Will It Take Off?

[dharvin]

A plane is standing on a runway that can move (some sort of conveyer belt). 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).

 

The question is:

 

Will the plane take off or not?

[Callipygous]

unless the conveyor belt has enough friction to get the air moving at about the same speed, no.

[Daecon]

Wouldn't it would be the equivalent of trying to take off from a frictionless surface, such as an ice-covered lake?

 

Is it possible to take off without any friction or traction under the wheels?

[padren]

Airspeed, specifically over the airfoils is what dictates lift. You can have a plane on the ground completely stationary, and a strong wind (100+mph or whatever is equal to liftoff speed) will cause it to lift - though, it will soon start to move backwards due to air resistance, and its relative airspeed will slow until it stalls and crashes.

[Bettina]

I'm a little confused about the question, because I have to assume an airplane, say a piper cub, has an engine running. If it does, and it is traveling at 80mph airspeed, it will get airborne even if the conveyer belt is moving at 80mph in the opposite direction.

 

Airspeed is not the same as groundspeed.

 

If we are talking about just groundspeed, then it won't. Thats my guess.

 

Bettina

[padren]

I'm a little confused about the question' date=' because I have to assume an airplane, say a piper cub, has an engine running. If it does, and it is traveling at 80mph airspeed, it will get airborne even if the conveyer belt is moving at 80mph in the opposite direction.

 

Airspeed is not the same as groundspeed.

 

If we are talking about just groundspeed, then it won't. Thats my guess.

 

Bettina[/quote']

 

The propeller is a propulsion aid to move the plane forward through air. If the conveyer underneath results in the plane not moving relative to the air and ground, it will not take off.

 

The only air moving over the wings would be from the propeller itself and be minimal, and the result would be as if you tied a plane to a pole and hit the throttle up to what you need to hit 80mph, but no real lift would be generated.

[smommer]

Aircraft wings work as the top side of the wing has a greater surface area than the underside of the wing (Square RAM-AIR parachutes work in the same way, theyre just inflatable wings). When air passes over and under the wing, lift is generated. A better description than i could write can be found here http://howthingswork.virginia.edu/home.html

 

When air flows past an airplane wing, it breaks into two airstreams. The one that goes under the wing encounters the wing's surface, which acts as a ramp and pushes the air downward and forward. The air slows somewhat and its pressure increases. Forces between this lower airstream and the wing's undersurface provide some of the lift that supports the wing.   But the airstream that goes over the wing has a complicated trip. First it encounters the leading edge of the wing and is pushed upward and forward. This air slows somewhat and its pressure increases. So far, this upper airstream isn't helpful to the plane because it pushes the plane backward. But the airstream then follows the curving upper surface of the wing because of a phenomenon known as the Coanda effect. The Coanda effect is a common behavior in fluids--viscosity and friction keep them flowing along surfaces as long as they don't have to turn too quickly. (The next time your coffee dribbles down the side of the pitcher when you poured too slowly, blame it on the Coanda effect.)   Because of the Coanda effect, the upper airstream now has to bend inward to follow the wing's upper surface. This inward bending involves an inward acceleration that requires an inward force. That force appears as the result of a pressure imbalance between the ambient pressure far above the wing and a reduced pressure at the top surface of the wing. The Coanda effect is the result (i.e. air follows the wing's top surface) but air pressure is the means to achieve that result (i.e. a low pressure region must form above the wing in order for the airstream to arc inward and follow the plane's top surface).   The low pressure region above the wing helps to support the plane because it allows air pressure below the wing to be more effective at lifting the wing. But this low pressure also causes the upper airstream to accelerate. With more pressure behind it than in front of it, the airstream accelerates--it's pushed forward by the pressure imbalance. Of course, the low pressure region doesn't last forever and the upper airstream has to decelerate as it approaches the wing's trailing edge--a complicated process that produces a small amount of turbulence on even the most carefully designed wing.   In short, the curvature of the upper airstream gives rise to a drop in air pressure above the wing and the drop in air pressure above the wing causes a temporary increase in the speed of the upper airstream as it passes over much of the wing.

 

So air needs to be travelling over the wing to provide the lift. If i got what you were saying right then what you are decribing is a bigger version of us running on a treadmill. we obviously remain stationary when this happens so there would be no air passing by us other than the wind that was blowing on that day. so no lift generated to get the airplane off the ground.

[Edtharan]

It is the speed of the air over the wings that cause lift, not the speed of the wheels on the ground. Regardless of how fast the weals are moving, if the speed of the air over the wings is not enough to generate the requiered lift then it will not take off.

 

Think of a kite. While holding the string the kite has no speed relitive to the ground. However the speed of the wind causes the kit to fly. If you let go of the kite then it quickly matches the air speed (or close to the air speed) and so the lift cause by that air is lost, but is is now moving relative to the ground, at rounghly the same speed as the air. The kite having lost the lift will fall to the ground.

 

So it is the speed of the air (air speed) that is important, not the speed compared to the ground (ground speed).

 

In a car you feel the air rush past you because you have speed relative to the air, you also have speed relative to the ground, but this is a different speed (think of driving into the wind as opposed to driving with the wind). In fact if you drove your car with the wind at the same speed as the wind then you would not notice the wind at all even though you are moving.

[Bettina]

The propeller is a propulsion aid to move the plane forward through air. If the conveyer underneath results in the plane not moving relative to the air and ground' date=' it will not take off.

 

The only air moving over the wings would be from the propeller itself and be minimal, and the result would be as if you tied a plane to a pole and hit the throttle up to what you need to hit 80mph, but no real lift would be generated.[/quote']

 

According to the original post the conveyer will match the planes speed, not the other way around and he says that the plane can move in one direction...which means it isn't tied down.

 

Since the propeller of an airplane screws thru the air and has no relation to the ground, it will move the plane forward until 80mph airspeed is reached in which then it will take off.

 

Bettina

[DV8 2XL]

The failure here is assuming that this system will permit the aircraft to remain stationary relative to the ground as the prop tries to pull it through the air. As Bettina point out, correctly: "the propeller of an airplane screws thru the air and has no relation to the ground", consequently the only effect of the conveyor would be to make the (free spinning) wheels of the plane spin twice as fast!

[J.C.MacSwell]

Aircraft wings work as the top side of the wing has a greater surface area than the underside of the wing[/b'].

 

This is not required although many designs are done this way.

[YT2095]

forget the over complication of the treadmill idea, just tie the back of the sucker to the ground, would it lift at all then?

 

the answer`s no, else it would have been used on aircraft carriers decades ago and billions would have been saved in VTOL engineering

[DV8 2XL]

Well I thought I had put this to rest with some finality two posts up but you managed to trump me YT!

[YT2095]

well 2 posts ago you mention about the wheels spinning 2x as fast??? that bit kinda lost me entirely, where did the twice as fast come from?

 

surely the max it would/could go would be full throttle at take off speed and no more?

 

eitherway the concept is flawed and will fail.

[DV8 2XL]

OK, it was an extension to B'tina's post.

 

If you think about it the prop pulls the plane through the air, so the aircraft will move relative to a fixed point off the belt regardless. Because the wheels are free spinning they are not coupled to the motion of the fuselage only to the belt (which is moving backwards) thus they will spin at an RPM relative to the belt plus the RPM relative to the forward motion of the plane (as if it were on a non moving serf ace). Your explanation was much more puissant than mine, I just wanted to give her credit for at least seeing the basic flaw in the model.

[padren]

According to the original post the conveyer will match the planes speed' date=' not the other way around and he says that the plane can move in one direction...which means it isn't tied down.

 

Since the propeller of an airplane screws thru the air and has no relation to the ground, it will move the plane forward until 80mph airspeed is reached in which then it will take off.

 

Bettina[/quote']

 

In that case yes. I thought the point of his post was that the conveyor would match the speed needed to nullify the forward effects of the propulsion, which is why I said it would not take off. But the plane would be pulling against air, and the conveyor would add very marginal resistance due to friction within the wheels even if the conveyor moved quite fast.

[YT2095]

if the plane does not move allowing that air to pass over the wings (for lift) you`ll just be wasting fuel! it will NOT take off.

 

a bit like a boat in a fast opposing current, if the water is traveling at 20 knots and the boat opposes that flow at the same rate, it`ll remain stationary.

[DV8 2XL]

Come on, the boat is working against the water, so that holds, the plane is working against the air - the ground isn't involved. Now if the original question was framed to stall the air over the wings, then sure, no flight - but that's not how it;s written.

[Bettina]

eitherway the concept is flawed and will fail.

 

I don't understand.

 

Bettina

[DV8 2XL]

Irregardless of what you assume the question to be saying the plane will not fly if no air moves over the wing OR the plane will fly because the belt will have no effect on the forward motion of the aircraft.

[Bettina]

Irregardless of what you assume the question to be saying the plane will not fly if no air moves over the wing OR the plane will fly because the belt will have no effect on the forward motion of the aircraft.

 

I understand what your saying, and your correct. However its not what the original poster asked. He asked "will the plane take off or not" (given the scenario he presented), so I am going to stick with my post #9 which I believe is right.

 

Bettina

[Cap'n Refsmmat]

Sure it will take off.

 

Remember that the ground and the wheels have no effect on the plane's lift. You may begin to move forward at 140mph, and the ground will move backwards--but unless you have pathetic bearings in your wheels, the plane will keep moving forwards.

 

edit: see padren's post below, he's right too, and he covers it better

[padren]

Just to play devil's advocate here, if the conveyor moved at say, 4mph, and the engine was off, and you held the plane in place with a rope tied to the nose while standing and holding it on the other end beyond the front end of the conveyor, due to friction, x pounds of force would be felt on the rope.

 

As you speed up the conveyor, more force is felt on the rope, and at some point, the speed of the conveyor will get high enough that the force on the rope is equal to the force of thrust generated by a piper cub's engine.

 

Since the original post asked us to assume that the conveyor automatically adjusted its speed to nullify the forward speed of the plane (that is effectively what he was asking, even if it was worded ambiguously) then wouldn't via friction force this still be possible if the conveyor moved at cartoonishly high speeds?

 

Those that said, "A conveyor will not prevent the plane from pulling through the air, so it will move forward regardless of the ground and take off" are correct because the conveyor is generally a flawed way of forcing a plane to not move forward.

 

But, if you take the conditions set by the opening post (in which the flawed method is assumed to actually stop the plane from moving forward) then the plane could not take off.

 

At this point its somewhat silly, since we all are on the same page and agree about the flaws in the system and the conditions of airspeed required for take off.

[Callipygous]

three cheers for arguing stuff that doesnt matter!

 

the question has been answered from the science perspective, and since i dont see this in the brainteasers section, i dont think all the semantics matter. if air is passing over the wings it has lift, otherwise it doesnt, regardless of the planes position with the ground.

 

question answered, yes?

[DV8 2XL]

B'tina - just so your clear on this my first post in this thread was in support of your interpretation.