Riddle Me This......... - Page 2

DOC · 30-11-2005, 04:36 AM

Quote:

Originally Posted by THORNSPAWN

I agree with everyone who thinks it WILL take off.
It's a plane not a car it doesn't use the wheels to accelerate it uses thrust, air, gasses
to push it forwards, the conveyor can be travelling as fast as it wants in the other
.

Not causing an arguement and i might be wrong, but picture your self sitting at the end of the wing while this is happening, now as it speeds up you still going no where and yes the jet is at full throttle but there will be no wind hitting you at all, the thrust is used to propell the plane forwrd yes but it is the air travelling over the wings that give it lift ( take off ) and there is none in effect

air travelling over the wing because of the shape causes a lower air pressure above the wing giving lift

as some one else said Mythbusters please.

THORNSPAWN · 30-11-2005, 04:45 AM

I fully understand the workings of an aircraft.
You keep saying it will not be moving, can you explain why not?
If the engines provide thrust therefore creating momentum why would
the ground, moving at any rate, effect it's speed through the air?
If the conveyor moving in the other direction could keep the plane stationary,
even with the engines at full thrust, there would need to be massive friction in
the wheels in order to hold it back.
You agree the thrust moves the plane forward, yes?, then how would the ground moving stop it?

DOC · 30-11-2005, 05:00 AM

Quote:

Originally Posted by THORNSPAWN

I fully understand the workings of an aircraft.
You keep saying it will not be moving, can you explain why not?
If the engines provide thrust therefore creating momentum why would
the ground, moving at any rate, effect it's speed through the air?
If the conveyor moving in the other direction could keep the plane stationary,
even with the engines at full thrust, there would need to be massive friction in
the wheels in order to hold it back.
You agree the thrust moves the plane forward, yes?, then how would the ground moving stop it?

I didnt mean to suggest that you didnt understand the working, sorry, but if the conveyer belt is keeping up with the thrust, looking at the plane from side on it would not move it would appear stationery, thus no air over the wings.

a bit like a runner on a tread mill running at 10K an hour with the tread mill running at the same speed from side on the runner would not move forward or backwards but appear to run in the same spot.

THORNSPAWN · 30-11-2005, 05:10 AM

A conveyor would only be able to keep up with a wheel driven vehicle.
Here's another experiment:
Put a bicycle on a treadmill and turn it on, the wheels will turn, then simulate
thrust by pushing the bike, will it move forwards?
Of course it would no matter how fast you had the treadmill it can't stop thrust from
a source other than the wheels.
Now if you were on the bike pedalling at the same speed as the treadmill it would
be a different story.

DOC · 30-11-2005, 05:17 AM

Quote:

Originally Posted by THORNSPAWN

A conveyor would only be able to keep up with a wheel driven vehicle.
Here's another experiment:
Put a bicycle on a treadmill and turn it on, the wheels will turn, then simulate
thrust by pushing the bike, will it move forwards?
Of course it would no matter how fast you had the treadmill it can't stop thrust from
a source other than the wheels.
Now if you were on the bike pedalling at the same speed as the treadmill it would
be a different story.

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 opposite direction).

I agree with what you say but the question states the conveyer tracks the planes speed ( relative to thrust ) therfore both peddling thier bike at the same speed going no where.

any way this could go around in circles and before we know it be all up in the air over a bit of fun so ill leave it to others to decide and make thier call, but would be good to get the official mythbusters version

cheers
Pete

mickiamo · 30-11-2005, 05:19 AM

It's hard to say for sure but I reckon the plane would get of the ground...dont know if it would stay up though.
Depends on Power and weight of the plane.. the plane would be revving its guts out and taking off from a standing start.
Put a jet engine under your armpits while your on a treadmill and I'd guarantee you would go flying.. once the revs were up!

THORNSPAWN · 30-11-2005, 05:29 AM

Quote:

Originally Posted by DOC

I agree with what you say but the question states the conveyer tracks the planes speed
( relative to thrust ) therfore both peddling thier bike at the same speed going
no where.

Ahh! but aircraft don't put power to the wheels, they push against a source
other than the ground.
It wouldn't matter that the conveyor tracked the planes speed if the plane was
MOVING at say 50knots the conveyor would be going the other direction at the
same speed with the wheels spinning at 100 but the plane would still be doing
50 ie. moving

Time to go to bed now, too much thinking.

Steffo · 30-11-2005, 07:19 AM

Quote:

Originally Posted by DOC

I agree with what you say but the question states the conveyer tracks the planes speed ( relative to thrust ) therfore both peddling thier bike at the same speed going no where.

Ah but plane's wheels don't put any drive to the ground. You can't dyno a plane, if you tried, it would constantly return 0kW to the wheels, because it puts no power through them.

So lets say you've got a jet with a take off speed of (just pulling a number out of my head) 600km/h. The pilot applies the required throttle to the engines to reach that 600km/h takeoff speed. The engines begin to move the air which propells the plane, but the conveyor senses what's going on, and begins to spin at 600km/h in the opposite direction. This will not affect the plane, as it is sourcing all its drive from air displacement, not putting any drive to the wheels. It will still take off, but the wheels, if they can sustain it, will be spinning at 1200km/h.

Then again, the easiest way to overcome this would be, as you said, to just get a Harrier Jump Yet or YAK 141 'Freestyle' and take off vertically

The Monty · 30-11-2005, 08:42 AM

If you walk up an escalator at the same speed as it is going down, do you get anywhere? No. Answer to thie aforementioned question, NO, the plane will not take off as there is no wind to make the plane go up. Its like saying can a stationary plane take off? Of course it cant, unless its one of the two above mentioned planes.

fauxpas · 30-11-2005, 09:02 AM

Very funny thread... I'll fill you in on some basic aerodynamics...

What causes lift in a place in not the speed it travels at, its the air travelling past the wing, namely over and under it.

A plane's wings are shaped in a way to have more surface area on the top than the bottom. Hence the curved shape top and usual straight underneath surfaces.

This is done so air travelling over the wing has further to travel than the air underneath, creating a low pressure system. That means the air under the wing is pushing the under side of the wing in an attempt to equalise the air pressure, causing lift.

Remember in school, you held 2 pieces of paper near each other and blew in between them thinking the pieces of paper would flap away, instead, they stuck together?

So no, a plane on a treadmill or conveyor belt going nowhere will not take off... unless there's a cyclone with high speed winds!

**Charliewool** · 30-11-2005, 09:10 AM

I've jumped ship!.. It WILL take off... The conveyor is irrelavent.
If say, the said conveyor is the width of the plane & length of the runway.. The thrust would still be propelling it forward until it reaches takeoff speed regardless of what it's wheels are doing.

:

falcon91 · 30-11-2005, 10:11 AM

Consider this!
The aircraft is landing on a conveyor belt at 125kts IAS with the conveyor belt also travelling at 125 kts in the opposite direction. Does the aircraft and all it's passengers come to a sceaming halt. I think not. All that would happen is the wheels would go twice as fast. So in other words it would take off.

Shonky · 30-11-2005, 11:01 AM

YES.. the plane will take off.

A seaplane on skis can still take off going against the current. Same for a wheeled craft on a conveyor belt.

look at it this way. Providing there was no friction, if the plane was sitting on the conveyor turned off, and the conveyor started up, the plane should not move because all the conveyor will be doing is spinning the wheels. But this requires that there is no friction...

A non powered, non braked wheel does not have any control over motion. If a plane was flying over a conveyor belt at 1000ft, it would not magically stop in mid air... if it was flying over the conveyor belt at 2cm it still wouldnt affect it. If the wheels touched the conveyor during this hypothetical flight it wouldnt stop the plane mid air. It would simply spin the wheels into oblivion!

Perana · 30-11-2005, 11:12 AM

It WILL take off...

basic physics there...

As others have said the ONLY way the conveyor will affect its speed relative to the earth is if it is driven by its wheels...

Consider this. The plane is in the air and moving... yet the wheels are still... Going by some of the logic posted this would be impossible!

Wheels have NO effect on the forward motion of a plane (unless brakes are on of course)

LTDHO · 30-11-2005, 11:22 AM

Young-un, the sea plane goes faster then the current to take off.

It is not the speed of the plane that makes it fly.
It it the pressure around the wings, which there is none.

The bearing in the wheels would explode before the aircraft gets any lift.

Casper · 30-11-2005, 11:27 AM

FLappist explained it. It will take off.The wheels are not powered so unless the conveyer belt can move the amosphere around the plane as well (as this it what is used to drive the plane forward) it will have no effect on the actual movement of the plane.

Just make sure those wheels have fresh grease cause those puppies are going to spin FAST!

flappist · 30-11-2005, 11:54 AM

This is why when aquiring a pilot's licence there are many THEORY exams and well as practical. Many have shown very little or no understanding of how why and aircraft actually flys or any of the forces involved (some quite aggressively).

Lift is generated by airflow with is the relative velocity between the wings and the AIR, not the ground or conveyor.
The relative velocity is acheived by pushing air/gas by way of a propellor or turbine (or other exotic device such as rocket or ramjet) not by apply power to the wheels.

Wheels are for taxiing and reducing friction while on the ground, they have no effect in the air other than to create drag.

If you apply brakes in the air you do NOT leave black marks in the sky.....

Dave_au · 30-11-2005, 11:56 AM

I've got my pilot's licence too and I was in the "it will take off" category, but now I'm not so sure. Obviously, the plane's engines give it thrust, but for a positive and negative pressure to form around the planes wings, you obviously need air movement. But, the amount of air that would travel over the wings of a stationary plane on full thrust would be less air than a plane that has accelerated to 250km down the runway.

That saying, yes, the engines provide thrust. Yes there is an air effect, but the air effect provided by the engines isn't great enough for a stationary plane to take off, it needs the relative speed of the surrounding air particles to cause lift.

Thats why you have long runways - the plane needs to get the speed of the airflow over the wings up to speed - something the engines can't do on full thrust without relative acceleration in the surrounding air, let alone the lack of wing in ground effect.

So my argument is, the engines alone do not provide enough thrust on a stationary wing for the plane to lift off.

Mr. CVE · 30-11-2005, 11:58 AM

The plane will not take off
the air speed going over the wings is what creates the lift.
the jets only provide the thrust to get it moving to pass the air over the wings,
so yes the plane will be stationary, as the jets propell it forward yet the treadmaill pulls it back, there will be no 100 mph + air speed going over the wings to provide lift.

Cool thread

in real life it would be tricky to demo, but in theory it works well.

And the photo of the plane on the treadmill !! excellent !!

Perana · 30-11-2005, 11:58 AM

The only thing that would prevent it taking off is the bearings in the wheels... if they fail then it probably wont take off!

flappist · 30-11-2005, 11:59 AM

Quote:

Originally Posted by Dave_au

I was in the "it will take off" category, but now I'm not so sure. Obviously, the plane's engines give it thrust, but for a positive and negative pressure to form around the planes wings, you obviously need air movement. But, the amount of air that would travel over the wings of a stationary plane on full thrust would be less air than a plane that has accelerated to 250km down the runway.

That saying, yes, the engines provide thrust. Yes there is an air effect, but the air effect provided by the engines isn't great enough for a stationary plane to take off, it needs the relative speed of the surrounding air particles to cause lift. Thats why you have long runways - the plane needs to get the speed of the airflow over the wings up to speed - something the engines can't do on full thrust without acceleration, it needs momentum.

Why do you think the thrust will not accellerate the aircraft, it is thrust against the AIR not the GROUND

RED_EL_XR8 · 30-11-2005, 12:13 PM

There is no hard and fast answer to this question, it is entirely dependant on the airframe design engine location of the plane.

The lift of an aircraft is provided by air movement across the wings, and while an aircraft sitting in a stationary position will generate a lot of air movement with engines at full thrust that air movement will not be across the surfaces of the wing for most airframe designs, so negligible lift will be created.

A jet can sit and run engines to full thrust without movement whilst the wheels are braked, releasing the wheels allows the thrust to generate forward movement of the plane, and the resultant air across the wings enables flight.

Heavy Jet aircraft can utilise this to allow full thrust to be obtained prior to forward movement and thus available for the entire take off duration on short runways.

I would suggest that any conventionally designed aircraft would not lift.

Dave_au · 30-11-2005, 12:15 PM

Quote:

Originally Posted by flappist

Why do you think the thrust will not accellerate the aircraft, it is thrust against the AIR not the GROUND

Yes and I said that, but you can't accelerate air particles to the required rotation speed just by having the aircraft stationary - likewise, a jet engine with it's brakes on and full power going through the engine generates hardly any lift at all.

Think about aircraft carriers - and their catapult systems. they fire jets forward on full thrust. .

Those jet engines do not provide lift, or direct airflow over the wings, they just provide thrust. If anything, a stationary jet engine on full power will draw air particles in away from the wings.

The coveyor belt theory *could be* more accurate and possibly work for a small, counter prop twin - I wouldnt agree to a large jet being able to do it however.

Doubt a single prop plane would be able to do it - given the corkscrew nature of the propellor wash, it would more likely flip the plane.

flappist · 30-11-2005, 12:46 PM

Quote:

Originally Posted by Dave_au

Yes and I said that, but you can't accelerate air particles to the required rotation speed just by having the aircraft stationary - likewise, a jet engine with it's brakes on and full power going through the engine generates hardly any lift at all.

Think about aircraft carriers - and their catapult systems. they fire jets forward on full thrust. .

Those jet engines do not provide lift, or direct airflow over the wings, they just provide thrust. If anything, a stationary jet engine on full power will draw air particles in away from the wings.

The coveyor belt theory *could be* more accurate and possibly work for a small, counter prop twin - I wouldnt agree to a large jet being able to do it however.

Doubt a single prop plane would be able to do it - given the corkscrew nature of the propellor wash, it would more likely flip the plane.

Ok I will rephrase the question.....

Why do you think that a reverse turning of the wheels, that is, causing then to spin on their axis will prevent forward movement of the aircraft?

As stated earlier, if you hold a pushbike on a treadmill and turn the treadmill on, the pushbike does not move.
If you wish to move the pushbike forward against the treadmill there is almost no force stopping you.

As you hold a pilot's licence you will be aware why you should take of into wind. If you do then the air speed will be greater than the ground speed and there will be less wear on the gear.
If you take off with a tail wind then the ground speed will be greated than the air speed and the gear will cop a flogging (as will you if CASA see you).

Now as this whole riddle is hypothetical then the conveyor must be of infinite linear length (as opposed to virtual belt length which is always infinite) or you would run off the end of it and must go at infinite speed and your wheels capable of infinite speed or you will outrun it or break something.

The botton line is:

As soon as power is applied the aircraft will move.
The laws of phyics and reality would support that the aircraft would accellerate faster than the conveyor as there would be too much friction in the belt and rollers.

This is a silly as "If I had I time machine could I kill my father when he was a child?"

Ghiadude · 30-11-2005, 12:57 PM

the question is confusing alot of you.... in order for the treadmill to move, the plane must first move...move relative to what?
the treadmill??the ground???
if the plane is "moving" its moving relative to the ground and air and the base of the treadmill...forget about the treadmill belt its irrelevant as any force exerted on the plane by the belt will be acted upon by the opposing force of the engine minus the loss of friction of wheels to road, if its moving relative to the air there is airflow.. if theres airflow, there is lift... if theres lift theres takeoff.
those of you who think that the wheels have any impact you are wrong. The wheels on a plane are only there so that the plane can overcome the friction with the ground... as long as the tyres hold up and the bearings hold up and the treadmill doesnt explode the PLANE WILL TAKE OFF!! the treadmill belt will keep on accelerating in speed until theoretical infinite the wheels on the plane will match this speed untill the plane leaves the belt.

its all about frame of reference...

i didnt get an A in physics for no reason...a C in engrish however...

Dave_au · 30-11-2005, 01:05 PM

Quote:

Originally Posted by flappist

As soon as power is applied the aircraft will move.
The laws of phyics and reality would support that the aircraft would accellerate faster than the conveyor as there would be too much friction in the belt and rollers.

My concerns with this theory just lie in the relative airflow - and I just can't stop thinking of a 747 won't generate lift while being relatively stationary in an enviroment.

Yeah I agree, the plane would have forward motion relative to the conveyor, but the aircraft would have no motion relative to the enviroment. The engines are deisgned for rearward thrust, and not lift. There would be a pressure differential in front of and behind the aircraft, however I doubt there would be a pressure differential over the wings.

You used the example of a headwind - and your exactly right - if you had a 45kt headwind then your Stork would have been airbourne and probably flying backwards at this time. If the Stork was on the conveyor, and the prop was able to provide whatever the v1 of the stork would be - what - 33kt or so, then the plane may gain lift. I highly doubt it would be uniform lift for safe flight however.

But the issue of this riddle comes down, in my opinion to jet engines, which in my opinion, would not provide enough movement of airflow of the enviroment to cause for lift on a jumbo.

Quote:

Originally Posted by tx3dude

if its moving relative to the air there is airflow.. if theres airflow, there is lift... if theres lift theres takeoff.

I think the argument now is, that yes the plane is moving relative to the conveyor belt, and yes the engines, providing thrust to the plane is creating some airflow, but the airflow should, for intensive purposes really be relative to the stationary ground.

The prop/jet would be sucking in air, however I have serious reservations over the amount of air the aircraft would be moving over it's wings.

Racecraft · 30-11-2005, 01:10 PM

I'm with this guy.. because he used equations and has drawn a cool plane

Dave_au · 30-11-2005, 01:22 PM

We need Mythbusters.

Quote:

Originally Posted by flappist

As soon as power is applied the aircraft will move.
The laws of phyics and reality would support that the aircraft would accellerate faster than the conveyor as there would be too much friction in the belt and rollers.

In a real world, I agree.

Racecraft · 30-11-2005, 01:23 PM

Quote:

Originally Posted by Dave_au

We need Mythbusters.

Best comment ever!!! hahahaha

**Charliewool** · 30-11-2005, 01:24 PM

Looks like we're not the only nutters pondering this!... A google search "Will the plane take off" produced a heap of other forums arguing on this.. the one below has FORTY SIX pages & they're still going round in circles! lol

http://community.discovery.com/group...6/m/7451937218