Question.
27 November 2012, 07:58 PM
#35
Scooby Regular
Join Date: Oct 2009
Location: Notts
Posts: 661
Likes: 0
Received 0 Likes
on
0 Posts
28 November 2012, 05:07 PM
#36
Scooby Regular
Join Date: Aug 2002
Posts: 39,877
Likes: 0
Received 0 Likes
on
0 Posts
Unless the ambient air is moving past the aircraft at the same speed as the moving runway, there is no relative airspeed so the wing cannot produce any lift.
It won't fly!
Les
28 November 2012, 05:25 PM
#37
As addi has pointed out, it doesn't matter what the conveyor belt does, it will not slow the plane's airspeed. The plane will continue to increase its airspeed at whatever acceleration its engines (propeller or jet) can generate. The conveyor will match the airspeed, but the only effect of this will be to make the plane's wheels spin faster. When the plane reaches the appropriate airspeed it will take off.
28 November 2012, 05:52 PM
#38
Scooby Regular
Join Date: Aug 2001
Location: Warrington
Posts: 4,554
Likes: 0
Received 0 Likes
on
0 Posts
+1.
Those assuming that the conveyor cancels the planes speed are incorrect. If the plane is doing 0 mph then so is the conveyor. The clue in the OP is "The plane moves in one direction ..." If the plane is not moving that is a different scenario.
Those assuming that the conveyor cancels the planes speed are incorrect. If the plane is doing 0 mph then so is the conveyor. The clue in the OP is "The plane moves in one direction ..." If the plane is not moving that is a different scenario.
28 November 2012, 05:53 PM
#39
Scooby Regular
Thread Starter
iTrader: (6)
Join Date: Nov 2004
Location: Deepest Darkest Kernow
Posts: 4,404
Likes: 0
Received 1 Like
on
1 Post
As addi has pointed out, it doesn't matter what the conveyor belt does, it will not slow the plane's airspeed. The plane will continue to increase its airspeed at whatever acceleration its engines (propeller or jet) can generate. The conveyor will match the airspeed, but the only effect of this will be to make the plane's wheels spin faster. When the plane reaches the appropriate airspeed it will take off.
29 November 2012, 03:34 PM
#42
Scooby Regular
Join Date: Aug 2002
Posts: 39,877
Likes: 0
Received 0 Likes
on
0 Posts
The OP needs to explain the situation more clearly.
Are the aircraft's engines running and producing sufficient thrust to accelerate the aircraft along the moving runway?
If the engines are accelerating the aircraft, as soon as it reaches unstick speed it will take off regardless of what the runway is doing beneath it, either in the same direction of the aircraft,ie not moving in relation to the aircraft, or in the opposite direct ie twice the unstick speed in relation to the aircraft as in this case.
It is the speed of the air in relation to the aircraft which affects its flying capabities, not the speed of the runway beneath it.
Les
Are the aircraft's engines running and producing sufficient thrust to accelerate the aircraft along the moving runway?
If the engines are accelerating the aircraft, as soon as it reaches unstick speed it will take off regardless of what the runway is doing beneath it, either in the same direction of the aircraft,ie not moving in relation to the aircraft, or in the opposite direct ie twice the unstick speed in relation to the aircraft as in this case.
It is the speed of the air in relation to the aircraft which affects its flying capabities, not the speed of the runway beneath it.
Les
Last edited by Leslie; 29 November 2012 at 03:36 PM.
29 November 2012, 03:45 PM
#43
Scooby Regular
Join Date: Aug 2000
Location: God's promised land
Posts: 80,907
Likes: 0
Received 0 Likes
on
0 Posts
As addi has pointed out, it doesn't matter what the conveyor belt does, it will not slow the plane's airspeed. The plane will continue to increase its airspeed at whatever acceleration its engines (propeller or jet) can generate. The conveyor will match the airspeed, but the only effect of this will be to make the plane's wheels spin faster. When the plane reaches the appropriate airspeed it will take off.
And i thought this was easy
Ok i get your point, but let's assume the plane, spinning on its conveyor belt, isn't actually going anywhere, but at a certain indicated airspeed in the cockpit it attempts to take off. Doesn't the fact that in reality, because it isn't actually moving relitive to the ground, it will never have any actual airspeed with which to achieve levitation, ie it would have to go from 0mph to 200mph instantaneously? Could it not be caught in a cycle of "attempted" take-offs?
29 November 2012, 04:50 PM
#44
Scooby Regular
Join Date: Apr 2000
Posts: 1,054
Likes: 0
Received 0 Likes
on
0 Posts
And i thought this was easy
Ok i get your point, but let's assume the plane, spinning on its conveyor belt, isn't actually going anywhere but at a certain indicated airspeed in the cockpit it attempts to take off.
The groundspeed indication can be what everDoesn't the fact that in reality, because it isn't actually moving relitive to the ground, it will never have any actual airspeed with which to achieve levitation, ie it would have to go from 0mph to 200mph instantaneously? Could it not be caught in a cycle of "attempted" take-offs?
Ok i get your point, but let's assume the plane, spinning on its conveyor belt, isn't actually going anywhere but at a certain indicated airspeed in the cockpit it attempts to take off.
The groundspeed indication can be what everDoesn't the fact that in reality, because it isn't actually moving relitive to the ground, it will never have any actual airspeed with which to achieve levitation, ie it would have to go from 0mph to 200mph instantaneously? Could it not be caught in a cycle of "attempted" take-offs?
Imagine you're at an airport that has moving walkways, and you encounter one that's going the opposite direction to you. Because you're a bit pissed from the airport bar you decide to walk alongside the moving walkway but lean over and wheel your suitcase along the moving walkway itself. You then walk along in your direction at the same speed as the walkway is going in it's direction - let's say 3mph.
Do you for one second think your suitcase isn't going to come with you? Of course it is. All that happens is while you are walking along at 3mph the wheels on your suitcase think they're doing 6mph, because they have the 3mph you're walking at plus the 3mph in the opposite direction of the walkway.
It's no different in the plane example, because the force that's making the plane move, specifically the engines, is unrelated to the surface the plane is on. In the walkway example, you are the engines, the walkway is the runway and your suitcase is the plane.
29 November 2012, 05:02 PM
#45
Scooby Regular
Join Date: Aug 2001
Location: Warrington
Posts: 4,554
Likes: 0
Received 0 Likes
on
0 Posts
"The conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same" If the plane speed is measured relative to the conveyor and not the airport then it doesn't matter what speed either is doing, their relative speeds will always be exactly opposite to each other and no control system is required.
QED
Yes Les we do have to assume that the plane is under its own power and not being pulled by a tug, but surely that's obvious. Otherwise the plane could be moving backwards and will never get airborne.
29 November 2012, 06:09 PM
#46
Scooby Regular
Join Date: Aug 2000
Location: God's promised land
Posts: 80,907
Likes: 0
Received 0 Likes
on
0 Posts
Beef, not sure about that. The suitcase is the plane, as you say. And once you reach the end of the walkway, the case comes with you. But it only does so because i am pulling it from an effective 0mph to my walking speed, at fairly high acceleration. But a plane needs to be doing the *full* walking speed before it can take off, it doesn't have the luxury of the acceleration phase. No?
Speedking, i'm reading it completely different. If you're saying the conveyor belt is within a runway for example, so that the plane's engine can make it move then that's a no-brainer, the thrust will act as usual, there will just be less effective friction with the runway. I can't believe that's what the initial scenario is suggesting though.
And i spelt relative wrong, most importantly.
Speedking, i'm reading it completely different. If you're saying the conveyor belt is within a runway for example, so that the plane's engine can make it move then that's a no-brainer, the thrust will act as usual, there will just be less effective friction with the runway. I can't believe that's what the initial scenario is suggesting though.
And i spelt relative wrong, most importantly.
29 November 2012, 06:21 PM
#47
Scooby Regular
Join Date: Aug 2001
Location: Warrington
Posts: 4,554
Likes: 0
Received 0 Likes
on
0 Posts
the plane, spinning on its conveyor belt, isn't actually going anywhere, but at a certain indicated airspeed in the cockpit it attempts to take off
You can't seriously be saying that you think airspeed is measured via the wheels of the plane! How would that work in flight, unless it was equipped with very long undercarriage legs.
So when the OP says "The plane moves in one direction", you read that as the plane doesn't move, but the belt moves in the direction of the back of the plane? How do you get to that interpretation?
You can't seriously be saying that you think airspeed is measured via the wheels of the plane! How would that work in flight, unless it was equipped with very long undercarriage legs.
So when the OP says "The plane moves in one direction", you read that as the plane doesn't move, but the belt moves in the direction of the back of the plane? How do you get to that interpretation?
29 November 2012, 09:14 PM
#50
Scooby Regular
iTrader: (13)
Join Date: Oct 2009
Location: here, there, everywhere
Posts: 3,111
Likes: 0
Received 0 Likes
on
0 Posts
ultimatley the ground speed of the aircraft is not relevant - saying that if there is significant air movement over/under the mainplane it could provide whats required to enable take off.
if example is meant to exclude all other airmovement generated then its not going to take off. if included is airmovment generated by the said conveyor, or even by propeller blades and/or any wind then it may be possible. the scenario isnt specific enough in its parameters
if example is meant to exclude all other airmovement generated then its not going to take off. if included is airmovment generated by the said conveyor, or even by propeller blades and/or any wind then it may be possible. the scenario isnt specific enough in its parameters
29 November 2012, 10:47 PM
#51
Scooby Regular
Join Date: Aug 2000
Location: God's promised land
Posts: 80,907
Likes: 0
Received 0 Likes
on
0 Posts
the plane, spinning on its conveyor belt, isn't actually going anywhere, but at a certain indicated airspeed in the cockpit it attempts to take off
You can't seriously be saying that you think airspeed is measured via the wheels of the plane! How would that work in flight, unless it was equipped with very long undercarriage legs.
So when the OP says "The plane moves in one direction", you read that as the plane doesn't move, but the belt moves in the direction of the back of the plane? How do you get to that interpretation?
You can't seriously be saying that you think airspeed is measured via the wheels of the plane! How would that work in flight, unless it was equipped with very long undercarriage legs.
So when the OP says "The plane moves in one direction", you read that as the plane doesn't move, but the belt moves in the direction of the back of the plane? How do you get to that interpretation?
No, now you're making assumptions, calm down. Obviously the wheel speed doesn't indicate airspeed. The conveyor belt simulates the plane moving through the air, so it replicates a take-off. You are taking the "moves in one direction" too literally, in my opinion. To an observer, the plane isn't actually moving at all, the conveyor is counteracting its forward progress so all you're left with is the thrust from the engine.
But at a certain level of airspeed, the plane would be able to take off IF it actually had the momentum to do so. My point is that because it has zero relative momentum, airspeed alone is insufficient to allow it to take off, as its realtive speed, once the wheels lost contact with the conveyor, would be 0.000001mph or as close to zero as makes no difference. So my point, to repeat, is that if it can gain absolute momentum fast enough after leaving the conveyor then sure, it might stay in the air. But unless its a VTOL plane i can't see how that would work, it couldn't accelerate fast enough, conventional planes need runway speed as well as thrust to take off. It would just keep falling back down onto the conveyor after a miniscule amount of time.
But the fact that there's any confusion about the nature of the experiment itself, then as Jef says, the parameters aren't defined enough to be able to tell. But i'd love to hear an authoritative conclusion from somebody who knows the answer, if the parameters are as i interpret them.
What have you done with Telboy?
29 November 2012, 11:17 PM
#53
Scooby Regular
iTrader: (13)
Join Date: Oct 2009
Location: here, there, everywhere
Posts: 3,111
Likes: 0
Received 0 Likes
on
0 Posts
the same question keeps popping up, but doesnt define enough of the exact parameters needed.
id say technically the plane doesnt need any speed atall, just enough airmovement above and below the mainplane to cause a significant difference in air pressure to create lift over the weight of the aircraft, lift could be acheived. but a simple question like the o.ps doesnt give enough info to show this, its an endless circle of possibilities
id say technically the plane doesnt need any speed atall, just enough airmovement above and below the mainplane to cause a significant difference in air pressure to create lift over the weight of the aircraft, lift could be acheived. but a simple question like the o.ps doesnt give enough info to show this, its an endless circle of possibilities
29 November 2012, 11:21 PM
#54
No, now you're making assumptions, calm down. Obviously the wheel speed doesn't indicate airspeed. The conveyor belt simulates the plane moving through the air, so it replicates a take-off. You are taking the "moves in one direction" too literally, in my opinion. To an observer, the plane isn't actually moving at all, the conveyor is counteracting its forward progress so all you're left with is the thrust from the engine.
But at a certain level of airspeed, the plane would be able to take off IF it actually had the momentum to do so. My point is that because it has zero relative momentum, airspeed alone is insufficient to allow it to take off, as its realtive speed, once the wheels lost contact with the conveyor, would be 0.000001mph or as close to zero as makes no difference. So my point, to repeat, is that if it can gain absolute momentum fast enough after leaving the conveyor then sure, it might stay in the air. But unless its a VTOL plane i can't see how that would work, it couldn't accelerate fast enough, conventional planes need runway speed as well as thrust to take off. It would just keep falling back down onto the conveyor after a miniscule amount of time.
But the fact that there's any confusion about the nature of the experiment itself, then as Jef says, the parameters aren't defined enough to be able to tell. But i'd love to hear an authoritative conclusion from somebody who knows the answer, if the parameters are as i interpret them.
But at a certain level of airspeed, the plane would be able to take off IF it actually had the momentum to do so. My point is that because it has zero relative momentum, airspeed alone is insufficient to allow it to take off, as its realtive speed, once the wheels lost contact with the conveyor, would be 0.000001mph or as close to zero as makes no difference. So my point, to repeat, is that if it can gain absolute momentum fast enough after leaving the conveyor then sure, it might stay in the air. But unless its a VTOL plane i can't see how that would work, it couldn't accelerate fast enough, conventional planes need runway speed as well as thrust to take off. It would just keep falling back down onto the conveyor after a miniscule amount of time.
But the fact that there's any confusion about the nature of the experiment itself, then as Jef says, the parameters aren't defined enough to be able to tell. But i'd love to hear an authoritative conclusion from somebody who knows the answer, if the parameters are as i interpret them.
- You are wearing a pair of roller boots.
- You are on a running tread mill.
- There is a length of rope attached to a wall in front of you.
- You pull on the rope to go forward.
- The tread mill matches your forward movement rolling the opposite direction.
- Are you saying you won't be able to pull yourself towards the wall?
30 November 2012, 12:10 AM
#55
The scenario that the OP's question is probably describing is one where the forward thrust of the airplane's motor(s), be that jet(s) or propeller(s), is exactly matched by the passive or active slipping motion of the conveyor-band runway (although admittedly the question is far too shoddily expressed for anyone to be certain). In that case, the airplane is static relative to the surrounding air, its wings therefore generate no lift, so obviously it isn't ever going to take off.
So far as any running-on-a-reverse-motion-conveyor-belt analogies are concerned, these are only relevant if you add another (so far missing) component, which addresses the wind/air movement factor. For example, if put on a soaking wet T-shirt then run along a reverse-motion conveyor-belt (effectively standing still relative to the surrounding air), will the T-shirt dry as quickly as if you were running on solid ground (actually moving through the surrounding air)? Ladies of scoobynet, your participation in the above experiment is vital to the solution of this important scientific conundrum, video and photographic evidence is of course mandatory
So far as any running-on-a-reverse-motion-conveyor-belt analogies are concerned, these are only relevant if you add another (so far missing) component, which addresses the wind/air movement factor. For example, if put on a soaking wet T-shirt then run along a reverse-motion conveyor-belt (effectively standing still relative to the surrounding air), will the T-shirt dry as quickly as if you were running on solid ground (actually moving through the surrounding air)? Ladies of scoobynet, your participation in the above experiment is vital to the solution of this important scientific conundrum, video and photographic evidence is of course mandatory
30 November 2012, 01:01 AM
#56
Scooby Regular
Join Date: Oct 2004
Location: If you're not braking or accelerating you're wasting time.
Posts: 2,684
Likes: 0
Received 0 Likes
on
0 Posts
If you are on an indoor running machine it dosen't matter how fast you run you will never get that 'wind in your face' feeling.
30 November 2012, 08:49 AM
#57
When there is significant engine thrust, this will provide the plane with a forward force.
The wheels will start moving.
Newtonian mechanics teaches us that the forward force generated by the engines will be equal to the push back from the wheels on the ground, hence the plane starts moving. (equal and opposite)
The treadmill will start spinning in an opposite direction, to generate a force at the wheels equal to the forward force, causing the net acceleration to be zero.
So rather than force at the wheels being transferred to the ground and causing ground acceleration, it is transferred to the treadmill.
No matter what accelerating force is applied by the engine, the treadmill will counter it with a negative force at the ground.
It will not move, and no lift will ever be generated.
Which bit of this is wrong?
Edit:
D'oh! The forward force from the engine comes from pushing against the air around it. The reverse force from the wheels would just spin them more, but do not much else.
The plane would move relative to the ground
The wheels will start moving.
Newtonian mechanics teaches us that the forward force generated by the engines will be equal to the push back from the wheels on the ground, hence the plane starts moving. (equal and opposite)
The treadmill will start spinning in an opposite direction, to generate a force at the wheels equal to the forward force, causing the net acceleration to be zero.
So rather than force at the wheels being transferred to the ground and causing ground acceleration, it is transferred to the treadmill.
No matter what accelerating force is applied by the engine, the treadmill will counter it with a negative force at the ground.
It will not move, and no lift will ever be generated.
Which bit of this is wrong?
Edit:
D'oh! The forward force from the engine comes from pushing against the air around it. The reverse force from the wheels would just spin them more, but do not much else.
The plane would move relative to the ground
Last edited by ReallyReallyGoodMeat; 30 November 2012 at 09:05 AM.
30 November 2012, 08:50 AM
#58
30 November 2012, 09:16 AM
#59
When there is significant engine thrust, this will provide the plane with a forward force.
The wheels will start moving.
Newtonian mechanics teaches us that the forward force generated by the engines will be equal to the push back from the wheels on the ground, hence the plane starts moving. (equal and opposite)
The treadmill will start spinning in an opposite direction, to generate a force at the wheels equal to the forward force, causing the net acceleration to be zero.
So rather than force at the wheels being transferred to the ground and causing ground acceleration, it is transferred to the treadmill.
No matter what accelerating force is applied by the engine, the treadmill will counter it with a negative force at the ground.
It will not move, and no lift will ever be generated.
Which bit of this is wrong?
The wheels will start moving.
Newtonian mechanics teaches us that the forward force generated by the engines will be equal to the push back from the wheels on the ground, hence the plane starts moving. (equal and opposite)
The treadmill will start spinning in an opposite direction, to generate a force at the wheels equal to the forward force, causing the net acceleration to be zero.
So rather than force at the wheels being transferred to the ground and causing ground acceleration, it is transferred to the treadmill.
No matter what accelerating force is applied by the engine, the treadmill will counter it with a negative force at the ground.
It will not move, and no lift will ever be generated.
Which bit of this is wrong?
