The old airplane/treadmill revisited

[JeffG78]

just a point that i made earlier anyone motion can be obtained if enough energy is exerted on the components. every example given has used an energy source that far exceeds the friction losses of the interface between the moving conveyor and the person airplane etc.

this is like saying that all rockets can go straight up. That only applys if the thrust of the engine exceeds the weight of the rocket. otherwise it will just sit on the launch pad and go nowhere.

Take the roller blades off of the man at the airport and put on ice skates and he will not move against the conveyor. because the engine does not deliver enough power to overcome the friction of the ice skates.

in any case that the engine has excess power then the conveyor interface and inertia requirements the airplane will take off at some point.

OK, but that's saying that a plane will struggle to take off without being on a conveyor. The friction between the tires and the tarmac is there either way. The ONLY difference is the speed of the tires. Rolling friction is not a significant force.

[master53yoda]

There is an electrical law called Kircoffs law that says that the sum of all energies coming into and leaving a point must add up to zero.

the test that i used in my previous comment is based on that law. in order to move the object forward the engine power must exceed the loss to the conveyor. The conveyor losses go up exponentially with the speed of the conveyor and at some point the conveyor speed would exceed the power that is being added into the airframe from the engine. the interesting thing is that the motor driving the conveyor would be delivering the exact power as the engine on the airplane.

engineers spend their lives figuring out how big the motors must be to drive a process.

[master53yoda]

OK, but that's saying that a plane will struggle to take off without being on a conveyor. The friction between the tires and the tarmac is there either way. The ONLY difference is the speed of the tires. Rolling friction is not a significant force.

they do struggle that is why they have far more powerful engines per lb then a car does.

[Alex Franke]

Once again, if we can have a huge, super-fantastic conveyor belt that can zip along at zillions of miles per hour, then we can also have super-fantastic wheels that don't impose any unrealistic friction.

The friction of the wheel in contact with the ground provides the opposing force - that's what allow a car to move. So the conveyor is providing a force that counteracts the attempted forward movement of the airplane, generated by the propeller.

This doesn't make sense to me -- There *needs* to be friction between the wheel and the ground, and lots of it or the wheels wouldn't be very functional at all when starting, stopping, or changing speed. It seems to me that the opposing force would come from the bearings, or maybe from the road/wheel interface if the tires are flat.

Even so, wouldn't you agree that with perfect frictionless wheels and a perfect super-fast conveyor belt, the plane would still take off?

[JeffG78]

I've changed my thinking. Planes are too heavy and can't possibly fly.

[master53yoda]

Once again, if we can have a huge, super-fantastic conveyor belt that can zip along at zillions of miles per hour, then we can also have super-fantastic wheels that don't impose any unrealistic friction.



This doesn't make sense to me -- There *needs* to be friction between the wheel and the ground, and lots of it or the wheels wouldn't be very functional at all when starting, stopping, or changing speed. It seems to me that the opposing force would come from the bearings, or maybe from the road/wheel interface if the tires are flat.

Even so, wouldn't you agree that with perfect frictionless wheels and a perfect super-fast conveyor belt, the plane would still take off?

only if the engine is big enough put a b17 ww2 bomber on your tread mill with one engine at an idle and it won't oppose the treadmill

[Alex Franke]

they do struggle that is why they have far more powerful engines per lb then a car does.

How is an airplane any different from a car when it's accelerating on the ground?

Wouldn't the goal of a high power/weight ratio be accelerate quickly to a speed required for lift-off? (as opposed to overcoming some kind of plane-specific takeoff friction?)

[Alex Franke]

I've changed my thinking. Planes are too heavy and can't possibly fly.

ROFL -- I'm with you, Jeff!

[Alex Franke]

only if the engine is big enough put a b17 ww2 bomber on your tread mill with one engine at an idle and it won't oppose the treadmill

Of course not -- but it also won't take off without the treadmill.

The point is that if you had perfect frictionless wheels and a perfect super-fast treadmill, then a plane that could take off without the treadmill could also take off with it, no matter how fast it (the treadmill) was going, or in what direction.

Right?

[master53yoda]

How is an airplane any different from a car when it's accelerating on the ground?

Wouldn't the goal of a high power/weight ratio be accelerate quickly to a speed required for lift-off? (as opposed to overcoming some kind of plane-specific takeoff friction?)

the lift of the wings reduce the weight of the plane and reduce the losses to the wheels until lift off. if they didn't you wouldn't get enough speed to get in the air.

[Alex Franke]

the lift of the wings reduce the weight of the plane and reduce the losses to the wheels until lift off. if they didn't you wouldn't get enough speed to get in the air.

Okay, but then doesn't that mean that they would struggle less than a car would? I though you were suggesting that an airplane had to have more powerful engines so that they could overcome some unique airplane-related "struggles".

Maybe I'm confused -- I'll go back and read it again...

[JeffG78]

the lift of the wings reduce the weight of the plane and reduce the losses to the wheels until lift off. if they didn't you wouldn't get enough speed to get in the air.

What?

If you've ever had to push a broken down car, you will know that getting it moving is much harder than keeping it moving or increasing the speed. The breakaway friction in tires and wheel bearings is a much greater force than the rolling friction. Pilots don't just accelerate down the runway and magically lift off due to reaching the proper speed. They have to pull back on the stick to create the needed lift once they are up to the take off velocity.

Air friction is a huge force that both cars and planes must overcome to reach high speeds. Mecanical friction is only a minor force once moving. Planes big and small need their power to overcome air friction and gravity to allow them to climb. Ground speed is not overly difficult to achieve.

[woodturner]

Pilots don't just accelerate down the runway and magically lift off due to reaching the proper speed. They have to pull back on the stick to create the needed lift once they are up to the take off velocity.

The reason pilots have to pull back on the stick is that they are pushing forward on the stick during the takeoff roll. They don't want the airplane to go airborn too soon and stall, so they use the stick to push the plane down toward the ground. When air speed is attained, they pull back a bit, causing the nose to lift, changing angle of attack on the wings and increasing lift under the wings.

[woodturner]

Even so, wouldn't you agree that with perfect frictionless wheels and a perfect super-fast conveyor belt, the plane would still take off?

By frictionless wheels, do you mean no friction with respect to the ground? So if I sneeze on the plane, it slides across the ground?

If so, then yes, because the airplane will move forward along the ground.

[LCHIEN]

rolling friction of wheels is very low, esp compared to the air resistance at 100+ Mph.
Rolling friction also goes up linearly with speed, air resistance goes up with the square of the speed.

Airplane wheels are meant to go at very high speeds. when taking off they go about 150 mph and the wheels spin freely after. When landing the wheels go from 0 to 150 mph in an instant. the tire squeals due to the angular momentum going from 0 to max, not from the resistance of the wheel bearings to turning.

If the plane can reach takeoff velocity with respect to the air, the friction of the wheel bearings even at 300 mph won't hold it back.