Physics Question for you.....

[backpacker85]

OK, I'm seeing that everyone agrees with the principle of this thing, that the "flight" of the plane is based on it's forward progress through the air. The only points being disputed now seem to be whether there will in fact be any forward progress made by the plane, and that can't be determined given the facts in the original question (and the "conveyor" may be the red herring).

If the assumption is that the plane is truly on a "conveyor", the only effect the conveyor would have is on the wheels of the plane, not its relative motion through the air.. THE PLANE WILL FLY.

If the assumption is that the "conveyor" is somehow able to counteract the plane's forward motion through the air, THE PLANE WILL NOT FLY.
This is the part I have trouble with.

To counteract its forward motion through the air, the plane would have to be firmly anchored to the "conveyor". In effect, the plane may as well be anchored firmly to solid ground!!

If you work the problem backwards assuming the plane will NOT fly, you are saying that there is no forward progress being made through the air; and if there is no forward progress being made, there is also no backwards motion being supplied by the conveyor, since it is assumed to move at equal speed but in an opposite direction than the plane through the air. The whole setup is sitting motionless.

Sounds like the only way the plane WILL NOT fly, is if it is firmly anchored down.

[LarryG]

Last night, sitting in the local community theatre, waiting for the play to begin, I suddenly understood what the "the plane will fly" contingent was arguing and was ready to admit they're right. This morning, I'm not so sure.

First, an analogy that illustrates why the conveyor moving backwards matters:

A river flows at five MPH. If you're in a rowboat, paddling upstream at five MPH, the current carrying you downstream will exactly cancel out your forward progress. Your boat will remain stationary relative to the river's banks, because your propulsion system -- the oars -- are "connected" to the current.

Now tie a rope to the boat, get out onto the river bank, and walk upstream at five MPH. Now your propulsion system is independent of the river's current. While it is true that you will have to expend sufficient effort to overcome the current, the amount of work you're doing doesn't matter. All that does matter is the end result. If you are able to walk upstream at five MPH on the river's bank, the boat tied to the other end of your rope will also move upstream at the same five MPH. It must.

The key phrase in the foregoing is you will have to expend sufficient effort to overcome the current and that, I think, is what we don't know about our hypothetical airplane: will its engines produce enough thrust to first overcome the conveyor's carrying it backwards AND THEN accelerate it to take-off speed? Or rather, can its engine produce enough thrust to do this? I don't think they can, and here's why:

Contrary to what some are claiming, there's a lot more to overcome than just the small amount of friction in the plane's wheel bearings. There's the weight of the plane; there's also the drag it will produce once it begins moving forward through the air surrounding it.

If you don't believe the plane's weight enters into this, consider this: if the plane was sitting with its engines off, and the conveyor started running at 100 MPH backwards, what would the plane do? Obviously it would move right along with the conveyor, at 100 MPH, backwards. Granted, if the wheels were not chocked or otherwise restrained, and the conveyor instantly accelerated to 100 MPH, the plane might initially stay right where it is -- think about the trick of jerking a table cloth out from under the dishes -- but soon the spinning wheels would coast to a stop and the plane would be carried rearward on the conveyor at 100 MPH.

If the pilot started the engines and opened the throttles, the plane would begin to attempt moving forward, since its propulsion is independent of the conveyor's influence (i.e., the rope tied to the boat). But since the engines were started when the plane was moving backwards at 100 MPH, the first thing it must do is generate enough force to cancel out that backwards motion. When that balancing point is reached, and the plane is back to being motionless relative to the fixed ground adjacent to the moving conveyor, its wings will also be motionless WRT the surrounding air. Motionless wings generate zero lift; and until the wings begin generating lift, the plane's full weight will still be pressing down on the conveyor, and the conveyor will be attempting to carry it backwards. The plane's weight is "connected" to the conveyor in the same way the boat's hull is "connected" to the river current.

If the plane is able to take off at 100 MPH, it engines at this point it is able to counteract the conveyor's rearward motion are producing only half the amount of thrust needed (100 MPH to get even, with another 100 MPH needed to take off).

And therein lies the rub. No matter how much thrust the engines produce to move the plane, the conveyor will compensate by speeding up, serving to keep the plane's wings motionless in the surrounding air. If the wings are motionless in the air, they are producing no lift. And as long as the wings are producing no lift, the conveyor is supporting the plane's full weight and is therefore attempting to carry it backwards with it. The faster the plane goes, the more behind it gets.

The plane won't fly.

[Stytooner]

I gotta be thinking to simple on this.
Any forward motion of the plane is countered by backward motion of the belt. No matter how big, how fast, how strong the engines are or what the tire pressure. It could be skid mounted too. Gravity is keeping the plane on the belt of course. It is the forward motion of the entire plane that is being negated by the belt. It ain't flying.

He didn't say the belt is turning in relation to thrust or how fast the wheels are turning. It's moving a millimeter back for every millimeter the plane moves forward, but also at the same time.

[niki]

OK, after re-evaluating the question, and second (or more) thought, I'm changing my mind, it will fly.

Yes, the tires will speen like crazy and we shall need a little bit more thrust or power to overcome the drag but at the end we shall takeoff.

Newton's Third Law states that for every action there is an equal and opposite reaction.

Thrust is created just by accelerating an air mass, the reaction to this acceleration is the force to the "opposite direction".
It can be created even in a vacuum (in space), if we shall shall supply the required fuel and air to operate the engine (I'm talking about Jet engine).

The conveyer can adjust it's speed to the tires speed but cannot oppose the foreward thrust created by the engine.

The conveyer is put in the question just to decieve me, and it did.

niki

[gjat]

I re-read the question and have changed my answer.

The plane will fly.

The plane motor (let's make it a jet), moves the body of the plane including the wings. The plane will move and take off like normal if it's on a non-moving runway. It will also take off on the conveyor belt because, the plane will still move through the atmosphere as observed by it's spcatial relationship to the Tower, FBO, and stray golfball.

The conveyor will only make the wheels spin faster and has no relationship with the movement of the plane body and wings through the atmosphere.

Final answer. Am I a millionaire, Regis?

[mmgross144]

Interesting Quoting

Hank,
Just wondering, are you a journalist? I reflect on you last post on gliding and the portion that you quoted from a previous post of mine (which was in part pulled from your prior posting), in which you did not include an interior paragraph where I addressed gliding...so I'll restate it here for posterity's sake:
"Of course, but wings without thrust are just nifty sun-shades. Wings cannot lift off by themselves without an outside force, Wind. Wings, properly balanced can glide if given sufficient airspeed to generate lift, but they won't do it from the ground by themselves."

Your examples of gliding stem from aircraft that had already achieved lift, via their own thrust (airliners) or external thrust the glider's tow aircraft. According to the law of inertia, an object at rest will stay at rest unless acted upon by an outside force, and an object in motion will remain in motion unless acted upon by an outside force (loose translation).

Your already flying wings will continue to fly until the outside forces, wind friction & gravity, overcome the lift (especially in airliners) forcing the craft down. Wings that have not achieved lift/flight will remain pinned by the same gravity until sufficient force is exherted upon them to generate the motion needed to gain the needed airspeed to gain lift.

So, how's Tokyo this time of year? (your post says that you are there.)

[Stytooner]

I think that all the guys that say it will fly have one thing in common. They all think the plane will be moving in relation to the Flight tower, the runway lights and that stray golf ball lost by one of the pilots.

The scenario and the reason for the inclusion of the conveyor is to cancel out forward movement of the plane and its wings includings its engines. Sure, air is flowing through the engines. It's being sucked in at the the front and blasted out the back. This is the force that is keeping the conveyor going. It actually has nothing to do with gravity or bearings at all. Its forward speed that is lacking because of the belt. Its a theoretical scenario of course, but theoretically there can be no acceleration of the wings if the belt keeps them from moving in a forward direction. Sure the wheels are rolling, but the plane ain't moving.

This is my last shot at this.
Say there is a tug on this conveyor. Attached to this tug with a long release line is a glider. The glider is sitting on the ground behind the conveyor. The driver puts the tug in gear and tries to drive forward. That's thrust at the wheels, but he ain't going anywhere because of the conveyor. He now has it floored at 100 MPH on his speedo and it still isn't going anywhere. The belt is going just as fast as his wheels are, but the other way. What's happening with the glider? Nothing. Still just sitting in the same spot.

Now the glider pilots starts yelling at the tug driver. The driver slows down the wheels on the tug until he's comletely stopped. He wants to back up and disconnect the glider. He puts the tug in reverse and starts the wheels rolling backward. This conveyor belt starts to go the other way. Dang, he still ain't moving. Its not gravity nor friction thats keeping him in position on the conveyor. Its an equal and opposite reaction to the energy of his wheels which in this case is directly proprtional to the direction they are turning.

You guys ever seen a machine that checks your speedometer for accuracy?
It's a set of rollers that you pull the car up on. Both drive wheels set in between these two rollers. When you put the car in gear, the wheels turn these rollers. No matter how fast the speedo says the car is going, the front wheels ain't moving.
This is the type scenarion That I think Mike intended when he set up the scene. A force that was acting against the thrust to virtually hold the plane still w.r.t. the ground, not the conveyor bellt.

If a bird was on this conveyor, it could easily take off at any speed due to them using their entire wing span to develop thrust and lift. They don't need speed of forward motion because they use speed of flapping wings to create lift and thrust.
Put an albatrose on this conveyor though and he will have problems. He won't be able to run fast enough to develop the forward motion needed to help him fly. He is much more like an airplane. He can't initially flap his wings fast enough to lift his body and uses forward speed to help him get the needed air flowing under his wings. With the coveyor holding him basically in the same spot on the tarmack, he's not lifting, not flying, he's running in place flapping his wings and looking even more awkward that usual.

[leehljp]

In response to:

"THRUST does NOT make the plane fly! The PURPOSE of Thrust is to get enough speed up so that there is a fast flowing of wind/air OVER the WINGS. It is WIND Speed over the Wings that create lift for flying.

Thrust will not make it "fly" until there is enough airspeed (wind) over the wings.

Since the natural winds are hardly controlable, nor convenient to our flight requirements, artificial wind must be generated, BY THRUST, to acheive sufficient airspeed.

OK, sorry to belabor the subject. Have a wonderful holiday season!

Are we forgetting about gliders? When the tow cable is released, gliders don't drop straight to the ground.

Some of you Canadians will remember the commerical jet about 15 years ago in which there was a mix up in liters/gallons for the American pilot. He ordered gallons/ they put in liters. The plane was somewhere near 30,000 feet or so and the engines quit - fuel starvation. No Fuel. They glided 50 or 60 kilometers and did a dead stick landing safely at an abandoned airfield where there was a lot of Sat/Sun afternoon crowd, jsut missing them.

Same thing when a 747 about 15 - 20 years ago flew through a erupting volcano cloud at night in SE Asia. No one knew the remote volcano was erupting. Pilot thought it was high clouds at 30,000+ feet. Engine failure and they glided something like 100 miles to Singapore or somewhere.

The Point: Air speed over wings create lift even when engine thrust is long gone. No engines to "pull" it forward. Although engines provide thrust to get it up to "wind speed', the thrust does not produce lift except in a guided rocket like situation - in which case it is not "flying" in the manner of Bernouilli. Thrust does not produce lift on a plane. It produces speed. When ennough WIND speed over the wings is reached, it will fly. Drop a well balanced light hand glider from the roof of a house and it will be flying in 6 to 8 feet due to wind speed over the wings.

As Stytooner said, in a hurricane and plane can take off and land due to wind speed over the wings - as long as there is not direction change of plane or wind.

Some might argue that gravity and huricanes provide the thurst. Gravity will provide "thrust" in the sense that it is causing accelleration downward. A hurricane's wind on the other hand will provide lift without the thrust. Wings provide the lift at a given wind speed, at which point "flying" takes place.

[BigguyZ]

The Air plane will not fly! OK, I think we can all pretty much agree that the plane's speed realative to the ground means nothing. It's the plane's speed in relation to the air around it. It takes a large mass of air passing over/under the wing for the Bernouli Effect to create enough of a pressure differential for the upward lift (caused by a higher pressure below the wing than above) to overcome the force of gravity.

Now, the engines themselves move air. That is, after all, how they create thrust. However, they do NOT move enough air by themselves to create enough lift to allow the plane to take off. In a prop type engine or a Jet engine (though especially a jet engine) they do not suck air over the entire wingspan. That's why when a plane moves forward through the air, which for all intensive purposes is static in relation to the ground, there is a greater amount of wind current over the entire wingspan than when the only airflow is generated by the flow of air through an engine.

It won't happen.

[tjmac44]

3 forces involved. Forward force of the plane, opposite force of the conveyor and downward force(gravity).
Wont fly. Dont forget about gravity acting on the plane, the downward force. The upward force, lift of the turbulence created by air moving over the wing, must overcome gravity. This wont happen because the planes movement, forward force, is cancelled by the conveyors opposite force.

[TheRic]

To make things simple we need to assume frictionless bearings on the wheels, frictionless conveyor belt (so it does not generate it's on wind speed), no wind (from mother nature).
The OP says the conveyor belt is moving in opposite direction of the plane.
I think we all agree the wheels of the plane do not make the plane move.
I think we all agree that the engine pushing against the air moves the plane forward.
I think we all agree that for a plane to take off air needs to pass across the wings generating lift, enough lift to raise plane off the ground.

The conveyor belt is moving opposite the speed of the plane, NOT the speed of the wheels, since they have no bearing on the speed of the plane. I think a few people are confused on this.

This planes engines push the plane forward, relative to a stationary post.
Since the plane is now moving forward, the conveyor belt is moving backwards, at the same speed, relative to the post.
The plane will keep moving forward at the same speed relative to the post, while the conveyor belt will move backwards at the same speed, relative to the post.
The wheels will freely move across the conveyor belt at twice the speed, Speed of plane relative to post + speed of conveyor belt relative to post.
Gravity makes little difference (other than the weight of the plane, so therefore less/more weight to overcome less/more lift needs to be generated).
As the speed of the plane (relative to the post) increases, the conveyor belt (relative to the post) increases, the wheels will spin twice as fast.
Air will move across the wings, since the plane is moving relative to the post. Once the air speed is great enough the plane will lift.

In the perfect world the plane will leave the ground at the SAME distance it normally would.

In the real world due to friction the plane would probably leave the ground about 5-10% further down the runway.

[Russianwolf]

I duck out to do flooring for the day and I come back to find 5 pages of responses? Sheesh.

Okay, I'll take one example that was used and try to model the situation. The treadmill. Take a matchbox car (with freespinning wheels) and put it on the treadmill. Now with your hand acelarate the car to 1 mph and turn the treadmill on to 1 MPH. Now the car is moving forward realative to items off the treadmill at 1 MPH and the wheels are spinning at 2 MPH.

The airplane will take off.

As was mentioned, the engines apply force to the air, not the conveyor. Therefor the aircraft will move in relation to items off the conveyor, which is what is needed to gain lift and airborn.

[mpc]

Okay, my two cents since everybody else has replied in this thread...

Walking on the treadmill/conveyor belt: you can go forwards or backwards depending on your walking speed relative to the belt. Because the Force in the "F=ma" equation to make you move occurs between your feet and the belt (traction/friction) the relative speed of the belt combines with your foot speed to create the total speed your body moves. Now replace the belt with a sheet of ice... your feet move a ton but you go nowhere (except downwards onto your butt) because friction/traction are nil. If somebody gave you a shove though you'd move across the ice. Even if you didn't move your feet. If the force is EXTERNAL to the belt (ice) then the belt has no effect.

Now stand on a conveyor belt again, with roller skates on. If the belt starts rolling, your skate wheels will go backwards. If there were no friction in the skate bearings you'd stay put. Now if somebody shoves you in the back, you WILL go forwards. And your skate wheels will be spinning at a rate that doesn't match the rate you're moving forwards. Their spin rate corresponds to the sum of your rate plus the belt rate. This is exactly what happens to the airplane too. The applied force is acting on your body or the airplane body - NOT on the part in contact with the moving ground/conveyor belt.

You can mimic this whole test yourself with a plain old kitchen rolling pin and a sheet of cardboard or plank of wood. The rolling pin is the airplane wheels (or your skates) and the handles represent the airplane body (or the dude on skates)... Push the handles forwards... the rolling pin rolls. Whee, duh. Now have somebody pull on the cardboard - what happens? The rolling pin rolls at a different rate BUT the whole thing WILL still move forwards as long as you keep pushing the handles forwards.

My initial gut response to the original question was that the airplane would stay still, spinning its wheels. But that's not what happens because the movement is NOT dependent on motive force being transmitted to the ground - traction/friction are NOT required. The folks saying the airplane's wheels will be spinning furiously are right. Now if the original question was about a CAR instead of an airplane then the answer would be "car does not move"; the wheels spin furiously because the F=ma force is tied to the ground. Another way to look at the CAR version of this: the conveyor belt doesn't need to be there - just replace it with ice again. No grip = no car movement. If the tires can't bite and generate force the car won't accelerate. The engine may be spinning like mad too - high RPMs - but the horsepower will be very low actually since NO WORK is being done by the tires - the engine will be revving in neutral basically. Just like revving on ice - you don't need full throttle to rev up a lot when the wheels are just spinning uselessly with no traction; not doing any WORK.


An airplane sitting on a real runway uses engine thrust - action-reaction principle - to make the Force in the F=ma equation and get it moving. There is some rolling friction though... about 15% is typical... i.e. for every 100 pounds of vertical force on the wheel & tire there will be 15 pounds of rolling drag. That subtracts from the engine's thrust so net F is really equal to "Thrust - 0.15*weight" initially; as the aircraft accelerates the wings start lifting making "weight" seem to decrease. Rolling drag will thus go down... but drag of air over the whole airplane plus induced drag (drag due to lift, you can't make lift for free!) will increase. This 15% rolling drag makes the aircraft accelerate slower than it would given engine thrust alone in the F=ma equaiton. That 15% rolling friction is a 15% "traction" effectively giving the conveyor belt SOME input (drag or pull) to the airplane... but it's nowhere near enough to keep it from accelerating. It'll still take off, using just a little more actual runway length than normal.

mpc

[bigfoot]

What is it about equal speed in the opposite direction that is confusing?
The experiment is exactly like a tread mill. When you try to run faster and faster forward, the conveyer runs just as fast the other way to effectively keep you stationary with absolutely no wind in your hair. Why do you guys still imply that the plane keeps on moving forward. Its stated in the beginning that its forward speed is negated by the conveyor. It really ain't going anywhere.

Again,

walking on a treadmill: feet impart thrust by pushing against treadmill

airplane engine on treadmill: engines impart thrust against AIR

if the treadmill runway is not powered by it's OWN engine/motor, then I would be surprised if the belt even moved.

example, picture an unpowered treadmill backed up to a wall

walk on treadmill, treadmill moves due to the thrust created by your feet, in the opposite direction, and you remain relatively motionless to the stationary wall

now stand on a powered treadmill (the runway) wearing roller skates (landing gear) and push off from the wall.

[bigfoot]

I just copied the question to have it in front of me

"A plane is standing on 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)."


Let's make an experiment.
Instead of the airplane put your car on the conveyer and just for the experiment lets stick a pole near by. Take your hand out of the window and hold the pole.

Now, "step on it", the car will accelerate to 100mph, but the conveyer will also accelerate to 100mph to the opposite direction at the same time. The relative velocity to the ground and to the air is - zero. your hand will still hold the pole.

Second experiment
Go to the gim and stand on the conveyer and start to walk on it.
Slowly slowly increase the conveyer speed until you run at your maximum running speed.
Are you advancing relatively to the ground? - no, you are running at the same place.
Do you feel any wind on your face (which indicates relative velocity between you and the air)? - I don't think so, you are not advancing relative to the air around you.

niki

The airplane is pushing against the air, your feet are pushing against the conveyor, and there is a big difference.