think about this...
 

There is a plane (Boeing 777, prop plane, whatever) on a moving, conveyor-type runway. The runway moves in the opposite direction of the plane at the exact same speed as the plane's wheels. There is no wind. Can the plane take off?

Re: think about this...
 

What kind of plane?

Re: think about this...
 

depends...is the pilot stoned?

Re: think about this...
 

If there is no wind speed, there is no lift. OTOH, you could be facing a strong headwind. Insufficient data to answer your question.

Re: think about this...
 

Airspeed = 0 the plane cannot take off. This would be the same if the plane had a tail wind that was = to it's forward motion.

Re: think about this...
 

No

Re: think about this...
 

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Airspeed = 0 the plane cannot take off. This would be the same if the plane had a tail wind that was = to it's forward motion.

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Good answer, but just seconds too slow. [img]/images/graemlins/grin.gif[/img]

edit:
I see Ray has altered the OP to provide some of the lacking information. My "insufficient information" answer was clearly the correct one. [img]/images/graemlins/wink.gif[/img]

Re: think about this...
 

this doesnt require much thought

Re: think about this...
 

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this doesnt require much thought

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so what's your answer? maybe it requires more thought than you're giving it credit for.

Re: think about this...
 

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this doesnt require much thought

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so what's your answer? maybe it requires more thought than you're giving it credit for.

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if im reading your question right, it will absolutely take off
maybe im stupid
[img]/images/graemlins/confused.gif[/img]

Re: think about this...
 

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this doesnt require much thought

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so what's your answer? maybe it requires more thought than you're giving it credit for.

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if im reading your question right, it will absolutely take off
maybe im stupid
[img]/images/graemlins/confused.gif[/img]

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and if im stupid, im stupid!!!

Re: think about this...
 

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Airspeed = 0 the plane cannot take off. This would be the same if the plane had a tail wind that was = to it's forward motion.

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this is not true.

Re: think about this...
 

If the plane is not moving, no air is being forced under its wings. If no air is being forced under its wings, there is no lift. So it stays grounded.

If a car were going 60 miles an hour and you stuck your head out the window, you'd feel a strong breeze in your face. Put the car on a treadmill that equalizes the speed, stick your head out the window, and you won't feel the wind rush through your hair.

Re: think about this...
 

The problem with this scenario is that the drive isn't created by the wheels, so this belt wouldn't result in the plane not moving, it would mean the wheels would be moving REALLY quickly, but the thrust would still push teh plane forward.

This scenario is impossible.

Re: think about this...
 

http://wwwsam.brooks.af.mil/af/files.../fig_28-10.gif

Re: think about this...
 

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If the plane is not moving, no air is being forced under its wings. If no air is being forced under its wings, there is no lift. So it stays grounded.

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When did I say the plane wasn't moving?

Re: think about this...
 

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If the plane is not moving, no air is being forced under its wings. If no air is being forced under its wings, there is no lift. So it stays grounded.

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When did I say the plane wasn't moving?

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you essentially gave us two velocity vectors of equal magnitude that work in exactly opposite directions. the plane has zero velocity relative to somebody standing on the ground observing it. it is going nowhere.


i think.

Re: think about this...
 

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If the plane is not moving, no air is being forced under its wings. If no air is being forced under its wings, there is no lift. So it stays grounded.

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When did I say the plane wasn't moving?

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you essentially gave us two velocity vectors of equal magnitude that work in exactly opposite directions.

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All I really told you is that the conveyor belt velocity and the wheels' tangential velocity are equal, essentially. But does this address the plane's velocity?

Re: think about this...
 

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If the plane is not moving, no air is being forced under its wings. If no air is being forced under its wings, there is no lift. So it stays grounded.

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When did I say the plane wasn't moving?

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you essentially gave us two velocity vectors of equal magnitude that work in exactly opposite directions. the plane has zero velocity relative to somebody standing on the ground observing it. it is going nowhere.


i think.

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Everyone seems to be figuring this from the standpoint that the plane's wheels are where the thrust is comming from. The wheels do not push the plane forward, the props do.

Re: think about this...
 

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If the plane is not moving, no air is being forced under its wings. If no air is being forced under its wings, there is no lift. So it stays grounded.

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When did I say the plane wasn't moving?

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you essentially gave us two velocity vectors of equal magnitude that work in exactly opposite directions.

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All I really told you is that the conveyor belt velocity and the wheels' rotational velocity are equal, essentially. But does this address the plane's velocity?

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i'm misunderstanding the way you're phrasing this, I suppose. the comments I made above assumed that the forward velocity of the plane was exactly canceled by the reverse velocity of the conveyor.

I thought your original post was implying that with a setup like this one could get the plane to take off vertically like a harrier jet without the vertical engines

Re: think about this...
 

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If the plane is not moving, no air is being forced under its wings. If no air is being forced under its wings, there is no lift. So it stays grounded.

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When did I say the plane wasn't moving?

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you essentially gave us two velocity vectors of equal magnitude that work in exactly opposite directions. the plane has zero velocity relative to somebody standing on the ground observing it. it is going nowhere.


i think.

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Everyone seems to be figuring this from the standpoint that the plane's wheels are where the thrust is comming from. The wheels do not push the plane forward, the props do.

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Why is that relevant? The thrust from the props = X. The "thrust" from the moving conveyor-runway = -X. The aircraft is not moving relative to the air. Hence, no airspeed, so no lift.

Re: think about this...
 

so do you get it now?

Re: think about this...
 

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The thrust from the props = X. The "thrust" from the moving conveyor-runway = -X.

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this is wrong.

Re: think about this...
 

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Everyone seems to be figuring this from the standpoint that the plane's wheels are where the thrust is comming from. The wheels do not push the plane forward, the props do.

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no. I understand what causes the plane to move forward.

Re: think about this...
 

What everyone is missing is that the plane is attached to a hook on a launching arm, and the runway is oiled and there is a very low coefficient of friction. The wheels are just spinning wildly, it just happens to be the same speed as the conveyor belt. I hope this makes it easier to understand. [img]/images/graemlins/grin.gif[/img]

Cheers,
Rube Goldberg

Re: think about this...
 

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If the plane is not moving, no air is being forced under its wings. If no air is being forced under its wings, there is no lift. So it stays grounded.

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When did I say the plane wasn't moving?

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you essentially gave us two velocity vectors of equal magnitude that work in exactly opposite directions. the plane has zero velocity relative to somebody standing on the ground observing it. it is going nowhere.


i think.

[/ QUOTE ]

Everyone seems to be figuring this from the standpoint that the plane's wheels are where the thrust is comming from. The wheels do not push the plane forward, the props do.

[/ QUOTE ]

Why is that relevant? The thrust from the props = X. The "thrust" from the moving conveyor-runway = -X. The aircraft is not moving relative to the air. Hence, no airspeed, so no lift.

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I agree, ground speed has nothing to do with it. It's all about airspeed (the speed at which air passes over the wing). Nothing else matters.

If an plane is moving forward at 50 MPH into a headwind of 100 MPH, the net airspeed is 150 MPH BUT the plane would actually be moving backwards at a rate of 50 MPH groundspeed....

Re: think about this...
 

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If the plane is not moving, no air is being forced under its wings. If no air is being forced under its wings, there is no lift. So it stays grounded.

[/ QUOTE ]

When did I say the plane wasn't moving?

[/ QUOTE ]



you essentially gave us two velocity vectors of equal magnitude that work in exactly opposite directions. the plane has zero velocity relative to somebody standing on the ground observing it. it is going nowhere.


i think.

[/ QUOTE ]

Everyone seems to be figuring this from the standpoint that the plane's wheels are where the thrust is comming from. The wheels do not push the plane forward, the props do.

[/ QUOTE ]

Why is that relevant? The thrust from the props = X. The "thrust" from the moving conveyor-runway = -X. The aircraft is not moving relative to the air. Hence, no airspeed, so no lift.

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I agree, ground speed has nothing to do with it. It's all about airspeed (the speed at which air passes over the wing). Nothing else matters.

If an plane is moving forward at 50 MPH into a headwind of 100 MPH, the net airspeed is 150 MPH BUT the plane would actually be moving backwards at a rate of 50 MPH groundspeed....

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too bad, i'm right.

Re: think about this...
 

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i'm right.

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this is true - you seem to be the only one grasping it so far. out of curiosity, what is your job/major?

Re: think about this...
 

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i'm right.

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this is true - you seem to be the only one grasping it so far. out of curiosity, what is your job/major?

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Computer tech/networking/misc

Re: think about this...
 

If a plane (specifically its wings) is traveling with zero speed with respect to the air, then there can be no lift. If the props are forcing air over the wings, there can be lift (though then the plane is not traveling with zero speed wrt the air, so we don't contradict the above). If you have some VTOL capabilities (really just VT) you could take off. I think in the scenario you described, you cannot, though I am not sure how much air the props push over the wings.

The discussion in this thread makes me think perhaps your phrasing is awkward.

Re: think about this...
 

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i'm right.

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this is true - you seem to be the only one grasping it so far. out of curiosity, what is your job/major?

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dude, i grasped it too!!!

Re: think about this...
 

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The discussion in this thread makes me think perhaps your phrasing is awkward.

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I agree with this, the OP is a bit convoluted, but everyone is seeing this as some sort of runnign on a treadmill scenario, when it really isn't.

Re: think about this...
 

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The discussion in this thread makes me think perhaps your phrasing is awkward.

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I think everyone here understands what makes a plane lift. The question is - can lift be created in this scenario?

My wording was intentional.

Re: think about this...
 

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dude, i grasped it too!!!

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my bad.

Re: think about this...
 

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The problem with this scenario is that the drive isn't created by the wheels, so this belt wouldn't result in the plane not moving, it would mean the wheels would be moving REALLY quickly, but the thrust would still push teh plane forward.

This scenario is impossible.

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Yeah, the exact speed I think will be determined by the friction in the axels given that the wheels are not skidding in any way. It's hard to picture the forces in my head on this one, so I am not sure if there can even be any force acting on the plane to counter the force of the thrust, but I don't know that it is impossible. I was assuming it was possible since I was reading it as a given.

Re: think about this...
 

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The problem with this scenario is that the drive isn't created by the wheels, so this belt wouldn't result in the plane not moving, it would mean the wheels would be moving REALLY quickly, but the thrust would still push teh plane forward.

This scenario is impossible.

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Yeah, the exact speed I think will be determined by the friction in the axels given that the wheels are not skidding in any way.

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I was thinking about this earlier, too. I think the wheel speed/conveyor speed would basically accelerate exponentially until the plane took off. Clearly this whole scenario is probably not physically possible, but it's an interesting theory to discuss.

Re: think about this...
 

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The discussion in this thread makes me think perhaps your phrasing is awkward.

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I think everyone here understands what makes a plane lift. The question is - can lift be created in this scenario?

My wording was intentional.

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So your basic question is "Is there a physical contradiction in the assumptions"? Thinking about it more, I think there is since you assume the wheels roll freely (i.e. no skidding). Angular momentum isn't a factor since those vectors will be perependicular to the direction of the plane.

Re: think about this...
 

This scenario is no different than if there was no conveyor runway but instead the breaks were locked on or the wheels were blocked. Sure there is some lift but not enough to take off.

Re: think about this...
 

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This scenario is no different than if there was no conveyor runway but instead the breaks were locked on or the wheels were blocked. Sure there is some lift but not enough to take off.

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No - it's actually more like if the wheels were locked, but the runway was basically frictionless. Or even better - if the plane had no wheels and was just able to hover in the air. And it can certainly take off in those scenarios, right?

Re: think about this...
 

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The discussion in this thread makes me think perhaps your phrasing is awkward.

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I think everyone here understands what makes a plane lift. The question is - can lift be created in this scenario?

My wording was intentional.

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The question fundamentally is what force is imparted on the aircraft by the conveyor-runway. You are saying that that force is less than the force of the props (are you saying that it is 0?). I don't see how that could be the case from a standing start, but I'm not an engineer.