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[Starver]

This forum has showed for me that we have here a lot of men with above average mental abilities (sorry for my English). And I have a challenge for you!
I want to test it on a simple question concerning physics and logics. The question itself is a meme and I want you to think it yourself, without using google.

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?

So, what do you think of it?

Yet to read the thread, but my take on this (waiting to proven wrong) is... Yes.

Assuming that the drag from the free-rolling wheels never becomes excessive or there's any other mechanical failure (when achieving minimum 'normal' take-off speed, one assumes that the wheels will be rotating at twice the speed they'd do on a stationary runway) and that the runway itself never exerts significant movement on the air (although this should actually assist take-off at a lower speed, methinks, with perhaps problems with stalling as/when the plane rises outside of this 'move ground effect' zone), then the absolute main thing that matters is that the engines push the plane forward through the air fast enough for the relative motion of air and wing to create sufficient lift.

If for some strange reason, the launch mechanism relies upon relative ground speed (e.g. catapult, or a wheeled trolley that launches a glider) then I see no problem if this device could be given twice the usual take-off speed.  But that's not relevant to a self-powered air-thrust propulsion system (jet or propeller or rocket or even flappy winged ornithopter)... leastwise, as I say, there's no mechanical problem with the craft/ground interface having to withstand double the 'normal' translational forces, or excessive rolling resistance due to this.

Anyway, you're on page three of this thread, by now, and I'm now going to read on and see if someone's thought of something that I haven't.  (And which I agree with, naturally.)


(Oh, and I've assumed you mean "tracks the speed w.r.t. 'stationary' ground and tracks the non-stationary conveyor in the opposite direction.  Tracking the speed w.r.t. the conveyor would... depending on how one interprets that, either allow the moving conveyor to be stationary and still obey the technical stipulation given, or cause a positive feedback loop that would be ridiculous.  Still, I'd say yes while the landing gear behaved.)

[PanH]

I think the one question about planes that really confuses me is how in hell do they fly upside down for extended periods without crashing?

They don't. You can slow it, but you'll always go down.

[Starver]

The same way that a plane going at more than minimum take-off speed[1] can go down.  i.e. angle itself so that (inherent 'lift' component' of the wings aside), the total vector of its thrust+wing aims above/below the horizon, according to what you're trying to do...

Think of a Stuka dive bomber...  Its going fast.  Its wings must producing a lot of 'lift', yet it's going down.  How?  Because its 'lift' is perpendicular-ish from the direction of travel, but it's aiming down, with both gravity and airscrew propulsion speeding it down.

A plane upside down just has to angle its nose to above the horizon ('down into the sky', that is, rather than 'up into the ground') enough that (stall conditions permitting) all the forces balance out in a skyward-direction (or totally horizontally for constant-altitude flight). 

IYSWIM.

Of course, unlike the Stuka, gravity is opposing this attempt, and there may also be problems with fuel pumps (or the pilot's own circulatory system) conking out if the forces on the plane(/pilot) are outside of the expected parameters, but it could continue for as long as the plane has fuel (and the pilot has control).



[1] Deliberately specified.

[misko27]

The MythBusters did this. The answer was yes. http://www.youtube.com/watch?v=0KsdMuhYJPw
 
Thread over.

[misko27]

And now the US is officially best place in world. For completely arbitrary reasons, as it should be.
 
http://nslog.com/2008/01/31/stupid_people_refuting_mythbusters_over_planeconveyor_belt
 
Sums up. Look up "plane on conveyor belt mythbusters".

[Facekillz058]

This is an awesome question.
Brb, doing math.

Fake Edit:

Spoiler (click to show/hide)

Okay, the plane wouldn't take off.
If you guys want to know how I figured that out in a few seconds, let me know.

[Starver]

And now the US is officially best place in world. For completely arbitrary reasons, as it should be.
 
http://nslog.com/2008/01/31/stupid_people_refuting_mythbusters_over_planeconveyor_belt
 
Sums up. Look up "plane on conveyor belt mythbusters".

My favourite quote (of a person with obviously no understanding): "Do you REALLY think that the NAVY would spend BILLIONS of dollars on catapult systems for aircraft carriers, if they could have just gotten away with having a treadmill instead."

Just got to see a video of it.

The plane was moving horizontally... I thought the point of it was that the plane DIDN'T move horizontally?

It isn't.  Read the premise again.  Read that aforementioned quoted article, as well, if you haven't already.  And read it again if you have.

[Facekillz058]

I really think this question is super simple if you aren't thinking about it that hard.
Example:

Spoiler: !!SCIENCE!! (click to show/hide)

Simple diagram tells it all.

[Aqizzar]

I think you guys are missing the idea of the question.

The issue that you're coming back to is: if the same amount of force it would take for a plane to begin flying were applied to the body so that the force is canceled, it would not fly because there would be no air movement to lift it.

This is true.

The counter argument demonstrated by the MythBusters is that because the wheels are spinning freely, the minute amount of friction generated by the conveyor under the wheels is easily overcome by the natural pull of the propeller and the plane takes off exactly as if the ground were stationary, just with its free spinning wheels freely spinning at a higher speed.  The effect described in the graph would be achieved by tethering the plane, not by moving the surface its sitting on.

[Starver]

This is an awesome question.
Brb, doing math.

Fake Edit:

Spoiler (click to show/hide)

Okay, the plane wouldn't take off.
If you guys want to know how I figured that out in a few seconds, let me know.

I'd dearly love to know.

I could do some maths of my own, but what's the point in making terms for absolute ground speed, relative-to-runway speed, air-speed, and positions relative to air/ground/runway?

Plane whirs engines,of whatever design.  These engines, on windless day on a stationary runway would propel the plane forward at V_TO, or the take-off speed necessary.  They do this by acting on the air.  If the runway is designed so that as the plane moves (say) in one direction by a given speed then the ground moves in the other direction by a given speed... well, it doesn't matter[1], because the engines aren't pushing against the ground.  The engines are pushing against the air, which is still assumed to be still.  The plane moves in the direction it should do, the rebellious runway moves in the other direction.  The wheels of the plane, free-wheeling, are rotating at twice the speed that they would do in a normal take-off, but unless the bearings catch fire or something else goes wrong this has no (*ahem*) bearing on the speed of the plane...  Even in a fully integrated RL example it might mean you'd have some marginal extra friction, but that should be well within the ability of the plane's engines (either through a percentage rise in engine speed, or just taking slightly longer with a smaller acceleration).  The aircraft is moving, though, and reaches V_TO, air speed.  A figure you'd double to work out the speed relative to the backwards-rolling runway.


Right, in the light of the two most recent replies (ignoring Aquizzar, who has additionally ninjaed while I was suffering delays in getting my preview), I'm going to put this in bigger text...

The plane is not stationary!  The plane is moving in one direction, the runway in another.

As someone else (several someone else's!) pointed out, if the plane were not moving, the runway would not be moving.  In the example of a car on such a runway going at a runway-relative speed of 30mph, the runway would have sped up to 15mph backwards in order to match the 15mph forwards travel of the car.  Or, to put it another way, whatever speed the runway things the car is going, relative to stationary, and thus what speed the runway is going backwards from stationary, the car is going twice as fast...

So, for a car, to go a certain speed, you'd be driving twice as fast as you normally would need to...  But you're not in car, you're in a plane.  The plane doesn't care about the runway[1], it just cares about the air.  Which is stationary.  So to go forward at a given speed it...  uses as much engine power as it normally needs to go forward at that speed.

(If the air were not stationary, it would be pushing against/with a wind-speed in its attempt to attain a similar relative velocity w.r.t. the air (stationary or not) in order to get lift.  The only problems here is if you run out of runway.  With a headwind, you'd need less runway anyway.  The maths actually comes out that if you had the wind going one direction as fast as the plane were going in the other direction, then you'd have the "car" example, again, of the plane going half the speed forward and the wind half the speed back in order that the plane's speed relative to the air is the full amount needed to attain lift.  Which it could do as easy as (if not easier than) doing so in still air.

Newtons don't really come into it at all[1], for all these examples.

[1] Any friction excepted.  Which is going to be largely irrelevant for any plane without jammed-up wheels in the first place.

[mainiac]

People get angry about this question because the originally popularized version of the question itself was logically inconsistent.  The phrasing usually implies two contradictory things, which boil down to:
1) The plane is held stationary because there is friction between the wheels and ground
2) The plane is not held stationary because there is not friction between the wheels and ground
...these assumptions are hidden under the unclear wording.

People get angry at each other as a result because we only notice either assumption 1 or assumption 2.  We should actually be angry at the fucking moron who came up with this bullshit question.

The inconsistency is not present in the particular version of the question that the OP gave.  In the OP version someone ironed out the logical inconsistencies by making a situation where the plane just goes half it's normal speed.  But one should never express disdain for those who see this question differently because it's the questions fault, not the person.

[Starver]

First of all, apologies if I've seemed angry, at all.  Mostly I'm just amused.

I don't believe the posited scenario has ever actually mentioned friction.  (I do, in my treatments, if only because there'd be some in a RL case, but I believe it would be largely inconsequential.  When not catastrophic. )  If it did mention friction, that still wouldn't make it a zero-sum situation.

I'm also surprised to see you say "by making a situation where the plane just goes half it's normal speed".  That'd be the car-on-the-conveyor example that did that.  AFAIAC, a plane that normally goes at 100kn to take off manages to go 100kn and takes off.  200kn ground speed, of course, relative to the dynamic airstrip, but 100 knots 'absolute', while the ground is hauled backwards at 100 knots in mirroring this.


You want a silly question?  I often recite an "IQ test" question I once tried.  Ok, so it was from a "page a day" calendar  with IQ(-ish) questions on each, so pretty low on any scale of authoritative IQ testing, but it was so not an IQ test question.

First of all, it gave four anagrams.  I won't go into why that's not a test of intelligence, and instead I'll give the words they (easily?) solve to....  "Tent", "Kennel", "House", "Cave".  The ultimate question being "Which one is the odd one out?"

Spoiler: If you don't want to know the 'answer', don't open this... (click to show/hide)

Which one is not the odd one out?  Actually, if there was one that wasn't the odd one out, then that word could automatically be argued as the odd one out.

They're obviously all residences, and there all have any number of reasons why they're different, but here's my 'best' reason for each.  YMMV.
1) "Tent" is the odd one out, because it's a temporary structure.
2) "Kennel" is the odd one out, because humans have routinely lived in all the others.
3) "House" is the odd one out, because it's the only one that has proper windows and doors/chimney/carpets/whatever you care to mention.
4) "Cave" is the odd one out, because it's not been constructed.

Other reasons include: "House" is the only word with an odd number of letters; A "Cave" is a negative space; "Kennel" has a repeated vowel; "Tent" starts and ends with the same letter; etc etc.

...but whether you peak or not, feel free to opine.

[mainiac]

I don't believe the posited scenario has ever actually mentioned friction.

You are right, it doesn't ever mention friction.  This is a big problem because friction is a huge part of the system being modeled.  Without friction we have no question.

Consider the question if we assume zero friction between the plane and the ground.  Then the conveyor belt doesn't matter.  The plane takes off as usual.  There is no question because the entire experiment was rendered irrelevant.

The design of the question usually implies that there is a non-zero amount of friction.  It is usually implied or stated that the conveyor belt could move the plane if the engines weren't on.  That means there is friction which is a terribly important detail.  But it's a terribly important detail that we are given no information about.

I'm also surprised to see you say "by making a situation where the plane just goes half it's normal speed".  That'd be the car-on-the-conveyor example that did that.  AFAIAC, a plane that normally goes at 100kn to take off manages to go 100kn and takes off.  200kn ground speed, of course, relative to the dynamic airstrip, but 100 knots 'absolute', while the ground is hauled backwards at 100 knots in mirroring this.

Yes, it would take longer to get to this speed (assuming it could indeed get up to speed under these new conditions), which I described as "goes half it's normal speed".  So we are saying the same thing.  But the ambigious as hell wording makes us suspect we aren't saying the same thing (or are we 0.o).

[Soadreqm]

Why are people suddenly talking about the plane again? I thought that was solved already.
1) Conveyor belt matches the speed of the plane: Plane takes off normally, conveyor belt moves the opposite direction at the same speed, wheels spin at twice the normal speed.
2) Conveyor belt matches the accelerating force of the plane: Plane remains stationary, conveyor belt moves CRAZY FAST, wheels melt, plane smashes to the conveyor belt. If the conveyor belt hasn't also melted at this point, the plane is propelled backwards with a shower of sparks. Some parts of the plane might take off, depending on how it hits the conveyor belt, and whatever is behind the conveyor belt.
3) Case 2) except both the belt and the wheels are frictionless: The centrifugal force tears the belt and the wheels apart. The plane takes some damage from shrapnel, smashes to conveyor belt as before.
4) Case 2) except both the belt and the wheels are massless and frictionless: Okay, I don't think anyone mentioned this before, and it seems kind of tricky. Wheels become tachyons?

[Sheb]

Well, if the wheel are firctionless, they won't turn at all. The conveyor belt will break speedlight in a fraction of a second, and collapse into a black hole from all the extra energy. Then suck the plane in.