Would It Take Off?
Discussion
The question is ambiguous, there are two possible scenarios.
In both cases let's assume the conveyor is infinitely long, air is still relative to the ground, and we are capable of making the conveyor go arbitrarily fast and ignore any practical limitations about how fast the wheels can go.
Scenario 1: For every inch the plane moves forward relative to the ground, the conveyor belt moves backwards an inch relative to the ground. What happens when the pilot tries to take off? The plane takes off, with the wheels spinning twice as fast as normal.
Scenario 2: For every inch the plane moves forward relative to the conveyor belt, the conveyor belt moves backwards an inch relative to the ground. What happens when the pilot tries to take off? As soon as the aircraft moves forward by the tiniest amount, the conveyor instantly accelerates arbitrarily fast in the opposite direction. Since the wheels do have friction, drag and inertia they can produce a backwards drag on the plane, although it will require immense accelerations and speeds to produce sufficient drag to balance the thrust of the engines, so the conveyor will accelerate as fast as necessary to counteract the thrust of the engines and the plane will remain stationary.
In both cases let's assume the conveyor is infinitely long, air is still relative to the ground, and we are capable of making the conveyor go arbitrarily fast and ignore any practical limitations about how fast the wheels can go.
Scenario 1: For every inch the plane moves forward relative to the ground, the conveyor belt moves backwards an inch relative to the ground. What happens when the pilot tries to take off? The plane takes off, with the wheels spinning twice as fast as normal.
Scenario 2: For every inch the plane moves forward relative to the conveyor belt, the conveyor belt moves backwards an inch relative to the ground. What happens when the pilot tries to take off? As soon as the aircraft moves forward by the tiniest amount, the conveyor instantly accelerates arbitrarily fast in the opposite direction. Since the wheels do have friction, drag and inertia they can produce a backwards drag on the plane, although it will require immense accelerations and speeds to produce sufficient drag to balance the thrust of the engines, so the conveyor will accelerate as fast as necessary to counteract the thrust of the engines and the plane will remain stationary.
All of it is moot point.If the conveyor belt is trying to match the speed at which the aircraft moves the wheels are going to be turning at double the speed at which the thrust component moves the airframe along.
The tyres will therefore go pop before it reaches flying airspeed. To get airborne at 150kts (for instance) the wheels would be turning
at a speed equivalent to nearer 300kts. Aircraft tyres aren't designed to cope with loads at those sorts of energy levels.
Tyres go bang, aeroplane goes out of control off the side of the conveyor and ends up a fireball.
There's not much useful point in theories which have to be so far abstracted from reality to work...
The tyres will therefore go pop before it reaches flying airspeed. To get airborne at 150kts (for instance) the wheels would be turning
at a speed equivalent to nearer 300kts. Aircraft tyres aren't designed to cope with loads at those sorts of energy levels. Tyres go bang, aeroplane goes out of control off the side of the conveyor and ends up a fireball.
There's not much useful point in theories which have to be so far abstracted from reality to work...
GreenV8S said:
As soon as the aircraft moves forward by the tiniest amount, the conveyor instantly accelerates arbitrarily fast in the opposite direction. Since the wheels do have friction, drag and inertia they can produce a backwards drag on the plane
The wheels cannot transmit the conveyor belt force into the airframe though....
m.c said:
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).
The question is:
Will the plane take off or not?
There is no forward movement of the aircraft relative to the surrounding air.
If the aircrafts airspeed is 0 then there can be no lift as there is no air moving over the wings.
No lift = No take off.
The wheels are doing nothing all the thrust is from the engines.
>> Edited by stovey on Saturday 4th March 13:50
try again...
the wheels are connected to the engines via the airframe so until the plane is airborn all forward propulsion generated by the jet thrust is transferred to the ground via the wheels. if the conveyor is exactly matching the speed and counteracting this forward motion, it goes no where.
analogy... moving walkway travels at 4mph, you get on at the wrong end walking at 4 mph.. does the scenary change?
ps. I really am going now.. this is tedious!
>> Edited by fozzi on Saturday 4th March 13:48
the wheels are connected to the engines via the airframe so until the plane is airborn all forward propulsion generated by the jet thrust is transferred to the ground via the wheels. if the conveyor is exactly matching the speed and counteracting this forward motion, it goes no where.
analogy... moving walkway travels at 4mph, you get on at the wrong end walking at 4 mph.. does the scenary change?
ps. I really am going now.. this is tedious!
>> Edited by fozzi on Saturday 4th March 13:48
fozzi said:
ps. I really am going now.. this is tedious!
Yes it is because you just keep reposting the same mistakes...
Your analogy is pointless - you obviously still have not read the analogy with the motorcycle which *should* allow you to see the answer......
Maybe you will never understand......
fozzi said:
try again...
the wheels are connected to the engines via the airframe so until the plane is airborn all forward propulsion generated by the jet thrust is transferred to the ground via the wheels. if the conveyor is exactly matching the speed and counteracting this forward motion, it goes no where.
analogy... moving walkway travels at 4mph, you get on at the wrong end walking at 4 mph.. does the scenary change?
ps. I really am going now.. this is tedious!
>> Edited by fozzi on Saturday 4th March 13:48
I'm no expert, but i'm pretty sure it's not like walking, it's like skating. As such you're analogy of the moving walkway doesn't work/help. If you skated onto a moving walkway at 4mph then you wouldn't stop instantly as you hit it...
Cheers,
Rob.
gorvid said:
As soon as the aircraft moves forward by the tiniest amount, the conveyor instantly accelerates arbitrarily fast in the opposite direction. Since the wheels do have friction, drag and inertia they can produce a backwards drag on the plane
The wheels cannot transmit the conveyor belt force into the airframe though....
If you allow the conveyor belt to have infinite acceleration and speed then some effects which are normally negligeable can become significant. The bearings and tyres introduce rolling resistance and drag, some components of this increase with speed. The wheels have significant rotational inertia which means you can produce a net thrust when you spin them up. Think of a wheel sitting on a table when you do the classic 'whisk the tablecloth away' trick. The angular inertia of the wheel will cause it to move slightly in the direction the cloth moves.
Edited to add: since this is a 'thought experiment' you could specify wheels which have zero inertia and drag. In that case the conveyor belt will instantly accelerate up to infinite speed, time and space will fold in on themselves and the universe will disappear in a puff of logic.
>> Edited by GreenV8S on Saturday 4th March 13:59
The aircraft must be moving forward RELATIVE to the surrounding air.
In this question there is no forward movement as the conveyor is matching the thrust of the engines.
There is NO LIFT here. In the aircraft the airspeed indicator would read zero.
It can't fly untill the airspeed( the speed of air over the wings) is whatever it should be on a normal runway.
It is simple aerodynamics.
Aerodynamically the aircraft is stationary.
>> Edited by stovey on Saturday 4th March 14:01
>> Edited by stovey on Saturday 4th March 14:04
In this question there is no forward movement as the conveyor is matching the thrust of the engines.
There is NO LIFT here. In the aircraft the airspeed indicator would read zero.
It can't fly untill the airspeed( the speed of air over the wings) is whatever it should be on a normal runway.
It is simple aerodynamics.
Aerodynamically the aircraft is stationary.
>> Edited by stovey on Saturday 4th March 14:01
>> Edited by stovey on Saturday 4th March 14:04
fozzi said:
try again...
analogy... moving walkway travels at 4mph, you get on at the wrong end walking at 4 mph.. does the scenary change?
Try this:
fozzi said:
try again...
analogy... moving walkway travels at 4mph, you get on at the wrong end cycling at 4 mph.. does the scenary change?
Now add this:
fozzi said:
try again...
analogy... moving walkway travels at 4mph, you get on at the wrong end cycling at 4 mph.. and there is a 500mph tailwind does the scenary change?
Now try this:
fozzi said:
does the scenary change?
YES!
The clearest way described to me is as follows.
Forget it's a plane for the time being. Imagine you're standing on a skateboard instead. The treadmill spins at 5mph, the skateboard's wheels spin at an equal speed. You have no forward motion, no lift, nada.
There are, however, parallel bars on either side of the treadmill. By pulling on these bars you can propel the skateboard to the front of the treadmill quite easily, ie you have acceleration. Now substitute your arms pulling on the bars for the jet engines pulling against the air and you'll appreciate that yes, the jet will move forward, airflow will form over the wing and the plane will take off.
It seems counter intuitive at first but there it is.
Forget it's a plane for the time being. Imagine you're standing on a skateboard instead. The treadmill spins at 5mph, the skateboard's wheels spin at an equal speed. You have no forward motion, no lift, nada.
There are, however, parallel bars on either side of the treadmill. By pulling on these bars you can propel the skateboard to the front of the treadmill quite easily, ie you have acceleration. Now substitute your arms pulling on the bars for the jet engines pulling against the air and you'll appreciate that yes, the jet will move forward, airflow will form over the wing and the plane will take off.
It seems counter intuitive at first but there it is.
bad_roo said:
There are, however, parallel bars on either side of the treadmill. By pulling on these bars you can propel the skateboard to the front of the treadmill quite easily, ie you have acceleration. Now substitute your arms pulling on the bars for the jet engines pulling against the air and you'll appreciate that yes, the jet will move forward, airflow will form over the wing and the plane will take off.
In the original question the treadmill will accelerate to match the new opposite force.
So there is no forward movement and no lift.
stovey said:
In the original question the treadmill will accelerate to match the new opposite force.
That would still work in the skateboard analogy!
The wheels would increase in speed as you pulled yourself forward...
But that speed increase would not affect your ability to pull yourself forward (just melt your wheels
0
Well I have got 6 legs and a pair of antennae stovey said:
In the original question the treadmill will accelerate to match the new opposite force.
So substitute 'plane' with 'person on skateboard' and 'engines' with 'arms' and then answer the question. The person pulls themselves forwards using the grab rails, the conveyor moves backwards as fast as they move forwards, it doesn't stop them moving forwards. If it does, then how?
What i love about this/these thread(s) is that there's always a section of people who think something *must* be wrong because they can't imagine it. Scarey, really... I mean i'm fairly confident with my position that it would take off, especially considering my skateboard analogy i put up earlier, but i wouldn't fight to the death about it like some of you guys would... 
Cheers,
Rob.
Cheers,
Rob.
Its great how some people get so animated defending the incorrect answer.
As a partially qualified aeronautical engineer, I can tell you that the aircraft will of course take off in this hypothetical situation.
One way to simplify matters (something engineers love doing) is to imagine the aircraft is being levitated by magnets, of course the runway will have no way of transmitting the rearwards force to the aircraft. I know what all you doubters are thinking, the wheels do indeed touch the runway but not to apply any horizontal force to the aircraft, only vertical. As someone stated before, they are effectively only there to hold the aircraft up.
As a partially qualified aeronautical engineer, I can tell you that the aircraft will of course take off in this hypothetical situation.
One way to simplify matters (something engineers love doing) is to imagine the aircraft is being levitated by magnets, of course the runway will have no way of transmitting the rearwards force to the aircraft. I know what all you doubters are thinking, the wheels do indeed touch the runway but not to apply any horizontal force to the aircraft, only vertical. As someone stated before, they are effectively only there to hold the aircraft up.
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