get air over the wings to generate lift. The plane is not moving forward. The movement is limited due to the wheels. The wheels are what allows the plane to move prior to enough lift being generated. The wheels are the ground contact point.

The thrust is pushing the plane forward. But how is the plane moving forward? If it is on the wheels then the wheels need to be moving faster then the conveyor belt. Your theory is correct once the plane if flying, but not prior to it flying.

The thrust of the engines will push the plane forward, and the conveyor belt will rotate the wheels in the opposite direction. The turning of the wheels will not exert any force on the aircraft, so it will continue to move forward

I disagree. How is this any different from a regular runway? Unlike a car where the engine provides thrust directly to the wheels. The engines on a plane apply thrust indirectly to the wheels. It is still a force acting on the plane. The force is pushing the plane forward on the wheels. If you think the wheels don't matter then try it with the landing gear up. If you put wings on a car and put it in the same scenario, would it take off? I would say no. The car might be getting its thrust through a different mechanism (engine moving the tires vice engines pushing the car) but the result is the same.

 

[FlyingLow]

Imagine this too. . . It's kind of opposite of this scenerio.

Stationary runway.
Stationary wheels. (locked)

With the engines at full thrust, the engines would DRAG/skid the wheels across the tarmac. Wheel speed is completely irrelevant to the engines creating thrust. Period.

Even then you would not get going fast enough to generate lift. Take the wheels off completely and let the airplane rest on the body. You might get a little movement, but not much. How is this any different then someone running on a treadmill or a car on a dyno.

What if you start moving the treadmill prior to starting the engines, would the plane stand still. No it would move backwards. Once you start the engines and apply power you would start to negate the effects of the conveyor belt. Apply enough thrust and you will get to a non moving thrust = drag situation. If there is enough thrust then the plane would start to move forward, however the belt would increase its speed also.

 

[ntw0rk]

You know, at first I followed Smoke's theory, it sounds plausible. But, I think that Jimi is right.

The wheels and runway would be acting independently of the engine thrust. If you were relying on the wheels to propel the plane, you would be stuck to the ground. But with the thrust of the engines pushing on the weight of the aircraft alone, forward motion should be achieved.

The wheels would be spinning twice as fast as the airplane was moving, but the aircraft would be moving forward. The runway would be going 120 mph in one direction and the wheels would be spinning at 240 mph in the opposite direction. Because the wheels don't have anything to do with propelling the aircraft, they merely allow the plane to move.


That would be cool to watch!


(edited)

 

[FlyingLow]

The runway would be going 120 mph in one direction and the wheels would be spinning at 240 mph in the opposite direction. Because the wheels don't have anything to do with propelling the aircraft, they merely allow the plane to move.

No because the runway increases speed as the wheels increase speed. They would both be moving at the same speed in the opposite direction.

(found on the internet a few months ago...)


This conveyor belt has a control system that tracks the plane speed and tunes the speed of the conveyor belt to be exactly the same (but in the opposite direction).
[/B]

 

[BigRed460]

The plane would take off, the only restance to the plane extra would be the wheels spinning twice the normal speed.

..to add to the tread mill...

...treadmill is going 5 mph your friend yanks the rope (because you are not producing power through the wheels, your doing it through "thrust"), when he yanks your going to go right off the front of the tread mill(with wind in your face)...



..the plane is going 200mph air speed to take off, that means the wheels are doing 400mph, 200 from the air speed and 200 from the tread mill working against it, the restance it's only what the bearings in the wheels are producing, which is easly over come by the thrust of the motors...

if your using wheel power then you will never over come the tread mill....




....another example would be landing with the tread mill going with you, in theroy you could land with out letting the wheels start to spin...

the wheels could spin 10,000 mph in the wrong directon, the restance of them is so little the planes thrust could easily over come it...

you can maintain an air speed of 200mph weather the wheel speed is 0 or 1,000mph...

 

[FlyingLow]

You guys keep talking about wheel power, what powers the wheels?

The belt increases speed as the wheels do. If the wheels are spinning at 200 mph then the belt is also moving at 200 mph as per the original problem.

 

[FlyingLow]

...treadmill is going 5 mph your friend yanks the rope (because you are not producing power through the wheels, your doing it through "thrust"), when he yanks your going to go right off the front of the tread mill(with wind in your face).

If you are not producing enough thrust then you will be moving backwards. What does your friend holding the rope represent?

Plane=you
Treadmill=treadmill
Engine=Holding a rope (causing you to not move)
Friend pulling rope=?

 

[BigRed460]

If you are not producing enough thrust then you will be moving backwards. What does your friend holding the rope represent?

Plane=you
Treadmill=treadmill
Engine=Holding a rope (causing you to not move)
Friend pulling rope=?

If you were in a car the yes Engine=holding rope, your in a air plane. your wheel speed has nothing to do with flight. How fast would the tread mill go to over come 15,000lbs of thrust through friction of wheel bearings?


.....if you stick a plane on a dyno, is it going to be affected at all by the rolling drum?

 

[FlyingLow]

How fast would the tread mill go to over come 15,000lbs of thrust through friction of wheel bearings?

Doesn't matter. The problem stated that the treadmill moves at the same speed as the airplane. The airplane is rolling on the wheels. The wheels are spinning, but not going anywhere due to the moving treadmill.

.....if you stick a plane on a dyno, is it going to be affected at all by the rolling drum?

Yes. As the plane tries to move forward on the wheels, the drum moves opposite. So there is no forward movement. As long as all the wheels are on drums.

 

[Roz]

If you are not producing enough thrust then you will be moving backwards. What does your friend holding the rope represent?

Plane=you
Treadmill=treadmill
Engine=Holding a rope (causing you to not move)
Friend pulling rope=?

Forgive me, my comparison in the beginning isn't an exact duplication of the process. . .

Treadmill = Treadmill
Roller Skates = Plane w/FREELY SPINNING WHEELS.
Engine/Thrust = Friend Pulling Rope

To actually duplicate "take-off" process, you'd need to start the treadmill, and have your friend the pull the rope the exact second the treadmill is started.

So technially, simply "holding the rope" is not a factor mentioned in the original equation.

 

[FlyingLow]

This disconnect here is that the wheels are free spinning. So is a car in neutral. Lets all stand behind the car and push it while it is on rollers that match the tires speed exactly. Could we push it off the rollers? NO. As the wheels start to move forward as we try to push it off the rollers, the rollers would start to move also. We are standing on the ground, the car is on wheels which is on the rollers. As we push the cars starts to go forward on the wheels, but as the wheels move then the rollers start to move also.

 

[FlyingLow]

I got to go to work. I will check on this thread later. Nice discussion though.

 

[ntw0rk]

This disconnect here is that the wheels are free spinning. So is a car in neutral. Lets all stand behind the car and push it while it is on rollers that match the tires speed exactly. Could we push it off the rollers? NO. As the wheels start to move forward as we try to push it off the rollers, the rollers would start to move also. We are standing on the ground, the car is on wheels which is on the rollers. As we push the cars starts to go forward on the wheels, but as the wheels move then the rollers start to move also.

Sorry Scott, this is not a good example because you are relying on the ground to provide leverage. An aircraft engine doesn't need the ground for leverage, it uses the density of the air. So, independently spinning wheels are merely the bearings that allow the aircraft to move forward, propelled by the engines which are pushing on the air, and therefore not affected by the speed on the runway.

I know that you have seen some of those old videos of the early aircraft carriers, when the wind was right, and the speed of the carrier was fast enough, the planes could get lift with very little forward speed relative to the deck, sometimes none. That is really no different than what we are faced with here, except that the wheels will be spinning faster as the plane achieves lift. The key is relative speed. The aircraft speed relative to the moving runway is not important, nor is it (necessarily) important to the relative speed of the ground, (depending on wind speed, as you stated earlier) it only matters relative to the air itself.

 

[SilvrSRT10]

Here's another wrench I'll throw in the mix. If the treadmills duty is to cancel the planes wheel rotation, as the plane moves forward under it's engines thust, then so to will the treadmill. Effectivley keeping the planes wheels from rotating until the plane achives liftoff. The plane and the treadmill will be doing identical speeds but the wheels on the plane will not move.

Nevermind. I just re-read the problem and it says that the conveyor moves in the opposite direction of the plane. So if the plane achives liftoff at 150mph then the conveyor would be going 150mph in the opposite direction. If you take the speed at the tires, it would be 300mph.

 

[Roz]

Also . . . Since the WHEELS are FREE SPINNING and are in no way connected mechanically to an engine (like a car/truck) . . . What caused the wheels to move in the first place????

I bet it was the movement of the plane.

 

[Roz]

AND....

Take this into consideration.

Lock the plane's wheels. The wheels won't turn - which means the treadmill won't turn.

Now put the engines to max output, the engines will drag the plane's locked wheels down the runway. Granted the friction created probably won't allow the plane to reach a speed high enough to allow for take-off. BUT, it does prove that the thrust/propulsion of the engines doesn't care what the wheels are doing.

The damn plane will fly.

 

[SilvrSRT10]

AND....

Take this into consideration.

Lock the plane's wheels. The wheels won't turn - which means the treadmill won't turn.

Now put the engines to max output, the engines will drag the plane's locked wheels down the runway. Granted the friction created probably won't allow the plane to reach a speed high enough to allow for take-off. BUT, it does prove that the thrust/propulsion of the engines doesn't care what the wheels are doing.

The damn plane will fly.

A plane is standing on a runway that can move (like a long, wide conveyor belt).
The plane moves in one direction, while the conveyor belt moves in the opposite direction.
This conveyor belt has a control system that tracks the plane speed and tunes the speed of the conveyor belt to be exactly the same (but in the opposite direction).

Can the plane take off?

According to the problem, the conveyor tracks the planes speed. Not the wheels rolling on the conveyor.