160 replies to this topic

[Corsair1138]

On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The 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 airplane ever take off?

[newflyer]

You must be a member of sp.com, huge discussion over there.  Anyway, the answer is no, in order to take off, there must be air going over the wings.  Essentially, the plane is moving forward at x speed, and backward at x speed the same time, thus producing no airspeed.  Thus, no flight.

It's the same as running on a treadmill, even though you're running really hard, you feel no wind hitting you, no airspeed.

Edited by newflyer, 29 November 2005 - 02:57 PM.

[C172pilot]

well, you are in a sense, producing a false sence of airspeed. You are trying to produce enough foward motion to get lift, but obviously, without any REAL foward motion you can't lift off.

It is a good question, but no. I wouldn't lift off.

It reminds of the time I flew backwards.. I was doing MCA(slow flight) and my airspeed was about 33kts. the winds aloft @ 3k however, were gusting to 40. So I had a false sense of lift as the wind acted as if I were flying through the air. It is SOMETHING VERY FUN!!!

[Corsair1138]

Newflyer, I'm not a part of sp.com. This came up on LS1.com and my brother is on this board. He asked me to bring it up for discussion here and see what everyone else would think, considering most should have some knowledge of aeronautic physics.

But, your analogy of running on a treadmill is incorrect.  You would stand in one place in relation to the ground because you use the ground for propulsion. An airplane doesn't use the ground for propulsion.

Edited by Corsair1138, 29 November 2005 - 03:32 PM.

[citationpilot]

You would take off. The tires have no effect on the airplane's propultion, they are only there to reduce friction. You are all thinking in the sense of a car, remember the wheels are spinning freely.

Edited by citationpilot, 29 November 2005 - 08:12 PM.

[Trent.]

You wouldn't be able to lift off. Even though your plane is going at t/o speed, you still need to factor in wind to create lift...which you won't have just moving in one spot.

[jetBlue 32]

It doesnt matter, you will never takeoff....lift is produced by the air flowing over the wing surfaces, with no air flowing, there won't be any liftoff whatsoever, not even close.

[motorman]

Beats the crap out of me, but I want the department of defense contract  so I can go out and buy a pilatus pc-12   (a real one).Here is another one if a 747 is full of
doves and the doves are let out of their  cages  does the planes weigth go down?
If a man walks down the asile of a 747 then walks back and hits another person going in the opposite direction is that a 1000 mph collision?
just food for thought
motorman

[citationpilot]

If you can't take off on this conveyor belt how would you be able to take off on ice in a ski equipped plane? Relative to your plane the ice is moving backwards as quickly as your aircraft is moving foward, the same as this conveyor belt. That's about as simple as I can put it.

[Majestyk]

err....I don't think so, not

[Hawker]

citationpilot, on Nov 29 2005, 09:10 PM, said:

If you can't take off on this conveyor belt how would you be able to take off on ice in a ski equipped plane? Relative to your plane the ice is moving backwards as quickly as your aircraft is moving foward, the same as this conveyor belt. That's about as simple as I can put it.

um....all that would happen on a belt is the tires would spin fast, the plane wouldnt lift at all.

[citationpilot]

You guys aren't using any logic, you just blurting out the first thing that comes to you mind. Read through this thread:

http://www.jetcareer...ead.php?t=22291

and this one

http://forum.physorg...topic=2417&st=0

Quote

Yes, the plane will take off, practically as it normally would. If you couldn't see the "runway" moving, you would see no difference to a normal takeoff, unless you could discern that the plane's wheels were spinning twice as fast as normal.

The reason: The plane, even if on the ground, accelerates relative to the AIR! Thanks to its wheels there is very little friction between it and the ground it stands on, so what the ground does is of almost no importance.

Now, if you would put wings on a car, that would be different. The takeoff would not work, because the car uses its wheels to push itself forward relative to the GROUND. It would not achieve any speed relative to the air, so its wings could create no lift.

That about sums it up.

Edited by citationpilot, 29 November 2005 - 10:07 PM.

[Prancer]

Maybe someone just needs to put this to the test. Build a big conveyor built and put a plane on it.

[wnfaknd]

Im with citationpilot on this one.  The airplane can indeed take off.
This is a trick question.
The statement says that the airplane is moving in one direction while the conveyor belt is moving on the opposite direction at the same speed.  The airplane can still move forward as the wheels spin freely, or better yet, they will not move at all.

In essense, you will have an airplane moving forward but the wheels will not be turning.

Once you reach Vr, you will indeed rotate and lift off... and you will not need to tap on the brakes before you retract the gears

Edited by wnfaknd, 29 November 2005 - 10:16 PM.

[citationpilot]

This is about as clear of an explination that I've heard.

Quote

It's not poorly worded. The treadmill will match the planes speed. So when the plane is at rest the treadmill will not be going. However, when it reaches 1mph, the treadmill will be going 1 mph in the opposite direction. Result? The plane is still going 1 mph forward but the wheels are spinning at 2 mph. When the plane accelerates up to 100 mph, the wheels are now going 200 mph.

Edited by citationpilot, 29 November 2005 - 10:23 PM.

[FL050]

citationpilot, on Nov 29 2005, 08:12 PM, said:

You would take off. The tires have no effect on the airplane's propultion, they are only there to reduce friction. You are all thinking in the sense of a car, remember the wheels are spinning freely.

True.  But if the wind is calm, there is no lift going over the wings which is the key factor in all of this.  It doesn't matter what airspeed your going, if there is no wind flowing over the wing creating lift then the plane will not get in the air.

[Trent.]

Exactly how I feel...because that is the whole reason how an airplane actually flies..lift.

Now i've been sitting here doing some serious thinking on this question..and it came to my mind that if you were to test this you could basically look at it like a wind-up toy plane. Holding the plane in your hand, you can wind the prop up, let it spin and let go...the plane will fly from the position that you let go from. Honestly I can argue for both ways..but until I actually see someone prove it or make it clear enough my answer stays idle.

[wnfaknd]

citationpilot, sorry for the bad name there, its been corrected.

FL050 and the others, you are missing the point.
The airplane will be moving forward.
About my statement that the wheels would not be spinning, i was wrong on that.  The wheels will be moving at twice the speed like citationpilot said.

Picture 100 really strong man pulling on the airplane with a bunch of ropes.  These dudes will be able to move the airplane forward, the conveyor belt will be moving backwards at the same speed and the net result will be the wheels moving at twice the speed.  But if these dudes, and believe me, they are really really strong dudes, can pull fast enough, the airplane will lift off.  Of course, these dudes will not be standing on the conveyor belt.

I hope that made it a little bit more clear.

[citationpilot]

Who said there is no airflow over the wings? The treadmill is moving as fast as the plane is, but that just means the tires will have to spin twice as fast as normal, the treadmill will not restrict the movement of the airplane, it will just make the tires spin twice as fast as normal. Read some of the quotes I have posted.

[FL050]

If the airplane is moving forward and the conveyer is moving backwards at the same speed, the airplane will not be moving forward.  How do you get there will be relative wind if a plane is sitting still?  Yes the plane will be moving forward on the belt but then the belt is moving backwards.  So essentially the plane is moving on the belt, but it is not moving through the air.