The case of the plane and conveyor belt
 

This isn't actually new, in fact it's quit old. But is has baffled everyone from web bloggers to pilots and Mythbusters will try to find out on Dec. 19.

Here's the problem: "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).

Take your time and think about this for a few moments.

Now, does the plane take off? :rly:

I wouldn't have thought so, as the air isn't as 'mobile' as it would be if the plane were hurtling through it, no?

Instinct tells me no, as I'd assume that there'd be no airflow around the wings and hence no lift could be created.

Is anyone else doing planes and bands with their hands?! :p

nah.. just you hahahaha

Hmmm ... did you consider all aspects?

Engines?

No, the conveyor is fine and there are no helicopter engines.
Why do you think the plane does not move forward?

Yes.

The wheels are just holding the plane up. It'll take slightly longer to get up to take-off speed, because of increased friction and rolling resistance losses in the wheels/tyres (which are rotating twice as fast as normal), but that's it.

Dave

female pilot.... she forgot to remove the handbrake

i like that theory alot

http://media.mlxxfc.net/Fly-NotFly.gif
Yep.

your both very smart.. well done you two.. that had never occurred to me

that's what I said, albeit not quite so straightforward :p

anyway, it would appear we're both wrong; Daveake is a bit of a brainiac by all accounts :D

Well you can't have looks and too much brains can you? :))

I just nearly choked on my sandwich! :shock:

couldda been worse.. couldda been your sausage

:whistle:

Yeah, I couldn't even spell 'engineer' when I went to university, but now I is one. :twisted:

Dave

this went a bit off topic, didn't it lads?

seeing as we're off topic anyway ;) you could read my blog :D

I think I know which side of that equation I sit ... :twisted:

Dave

Ok, I don't get that, I'll admit to not being the brightest bulb in the box but that goes straight over my head.

Now if it was propeller driven I'd understand it (wind flow over the wings creating positive uplift and all that) but not if it was jet engined........or have I got my muppet head on?

Nothing to do with jet vs prop propulsion.

Think of this scenario ... plane is sat on this conveyor belt, engines off, brakes off (that's the important bit), and someone turns on the belt which is now running backward at 200mph. What happens to the plane?

Dave

Inertia of the plane would make it go backwards?

Sorry, but I sense this could go on a while ...

No. Plane has no inertia 'cos it's not moving. When the conveyor starts up, the plane stays not moving*. So it's just the same as being on the runway. So if the engines are now turned on the plane can take off as normal.

* Actually it will start moving backwards a little. But not a lot. The wheels/tyres/bearings aren't perfect and will allow some of the conveyor movement to push on the plane a little.

I'm kinda out of explanation, so if you don't "get it" I think you'll stay not getting it.

Dave

Actually, I'll have one more go. Imagine you're wearing roller skates. Imagine that you're at the start of a runway and you get one helluva kick up the backside. You're now doing 200mph down that runway. Do you stop immediately? Nope. You keep going. Eventually wind resistance slows you down and you might eventually come to a stop but probably not before you run out of runway.

Now imagine the same thing, but instead of you moving at 200mph, you're stationary on a conveyor that happens to be moving 200mph backwards. You'll start moving backwards, but only a little (unless you bought very cheap roller skates). If the belt is really really long you'll eventually get thrown off fairly quickly to an untimely death, but the point is that you don't suddenly get chucked backwards at 200mph. You get saved by the wheels on those skates, which spin away like bug-gery.

Having survived that experience, you metamorphosis into a big jet plane. The belt starts moving, and you feel a little tug on your wheels. But you've got some BFO jet engines which are much more powerful. So you turn those on and easily overcome the resistance of the wheels. You roll forwards, and then take off to that holiday in the Galapagos that you've been saving up for.

The actual scenario in the root message was easier to overcome - the belt only starts moving backward when the plane moves forward. The trick though is that it starts making people think that the plane can't take off because the belt matches the plane's speed, like a belt down at the gym. Fortunately planes have unpowered wheels and not legs.

Dave

Can everyone see the lightbulb go on above my head?

Cheers Dave

"we have lift off" lol

So, could that be the solution to mega long runways for the A380?

I would patent that idea!

(Holy Thread Resurrection, Batman!) ...

And here's the result from those Mythbusters guys.

YouTube Video

I didn't spot this first time it was posted, think it's a cool conundrum, and am now going to irritate everyone I know with the question! ;)

Yeah, I skipped over this one cause R. usually posts videos which I can't watch at work. However, then I missed it from home too...nothing much to say but I amazed myself by actually understanding what Dave said first time around...on the other hand...he started to scare me even MORE...

dave can be quite scary, especially when hes driving :lmao: :p

You didn't get into the car he was driving, did you??? :shock:

Try being in Hennie's car next time then. He thought he was Michael Schumacher!

Why not? It's the best way to avoid being run over ;-)

Dave :twisted:

What, you mean desparately trying to keep up with the English guy in front? http://i19.photobucket.com/albums/b1...ck/driving.gif :lol:

Dave

Good argument, very eloquently presented...:up:

I did eventually figure this out lol