Calling all avaitors!!!!!

[JC]

Check out this Question and post what you think.

This came from avweb. com


Conveyer-Belt Runway



What I learned from Old Hack was that an updated version of a question aimed at confusing folks over relative measurements of airplane motion and the medium in which it operates had shown up on the Internet, and it was causing the fracas in the Lounge.

The question that has been going around is not particularly artfully worded, and I think that has caused some of the disagreements, but I'll repeat it as it is shown: "On a day with absolutely calm wind, a plane is standing on a runway that can move (some sort of band conveyor). The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?"

My comment: Notice that the question does not state that the conveyor's movement keeps the airplane over the starting position relative to the ground, just that it moves in the direction opposite to any movement of the airplane.

Initially, about a third of the folks here said that the airplane could not ever takeoff, because the conveyor would overcome the speed of the airplane and it could never get any airspeed. The rest said the airplane would fly.

The "It won't fly, Rocky" group said that the conveyor would hold back the airplane. They asked us to imagine a person running on a treadmill. As he or she sped up, the treadmill would be programmed to speed up, just as the conveyor in the problem, and the person would remain over the same locus on the earth, while running as fast as possible.

The argument was that if the airplane started to move forward, the conveyor program was set up to move the conveyor at exactly that speed, in the opposite direction, thus, the airplane would never move relative to the ground, and, because the air was calm, it could never get any wind over its wings. One of the analogies presented was the person rowing at three mph upstream in a river on a calm day. However, the current was flowing downstream at three mph, so the resultant speed with reference to the stream bank and air was zero, and thus there was no wind on the rowboat.

I watched and listened to the disagreement for a while and was fascinated to see that the argument seemed to split between those who had some engineering or math background, all of whom said the airplane would takeoff and fly without any problem; and those with some other background, who visualized the airplane as having to push against the conveyor in order to gain speed. Because the conveyor equaled the airplane's push against the conveyor, the airplane stayed in one place over the ground and in the calm air could not get any airspeed and fly.

It was an interesting argument, but as things progressed, more rational heads prevailed, pointing out that the airplanes do not apply their thrust via their wheels, so the conveyor belt is irrelevant to whether the airplane will takeoff. One guy even got one of those rubber band powered wood and plastic airplane that sell for about a buck, put it on the treadmill someone foolishly donated to the Lounge years ago, thinking that pilots might actually exercise. He wound up the rubber band, set the treadmill to be level, and at its highest speed. Then he simultaneously set the airplane on the treadmill and let the prop start to turn. It took off without moving the

[Prey521]

I doubt it could take off since there's no lift being created on the wings.

[ScottE]

Plane will fly with no problem. The tredmill doesn't matter at all.

[Prey521]

Plane will fly with no problem. The tredmill doesn't matter at all.

How's that?

[JC]

I really don't see how it would fly either.

[YeOldeStonecat]

It was an interesting argument, but as things progressed, more rational heads prevailed, pointing out that the airplanes do not apply their thrust via their wheels, so the conveyor belt is irrelevant to whether the airplane will takeoff.

I got to this conclusion within a few sentences of the article.

[YeOldeStonecat]

How's that?

Read the last paragraph again. A plane moves from thrust...from engines with props or turbines. Unlike a car, which uses wheels for propulsion. Thus...the treadmill is only affecting free spinning wheels. The plane could have a landspeed of 120 knots...in the above, the wheels would be thinking that they're doing 240 knots..but who the heck cares. What counts..is the actual ground speed..and the air still hits the airfoil shaft, thusly creating life, at 120 knots..just as it if were going down the normal tarmac.

[downhill]

The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction). Can the airplane ever take off?"

Notice also that it says that it "tunes" the speed of the conveyor to be "exactly" the same..but in the opposite direction.

I would take it to mean that the plane wouldn't propel the conveyor belt, that it would be something else that would. If so, the plane wouldn't move.

That would be a different story with no wind over it's wings.

[brembo]

I would take it to mean that the plane wouldn't propel the conveyor belt, that it would be something else that would. If so, the plane wouldn't move.

.

The speed of the conveyor does not matter...well it does in terms of friction on the wheels to some extent, but for argument purposes lets just say it does not. The conveyor could be going and the wheels of the plane turning and you could keep the plane in one spot with a rope and your feet most likely. Fire up the props/jets and match the froce to overcome the rolling resistance and the plane will essentially "sit still", overcome that small resistance and the plane will begin to accelerate despite the speed of the conveyor. Sudden shifts in the conveyor would effect the plane some as the mass of the wheels would come into play. But, overall the conveyor could go 1000x times faster than the speed of the plane relative to what would be stading still next to this fabulous contraption and the plane would still have no problems taking off.

Why? The plane does not use it's contact with the ground for anything in terms of forward motion, the wheels just make landings smooth and scrape free.

[downhill]

The speed of the conveyor does not matter...well it does in terms of friction on the wheels to some extent, but for argument purposes lets just say it does not. The conveyor could be going and the wheels of the plane turning and you could keep the plane in one spot with a rope and your feet most likely. Fire up the props/jets and match the froce to overcome the rolling resistance and the plane will essentially "sit still", overcome that small resistance and the plane will begin to accelerate despite the speed of the conveyor. Sudden shifts in the conveyor would effect the plane some as the mass of the wheels would come into play. But, overall the conveyor could go 1000x times faster than the speed of the plane relative to what would be stading still next to this fabulous contraption and the plane would still have no problems taking off.

Why? The plane does not use it's contact with the ground for anything in terms of forward motion, the wheels just make landings smooth and scrape free.

Agreed except for one thing and maybe I should have been more insightful.

The conveyor belt......my point being that to me, the way it's worded is that the plane has nothing to do with the belt moving. That the belt is (for lack of a better word) programed to move, relative to the motion of the plane to keep the plane stationary.

[brembo]

Agreed except for one thing and maybe I should have been more insightful.

The conveyor belt......my point being that to me, the way it's worded is that the plane has nothing to do with the belt moving. That the belt is (for lack of a better word) programed to move, relative to the motion of the plane to keep the plane stationary.

Theoretically if the conveyor had enough acceleration to make the plane use 100% of its power to match the wheel speed to conveyor....it could keep the plane still. But keep in mind that once the conveyor STOPPED accelerating(constant state) the plane would begin to accelerate at that point.

[Bastid]

if the plane does not move how does it get wind over its wings to create the lift required to fly?

[vinnie]

The plane moves just fine.

Provided that the friction of the wheel bearings isn't enough to overcome the forward thrust produced by the propellor, the plane will still move forwards through the air regardless of the speed of the conveyor.

That's all there is to it, nothing to do with acceleration rates or matching speeds, provided the wheel bearing friction isn't excessive the plane takes off. The question is just meant to create confusion, it's just like 'if a rooster lays an egg on the roof which way will it roll?'. It will of course not roll as roosters don't lay eggs. Non GM ones anyhow

[Bastid]

[Stewie/]DAMN![Stewie/] i hate when you ppl make me think....lemme get back to you...

[YeOldeStonecat]

Agreed except for one thing and maybe I should have been more insightful.

The conveyor belt......my point being that to me, the way it's worded is that the plane has nothing to do with the belt moving. That the belt is (for lack of a better word) programed to move, relative to the motion of the plane to keep the plane stationary.

If the plane were propelled by the wheels, same as a car, this would be true. But the plane is propelled by engines suspended under the wings, or in the tail section. The engines on planes propel the craft by using the air, not the wheels like a car. So the conveyer belt can do whatever it wants..it's irrelevant. The wheels of the plane are free spinning.

The only exception to this that I thought of matches what Vinnie mentioned...if the friction of the bearings of the wheels gets to be too much..which I don't see happening in realistic speeds. It's not like the plane will be doing mach 3.5 on the ground. We were never told what kind of plane that it is, so we don't required ground speed before lift...lets say it's some Cessna 170. Needs what Mountainman, 90 knots ground speed before takeoff? So the wheels would be doing 180 knots worth of rotation...they'll live.

[Kip Patterson]

This has gone on for days if not weeks on several forums. Of course the plane moves and takes off - there is no connection between the plane and the conveyor other than the friction in the wheel bearings, and no other source of forces to resist the thrust of the engines.

[brembo]

Friction of wheel bearing AND energy required to spin up the wheels. In my theoretical setup, the conveyor is unlimited in acceleration and speed, which we all know is not possible. IF it were the act of accelerating the wheels could keep the plane in one spot. Friction is a constant of sorts, so once that hurdle is overcome any further forward thrust will simply move the plane, whereas inertia forces on the wheels apply at anytime. It would have to be a mother of a conveyor tho....

[YARDofSTUF]

MYTHBUSTERS!!!!! I want life size 747 demonstrations LOL


I agree, the plane will fly.

[Kip Patterson]

Yes, the inertia counts - but the conveyor does not move unless the plane does. In reality, what happens is that the plane accelerates to takeoff speed a little slower than normal because the wheels are spinning at twice their normal speed at takeoff. Fortunately, the inertia of aircraft wheels is kept as low as possible to reduce tire wear adn so that would not be much of a factor.

[YeOldeStonecat]

Size/type of plane was not specified...so the arguement of "interia of the wheels"...is simply too vast of a spread to be debated at this point of "lack of information". While technically it's true, in reality..well, we don't know if it'll come much into play, because we don't know what kind of plane is to be used.

We don't know if this is a Piper J-3 (in which case it's really not much at all), or a C-5 Galaxy, or an AN-225 (in which case...yes...it'd factor in just a bit more)

Clearly a difference in wheels and inertia required...

Piper:low takeoff speed, load drag of the wheels

Anotov 225:I don't have the time to lookup takeoff speed..even unladen, but substantially higher, and obviously more wheel drag

[downhill]

Theoretically if the conveyor had enough acceleration to make the plane use 100% of its power to match the wheel speed to conveyor....it could keep the plane still. But keep in mind that once the conveyor STOPPED accelerating(constant state) the plane would begin to accelerate at that point.

I think most are still missing my point .

Most of you are assuming that the plane would control the movement of the belt..(or in other words, it really doesn't which I would agree with. )

I'm assuming that the way it's worded, it doesn't. That the belt is powered by another means and that it's controlled so to keep the plane stationary no matter how much thrurst it's propellers generate.

Again, the play on words in the question is the kicker.

[YeOldeStonecat]

I'm assuming that the way it's worded, it doesn't. That the belt is powered by another means and that it's controlled so to keep the plane stationary no matter how much thrurst it's propellers generate.

Again, the play on words in the question is the kicker.

In no place does it state that the movement of the belt would keep the plane stationary. The laws of physics dictate that it cannot. The trickery that this question is based on...is that it can lead some people to believe that "matching speeds opposite the planes speed" would hold it stationary. This cannot physically happen, assuming that the planes wheels are in a normal functional state, that the brakes are released, that the bearings have not melted down and turned them into a solid, non-rolling glob.

In reality..all the happens in the wheels end up rotating twice as fast as the true landspeed of the plane. If the plane is actually doing 50mph down the runway..the wheels, on this theoretical conveyer belt of a runway, would be spinning the equiv of 100mph. But if you were stand beside the runway with a radar gun aimed at the plane, you'd still read 50mph. The planes speedo would also register about 50mph, since they're pitot based (well, old ones, now I'm sure some are GPS)...and not wheel sensor based.

[YARDofSTUF]

I think most are still missing my point .

Most of you are assuming that the plane would control the movement of the belt..(or in other words, it really doesn't which I would agree with. )

I'm assuming that the way it's worded, it doesn't. That the belt is powered by another means and that it's controlled so to keep the plane stationary no matter how much thrurst it's propellers generate.

Again, the play on words in the question is the kicker.

Inorder for hte conveyor system to track/find out the planes speed means that the plane has to accelerate first. Now the plane is moving, the conveyor starts to move against it, the planes wheels turn faster, plane keeps moving.

[brembo]

In no place does it state that the movement of the belt would keep the plane stationary. The laws of physics dictate that it cannot. The trickery that this question is based on...is that it can lead some people to believe that "matching speeds opposite the planes speed" would hold it stationary. This cannot physically happen, assuming that the planes wheels are in a normal functional state, that the brakes are released, that the bearings have not melted down and turned them into a solid, non-rolling glob.

In reality..all the happens in the wheels end up rotating twice as fast as the true landspeed of the plane. If the plane is actually doing 50mph down the runway..the wheels, on this theoretical conveyer belt of a runway, would be spinning the equiv of 100mph. But if you were stand beside the runway with a radar gun aimed at the plane, you'd still read 50mph. The planes speedo would also register about 50mph, since they're pitot based (well, old ones, now I'm sure some are GPS)...and not wheel sensor based.

I will quit bickering after this.....

You CAN prevent the planes takeoff, by tieing up all the energy in accelerating the wheels. Granted that is one helluva conveyor and it would have to constantly be gaining speed(inertia of wheels) to prevent forward movement of the plane. So in a purely mathmatical world, yeah I could stop it. In the real world, the plane is just gonna take off like nothing is happening.

[downhill]

In no place does it state that the movement of the belt would keep the plane stationary. The laws of physics dictate that it cannot. The trickery that this question is based on...is that it can lead some people to believe that "matching speeds opposite the planes speed" would hold it stationary. This cannot physically happen, assuming that the planes wheels are in a normal functional state, that the brakes are released, that the bearings have not melted down and turned them into a solid, non-rolling glob.

In reality..all the happens in the wheels end up rotating twice as fast as the true landspeed of the plane. If the plane is actually doing 50mph down the runway..the wheels, on this theoretical conveyer belt of a runway, would be spinning the equiv of 100mph. But if you were stand beside the runway with a radar gun aimed at the plane, you'd still read 50mph. The planes speedo would also register about 50mph, since they're pitot based (well, old ones, now I'm sure some are GPS)...and not wheel sensor based.

Reread it.......at least to me, it does say that.

Forget the wheels of the plane. They serve no function in the whole question as the way I read it other than to roll on the belt.....Let's tear it appart.

a plane is standing on a runway that can move (some sort of band conveyor).


The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).

The way I read it, the plane is only trying to take off...The conveyor moves at the speed the plane trys to move. (control system)...

The conveyor is powered by it'self, and has nothing to do with the plane other than it does have a control system to keep the plane in a stationary place. Why would physics dictate that can't happen?

Powered treadmills work in a somewhat same fashion...



brembo...no bickering here....

[downhill]

Inorder for hte conveyor system to track/find out the planes speed means that the plane has to accelerate first. Now the plane is moving, the conveyor starts to move against it, the planes wheels turn faster, plane keeps moving.

We can send a man to the moon with a computer onboard that was less than the processing power of a p200.

You couldn't control the speed of a conveyor belt with that monster you built with a few sensors?

[YARDofSTUF]

We can send a man to the moon with a computer onboard that was about equal to a p200.

You couldn't control the speed of a conveyor belt with that monster you built with a few sensors?

In order for the conveyor to counteract the planes movement the plane must move. The conveyor could move at twice the speed of the plane and the plane will still take off.

[downhill]

In order for the conveyor to counteract the planes movement the plane must move. The conveyor could move at twice the speed of the plane and the plane will still take off.

True if it isnt' powered on it's own. How's that?

If it's powered and contolled...then the plane is wasting fuel.

[YARDofSTUF]

The wheels and the affect of the ground on the speed of the wheels dones not transfer to the plane or affect it.

If you put a car there, the car will not travel, but a plane is different, and I dont know how to discribe it any better than the others in this thread.

[YeOldeStonecat]

I will quit bickering after this.....

You CAN prevent the planes takeoff, by tieing up all the energy in accelerating the wheels. Granted that is one helluva conveyor and it would have to constantly be gaining speed(inertia of wheels) to prevent forward movement of the plane. So in a purely mathmatical world, yeah I could stop it. In the real world, the plane is just gonna take off like nothing is happening.

That's the one exception I noted....in my above linked pictures. Granted you'd have to have a damn wimpy plane, powered by rubberbands, and wheels with bearings as heavy as that AN-225 that have nearly rusted together.....

But I don't see, in the real world, any planes that have been designed by someone with partial aviation engineering skills.....a plane that's skewed that far.

[Kip Patterson]

The belt does not have a control system to keep the plane in the same place. It has a control systme to move the belt backwards at the same speed the plane is moving forwards. That does not keep the plane in place. There is no connection between the belt and the plane to allow the belt to apply any significant amount of force to the plane.

If you are concerned about how much friction and inertia add, think about what an aircraft does when its wheels touch down. Regardless of the aircraft, the wheels come up to speed in less than a second and there is no noticeable deceleration of the plane when that happens.

[YeOldeStonecat]

I read it once (all I need) but read it some more just to check.

OK Lets drop the car example since people aren't getting the difference in propulsion via wheels on a moving surface, versus propulsion from a separate means independing from this supposedly neutralizing influence.

Lets compare boats.....boats in...a strong current.

Boat goes along neutral (still) water...goes at 5mph...with a fixed throttle
Now it comes into a 3mph current in the opposite direction, throttle still fixed the same, we could say that the boat is now really moving at only 2mph, right? Propeller in this water.
Now the boat encounters a 7mph current..same throttle position, same propulsion system (screw in the water)...now the boat goes 2mph backwards.

That's how you guys are seeing it.

Now...instead of a water screw driven boat...lets use an air boat as an example. You know...those swamp boats with a big air pusher prop up in the back?

Would you still agree that the boat gets the same changes in movement as the water screw propelled boat? It's only a "yes or no" question, partially...only partially related to the plane on the belt.

Reread it.......at least to me, it does say that.

Forget the wheels of the plane. They serve no function in the whole question as the way I read it other than to roll on the belt.....Let's tear it appart.

a plane is standing on a runway that can move (some sort of band conveyor).


The plane moves in one direction, while the conveyor moves in the opposite direction. The conveyor has a control system that tracks the plane speed and tunes the speed of the conveyor to be exactly the same (but in the opposite direction).

The way I read it, the plane is only trying to take off...The conveyor moves at the speed the plane trys to move. (control system)...

The conveyor is powered by it'self, and has nothing to do with the plane other than it does have a control system to keep the plane in a stationary place. Why would physics dictate that can't happen?

Powered treadmills work in a somewhat same fashion...



brembo...no bickering here....

[ScottE]

Look at it this way. Take a mini car and set it on a treadmill. Hold your hand on it and start the treadmill the wheels on the car turn and the car does not move. Now move the car forward on the treadmill, what happens? The wheels of the car just turn faster and the car moves forward relative to it's previous position. Even if the tredmill accelerated at some rate in an attempt to counteract the forward motion of the mini car. It would just increase the rate at which the wheels were spining, but it would not prevent forward motion. The thrust is being applied not through the wheels! The plane is pushing the air! Yeah there is some friction between the plane's wheels and the ground, but it's negligable.

[YARDofSTUF]

Bump

[SpareX]

If the plane is held still on the ground.. how is it generating air speed? the props or turbines propel the plane forward.. its the physics of lift that move it up, BUT to create such lift the plane MUST be moving forward.. if the belt under it is so designed to prevent it from going forward as it was programed, then guess what.. there is NO lift and the plane Cannot fly in its current state..

Turn off the conveyor after it reaches a high speed and it will fly quite quickly.

here is another thought... while exercising on a treadmill I dont feel any extra wind against me even at a full sprint...


BTW on a runway.. an airplane is the same as a car.. until it gains enough lift to move it up.

[YARDofSTUF]

If the plane is held still on the ground.. how is it generating air speed? the props or turbines propel the plane forward.. its the physics of lift that move it up, BUT to create such lift the plane MUST be moving forward.. if the belt under it is so designed to prevent it from going forward as it was programed, then guess what.. there is NO lift and the plane Cannot fly in its current state..

Turn off the conveyor after it reaches a high speed and it will fly quite quickly.

here is another thought... while exercising on a treadmill I dont feel any extra wind against me even at a full sprint...

The plane is not held still because the conveyor does not stop the planes movement. It only accellerates the wheels.

[ScottE]

BTW on a runway.. an airplane is the same as a car.. until it gains enough lift to move it up.

An airplane is never a damned car. The forward thrust is being applied to the fuselage not the damned wheels. The only thing that changes because of the treadmill is that the wheels turn 2x as fast.


This thread is proof that the education system has failed at least 1/2 the people in this country.

[YARDofSTUF]

An airplane is never a damned car. The forward thrust is being applied to the fuselage not the damned wheels. The only thing that changes because of the treadmill is that the wheels turn 2x as fast.


This thread is proof that the education system has failed at least 1/2 the people in this country.

Give downhill a break, alot has changed in the last 700 years since he was last in a classroom.

[downhill]

Hey...no need to be grumpy....

It was only 30 years ago. ScottE....when half the people in the country have responded to this thread...I throw the hat your way.

You just made my most honorable smart list and when I grow up...I aspire to be just like you.

All that said...and even though I can still see some of your points......I'm still not sold. No biggie.

[ScottE]

Hey...no need to be grumpy....

It was only 30 years ago. ScottE....when half the people in the country have responded to this thread...I throw the hat your way.

You just made my most honorable smart list and when I grow up...I aspire to be just like you.

All that said...and even though I can still see some of your points......I'm still not sold. No biggie.

The forward moving force is being applied to the fuselage of the aircraft via the engine(s). The wheels are free turning. Like I said before the only thing that is going to change about the whole thing is that you'll lose some efficiency because a minimal ammount of the engine's power is needed to counteract the force of friction on the wheels, and the wheels will be turning 2x as fast at takeoff.