Andthen

O,

I'll spoil the fun.

Aircraft take off and stay in the air due to the difference in speed between the air travelling over the top of the wings relative to the bottom of the wing. Hence why they have all the different flaps on their wings.

So if the plane is moving on a conveyor belt, the wheels might be turning underneath it, but the air around the plane is not moving. (Save for the air entering and exiting the engines) ergo no take off.

Oakvillian

I still maintain that the scenario is impossible, but in your version, Andthen, have you considered that if the conveyor is moving fast enough, the friction between a runway-long, runway-wide conveyor and the air around it will be sufficient to get the air moving fast enough to generate the required pressure differential across the wings to provide take-off lift?

snowbunny

There's no conveyor belt/treadmill in my example above...... however, it still won't take off, because as soon as the plane lifts, gravity with the ground may temporarily be broken, but there won't be enough air speed to sustain the lift, even if one could magically turn on the thrust the instant the plane began to lift.

Bill_S

The aircraft's engines are creating thrust. The thrust is being applied to the airframe. The motion of the runway/treadmill cannot counteract that force.

Let's go back to the treadmill at the gym. Suppose it's a turbo deluxe extreme model, and it can drive its belt at 1,000 miles per hour. Hold a toy car in your hand and place it, wheels down, on the belt. The car's wheels are spinning 1000 miles per hour backwards. Apply foward force (thrust) to the car. Can you push it towards the front of the treadmill? If the treadmill were to increase speed as you were pushing on the car, could it stop you from moving the car forward relative to the air around it?

bazzz

I concur.

Bill_S

Absolutely correct, mate. The rest of this lot are going to owe us drinks

Surrey Expat

Interesting topic of discussion!

tom17

I have found through experience that it is often futile trying to convince the 'no-fly' people that it will indeed take off. Fun trying though

Sorry everyone

snowbunny

Discussion is fun!

My cousin was one of the first pilots to take off from an aircraft carrier -- in a BOMBER -- in October 1942.

iaink

Hmm. I can see where you are coming from.

At what point would the bearings sieze as the wheels would be running backwards at twice their normal speed due to the two forces (thrust and treadmill) acting in tandem.

I wonder at what point the friction of the rubber looses the fight against the forward motion of the airframe, if any. Aircraft rubber is pretty sticky, it has to be to stop 400 tons of jumbo landing at 200mph in a pretty short distance

Newtons laws still apply. Turn the engines off...and they dont act through the wheels remember... and the plane will go backwards with the treadmill.

Let go of the car and it will shoot backwards at 1000mph.

You have to take momentum and inertia and friction and all that other boring real world stuff into account.

OK, so whos going to build a treadmill runway to settle this for once and for all?

Oakvillian

suppose you tied a piece of string to the front of the aircraft and held on to it... all the treadmill will do is to make the wheels rotate. The Newtonian laws act on the bottom of the wheel, causing a moment to be applied about the axle which will be easily overcome by the thrust of the engines.
I know, life would be so much simpler if reality was like an O-level textbook - point masses, zero-friction bearings, no air resistance (hey, that would make my ergometer workouts a lot easier...), perpetual motion pendulums and all.

If you build it, they will come...

Andthen

I give up, it's a totally impossible situation!!

Bill_S

OK, since we've got round to Newton, I'll have another go...

3rd Law means for every action, equal and opposite reaction.

We have an aircraft stationary wrt the air around it. The action = application of thrust to the airframe. The reaction = the aircraft moves. The magic treadmill can spin the aircraft's wheels forward, backward, fast, slow... it doesn't matter what the wheels are doing. If a force is applied to the aircraft, the aircraft MUST move. Spinning the wheels CANNOT stop the aircraft from moving. (Unless the force applied is very tiny and not greater than the friction from the bearings, but that's hardly what we're talking about here).

koogar

Of course it will take off. The only thing that would stop it taking off is if the tyres/tires exploded due to rotating too fast.
It is, of course, an impossible scenario because, as soon as you provide thrust to move the plane forward through the air, the treadmill will never be able to match the speed of the wheels. As the plane accelerates through the air (which is of course what planes do, ground speed has nothing to do with take off speed, why do you think they always take off into the wind?), the treadmill will try to accelerate to match the speed of the wheels, but in doing so, will be increasing the speed of the wheels itself by the same degree, so the treadmill will fall increasingly behind, until the plane takes off or the tyres/tires explode. If the tyres, wheels, bearings etc can rotate up to infinite speed, the plane will take off.
If, however, you put a giant fan on the back of the plane that blew air over the wings at the same speed and in the opposite direction to the planes movement, then it wouldn't take off (presuming zero wind speed).

Er, no.
MOST planes require much less distance to land and stop than they need to take off.
Model: Boeing747-300
Engines: (1)JT9D-7R4G2(24,856kg)
(2)CF6-80C2B1(25,040kg)
(3)RB211-524D4(23,850kg)
Width: 59.64m
Length: 70.66m
height: 19.33m
Wing Area: 511.0m2
Maximum Take Off Weight: (1)(2)377,840kg
(3)371,945kg
Weight: (1)174,179kg
(2)175,994kg
(3)178,715kg
Maximum Cruising Speed: Mach 0.85
Maximum Range: 12,400km
Required Take Off Distance: 3,185m
Required Landing Distance: 1,942m

Hobbess

Has anyone questioned why the plane is even on a treadmill? Surely if it's in Canada (and why wouldn't it be as this is a Canadian forum) then it's probably a reasonably healthy plane and there are lots of nice wide open spaces to go out for a run that the use of a treadmill becomes a moot point...