airplane on convayor

[jraquet]

How is this...


You're standing on a very long treadmill wearing a pair of roller skates/blades holding onto a rope (the rope is the "air"). You start pulling on the rope (thrust): the air isn'tmoving but you are moving by pulling yourself through the air.


At the same time the treadmill begins to roll at the same speed you are pulling yourself along with the rope. What happens?


The wheels spin faster but you are still moving relative to the thrust you are creating by your arms with the rope. Once your airspeed (ropespeed) reaches that speed required for your lift to overcome your weightyou will take off.


Now, if you were using a hang-glider (an aircraft requiring initial take-off speed generated by ground contact not props/engines) you would have a hard time getting off the ground unless you could actually run faster than the treadmill: your airspeed is dependent upon how fast you can move forward relative to the ground/floor not to the treadmill.

 

[verge]

I guess the hardest part of this question is simply visualizing it. As long as the plane is moving in relation to the ground, the whole thing is easy to visualize. I'm obviously messed up on this thrust thing - no-one's been able to explain it to me in a way that's easy to visualize. I'm not pulling air over the wings, I'm pushing matter out the back, which is causing me to move forward in relation to theair - correct? Yes, I am an engineer, but fortunately not an aeronautical engineer.


I'm a drummer, and I daresay a very good one, and when I play I don't think of notes, I think of images. Can't understand a darn thing if I can't visualize it.


I'm getting close with this roller blade thing.

 

[dross]

You need airflow over the wings to create lift. If the plane is not moving through the air, you have no lift, no matter how fast the wheels may be spinning.

 

[verge]

jraquet,


I keep visualizing the engine "pulling" the air over the wings, and your description is another "pulling" visualization for me

. I guess I should be remembering that thrust works on the principle of equal and opposite reaction - the engine shoving the air, and the air shoving back hard enough to move the plane.


What about a prop-driven plane? (Don't laugh, I went to Michigan Tech, deep in the northern reaches of Michigan's Upper Peninsula. They have one plane that flies back and forth to Minneapolis four times a day - it ran on props!) Same principle or different as far as thrust is concerned?


All right, so I'm just fishing for free engineering knowledge. Whaddaya think I come to the forum for?

Maybe I should just stick to drumming...hit something, make noise.

 

[SmWave]

Question:
A plane is standing on a runway that can move (some sort of band conveyer). 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).<BR style="FONT-STYLE: italic"><BR style="FONT-STYLE: italic">Will the plane take off or not? Will it be able to run up and take off?

The problem with this hypothetical question is that a plane's speed is not measured by ground speed. There's not speedometer tied to the wheels of a plane. The plane measures it speed from the pressure of air and speed of the air flowing past the wing.So the belt or conveyor wouldn't know how fast the plane was going until it was moving. As long as the planes not moving it won't get airborn. Lift = Thrust - Drag. Amatuer RC pilot.
Edited by: SmWave

 

[magneplanar]

in any hypothetical question there will be aspects that confuse or require some measure of faith. You might question how something is measured or how something can move. The question finally becomes can you imagine the scenario. It is a test of your mind. All the issues, including mine, for why it would or would not work are based in some experiences ( I said no at first ). I doubt they would make a conveyor belt for this purpose since the power to run it would be far more than the savings gained.

The plane would take off in this scenario. The speed on the ground could be measured by some other means than the aircraft airspeed indicator. Laser speed detection would work fine, but this is a non-issue. The capability to make a conveyor belt or measure the speed is not part of the equation. The equation is: would the plane at rest be able to take off if the belt matched the speed. The answer is yes. Simply put, the planes wheels would provide almost no resistance, there is no gear, or drive shaft to provide an opposite force since they spin freely. So that means the belt speed does not matter because the belt can only affect the planes wheels. It would be a much harder scenario if the wheels were removed and the belt could actually affect the body of the plane. I doubt many planes could take off if they were resting on their fuselage.

In any case, the engines push aft against the standing air to provide thrust/forward movement. That air is always there so the engine thrust would be effective. The wings would reach a speed to produce enough lift because the airplane would move forward via the thrust. Since the thrust is effective and the wings provide lift the equation is solved and liftoff would occur. Nothing that the conveyor belt does would change that.

try some experiments yourself...for those that still can not visualize it. Believe me its a trick question meant to get you thinking down a specific train of thought. So step back, evaluate from another angle and see if you can find it...just remember that the aircraft wheels spin freely and the conveyor can only affect the wheels.

cheers,

M

 

[1ntegr8r]

Try this: get a wheeled cart (like a shopping cart or whatever) and put it on your treadmill at home. Then turn the treadmill on and stand behind it on the ground. Now push the cart forward. Is the cart standing still? Are you? This just illustrates the concept of an inertial refeference frame. In this case, the ground around the treadmill stands in for "inertial space". With respect to inertial space or the Earth at large, the plane will move, air will pass over its wings and it will take off. The wheels will simply be spinning twice their normal speed. So, this would even work in a vacuum (of course the plane wouldn't take off, but it would stil move forward) because the engines are not even "pushing against" air, it's simply Netwon'sThird Law.

 

[AHA-D]

... ahum ... in a vacuum the engines wouldn't even be running since they use the surrounding air to burn fuel.


But now we're really tearing this problem apart.


Alex

 

[1ntegr8r]

I meant for rocket engines in a vacuum, not turbofans

 

[verge]

The engines push aft against the standing air - now that's an effective visual. That hasn't shown up yet in this discussion. Nice.

And if I'm not mistaken, the reaction force of the air pushing back is what moves the plane forward. This has always been one of the most difficult things in physics for one to visualize.


My first though when initially "taught" this in school was something like "When I push against a wall it pushes back? What, are you nuts?"

Anyway, nice stumper. I must say this thread has been a blast! And no harm done! We need more of these...

 

[1ntegr8r]

You are mistaken. The air"pushing back" does not move the vehicle. What moves the vehicle is the reaction force of the exhaust gases moving out the back of the engine. This is why it would work in a vaccum with rocket engines. Haven't you ever wondered what there is in space for a rocket to "push against"? Nothing! F = m*a. The force of the ejected exhaust moves the vehicle pure and simple.


OK now I'm done with this thread

 

[verge]

My mistake. I guess I'm no rocket scientist.

Although I do have an acquaintance who's a rocket scientist. He wears a cool t-shirt that says "As a matter of fact, I am a rocket scientist!"


I have thought about this in the past, and then I'd go play my drums for a while to keep my brain from exploding!


I'm trying to be done with this thread, but I'm waiting for everyone to finish with correcting me.

Thanks, 1ntegr8r, for solving this mystery for me. Now I can play my drums in peace!

 

[magneplanar]

integr8r,

interesting thoughts...do recall that rocket engines have their own oxidizers and that is why they can burn in space...as for movement, the rocket engine pushes and the craft moves opposite as an effect of conservation of momentum. which is the same thing you feel if you shoot a shotgun, as a recoil. So the rocket in space only needs to push against the spacecraft to make it move, by pushing against the engine itself, since there is very little else to keep the spacecraft from moving.
How did we get onto rockets?

Suffice to say that any engine in the atmosphere will have different properties than in the vacuum of space(if they even work...747 will never fly in space with jet engines). In the atmosphere, the air is dense enough to allow lift over wings and thrust from jet engines to be effective (which relates to this original question). Rocket engines work too, as we all know, but their actions change when the craft leaves the atmosphere.

Here is a question then: can spacecraft be heard in space if you are not on the spacecraft?

cheers,

M

 

[jbuckl]

Question:
A plane is standing on a runway that can move (some sort of band conveyer). 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).<BR style="FONT-STYLE: italic"><BR style="FONT-STYLE: italic">The question is:<BR style="FONT-STYLE: italic"><BR style="FONT-STYLE: italic">Will the plane take off or not? Will it be able to run up and take off?


New challenge: design this conveyer system!


Its not possible for the (a) conveyer to be designed to keep up with the plane because the plane will move forward. Then there will be a wheel speed to conveyer speed difference which the control system with try to correct (but cannot) this means that damage will occur before takeoff due to excessive wheel and conveyer speeds.


But 'if' it was possible for such a conveyer, the tyres / wheels and conveyer would be damaged in the event of takeoff,


leading to a plane that needed to land without landing gear intact.


Perhapsthe pilotcould locate a suitable rotating conveyer to land on, thus no longer needing to rely onhis planesundercarriage being intact.........


regards


Jbuckl







Edited by: jbuckl

 

[prohammy]

integr8r,

Here is a question then: can spacecraft be heard in space if you are not on the spacecraft?

cheers,

M

Here's an interesting tit-bit from the wonderful world of the internerd...


http://www.space.com/scienceastronomy/blackhole_note_030909. html


Should answer all your questions and open up a whole new world of musical interpretation. I have already composed a whole symphony based on he note (unfortunately due to its range most of you people will be never truly appreciate its complexity....Ahh well


Kev


EDIT


Actually, maybe this where Soundgarden got their idea for 'Black Hole Sun'
Edited by: prohammy

 

[Dell_Boy]

From basic physics

In order for the conveyor to stop the plane from accelerating up to lift off air-speed the reversing force it applies to the plane must equal the static thrust of the engines.

The reversing force the conveyor can apply has two obvious sources.

The mass of the plane x the coefficient of the wheel bearing friction (plus a bit of rolling resistance)

The force required to angular accelerate the wheels.

Theoretically, if you can (continuously) accelerate the conveyor fast enough the angular acceleration force could be made large enough to counter the static thrust of the engines. In practice, not a chance in hell.

I haven't crunched the numbers out because I know the linear acceleration of the belt would have to be ridiculously fast because the rotational inertia is so tiny in comparison to the plane. Also if the conveyor could keep up this acceleration, even only briefly, the tyres and wheels would disintegrate, as jbuckl mentions, through overspeed which is often not much above maximum allowed take-off speed.

So the likely result is that the plane would get up to a reasonable percentage of take-off speed before it's undercarriage fails causing it to slew off the runway and kill all the spectators assembled to watch the experiment.


DB

 

[fiebigc]

What does this have to do with Pro/Engineer?

 

[vlad1979]

What does this have to do with Pro/Engineer?

Nothing and everything depending on how you look at it.

Here is a question then: can spacecraft be heard in space if you are not on the spacecraft?

cheers,

M

I believe it can, because sound travels thru space, otherwise those guys with the dishes (Arecibo&co and the others in New Mexico)that listen to the sky all day long are laying to us saying they can hear sounds coming from space.
Edited by: vlad1979

 

[magneplanar]

so, according to prohammy

someone has a big subwoofer playing a b-flat tone making the whole galaxy shake. This in turn causes friction in our atmosphere, and is the real cause of global warming? ( some creative license of course)

Because of that my computer is overheating due to todays high temperature and causing pro-e failure diagnostics to ...fail...and now I am unable to finish my mechanica analysis of the 3d conveyor runway stresses applied by the weight of the aircraft.

how is that for connecting the dots...

cheers,

M

 

[prohammy]

so, according to prohammy

someone has a big subwoofer playing a b-flat tone making the whole galaxy shake. This in turn causes friction in our atmosphere, and is the real cause of global warming? ( some creative license of course)

Because of that my computer is overheating due to todays high temperature and causing pro-e failure diagnostics to ...fail...and now I am unable to finish my mechanica analysis of the 3d conveyor runway stresses applied by the weight of the aircraft.

how is that for connecting the dots...

cheers,

M

Superb interpretation and analysis of one piece of web-based information.


If I may suggest, there aresome far reaching implications of the B-Flat note that I think you may have overlooked. The B-Flat note will be consistantly 'playing' for the life of the black hole. Therefore, for the life of the black hole you will not be able to finish my mechanica analysis of the 3d conveyor runway stresses applied by the weight of the aircraft. By extension, planes will no longer be able to take off and our whole planet is now effectively grounded.......Damn black holes, ultimately the ruin everything


Kev