Physics question/riddle. This is a good one, read it.

[subzali]

Okay, my FBD. I separated the plane and wheel (which is representative of ALL the (trucks I guess??)) to make it easier to illustrate. As long as ALL forces are present and equalized it don't matter. We won't worry about w and N, they don't affect the point I'm trying to make. I'm trying to show that the plane accelerates with respect to a stationary reference point. T is the thrust from the engines. f is the frictional force from the conveyor onto the wheels. Consider the plane+wheels as a system. The ONLY external horizontal forces to the system are T and f. There are NO other forces acting on the plane in the horizontal direction. We all know what T does, so it comes down to what does f do?

f turns the wheel. Take a bicycle tire and hold it at the axle and run it along the pavement, this is exactly as if you were holding it stationary and the conveyor was running underneath it. Feel any significant forces that try and keep the wheel from moving in translational motion? You shouldn't, unless your bearings are shot. *ALMOST* all the force from f produces the moment for the wheel to spin, *ALMOST* none of it gets translated to the axle, which is why I didn't include it in the drawing. But go ahead, add a force acting against the thrust on the axle. What's it gonna be? 5 lbs.? 10? Vs. 25000 for T? If there is nothing (or next to nothing in comparison) acting against T on the axle of the wheel, it's NOT getting translated to the plane, so it comes down to the point that there's a NET thrust of CLOSE to T in the forward direction. Remember we've already accounted for the conveyor, so what happens when there's a net force? F=ma, and the plane accelerates with respect to a STATIONARY reference frame. It will displace itself from its starting position JUST ABOUT the same distance a plane does on a normal runway, it will pick up speed as it does this (remember this STILL takes into account the conveyor, so don't even try it ), air will flow over the wings, lift will be generated, and the plane will take off just about like any other plane that takes off from runways around the world.

Summary: the conveyor makes little to no difference on the takeoff characteristics of the plane. It will need runway space, it will take a few seconds to gain speed, it will have speed when it takes off and keep flying on itw merry way. The ONLY thing that the conveyor does by travelling backwards is it makes the wheels go faster.

[Hawke]

There better be a verifiable answer to this question or I suspect a frusterated group of Mudders will hunt Arya Ebrahimi down and run him over. Maybe on a conveyer belt.

[sisukid1975]

Originally Posted by subzali

Okay, my FBD. I separated the plane and wheel (which is representative of ALL the (trucks I guess??)) to make it easier to illustrate. As long as ALL forces are present and equalized it don't matter. We won't worry about w and N, they don't affect the point I'm trying to make. I'm trying to show that the plane accelerates with respect to a stationary reference point. T is the thrust from the engines. f is the frictional force from the conveyor onto the wheels. Consider the plane+wheels as a system. The ONLY external horizontal forces to the system are T and f. There are NO other forces acting on the plane in the horizontal direction. We all know what T does, so it comes down to what does f do?

f turns the wheel. Take a bicycle tire and hold it at the axle and run it along the pavement, this is exactly as if you were holding it stationary and the conveyor was running underneath it. Feel any significant forces that try and keep the wheel from moving in translational motion? You shouldn't, unless your bearings are shot. *ALMOST* all the force from f produces the moment for the wheel to spin, *ALMOST* none of it gets translated to the axle, which is why I didn't include it in the drawing. But go ahead, add a force acting against the thrust on the axle. What's it gonna be? 5 lbs.? 10? Vs. 25000 for T? If there is nothing (or next to nothing in comparison) acting against T on the axle of the wheel, it's NOT getting translated to the plane, so it comes down to the point that there's a NET thrust of CLOSE to T in the forward direction. Remember we've already accounted for the conveyor, so what happens when there's a net force? F=ma, and the plane accelerates with respect to a STATIONARY reference frame. It will displace itself from its starting position JUST ABOUT the same distance a plane does on a normal runway, it will pick up speed as it does this (remember this STILL takes into account the conveyor, so don't even try it ), air will flow over the wings, lift will be generated, and the plane will take off just about like any other plane that takes off from runways around the world.

Summary: the conveyor makes little to no difference on the takeoff characteristics of the plane. It will need runway space, it will take a few seconds to gain speed, it will have speed when it takes off and keep flying on itw merry way. The ONLY thing that the conveyor does by travelling backwards is it makes the wheels go faster.

Yeah, I drew one too that looks a lot like that... but I didn't post it cause I thought I could logic it out... Just trying to make clear why the plane moves in the first place, which I though the magnetically levitated thing was pretty clever for. I figured once it was clear why the plane moves at all that the rest would just click, but sometimes the first part is the hardest.

Oh, well....

Time for bed anyway.



Fred

[srafj40]

I don't think that is how you spell plene

On speeelling alone, I don't buy it.

[subzali]

I think people were thinking it would levitate off the conveyor. It wouldn't. It would fly off the conveyor. BTW Physics undergrad (my roommate) thought about it for less than 5 seconds and could explain everything about it. That's TWO Mines kids you're messin' with now

[Cube Dweller]

f you take your cruiser off a sweet jump, it loses velocity when it's wheels are not in contact with the ground. Because it 's forward motion comes from the wheels rotation aginst that ground. But what happens when a plane becomes airborn? It doesn't lose speed, it usually gains it. Why? Why does a plane not need the runway in order to maintain it's speed?

I'm not arguing the plane will not exert thrust against the still air and want to move forward. I'm arguing this initial forward movement will cause the wheels of the plane to turn which will cause the roller or conveyor to turn so the relative velocity of the air over the wings will be nil(since it's a still day) + whatever boundary layer turbulence e9999 thinks might happen. But that will not be enough to generate the lift needed for the plane to fly. Without lift the plane sits on the conveyor and continues to accelerate until it reaches it's terminal velocity. Assuming of course it's a regular fixed engine jet with the nozzles close to parallel to the ground.

subzali, I don't think your solution includes the lift that's needed to get a plane off the ground. To me it seems like your treating this like a rocket rather than a plane, where the fins on a rocket are for stability and direction rather than lift. I think by your logic then the wings on a plane are irrelevant for lift? If the conveyor can exactly match the speed of the wheels of the plane, how does the plane ever move relative to the still air of the earth, generating lift(other than the initial minute amount to start the conveyor moving)?

EDIT: I didn't look at your FBD yet, maybe tomorrow.

[sisukid1975]

Originally Posted by subzali

I think people were thinking it would levitate off the conveyor. It wouldn't. It would fly off the conveyor. BTW Physics undergrad (my roommate) thought about it for less than 5 seconds and could explain everything about it. That's TWO Mines kids you're messin' with now

Right, cause it moves through the air, it doesn't get it's forward motion by imparting force on the pavement, it get's it's motion by imparting force on the air, which we are assuming is still.

I have to admit, I first thought no, then I slowed down and rethought it and found my mistake....

Anyway, yeah, two mines students, one Helsinki student.... It's all good.

Take care.

Fred

[srafj40]

I need to ask this.

If the plane does fly wouldn't the plane stall at the low airspeed?

[IdahoDoug]

For you "no fly" types. Remember that it is the plane's FORWARD MOVEMENT that starts the conveyer. That's movement over the ground (and of course, through the air) So, the plane is moving at 100mph relative to the ground. And the conveyer belt is moving at 100mph rearward. When the plane is moving forward at 100mph it is indeed moving truly forward. Yep, the conveyer is whizzing the opposite direction, but all that's doing is spinning the tires. Close your eyes and envision an airplane going down a runway from right to left at 100mph. That's indeed what's happening in this problem. That the runway is moving the opposite direction at 100mph under it is near meaningless.

Here's another fascinating bit of airplane trivia that amply demonstrates the raw power of a modern passenger jet's engines. No names here. It was related to me that someone NOT in my family of pilots once piloted a fully laden 747 down a wet runway. After clearing the runway, the copilot brought 3 red lights to the pilot's attention. The brakes were fully locked. Yes, Virginia a 747 generates enough thrust to drag all of its locked tires down a wet runway and take off. Now that's power. Unfortunately, they had to immediately return to the airport under emergency landing conditions and have all 20 something tires replaced....

Nothing really to do with the problem here but interesting nonetheless.

At any rate, the airplane in the problem will accelerate to takeoff speed and nip out of there.

DougM

[sisukid1975]

Originally Posted by Cube Dweller

I'm not arguing the plane will not exert thrust against the still air and want to move forward. I'm arguing this initial forward movement will cause the wheels of the plane to turn

You admitted that thrust against the air causes it to move initially, that's a start.

But after it starts rolling, why does it keep accelerating? Because of that same thrust through the air.

The runway means nothing to the plane, since it is getting acceleration from imparting force on the air.

Fred

P.S.: I'm really going to bed now. I'm sure this albatross will still be alive tomorrow morning, and possibly for many days to come.

[subzali]

Well sure I'm leaving the lift of the wings out, once it's shown that the plane accelerates relative to a stationary reference frame, (and referencing my earlier post about how the air is stationary relative to the stationary reference frame - thus the plane is accelerating with respect to the air - this is the KEY), then the plane will eventually gain enough speed with respect to the stationary reference frame and the air that lift is implied to happen. Once it's shown that the plane accelerates, everything else follows suit. It's not really that much different. The wheels and conveyor have zero effect (or close to that) on the acceleration of the plane (with respect to the stationary reference frame).

[Cube Dweller]

Originally Posted by ary

the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction

Originally Posted by suzali

It will displace itself from its starting position JUST ABOUT the same distance a plane does on a normal runway, it will pick up speed as it does this (remember this STILL takes into account the conveyor, so don't even try it )

If Ary's magic conveyor can exactly match the speed in the opposite direction, how can you negate this and claim the plane moves almost the same distance ? 'splain pls.

We need an aeronautical guy to chime in(I'll send this to my buddy manana), but I seem to recall the thrust is generated from the expanding gases pushing off air molecules behind the nozzle. This is kinda vague since it's been more than 10 years, but if so, I think your FBD lacks this reaction which would account for the T.

[LoveTractor]

Dammit. I couldn't sleep. Had to google the damn problem. I want to change my vote.
The conveyor belt is irrelevant. The wheels are, for the most part, free-spinning. The engine/prop/jet will eventually produce the critical amount ot force to put the plane into "flight."
I have an awful example I came up with, but I understand it.
Let's say the Earth rotates at X speed along the equator east to west. A plane travels just above the equator at X speed along the equator west to east. Is the plane flying? Yes. If the plane (theoretically) lands along the equator on its "free-spinning" wheels, does it just stand still? No. The wheels are just spinning at 2X speed. If the plane pulls up, is it flying again? Yes. Sort of a grandiose example, but the best I could come up with.

[subzali]

Originally Posted by srafj40

I need to ask this.

If the plane does fly wouldn't the plane stall at the low airspeed?

You're making an assumption in that question. The plane needs to have a certain velocity relative to the air to take off, and once that's reached it won't stall. There's no other way that the plane will take off, it will not "levitate" off at a low speed (relative to the air). It will need a full takeoff-length, reach full speed of a normal aircraft, but since the conveyor/wheels have nothing much to do with counteracting the thrust of the engines, all that happens about the same as a normal plane.

Think about this: Consider the plane is stationary relative to a stationary reference frame, but the conveyor is moving underneath it. Does the plane move? Maybe slightly backwards due to real friction in the wheels/axles whatever, but as soon as you add 25000 lbs. of thrust in the opposite direction which way do you think it will go? It will reach the speed of the conveyor (albeit in the opposite direction) and then surpass it (in the opposite direction) and then just keep going until it takes off. Wheels and conveyors, no matter what speed they are running, have a "negligible" effect on the plane.

[IdahoDoug]

Cube,

The answer lies in your own reply. The magic conveyer exactly matching the speed in the opposite direction. The plane MUST move relative to the ground in order to start the conveyer moving. So, the plane gets to 5mph over the ground and the conveyer matches it by moving under the wheels at 5mph in the opposite direction. A few seconds later, the plane is moving at 50mph over the ground and has travelled perhaps 500 feet and the conveyer is whirring away under its wheels. A few seconds later the plane has travelled 1000 feet and is moving at 100mph.

Briefly taking the opposite view. If the plane does not move relative to the ground, the conveyor will not move either. It's a brain teaser until the light goes on. By definition, the plane MUST move relative to the ground for the conveyor to start moving also. It is this movement relative to the ground that is the key - the plane will take off normally.

DougM

[LoveTractor]

Another stupid example. A plane is flying at 20,000 feet. This conveyor belt is (theoretically) built onto a flying carrier which flies under said plane and turned on. Plane lowers it's free-spinning wheels, and "lands." The conveyor belt is only moving the wheels. The plane(fuselage and wings) is still flying.
Bad example? Don't care. I'm going back to bed. I'll dream of planes, conveyor belts, wheels.......

[iron_giant]

Im a novice but I'll give it a shot-
imagine a plane "Touching Down" on said conveyer which matches (counters) its speed while its in the air and adjusts as necessary. As soon as the wheels touch they would spin forward twice as fast as the plane was moving, but would exert little to no force affecting the planes forward movement. If the conveyer did affect the plane then the plane would suddenly stop its forward movement as soon as the wheels touch the conveyer. Long story short the plane would run off the conveyer because the plane is moving forward.

[subzali]

Originally Posted by Cube Dweller

If Ary's magic conveyor can exactly match the speed in the opposite direction, how can you negate this and claim the plane moves almost the same distance ? 'splain pls.

So what if the conveyor matches the speed? All it does is turn the wheels faster, it doesn't have any real effect on the plane itself. Again try the bicycle tire. Have a friend hold the axles, and then take a belt sander to the rubber on the tire. It will turn the tire, but I bet your friend won't feel a WHOLE lot of force trying to take it out of his/her hands...so back to the plane, the horizontal force from the conveyor doesn't really counteract the thrust from the engines enough to make that much of a difference.

Originally Posted by Cube Dweller

We need an aeronautical guy to chime in(I'll send this to my buddy manana), but I seem to recall the thrust is generated from the expanding gases pushing off air molecules behind the nozzle. This is kinda vague since it's been more than 10 years, but if so, I think your FBD lacks this reaction which would account for the T.

Yes that's true that it's pushing the air molecules. But at that point the force is outside the system and not cancelling anything, so I don't see your point? T is the force from the air on the engines of the plane (created by the turbine/nozzle etc.), which pull the plane forward.

[bad_religion_au]

Originally Posted by Lashcat

Yes it will take off. Planes gain lift from air speed not ground speed. The prop would still move the plane forward due to the thrust produced by the prop. The speed of the convayor would only make the wheels thurn faster. If this were to be a real life situation i would hate to be the pilot tring it due to the fact of trying to control the plane until it gained enough speed to gian flight. Too much brake on either pedal would end up in a bad situation.

agreed with lashcat. jet propulsion will move the jet relative to the scenery, the conveyer belt won't be stopping it from movine... good question

[phlyfish]

Afirm, without air movement over the wing nothing will happen, ground speed has no bearing on the situation the wings are in. the aircraft's wing needs the air movement to generate high presure (below the wing) and low presure (above the wing) to generate lift. Without air movement the plane cannot take off.

How about the opposite. Place this plane in wind tunnel, NO ground speed but generate enough wind velocity and teh aircraft will lift off.

I have seen pictures of Piper Cubs, when parked, chained down, flying from wind.

[bad_religion_au]

oh and to those that are stuck on the conveyer belt... get over it. if the jet's AIRSPEED is 100 mph, then the conveyer belt is going 100MPH in the opposite direction... meaning that the wheels are spinning 200MPH.

[89GASHOG]

Consider this: The rotational speed of the earth at the equator is over 1000mph!

[airon23]

so if this conveyor thing works why don't they use it like on aircraft carriers?

[jwest]

all things are equal, ...that being the tires can take the rpm's and the engines are powerful enough to overcome the extra load caused by the conveyer. I don't see a light aircraft doing this.

The conveyer and tires are meant to throw you. They are nearly irrelevant and could conceivable spin at 10 or 100 times the so called speed in rpm's needed for the aircraft to attain lift.

This is the right answer.

If you were running on a treadmill at 10 mph or 50 mph or if you were standing still, the ouside force needed for me, standing on the ground, to push you forward is relatively the same.

Now, in the aircraft analogy, that outside force is the aircraft's engines.

Now buckle up and have a good flight.

.

[airon23]

you are now free to move about MUD.