Critical thinking puzzle.

[Hazen]

Without foward motion there's no air being pushed over\under the wings (besides ambient)
Without air motion over the wing surfaces there is no lift
No lift = plane not taking off

Agree with this: airflow over the wings generates lift, hence the reason aircraft take off into wind, to maximise the effect for forward ground speed. Same goes for prop powered

[Admin]

I think the trick in the question is that it leads us to assume that because the belt is running backwards that it prevents the plane from moving forwards. In reality it will only make the plane's wheels spin twice as fast.

If we had Chitty Chitty Bang Bang trying to take off under those conditions it would never move forward as its propulsion is via its wheels. I think that's the picture that we get with this puzzle - the belt preventing forward motion.

A plane, however, does not use its wheels for propulsion. It uses a jet or propellers to force air backwards (and itself forwards) and that force is not cancelled out by the conveyor belt.

An analogy is to have a rocket at one side of the runway which is fired forward. The belt will have no impact on the rocket as it's thrusting through the air above it.

Now, put the rocket on wheels (an ACME rocket :D) and fire it. It will thrust forward in exactly the same way as before and the only effect on the wheels is that they will spin faster than they would have done if the belt weren't moving.

I don't know if that helps visualise it better, but the plane will take off as it thrusts forward in the same way as a rocket, not a car. Trust me, I'm a smart arse.

[Hazen]

Good point. Engines push against air not ground.
Conceded.

[misquote]Trust me, I've a smart arse.[/misquote]
And a very clever backside it is too.

[Mongrel]

OK - my answer (not that I'm hedging ) was based on the following assumptions;

The original question is aimed at laymen, thus imprecise language therefore I read it as "as the plane applies more power the conveyer belt moves faster to give a net ground speed of 0". From a standing start the pull\push from the motor still has to use the wheels and their friction Vs. the ground to achieve foward movement > increased air flow > lift. All the while the lift is less than the weight of the plane it's not going anywhere.*

I think of it as a a glider strapped to a big truck. Get the truck up to 80MPH (not real number) and the glider will have enough lift to take off. Put the same glider and truck combo on the conveyer and accelerate the truck to 80 and the glider pilot is in for a long afternoon...

*And yes, helicopters, harriers, Osprey tilt rotors and the few other VTOL craft are disallowed in this experiment*

[Admin]

It's just a game and I haven't done it yet but I thought I'd post it before I forget: http://www.theconspiracygame.co.uk/conspiracy/

'Tis a conspiracy!!! :o

[seren]

There are various different wordings of that plane puzzle and people do seem to agonise over the language.

I think the consensus is that it would take off (for the reasons John has mentioned), and in fact some small scale experiments have taken place to prove it. But there are some who won't be swayed and say it isn't possible.

Someone must have enough money to build a giant conveyer belt and finally put this one to bed! :)

[Admin]

I think that it's necessary for the wording in these puzzles to be less than perfect otherwise spelling everything out would give it all away.

I'll do this experiment when I take delivery of my first Lear Jet. 8)

[Skepticus Rex]

if the conveyor belt adjusts it's speed to be the same as the plane (not sure how that would work, but for the purposes of the problem lets assume it can do that), then it doesn't matter how the plane is moving, it's speed will be adjusted to.

now, the plane is sitting on the conveyor belt on it's wheels, so as the conveyor moves, the wheels (and therefore plane) are pulled backwards. the engines pushing against the air are pushing forward against this motion, so it stands still.
remember,
"Without foward motion there's no air being pushed over\under the wings (besides ambient)
Without air motion over the wing surfaces there is no lift
No lift = plane not taking off"

so the plane will not take off.


it makes sense if you don't think of the plane as being held stationary, think of it as being pulled backwards and pushed forwards at the same time. this is exactly the same as being held stationary, but you remember why it's held stationary, and so don't get distracted by how the plane moves, which it doesn't matter.

[Admin]

A conveyor belt spinning at, say, 150MPH will not push the plane back at 150MPH it will simply spin its wheels at 150MPH.

The belt will offer no resistance (to all intents and purposes) to the forward motion of the plane so as soon as thrust is applied it will move forward as normal.

The plane will take off as normal but its wheels will be spinning at twice the speed that they would have done had the belt not been turning.

[Skepticus Rex]

but the plane is actually resting on the conveyor belt. the axle can't be frictionless, so the conveyor belt wouldn't spin the wheels backwards, it would move the plane backwards, at the speed it moves.
the plane engine will cause the wheels to roll forward, at whatever speed it can push them (which is the speed the conveyor belt matches), and that will move the plane forward over the belt.
however, the belt is moving backwards, so the two motions cancel out.

[Mongrel]

The way I got my head around it eventually is to think of the fuselage as having a long threaded bar through the middle, due to the way it's fixed at the ends the bar rotates and the plane travels along it's length. This represents the Jet\propellor pulling the craft foward.

Now stick this contraption above a conveyor belt - no matter how fast that conveyor is going the body will still move along the threaded bar.

[Ginger Rogers]

Actually I thought it was going to be more likely to be another boy.

As I recall from genetics at school (oooh that was a while ago now) Boys are Y chromosomes and girls are X. Men have only Y chromos, women have X's and Y's therefore the odds are already higher of a child being a boy.

You see more families with several boys than girls don't you?

some people keep having kids cos they want a girl but keep getting boys. Like my aunt who had 3 boys and victoria beckham.

Please correct me if i'm talking cobblers!

[Admin]

but the plane is actually resting on the conveyor belt. the axle can't be frictionless

Yes there will be a small amount of friction caused by the spinning of the wheels' axles; but this is the only force the forward thrust has to counter.

The key to this puzzle is in understanding that the plane's propulsion is independent of the conveyor belt (i.e. it's not pushed forward by its wheels).

[Skepticus Rex]

The key to this puzzle is in understanding that the plane's propulsion is independent of the conveyor belt (i.e. it's not pushed forward by its wheels).

when a plane is sitting on the ground, it's weight is on the wheels, yes?
when a plane taxis along the ground, it's weight is still on the wheels, but it is pushed forwards by the engine, so it moves along, but at a steady height, so the weight is still the same on the wheels (lift will negate some of the weight, but only at higher speeds).

the conveyor belt moves backwards, and anything on it will move bodily backwards, if no other force is on it. so a plane sitting on it will move entirely backwards (as long as the conveyor belt has enough power to move that much weight), the wheels will not turn.

when the engine is pushing forward that will push on the wheels, making them roll forward. but the surface that the wheels are resting on is moving backwards at the rate that the engine is pushing them forwards (that is the only way of thinking of the conveyor belt matching the speed of the plane, because if the plane was not moving, it would have no speed, and so the conveyor belt would stop).
therefore, the two movements will balance out, i can't find the words to explain exactly what happens to the wheels. i might make a flash animation later to demonstrate.
this means that the body of the plane will remain completely stationary, even with engine on full tilt. just the wheels will rotate on the axle.
and because the plane isn't moving, there won't be any extra air passing under the wings, so there won't be any lift.

[vbloke]

Yes, the plane can take off. The key is that the plane's wheels *freewheel*, they are not driven. the conveyor belt therefore provides NO force to the plane (OK, there's a little friction in the bearings which could provide a couple of pounds force to a normal aeroplane, but that's insignificant compared to the thrust. For a RC plane, the situation should be similar). The prop pushes the plane through the air until it reaches takeoff AIRspeed, which is the same no matter how fast the wheels are going. So if you were inside watching the instruments, you would observe that the wheels are spinning at a GROUNDspeed different than the indicated AIRspeed, but that does not matter, since it is AIRspeed you need in order to take off. (Caps added for emphasis).