Spark123 said:
For an aircraft to take off (i.e. have lift) it must have forward momentum through the air which is theoretically still. This forwards force is caused by the engines thrust pushing the aeropane forwards against the still air (not the moving runway).
True that there is no force against the runway, but not true that there is a force against the air, since that's not how jets work.
The thrust comes from the principle of conservation of momentium - you bung a bunch of hot gas out the back of a jet engine, at high speed, and the engine reacts by being forced in the other direction (equal and opposite reaction). The engine is attached to the wing, which is attached to the plane, so the whole kit and caboodle moves forward.
Spark123 said:
This force needs to overcome the air resistance and any other forces acting against it for the plane to move forwards. Now comes the tricky bit when you consider what was being said about the runway moving, it depends on how you calculate the speed, wether it is done relative to a still point (the air or terra firma) where the wheels will spin at twice the rate which they would normally or wether it is calculated between the wheel and the moving runway. It is an impossible concept for the runway to be moving at the opposite but equal speed to the wheel and for the aircraft moving forwards, the only time this holds true is when the aircraft is stationary so this is where the treadmill falls down. As soon as the wheel moves forward say 1m, the treadmill will have to move backwards 1m to compensate causing the wheel to move 2m which means at the same time the treadmill has to move 2m to compensate etc etc etc....
This is all very nice, but irrelevant. The plane will move forward if the jet thrust overcomes the friction at the wheels.
johnny_t said:
It is kind of the same difference as you would get between having a car in neutral on a moving conveyor belt, and a car in 1st gear. The Car in neutral would not move backards at the speed of the conveyor belt, even though the wheels were turning at the same speed as the conveyor was moving underneath it.
The car analogy is a poor one, because the motive force requires friction between tyre and ground, therefore the car velocity is non-zero relative only to the conveyor belt, since we're told that the conveyor always moves at the same speed as the car, but in the opposing direction.
big-all said:
rubber wheels on rubber conveyor would give you probably about 90% of the weight as avalable grip for acceleration and deceleration
Grip between airplane tyre and runway has nothing to do with take off. It is vital when landing because the plane is slowed down using brakes on the wheels.
empip said:
The (unladen) aircraft having surplus power is well placed to overcome the added friction.. it accelerates through the air, relative to terra firma up to take off velocity then leaves the runway/ belt, with wheels spinning very quickly.
Utterly correct, but really only a long-winded repeat of what I wrote earlier. The wheels will be spinning precisely twice as quickly as if the conveyor was not moving at all, and the friction will be precisely twice as great, assuming that the increase in friction of the bearings and tyres is linear.
Spark123 said:
I thought in order for the plane to move forward the wheel speed must exceed that of the conveyor??
No. No no no no no. No.
A positive net forward motion of the plane on the conveyor requires only that the thrust from the engines can overcome twice the frictional force that arises at the wheels, which is twice as big as when taking off from ground that is stationary.
