Can the plane take off?

[Da1andonly]

the conveyor belt senses the speed of the plane's wheels and moves at exactly the same speed in the opposite direction.

I'm with thesearcher on this one, aswell as Dameon "Kurt" Angell. The treadmill is moving exactly the same speed in the OPPOSITE direction, which means the plane is "standing still". Fuck you, da1! We'll get a bigger engine! Well, the treadmill still senses the speed and the plane stands still.

If you take, say, a large chunk of goddess and put two engines on the sides and leave it on a concreteschlyrd, it wouldn't go anywhere either. It would need something to move swifter, say a pontoon -- or wheels.

I'm hungover.

[ZeUs!!]

The wings will still push the air down, and as soon as it gets an inch off the ground it'll shoot forward at 500 mph

[Doc Flabby]

just turn the plane around.

[Fluffz]

oh holy god how hard is that to get! imagine a gigantic hoovercraft with a guy holding a stick to the ground, at the end of the stick there is a wheel. why should the hoovercraft not be able to move totaly unrelated to what the ground does? the ground would have to move in the opposite direction at impossible speed to create a bigger pulling force (by friction) than the hoovercrafts gigantic engine can push



that being said and understood what is the grounds speed in our case? assuming that the grounds speed is the speed of the wheel aka the wheels rotation and wheel speed = ground speed + plane speed

plane speed: 5
ground speed:5
wheel speed: 10, ERROR must match ground speed, increase ground speed

plane speed: 5
ground speed:200 000
wheel speed: 200 005, must match ground speed, ERROR, but getting closer

plane speed: 5
ground speed:300 000
wheel speed: 300 000, SUCCESS

Conclusion:
a.) the gound speed/wheel speed is fucking HIGH, the plane is melting due to friction. melting planes cant fly. its not moving at all
b.) the wheel's speed is not the rotating speed but the speed at which the plane and the wheel move forward. the plane takes off as always.

[Rudy]

I suspect this is the thread with most replies in the shortest space of time ever to grace these forums. Congratulations everyone. You turned a yes or no thread into a record breaker.

[HeavenSent]

Trick questions don't count as simple yes/no answer threads. :P

[Pressure Drop]

Ok, here's my 2 cents...

The conveyor belt is irrelevant.
The plane's engines are working against the air, not the ground. The plane is gonna move forward no matter what the conveyor belt's doing. B)

not true. The conveyor belt isn't irrelevant ...

yes the engines work against air and not mechanically drive the wheels, but the plane is still connected to the ground ... and this means it applies a force to it, it is relevant. The engines are moving air to create enough force to overcome these forces INCLUDING THE FRICTIONAL FORCE PROVIDED BY THE GROUND OPPOSING THE PLANE GOING FORWARD

being on a treadmill actively putting in energy to the system by matching the speed of the wheels is going to increase this force.

If the wheels moved the should then exponentially accelerate on the spot till they fail/ super wheels that don't fail - reach a speed where by the frictional force provided by their action on the treadmill = the thrust of the engines.

[HeavenSent]

eventually the plane is going to get enough thrust to make the converor belt irrelevant. Initially the belt will keep the plane in place but once it reaches enough power to "grab" the air, the belt becomes irrelevant and the plane moves forward.. picking up enough speed to take off.

[Galleleo]

I think the plane will always have a forward movement cause the conveyor belt will behind on the speeds of the wheels. Cause a plane doesn't drive it's wheels by engine but only by movement. The plane engines start moving air, moving the plane forward, THEN the wheels start rolling THEN the conveyor belt can start matching the speed of the wheels, but the plane is already in a forward motion here, I think the belt will lag behind on the planes speed.

EDIT: Look at this way, you have those things you can put a car on and then let the car drive without moving (those rollers under the wheels that just match the speed of the wheels) so you can test the wheels, or the speed of the car or whatever. If you were to make such a thing for the wheels of the plane, it wouldn't work because the plane doesn't move the wheels by it's engine power, but by it's forward movement.

I am talking about something like this:

[Displaced]

that gall is a dyno, and i have to agree with Da1 here.

just going against fluffz hovercraft point, you cant compare a hovercraft to an airplane, beucase they dont work the same way.

in a hovercraft, the engines create thrust (i.e. forward thrust), AND they create lift (they fill the hovercrafts skirt with air, thus creating lift), so it doesnt matter if it were on a tradmill, because it isnt actually connected to the treadmill in any way, shape or form.

now an airplane IS connected to the treadmill, why?
because, thebody of the plane itself, is connected to the landing gear, onto which the wheels are bolted.
therefore, all of the planes mass is restng on its wheels when it is standing still, and does so before it has left the ground.

so, you have ur little treadmill, which moves in a left to right direction at lets just say 900km/h.

and ur little plane is using its engines to create enough thrust to move at 900km/h right to left.

in essence the plane isn't actually moving anywhere, because all its doing is matching the equal and opposite force being made by the treadmill, hence there is no air passing over its wings (im negating wind here), hence it has no lift, hence its not going to take off.

now before you start debunking that, IF the wheels and whats happening on the ground has absolutely no effect on an airplane, why do we have runways and wheels in the first place? cant the plance just start its engines and take off immediately, no, because it needs to achieve a set speed, where it gains enough lift to actually take off, so if its not moving forwards, it isnt creating any lift. (unless were talking about Harriers or whatever, but that is completely different)

[Ewan]

Big edit: It's a big yes. I re-read it and didn't notice the bit about "sensing" the speed of the plane. This means it starts from a stand-still, so the only thing that is going to happen is that the runway will have to be very slightly longer (a few metres) or slightly more thrust will be needed. I'll leave in what I originally wrote

The plane will be able to take off.

It, will, however, take a longer time as the engines will have to accelerate the plane from whatever speed they are pulling it back to the required take-off speed.

You'll also have to take into consideration the tyres. I'm not too clued up on the regulations regarding tyres, but it's probable they will be able to withstand the increase in heat from the faster rotation speed.


Also you have to consider the way turbofan engines work. The require a starter motor to get them going to a high enough speed that they can "suck" in the air from the front and be self sufficient (ofc they have to burn fuel). If you're starting the conveyor belt at the same time as the engines, this probably won't be an issue. If, however, you wait until you're travelling backwards at some speed before attempting to start the engines, it's likely you won't be able to get the compressor (intake) spinning enough to "suck" any air in, and thus won't be able to get started.

[Ewan]

well if it's a jet engine it draws air in and around it even when it's not moving, so you could theoretically get enough airflow over the wings with a powerful enough engine

What?

The wings will still push the air down, and as soon as it gets an inch off the ground it'll shoot forward at 500 mph

Uhh...

now an airplane IS connected to the treadmill, why?
because, thebody of the plane itself, is connected to the landing gear, onto which the wheels are bolted.
therefore, all of the planes mass is restng on its wheels when it is standing still, and does so before it has left the ground.

so, you have ur little treadmill, which moves in a left to right direction at lets just say 900km/h.

and ur little plane is using its engines to create enough thrust to move at 900km/h right to left.

in essence the plane isn't actually moving anywhere, because all its doing is matching the equal and opposite force being made by the treadmill, hence there is no air passing over its wings (im negating wind here), hence it has no lift, hence its not going to take off.

now before you start debunking that, IF the wheels and whats happening on the ground has absolutely no effect on an airplane, why do we have runways and wheels in the first place? cant the plance just start its engines and take off immediately, no, because it needs to achieve a set speed, where it gains enough lift to actually take off, so if its not moving forwards, it isnt creating any lift. (unless were talking about Harriers or whatever, but that is completely different)

Ok. This doesn't work.

Your explanation would only work if the wheels where unable to rotate. The thrust from the engines pushing the aircraft forwards will overcome that of the treadmill pulling it back. Let's make a simplifying assumption:

1. The wheels rotate at zero friction. Imagine the perfect frictionless axel has been invented. This means that the treadmill moving backwards will rotate the wheels, with the plane staying where it is. Now, the engines turn on, pushing the plane forwards, the wheels just rotate at whatever speed is needed, they don't care. The plane takes off like any regular plane.

Now, of course this isn't the case, but it should give you an idea of where I am coming from.

The wheels will have some sort of friction when they rotate, and so the plane will start to move backwards (if the treadmill was started early) but it would take a long time to reach the speed of the treadmill. Now, assuming you did wait until the plane was moving backwards at 900km/hr before starting the engines and say it has to reach a speed of 900km/h to take off. It would simply take approximately twice as long to take-off (ignoring the engine problems I stated in my last post) as it has to accelerate from -900 - 0 then 0 - 900. The backwards rotation on the wheels will have a negligible effect.


Now, in the original problem, it states that the treadmill is always moving in the opposite speed. So therefore, the plane is starting from a standstill. The treadmill is only applying (a tiny tiny, remember THE WHEELS ROTATE UNDER THE PLANE) backwards force once the plane starts moving, and thus only a very small extra amount of runway is needed.


EDIT: Thought of an analogy! Imagine you're playing with a toy car you push it with your arm forwards at 2m/s. Now, imagine you're doing the same on a treadmill, it really doesn't take much extra force to push the car forwards at the same speed. Engines on a plane are doing the same as what your arm is doing to the toy car.

[Displaced]

ah, i do see where you are coming from ewan, didnt think about it like that.

however, i am assuming that if the treadmill were to be moving backward at enough speed, the friction between the treadmill and the wheel, and the wheel and its bearings, would at some point become significant enough to retard any acceleration.

[Hero*]

You can't say "just make the plane go faster so the treadmill will be irrelevant". Fuck its always the same speed against the same speed (that of the plane and treadmill). Stop talking like scientists about 'friction' and blahblahblah. I don't think its possible cause that shit aint moving.

[thesearcher]

but doesnt the question state that the treadmill is always matching the wheels speed, you are all saying that eventually the wheels will just rotate faster than they usually need to, but that wont matter, because the faster the wheels rotate, the faster the treadmill will go to match it.

what you said about the car ewan makes sense, but think of it like this, as you push the car slightly more, the treadmill moves faster to counter the extra force instantaneously. the treadmill is always going as fast as the wheels, i know this wouldnt be possible in reality, but thats why its a hyperthetical question.