Airplane on a Conveyor conundrum

[Apache]

No they dont!

They are just very good with hypnotism. They walk everywhere, but convince us that they're flying.

Next time you see a bee, stare at it and listen...

"Look into my eyes not around the eyes into my eyes"

[Andy Dawson]

Hum, will try and explain my reasoning again!!

The thrust of the engines try to push the aircraft forward

In order for the aircraft to move the wheels have to rotate, this rotation is countered by the action of the conveyor-belt.

Thus the aircraft does not move forward and thus generates no air flow over the wing and thus no lift is generated – no lift, no flight

The means of getting the aircraft to move forward is irrelevant; air will still be sucked into the engine, its velocity increased and flow out of the tail pipe regardless of whether the a/c is moving or not!

The only factor that is important is forward movement know in aviation circles (so I believe) as ‘Airspeed’ – no airspeed, no flight!!!

Imagine you are on a conveyor belt, if you walk in the opposite direction to that of the belt with equal force you will remain stationary relative to your surroundings. Providing you and the walkway maintain the same relative speed you will be going no where fast or slow!! The action of the conveyor belt vs the a/c attempt to move is the same.

[Slugsie]

Slugsie, Nero, this was the premise.

"You have an airplane sat on a huge conveyor belt. The conveyor belt is computer controlled such that its speed is always the same as that of the aircraft - only backwards. I.e. If the aircraft is moving forwards at 10mph, then the conveyor will move backwards at 10mph"

1) No-one stipulated air or groundspeed.

However, it is reasonable to assume that when "its speed is always the same as that of the aircraft - only backwards" that it means there is no net speed differential. Therefore, there can be NO LIFT. Therefore the f***ing aircraft WILL NOT FLY!!! Not in my universe where the laws of physics apply!

Unfortunately that is not a reasonable assumption. Like I said, for the conveyor to halt the forward motion of the aircraft, it will have to excerpt a pulling force - acting through free spinning wheels - on the aircraft. Is that possible?

Hands up all those who have designed and bolted stuff on aircraft?

What about those who have flown aircraft of their own design (albeit models - same rules apply) and had such designs reviewed by peers and published?

No? Then shut the f*** up with your nonsense!

Again, not relevant. When mythbusters did the experiment, the pilot who was flying the aircraft thought it wouldn't take off. He was proved wrong.

The problem is that most people are using their everyday experience of cars. A car doing this would indeed not move. But it's not a car, it's an airplane. The move through the air using a different method of propulsion.

(PS - really no offence intended, but what a great topic after months of 'here's a spammer' and 'hi I'm a newbie should I buy a Surf' type threads!)

None taken, it's fun to have a good intellectual debate occasionally.

[Apache]

Andy, you're approaching from the same angle as me. Slugsie take note.

Where the problem arises is speed needs to be relative to something. In the original premise, speed relative to the conveyor (the assumption I made) - no flight.

Speed relative to some fixed point - possible flight.

Speed relative to airflow (ie Airspeed) - the only one you can definitely say there would be flight.

Slugsie, to take your car analogy, its the same. If the cars speed is relative to the conveyor, it wouldn't move. If its relative to an external fixed point then it would.

The Mythbusters pilot is a fool and probably should not be flying planes. However, its not the same as the original question because his and the conveyors speed are not limited by the original premise. His speed is not relative to the conveyor.

[KERRSURF]

I voted.

It will fly.

[gwh200]

kinematic equations explain,with a touch of calculus,every thing ,in the macroscopic world that has motion.they can describe motion and its effects in gret detail,so far from being irrelevant they describe what is happening to to the plane and belt far better than some bloke wearing a beret !!
i covered this sort of stuff whilst in the first year of my physics degree and its an old lemon,it is all to do with moments,kinematic equations

(try them all dude you get the same result)

and momentum(nominal and angular)

in an ideal premise as forwarded by yourself,it wont fly.in a myth busters
premise it will,but it aint great science that fails to duplicate the lemmas proposed

[da SLUG man]

The problem is that most people are using their everyday experience of cars. A car doing this would indeed not move. But it's not a car, it's an airplane. The move through the air using a different method of propulsion.

yes, but the wheels need to roll to get it moving...

to get any forward motion, the wheels must be travelling faster than the conveyor... which is impossible.

but without forward motion, there would be no wheel movement!!... so the conveyor wouldn't move either!!... so you'd get forward motion... and the conveyor would start... but then stop again... it would never end!!!


i've gone from yes, to no... and now i'm back at yes again!!... the plane WOULD take off!!

[Apache]

far better than some bloke wearing a beret !!

Dont forget the extremely dodgy facial hair. No way I'd trust him!

[Apache]

to get any forward motion, the wheels must be travelling faster than the conveyor... which is impossible.

No it isn't. The thrust is supplied by acting on the air (a planes wheels aren't turning when its in the air, but it moves forward)

The issue is simply with the wording of the premise.

1) If the stated planes speed is relative to the conveyor, it wont fly.

2) Relative to a fixed point outside of the conveyor, it might, depending on windspeed.

3) Relative to the surrounding air, it will.

Anyway, I've whored enough in this thread! It's been a blast of fresh air (moving with enough velocity to create lift)

[soramad]

so then by the same token, if you fire a gun attached to the outside of a aircraft the bullet of which travels at 600 ft per second and the aircraft is travelling at 601 ft per sec, both are facing the same direction, would the bullet leave the barrel?

[nero279]

C'mon guys think about it, the fact that the wheels are freewheeling means there's no transfer of force between the conveyor and the body of the plane, the conveyor's force is acting on the wheels.


Conversely the propulsion generated by the planes engines does act on the body of the plain, meaning the plane must accelerate.


Relative to a fixed point or relative to the conveyor, airspeed/groundspeed, whatever, it has no bearing.

[Chillitt]

Can we just go back to basics for a moment?
When the plane attempts to move, it is powered by its own engines, yes?
And the conveyor belt. it moves in the opposite direction to the plane, but at the same speed, yes?
If so, then the plane will take off, but the wheels will be turning at twice their normal speed.. Or have I oversimplified things?

[gwh200]

there does not need to a direct mechanical link beween the propulsion and
and the wheels.you frewheel down a hill,what would happen if halfway down the hill the road turned into a belt and matched your road speed,,you would slowly descend (by v=u+1/2 at^2) the hill as your mass would over come the negative net effect of the reversing road.

ie,
if the car is x and the road is y and we know x = y (car velocity and moving road speed) then adding something to x (the cars mass, acting not now downwards,but towards the front of the car,albeit at an angle) and we call that m

then we know x = y
but xm doesnt = y and can over come it , till x tends towards zero(when the moment of force once again reasserts itself straight down

[Slugsie]

OK, the original premise stated that the conveyor matched the aircrafts speed. I will admit that there is a problem of which speed the conveyor matches. Does it match the airplanes speed relative to the fixed ground, or the conveyor its self. This was not an attempt at tricking anyone, just poor wording. My intention was that the conveyor would match the speed relative to the fixed ground.

But let's quickly examine the other one - relative to the conveyor.

So, the aircraft sets off, and achieves a speed of 5mph relative to the conveyor. So the conveyor moves at 5mph in the opposite direction. But this now means that the airplane is moving at 10mph relative to the conveyor (5mph from the airplane, 5mph from the conveyor). So the conveyor must speed up to 10mph. But now the aircraft is moving at 20mph relative to the conveyor, and so on. This quickly leads to a conveyor that is moving at infinite speed. This is impossible.



Going back to my restating of the problem using the analogy of tsanding on a skateboard pulling on a rope. I'll ask again, does anyone truly believe that you will not be able to pull yourself forward?

[Apache]

Relative to a fixed point or relative to the conveyor, airspeed/groundspeed, whatever, it has no bearing.

Of course it does! 10mph forward RELATIVE TO THE CONVEYOR travelling 10mph the other way results in 0. Its irrelevant how the force is delivered in this case, whether through wheels or through air thrust.

Soramad, the bullet will travel at 1201ft/s initially but will slow quicker due to the extra 600ft/s worth of drag.

Anyway, I said no more whoring for me, so I'm off to watch telly!