Riddle me this batman

[Kipper]

MA'a theory sounds the most correct to me. It MAY rise into the air theoretically, but I ain't buying no ticket for that flight!
Ed

Ed, if it has sufficient lift to rise into the air at all then that means the plane is moving forward. The only thing the conveyor affects is the wheels, not the fuselage, wings, or the air around it. Book the flight! You only live once.

 

[DVINNY]

I'm being serious.

The plane would take off at the exact speed and acceleration it normally would.

However, the wheels would spin twice as fast on the conveyor as what they would on non-moving pavement.

 

[EdinNO]

The plane is moving opposite the conveyor, but at the same speed. How do the wheels spin twice as fast?

Ed

 

[DVINNY]

The plane is moving opposite the conveyor, but at the same speed. How do the wheels spin twice as fast?
Ed

Are you messing with me?

or do you really not get what I'm trying to say?

You're just eggin' me on aren't ya?

 

[Kipper]

sapper:

do you read my posts at all? FWIW: I'm an ME with a structural PE.

Planes don't need thrust to fly they need a relative velocity between the plane and atmosphere sufficient to produce enough lift under the wings to make the plane fly (ever heard of a glider?)

It has nothing to do with sailboat. Of course that powered by the realtive velocity between the air and the ground.

Using the same example if you had the wind blowing upstream at the same speed that the river is flowing down stream, you could theoretically have a sailboat that is filled with wind but making no headway upstream.

Just like uncles and cousins, it's all relative.

 

[DVINNY]

 

[DVINNY]

Essentially, the plane is moving twice as far relative to the conveyor, but not twice as far relative to the earth, wind, atmosphere whatever. The plane still takes off in the same distance as it would relative to the air. But because the conveyor is moving the same speed as the plane, the plane is twice as far from the starting point on the conveyor than it is releative to the air, and the wheels have to travel twice the distance in the same amount of time, causing them to spin twice as fast.

You're dead on brother.

 

[civengPE]

with our technical skills we should be able to do just that DVINNY.

I am thinking we could build a conveyor belt powered by a lawn mower engine. It should be around 50 feet long. We could then get one of those cheap battery powered R/C planes and away we go.

What do you think?

Do you think it would finally convince the non believers?

 

[DVINNY]

I'm just thinking that it would be impossible to have the speed of the conveyor match that of the plane's thrust.

Or I'd be game.

 

[civengPE]

I agree on matching the speed, but we could set it at the maximum speed of the plane.

ie. the worst case scenario.

we could just use some sort of backstop to hold the plane in place prior to hitting the gas. After that the plane should just pull away from the backstop.

One problem we have not discussed is controlability. I know I have enough trouble keeping the plane straight down the runway on a stationary surface. all control inputs on the conveyor will be multiplied until the control surfaces take ove and you are not steering by the nosewheel anylonger.

 

[Hill William]

If we are talking frictionless hubs, once the conveyor started, the plane would sit still (relative to the ground) unless you pushed it, right? No matter how fast the conveyor. Crank up the engines and it would apply a force using the air, right? Plane moves forward relative to the ground and takes off. Just my $0.02

 

[MA_PE]

You guys are killing me.

I interpret the problem statement as the conveyor belt is in fact a driven treadmill.

Draw dotted lines indicating a controlled volume (CV) around an image of the plane and the conveyor. Consider ythe conveyor as an infinte movable horizontal surface, one line entering the CV in the front and exiting in the back.

The jets start , they move air from the fron of the CV compress it and exhaust it out the back of the CV. They hit cold stagnant air and exert thrust that wants to push the body of the plane forward. If it were on firm ground it would translate in the x-axis. As soon as the jet overcomes static friction and starts to roll forward (on its wheels) the conveyor moves to the rear so as a point on the wheel rotates forward the contact points on the conveyor roll backward. Net is that there is no translation of the plane body. Resistance to the thrust is not constant because you have a compressible gas behind the jet that may or may not be able to resist the weight of the plane being pushed back by the conveyor. Da plane does not advance on the x-axis.

The exhaust gasses have displaced the static air immediately behind the jet (how far???) we don't know. Air sucked in by the jets is replaced by static air in front of the CV due to atmopsheric pressure.

As the thrust increases the plane body wants to move forward faster. The counter action of the conveyor keeps the plane body stationary. Simultaneously air is entering and exiting the CV at a faster rate.

My previous post lists the three outcome scenarios for this model.

the rotating speed of the wheels is completely irrelavent. What is important is 1) Can the air behind the CV keep exerting resistance to the thrust? If it doesn't then the whole thing reaches equilibrium and the flow throught he CV is not enough tot lift the plane so the plane and conveyor roll merrily along until you trun it off. 2) Will the air passing through the CV reach critcal velocity and provide lift? Note: the CV has a top boundary, too. Once the plane is outside of its own little "wind tunnel" what happens? dunno either it flys or see 3. 3)Can the air behind the CV keep exerting resistance to the thrust? If it does then it should lift off of the conveyor What happens? dunno see 2.

Kipper:

what the H-E-double hockey sticks are you talking about? The sailboat was for slapper's benfit that you can have a sailboat working it's ass off and go nowhere too.

DVINNY sorry dude, I don't know where your going with wheel speed but it is completely irrelavent to this problem in my mind. FWIW: planes need wheels to reduce friction so that they can attain thier critical velocity with minimal thrust. Plus it makes for nice landings.

 

[MA_PE]

Crank up the engines and it would apply a force using the air, right? Plane moves forward relative to the ground and takes off.

Air is a gas, it flows. think of a boat propeller in a running stream. not to mention gasses are compressible.

jet thrust is in thousands of pounds atmospheric pressure is 14.7 psi (2116.8 psf) you push more than that and you need to resistance from somewhere.

 

[Kipper]

The sailboat was for slapper's benfit that you can have a sailboat working it's ass off and go nowhere too.

:beerchug

But the plane isn't working its ass off, it is taking its ass off.

:beerchug

 

[civengPE]

:rotfl:

 

[DVINNY]

DVINNY sorry dude, I don't know where your going with wheel speed but it is completely irrelavent to this problem in my mind.  FWIW: planes need wheels to reduce friction so that they can attain thier critical velocity with minimal thrust.  Plus it makes for nice landings.

The wheels are VERY relevant because they are what makes the whole problem.

The planes thrust (acceleration) is relative to the air. The conveyor is relative to the planes speed. As that plane goes faster in the positive direction, the conveyor moves at the equal rate in the negative direction.

the (very important) wheels in between spins between the two, at the combined speeds.

If the wheels were fixed to the plane, and DID NOT spin, then they would create a force in the negative direction for the plane that would counter the thrust, but that is not the case. The plane is not fixed to the conveyor, it has wheels in between.

BIG DIFFERENCE

 

[DVINNY]

FWIW,

I'm having fun too. :thumbsup:

 

[MA_PE]

If the air moved at a rate of speed equal and oppostite the plane, then the plane wouldn't move forward because there would be a net force of zero of thrust against the air.

That is exactly where you and DVINNY go off track. Fluids CAN move past objects. Ever hear of a wind tunnel? How about the new craze of indoor sky diving.

The plane CAN fly without moving forward as long as the windspeed relative to the plane is greater than the critical velocity needed to provide enough lift.

How a bout a bird hovering over the ocean in the same spot or a kite flying?

It has nothing to do with forward motion.

DVINNY:

Don't be pullin' this stuff on me.

Hell, I say cut the damn wheels off, lay that airplane on its belly on the conveyor belt, fire that belt up, and the thrust from a jet engine will STILL propel that sucka up the road a piece. It will light that conveyor on fire, but that plane will still move forward.

then you go agreeing with me and trying to make it look like I'm inconsistent.

The wheels are VERY relevant because they are what makes the whole problem.
The planes thrust (acceleration) is relative to the air. The conveyor is relative to the planes speed. As that plane goes faster in the positive direction, the conveyor moves at the equal rate in the negative direction.

AAAAAH :suicide:

I believe that we're all converging on this one. good time, but I really should get some work done.

 

[Hill William]

Somebody should post this on "the other board". Im sure they would know whats up.

:"the other board":

 

[petergibbons]

If you were sitting in a wheel chair and had big wings strapped to your back, and the wheel chair were on a conveyor belt, And I was walking along side the belt on solid ground and pushing you, then you would move. If I pushed you fast enough the wings would provide lift and you would take off.

Sapper,

Are you saying this is equivalent to what we have in the problem statement?