[Closed] plane and conveyor belt

So... can a sea plane take off from a fast flowing river? 🙂

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Probably a little less for these beasts 🙂
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More worryingly than this being debated on a bike forum...
This question caused a very long debate over on both Pprune and airliners.net with even professional pilots arguing that the plane wouldn't take off. I might ask the captain next time I board a Ryanair flight - if he says no I'll re-book!

You'll have to pay £20 to talk to him though, and another £30 to get an answer.

Flaperon - Member
YES, for crying out loud, YES!

With a name like Flaperon, if he says yes, then YES!

You'll have to pay £20 to talk to him though, and another £30 to get an answer.

when I read the question I assume that the plane is not moving in relation to the ground (as a person running on a treadmill is not moving in relation to the floor of the gym). Therefore there is no air moving over the wings, therefore no take-off.

eckinspain - a person requires their feet to be in contact with the ground to move - hence, if the ground is moving at the same speed but opposite direction to that which they are trying to run at, then they will remain stationary - same as a car on a rolling road.

Ask yourself whether the plane uses it's wheels to propel itself forward while attempting to take off or whether it uses the big spinny thing attached to the engine and whether that big spinny thing is concerned about what's happening to the wheels? 😀

No, the tyres would explode.
Edit: With LHS et al...

what if both plane and treadmill were inside a vacuum?

Terrible news, all you stats/offspring puzzle fans:

I think (allowing for bearing friction & safe tyres) the plane will take off, therefore clearly that is the wrong answer 😕

Now, if the pilot was a traveller and the passengers all bankers or MPs, would the weight of stw opprobrium allow it to take off ?

Stumpy - Ask yourself if the plane is flying from the moment it starts to move forwards or has to reach a certain speed before the effect of lift can overcome the force of gravity?

I find these threads immensely entertaining.

Ignoring peripheral silliness about wheel bearings and tyre ratings etc. I really can't understand how anyone would think the answer is no.

Stumpy - Ask yourself if the plane is flying from the moment it starts to move forwards or has to reach a certain speed before the effect of lift can overcome the force of gravity?

Eh?

The plane will not fly until the air moving over the wings reaches a certain speed. It is almost immaterial as to whether the plane is actually moving (e.g. stationary plane, very strong head wind).
Assuming this 'experiment' takes place on a totally still day, then the aircraft will have to obtain a ground speed high enough that the airflow over the wings creates enough lift to overcome gravity.

EDIT - should have said that the wing will start to overcome the force of gravity from the moment that the aircraft gains enough speed for it to generate 'some' lift. At this point, the plane will start to become 'lighter' (if we imagine a runway that not only has a conveyor attached but a giant set of scales under it). Eventually the plane will be going so fast that the lift=the weight of the aircraft and any further increase in speed will cause the plane to fly.....

So how does it reach that speed if the ground is always moving back at a speed greater than the speed at which it can propel itself forwards?

The Southern Yeti - Member

So how does it reach that speed if the ground is always moving back at a speed greater than the speed at which it can propel itself forwards?

why does the speed at which the ground is moving backwards have any effect whatsoever on the speed at which the plane can propel itself forwards?

FFS, THE GROUND SPEED IS TOTALLY IRELAVANT!

A plane will only fly given sufficient AIRSPEED.

Gravity?

Why won't my jet propelled bike link work? Anyhow, will a jet propelled bike go forwards on a tread mill going backwards at the exact same speed or stay still?

Forward

(and if it doesn't then you really need to grease your hubs).

If you tie a bit of string to the front of the plane, then [b]pull it forward [/b]by that string (imagine God is pulling on the string...) - does it matter one bit which way the wheels are moving? Forwards/backwards - as long as the plane is moving forwards, the wheels don't matter (neither does the floor).

Now - imagine this - it's [b]NOT[/b] God pulling the string, but [b]propellers/jet[/b] pulling the plane.......

Geesh - how hard is it to get!

DrP

[i]Put a model car on a treadmill and push if forward with your finger. Does it move forwards? Of course it does.[/i]

That explains it perfectly IMO. Unless there enough friction in the bearings it doesnt take any more effort to push either. Put plane in place of model car, and plane engines in place of your hand. Or am I getting it wrong?

Edit: Dr P's explanation is good.

Oh no - I tried for so long not to post on this thread & now I can't stop.....

The Southern Yeti - Member
So how does it reach that speed if the ground is always moving back at a speed greater than the speed at which it can propel itself forwards?

You've kind of answered your own question there.....'propel itself forwards' is what you wrote....
How does it do this??
Does the plane rely on:
a) the wheels which are just fitted to free spinnig hubs so the plane has something to sit on & move around the ground on, or
b) the great big propeller/jet engine that has no contact with the ground and pushes heaps of air backwards

to propel itself along.....??

If your answer is a) you are wrong.
If your answer is b) you are correct.

Considering that you went for b), then the little wheels can be doing whatever they want against a conveyor/hundreds of marbles/zilions of ants running the wrong way but the propeller/jet engine is still gonna be pushing air backwards causing the plane to move forwards.........

HAHAHAHA, can I ask another question?

no.

Right people - stop being thick. Newton's 3rd Law explains it all.

.....?

Strain = (L'-L)/L

or am I looking in the wrong book? 😉

The every action having an equal and opposite reaction one... If people the people on here that say the plane wont take off are correct how are vertical take off and space travel possible?

The whole reason Hovercraft can't fly but can only get a little bit off the ground is because they don't have any wheels.

But would it make a sound?

Depends on the Doppler effect. How fast is it moving? And how fast are you moving?

Vertical take off and space travel do not rely on airspeed in the same way as an aircraft does. They basically use brute force from the engines to overcome the gravitational pull of the earth.

They use thrust from the engines to move the plane against the [b]AIR[/b]

Just for anyone who missed that

[b]ITS THE REACTION AGAINST THE AIR[/b]

for clarity

[b]THE WHEELS AREN'T POWERED[/b]

🙄

wait - so if the plane and conveyer belt was in a vacuum it wouldn't be able to take off? yeah, right!

What about if the plane is full of Americans on their way home from Disneyland?

The rocket itself is 'pushing' against the inertia of the propellent which is being ejected.

Mintman - what about when they are outside of the earth's atmosphere - how do they accelerate then?

[i]The rocket itself is 'pushing' against the inertia of the propellent which is being ejected.[/i]

I've had mornings after a vindaloo like that.

FFS, THE GROUND SPEED IS TOTALLY IRELAVANT!

A plane will only fly given sufficient AIRSPEED.

I agree, but surely the airspeed is zero? The treadmill is going round, the wheels are moving but the plane is staying still, no?

No. the wheels will be spinning twice as fast. The plane will still go forward at the same speed. Unless there's enough friction in the wheel hubs to stop it moving forward. In which case it won't take off.

It's easy to see why it causes confusion though.

I thought in the original problem the conveyor belt continually matched the speed of the plane, so the plane goes forward at 10 mph, conveyor moves back at 10 mph therefore overall speed of plane through air (wind over wings) equals 0. Plane accelerates to 20mph, conveyor matches, plane through air = 0 and so on?

It has no affect the air movement though.

Can a Swan take off on a treadmill?

After it's broken your arm.

I think some trolls have crept into this thread! 😉

The arm is irrelevant. Can the swan take off?