Will it take off ?!

Post by **Zeus** » 22 Feb 2006, 15:38

Ok, last night whilst in the pub I was arguing with a mate over a problem that his physics lecturer had set him. By the end of the evening, neither of us had persuaded the other that he was right, but had managed to confuse about twenty people who we explained it to. Now before you vote or add your comments, just take a moment to think about it !

The problem goes like this:
An aircraft is standing on a runway that can move (a conveyor belt). The aircraft moves in one direction, while the conveyor moves in the opposite direction. This conveyor has a control system that tracks the aircraft's speed and tunes the speed of the conveyor to be exactly the same, but in the opposite direction. There is no wind. The pilot begins to add thrust to the engines...

The question is:

Will the plane take off or not?

Post by **murf** » 22 Feb 2006, 15:42

Doesn't it need the wind/air on its wings to provide lift??? Therefore could only theoretically take off if into a very strong headwind!

I think.

Post by **The Ledge** » 22 Feb 2006, 15:42

Will it take off? In reference to this thread... NO!

Time for the smartypants of FISO to show their own.

Post by **Zeus** » 22 Feb 2006, 15:45

murf wrote:Doesn't it need the wind/air on its wings to provide lift??? Therefore could only theoretically take off if into a very strong headwind!

I think.

This was my mates argument, and I agree somewhat. Yes it needs the air flowing over the wings to take off, but I say that it will take off as it will get the necessary wind. I say that the aircraft will take off

Post by **Bunners** » 22 Feb 2006, 15:46

no it wont, no physics knowledge at all, call it instinct

Post by **thelip** » 22 Feb 2006, 15:46

IMO it won't take off as, no matter how much thrust he applies to the engines, the airplane will be standing still.

To take off it needs a negative force under the wing which can only be caused by wind passing over (and under) the wings.

I think.

Post by **Madchester** » 22 Feb 2006, 15:49

it'll take off - it's a helicopter

Post by **Spencer4** » 22 Feb 2006, 15:50

Tell you one thing, I certainly wouldn't be on it. It'll be going nowhere I reckon.

Post by **Madchester** » 22 Feb 2006, 15:51

especially if it's got ryanair staff on it (fast asleep)

Post by **Zeus** » 22 Feb 2006, 15:52

So you dont think that the airplane will accelerate as normal ?

Think of where the propulsion of an aircarft comes from, not through its wheels as a car does, but from a jet engine mounted somewhere on the body. Thus the power needed to take off comes from the jet, not the wheels, the wheels are simply to counter the force of gravity.

Post by **The Ledge** » 22 Feb 2006, 15:53

It won't have to take off if the conveyor belt goes all the way to the aircraft's destination, therefore saving on expensive aviation fuel. I think we could be onto something here

Post by **thelip** » 22 Feb 2006, 15:53

Madchester wrote:especially if it's got ryanair staff on it (fast asleep) - alledgedly

Is this what you meant to say Mad?

Post by **DixieDean** » 22 Feb 2006, 15:53

I was crap at physics but I've seen enough of the Dicscovery Channel to know that it's speed and forward motion that generates the necessary air pressure over the wing that lifts it into the air.

So no, it wouldn't take off.

Post by **Madchester** » 22 Feb 2006, 15:54

thelip wrote:
Madchester wrote:especially if it's got ryanair staff on it (fast asleep) - alledgedly
Is this what you meant to say Mad?

ofcourse

Post by **Zeus** » 22 Feb 2006, 15:57

DixieDean wrote:I was crap at physics but I've seen enough of the Dicscovery Channel to know that it's speed and forward motion that generates the necessary air pressure over the wing that lifts it into the air.

So no, it wouldn't take off.

But that doesnt matter, its all about Newtons Third Law, each action has an equal and opposite reaction.

Think, If the aircraft accellerates to 200mph forwards, the conveyor belt will be going 200mph backwards, the wheels will thus be doing 400mph. But the aircraft will accelerate as the driving force behind the motion is not related at all to the conveyor, and does not act on it, but on the body of the aircraft.

Post by **Karrde** » 22 Feb 2006, 16:07

Zeus wrote: But that doesnt matter, its all about Newtons Third Law, each action has an equal and opposite reaction.

Think, If the aircraft accellerates to 200mph forwards, the conveyor belt will be going 200mph backwards, the wheels will thus be doing 400mph. But the aircraft will accelerate as the driving force behind the motion is not related at all to the conveyor, and does not act on it, but on the body of the aircraft.

Thats all well and good, but nothing is moving the air around - or through the air in order to get the lift.

The plane would go nowhere because all the air above and below the wing is static.

Post by **murf** » 22 Feb 2006, 16:08

Zeus wrote:
DixieDean wrote:I was crap at physics but I've seen enough of the Dicscovery Channel to know that it's speed and forward motion that generates the necessary air pressure over the wing that lifts it into the air.

So no, it wouldn't take off.
But that doesnt matter, its all about Newtons Third Law, each action has an equal and opposite reaction.

Think, If the aircraft accellerates to 200mph forwards, the conveyor belt will be going 200mph backwards, the wheels will thus be doing 400mph. But the aircraft will accelerate as the driving force behind the motion is not related at all to the conveyor, and does not act on it, but on the body of the aircraft.

I was assuming the plane wasn't moving forwards. e.g. the conveyor and the wheels were moving at 200mph.

Post by **Tony Towners Tache** » 22 Feb 2006, 16:10

Zeus

I accept your theory regarding thrust. But..

No actual physical movement, no air flow, no take off.

The reaction created by the conveyor is equal and opposite to the action created by the wheels resulting in equilibrium which is what Newton's third is pretty much about. Isn't it?

Also surely if this was possible it would already have been implemented.

Post by **thelip** » 22 Feb 2006, 16:11

I think the thing is standing stock still. Just like accelerating like mad on an icy road.

Post by **murf** » 22 Feb 2006, 16:14

Q2. A plane sits empty and pointing forwards on an aircraft carrier which is travelling quite fast into a very fast headwind. Could we see empty planes lifting into the air?

Post by **Zeus** » 22 Feb 2006, 16:23

Tony Towners Tache wrote:Zeus

I accept your theory regarding thrust. But..

No actual physical movement, no air flow, no take off.

The reaction created by the conveyor is equal and opposite to the action created by the wheels resulting in equilibrium which is what Newton's third is pretty much about. Isn't it?

Also surely if this was possible it would already have been implemented.

But the conveyor is not exerting any force of the body of the aircraft as the wheels, which despite a small amount of friction, are not attached to the aircraft by any gearing mechanism, they just freewheel.

Its the same idea as a plane that takes off on a frozen lake with ski's on. The pilot accellerates and the plane slides forward, why? because there is very little friction between the palne, and the ice. Now imagine that the ice could be accelerated (I know, its all theoretical though). The ice wouldnt slow down the aircraft even if it was rushing underneath the skis at 1000mph because of the lack of friction affecting the plane. The plane is purely affected by the force of the jet engines, it accelerates, gets the required lift and flies away.

Now bring that same thinking to this problem, remember that apart from the tiniest friction of the wheels, the aircraft is propelled by the jet, it will move forward, and take off. I agree that the conveyor will be going extremely quickly, but its all about the effect that it has on slowing the aircraft down, ie very little indeed

Post by **Zeus** » 22 Feb 2006, 16:25

murf wrote:Q2. A plane sits empty and pointing forwards on an aircraft carrier which is travelling quite fast into a very fast headwind. Could we see empty planes lifting into the air?

Yes, without a doubt. Thats why they anchor them all down to gurad against unexpected gusts of wind. If you were silly enough to be out in a hurricane, and happened to see an unmanned aircraft sitting, facing into the wind, you could see it fly-on-the-spot.

Post by **Zeus** » 22 Feb 2006, 16:27

thelip wrote:I think the thing is standing stock still. Just like accelerating like mad on an icy road.

See my post above. In a car, the driving force behind your movement is through the wheels of your car, thus the conveyor could negate that motion by matching the speed of the wheels. But in an aircraft, the force is supplied not through the wheels at all.

Post by **thelip** » 22 Feb 2006, 16:27

Ok Ok where can we find a jet and a conveyor belt to sort this out once and for all?

Post by **Karrde** » 22 Feb 2006, 16:28

Zeus wrote: But the conveyor is not exerting any force of the body of the aircraft as the wheels, which despite a small amount of friction, are not attached to the aircraft by any gearing mechanism, they just freewheel.

Ok, but if the air isnt moving, how is the plane going to lift?

Post by **Tony Towners Tache** » 22 Feb 2006, 16:30

Zeus

My head hurts

Post by **Zeus** » 22 Feb 2006, 16:34

This a mirror of the argument last night.

The air will be moving because the aircraft will accelerate, allbeit slower than it normally would, then gain the required velocity and lift.

Post by **murf** » 22 Feb 2006, 16:34

I think we need to anchor our plane to a runway (extra strong Pritt Stick?) and turn the thrust on. We'd need a kind of vertical conveyor/anchor thing (a big tube of pritt stick!) to see if it lifted off vertically jump-jet syle.

Post by **Karrde** » 22 Feb 2006, 16:37

Zeus wrote:This a mirror of the argument last night.

The air will be moving because the aircraft will accelerate, allbeit slower than it normally would, then gain the required velocity and lift.

The plane wont accelerate if its going 200mph on a belt going 200mph in the other direction. There is no forward momentum at all. So the air wont move.

Post by **DixieDean** » 22 Feb 2006, 16:37

If they were both going 200 mph in each direction and as you say, it lifted off then it would surely shoot off backwards once in the air???

Or it would just sit in mid air and then crash

Einstein's theory or something like that

or have I got that wrong...

I'll try an experiment on the escalators at Liverpool Street in the morning just to make sure

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Will it take off ?!

Will it take off ?!

Will it take off ?!