Imagine a plane is sitting on a treadmill

@sgb27 and gideon

The parameters of the question stop it from moving forward. the conveyor is going the same speed as the planes wheel.

@ JerseyBoy

Correct, it's not rate, strictly speaking, it's force instead. I think, if I remember my physics correctly. But that's how jet engines always work, the fact that it's on a giant conveyor makes no difference.

I get the plane air speed as 350mph as there is nothing restricting it from accelerating to that speed.

edit: Thanks DW, this is a fantastic thread

The convayor is as long and as wide as the runway.
It matches the planes speed but in the oposite direction. This doesn't matter. All that matters is that the planes thrust moves it down the runway/convayor at an air speed that creates lift.
Yes, given the information in the question, the plane will take off.

Except the conveyor belt moving in the opposite direction.

And what does that have to do with anything?

How does the conveyor slow down the plane though?

Remember the wheels are not driven like a car, it's freewheeling.

Anyway taking off boring



landing parking is so much more fun.

Repeat: The engines on any aircraft are used to propel the craft forward, so that lift on the wings can be generated. If the aircraft is not moving forward, due to the conveyor belt, there is no airflow over the wings. No airflow, no lift.

Nothing is stopping the plane from moving forward on its wheels. The conveyor belt just keeps the craft in place with respect to the person not standing on the conveyor belt.

EDIT: I am assuming, of course, that the airflow is stationary w.r.t. the observer.

But it wont move down the runway! As the runway is moving in the opposie direction at the same speed as the Aircraft is attempting to move down the runway. Like running on a tread mill. As I read it!

How steve, the plane is stationary from any perspective except if you are standing on the conveyor belt. The air around the plane isn't movuing (except the bit going through the jet), if no air is moving then no lift generated, no lift, no take off, no take off and the answer to a question on TT is NO.

AARRGGHH!!!

Of course, I am assuming throughout this thread that we are dealing with a fixed-wing aircraft, without the ability to redirect thrust.

The Harrier being shown has the ability to redirect thrust and has a thrust to weight ratio more than one.

EDIT: I am also making one large assumption which I haven't stated: the flow is inviscid, so that the moving conveyor brings no air along with it.

Sorry, what's so difficult to understand???

The conveyor belt is irrelevant, as long as the tyres will take the centrifugal forces exerted on them..

so, yes it will take off.

If this question was on your application for a job, half of you would be unemployed.
Why am I unemployed?

That's your problem, it's NOT like running on a treadmill because a plane doesn't use the "treadmill" to provide it's forward force, it uses the air. It is just physically impossible for any conveyor to prevent a plane from moving forwards once the jets are fired up.

Here's an analogy that might help. Imagine a conveyor going *UP HILL* like one of those ones in the airport. Stick your luggage trolley at the top and let go. What happens? Does it go up or down the hill? Exactly the same for a plane (except engine thrust instead of gravity).

EDIT: assume the luggage trolley doesn't have brakes of course...

Due to the conveyor belt doing what? The jet engines push against the air, not the ground. So the wheels go round very quickly - so what?

Your logic There is nothign stopping the plane from gaining air speed. How exactly is the conveyopr belt preventing the plane from moving forwards relative to the air?

Funny, when I run at the gym on the treadmill I run on a 2.5% grade. I still don't move anywhere.

yes they are freewheeling. but they do not while in contact with the ground move at an independant speed realtive to the planes thrust.

the plane itself can not achieve "airspeed" as such until it has taken off. it will not take off if its forward momentum is countered by a conveyorbelt traveling at exactly the same speed as the plane but in another direction. as the original quote says. it has technicly an air speed of 0mph and a ground of 350. if we were stood beside the plane we would not percieve it as moving.


no mention of wheels btw.

Bloody hell - even my elementary IBH Physics taught me that!

@ JerseyBoy

That's because your propulsion is pushing against the treadmill .The plane's isn't, it pushes against the air.

ok so I don't see how people are saying the plane can move...the conveyor belt keeps it stationary, sure the wheels are going nuts and the engines are having a ball but the wings are still in the same position as they were to start with...so no air going over the wings...how the heck is it ment to take off?

The air isn't moving, the conveyor is.

The conveyer belt is very relevent as it is stopping the aircrafts motion through the air. Which is what it need to do to generate lift to take off. The wheel are irrelevant, it could be on skies, bearings or what ever you want.

btw Great question to get the gry matter working on a Monday.

The jet engines move air backwards, the plane accelerates in the other direction. Once again, the wheels have no effect, assuming the brakes are off.

Erm, I don't understand your logic. What is causing the plane to get airspeed to begin with? The jet engines (on a fixed wing aircraft) only act to accelerate the aircraft on the conveyor belt. Therefore, the airspeed is zero.

Yet again, what exactly do you think is stopping the plane movign forwards under this situation? What force?

I recon its a cleverly disguised troll by DW...

the engines are for thrust, the wings for lift. if the engines give lift too, what are the wings for ? erm, steering ?

can someone please tell me what force the treadmil is exerting on the plane to prevent it from moving forwards?!?!?!

(other than the miniscule amount of frictions in the tyre carcass and in the wheel bearings. Oh and wind resistence. All of which are countered for by the thrust in the engines)

The plane takes off when there is a relative speed between the wings and the air.(The lift force is proportional to the square of relative speed)The jet engines only provide forward thrust. When the conveyor moves back the plane exaclty with the same speed as the jet engine provides to the plane, the wings(and body of airplane) stay stationary relative to surrounding air and so no relative velocity=no lift=no take off.

The jet engines produce a forward force on the plane. The only backwards force is through the friction in the wheel bearings. Net force is forwards, which means the plane accelerates, pretty much as normal.

No they don't. That would only happen if the pilot had the brakes on. The converyor moving back will *NOT* pull the plane back, because the wheels are free to spin.

Dear me, how many more times!

OK, Allright, Anybody that argues my point is a mental moron.
Go back and read the question.
Can I say that without getting deleted from the system?
Really, go back and read the question.

The plane IS moving forwards.

It's just moving forwards on the conveyor belt, which is, in turn, travelling in the opposite direction.

No the planes relative motion is to the air, caused by its engines.

how can the plane accelerate? it says the conveyor belt is the same speed as the plane..plane speeds up, conveyor belt speeds up. so no matter what the plane doesnt move

Aaaaaaaaaaaaarrrrrrrrrrrrrrrrrrrrrgggggggggggggggghhhhhhhhhhhhhh!!!

Unless the plane is held back by cables or somesuch, nothing is preventing it from launching off the conveyor belt with wheels spinning exceptionally fast

Yes, but you're not farting...

The plane is moving forwards relative to the air, that's what jet engines do. To stop the plane moving forwards relative to the air there needs to be an equal force pulling the plane backwards. A converyor can't do that because the wheels are free to spin.

Put a model car on a piece of A4 and pull the piece of paper, notice the car stays pretty much where it is.

it says same speed as the plane not the stupid wheels...geeeeeezzzz

haha sgb27 googled it because I so just read that webpage.

the plane is moving forward.

but for every meter it is moved forward it taken one back by the converyor belt. it has nothing to do with powered wheels jet engines or anything else. as long as gravity is acting apon the plane, with no moving air lift can not counter it and the plane remains on the ground.

you have two forces acting on the palnes position. they counter themselves exactly. therefore no change in airflow.

i am seriously wondering what nasa does with its budget.

What "takes" it back? There is no appreciable force between the treadmill and the plane to do any "taking"

How? The wheels are free to spin, and they do. That doesn't slow the plane down.

Putting such questions out on the net to fish for the next generation of crack engineers

realtive to the conveyor belt. not the observer.

in which direction would the plane go if the engines were switched off?

IT CAN'T!! A conveyor belt cannot pull the plane back because the wheels are free to spin!