UKC

A freind emailed me this, just wondered what you guys thought?

A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction). Can the plane take off?

What do you think? I think it would, but not telling why

In reply to pale ale: No, because it's airspeed thats important, not ground speed. That's why planes take off into the wind. I would have thought this was fairly obvious.

In reply to nick nc:

And where does a plane get it's airspeed from?

In reply to pale ale:

Yes, of course it will - the wheels can be spinning backwards for all the plane cares!

In reply to pale ale:
This was done to death several months ago, resulting in a thread several hundred responses long. The correct answer is this:

The plane will NOT take off, because the question states that the speed of the runway will be exactly the same as the speed of the plane (presumably, the rotational speed of the wheels). Thus, no matter how much thrust is applied, the wheels cannot move forward - it is as if they have a brake of infinate power applied.

HOWEVER, many people don't understand the question. If the runway is merely 'on rollers' i.e. not powered, then the plane will take off with the wheels moving at twice the speed the would usually; because the thrust is coming from the air, not the ground.

That's all there is to it. Anybody disagrees, you're wrong!

In reply to pale ale: we've done this before!

Unless the plane can move forward it can't take off since the conveyor prevents the plane moving forward therefore it can't fly.

That is all.



(unless the engines are cabable of moving the entire atmosphere backwards at 150mph....)

In reply to Matthew B:

The plane pushes against the air, not the conveyer belt, so as long as there is no friction of course the plane will take off. If there is friction and the conveyor belt is capable of rapidly accelerating to near-infinite speeds, and the bearings of the wheels are capable of taking such speeds, it's possible to conceive of there being supfficient frictional drag to hold the plane back.

In any realistic, physically limited interpretation, the plane will take off.

In reply to pale ale:

>Can the plane take off?
No, of course not. Planes take off as a result of lift generated by the movement of air above and below the wings. As you've described it, the plane is not moving through the air, so there is no air flowing over the wings and so no lift is generated and the plane stays on the conyeor belt going nowhere.

In reply to pale ale:
Been done to death......

http://www.ukclimbing.com/forums/t.php?t=162217&v=1#2310749

IIRC it all depends if you do it as a perfect thought experiment, or if you allow nasty things like friction to get in the way.

In reply to pale ale:

No, it won't take off. You need air flow over the wings. The plane is to all intents and purposes stationary, so there is no air flow to create lift.

In reply to Matthew B:

That is wrong. If the wheels have infinate high friction with the surface, and there is an infinately high brake applied, the plane cannot move. Despite the fact that forward momemtum is applied by thrust, if the plane is anchored to the ground, it will not move, and so won't take off.

In reply to pale ale: no it wont its the air rushing over the wings that produces the pressure change around the wings that gives the plane its lift if its on a conveyor its not moving through the 'air' so no pressure change occurs

In reply to Richard:

The perfect engineers answer! (and I mean that as a compliment!)

In reply to Alun:

Thanks! Of course that's what it's all about, isn't it? Someone actually wants to know what would happen in the real world...?"

In reply to Alun:

Where did infinate[sic] high friction come from?!

Nothing in the original question indicated the plane is anchored anywhere.

In reply to tatty112:

So if you push a toy car at 5 mph along a conveyor belt going in the opposite direction at 5 mph, your saying the car would not be moving? I think it would
After all, it's not the wheels that are pushing the toy car, its your hand, and your hand doesn't know about, let alone care, about the conveyor belt. Same for the plane, the engines pushing the plane forward aren't really that bothered in what the wheels are doing.

In reply to Alun:

Agreed, with both responses. I used to think otherwise so I can't mock anyone that thinks it won't.

In reply to Richard:

Well in the real world you wouldn't stick your flight to Malaga on the conveyor belt in the first place

In reply to Matthew B:
Sigh. It's supposed to be a thought experiment - or do you think that building a conveyer belt with an infinately fast feedback loop so that it can travel at the same speed as the plane is realistic? :P

Richard's answer explains very well in 'realism' terms.

In reply to pale ale:

nope. not unless the air is glued to the conveyr belt and therefor moves aswell.

In reply to Kenny:

lol. Sod off Kenny.

In reply to mik:

Inspired thinking!

In reply to mik:

air glue - my god this would help my climbing

In reply to Alun:

It's not realistic, but it *is* given in the question... on the other hand, even the likes of Easyjet manage to lubricate their wheel bearings occassionally

I still reckon it takes off.

In reply to pale ale: If you push a car along at 5mph and the conveyor belt is going the other way at 5mph, although the car wheels are moving, the car is STATIONARY! Any stationary object cannot have an airflow (have you felt any breeze through your hair on a treadmill? no? perhaps that's why they have fans in gyms).
Wings need an airflow to create lift. No lift = brick. To get this to work the conveyor belt would need to be in a wind tunnel.

Oops - time to get off my soapbox. I blame the heat. or the government. Or the young people of today.

In reply to mik:

Now you mention it, air does have a viscosity and therefore momentum will be transferred to the air, creating a flow over the aircraft.

So it may take off vertically after all...

In reply to pale ale:

Does the plane have one of those cords to attach to it's waistband like treadmills do for runners?

In reply to pale ale:

what is providing the lift for it to take off?

is it air rushing forward to replace the air sucked in by the engines, thus generating airspeed and differential flow over the upper and lower wing surfaces ?

In reply to pale ale:

In the interests of maintaining the standards of pedantry on these fora, please note that it's an aeroplane, not an airplane..

In reply to Twig:

<pedant>
If you're pushing a car at 5 mph, the car is moving at 5 mph. It makes no sense to attempt to define 5 mph as stationary...

Think in terms of forces and thus accelerations, not speeds.
</pedant>

In reply to Richard:

What are you talking about? It won't take off.

The flight of a airplane is caused by a pressure difference caused by differing air speeds over the aerofoil that are the wings, causing the plane to be pulled of the ground.

The key phrase here is "air speeds", the air needs to be moving over the wings.

In the case stated by the OP, the speed of the airplance relative to the air is zero, no air flow over the wing, no pressure difference, no flight.

In reply to Ridge:


... and the correct abbreviation is thus 'plane, not plane.

In reply to Twig:

I warn you, do not be so certain or superior. It may come back to haunt you. It WILL take off.

If the car was being driven by it's engine then no it wouldn't move but it isn't. It's a toy car and you're pushing it. Are you saying that if you hold a toy car on a moving treadmill that it will go backwards? If you begin to then move the car at the speed of the treadmill will it stay still?

I don't know why I'm getting involved, again.

In reply to Twig:

In both the examples you've given (car, running) the forward motion is achieved through friction with the ground, and therefore not the same thing at all.

In reply to galpinos:


Aie! Think about the forces involved.

The engines apply force to the air, not to the track. That is, they drag air past the wings and, by throwing that air backwards, generate thrust.

The only way the conveyer belt can stop the plane from moving is if it applies an equal and opposite force, through the wheels. If there is no friction, this cannot happen regardless of how fast the conveyer belt is moving. It is only if you take friction into account that you can imagine there being any braking force on the plane at all.

In reply to galpinos:

Wrong. Just because there is a conveyor belt spinning the wheels at some crazy speed, does NOT mean that the plane is stationary. Airplane engines provide forward aceleration, which result in airspeed, which results in take off. Again, what the wheels are doing has no effect on this situation. The engines will pull the 'plane forward whether the wheels are going forward, backwards, or have been replaced by a mule train.

In reply to Eric the Red:

Because it's fun wathcing people who are 110% certain that the plane won't take off be proven wrong. When they could just go to google and find the proof explained in various degrees of technicality

In reply to Richard & Smitz:

Good point well made, not really thought this through. I guess the wheels just keep the aeroplne of the ground.

I stand corrected gentlemen.

It's a 'trick' question. When you first think about it the question makes you (well it did to me) forget that there's an engine pushing air past the plane. You initially just concentrate on the wheels.

Bernouilli basically.

In reply to Smitz:

thought the engines is pushing it forward.
But no matter how much thrust the engines gives, the plane is not moving forward other than in comparison to the converybelt. and i think lift is quite ignorant to what goes on below it. It will only care wheter air is moving over/under wint at big enough speed to create the lift needed.

In reply to pale ale:

I've been reading this now going back and forth and i beleive there is only one way to sort it, we'll get a plane............. fly to cuba, drink rum and fight over it on a beach. Skins (or non-beleivers) vs Shirts (or bee leavers) the last swat standing wins.

who's up for it? say I

In reply to pale ale:

OK.

Assume everyone agrees an ordinary plane at rest will not take off.

Now in an ordinary runway, engines are used to accelerate airflow through the engines to create thrust. Thrust, or force, will accelerate the aircraft down the runway (Newton's first) causing airflow over the wings. The Bernoulli's principle then suggests that the wings will produce lift and the aircraft may take off.

Now in this case, we have thrust, a moving runway, and zero velocity relative to the air and the earth. An observer stood in the tower would observe an aircraft at rest. Ergo the aircraft will not take off as there is no airflow over the wings to produce lift.

Put another way, the moving runway is equivalent to the engine tests all aircraft perform prior to takeoff. The engines are powered up while the brakes are applied, and low and behold, with zero airspeed the aircraft has zero lift and will not take off. The brakes and the moving runway are equivalent, and all this talk of friction in the air causing it to move is irrelevant. It does not take off with brakes applied and sure as hell will not take off with a rolling runway.

Regards

Goose

In reply to Richard:

Thank you. I will do better in future...

In reply to Matthew B:
sounds like this one has been answered now! the wheels on an aeroplane are not "driven" so do not provide the forward motion (which is just as well really otherwise you'd get the thing in the air and it would suddenly start loosing it's forward momentum, leading to a suprisingly short flight)
'plane is sitting on the belt, belt starts moving backwards with the plane
pilot applies enough throttle to keep the plane steady in relation to the ground (not the belt) air flow over the wings (unless plane is pointing into a 150mph wind) will be insufficient to provide lift.
engines will have to provide enough thrust to counter and action of the belt trying to push the 'plane backwards and generate enough forward velocity against the airflow to generate lift on the wings (previous posts provided a decent enough explaination as to how that works)
in practical terms i imagine that the plane could be coaxed into leaving the ground if it had enough thrust to do the above....
now go away and leave us alone

In reply to SARS: In reply to the pedants regarding forces etc etc: engines in aeroplanes use newton's law to push the body of the plane forwards. Until the body of the plane is moving fast enough, the wind speed over the wings is not enough to create lift.
If the engines could push enough air over the wings then planes could take off vertically without runways.
Jet engines can be sited anywhere to provide the thrust for the aeroplane to get it moving forwards.
If having an aeroplane on a conveyor belt could generate lift, then formula 1 would be testing cars on rolling roads to check wing positioning rather than wind tunnels.

In reply to pale ale:

FAO moderators

Can we finish this tedious gunk please? Or start a separate forum for misled punters.

In reply to Twig: Sorry Mr Goose, you can type quicker than me!

In reply to Mr Goose:

Nicely put.

The explanation i found on google for why it do take up was funny.
Especialy since they seems to belive the Engines create the airflow over the wing.
And also they mention the length of the convery belt as somthing which matters, hahahahaahahahahahahah

In reply to mik:

Why what's stopping it? the engines are pushing forward through stationary air. the conveyor belt is simply a red herring designed to make us think of cars which are propelled forward by friction between the wheels and the ground. A plane (sorry, a 'plane!) only uses it's wheels to decrease the friction between the earth and itself. Whether the wheels are spinning in one direction or the other, they are still only friction removers, they are not providing any forward force to the 'plane. Therefore, the wheel's speed and direction is irrelevant to the forward thrust provided by the engines. Therefore, the plane accelerates forward, and eventually takes off. If you still don't believe me, just go to google

In reply to pale ale:

go read Gooses post, if you cant understand it there u never will.

In reply to EdEd:

I'm up for that ... but isn't Cuba in the wrong direction ? According some of the replies above, there's no way you'll get the plane to take off in an east-west direction considering the rotation of the earth!

HP

In reply to SARS:

So if the engines can drag enough air over the wings to provide enough lift, why do they bother building runways?

In reply to mik:


Well... if I have some big f*ck off prop engine which weighs nothing at the front of my wings, that's exactly what will happen. So it all depends upon whether you're talking about theoretically or realistically...

In reply to pale ale: the point you are missing about aircraft wheels is that although they are not powered the aeroplane is attached to them, and your setup suggests a perfect feedback between the weheels and the runway. That is to say that no matter howfast they spin they will not move forward relative to an observer not on the runway. If the wheels do not move forward relative to our observer then neither can the aeroplane, if the aeroplane is not moving forwards relative to our our observer then it cannot generate lift, without lift it can't fly.


please demonstrate to me how the aeroplane can move forward without it's wheels.

In reply to Mr Goose:

Incorrect assumption. I'm not going to post any more, this is a well known, old, and well answered conundrum, the solution to which can be found on google in varying levels of techno speak.

In reply to mik: Again, maybe my explanations are at fault, but google this for a better explanation with images etc.

In reply to pale ale:
If you still don't believe me, just go to google

and for google i just did, see my previous reply

In reply to pale ale:

I don't know why I'm getting involved again either


Except their not. I'll say it again, for the benefit of those not convinced. In the real world, of course it'll take off. But as a thought experiment (assuming infinate friction in some places, zero in others, and an infinately fast feedback loop between wheels and conveyor), it won't take off, cos it won't move.

See my first post on the thread, and better still, Richard's first post. That's all there is to it.

And I'm not going to look at this thread again. Honest.

In reply to pale ale:

the google explanation is full of crap just like yours.
So your doing as good as them.

In reply to pale ale:

Many of the google answers are written by engineers though, who are incapable of comprehending phrases such as 'infinate friction' and 'zero friction'. :P

In reply to Alun: http://www.straightdope.com/columns/060203.html

So who's right then? I think the question needs to be refined to be more explicit.

Anyone?

In reply to Mr Goose:

That was my first thought, but as someone pointed out, engines don't apply thrust through the wheels. Imagine the 'plane is on an incredibly slippy surface. If you tried to tow it it wouldn't move, as the tow trucks wheels would spin, just like the wheels on this imaginary conveyor belt. Apply thrust via the engines and it would slide forward and eventually get enough speed to take off.

In reply to Ridge:
we have been had.
guess because our minds are not usualy spending time on silly stuff like this.
But wheter pale ale think the engines create the airflow on wings or is fully aware she is just leading os on when we think that THEY want the airplane to stand still compared to anything but the convery belt.

In reply to Ridge: slippery surface isn't moving.

The setup in the op requires friction between the wheels and the road surface (otherwise neither would turn) think of it another way, turn off the aircrafts engines all together, attach a tow rope to the nose of the 'plane then attach the tow rope to a winch in front of the conveyor.

turn on the winch.

does the aeroplane move?

if yes, why?

if no, why would the aircrafts engines have a different effect to the winch?

In reply to mik:

My excuse exactly.
Lets talk about important issues, like nice arses in tight harnesses...

In reply to mik:

Oh definately not, particularly as the engines usually hang underneath the engine
It's a stupid thought experiment, see my next conundrum post here: http://www.ukclimbing.com/forums/t.php?t=192665&new=2800539

In reply to pale ale:

These easiest way to convince yourself of the correct answer, I reckon, is not to try and think about the physics but to look at the real world:

1. Are there any tiny islands or aircraft carriers with conveyer belt runways? No.

2. Do aeroplanes always accelerate down a runway until they reach the correct air speed to take of? Yes!

Therefore we can be pretty sure that it is impossible for a plane to take off from stationary* - the plane on the convey belt will not take off! Case closed.

*VTOL are a special case and are excepted here!

In reply to pale ale:

yea i know.
but its not realy a thought experiment, its tricking people into thinking certain things

In reply to pale ale: What if it's really windy?

In reply to Ian Straton:

I take it we're assuming infinate friction between the wheels and the stationary conveyor, for the aircraft not to move?
Otherwise yes, the winch would pull the aircraft forward off the conveyor.

To all non-believers.

The conveyor does not exert a braking force on the plane.

Why? Because the wheels are not connected to the drive system. They are free. If you lifted the plane in the air you could merrily spin the wheels for all your worth.

Therefore the engine accelerates the plane forwards and thus it takes off.

I'm not sure where you're coming from Alun, by the way.

In reply to all RTFM

or RTFQ

it clearly states "The plane moves in one direction," the only assumption left open is that moves in a forward direction.

In reply to Mr Goose:


The problem here is that you are defining the plane as being at rest relative to the ground (and hence the air) and not explaining how that comes about. It can be done, but requires friction to do so (and also, now that I've thought a bit more about the problem, angular momentum to accelerate the wheels - the consideration of which allows the plane not to take off even without friction). However, this is as thought experiment only, and requires almost infinite accelerations (and hence power), unworldly materials to take the stresses, and an unobtainably perfect feedback loop. So the question is constructed, from the ground up, to give the answer that the plane will not take off. If any of the above are not assumed, though, the plane will take off.

In reply to Ridge: no just normal friction, the point I am making is that despite the fact that the wheels are not powered unless they move forwards the aeroplane will remain stationary relative to everything except the conveyor.

If there is perfect feedback between the conveyor (which is moving) and the wheels (which turn due to contact friction with the conveyor) it doesn't matter where any motive force is applyed to the aircraft, it can't move relative to the airmass around it, therefore it cannot fly.


(the example of the winch would actually result in the winch being pulled towards the aeroplane)

In reply to pale ale:

Basically you, and possibly also Google (I can't be bothered to look), are wrong.

Sorry to put it bluntly, but having explained it in terms of secondary school physics (Newton etc), examples from real life (the engine tests) and confirmed the relative velocity point (with the observer in the tower) if you still can't understand, please leave it.

One final example: a Tristar aircraft - in service with HM RAF. In this example the engines are located on the rear of the aircaft body, in a cluster around the tail. If you accept that the lift comes from the main wings and not the tail wings, then you must surely see that the engines can not possibly draw enough airflow over the wings (given the distance and width of the wings) to create lift and allow take off.

On the concept of the relative velocity of the aircraft to the earth, I have defined my frame of reference (the guy in the tower) so there should be no confusion there. Now you seem to accept that the wing must have velocity relative to the air to produce lift (Secondary school physics classes). However with an assumed stationary airflow in the beginning of the test (no wind), I fail to see how the relative velocity is achieved, unless you believe the engines are moving a sufficient quantity of air. If you believe this, please read the engine test and Tristar examples above and before, and reconsider.

Regards,

Goose
(Ex RAFVR)

In reply to Ian Straton:

But there must be friction in the bearings (or, as I stated, a consideration of angular momentum) to transfer momentum from the conveyer belt to the plane through the wheels. Doesn't anyone do force-balance diagrams anymore?

In reply to Richard:

I amd defining the aircraft as being at rest relative to the tower as that was the original question.

I.e. the speed of the runway and the aircraft are equal and opposite and thus cancel each other out.

So if the aircraft, on a normal runway, would achieve 60mph (relative to the ground) for a given level of thrust, the moving runway moves at 60mph backwards => ergo the relative velocity of the aircraft to the ground is ZERO, and hence the tower observer would see an aircraft at rest.

Replace the belt with brakes and the effect is the same.

In reply to pale ale:
Why would the plane move forward? The moment the plane begins to roll forward the conveyor belt moves in the other direction at the same speed.

Assuming this relationship is instantaneous, the aircraft never has any airflow over (and under) the wing, therefore no lift is created. The aircraft never takes off.

Surely this is simple?

In reply to Richard: excuse me but where did I say there was no friction?

In reply to Mr Goose: i hear you - but the thing is when the plane takes off it doesn't take off vertically it takes off completely normally by accelerating down the 'rollers' exactly like it would on a normal runway

In reply to David Martin:

Oh dear....

In reply to Mr Goose:


But you cannot do that - you must explain where the force with that result comes from.

I'm not saying that you are wrong - I've given several explanations of where they might come from - but it is tautologous to say, "The 'plane is defined as not moving and therefore the plane cannot take off."

In reply to jamiemoss:


tiny islands are served by seaplanes

In reply to Ian Straton:

Sorry, you didn't. Just getting carried away...

Are the people in this thread who believe it takes off actually serious or just having a laugh?

Thats the only thing not clear in my mind.

In reply to rich: how does it move down the rollers if there is perfect feedback between the motion of the wheels and the motion of the conveyor?

In reply to Richard: so are you now accepting mine and goose's explanation that the aircraft cannot take off?

In reply to rich:

It never accelerates. To accelerate requires a change in velocity. The velocity (relative to the tower) is always zero otherwise the speed of the aircraft and belt are not matched as was originally defined.

Opposing forces are the aircraft engines and the motor for the conveyor belt working in opposite and opposing forces as a system.

In reply to Richard: the aircraft is originally at rest, your start the engines, the wheels start to turn in response to this thrust, this movement is countered by an instantaneous feedback from the conveyor, so now the wheels and conveyor are moving but their result forces cancel leaving the aircraft body at rest relative to the observer in the control tower.

In reply to Ian Straton:

Can I refer you back to my first post?
http://www.ukclimbing.com/forums/t.php?n=192652#2800221

In reply to Chris Harris:

>So if the engines can drag enough air over the wings to provide enough lift, why do they bother building runways?

Not needed in some cases.

Saw a tiger moth trying to take off into a gale, went forward ten feet, lifted off, then flew backwards.

The thrust of the engine against the air will let the theoretical flight take off, on the other hand, if it is a Ryanair plane going to Norway it will not take off, but will be cancelled.

In reply to Mr Goose:

the plane goes forward due to the jet engines
the rollers go backwards at the same speed

the wheels (while they're in contact with the rollers) go 'backwards" as well but this does't affect the plane's speed

In reply to Ian Straton:

I think I see where the confusion about my position is coming from - Goose's post was not an explanation: it was tautologous. He had defined the plane as stationary, which of course means it will not take off. What I was trying to explain to him was why it was stationary.

In reply to pale ale:

The big problem with this is that the tyres would blow and the plane would crash before becoming airborne.

The 'groundspeed' relative to the runway will be twice* the normal speed at which the plane could rotate which would likely cause a tyre blow out a la Concorde.

* actually somewhat less than twice as fast as a moving road/runway would entrain a fair amount of air generating a significant additional headwind and additional lift due to the ground effect

In reply to Richard: you could if it made sense, you seem to have mixed up the role of friction in it however...

the only "real life" situation that would allow the aircraft to take off is imperfect feedback.

That is to say it is impossible (in real life) to to have the conveyor moving at the same speed as the wheels at every instant, this would allow take off but over a truly huge distance.

if you assume perfect feedback for the conveyor and normal physics for everything else then the aircraft remains stationary and does not fly.

In reply to sutty:

>on the other hand, if it is a Ryanair plane going to Norway it will not take off, but will be cancelled.

LOL!

In reply to rich:

the rollers are in feedback to the wheels not the engines


no they don't, the wheels rotate in the opposite direction to the conveyor. Go play with some gears.

In reply to jkarran:

I'm a complete dunce with regard to these techy things so this might seem real stupid, plus I haven't a clue about anything else you're on about, but in the OP it says the plane moves in one direction (presumably forwards if it wants to take off!), and the conveyor belt moves correspondingly in *the other direction* (i.e. backwards on the surface). Forgive my ignorance, but would this not cause the wheels to rotate *forwards*, and if anything increase the planes speed?

In reply to pale ale:

Why are people still arguing over this, when Richard gave the correct answer right at the start of the thread:

"The plane pushes against the air, not the conveyer belt, so as long as there is no friction of course the plane will take off. If there is friction and the conveyor belt is capable of rapidly accelerating to near-infinite speeds, and the bearings of the wheels are capable of taking such speeds, it's possible to conceive of there being supfficient frictional drag to hold the plane back.

In any realistic, physically limited interpretation, the plane will take off."


Of course, the question doesn't state what type of airplane is involved. If it has VTOL capabilities then we can forget about the wheels and runway altogether.

In reply to Ian Straton:


i quote

This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same

In reply to mostly cloudy:

Makes you wonder why they spend millions on creating long runways. All the controversy about Stansted's second runway cutting through historic villages would be eliminated overnight: 100 metres or so of "conveyor belt" would sort things nicely, without the need to knock down any 600 year old farmhouses.

Now I *wonder* why it hasn't been done?

In reply to all:

I officially resign. No more from me sorry.

In reply to Mr Goose:

You are very funny.

Try, please, really try, to understand the other side of the argument with an open mind.

Accept that people holding these views are intelligent people and might actually have a point.

Maybe, just maybe, they are more qualified to answer the question than you, or perhaps have thought about it more deeply or even, shock horror, are just plain cleverer.

In reply to Ian Straton:

Eh? How?

Actually, the feedback loop performing as assumed in the original question is logically impossible. It relates the conveyer speed to the speed of the plane; if the plane is stationary then the belt is not moving - so the plane must be moving for the conveyer belt to move. If the plane can move - and thus the belt can move - the plane must be able to take of...

The feedback loop makes sense if the conveyer speed is linked to the speed of the wheels. Even in this case, force can only be transferred to the plane if there is friction between the wheels and their bearings, otherwise the force balance does not occur (there is no resistance to the wheels being accelerated, except for angular inertia as I mentioned earlier but this appears to have been universally ignored) and there is nothing slowing the plane down - the wheels can spin infinitely fast but this will have no effect on the behaviour of the plane.


... or the conveyer not being able to accelerate fast enough, having a top speed, having limited power...

In reply to pale ale:

Er... ignore my previous post. I just thought about the question! I was thinking of a plane on a treadmill!

Yes it will still fly though, ground speed has little to do with lift, only airspeed and the thrust vector.

Think of an F15 sat on it's catapult on an aircraft carrier (the catapult is the moving runway) and the plane takes off fine (usually!). With a slight modification (sit the plane on a big tray attatched to the catapult) it's tyres need not turn but it will still take off.

Yes, strictly speaking the catapult 'throws' the plane but it needn't. With the correct control and enough space it could just track the plane's (thrust driven) acceleration and the plane would still take off.

In reply to Factual:

Nope, I'm the dunce, didn't read the OP carefully enough befor starting! See my follow up 'apology'

In reply to rich: I'll ask again, how does the plane move if the wheels it is attached to do not?

the plane speed is the result of the wheel movement on the surface of the runway, if that movement is cancelled by the conveyor you can run the engines at full power and the plane will not move.

In reply to rich:

Or alternatively the wheels rotate at twice the speed that the plane body and conveyor are moving at...

In reply to Ian Straton: have you ever run up a down escalator or vice versa?

do you think you could run the wrong way along an airport travelator and get from one end to the other?

In reply to Richard:
the realworld/imaginary world problem is to do with the feedback loop, there is no possibilty of perfect feedback whether you track the wheels or the plane body, if you allow perfect feedback on the wheels then you can also allow perfect feedback on the plane body.

the reason everyone ignored your discussion of the role of friction in the bearings is because noone has ever suggested that it would be absent, that is we are all making the same assumption about friction being present.

as for this:"> ... or the conveyer not being able to accelerate fast enough, having a top speed, having limited power..."

surly those can be included as part of the concept of imperfect feedback

In reply to Ian Straton:

"the point you are missing about aircraft wheels is that although they are not powered the aeroplane is attached to them, and your setup suggests a perfect feedback between the weheels and the runway. That is to say that no matter howfast they spin they will not move forward relative to an observer not on the runway. If the wheels do not move forward relative to our observer then neither can the aeroplane, if the aeroplane is not moving forwards relative to our our observer then it cannot generate lift, without lift it can't fly. "

I think that is it. Different people are making diferent interpretations of the setup - those who interpret it as allowing the thrust to move the A/C forward achieve take off, those who interpret the wording as meaning as constraining o that that no forward motion is achieved have no take off

In reply to pale ale:

This is brilliant. People so stubbornly wrong!

Plane moving at 100mph, wheels turning at 200mph, conveyor going backwards at 100mph.

Plane takes off.

I have a masters in physics and I'm pretty sure a lot of other people in here are as or more qualified as me. Try and think "why would it take off?" rather than blankly refusing to accept anyone else's points. For the record, I used to think it wouldn't until I thought about it.

In reply to Ian Straton:

The weels run on bearings causing little drag what with them being wheels not skids. The wheels' rotational speed therefore has no significant influence on the aircraft's acceleration relative to a third reference (the air).

In reply to chris j:
That's what I think too! Must be a mistake in the original puzzle, but you'd think all these hotshots would have noticed it.

In reply to rich: yes but neither of those are in a feedback loop to me, they are in constant motion ie their motion is unaffected by mine.

That is not the case for out hypothetical aeroplane.

In reply to Eric the Red:

do we therefore have a system where airspeed is not greatly affected but groundspeed is doubled (if the conveyor is defined the ground)

In reply to pale ale:

People are now arguing about 2 different scenarios as one and can therefore 'both be right'.

There is one scenario relating to a thought experiment where the conveyor matches the ground speed of the aircraft. In this case the plane does not take off.

There is another scenario where the conveyor does not match the ground speed but is simply free to move in an opposite direction. In this case the plane will take off with the ground speed relative to the conveyor of twice the airspeed and the ground speed relative to a seperate static point equal to the airspeed.

The slight ambiguity of your original question/scenario leads to the confusion.

You would get 9/10 for trolling but it's beeen done before so a 'me too' 3/10.

Try harder.

In reply to Ian Straton:


Not those who insist that the plane might take off, or those who insist that since the wheels don't move relative to the conveyer belt, the plane cannot either - they're just defining it that way rather than explaining it.

FWIW, I agree with you; I just think your explanation (although better than Gooses) is incomplete.

In reply to Ian Straton:

Hi,

In the same way it manages to do so at 30000 feet. The wheels just provide a reasonably low friction way for the 'plane to move along the ground's surface.

Jon.

In reply to 'Rock'DJ:

Alun covered both these points in his first reply to this thread. Physicists clearly don't read when there's a chance of a good argument.

In reply to Ian Straton:


The bearing friction doesn't matter either way, the plane doesn't moove relative to the conveyor so could therefor have it's brakes on and still take off. In fact it could take off without wheels.

Landing might be tricky though!

In reply to Ian Straton: Eric the Red's 11:53 offering is the answer

i was going for a perosnal example of the 'surface moves backwards slowly - you move forward slowly - connector (your legs) have to move really fast' scenario

In reply to jkarran: except for that whacking great big axel through the middle of the wheel attaching it to the landing gear.....

the feedback loop prevents the wheel moving forward relative to our tower observer, if the wheel cannot move then neither can the aircraft to which it is attached.

You are correct in that the wheel speed has no impact, you could spin it at light speed for all the difference that makes, but unless the wheel moves forward the aircraft cannot.

In reply to l21bjd: no that doesn't answer the question.

If the wheel cannot move forward relative to a an outside observer then how can the aeroplane move forward?

In reply to jkarran:

Then why don't aircraft carriers have conveyers instead of catapults and runways? Obviously because the convey belt idea does not work and no planes could ever take off this way.

In reply to rich:
no it doesn't, he state the aircraft is moving at 100mph, he offers no explanation as to how it has started moving forwards without it's wheels.....

In reply to jkarran:

Where did that one come from?!

In reply to Ian Straton:

Perhaps a lot of grease or a non-stick frying pan coating on the bottom of the plane??

In reply to chris j: Haaaa Haaaa ha! Like it.

It's lunch time. TTFN

In reply to Ian Straton:

it has it's wheels

the wheels are spinning faster than they'd need to if the runway wasn't moving backwards

the plane is not being driven by its wheels


if you ran down an airport travelator the wrong way pushing a supermarket trolley . . .

In reply to Ian Straton:

Why can't the wheel move forward relative to an outside observer? There is feedback, granted, but no feedback loop.
As has been said, the engines provide thrust against the air. If they were used to power the wheels, like in a car, then the plane would remain stationary wrt the outside observer.

Jon.

In reply to jamiemoss:

A catapult can be thought of as a section of an incomplete conveyor. Therefore they do and they do work, watch TopGun

In reply to jamiemoss:


You could have a conveyor belt if you really wanted, but it would have to be the same length as a normal runway.

Jon.

In reply to chris j:

One of the scenarios seems to be plane moves forward - ground mooves back. This best fit's the OP scenario. Ground speed is essentially double airspeed, plane still take's off (neglecting tyre failure)

The other scenario: Runway moves forward, plane moves forward at the same speed. Therefore: plane doesn't move relative to the belt (until takeoff)

The plane flies in either scenario.

Of course if you're simply refering to the double O in 'moove' that came from the 'ooooo' key!

In reply to l21bjd:


Or shorter if you left the brakes on. See the catapult analogy.

Still needs to be long to land of course, cable arrestors and 100 ton comercial airliners don't mix.

In reply to jkarran:

Please explain. As far as I can see, the catapult pushes on the aircraft, increasing its airspeed until it can take off. The conveyor belt does exactly the opposite, keeping the airspeed of the plane are zero so it never takes off.

In reply to Ian Straton:


It used it's engines.

Forget the conveyor for a moment.

The engines impart a thrust on the plane. An acceleration is then observed. This thrust causes movement forward, this movement is eased by the wheels reducing friction between the plane and the ground. At no point does the plane need the wheels to turn. If you put the brakes on you just need a bigger thrust.

Introduce the conveyor.

As the engines propel the plane forwards, the conveyor matches the acceleration but in the opposite direction. However, the force from the conveyor is imparted onto the wheel. As the wheels are free to rotate this causes them to spin but does not cause the plane to slow down.

In short, the force from the engines moves the plane forward and as a result of friction betwixt wheel and conveyor, the wheels spin. The force from the conveyor spins the wheels because of said friction, however and here is the crux, there is no mechanism to transfer this force back to the body of the plane and so it has no effect on the forward motion.

QED

In reply to pale ale:

It might if it's a Harrier jump jet...

In reply to l21bjd:

from the OP: "This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in the opposite direction)."

sounds like a feedback loop to me.....

In reply to pale ale: Firstly it depends on the type of plane because sone can take of vertically like the harrier and F25 JSF, however if its not then no it wont take off. Secondly what happens if the plane turns around???

In reply to Eric the Red:


Yes there is. To keep the wheels stationary while simultaneously applying a torque two opposed forces must be applied to the wheel.

The way I see it, the question is badly phrased.


So does this mean the ground speed of the wheels, ie the speed of the plane relative to the conveyor, or the speed of the plane relative to the control tower?

If the latter, the plane will take off.

If the former, we can say that the conveyor's job is to keep the plane stationary relative to the control tower, and so prevent the plane from taking off - that would be the effect of that condition. Can the conveyor do this job? In the case of any friction in the wheel bearings, clearly yes. In the case of no friction, the answer is still yes, due to the torque effect described above.

And people shouldn't believe everything they read on the tinterweb. I read somewhere that Russian brides were waiting for my call, but that turned out to be a lie.

Nick

In reply to Ian Straton:

I don't think it's a "loop": the plane's engines don't push against the conveyor belt.

Jon.

In reply to Eric the Red:
So you are invoking a rocket then? you are presumably aware that aircraft wheel brakes can prevent the 'plane moving even under full power?

This is where I think you are wrong. The mechinism for transmitting the force back to the aircraft is the axel. The aircraft cannot move independantly of it wheels, the 2 are physically attached, if the wheel cannot move forward due to the conveyor spinning then the aircraft cannot move unless it becomes detached from the wheel.

as you say....

In reply to Eric the Red:
That's key though, isn't it? The plane does need the friction-reducing properties of the wheels in order to achieve take-off. Otherwise commercial jets could just drag themselves off the runway from skis or their bellies.

If the wheels are immobile due to the action of the conveyer, the effect is that the plane would have to overcome the friction between tyre and ground in order to achieve forward motion, making takeoff impossible.

In reply to l21bjd: they don't have to. Consider a normal 'plane on a normal runway. The wheels are unpowered therefore they cannot spin any faster or slower then the whole aircraft is moving. The rotational speed of the wheel exactly matches the forward speed of the aircraft at all times.

With the conveyor in place the engines push the body of the plane forward, the wheels respond to the torque applied by rotating, the conveyor matches this rotation, the wheel stays in the same place relative to an outside observer. Adding more thrust from the engines creates more torque on the wheel (by trying to push the axel through the wheel), causeing it to spin faster but so does the conveyor. Since the wheel is physically attached to the plane the plane cannot move forward relative to our external observer, therefore there is no relative movment to the surrounding airmass and no lift.

In reply to Ian Straton:

Hmm. Are you resolving your torque about the axle? If so, the plane provides no torque to the wheels. Instead, the torque is applied at the point of contact between the tyre and the conveyor, and arises because of friction between the two.

If the plane is on a powered conveyor, this conveyor applies torque to the wheels. This makes them spin faster, but doesn't affect the plane itself. (Until you get to speeds where the wheels are spinning faster than they're designed to, at which point they'll probably fail).

(You could also resolve your torques about the point of contact between the tyre and the conveyor, but this only muddies things).

Jon.

In reply to l21bjd: given that the feedback loop purly counters the "drive" from the wheels and is not itself powering the system I don't think it matters whether you resolve the torque at the axel or the point of contact does it? (I don't know the answer here, if it does matter please explain why and we will see if it changes the system..)

I suppose the point I am trying to get accross is that although the wheels are not powered they are still subjected to torque from the aircraft engines (indirectly) and that given this motion is countered by the conveyor there can be no forward motion of the aircraft, so long as it remains attached to it wheels.

In reply to pale ale:

come on people Cuba! lets not fight here lets do it there!

In reply to pale ale:

The only way the plane could take off would be if an *extremely* strong wind was blowing down the runway.

Wangy

In reply to Twig: the F1 wind tunnels have a rolling floor to eliminate the boundary layer

In reply to Wangy: It would take off. The wheels have nothing to do with getting the plane in the air, just the engines. But don't take my word for it, go to http://www.straightdope.com/columns/060203.html for the final answer.

In reply to all

I'm about ready to give up, you won't listen.

A plane will take off with the brakes on, you just need a proper big engine, like a rocket. That was my point. The wheels mean you can do it without such a big rocket.

The thrust from the engine acts on the plane. Friction with the runway causes the wheel to roll rather than slide. The centres of the wheels are now moving at the same speed as the plane, they are attached. This is obvious.

The runway is now made to move in the opposite direction to the plane with the same velocity. This results in the wheels now spinning at twice the velocity they were spinning at. However, the plane is still moving at the velocity it was before, as are the centres of the wheels.

Would you care to explain how spinning the wheels of the plane makes it move?

In reply to Ian Straton:

No it will not matter where you resolve the torque - it's just easier to envisage if you resolve about the axle. You should always end up with the same outcome wherever your origin it.


Well, if we resolve about the axles, then the engines do not provide torque to the wheels. The conveyor causes the wheels to turn. The friction in the wheel bearings would act to slow the plane (relative to an outside observer) more than if it was on a normal runway, as they'll be spinning faster than normal. So, for the plane to be able to take off, say at 150mph, the wheels must be capable of turning twice as fast as normal, as the relative speed between the plan and the conveyor will be 300mph.

Jon.


Jon.

In reply to pale ale:

Ok, another idea- when the 'plane takes off on a normal runway, it releases the brakes on its wheels, and they then freewheel as the plane gathers speed and takes off. If the runway is moving in the opposite direction at the exact velocity of the plane, then the wheels will just freewheel at double the speed.

I think

Dave

In reply to dave_strachan:

Oh yes and it will take off as normal (excluding a small amount more friction in the wheel bearing joints)

Dave

In reply to pale ale:

Having just spotted that Jon, who clearly knows that upon which he is commenting, has said exactly what I intended to say, but with the addition of long words (like torque), of which I have little knowlege. Thus, look at his description instead of mine.

Dave

In reply to dave_strachan:

This whole free wheel thing doesn't make sense to me.

Imagine the plane was switched off, but the runway was going backwards at 100mph - the result is the plane goes backwards (-100mph).

Now imagine the plane has forward thrust equivalent to 1mph on a stationary runway. Given a 100mph backwards runnway, the obvious (and in my opinion correct) expectation is that the plane will still be going backward, but at -99mph.

However, with this freewheel answer, there appears to be no relationship between the wheels and the forward momentum of the plane, so I guess it is going 1mph forward, and the wheels are spinning pointlessly at 101mph. I consider this to be crap until someone explains otherwise.

In reply to Eric the Red:
I refer to my post of 11.56

In reply to dave_strachan: It probably depends on where the feedback of the plane's velocity is measured, if it is measured purely from the speed of the wheels then as soon as the plane starts to move away from stationary the runway will increase its speed to match the speed of the wheels but this means the wheels will now be going faster and the belt will speed up to accommodate and so on....................until the tyres melt, bearings blow up etc. etc. the plane would not take off (as the friction of the conveyor belt going infinitely fast is infinite).

It is basically a no-feedback control system with a ramp input.


However, if the plane velocity is measured from the ground and the conveyor belt was controlled to go exactly the same speed, but in the other direction, as the plane, it would take off nae bother as the wheels would just be going twice as fast.


The question is basically flawed, so a proper answer cannot be given.

In reply to jamiemoss:


Come on, think about this logically. How can the conveyer belt effect airspeed. Huge 'tailwind' fans would indeed prevent takeoff but that's not what we're discussing.

Wheels aren't needed for takeoff, they are just convenient, look at a float-plane or ski-plane, neither has rotating wheels, either can take off from a moving surface e.g. a river.

In the OP the conveyor only effects wheel speed, theoreticall it has no impact on the acceleration/velocity of the aircraft relative to the air (the important parameter)

As for a catapult being like a section of conveyer... draw a sketch of both and look for similarities.

In reply to Wangy: the bit youare missing is the feedback loop, youare talking about a runway in constant motion, the op is not.

In reply to hutchm:


Noooooooo in this case the wheels don't have to turn to get the plane up to take off speed because the conveyor moves instead of the wheels. The plane still moves through the air.

Aerodynamically this is equvelent to a stationary plane in a takeoff-speed headwind, it'll still take off give or take the relatively small effect of the non-moving floor as a couple of people have already pointed out.

In reply to Eric the Red:
Yes

Yes

IE zero mph, the feedback begins instantaneouly (according to the OP) therefore the only movement induced is the spinning of the wheels and the runway.


it doesn't that is my point! You still haven't explained how you get the wheels to move from their starting position relative to an observer not on the runway, without this movement there is no flight.

In reply to pale ale:
this simple way to look at this is that:
- the wheels on the plane do not have any motor etc in them they are free rolling.
- the conveyor belt moves at the same speed but in the opposite dirn to the PLANE (not the planes wheels)
- the thrust of the plane comes from the jets.
- therefore as the thrust of the plane moves the plane to say 10 mph the conveyor belt is moving in the opposite dirn at 10mph and the wheels on the plane are simply turning as fast as if the plane was doing 20 mph.

Therefore the plane can take off, the wheels will just be going around twice as quickly as normal.

In reply to Alasdair Fulton:


The question isn't flawed, it's been mis-read by some, me included but there is no way a moving runway will prevent an aircraft taking off. It will change the wheel speed up or down depending upon which way it rolls and slightly alter the aerodynamic performance.

It doesn't matter what you feedback is used to control runway-conveyor speed.

In reply to Alun:
your wrong, the speed of the runway is stated as being the same speed as the speed of the plane not the rotational speed of it's wheels.

In reply to pale ale:

I think I've got it sorted in my mind. It is all to do with the quality of the wheel bearings and the friction between the wheel and the runway. There is no correct answer.

Wangy

In reply to Ian Straton:
Dude the plane is not driven by it's wheels, it's driven by it's jets the plane moves forward beacuse it's jets push it. Think about it as the plane moves from stationary to 0.0000001 mph the converyor belt moves from staionary to 0.0000001 mph in the opposite direction. As the plane is not driven by it's wheels (and as there is very low friction in the wheels) the wheels just spin twice as quickly as they would have done, the plane stays moving at 0.0000001 mph. This idea can be continued until the plane reaches take off speed.

In reply to Wangy:
The take off speed of a plane is only around 150 mph, I guess the question is can the wheels turn at 300 mph without significantly higher friction or malfunction. If they can, the plane can take off. It seem plausible to me.

In reply to CurlyStevo: No-one read the link I pasted on then.... hey ho. To finally answer this I'll cut and paste the definitive answer to the question, enjoy! "The unwary tend to reason by analogy to a car on a conveyor belt--if the conveyor moves backward at the same rate that the car's wheels rotate forward, the net result is that the car remains stationary. An aircraft in the same situation, they figure, would stay planted on the ground, since there'd be no air rushing over the wings to give it lift. But of course cars and planes don't work the same way. A car's wheels are its means of propulsion--they push the road backwards (relatively speaking), and the car moves forward. In contrast, a plane's wheels aren't motorized; their purpose is to reduce friction during takeoff (and add it, by braking, when landing). What gets a plane moving are its propellers or jet turbines, which shove the air backward and thereby impel the plane forward. What the wheels, conveyor belt, etc, are up to is largely irrelevant. Let me repeat: Once the pilot fires up the engines, the plane moves forward at pretty much the usual speed relative to the ground--and more importantly the air--regardless of how fast the conveyor belt is moving backward. This generates lift on the wings, and the plane takes off. All the conveyor belt does is, as you correctly conclude, make the plane's wheels spin madly.

In reply to fijibaby: I'm sorry, I have read this and i still don't see how the wheels leave their starting position? if they don't move relative to anything except the conveyor then how does the aircraft move forwards?

In reply to fijibaby:


Who says it's definitive? Some dude on the internet? Right...

In reply to Ian Straton: The wheels are attached to the plane, that's how they leave their starting position. Think of them as skis, they serve no function other than to keep the plane off the ground.

In reply to Nick Lambert: Yeah, but what a dude! Have a look at the site, thorny issues solved with humour!

In reply to fijibaby:

I still claim my first post in this thread is definitive, and that was hours ago.

In reply to fijibaby: yes fine, but how do they leave their starting point? the motion of the wheels is completly countered by the motion of the conveyor, so how do they move from their relative starting point?

In reply to Richard: Just read your post. Pretty much on the nail. Didn't stop another 100 post that were waaaaaaay out. Internet and its affect on productivity at work conundrum......

In reply to Nick Lambert:

It is just some dude on the internet- but it's a dude that either by luck or reason has nailed it on the head. Imagine someone was on ice and you were pulling them by a rope- it wouldn't matter how fast the ice was moving away from you- you'd still be pulling them towards you- the only significant force comes from the rope. In the plane story, the only forces are the huge thrust of the engines forward being countered by air resistance (present whatever the case), and the increased friction of the wheels on the runway (tiny in comparison)

In reply to arby:

I preferred my infitesimally light propellor plane answer personnally.

In reply to arby:

Screw this significant crap, the situations not the same anyway as the ice cannot accelerate while the conveyor can.


Not if the conveyor was accelerating fast enough. In practise, yeah, sure, course the plane would take off, but that's not really the point of such things. A theoretical conveyor belt could impart an arbitrarily large force to the plane, and so could stop it taking off. What isn't clear is whether the wording of the conditions in the original question leads to this happening.

In reply to Anonymous:

No. The wheels are spinning at twice the rate they would if the conveyor wasn't there but the plane isn't moving any faster.

In reply to Nick Lambert:
This is quite basic stuff, A-Level newtonian mechanics covers this. I don't want to state my qualifications in a vain attempt to try and justify my answer by 'proving' i'm better than others as this is incorrect and also isn't the point, it's just something for everyone to think about and have a bit of a laugh with.

so as someone said, if you are pushing a shopping trolly down an airport travellator, but you are running along next tot the trolly to push it (not on the conveyor) then the trolly moves at whatever speed you can run at regardless of the conveyor (assuming no friction in the wheels) this is the plane with engines seperate from wheels.
If however you are pushing the trolly by running along the conveyor then you will go at your running speed-conveyor speed this is like a rolling road.
in example 1. the plane takes off,
in example 2 it doesn't
i finally understand, now to see if Rhino 3d has meshed my model

In reply to Southampton Tom: yes! i knew my pushing a trolley the wrong way up a travelator analogy would do the trick :¬)

thanks for adding the 'you're not actually on the travelator' bit

In reply to arby:

I quite agree. But you're still wrong

If the conveyor belt can exert even a very small force on the plane, then it can also exert an arbitrary large force, because we haven't said, at any point, that the conveyor has any limitations. I think it all comes down to whether the conveyor is matching the ground speed or the wheel speed of the plane.

Nick

In reply to Southampton Tom:

Yes, I was wrong earlier. Too busy wondering who the spoof Norrie was on the other thread.

In reply to Southampton Tom:

Even if there is no friction, the conveyor will exert a force on the wheel axle. This no friction thing is a red herring. IMNSHO.

Nick

In reply to Eric the Red:

what, not relative to the surface on which they are rolling?

I'm taking that to be the ground

In reply to Ian Straton:
the converyer belt only moves as fast as the plane is moveing it is not set to move at the same speed as the planes wheels turn around

In reply to Nick Lambert:
the question clearly states the conveyor belt moves as the same speed as the plane!!!! this has to be relative to a stationary point on the earth right!!!

In reply to CurlyStevo:

Well, you'd have thought so. But that's the crux of it.

To the non believers:

I suggest you get a toy car and a piece of paper, place the car on the far right of the paper push the car leftwards with one hand and move the paper rightwards with the other and observe what happens (the car does indeed move and the wheels turn about twice as quick!)

In reply to Nick Lambert:
when people say my car goes 100 mph that's what they mean. To take it as anything else (ie it goes 100 mph when compared to a car going 30 mph in the other direction) is just manipulating the question and throwing your toys out of the pram cos you got the "wrong answer" (best said in a USA ascent).

In reply to CurlyStevo:

surely travelling in a car only marginally changes our actual rotational speed on the earth's surface

In reply to CurlyStevo:

No, I didn't. Read my first post, in which I carefully hedge my bets to avoid precisely this accusation. Anyway, don't you think you can bully me into being wrong...

In reply to Nick Lambert:
soz wasn't meant as a personal attack I mean't you as being anyone.

In reply to CurlyStevo:

That's cool, forgot the

Oh, I wish I could understand this! If the plane is not moving relative to the ground, how can it be moving relative to the air? And if it's not moving relative to the air, how can it take off????

In reply to gingerkate: From what I can tell the question is badly written and can be interperated in slightly different ways. Each way can be proved with basic physics one way or the other depending on how you've understood the question.

In reply to gingerkate:
it is moving relative to the ground the wheels are just turning twice as fast (for the speed its moving relative to the ground) as the converyor belt is moving the same speed as the plane in the opposite direction.

Try my experiment with a toy car and I think all will become clear!

This thread was started in the wrong forum and has now been moved.
Please could you try and post in the correct forum, it makes life easier for both users and moderators.

Forum descriptions - http://www.ukclimbing.com/forums/info/forums.html

In reply to UKC Forums:

Brilliant it took 200 message for them to notice......

In reply to Beaver:
diagree all the information is available at a reasonable level in the question to say it isn't is quite convoluted IMO.

In reply to CurlyStevo:
Oh, I'm with you now. That's what the 'non-believers' are not understanding .... I was thinking that if the conveyor belt matches the plane speed the plane is still relative to the ground, but that's not right is it.

In reply to arby:

Just considering one more scenario...

Ignoring a few basic details like tyre failure then it is theoretically possible to impart a sufficiently large force to the aircraft axels to counter the thrust and prevent it from moving forward simply by continually accelerating the belt at a massive rate toward the aircraft.

The tyres have a finite and non zero moment of inertia and will produce a torque response at the contact point resisting their rotational acceleration due to that moment of inertia, that response will also cause a small backwards force on the wheel's axel.

If the tyres are heavy (and strong) enough and the belt capable of a MASSIVE constant acceleration then it would theoretically be possible to match the aircraft's thrust and keep it stationary relative to its surroundings without ever considering brakes or bearing friction.

In a real world this scenario is impossible and is not the one implied by the OP.

The answer cribbed from the internet is the correct one for the correct reasons.

jk

In reply to jkarran: All the talk of super high friction in the wheel bearings etc is irrelevent. The question says that the conveyor belt matches the speed of the aircraft not the spinning of the wheels. If the aircraft is moving at 50knots forward then the conveyor will be moving at 50 knots backwards and the wheels will be spinning at a rate equivelent to the aircraft moving at 100 knots. As long as the wheels can spin at twice the rate usual at take off speeds, which is reasonable to expect they can, the conveyor belt makes no difference.

It is frightening to see so many people have no idea how an aircraft works.

In reply to Anonymous:

I'd agree with this. But last time this question was 'done to death', as linked above, it was phrased;

"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?"

I would submit that, in this case, the answer is no - there are two different problems.

I'm sure people do really. They're just getting embroiled a bit. Anyway, even if they don't, it's not that scary, is it? Presumably none of them are aerospace engineers.

Nick

In reply to jkarran: This is doing my head & patience in!
Ignore wheels - they are irrelevant. The answer on the website states that the plane moves forward over the ground - if it does this then it goes off the end of the conveyor belt.
If the plane stays in the same position on the conveyor using its engines to provide thrust to stay in that position then there is no air flow over the wings.
Imagine running along an airport conveyor belt the wrong way. Your legs are doing the same job as an aeroplane engine - providing a force to keep you in the same place.
There is no airstream going past you. No airstream = no lift.
I would just like those who say the aircraft will take off to clearly explain how a non-moving object can achieve an airstream that will enable lift please?

I'm off back to the land of the only slightly insane.

In reply to Twig:

I, too, was earlier today in the same position of unenlightenment as you.

The answer above about the shopping trolley on a travellator is the one you should read. Repeatedly. Until your brain hurts.

In reply to Twig:


When you run you move forward because your legs push against the ground through your feet. An aircraft doesn't move forward by pushing against the ground but by sucking on the air.

In reply to Twig: "Your legs are doing the same job as an aeroplane engine - providing a force to keep you in the same place." - WRONG - A jet engine provides thrust that's independant of the floor... read shopping trolloey answer as stated above.

In reply to Twig: Twig, imagine you are standing on a treadmill in rollerskates and the treadmill acts in the opposite direction to your skates. If there is a rope tied to the wall in front of you and you pull on it what will happen?

Also, you're in correct in using the analogy of a person running along a travelator, that is the same as a car on a rolling road...i.e the power is driven through the wheels/ legs and therefore is negated by the travelator/rolling road. But the planes engines pull the wings along and there is no power through the wheels. As long as there are no brakes applied, they are irrelevant to the planes movement, they just stop it from dragging along the ground on its belly and offer far less friction. Also, the rolling runway offers no resistance to the plane, therefore the plane will move foward as normal but the wheels will be turning far faster than on a fixed runway.

In reply to pale ale:

that was possiblly the most ridiculous question i've heard. there are some very special people on this forum.

In reply to pale ale:

Hello Pale Ale

You've done well on this ione - I got just under 200 lured in last time:

http://www.ukclimbing.com/forums/t.php?t=162217

The issue is about how the question is phrased. If the conveyor matches the speed of the plane, it takes off. If the conveyor is supposed, somehow, to completely counteract the speed of the plane, it can't but there's an impossibility arises.

Anyway good to know folk are still so passionate about entirely theoretical things!

J

In reply to Twig:

Wrong. Sorry

In reply to Moacs/All


It can counter the thrust of the plane's jets keeping the aircraft but not the wheels stationary in theory - see my post of 17.10 and Nick Lambert's 15.47. The plane wont fly in this case but the belt does have to continue accelerating (at a much higher rate than the plane could) to keep the aircraft stationary. This is not possible worded as the OP is. This is a ridiculous scenario taken to it's conclusion purely for my entertainment and the frustration of those who don't believe. I can prove this one in a more intuitive way if people would like.

If the belt acted to keep the wheels from rotating (entirely possible and not a hard control problem to solve since the wheels aren't powered) the plane would still fly. See my catapult analogy and earlier pushing a trolly along travelator (walking alongside) analogies.

If, as in the OP the belt spins the wheels twice as fast as they would normally the plane still flies. Again, see trolly travelator analogies This is the simplest scenario to understand once you realise the wheels on a plane aren't powered. The plane still flies.

jk

In reply to jkarran: I wonder if all these people think that the plane is propelled by the wheels when it is in the air? As they clearly think the wheels propel it on the runway!!!! ;@)

In reply to jkarran:

"If the belt acted to keep the wheels from rotating ..."

Which is the same as having the brakes on ....? Don't planes often sit at the end of the runway with the brakes on and the engines spooled up to full wallop and go nowhere?

(Sorry didn't notice this in time to join in with the main ... errr fun ... earlier).

In reply to pale ale:

You could prob get it off the ground into a mean headwind...

In reply to Alasdair Fulton:
No they don't think that.

The extraordinary thing about this thread is how incredibly badly most of the people who understand it attempt to explain it.

Twig, read CurlyStevo's posts to me, then play with your mobile and a sheet of paper... pull paper one way and mobile the other. It all becomes clear...

In reply to stoo:


Similar, but different the belt can keep the wheels from rotating by accelerating under the plane at the same rate as the jet propelled plane. The brakes could be on or off, it matters not a jot.


Correct, brakes can hold back take off thrust. There's a pilot posting here somewhere, I'm sure he'll confirm or deny this. However, I refer you to the statement above. The wheels don't have to turn for the plane (on a moving belt afterall) to reach takeoff airspeed.


It's kept me 'amused' all day, you've not missed much
jk

In reply to jkarran: the plane cannot because the aircraft is always moving at the smae speed as the conveyor, it would have to be moving twice the speed at least to take off. that is the correct answer, think about it !!!

mike

In reply to nolo:


Ok, will do. Are you really 11?

In reply to All posters:

You're all answering the wrong question. The OP asks if the plane "can" take-off and you're answering as if the question was "will" or "would" the plane take-off. The OP does not give sufficient information to answer with a simple yes or no. Call me pedantic if you like.
I could tell you the real answer, as we covered it during the first week of aerodynamics during the groundschool phase of my pilot training just over 20 years ago - physics hasn't changed since then.
Another question answered was the following (you may have seen it before..)

A parrot sits on a wooden perch which is attached to the inside wall of the trailer of a large truck. The trailer is enclosed and airtight (yes it has enough air to breathe whilst we think of the answer !!) and its wheels rest on a weighbridge sufficiently sensitive to detect ANY change to mass/weight. The parrot flies from its perch to another, similarly positioned at the other end of the hold, and lands on it. Here's the question - Do the scales detect any change of weight during the time the bird is in the air?

In reply to Iain Watson:

Pedant Come on then what's the answer?

I think we need to assume the plane is capable of flight were it not for the runway part of the question although in fairness that was never stated.

In reply to jkarran:

If the wheels are not going round the only way the plane can move forward is by pushing the stationary wheels across the surface of the moving conveyor. This is exactly the same as the brakes being on. Brakes work by applying enough friction to the wheel to stop it from turning. By your own description the conveyor would be doing exactly the same.

The answer, as I see it, based on the wording of the original question, is that the plane will go up, exactly because there is nothing stopping the wheels turning.

Those that are saying it won't go up are assuming that the question means that conveyor is moving at the same speed that the wheels are turning, and thus the plane would behave as though it was parked on a normal runway with the brakes applied.

In reply to Iain Watson:


No.

J

In reply to pale ale:


this has been on here before. it doesn't matter about the speed of the wheels. the plane will not take off, the air speed over the aerofoils is the important part. there will be no airflow over them because the jet isn't moving.

Centurion05

In reply to Moacs:
If the conveyor matches the speed of the plane, it takes off. If the conveyor is supposed, somehow, to completely counteract the speed of the plane, it can't but there's an impossibility arises.

you're a tit, i work on aircraft and it won't take off. the question is reading, if the aircraft is stationary, but the wheels moving, will it take off, of course it f*cking well wont.

centurion05

In reply to centurion05:

The plane _is_ moving. Running the conveyor belt in the opposite direction does not stop the jet moving forward (relative to the ground, thus also relative to the air) because the jet isn't propelled by its wheels. So it speeds up same as usual, just with very whizzing wheels.

In reply to centurion05:

But it isn't stationary. I understand why you think it will be, I did too, but it won't.

In reply to gingerkate:

here's the answer.

the aircraft is stationary, if the conveyor belt is moving at the same speed as the wheels, its not going anywhere.

now picture the aircraft in chocks, which will replace the conveyor belt and moving wheels. no matter how much forward thrust you put out the engines, it won't move. i've seen a jet with afterburners on in chocks, funnily enough, it never went anywhere.

Centurion05

In reply to centurion05: yes i am 11 look at my gallery its a pic of me!!!

mike

In reply to centurion05:
The chocks aren't an accurate analogy.

Try this: get your mobile phone out of your pocket. Lay it down on a sheet of paper. Now pull the paper one way, and the phone the other, both at equal speed.

You see?

In reply to gingerkate:

how does that work. and why are the chocks not accurate?

if the conveyor is moving directly proportional to the speed of the wheels in the opposite direction, the jet is moving on the conveyor, but staying in situ of the ground.

its not going anywhere.

Centurion05

In reply to centurion05:

The question doesn't say anything about the conveyor belt moving at the same speed as the wheels are rotating, it talks about the conveyor moving at the same speed as the plane.

Plane moves one way at 100 Knots, conveyor goes the other way at 100 Knots and *the wheels spin as though the plane was travelling along a conventional runway at 200 Knots*.

V1 and the plane goes up in about the same distance as it would on any normal runway.

In reply to gingerkate:

and pulling them at equal speeds will require the phone to stay in situ, moving it away will annotate the aircraft moving at twice the speed of the conveyor.

Centurion05

In reply to centurion05:



Thing is though, you're making assumptions about the friction; chocks are guarenteed to increase the friction whilst a hypothetical conveyor belt is not.

If you assume that there is no friction, then the aircraft must take off because there is nothing to stop it moving through the air. Paradoxically, the aircraft moving will violate one of the rules of the puzzle (the feedback loop in the conveyor belt), and thus can't happen by the rules given.

Basically, it is a puzzle that doesn't have an answer.

Mark

In reply to stoo:



so if the conveyor is moving in the opposite direction as the plane at the same speed then the aircraft isn't moving at all in respect to the ground.

so no it won't take off.

Centurion05

In reply to centurion05:

Wow that's scary!

and it won't take off. the question is reading, if the aircraft is stationary,

It is a question of reading. I keep reading the bit that says "The plane moves in one direction,".

hope you can read the service bulletins better than these posts.

In reply to stoo:


Yes, but it should do; otherwise it isn't much of a puzzle!

Mark

In reply to Anonymous:

it won't fly, the aircraft os going no where, ur just trying to blame a simple answer on friction and shit.

centurin05

In reply to centurion05:

Yes it is, courtesy of those things that make all the noise pushing against the air which couldn't care less what the conveyor belt is doing, *as long as the wheels are free to rotate*.

In reply to Hairy Pete:

the plane does move in one direction, but then it is immediatly counteracted by the conveyor belt, so it isn't moving at all.

and why is is scary that i work on a/c? whats ur job?

centurion05

In reply to stoo: those things that make all of the noise push the aircraft forward on the wheels, the aircraft requires lift from the mainplanes which is generated my forward movement of the whole aircraft, this isn't happening if the conveyor belt is moving rearwards.

Centurion05

In reply to centurion05:
I wouldnt place too much in your intellectual rationale.

A quick search utilising the problem in point returns with the following answer...

You are wrong...

AGAIN.

In reply to centurion05:


Sometimes it helps to vusualise the aircraft being towed by a truck which is beyond the conveyor belt to see why the aircraft must move (given frictionless wheel bearings).

mark

In reply to centurion05:

Because if it's in chocks the plane isn't moving relative to teh ground.

Not speed of wheels... re-read it... the conveyor is just moving at the same speed as the plane. (The wheels are spinning as whizzy as can be).

>the jet is moving on the conveyor, but staying in situ of the ground.

No it isn't. It's moving relative to the ground too. Try it with your phone!

You tried it yet?

In reply to Ste Brom:

give me your reasoning why and how the aircraft is moving forward in proportion to the ground, because its that what creates lift.

Centurion05

In reply to centurion05:
Sorry, I dont reason with semi naked helmet wearing homophobes.

I use this website to answer my queries.

www.google.com

I find its handy to be able spell.

In reply to gingerkate:



I think you need to assume that what was meant was that the conveyor matches the wheel speed (that's how I've always seen it phrased before). If not, the answer is too obvious. Well, to most people anyway.

mark

In reply to centurion05:
Did you try it? Stop thinking about it and just try it. Ignore all the stuff about friction, it just makes it confusing. Just pull the paper one way and your phone the other. Now did that phone stay still relative to your desk? It did not! And you can't tell me it did. If you say it did you've not tried it!

In reply to centurion05:

It's a trick question C, to do with the way the question has been phrased. You are right; there has to be forward motion against the airflow for there to be lift. If the aircraft is stationary in relation to the air and all that's happening is that the wheels are spinning against the conveyor there won't be any lift. But the question says that the aircraft is moving forward. It's irrelevant how fast it's wheels are spinning; they aren't powering it anyway. If it is moving forward at the same speed that the conveyor is moving backward, it is still moving forward it's just that the wheels are spinning twice as fast. So it will take off.

In reply to gingerkate:

i tried it, the reason it moves is because the phone is moving at twice the speed of the paper.

In reply to centurion05:

It's stated in the question, that it moves in one direction. Therefore the brakes are not on, therefore the aircraft can move backwards or forwards along the conveyer regardless of the motion of the conveyer. The engines act on the air mass, which has no connection to the conveyer. Therefore the plane goes in whichever direction the engines push it.

In reply to Ste Brom:


Do you know how Google works? Regardless of the particulars of this debate (by the way, the 'plane won't fly); you should be wary of the answerrs it gives to this type of question.

Google is a "popularity contest", and therefore not infallible.

In reply to centurion05:
Why did you move your phone twice as fast as your paper? Try it again, slow this time. Move them at the same speed.

Like Alison says, it's just a trick question. It cons you into thinking the forward motion of the plane is being counteracted by the conveyor, but it isn't.

In reply to KeithW:

Keith, it's one of those things that once you've twigged it, is absolutely obvious. It's got ef all to do with aeronautics. All the fluff on teh thread about this and that and the other makes you think it's something subtle and probably mad, but it isn't, like Alison says, it's just a trick. Do the phone thing, then you will get it.

In reply to stoo:


No... Brakes and conveyor do totally different things to stop wheels turning.

Brakes provide a torque equal and opposite to that applied externally to prevent the wheel turning.

The conveyor moves with the wheel so no torque is applied therfore the brakes become irrelevant.

In reply to pale ale: it wont take off, IMHO if it did they would use that method on aircraft carriers

In reply to brendanhanratty:

No, because the conveyor belt would need to be as long or longer than a normal runway. In effect, it's simulating a tail wind; exactly the opposite of what aircraft carriers do (i.e. steam into the wind).

mark

In reply to gingerkate:

Sometimes I think (some) men are on a different planet

In reply to centurion05:


God help everyone if you work on aircraft.

Read the post yourself... "the aircraft moves".

this is getting way past stupid now. The aeroplane accelerates as if it's a normal runway. As far as the passengers are aware there is no conveyor belt. The plane takes off- again totally as normal. 9 hours lt plane lands in los angeles with the passengers blissfully ignorant to the fact that the onlokers at heathrow as they took off are still raging about how the plane ever managed to move- despite there being frickin' huge jet engines strapped to the thing that should make it quite obvious why it moved.

Next question, what happens if the runway at LAX is moving backwards with a speed equal to that of the plane?
And secondly, how about forwards equal to the speed?

In reply to centurion05: What do you do when you work on planes?

In reply to jkarran:

"conveyor moves with the wheel" - and so the wheel is going round, which aint what you said originally

In reply to all: I'm amazed this is still going, I thought it was dead and buried by a couple of rational replies around lunchtime. Am impressed it took the powers that be 7 hours to move it out of Rocktalk!

In reply to jkarran: the aircraft isn't stationary. You might work on aircraft, but let me guess that you didn't study aeronautical engineering? or even basic mechanics?

In reply to brendanhanratty:

No, this is where the trick in the question really is. People assume that the aircraft remains stationary because it is on a treadmill. This is because they are used to running machines and maybe seeing cars testing on rollers or similar. An aircraft on a teadmill will not remain stationary with the engines running. It will move forward along the treadmill.

In reply to arby:

I think that was addressed to centurian not jkarran. And you are being a teensy weensy bit unkind :-P
The question is phased in such a way as to trick people into thinking the aircraft is stationary.

In reply to chris j:

Aircraft toilets need cleaning as well.

In reply to Alison Stockwell:
Well I was completely bamboozled by it myself until I tried it with the phone, so I have a great deal of sympathy for everyone who thinks it is mad

In reply to gingerkate:

I do agree with you - the aircraft will definately get airborne - there is not one doubt in my mind.

However, the pushing the phone thing may confuse some people, as the phone doesnt have the 'free-rotating' wheels that the aeroplane has.

If an aircraft, flying at 100kts, comes in to land on a rolling runway moving at 100kts in the opposite direction, the aircraft will not skid to a halt. It will continue to fly at 100kts (ok, maybe 99kts, if you take into accoutn friction). The wheels will rotate as if the aircraft is landing on a static runway at 200kts, so will be going twice as fast, but the aircraft will still maintain its forward motion in relation to the air.

Same applies as the aircraft gets airborne - its simple:

Aircraft's engines push it forward, regardless of how fast the ground is moving
Wheels spin faster than usual, as they have both the aircrafts velocioty and the movement of the 'runway' to spin them
Aircraft continues to move forward at same speed
Aircrafts wings through air generate lift, and it takes off.

IF the aircraft was powered through its wheels (as in a car), it would be entirely different.

There you go - a definitive answer. Argue, disagree, whatever - its physics!!

In reply to gingerkate:

I had to re-read the question to get it clear in my head.

In reply to brendanhanratty:

They do albeit in reverse, it's called a catapult.

In reply to Alison Stockwell:
Yeah, I think they are being unkind too.

In reply to Enigma:
i thought the question about landing might make things clearer. The plane doesn't just screech to a halt (or go backwards), in fact it's unaware the runway is moving. This seems easier to grasp for most people, the taking off part is pretty much the same

In reply to Enigma:
Well my phone is pretty frictionless so it worked for me ... but I take your point.

In reply to stoo:

Sorry Stoo, I have two different discussions going on here from earlier, we were discussing the belt moving with or against the aircraft. Neither matters.

Basically, it matters not a jot what the conveyor does (within limits) it's all to do with the engine thrust and airspeed.

In reply to arby:

Until it stops. Then everyone gets a horrible shock

In reply to gingerkate:

Must admit; I read the original question ages ago, and assumed it meant the aircraft was stationary with respect to the air around it.

I'll stick to my point about Google though; many people think it's infallible but ultimately it measures "most linked to" pages, which is not a guarantee of accuracy.

In reply to gingerkate:
I admit, that was unkind. I'm just not keen on people stating qualifications as a means of 'proving' they're right. Especially if they're wrong, but that does make it more fun. But there were nicer ways to say it than what i did so I should apologise.

In reply to gingerkate:


It's a bloke thing. Scoring points and all that

Good conundrum though. I'll try it out on my husband later.

In reply to gingerkate:

A frictionless phone, lol. Mine is too. Actually, thats just my excuse for dropping it when drunk...

The landing scenario does make it easier to understand, and its the same forces being applied!

In reply to KeithW:

That's what I thought ... I read virtually the entire thread thinking everyone had gone stark raving mad, until curlystevo (I think it was) pointed out it wasn't stationary.

In reply to Alison Stockwell:
It caught mine out

In reply to centurion05:

Gingerkate and a few others are talking sense from what I can make out of this shipwreck of a thread.
Think about where the thrust is coming from, the wheels will act like a bearing regardless what way the feckin conveyor belt is turning.
If it were a car with wings then it would be a totally different ball game as the main thrust would be transfered through the wheels and this would be counteracted by the conveyor.
As the forward thrust is generated by the jets which is independant of the conveyor, the wheels don't come into play the same way, the only effect it will have on them is that they will rotate twice as fast in the opposite direction.

Am I right, or do I need locked up?

Ewan, physics expert after 3 beers.
As my boss likes to say, "Think outside the box".

In reply to ewang:

any idea what speed a model aeroplane needs to take off?
that and a treadmill should soon sort the question

In reply to pale ale:
ah.. pale ale stretches the cerebal cotex, but the answer to the leading question is in the leading edge, which most aeronautics 101 students would understand. Google is for gagas.
Reality is about relative position and speed of trajectory. And so which is your position pale ale? Are u Cartesian or rotational tensor ? Do u have a singularity that needs investigating ?
And back to the posed question. I suppose if you take the length of a well known tick from a well known website you may discover that 2001 was in fact the answer to your question !

In reply to dissonance:

Would need a long treadmill though, as the aircraft DOES move forwards.

Maybe one of the ones at the airport would do it... hmmmm...

In reply to gingerkate:

OMG!

My husband and I have just had a HUGE row over this.

He just said "I've got an A level in physics so I know these things."

This is the first argument we've had in years.

In reply to Alison Stockwell:

So what was his answer?

In reply to ewang:

He thinks it will not take off and he says he doesn't want to discuss it any more.

In reply to arby:

I don't think any apology is necessary (assuming we're talking about the same poster) since he/she used their 'qualifications' as justification for derriding another poster.

In reply to Alison Stockwell:

I've been there when two physics PHD students were arguing about this so 'intelligence' doesn't seem to solve this problem! Depends on your initial viewpoint and once it's fixed everything else seems absolutely implausible

In reply to Alison Stockwell:
LOLOLOL!!! That is so funny!!!!

You know, I think this is harder for people who 'know about these things' than it is for everyone else. Because we/they assume we know what's going on, and we don't.

What's your background Alison? Are you arty or sciencey? I'm mathsy, and as I've said it took me ages for it to click.

In reply to arby:

Yes indeed. That's why it's such a good conundrum.

In reply to Alison Stockwell:
But even if you let him 'win'- at least you can go to bed tonight safe in the knowledge that not only were you secretly right, but that you were also good enough not to keep trying to prove it (unlike the rest of us on this thread)

In reply to Alison Stockwell:

I think we should all storm the HQ of Boeing and demand an investigation!

In reply to Alison Stockwell:

I've got my masters in physics and my girlfriend is a professor in graphic design. this was our first ever argument as well! Things still go a little silent when the conversation comes round to runways....

In reply to gingerkate:

Well, I started off arty. That might be because I'm OK at English and I went to a school that was useless for science. But I work in IT, and I have got more sciency as I've got older. So I suppose I'm bang in the middle now.

I have a suspicious nature though, and I thought there must be a trick somewhere, so I focused on working out what the trick was. That's when I got it.

In reply to arby:
You're too nice, I think she should keep him awake until he admits he was wrong

In reply to Alison Stockwell:
I've done the reverse, started off in maths, now writing. But as soon as anyone says anything that sounds like maths my brain goes into maths mode... and this time got it completely wrong :oS

In reply to Enigma:

dont think you would, the simple act of moving (excluding the argument the wheel bearings etc would overheat and melt)would prove, assuming it wasnt down to mismatched speeds that it would take off.

kev

In reply to gingerkate: he won't though. one of the beauties about being a man. I've seen quite a few women be 'converted' on this topic but never a man

In reply to arby:

Tee hee. I like that thinking.

He has persuaded our daughter (who really should be in bed by now!) that he is right. But I will sort that out in the morning

In reply to gingerkate:

I've been writing recently too! Good luck with that

The most challenging sciency thing I've done recently was my amateur radio exam. It was the hardest subject I've ever studied and it literally made my brain hurt; but I passed anyway.

It's good to be a bit of both.

In reply to gingerkate:


For those of you wanting a less technical explanation I offer the follwing:

Shortly after take-off from Vienna the g/f asks 'what makes the plane go up ?' So I told her, 'It's like this, there's a bloke, near the pointy bit at the front, called a pilot. He makes it go up'

HP

In reply to Hairy Pete:

Did she slap you?

In reply to centurion05:

You're a tit, i work on aircraft and it won't take off.


Working at "Thomas Cook" does not mean you work on aircraft.

In reply to arby:

Incorrect - I design industrial hygrometers


Correct ish - depends to what level you mean, my masters degree specialised in aeronautics and flight control but you're right, I'm no expert on A-Level physics

In reply to jkarran: I think the pilot would have got bored by now and gone home, that or his shift has ended or his union has stepped in.

In reply to pale ale:

Blimey, is this still going?

A final thought:
Even in a thought experiment, there can be no such thing as a "perfect feedback loop". Feedback is limited to the speed of light and therefore there must be a lag between the action of the plane and the result, the movement of the conveyer belt. A synchronous action/feedback mechanism is, at any rate, a causal absurdity.

In reply to pale ale:

So what was the answer? Can't be bothered to read 300 plus posts. I would have thought that if a plane on a conveyor belt could achieve vertical take off, there would be quite a few people doing it.

In reply to dark_star:


He's probably still trying to manoeuvre his jumbo jet onto the travellator at Heathrow to obtain final proof...

In reply to Steve Parker:

Yes, it takes off. The wheels are merely friction removers and have no bearing on the forward thrust imparted by the engines. Interesting to watch people hash this out though

In reply to Beaver: Ah, so you have read his previous posts too?

In reply to Ste Brom: haha, yeah!!

In reply to Smitz:

Ah, gotcha. I guess it's an easy assumption that the wheels are doing the driving, when of course they're not driving anything and are pretty much irrelevant to the forward motion, apart from not braking it.

In reply to Richard:

I agree with your initial answer (at least once I had read the question properly). But what's all this about the conveyor belt having to be able to accelerate to near infinite speeds? Initially the plane and the conveyor belt are stationary. If the plane ever reaches a certain speed (say 150mph) then it takes off and the experiment ends. The plane's speed is a continuous function of time presumably, and therefore it the speed of the plane on the conveyor belt is bounded between 0 and 150 mph. Thus the conveyor belt's speed is bounded between 0 and 150 mph for the duration of the experiment.

In reply to pale ale:
The wheels will be turning at twice the speed of the conveyor belt / plane, which takes up the opposing motion

In reply to pale ale:

This is still going? Incredible.

In reply to pale ale: Excuse me, harriers take off (not in vertical mode) when the carrier is going in the opposite direction, the ground (sea) momentum has nothing to do with the plane taking off.

In reply to crossdressingrodney:

The question is badly worded, admittedly - but what some people are arguing is that the conveyor belt moves backwards to counteract all of the plane's forward movement - not just to equal its speed. In this case, to supply sufficient friction to prevent the plane from moving against the thrust of the engines, the conveyor belt would need to be moving very very fast indeed.

Think of a model plane, say, on buttered skis on a conveyer belt. The butter reduces friction but not by much, so the conveyor belt can balance the force from the engine and the plane can stay stationary. Wheels reduce friction but do not eliminate it entirely (and also require additional force to accelerate them angularly). A jet engine is powerful but not almighty. Consequently the forces can again theoretically be balanced (to hold the plane still), but it requires a very fast-moving conveyor belt.

As someone else said upthread, if the conveyor can apply a small force to the plane, it can apply an arbitrarily large one. And if it can apply an arbitrarily large force, it can prevent the plane from taking off.

In reply to Eric the Red:

Yep, the conveyor belt bearings are getting hot though.

In reply to Steve Parker:

The answer is that the aircraft takes off, although not vertically, as your post suggests.

People are thinking that the aircraft will not move in relation to the air. It DOES move forwards, at the same speed it would do if it was on a normal runway (minus a small fraction due to friction on the wheels).

Therefore, the reason this will not work on an aircraft carrier, or allow VTOL of a regular aircraft, is that the aircraft would still need the exact same length of runway, no matter what speed the ground is moving.

I cant believe this is still an ongoing descussion! This post, and my previous post, explain the way it works!

Its physics, no matter how long it is debated on here, thats the way it is. The aircraft gets airborne from the runway, and then can land perfectly normally back on it too. Although taxiing off a moving runway onto a static taxiway could get interesting.

In reply to Enigma: I think we agree about the carrier dont we. Also I have another observation. I have watched Harriers take of and land in vertical mode on a carrier at sea. I asked the pilots if it was difficult landing in vtol on a moving ship. They said no, the ship automatically becomes stationary ground once they hover above it. Now they may be wrong, maybe the planes are programmed and they didnt care why, but thats what they said.

In reply to Al Evans: i think what they mean is water (just for this example) is still boat is moving at say 25knts to hover over the boat, the harrier needs to bemoving forward in realtion to the sea, 25knts it is therefore matching the speed of the boat and so appears to hover

also the harrier is now one of the very few navy aeroplanes that takes off without the aid of a steam catapult

In reply to rallymania: They dont use vtol though, the run up the ramp.
Actually I have the usual boring Al Evans story here.
We were flown out to the ship from Benbecula by helicopter, thats another story.
As we landed on the deck and unloaded our gear our 'assigned officer came running up ans said
"The Harriers are taking off in 5 minutes, and this will be the only time they take off in daylight while you are on board."
"Ok " I said, "initial shot from the edge of the ramp for the first one and I'll film it coming and whip pan round with it as it leaves the ramp and flies off"
Turns to officer.
" Then I'll assess how that worked and reposition if neccessary, how long between each take off?"
" 4 seconds"
cue 4 identical shots

In reply to Al Evans:


Did you find the helicopter initially really exciting, and then five minutes in realise everyone else was asleep and start to wish you'd brought a book?