UKC

This is a laugh - seems to divide people pretty cleanly down the middle.

"Imagine a commercial jet plane standing on the runway. Except the runway is an enormous treadmill. A sensor on the plane's wheel feeds-back to the runway/treadmill so that, as soon as the wheel starts to turn, the treadmill exactly matches the turning, but in reverse. Will the plane take off?"

Me, I'm firmly in the "yes" camp.

It'd be good to get some Friday disagreement going.

John

In reply to Moacs:

Of course not. It's wind speed across the wings which provides the lift, not wheel speed.

In reply to Moacs:

Nah because it would never get enough lift, however it would drop of the end - since no way it could match speed perfectly.

In reply to Moacs:


nutter

In reply to Moacs:

i vote for no. What's wheelspeed got to do with it?

In reply to Moacs:

no, as it's not really moving.
If you jam on the brakes it maght just "take off" the back of the treadmill though

In reply to Moacs: 'but in reverse': you mean the treadmill is moving in the same direction as the plane, unlike a usual treadmill. If yes, yes, the plane will take off. I m wondering if it would do it in half the distance required normally.

In reply to brothersoulshine:

Exactly - nothing.

I think yes - because the engines thrust on the air to move the plane...conservation of momentum. The wheels jut hold the plane up. Fundamentally different to a car where the wheels act on the ground - a car wouldn't go forward, the plane will. It's the air across the wings from going forward that will make it take off. JMHO

John

In reply to Moacs:
That seems rather alarming - that half the people who travel by air have no idea what is keeping them airborne.

In reply to Gordon Stainforth:

Well we've 5 no's and 2 yes's here - and I'm a yes.

J

In reply to Moacs: Erm, no - as someone has said previously it's the velocity of the onflow to the wings that matters.

However, maybe if you kept it on the treadmill and fed the same data to a fan infront of the aircraft you could get vertical take off from a passenger airliner. Though it would be a very quick flight, for as soon as it left the area covered by the fan it would stall and hit the ground again (I suppose this would mean you'd have vertical landing as well!)

In reply to Moacs:
Yes.
The jet engine on the body/wing will push the whole plane foreward and so create air flow across the wing which will cause lift.
What the wheel/treadmill speed will be beats my small mind at the moment, but this is irrelevent to the plane taking off.

If the plane were to be propelled via the wheels, then no, it wouldn't take off.

In reply to Moacs:

You're really not very bright, are you?

What air moving across the wings?
If its on a treadmill then it (and the air over the wings) is not moving - hence no lift.

If it was a propellor driven plane (on the nose, not under the wings), then you might get some lift from that, but unless there is movement of air over the wings from the plane moving forwards then it had as much chance of getting airborne as a dead parrot.

In reply to Moacs:

don't forget the 'momentum' of the treadmill

In reply to Moacs: No as the plane won t be moving forward. It s like if its breaks were on while on full power, it would still not take off.

In reply to Moacs:

Durrr... i read the question wrong! I thought it said "landing" on the runway, rather than "standing".

What was the question again? You mean as if the plane had frictionless skis instead of wheels? It'd take off then.

In reply to Moacs:

3 words:

frame of reference

In reply to Moacs: Sat here in my office, with my Spares kit for a Fuel pump for an aircraft, i an firmly in the NO camp, as basic avionics is to do with how the wind going over the top of the wing has further to travel, so moves faster, creatng lift etc.... so in the no camp due to the wheels only being there to give the plane something to roll allong the ground on, and nothing to do with the principle of flight, ask the Wright brothers!

Jess.x
(Apprentice Engineer Aviation test and control!)

In reply to brothersoulshine:

Now that is a more interesting and challenging question. My guess is it would slide out.

In reply to Gordon Stainforth:

Everyone can't know everything about everything though can they?

I have absolutely no knowledge about aerodynamics and the likes ...

In reply to Little Brew:

I believe you for some reason...

;0)

In reply to Moacs:

The problem is poorly specified as you do not say what "in reverse" means with regard to the treadmill. Do you mean that the upper surface of the treadmill moves in the opposite direction to that which the plane is "moving", i.e. as a normal treadmill operates. Or do you mean that the treadmill acts in the reverse manner to a normal treadmill?

If the former then no, if the latter then yes (with one caveat).

Reasoning: in the former case, as the plane does not actually move there is no airflow over the wings to create lift to allow the plane to take off. In the latter case then the plane is effectively catapulted forward as the treadmill will double its actual speed through the air. However once the end of the treadmill is reached or the pilot initiates lift-off, the plane must be travelling under its own power at its minimum airspeed otherwise it will come back to earth with a bump.

boB

In reply to Moacs:
Can you give some reasoning behind why the plane would take off?

Air would be forced back from the plane's engines and the wheels would be turning quickly but with no air flow over the wings how can the plane possibly take off? Unless you have the strange fan type apparatus described above.

In reply to Simon: Why thank you! =)

and why might that be?

In reply to Moacs:

NO, NO, NO, NO, NO!

GCSE physics says NO!

In reply to Moacs:

The engines just accelerate the air through them - they are not designed to directly force airflow over the lifting surfaces.

In fact if you look at the flow through jet intakes at zero forward velocity (as we have in this case), then the flow is even drawn from behind the front of the engine, and so possibly creating a flow in the opposite direction over the wing and potentially forcing the craft more firmly into the ground.

This effect is only changed when an non-zero onflow is imposed upon the engine (as is the case when the craft starts moving) and the net effect is for the flow to be drawn from infront of the intake.

In reply to Moacs:

As has been said by many above (so I'm just casting my vote), the plane is stationary wrt the surrounding atmosphere (the only pertinent frame of reference), this means no airflow across wings, so no pressure differential between above and below the wing, thus no upwards thrust and [finally] no take off.

In reply to Liam M:

I understand that the plane won't take off unless it has sufficient velocity.

I guess what I don't understand is why spinning the wheels backwards will have any big effect on it's velocity.

In reply to brothersoulshine:

(and my apologies for the extra apostrophe).

In reply to Moacs:

NO!

No way, it's not possible. Moving air on wing surface required.

Anyway, if a treadmill could easily replace a long tarmac runway, why then do we have long runways at airports which are frequently built in areas where land prices are high?

We could have much shorter air-craft carriers too, right?

In reply to brothersoulshine: Spinning the wheels will have no effect - my understanding of that idea was that it merely allowed the engines to be spun upto a high speed whilst the vehicle was stationary, and thus instigate a discussion of the effects of flows through the engines on flows over the lifting surfaces.

Of course the plane will take off

Feedback = delay. The treadmill will chase wheel speed which is accelerating.

The plane will move, the wheels will spin a lot faster due to the treadmill and while not frictionless will provide less drag than the thrust of the engines.

The plane would take off just needing a longer runway.

Cheers

Toby

In reply to Moacs:

An alternative take on this would be to consider the plane being held stationary on a flexible arm that allows it to move up and down but not backward or forward. If a large enough fan is placed in front of the plan and blows air past it, the plane will indeed rise with the wheels not moving at all and the engines switched off.

You can do this with a piece of paper. Maybe A4 would be a bit too heavy, so try cutting / tearing a strip about say 5cm wide and 15cm long. Hold one corner of one of the short sides between the finger and thumb of your left hand and the other between the finger and thumb of your right hand. Bring it up to your mouth and put the short side on your bottom lip. Blow over the paper and it will rise. The same thing is happening in a plane wing, the shape of the wing makes air move more quickly over the top than the bottom. Faster moving air has less pressure so there is an upwards thrust.

In reply to Joe G:
Assume that the only job of the wheels is to keep the body of the plane off the ground and they operate with no friction.

Assume that all motive power of the plane is provided by the engine pushing air.

Assume that the treadmill has no effect on the air currents around the wing.

When the plane is stationary, the air flow over the wing will be the same as that over any other part of the airfield.

As the engines provide thrust the wing will move and so will provide a degree of lift. The rate at which the wheels are moving is irrelevant as they are on frictionless bearings.

The plane will take off as normal (relative to an observer not on the runway).

If there is some friction in the bearings, more thrust may be required, but the plane will still take off.

In reply to Moacs:


Only if there is a 200mph headwind

In reply to Toby_W:

I did semi cover that in my first response of it dropping of the end. however the length of the runway would be insane.
Also if we are using assumptions like that then it could also be that the system overcompensates and therefore the plane starts going backwards.

In reply to Toby_W: I think you're trying to make it into a practical problem - I believe it's meant to be more of a thought experiment, so ideas such as lag in the system can be neglected, and it can be thus assumed the craft remains stationary on the treadmill

In reply to dissonance:

Spoilsport

Cheers

Toby

In reply to dissonance:

Ummmmm.... use negative feedback?

Andy

In reply to markAut:


Wrong. Thrust and lift are two separate entities.

In reply to Liam M:

If it remains stationary then of course it won't take off. If it moves forward then it will eventually gain enough speed to take off.

It's hard to see why it won't move forward. Let's say the treadmill is set to always run at a speed faster than the wheels are spinning. All that the treadmill is doing is increasing the angular momentum of the wheels. The only loss is in the friction of the wheel bearings.

The plane will move forward on the treadmill.

In reply to Moacs: the plane will take off, but the treadmill will have no affect on this. As has already been pointed out, the wheels are free to turn. the treadmill could be running in either direction.
Tree

In reply to Moacs: John,
I think the plane "may" take off for the following reasons.

The wheels on an aeroplane are free running and have nothing to do with the planes acceleration - that is caused by the action of the jet engine on the air. Therefore although the wheels are going faster and faster there is nothing to stop the plane going forward and gaining airspeed allowing take off.

However as the plane gains forward speed then the runway reacts to this and the wheels will spin ever faster - would this be logrythmic increase? - with the possibility that the bearings overheat and fail at this point, should it happen, then a real connection will exist between the runway going backwards very quickly by now and the plane going forward with bad consequences.

cheers - chosfel

In reply to Craig_M: Indeed, the thrust in most aircraft is designed to provide a very focused force to move the aircraft through the stationary air - not to move draw all the required airflow over the relevant lifting surfaces (see my post above about the flow potentially going backwards across the wings with the aircraft at zero onflow velocity, e.g. a stationary aricraft)

In reply to Moacs:

Dear god - the government should see this thread and use it of an example of dropping standards in science teaching and critical thinking. Sod this being a recent occurance!

In reply to James Jackson: And your answer to this is?
Tree

In reply to Moacs: i realise its been said but it wont take off! this is due to airspeed over wings!

if it could work then whats the point of runways?? every airport would have these imaginary treadmill things.

Al

In reply to Moacs:

Is it worth splitting this into two parts: -

a) If a plane is stationary wrt to the surrounding atmosphere, can there be any airflow over its wings and therefore any upward thrust? Answer: 100% no.

b) Does the mechanism of spinning the treadmill backwards as the wheels start to move forward satisfy keeping the plane stationary wrt to the surrounding atmosphere? Answer: I guess it depends on what assumptions you make about how the plane's wheels work.

Again in the spirit of the thought experiment, if we assume that the mechanism can and does work, then no take off. If the mechanism cannot work for either theoretical or practical reasons, then the plane is not stationary wrt to the surrounding atmosphere and can take off. However this seems rather contrary to the spirit of the OP.

In reply to Craig_M:

Well Craig, not sure how to respond to that.

If it's just a random insult from someone that's never met me, then fair enough, I understand the way you work.

If you think that posting this conceptual brainteaser and having a rationale for the plane taking off makes me dim, then I think you're wrong.

Cheers

J

In reply to Tree:

Of course it's not bloody going to take off - those saying (and the reasons they give, taking the abstract nature of the problem into account, are correct).

In reply to Moacs:

No, I think that thinking the plane will take off makes you dim. I don't need to have met you to reach that conclusion.

In reply to brothersoulshine:

For the aircraft to move forwards the wheels need to rotate (ignoring any potential to slip) relative to the ground. If the treadmill operates so as to oppose any rotation of the wheels (ingonring any lags and mechanical limitations) then it does not matter how fast they spin, there is no net difference between these velocities, and the aircraft's lifting surfaces have no onflow, so no lift is created.

In reply to Liam M:
But will the jet engines move the plane forward in relation to the air, regardless of the greater speed of the turning wheels?

If so it will take off.

In reply to TRNovice:

Definitely agree with your 2 part analysis.

I think we all agree that planes work by air moving over the wings and essentially reducing pressure above them to create lift.

So the question becomes "can the plane move forward against the flow of the treadmill?" - do the wheels make a difference?

If they are friction-free bearings, then it will make no difference what speed the treadmill turns, or indeed in what direction.

If the bearings have friction the quesiton is whether the force created on the axle through the friction in the bearings as the wheel turns is great enough to counter the thrust of the engines.

The spirit of the question assumes free-turning wheels.

Therefore I think the thrust will be enough to move the plane forward...and take off.

John

In reply to Moacs:

But you can't use an argument along the lines of "if the plane was in flight at landing-gear height then you could just lift the wheels and it would be the same as what I am arguing". The fact is that to be flying ***in the first place*** it must be moving relative to the surrounding air to generate lift by pressure difference in the airflow over the top and bottom of the wing. But while its weight is on the treadmill at first (i.e. at rest), then moving the treadmill backwards to counter any forward motion that the engines try to provide will mean that the plane hasn't moved (because the fact that the weight is on the treadmill means that the plane will want to move with it - as it would do if you turned the treadmill on with the engines still off).

Think of it this way, if you are on the treadmill in the gym then are you not trying to thrust forward (c.f the engines), but not moving anywhere?

So... it is still at rest relative to the (assumed still) air and hence has no lift. This is self-sustaining.

If just having the engines to give thrust running provided lift then why not just slam on the anchors and run the engines until the plane takes of on the spot?

Andy

In reply to brothersoulshine:

I agree - better put than I managed.

J

In reply to Moacs:

Oh for f*ck's sake, read up on frames of reference, as Kenny said way up there ^

In reply to Anonymous:


But a gym treadmill relies on me pushing against the ground (treadmill) not the air. I agree a car in htis situation wouldn't move, but it'slike having the front wheels of a rear-drive car on a treadmill and the rear wheels on the road. The car will go forward because it's pushing on something that's not counteracting it.

John

In reply to Moacs:

I’m stunned.

The wheels have everything to do with it, they don’t just hold the plane off the floor. They transfer the thrust to the runway so that the plane can move. Yes, the jet engines generate the thrust but if the plane had no wheels it wouldn’t go anywhere. Unless of course you’re assuming zero friction in which case the whole treadmill aspect is a complete waste of time. What we’d then be asking is, assuming zero friction, will a plane take off on a treadmill? Which is clearly a yes. But we’re not asking that.

There will be no relative wind over the wing, there will be no takeoff.

That is my final answer.

In reply to James Jackson:

Could you summarise the "frames of reference" input?

Ok. Try again:

Imaging the plane has an enormous broom up it's backside, wedged against something immobile behind it. You can run the treadmill as fast as you like and the plane stays still, pushing back on its brrom.

Now push the broom forwards - plane moves forwards?

Now push against the air, not the broom.

J

In reply to Moacs:

No need to. Eric, in the post above your last one, has explained it (again).

In reply to Eric the Red:

I think we agree - if there are free-wheels then the treadmill is irrelevant.

And in a friction-free envirnment, where you have agreed it will take off, how will hte plane transfer thrust to the ground through the wheels?

John

In reply to Moacs: I don't think the plane would remain stationary. The engines act against the air. Sure, the treadmill will start to move as the wheels move, but the plane will still be moving in a forward direction through the air. If it does take off, the wheels will be spinning much faster than they normally would because the treadmill is acting against them. However, the plane would probably take off before they exploded.

I will say yes, the plane would take off.

In reply to Moacs:

65 messages in 1hr and 15 minutes...and we got almost 3/4 of the way through them before people got abusive.

'sfunny. I'm off to eat goat.

J

In reply to Moacs: But in the second case you consider the broom doesn't move forward. As you push forwards, you increase the speed of rotation of the wheels (initial speed + extra rotational speed from you pushing forwards), and the treadmill detects this increase in rotational velocity of the wheels and speeds up accordingly, effectively pushing you backwards. So whoever is pushing the broom justs ends up running on the spot.

In reply to Moacs:

Not sure how pertinent this is to the question, but are a plane's wheels wholly free turning? I.e. even when taxiing is it always the engines that are providing the motive force? Not trying to argue one way or another, just ignorant of the engineering.

For example if they had motors (used in taxiing say) then these could presumably help to start the plane moving and then be progressively geared-down then disengaged. I suspect that this is not actually the case, just wondering if anyone else knows.

Think of it like this. Put a skate bourd on a running machine with the front attached to a piece of elastic. As you turn up the speed does the elastic stretch?

If the wheels are fricionless it won't, (the plane would take off)

If the wheels aren't frictionless it will unless you pull the elastic to keep the board in the same place. (the plane won't take off)

Cheers

Toby

In reply to TRNovice:

Plane wheels are driven - but I think this is to pre-spin them for landing, I don't think it's used to taxi the plane.

In reply to Moacs:

Just logged on to this thread and haven't read it all but I'm in agreement with you John. The plane is driven forward by the thrust of the engines, not by the turning of the wheels. The wheels are only there to allow the plane to move forward under that thrust. They are a friction-reducing device. Introducing another such device such as a treadmill (although I prefer the mental picture of a free-wheeling conveyor belt) will not stop the forward movement of the plane.

Got any more?

Simon

In reply to Toby_W:

Agree. So depends on the friction in the wheels?

J

In reply to Eric the Red:

This fact needs repeating:

The wheels transfer the thrust to the runway.

If the treadmill moves at same speed as wheels, then the wheels can't move, and the plane goes nowhere. The proportional treadmill idea is like putting an infinitely powerful brake on the wheels.

In reply to TRNovice: I think for weight saving purposes all the wheels are free running (unpowered) and use the jets and control surfaces on the ground. The front wheels may be steerable but that's about it.

Hence the use of tow trucks for moving them on the apron where they may be too powerful to accurately position themselves.

In reply to Jon Dittman:

If the aeroplane moves, then the rules of the game have been broken. i.e. "that, as soon as the wheel starts to turn, the treadmill exactly matches the turning".

It's fairly intuative if you think about it. For the aeroplane to have moved on the treadmill, the wheels would have to have moved rotated a greater distance than the treadmill (that much is self-evident). If that has happened, the rules of the game, as stated, have been broken.

Therefore, the aircraft can't move, and thus can't take off.

Mark

In reply to Moacs: the thrust of the aircraft is not mechanicly linked to the wheels, therefore the wheels do not affect the aircrafts behaviour in any tractive way. Therefore assuming perfectly frictionless bearings on the wheels the body of the aircraft would move forwards in accordence with newtons third(?) law, the aircraft would reach take off speed in exactly the same time as normal, the wheels however would be spinning incredibly fast (in fact I have an incling they would be spinning infinitly fast!)

so as a thought experiment yes in the real world no.

In reply to Alun:

So as the spin speed increases the friction increases? I get you.

So the disagreement is actually between folk who imagine a frictionless system and those who imagine one with friction.

Anyway...I can't stay and fuel disagreemnts all day. Got to fly!

J

In reply to Ian Straton: That is my kind of thinking and I think you put it much more elegantly than me. I like your use of Newtons 3rd law. No idea what it is, but it sounds good!

In reply to Alun:

You can buy the answer for two bucks here: http://answers.google.com/answers/threadview?id=428718

In reply to Ian Straton:



In that case you believe that the aircraft has moved forward on the treadmill. If so, you must accept that the wheels have covered a greater distance than the treadmill.

How then do you reconcile this with the statement that: "as soon as the wheel starts to turn, the treadmill exactly matches the turning"

Mark

In reply to Moacs:
"So the disagreement is actually between folk who imagine a frictionless system and those who imagine one with friction."

I believe so.

Frictionless : takeoff as normal

Friction: carnage

I think I know why people are getting confused.

Firstly, we shall assume friction exists. In fact we're not assuming. We KNOW it exists. Without friction the treadmill is irrelevant, the question is irrelevant.

Secondly, the argument for the plane taking off seems to be that the jet engines "push against air" and so the wheel speed doesn't make a difference. This is wrong.

Whilst the plane is in contact with the ground, the wheel speed is directly linked to the velocity of the plane. If the treadmill is counteracting the spin of the wheels then the plane will not move forward.

To put it another way. Imagine a wheel on a treadmill. The wheel is being driven at 100mph. The treadmill is spinning backwards at 100mph. An observer on the stationary ground sees a stationary wheel. Now, attach a jet plane to the wheel, the wheel is still being driven at 100mph only now it has a jet driving it. The wheel is attached to the plane, the plane can only travel as fast as the wheel. Now an outside observer sees a stationary plane on a treadmill. If that stationary observer stands in front of the plane and is, let's say a bit of air, the plane won't get any closer. Now say the bit of air has lots of friends, the plane doesn't get any closer to any of them. There is no flow of air over the wing because the plane isn't moving relative to the air. No lift, no take off.

In reply to Moacs:

Is this a blindingly obvious and easy question, or a very sophisticated and intelligent troll? Answers on a postcard...

This is nearly as good as that old left and right handed suger molecule one. Did you know if you eat only left (right) handed ones you won't put on any weight?

Cheers

Toby

In reply to Patrick Ruane:

Or read the title and framing of the OP..

In reply to Eric the Red: As i mentioned before, if it could take off, then why do airports have runways???they would just have these "theoretical" Treadmills!!

Also when a plane takes off, its nose lifts for a few seconds and then slowly takes off, so how would this work on a treadmill, would it slowly raise, the plane still stationary, and then ,,,whooosssssssshhhhhhh, instantly hitting about 300 mph as it takes off??

i dont thinks so,no aor moving over wings, no take off

A;

In reply to Moacs:

The question of what is happening to the wheels and the converor belt is a tricky one but has nothing to do with whether the plane leaves the ground. For example one contributor has suggested that the wheels transfer the thrust of the engine to the runway. If this were so, how would the plane keep flying when it left the ground?

I think there is a deliberate? mistake in the original question.

Consider if the wheel brakes were left on and the wheels couldn't turn. The conveyor belt would have to move forward on its mathematically frictionless bearings. As the brakes are gradually released, the wheels start to turn and the conveyor belt slows its forward motion as there is friction between the belt and the tyres. So the original question which supposes that the conveyor belt rotates backwards is a red herring.


Simon

In reply to Moacs:

In fact to quote me...

"The wheel is attached to the plane, the plane can only move as fast as the wheel."

The question states that the treadmill counteracts any movement of the wheel so the wheel is stationary. So the plane is stationary. No lift. No take off.

I really am shocked at people's lack of understanding. Introducing Newtons 3rd law is irrelevant. In fact, the last time I saw science so blatantly misused on the internet it was on the answersingenesis.org website.

I despair.

In reply to Moacs: If the system was frictionless the engines wouldn't work either so the plane still wouldn't take off.

In reply to Moacs:

err definately no as you need to have the windspeed across the wings to acheive liftoff,,,,

In reply to Simon White:

The plane in the air flies because the thrust from the plane no longer needs to overcome the friction generated by contact with the ground.

When it is in contact with the ground it must overcome that friction with a special device. The device is called a wheel.

Flight only occurs above a certain velocity, below this the plane is merely a glorified car.

In reply to gingerdave13: If we forget engines for a minute and imagine that this is a model plane. Presumibly given the original set of circumstances you would not be able to push it forward. Would would be able to lean your whole weight against it and it wouldn't move. In fact, you would attach it to a space rocket and it still wouldn't budge??

No????

In reply to Anonymous: well my argument requires frictionless bearings capable of spinning infinitly fast, if they can manage that then we basicly can ignore the physics of the treadmill and consider the effect of moving several thousand pounds of air through the engines as an entirely seprate frame of reference from the wheels on the treadmill.

If however you introduce any element of friction or a requirement that things cannot spin infinatly fast then you are spot on, the aircrafts lifting surfaces will remain stationary wrt the surrounding atmosphere and all the movement of air through the engine simply goes into frictional losses at the wheel bearings.

to explain by analogy: if you put a ball on an infinatly long frictionless slope in an environment with no atmosphere and release it it will not roll down the slope, it will slide. An observer on the ball would percieve no motion at all (this is our aircraft sitting on it's wheels) and is free to move however he wishes. An observer at the bottom of the slope would see the ball coming towards him infinatly fast (this would be the surface of the wheels on our aircraft). Our two observers are in different frames of reference and observe different results from the same process.

In reply to Eric the Red:

But you can't get over the point that the original question is wrong. There is no force that would rotate the conveyor belt backwards against the movement of the plane, unless, like a car, the wheels were driven by the engine and had to use their rotation to overcome the momentum of the plane. The belt would move forward, not backward, at a rate governed by the amount of friction between the belt and the tyres.

Simon

In reply to Moacs:

What if we mount the treadmill on a carpet of ball bearings?

In reply to Eric the Red: If it makes you feel better you are absolutly correct, in our physical universe the aircraft will not take of. However as a thought experiment it is much more fun to allow elimination of friction!

In reply to Eric the Red:


At least we agree on that, if for apparently contrary reasons!

Simon

In reply to Simon White:

Why? If the wheels aren't turning, the treadmill will move backwards at the velocity that the plane is sliding forward, assuming of course that the jets can even make it move.

In reply to Simon White:

They didn't suggest it, they stated it because that's the laws of physics!


Because once it is moving fast enough the flow of air over the wings generates lift!

Here's another analogy. You put a small, four wheeled tea trolley on a treadmill in a gym. You set up the treadmill so that it always moves as fast as the wheels of the trolley. Now you can come along and push or pull that trolly as hard as you want, but you'll never be able to push it off the treadmill because the instant you give it any momentum there will be an equal momentum (the movement treadmill) pushing it in the opposite direction.

In reply to Moacs: I changed my mind! If an helicopter in stationary(sp?) flight drops a missile, the missile will starts to move forward as soon as its engine kicks in. I suppose the same principle applies with the plane: it will still move forward as its own engines who make him forward , not the energy applied on the ground.

In reply to sebastien: The wheels will just turn double of the speed.

In reply to Moacs: A wordy article on this. http://www.avweb.com/news/columns/191034-1.html

In reply to sebastien: And the runway would not be any longer.

In reply to Eric the Red:

Sorry can't explain any better than this: Imagine putting your hand on a treadmill (one which has frictionless bearings but a sticky surface, as proposed in the brainteaser), and pushing forward. Your hand is imitating the plane being thrust forward by its engine but with the wheel brakes on. You will rotate the treadmill in a forward direction.

If that doesn't convince you I can only suggest trying it with, say, your hand on a table mat on a polished table. The mat will slide forward as you push.

Simon

In reply to Alun:

Sorry but I think lift is what keeps it up, not what keeps it moving forward. So I stand by my question: if there is no "transfer of thrust to the runway" once the plane is off the ground, how does it keep moving forward?

Simon

In reply to Simon White: Exactly, put a plane on skies or water, it will still take off

In reply to sebastien:
Yes

The fact that the plane wheels are on a counter rotating treadmill won't stop the plane moving forward under its own thrust, gaining lift and taking off.

Th feedback loop mentioned in the question would not keep up, it is bound to have some feedback delay; otherwise the treadmill and wheels would instantly accelerate to infinity (which would be daft).

The treadmill and wheels would spin faster and faster until either one or the other explodes under its own centrifical force, hopefully by which time the plane would be airborne

In reply to Jon Dittman:

What's wrong with all you people? The set-up is:

"Imagine a commercial jet plane standing on the runway. Except the runway is an enormous treadmill. A sensor on the plane's wheel feeds-back to the runway/treadmill so that, as soon as the wheel starts to turn, the treadmill exactly matches the turning, but in reverse. Will the plane take off?"
Let's recap: the plane's engines are pushing against the air, not the treadmill, and thus the plane will experience a forward thrust, and so will accelerate forwards in the absence of any counteracting forces.

Those forces can only come from two things - friction (let's neglect that for the moment, as it's fun to) and the force required to accelerate the wheels. IFF the lag time between the wheels starting to turn and the treadmill accelerating to match is genuinely zero (which is unphysical; it will be ultimately limited to the speed of light at the most) then the engine forces can be balanced by accelerating the wheels. Introducing friction will heighten this effect, particularly after the first few seconds as the wheels approach light-speed themselves.

If the wheels have a theoretical diameter of zero, though, then they will have no angular momentum regardless of speed and the plane will take off (assuming no friction).

In reply to Ian Straton:

The friction of the bearings has nothing to do with it because the axel in the bearings can act with a force in the opposite direction to the thrust of the aircraft even in a frictionless bearing. Friction is only required between the treadmill and the wheels as without this the wheels would slide and the sensor would record a zero wheel velocity and hence the treadmill would not move. In this case the plane would be sliding relative to the air and the treadmill and would generate forward speed through the air and lift off.

The plane does not have frictionless tyres and therefore would not take off.

In reply to bernard:

It doesn't consider the same question. In the example used in the article:

"[the plane moves forward] at about 10 mph. At that moment the conveyor is moving at 10 mph [in the opposite direction] and the tires are whirling around at 20 mph because the prop has pulled it to an airspeed, and groundspeed, of 10 mph"

This is wrong as in the question stated by the OP, the conveyer moves at the same speed as the wheels are turning. The example above clearly states that the conveyer is moving at 10mph, same speed as the plane.

If the conveyer moves at the same speed as the plane in teh opposite direction, the plane will take off fine, though the wheels will be rotating double fast.

But if the conveyer moves at the same speed as the rotational speed of the wheels, the plane will never move.

In reply to Alun:


The plane is pushing against the air. How does the conveyer, however fast it's moving, transfer force to the plane to counteract that force and thus prevent the plane from accelerating?

In reply to sebastien:

Phew some cracking answers and some real fuzzy thinking on this thread.

Of course the plane will take off, if it was on water or ice it would move forward due to thrust and overconm the slight resistance due to friction.

Think of the plane on a treadmill the length of a runway. Now apply the brakes to the wheels and apply thrust and the plane will drive the treadmill forward till it reaches takeoff speed and then rise into the air, with its wheels not having rotated at all.

Planes fly by thrust to achieve a speed that will give them lift. If they relied on the wheels for anything as soon as they took off they would land again.

I hope no aeronautical engineers got this wrong. If so better look up their text books.

In reply to sebastien:

Except that I've just gone back and re-read the OP. Always a good idea!

The OP actually suggests that the runway conveyor is driven backwards by an independent power source. If this is so, and it is driven to carry the plane backwards at exactly the same speed as the engines try to thrust it forward, then of course the plane remains stationary and cannot take off.

So the answer depends on whether the runway is driven backwards, in which case the answer is "no" or allowed to rotate freely according to the friction with the plane and a set of frictionless bearings, in which case the answer is "yes".

I was answering the latter question in my posts. Apologies to any who thought I was arguing against the former.

Simon

In reply to Simon White:

your answer would only be correct if the plane was standing on props, not on wheels

In reply to sutty:

Exactly what I've been saying Sutty, but look again at the OP. The treadmill independently driven backwards is like a tailwind.

Tailwind equal to forward engine thrust = zero net airspeed = no lift.

Simon

In reply to Simon White:

but the force of a tailwind acts directly on the aircraft

the aircraft is isolated from the treadmill by its wheel bearings

In reply to Simon White:


Nonsense - regardless of whether or not the conveyer is driven, it must still, somehow, transfer a force to the plane. As I said, that can only be friction, or the force required to accelerate the wheels, and unless you're accelerating the wheels very very fast indeed, you're not going to be able to counteract the force generated by a jet engine.

If we assume a conveyer capable of infinite acceleration, and a jet engine capable of infinite thrust (this is a thought experiment after all) then a paradox is generated. In any remotely realistic situation, the conveyer belt cannot apply enough force to the plane to prevent it from accelerating.

In reply to Moacs:

The wheels provide rolling support on the runway whilst the wings are pushed forward through the air. If you compensate for the rolling support, no matter how much air you push behind you, you cannot achieve take off velocity, because you are not moving forward. The air behind you would be red hot and possibly cause the tailgate and rolling runway to ignite, causing the aeroplane to burst in to flames.

In reply to Simon White:

A perfect summary, I agree completely - this is the confusing crux of the question!

In reply to kevin stephens:

No. Imagine a plane taking off from an aircraft carrier. If the carrier is moving backwards then the plane will need to overcome the backward movement before generating its own forward movement against the air.

Of course if the wheel bearings on the plane were perfect and frictionless and the brakes were off, it would have slid off the carrier ages ago, but the principle holds good.

In reply to Alun:

Thank you Alun. How do we convince the non-believers?

Or is that a question for another Friday afternoon!

Simon

In reply to Moacs:

thanks James Jackson (and vicariously Eric the Red) for acknowledging my 'frame of reference' reference.

It's not rocket science.

Hang on...

In reply to Simon White:

wrong analagy, the carrier would already be moving backwards relative to the air before the plane starts, The carrier's rear ACCELERATION would not be equal and opposite to the plane's ACCELERAION

In reply to Richard:

To all those who still think this will make the plane move:

What do you think happens when the plane's engines are at full power, but the brakes are on the wheels?

Does the plane take off? Yes but only when the thrust is enough to overcome the friction between the tyres and the runway - until that moment the brakes stop the wheels rolling and thus the plane does not move.

The point I'm trying to make is that the wheels on the ground do have an affect on the speed of the plane!

In reply to Alun:


Exactly, with brakes on the treadmill imparts momentum to the plane

But during take off the brakes are NOT on!!!!

In reply to Simon White: But there is nothing that prevent the plane moving forward.
If your bike is on a treadmill, nothing prvents you to move it forward holding it by the seat. Now swap your hand for two rockets, it will push the bike forward.

In reply to kevin stephens:

According to the original post, that's exactly what is being proposed:

"A sensor on the plane's wheel feeds-back to the runway/treadmill so that, as soon as the wheel starts to turn, the treadmill exactly matches the turning, but in reverse"

In reply to Simon White: Please read above: frictionless bearings can still exert a force between the axel and the wheel perpendicular to the frictionless surface. The wheels will therefore turn as long as their is friction between the treadmill and the tyre. If the tyre moves, the treadmill will act in the opposite direction to counter act the movement. The tyre will spin but the axel will hence the plane will remain stationary compared to the surroundings. There will therefore be no airflow over the wing and the plane won't take off.

In reply to Simon White:

idiot, if the aircraft carrier is moving backwards then the aircraft has no reverse movement to play against has it? it stil moves forward at the same speed as if the carrier would move forward at. the speed the aircraft takes off is vastly larger than the speed of an aircraft carrier so the carrier going rearwards won't have diddly squat effect on the a/c taking off

In reply to Simon White:
http://en.wikipedia.org/wiki/Aircraft_carrier#Flight_deck_configuration
explains in the first few lines how aircraft carriers move forward to create apparent speed. So as you suggest moving backwards would make take off impossible, as would a conveyor belt.

In reply to sebastien: But you push harder, if I increase the speed of the treadmill it will push your arm back until eqm is reached (stationary bike) or your arm is forced backwards.

Cheers

Toby

In reply to centurion05:

Let's not use expressions like idiot, please.

Waiting for a retraction.

In reply to Simon White:

but (see my earlier post), instantaneous feedback is not just a simplification, but a physical impossibility

If it were possible the whhels and treadmill would instantly accelerate until the centrifical force overcame binding of the molecules in the metal, wheels would explode

In reply to Simon White:

In reply to Richard: I sort of agree with you and sort of don't. The point the OP infered IMO was that if the engines were used to propel the plane 10m forward, the treadmill would move the plane 10m backwards. As you stated it depends if we are talking about friction or not. If we are in the surreal world of 0 friction, then yes, the plane would take off and the wheel would probably be spinning at infinity RPM.

However back in the real world, the plane would remain stationary because the friction in the wheels would prevent it from moving forward.

The answer is therefore yes or no depending on whether friction is part of the equation.

In reply to kevin stephens:

Kevin, please, this is theoretical puzzle, if I wanted to be unrealistic I could quite easily say "well you never get a treadmill big enough for the new Airbus, so the whole problem is nonsense"

In reply to Toby_W: No, it won t push my bike bakwards because the wheels are free to accelerate. You can stand beside the working treadmill and hold your bike still. If the treadmill accelerate, you can still hold your bike still or move it forward: the wheels are independant of the frame.

In reply to centurion05:


Err, an aircraft will take off at somewhere in the region of 110kts, an aircraft carrier will steam at about 30kts. That isn't a huge difference. In fact if you suppose a 20kt headwind you've nearly halved the relative speed that the aircraft needs to take off with.

Mark

In reply to Simon White:

I'm really glad you cleared that up. I didn't realise we were answering two different questions! You're arguments now make more sense now, but then again I read the question so I'm right

We now agree that if the treadmill is powered and opposes the motion of the wheels then it won't take off. Good.

Now, an unpowered treadmill...

In reply to sebastien: put the bike on elastic, increase the treadmill speed.

The elastic stretches.

Increase the speed of the treadmill enough you'll snap it.

Why.

There is a certain amount of friction in the wheels.

Try it.

Cheers

Toby

In reply to Toby_W: Come on, the friction thingy is a practical argument. The OP is purely theoretical...

In reply to kevin stephens:

I think the instantaneous feedback hypothesis is taken as read in the world of mathematical brainteasers such as this.

I'm having another rethink and in the mathematically perfect world (as opposed to the commercial airliner in the OP), maybe the whole rotating runway is another red herring. If the wheel bearings on the plane are "perfect" then in fact whatever the (independently driven) runway does is "ignored" by the plane. A bit like trying to shake your fried onions to and fro in a non-stick pan. As the pan moves back and forth, the onions stay still relative to, say, the hob. So if the wheel bearings are perfect, the movement of the runway can never change the momentum of the plane, because the wheels simply turn to keep the plane stationery.

Therefore the plane, which is powered forward by jet engines, can generate forward movement with impunity.

In reply to Jon Dittman:

Actually, as I also said, the answer can be "no" even in the frictionless set-up.

A spinning wheel has angular momentum; thus it requires force to accelerate the wheels. If the wheels can be accelerated at a sufficiently high rate, enough force can be applied to counteract the engines.

However, there's still a singularity at the start of the experiment: the wheels cannot turn until the (initially stationary) plane moves wrt the treadmill, so to start the wheels turning (and hence the treadmill moving), the plane must move at some point...

In reply to Eric the Red:

See my new cat amongst the pigeons...

In reply to sebastien: Theoretical question but with friction. Check the OP.

Powered treadmill.
Normal plane.

Perfect feedback (although you wouldn't need this to keep the plane on the ground so long as it was fast enough to keep it devoid of speed (lift) to take off.)

Cheers

Toby

In reply to Richard: I am sure you are right, but at the end of the day, this is all bullshit and such a system could never be created. Also, why would you put a plane on a treadmill? I mean like durrrrrrr!

In reply to Jon Dittman:

Actually, I don't know what I'm arguing anymore.

The friction/non-friction thing is a red herring compared to the basic physics. In the thought experiment you could set it up so that the plane does not take off (because of the angular momentum of the wheels), but you'd have interesting effects that I'm not even going to try guessing at as bits of the system rapidly approached lightspeed.

In reply to Richard: I'll agree with the idea that the aircraft must start rotating forwards to allow the treadmill to operate. However, as the lag time tends to zero, then the forward motion of the aircraft also tends to zero.

And I think people need to be more explicit in what they are meaning when they say frictionless. My assumptions from the OP where that the friction in the wheel bearings tended towards zero, and that the friction between the wheel surface and the ground tended towards infinity (zero slip).

I think most of the debate here is due to people answering different questions as the OP is quite ill defined (probably intentionally so)

In reply to Jon Dittman:

it might need exercise ever heard of rehabilitation for when it is not quite up to flying.
I guess you could build it for a model plane

In reply to Liam M:


It's the best way.

My little sister came home from school just after she'd discovered algebra and asked, "x + y = 15. What's y?"

In reply to Moacs:

Right, I just went into the carpark and did this experiment.

The final answer is that the plane doesn't fly.

Then, however, I also attached wheels to the treadmill and towed it down the M25. At 130mph I experienced some lift. My car wouldn't go any faster.

In reply to Richard:

10.

Prove me wrong.


Cheers

Toby

In reply to Eric the Red: Did you try driving down a hill?

In reply to Toby_W: Good try but we all know it is 42

In reply to Guy:



Cheers

Toby

In reply to Richard:

y = 15 - x

Am I really so sad that I couldn't resist posting? I mean, what's happened to me? I had such high hopes of not being a geek when I left school. My whole life was ahead of me, a blank canvas on which I could paint life's colours...

In reply to Patrick Ruane:

When in fact the paints dried out and the brush has lost it's bristles.

Cheer up.

Cheers

Toby

In reply to Eric the Red:

Now I know your lying Eric, it is impossible for anything to move faster than 35mph on the M25.

In reply to Alun:

Oooh no, my sister phoned from the M25 the other night saying she had reached 80mph on it. It was at 9pm though.