Physics Question for you.....

[niki]

OpaDC

Welcome to the forum

All the airplane speeds are "Airspeed" that means the airplane speed relative to the air...the "ground speed" (the airplane speed relative to the ground) does not have any importance on the airplane performance except the flight time

The speed that the pilot is pulling the nose up to lift off the runway is greater than the stalling speed.

During the takeoff roll, the airplane speed is increasing more and more at every second and it is controlled by the pitch attitude...of course if the pilot will pull the nose up to 45° the airplane will stall and Newton's gravity low will take place...

All the takeoffs are made at a shallow angle of climb (except the F-16 with after burner) to allow the airspeed to increase...Airspeed = control

niki

[JimD]

Nicki,

I think your rocket example is a more colorful way to say what I am saying. If the conveyor moves at a constant velocity (500mph) and the plane has constant thrust and thus acceleration, eventually the airplane flies. If the wheels slip against the conveyor, it just happens sooner. The part I am unsure of is the drag represented by tires that have to skid on the conveyor. If this force is great enough, the airplane may not get to its take off velocity.

My comment about brakes is because I think that the friction of a large commercial jet's tires against the pavement is small compared to its thrust. I think it can skid it's locked tires down the runway and take off (assuming the runway is long enough). I doubt a little piper can. A lear jet has a lot of thrust for its weight and I think it can. Cars have a lot larger ground contact for their mass which prevents the engine from overpowering the braking force. I think jets have too little tire to offset their thrust. If this is true, and sufficiently true, the conveyor does not matter. It can accelerate as fast or faster than the thrust of the airplane but eventually the tires will slip against the conveyor and the plane will move forward. If the thrust is high enough to get the plane to its lift off velocity, even with skidding wheels, the plane takes off.

If you assume (or know) that the thrust if not enough to get the plane to take off velocity with wheels skidding then the plane probably does not take off. The drag of wheels skidding on the conveyor will require additional thrust to overcome (regardless of how fast it is moving and whether it is accelerating).

Another way to summarize this is to compare the thrust of the engine(s) to the friction force of the mass of the plane through its tires against the conveyor. The friction force is the maximum force the conveyor can apply to offset the thrust of the engines. If there is a large enough net force forward, the plane takes off.

Jim

[niki]

Hi Jim

I'm sorry but I cannot understand why you insist to lock the wheels and skid the aircraft on the conveyor while it's much easier to let the wheels to rotate free.

The engine thrust or power required to move the airplane from stand still is very low.
When you go to fly next time, please notice what is happening after the engines start (and push-back).

The airplane is on the taxiway center-line with the engines at idle.
The mechanic gives the thumb up signal and the pilot gets permission to taxi to the runway and then, the pilot increases the thrust only a little bit till the airplane starts to move.
Ones the airplane starts to move, the pilot pulls the thrust levers back to idle and continuos to taxi with the engines at idle...that, can tell you how much thrust is required to overcome the tire drag/bearing friction.

If we put the airplane on this theoretical conveyor like in the question...
At the beginning, the tires are not rotating so the conveyor dose not move...
We push the thrust levers and increase the engine thrust. the airplane starts to move forward and the conveyor backward but...the engines are pushing the airplane in reference to the space and not in reference to the conveyor.

To explain it better, the jet engine or the prop, are developing the thrust or power according to Newton's 3nd law;
"For every action there is an equal and opposite reaction."
The "action" is accelerating an air mass backward and the "reaction" is the forward movement of the airplane (the rocket is working also on the same law)
If you assume that the airplane will not move forward relative to the ground or space, the Newton's law does not make any sense...and we know that it does...

Of course part of this "reaction" is lost (or more correctly, transformed) because of the wheels drag/friction and the question is how much is lost or how much thrust we have to develop just to overcome the wheel drag/friction.
As I gave you the example form "real life", the thrust required for taxiing the airplane on a normal taxiway, or in other words, the waisted thrust to overcome the wheels friction/drag is very, very small and the rest of the "takeoff thrust" will push the airplane forward relative to the space or if you want, relative to the air.

The wheels speed will be double than the airplane speed relative to the air as the airplane accelerates on the conveyor and it will come maybe to 400 MPH but, at this speed of 200 MPH the airplane will already lift of (I'm talking about the big jets like 747).

Regards
niki

[JimD]

Niki,

The wheels cannot go backward and forward at the same time. The rotation direction to go backward is opposite of the rotation direction to go forward. They do not turn twice as fast, they have to slip. If a plane tries to move forward on a conveyor moving the opposite direction, the limitation is either the thrust or the friction of the conveyor with the tires or the power of the conveyor. If the conveyor has plenty of power, I think the weak link is the friction of the tires to the conveyor. At the point that the forward thrust exceeds the force the conveyor can apply through the friction of the tires, the plane moves forward. For this to occur, the tires have to skid on the backward moving conveyor. That will create drag reducing the rate of acceleration. As long as the engines have sufficient power, the plane flies.

The only reason I write about the brakes is because I think it is easier to understand and is the same point. Brakes usually have sufficient friction to skid the tires. If airplanes are this way, and we assume the coefficient of friction of the tires against the conveyor are the same as the tires on pavement, then its the same thing. The moving conveyor has the same effect on the airplane's ability to take off as locking the brakes and skidding the tires down the runway. If there is sufficient thrust, the plane flies.

Jim

[Kristofor]

The wheels cannot go backward and forward at the same time. The rotation direction to go backward is opposite of the rotation direction to go forward. They do not turn twice as fast, they have to slip.

Well, that's a helpful mistake as it explains where the confusion is coming from...

Take a toy car and set it on a piece of paper with the front pointing to the right. Now push the car forward. Notice that the wheels on the side facing you are rotating clockwise.

Now put the car back in the middle of the sheet of paper. Hold the car motionless and pull the paper to the left (like a conveyor moving against the forward motion of the car) and look at the wheels. They are still rotating clockwise. The wheels spin the same direction if the vehicle is moving forward or the conveyor backwards!

Now it's probably clear why the wheel speed is 2x the ground speed. If you're pushing the car to the right at 1 inch per second (your finger acts like the prop/jet) and pull the paper to the left at 1 inch per second the wheels spin at the same speed as holding the car motionless and pulling the paper at 2"/sec or holding the paper motionless and pushing the car at 2"/sec.

Kristofor.

[niki]

Hi Jim

Kristofor example is very good but I made it "in colors"

I don't like so much to "visualize" things in my mind. I love to see it "in action".

I took a piece of ply, and screwed to it 2 wheels


I put it on the belt sander


I turned the belt sander ON, put on it the "Thingy" with the wheels and as you can see, I hold it with my hand on the spinning belt. The only power or force that I need is to overcome the wheel and bearing (in this case hinge) drag.


As you can see, I can push it forward very easy...



Now, put on this "thingy" a Jet / prop / rocket and push the throttles to max thrust / power.
The "thingy" will "fly" on the belt (conveyor) relative to the air/ground/space.

Yes, the wheels speed will be very high and probably the tires will blow-up before airborne but, the question is theoretical so, there is no tire speed limit.

Regards
niki

[JimD]

One of us (or more) is missing something. I just took one of my son's matchbox cars and rolled if from the left to the right. Tires rotate clockwise. I then rolled it from the right to the left, tires rotate counter clockwise.

If the conveyor is opposing the forward motion of the plane, it must be rotating the tires in the opposite direction of the direction they want to go due to the engine thrust. If the tires were rotating the same direction as the thrust is pushing, the conveyor would be helping the airplane take off.

Nicki, I admire your experiment but I have found it sometimes fairly easy to move wood I am sanding against the roatation of the belt. Could it be that you are sanding off the wheels a little and thus you feel little resistance?

Jim

[niki]

Hi Jim

I think that if you'll put this "thingy" with the two wheels, that I made for the experiment, on the right edge of the belt sander (like on the 2nd pic) and you'll push it to the left, (with the belt stationary), the wheels will turn counter-clockwise (CCW).

Now, switch ON the belt sander (on the pics the belt is rotating clockwise) and hold the "thingy"...the wheels will still rotate CCW and if you'll push the "thingy" forward, the wheels still will be rotating CCW but at higher speed.

When I have some confusions about all those CW and CCW, I like to make a mockup and see the motions directions...

Maybe, you can make your experiment with your son's matchbox car...
Put the car on the right side of an A4 paper with the car front facing to the left.
Push the car to the left and at the same time pull the A4 to the right (opposite direction of the car) and look to which direction the wheels will turn. I think that the wheels will turn CCW....

Regards
niki

[Russianwolf]

One of us (or more) is missing something. I just took one of my son's matchbox cars and rolled if from the left to the right. Tires rotate clockwise. I then rolled it from the right to the left, tires rotate counter clockwise.

If the conveyor is opposing the forward motion of the plane, it must be rotating the tires in the opposite direction of the direction they want to go due to the engine thrust. If the tires were rotating the same direction as the thrust is pushing, the conveyor would be helping the airplane take off.

Nicki, I admire your experiment but I have found it sometimes fairly easy to move wood I am sanding against the roatation of the belt. Could it be that you are sanding off the wheels a little and thus you feel little resistance?

Jim

So how do people run on a treadmill?

If you stand to the left side of th treadmill and watch someone, their foot will contact the ground and move backwards on the belt, then lift and go forward. Their foot is acting as if were one tread on a tire. the circular motion (somewhat elongated) is still counterclockwise. The same as if they were walking accross the room, but the belt of the treadmill is negating the forward motion they would normally have.

[Alex Franke]

If the conveyor is opposing the forward motion of the plane, it must be rotating the tires in the opposite direction of the direction they want to go due to the engine thrust. If the tires were rotating the same direction as the thrust is pushing, the conveyor would be helping the airplane take off.

If you watch a plane take off to the left, it's wheels will go counterclockwise/anticlockwise. If you want to try to slow the plane down by moving the runway, you'd move the runway to the right as the plane tries to take off to the left. This makes the wheels spin counter/anticlockwise even faster.

If you accelerate the runway to the *left* at the same rate the plane is accelerating, then the wheels won't turn at all. It might seem like this is helping the plane take off, but because we're dealing with wheels, it really isn't.

If you accelerate the runway to the left *even faster* than the plane is moving, then the wheels will spin clockwise -- it will be rolling *backwards* on the runway, but it'll still take off normally, and you still really not helping it take off. So it really doesn't matter what the runway is doing.

Part of the trick of this problem is that the plane never really appears to stay stationary to an observer who is not on the conveyor -- it always moves in the direction it's being thrusted by its engines, no matter what the wheels (and conveyor) are doing.

So how do people run on a treadmill?

I typically collapse and get flung off the back. Clearly there is waaay too much friction between my belly and the tradmill.