Plane and a conveyor belt |
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Scotty 
Just Flight Staff 
Joined: 01 Apr 2008 Location: Oxford, UK. Points: 314 |
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 Posted: 05 Jan 2010 at 4:34pm |
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I think we should lock this topic now..
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Scotty
Just Flight Ltd. |
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dmr100
Chief Pilot
Joined: 02 Apr 2008 Location: Sheffield, UK Points: 1571 |
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Posted: 05 Jan 2010 at 4:39pm |
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Right, that's it, I'm building a giant conveyor belt. Someone get me a plane and a pilot and we'll end this discussion once and for all.
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VulcanB2
Chief Pilot
Joined: 02 Apr 2008 Points: 13365 |
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Posted: 05 Jan 2010 at 5:18pm |
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If we say the aircraft is a fully-loaded 747 thereby negating just about any wind there might be short of a hurricane, the answer is a definite NO. Best regards, Vulcan. |
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dodgy-alan
Chief Pilot
Joined: 16 May 2008 Location: bognor regis Points: 2994 |
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Posted: 05 Jan 2010 at 6:07pm |
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The same question could be asked about a fly in your car, you could be doing 70mph on the motorway when the fly inside will still keep trying to get out through the windscreen. HOW? the fly is not sitting on the car so cannot be travvelling at the same speed. By rights it should be in a heap against the rear window unable to move. But there it is dancing inside the windscreen though not attached to your car, merely inside it. If it moves outside the car it gets left behind rapidly. therefore the fly by default MUST be flying at 70 mph to keep pace with the windscrren its trying to escape from, yet science and nature say a fly cannot move at 70mph, so whats going on? Its puzzled me for years......and BTW, a plane must have sufficient air movement over the wing to become airborne. !!
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The light at the end of the tunnel is a freight train coming the other way !
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Slopey
Moderator in Command
AirHauler Developer Joined: 11 Jun 2008 Points: 8280 |
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Posted: 05 Jan 2010 at 6:15pm |
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That's easy - the fly is flying inside an airmass, inside your car, which is doing 70 mph.
Unless it's on treadmill! |
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For AH2 queries - PLEASE USE THE EA Forums as the first port of call. |
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VulcanB2
Chief Pilot
Joined: 02 Apr 2008 Points: 13365 |
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Posted: 05 Jan 2010 at 6:28pm |
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Relativity. The fly relative to the car is stationary if it is on the dash, or just a bit faster (or slower) than the car if it is flying around the cabin. Best regards, Vulcan. |
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dale_tem
First Officer 
Joined: 03 Nov 2009 Location: Berkshire, UK Points: 352 |
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Posted: 05 Jan 2010 at 9:17pm |
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Got there in the end. It will always take off (with both variations).
There are 2 variations - matching the speed of the wheels and matching the speed of the plane. The matching the speed of the plane is simple as it will just accelerate and the conveyor belt will match. The matching the speed of the wheels is a little more complex, but you basically get 2 results, the wheels, plane and conveyer aren't moving OR the conveyor and wheels are travelling at infinite miles per hour and the speed can do any speed it wants.
Its a simple mathmatical formula
A = planes forward velocity
B = speed of conveyor belt
C = speed of wheels
C = B+A
As the rule for the conveyor belt matching the speed of the wheels
C = B
Then they are either 0 (and the planes speed is zero) or they are both infinity. They would accelerate to infinity as soon as the plane started moving.
Back to the hovercraft
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Dale
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BigTex
First Officer
Joined: 04 Dec 2008 Location: RKSI - Seoul Points: 325 |
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Posted: 05 Jan 2010 at 10:32pm |
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They already tested this on Discovery Channel's Mythbuster's. Check it out:
Rick
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Slopey
Moderator in Command
AirHauler Developer Joined: 11 Jun 2008 Points: 8280 |
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Posted: 05 Jan 2010 at 10:43pm |
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Big - if you read up, that's a different scenario from the original question in that it had a non-infinite, fixed speed, or limited speed, belt!
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Magic Man
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Joined: 02 Apr 2008 Location: South Wales Points: 5336 |
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Posted: 05 Jan 2010 at 11:03pm |
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The experiment was close enough though, obviously they couldn't test a belt of infinite length...
It doesn't matter anyway, the result is the same. People get hung up on assuming the plane will remain stationary when it wont.
No reason to assume otherwise since we are talking about a standard plane that could take off normally anyway. The conveyor means that its wheels just need to rotate twice as fast for it to achieve take off speed.
Umm, yes it can, there is no difference assuming we are dealing with a normally operating aircraft (as opposed to something with flat tyres, jammed wheels, underpowered engine, i.e. if it can take off normally then it can take off on our conveyor - where does the wind come into it?
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MartinW
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Joined: 31 Mar 2008 Location: United Kingdom Points: 26722 |
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Posted: 06 Jan 2010 at 9:17am |
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 Vulcan gets it wrong... as usual
It's quite simple, it only becomes difficult when we make it more complicated than it is.
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VulcanB2
Chief Pilot
Joined: 02 Apr 2008 Points: 13365 |
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Posted: 06 Jan 2010 at 7:43pm |
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OK
So the faster the aircraft is detected to move, the faster the conveyor moves...
OK...
So the aircraft moves forward, the conveyor drags it back again. AIRCRAFT REQUIRE FORWARD AIRSPEED THEREFORE IF THE CONVEYOR PREVENTS THE AIRCRAFT FROM ACCELERATING FORWARDS IT WILL NOT FLY. Aircraft ---> Conveyor <--- The only thing that could happen is the aircraft still out-runs the conveyor and still accelerates to takeoff speed, only the wheels are spinning like crazy and far in excess of maximum permissible rotational speed. Best regards, Vulcan. |
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Magic Man
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Joined: 02 Apr 2008 Location: South Wales Points: 5336 |
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Posted: 06 Jan 2010 at 9:46pm |
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The conveyor can't prevent the aircraft from accelerating.
The aircraft could be travelling at 100mph ground speed, the conveyor would be travelling at 100mph in the other direction, the aircraft's wheels would be spinning at 200mph.
The wheels are not driven, it's not a car requiring its acceleration to be achieved through turning its wheels.
Put the aircraft on a frozen lake bed, lock the wheels with pins to prevent them turning. Power up and as long as it's not stuck fast then the aircraft could still move forward to rotate speed. Do the same with a car, it's not going to move anywhere.
Nope, it will take off without a problem. The wheels would only reach twice the normal rotational speed since the aircraft will take off at its normal v rotate.
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Slopey
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AirHauler Developer Joined: 11 Jun 2008 Points: 8280 |
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Posted: 06 Jan 2010 at 9:58pm |
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No, the wheels spin faster, but the aircraft, which is accelerating due to thrust (which has nothing to do with the wheels they're merely spinning as they're in contact with a surface which doesn't apply any force onto the fuselage) will move forward. You could only move the aircraft backwards in position if you could exert a large enough force through the wheels to do so. As they spin freely, you can't exert any force on the axle they're attached to, they merely spin faster. |
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Kelvin_C-J
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Joined: 02 Apr 2008 Location: Fareham Points: 620 |
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Posted: 06 Jan 2010 at 9:59pm |
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I understand what you're saying..
The aircraft remains stationary.. the wheels spin at 200mph.. and 'it flies'.. how?
The major factor missing is air flow - where does this come in..?
Sure, it may move.. but not at the required speed to get enough air over and under the wing to generate the required amount of lift, if it is in effect being held stationary by the counter-acting conveyor.
As Vulcan put:
-->
<--
No movement.
- Equal and opposite - wheels may spin at double the speed however, regardless of any negligible movement, the aircraft remains on the ground as no air flow (atleast in the amount required to lift an aircraft) will have been generated.
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Best Regards,
Kelvin Chilvers-Jones |
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Magic Man
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Joined: 02 Apr 2008 Location: South Wales Points: 5336 |
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Posted: 06 Jan 2010 at 10:00pm |
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It doesn't remain stationary.
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MartinW
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Joined: 31 Mar 2008 Location: United Kingdom Points: 26722 |
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Posted: 06 Jan 2010 at 10:01pm |
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the conveyor drags it back again.
No it doesn't drag it back, the aircraft accelerates [in relation to the ground/ surrounding body of air] right up to it's take off speed, as if the conveyor wasn't there. The wheels may as well not be there. The wheels will spin at whatever is required, what ever the speed of the conveyor plus the speed of the aircrafts [as a result of thrust] dictates.
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Slopey
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AirHauler Developer Joined: 11 Jun 2008 Points: 8280 |
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Posted: 06 Jan 2010 at 10:03pm |
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For the conveyor to prevent the aircraft from accelerating it would need to exert a force greater than that of the acceleration to keep it in check. It can't do that, as the only place via which it could exert that force are the wheels. The wheels contact the belt, but spin freely around the axle, so speeding up the conveyor will speed up the wheels, but have no effect on the axle/fuselage they are attached to. Therefore, the conveyor can't exert any force onto the fuselage, so it will move forward. The only way the conveyor could exert a force through the wheels is if they don't rotate freely - i.e. they are braked, or rusted or some such exerting a friction force on the axle/bearings. However, that's unlikely in any normal aircraft (at least such a force as to have any effect), and even if there were some minuscule friction action, you'd need to run the conveyor at an order of magnitude faster than the aircraft motion to arrest it - which you can't do as the control system locks the conveyor speed to the aircraft's speed. But the whole conveyor/wheels & and their speeds are irrelevant as they have no force/action on the fuselage. |
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Kelvin_C-J
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Joined: 02 Apr 2008 Location: Fareham Points: 620 |
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Posted: 06 Jan 2010 at 10:05pm |
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What gets me confused about this:
I imagine someone on a treadmill - they match the speed and they don't move forward, unless they run faster.
Yes, their legs are controlled by their muscles - i.e directly proportional.. and it's the same with a car's engine and it's tires..
So, the aircraft is effectively 'free-wheeling' however, the whole point was that the speed of the aircraft is matched.. i.e +100 for the aircraft and -100 for the resistive force.. therefore, the overall force is 0 i.e no movement.
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Best Regards,
Kelvin Chilvers-Jones |
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MartinW
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Joined: 31 Mar 2008 Location: United Kingdom Points: 26722 |
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Posted: 06 Jan 2010 at 10:05pm |
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The aircraft remains stationary.. the wheels spin at 200mph.. and 'it flies'.. how?
As Magic said, the aircraft accelerates normally to take off speed.
If there were no wheels and the aircraft was hovering above the conveyor, I'm sure you can see how the aircraft would accelerate and take off.
well the wheels have some friction, but no where near enough for them to impede the aircrafts accelartion... hence, they may as well not be there, they just spin as fast as required.
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This conveyor has a control system that tracks the plane speed and
tunes the speed of the conveyor to be exactly the same (but in the
opposite direction). Can the plane take off?