Airplane physics question

The wheels do not provide thrust. The jet engine or propellor does. This thrust acts on the wheels through the landing gear to rotate them. If the landing gear was skis, they would slide forward. Since they are wheels stuck on at their center point with an axle, the forward thrust changes to rotational thrust. The wheels rotate. Simple as that.

No need to overcomplicate it. Whether your propulsion comes from a car (not on the belt) pushing or pulling the plane, or a jet engine attached to its wings, or a propellor stuck on the front, or a combustion engine powering the landing gear wheels directly, the rotation of the plane's tires on the ground moves it forward in relation to the ground. This causes airflow over the wings and creates lift. If the plane is on water flowing backwards or on a belt that moves backwards at the same speed, while the plane moves forward, then the plane is not moving with respect to the ground or the air. It does not matter how the wheels are made to spin, or what speed they are spinning at, if the surface they are on spins backwards and matches their speed.

If the conveyor belt matches the wheel speed in the opposite direction, then the wheels are spinning but the plane is not moving. Additional thrust from the jet or propellor as the throttle is applied will result in additional rotational acceleration on the wheels. This will be matched by the conveyor accelerating in the opposite direction. If it's a perfect match, the plane will not move forward. No movement of the plane means no airflow. No airflow over the wings means it does not take off. Simple.

 

You are right about the drag and wrong about the take off. The wheels rotate fast enough to take up all the thrust energy of the jets. If they are weak, they will overheat or break. If they are strong enough, the plane will sit there, with the wheels rotating like crazy, and the conveyor belt rotating at exactly the same speed but in the opposite direction. That would be fun to see but it would get boring after some time because the plane would not take off.

 

Got a belt sander Art? Again, watch you fingers.

 

Props to both of you as well!

 

not even close. sorry.

 

Art,

With all due respect, the thrust does not act on the wheels in any way, shape, or form. Nor is the thrust "taken up" by the wheels. Sorry, this is not a rear wheel drive airplane! This may come as a surprise to some on ths thread, but most airplanes have enough thrust to easily overcome a locked or fully braked wheel, let alone freely spinnings ones.....

Rick

 

I think we might all be invalids, for taking this question this far.

Not so simple Art. You just said the plane does NOT move. The only way a wheel turns on a plane, since there is no direct drive to the wheels, is through a forward motion of the plane being translated to a rotation of the wheel. Since in you statement there is NO forward motion of the plane, there can be no rotation of the wheel, and no offsetting motion by the conveyor. An impossible situation.

This statement is way out there. You are still taking the power from the jet engines to rotate the wheels, with no drive mechanism to do so. Impossible.

Nope, can't work. See above.

Not a good example. The runner's engine is direct drive to his feet.

Doesn't work. A catapult is directly thrusting upon a FIXED member of the plane. See my post above about the catapult misunderstanding.

Not if you are on wheels like a plane. It wouldn't matter what the conveyor did. Your body wouldn't move, the wheels would simply turn.

Hee hee.

 

The only solution which holds together logically requires that the plane's wheels, which are initially not rolling, do not start rolling at any time during the experiment. For this to happen, the treadmill must move in synch with the airplane as thrust is applied. The forward motion of the treadmill must precisely match the forward motion of the airplane until it flies off. Then there will be no need for the plane's wheels to roll and no logical inconsistency will arise.

 

Thrust does act on the wheels and they do take up the thrust until they lift off the ground. Lock them up and see the forces acting on the blockage...where is the force coming from if not from the thrust?

 

In general you are correct, with this specific question the conditions propsed are impossile to satisfy.

It's just that the convery and wheel can't do what they are asked.

 

Art, do you ever see the day you will move up to a plane without props? An Eclipse 500 perhaps?

 

The brake pads clamped on the rotors. An entirely different situation than a free spinning wheel. In other words; "I object, your honor.".

 

Aghhhhhh...... time to unsubscribe again! Check in again in a few days.

Rick

 

Only if you assume the conveyor matches the translational, and not the rotational, speed of the wheels. In your statement there is NO rotational speed, which is one of the results of a poorly worded question. In fact I came up with this idea you have many pages ago.

 

Art's just wearing you down, Rick. Then he goes in for the kill.
And again, I think most who have followed this thread all the way through understand that the wheels rotation has nothing to do with a planes acceleration.
I submit again that the way the question is framed it is invalid, a conundrum that is impossible to satisfy.
If the original question had been: Can a plane take off on a conveyor, no matter which way or at what speed the conveyor is moving? The answer would be a simple YES.

 

That's precisely the point. The treadmill is designed to match the rotational speed of the wheels. That includes a rotational speed of zero. I don't think the question is poorly worded at all.

 

The plane does not move, correct. I am talking about linear, forward motion. The plane does experience a thrust force from the engine. The whole plane is a rigid body. Except for teh wheels. The plane's rigid body acts on the wheels like a thrust bearing, pushing them forward, but because they are pinned at the center with the axle, they rotate about the axle. This allows the plane to move forward smoothly. As they move, the surface of the conveyor moves an equal and opposite amount. So the plane does not move forward, even though the wheels are experience angular acceleration from the thrust of the engines. Simple.

 

If the rotational speed is zero. Then the treadmill speed is zero. The plane doesn't move.
Impossible.

 

It does not stay at zero. As the wheels accelerate (which they must if the plane is experiencing thrust from a standstill while resting on the wheels) the treadmill accelerates in the opposite direction in an exactly equal amount. No conundrum there.

 

The wheels can't experience angular acceleration if they don't rotate.

You also just said "This allows the plane to move forward smoothly"
and "the plane does not move forward"

Cool plane if it can do both at once.

 

The wheels do rotate. Faster and faster. The thing is, the treadmill goes faster adn faster in the opposite direction.

On regular ground, when the wheels rotate, the plane moves forward smoothly. On a treadmill, when the wheels rotate, the plane sits still smoothly. If the wheels were not perfectly round, or the treadmill surface was not smooth, then the plane would experience bumps, even though it is not moving forward in relation to the groudn and appears to be still.

 

Are you saying the conveyor is accelerating the same direction the plane is (forget wheel rotation for a minute) , or the opposite?

 

The rollers of the conveyor are rotating in the opposite direction to the rotation of the plane's wheels. Like a car sitting on a dyno.

 

Taking it back to basics for a minute........

thrust vs drag

On takeoff the plane has to have enough thrust to overcome the aerodynamic drag and whatever friction the wheel bearings might have in order to cause the plane to accelerate. I'm not sure just how to quantify the friction of the wheel bearings but it isn't that much since it is quite easy for one person to push a small plane around. Even if a plane were sitting on the proposed conveyer belt and the wheels (which spin freely) had to spin twice as fast I highly doubt there would be enough friction to cause much increase in takeoff distance. If the friction of the wheels were that significant you would feel a significant decelleration as soon as the mains touched down.

BTW.... I sent you a PM and called yer cell

 

Disagree. The only way the treadmill can maintain zero rotational speed of the tires is by moving in synch with the airplane. Until the plane takes off...

It looks like we just have a difference of opinion about how the treadmill is programmed!