The "Plane on a Treadmill" Question

[lumberjim]

the air has to go faster above than below, so the engine position below the wing prevents that from ever happening. right on

[Torrere]

I think I'm getting a clearer picture of the car / plane confusion. The wheels on a car rotate, grab the road (with friction), and 'push the road back'. Since the road doesn't move, the car moves forward instead. So, you could change the speed of the car relative to the earth by moving the road like a treadmill.

With a plane, however, the the only purpose of the wheels is to keep the plane above the ground and reduce friction. The plane pushes itself forward by pushing the air back (hence it is easier to take off in a headwind).

Probably the only way to keep the plane from flying would be to immerse it in a tailwind that matched the plane's velocity, or to lash it down and prevent it from rolling.

I also think that people might be confusing velocity and force. Only force can keep the plane from moving, and the treadmill cannot apply enough force.

[lookout123]

If you replace the word "force" with "Chuck Norris" you may be on to something.

[tw]

Originally Posted by Torrere

With a plane, however, the the only purpose of the wheels is to keep the plane above the ground and reduce friction. The plane pushes itself forward by pushing the air back.

Probably the only way to keep the plane from flying would be to immerse it in a tailwind that matched the plane's velocity,

As was posted previously:

Originally Posted by tw

So that velocity of a plane is totally irrelevant to ground. Wheels will spin as slow or as fast as necessary so that velocity of the runway and velocity of plane stay totally independent.

Posting by Torrere is that airplane wheels make that plane 'separate from' (independent of) both runway and treadmill. Treadmill only affects how fast those wheels spin. As MaggieL posted, otherwise those wheels would burn up; function instead like brakes.

Meanwhile, tailwind does not change the problem. Remember the two items that a force - the engine - applies between. Engine force pushes between airplane and air. Tailwind or headwind - still that engine applies same force causing the airplane to have a same velocity relative to air (also called airspeed). Still that same F=ma equation applies. No matter how fast the tailwind is blowing, F=ma makes the plane move a defined speed faster than air because same force (F) is remains between air and the airplane.

Airplane speed relative to runway is speed of airplane created by its engine plus speed of air created by tailwind.

[xoxoxoBruce]

Headwind/tailwind changes the airspeed by the velocity of the wind. A tailwind will make it harder for the plane to reach the airspeed necessary to lift off. And a headwind will make it easier.

[Torrere]

It only took us 18 pages, but lookout123 has solved the problem. The giant supernatural treadmill is irrelevant; only Chuck Norris prevent the airplane's movement. Since the problem did not specify that Chuck Norris was holding it back, the airplane flies.

[MaggieL]

Originally Posted by tw

Treadmill only affects how fast those wheels spin. As MaggieL posted, otherwise those wheels would burn up; function instead like brakes.

I think they're in danger of burning anyway. Their friction is the only path for the treadmill to transfer energy to the airplane, and they've been specifically designed *not* to do that, whereas the engines are designed specifically to do exactly what they're doing. So the proposed takeoff failure scenario has a power transfer through friction in the wheel bearings equal to the power output of the engines (less engine system losses, of course). So even in a smallish airplane we're talking 100-200 horsepower...which is about 150 kilowatts. How long before the wheel bearings fail from heat?

[tw]

Originally Posted by MaggieL

I think they're in danger of burning anyway. Their friction is the only path for the treadmill to transfer energy to the airplane, and they've been specifically designed *not* to do that, whereas the engines are designed specifically to do exactly what they're doing. So the proposed takeoff failure scenario has a power transfer through friction in the wheel bearings equal to the power output of the engines (less engine system losses, of course). So even in a smallish airplane we're talking 100-200 horsepower...which is about 150 kilowatts. How long before the wheel bearings fail from heat?

My auto inspection shop complains about the cost of their treadmill. $20,000+ to test hundreds of horsepower cars (a machine required by the state required and that has not purpose only years later). Same treadmills exist for Indy and F1 racers designed to test 500 and 800 HP engines. I think we can find those bearings.

Maybe we and the Russians could cooperate on buiding one for airports? Anything for world peace. And since we are using labrat's ass, then it would be methane powered. Reduced global warming. No tailwinds. Having tapped chuck norris, the reduction in bullshit story lines means no additional landfills. Clearly there are plenty of spinoffs from this original proposal including 18 pages of deep technical discusion that would make any Congressman proud.

[MaggieL]

Originally Posted by tw

Same treadmills exist for Indy and F1 racers designed to test 500 and 800 HP engines. I think we can find those bearings.

Not quite the same thing. The dynamometers you speak of are purpose built to carry the load of a race car in normal operation. The "no-fly zone" treadmill must run fast enough to dissipate the equivalant of all the power of the aircraft engine[s] through friction at the landing gear bearings alone. (I grant you can postulate treadmills of any desired power handling capability; Heinlein certainly did--convincingly--in "The Roads Must Roll").

The bearings in question are the landing gear wheel bearings, not anything in the treadmill. They weren't built to do anything more than handle relatively short takeoff and landing rolls near the stall speed of the aircraft, and some taxiing around at speeds about as fast as a man can walk.