12/12/2007 - the Conveyor Belt myth will be busted

[Greebo]

Ken:

If airplane goes ---> at 100kts
and conveyer belt goes <--- at 100kts

In a car, the wheels turn () () () at 100 kts
in a plane the wheels turn () () () () () () at TWO HUNDRED kts.

You're thinking of a plane like a car.

 

[Greebo]

There are two ways to interpret the question as I've seen is posited.

In both cases, the conveyor belt is as long as a runway. The question posts that the runway moves backwards at a speed that matches the airplane moving forwards.

Now, in one scenario the belt matches the speed of the mass of the plane's forward momentum. The plane moves forward and the wheels turn freely so the plane develops airspeed equal to 1/2 groundspeed (ground being the conveyor) and takes off.

In another scenario, the belt rotates the wheels fast enough to create enough reverse friction as to halt the plane's forward momentum. In this scenario, because lateral friction of a free spinning wheel is negligible. the belt and wheels must move so fast that something will break - an impossibility with today's materials, and so untestable.

A third scenario could have the belt match the rotation speed of the plane's wheel surfaces. This is an impossibility because the wheel speed is the result of the combination of forward motion of the plane plus reverse motion of the belt.

W = P - S
In scenario 3, W=S is also true
IF W = P-S and W=S then S = P+W. This is only possible when all three values = 0.

So only scenario one is possible, in which case the plane MUST take off.

 

[KennyFlys]

I don't care.

 

[EdFred]

It seems the terms of the argument have been changed. Whatever happen to a conveyor no larger than the footprint of the plane AND the conveyor speed matches whatever airspeed may be acheivable by a given amount of thrust?

That was NEVER the original argument. Even if it were, the plane would just move forward off the conveyor belt anyway. But I don't know any plane that can take off in 10 feet. (Osprey excluded)

 

[Frank Browne]

That was NEVER the original argument.

Then what was? I asked last night and got no reply.

 

[EdFred]

The original argument was:

If a standard airport runway were replaced with a large conveyor belt -- one moving in the opposite direction (and equal speed) that a plane intended to take off in -- would the plane ever leave the ground?

 

[jason]

It seems the terms of the argument have been changed. Whatever happen to a conveyor no larger than the footprint of the plane AND the conveyor speed matches whatever airspeed may be acheivable by a given amount of thrust?

Where did that come from? I've never seen it posted that way.

That said, it still doesn't matter. If the plane is "thrusting" forward at 100 kts. and the treadmill is moving aft at 100 kts. the wheels will be spinning 200 kts while the plane moves forward off of the tiny conveyor and takes off.

Kenny, you still didn't answer my question. If you contend that the aft movement of the conveyor will make the airplane move backwards, then why does the ground remaining stationary (on a normal takeoff) not result in the airplane remaining stationary?

 

[EdFred]

Now, I want to go step by step on this.

What moves an airplane? The propeller, right? There aren't drive shafts going to the wheels on a 172, so it is the force of the propellor that moves the plane.

Air is independent of a conveyor belt. If there were no plane on the conveyor belt, what happens to the air over the conveyor belt. Not 2" above the belt, but 3'-8' above the belt? Nothing at all, right?

So the propeller is acting on a medium independent of the conveyor belt correct?

Now, lets remove the propeller, and replace it with a large rope, and tie it to a vehicle that is off the far end of the conveyor belt. This vehicle is independent of the conveyor belt, just like the air is independent of the conveyor belt. Still with me?

The conveyor belt is started, and the vehicle that is off the conveyor belt starts towing the plane. The plane moves because the wheels spin freely since they have no drive shafts to them. The tow vehicle acts just like the propellor and pulls the plane forward because the rope and vehicle are completely independent from the conveyor belt.

The plane moves forward, because the wheels DO NOT DRIVE THE PLANE. The drive mechanism for the plane is 100% independent from the conveyor belt. Just like the tow vehicle that is not on the conveyor belt.

 

[jason]

Then what was? I asked last night and got no reply.

This is how I originally saw the question posed.

A plane is standing on a 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 the opposite direction). Can the plane take off? (Source: www.kottke.org)

Unfortunately, the specific wording of that question makes it even harder for people that believe that the plane won't take off to get past the fact that the conveyor belt is moving.

 

[stingray]

It has nothing to do with intelligence or knowledge of applied physics or anything else, it is about how one's brain is wired. You know, like those optical paradox illusions where some see the cube one way, some see it the other way.

Heck, I'm a frickin' genius and I got burned by the treadmill for a while (as some of you might remember.) That is why it is fun talking about it, it is not automatically intuitive for everyone.

That is why you move the plane first and have the treadmill match it in the opposite direction. For most people they can see the plane start to move even though the treadmill is going back wards. For those that cannot see that move the plane and they will immediately see that there is no force to stop the plane. It is like you said it is an illusion that you just have to break. I remember back on the red board they would use stations to watch (Like telephone poles or what ever) It totally would change peoples minds that had it right in the first place. They would either have the plane stopped or the belt moving backwards to start out. In your mind it was hard to get that darn plane going forward because the belt just kept it going backwards (In your mind). Once you put the ball in their court and have them stop a moving plane it is all over, it cannot be done. You change the illusion now it is so easy to see.

Dan

 

[mikea]

The one guy stated exactly what I've been saying all along. There's no air over the wings so it ain't gonna go no where! If it did, we could abandon the concept of airspeed.

I wonder how hard they had search to find a pilot that clueless.

Where in the problem statement is any mention of air over the wings?

Here we go again. The "not fly" crowd keeps assuming factors not in the problem.

Besides, that's not a plane. It's an LSA.

As I said, the show isn't going to end the endless babble.

 

[Dave Siciliano]

Ooops! Didn't realize folks were so worked up 'bout this <g>. Hope y'all get to see the episode. BTW, this show seems to be as much show as science. Hope they get the science part correct.

Best,

Dave

 

[stingray]

This is how I originally saw the question posed.



Unfortunately, the specific wording of that question makes it even harder for people that believe that the plane won't take off to get past the fact that the conveyor belt is moving.

That is the original question good job. You notice that the plane moves first. (To any doubters) stop the plane. Explain how you got the plane to move, then explain how the belt stopped that force. Even in your wildest dreams friction will not stop it @ full throttle full brakes it is hard to keep my Viking still.

Dan

 

[mikea]

If the floatplane is pointed upstream and the river is flowing at 80 knots, and the power is set to maintain 0 relative motion, is it not pulling 80 knots worth of air? But what's the airspeed show? Will the airplane take off? No! No air movement over the wing, therefore no lift.

Greg, when the floatplane is using "80 knots worth of thrust to counteract the current, exactly what force is propelling it? Did the plane drop a outboard prop into the water somewhere and not tell us? It's amazing that you set up the PERFECT analogy as to why the plane DOES fly and use that to deny it. The THRUST holds it by moving AIR backwards. THAT IS YOUR AIR TO FLY WITH.

The perfect analogy is that a floatplane CAN take off into a opposing current. You have to move your frame of reference from the shore (where it won't move) to the relative air (where it IS moving.) Even better than the conveyor belt, whre the wheels have no friction, the skis have lots of friction and water drag to overcome.

 

[KennyFlys]

I don't care.

 

[Greg Bockelman]

Here's one version of the question:

http://www.geekculture.com/joyoftech/joyarchives/903.html

Here's another version with the same outcome but better worded from Rick Durden:


In the version from Rick Durden, only advancing thrust beyond any drag by standing on the conveyor will move the airplane. What is not clear is if the plane is moving in relation to only the conveyor belt or in relation to the conveyor base.

Ken, lets leave the wording of the scenario clear out of it. We all agree that the wording can be interpreted in two different ways, depending on the outcome that is being argued.

Please answer the following questions and supply your reasoning behind the answers. Keep in mind the objective is to actually fly the airplane, NOT to adjust the power to maintain a position on the ground.

In the REAL WORLD, will an airplane take off from a conveyor belt that is moving backward as fast as the airplane is moving forward?

What sort of outside force would there have to be to prevent the airplane from taking off?

 

[Greebo]

What is not clear is if the plane is moving in relation to only the conveyor belt or in relation to the conveyor base.

It doesn't matter.

The plane can still overcome the coefficient of drag.

But the first option is impossible, because if the plane starts moving forward at 1kt, the belt must start moving backwards at 1kt, but now the relative speed to the belt is 2kts, so the belt must accelerate to 3kts.

In the first option, Belt Speed = - Plane Speed + (Belt Speed). In the second option the formula is Belt speed = - Plane Speed. (Belt speed is always negative)

Given that the plane will move, plane speed will not = 0, so the formula is impossible. X = -y + X is the same as saying x - x = -y, or y = x-x. x-x must equal 0, but y won't be 0.

 

[jason]

Here's another version with the same outcome but better worded from Rick Durden:

"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?"

Sounds familiar. Did somebody post that an hour and a half ago?

In the version from Rick Durden, only advancing thrust beyond any drag by standing on the conveyor will move the airplane. What is not clear is if the plane is moving in relation to only the conveyor belt or in relation to the conveyor base.

IT DOESN'T MATTER. If we're assuming that it is possible for the conveyor belt to match any forward speed of the airplane, we have to assume that the wheels will be able to spin at whatever speed they need to. If we wanted to bring drag into this, then we would have to question the entire premise of the question as it would be impossible to have a 3,000 ft. runway that moved in a conveyor like fashion in the first place.

Ok, let's ask this. Assuming the best available wheel bearings (unlimited friction), would one agree that having normal wheels for landing gear without breaks applied is no different than having "hoverboards" for landing gear (ala Back to the Future)? If the plane were hovering 6 inches above the conveyor belt, would it change your answer?

Why are you not responding to any of the questions or points raised in the thread?

 

[KennyFlys]

I don't care.

 

[KennyFlys]

I don't care.

 

[Greebo]

Drag. Actually, drag in the form of friction.

In order for any airplane to begin the takeoff roll, it must overcome the drag of the wheels on the runway. That drag varies only by how much weight is placed on the wheels and the traction the wheels have on the runway.

As the plane begins to roll on the conveyor and the conveyor is moving the opposite direction the initial drag is overcome but there continues to be drag. Eliminate the thrust, drag will stop the plane. In order for the plane to take off, there has to be sufficient thrust to overcome not only the initial drag but the continued drag as the conveyor rolls the opposite direction.

The increase in drag caused by rolling friction is far far lower than the thrust. Thats kinda the point of wheels.

 

[Greg Bockelman]

Drag. Actually, drag in the form of friction.

We've addressed that. There isn't that much friction, either in the wheel bearings or between the wheels and the surface. Proof is that on a normal takeoff, you could hold the airplane on the runway to twice or even three times its normal takeoff speed and it will still fly.

In order for any airplane to begin the takeoff roll, it must overcome the drag of the wheels on the runway. That drag varies only by how much weight is placed on the wheels and the traction the wheels have on the runway.

Ok. For the time being.

As the plane begins to roll on the conveyor and the conveyor is moving the opposite direction the initial drag is overcome but there continues to be drag.

I will assume you are using drag and friction interchangeably from here on out.

Eliminate the thrust, drag will stop the plane.

OK. But nowhere in the scenario does it mention eliminating the thrust.

In order for the plane to take off, there has to be sufficient thrust to overcome not only the initial drag but the continued drag as the conveyor rolls the opposite direction.

But that also applies to a takeoff from a normal runway. So far this line of reasoning is faulty.

I don't know if anyone is considering fluid drag or not. The force I'm looking at is friction which must be overcome with ever increasing thrust.

Fluid drag is induced drag in this case. The higher the airspeed the higher the drag by a factor of 4. But in your scenario, that is not a factor because in your scenario there is no airspeed, therefor no fluid, or induced drag.


And you never answered my first question. In the real world, will the airplane fly?

 

[Skip Miller]

In order for any airplane to begin the takeoff roll, it must overcome the drag of the wheels on the runway. That drag varies only by how much weight is placed on the wheels and the traction the wheels have on the runway.

Actually no, traction has nothing to do with it. On a windless day, a plane can take off on a long sheet of ice that has been polished to perfection - essentially a tractionless surface. The takeoff distance would be no more nor less than on dry pavement. The drag of the wheels on the runway you speak of includes the rolling resistance of the tires and the friction in the roller bearings in the wheels - a smaller amount than the rolling resistance of the tires (that's why they put them there).

The tires need no traction on the runway to take off. They need traction to steer (until the airspeed makes the rudder effective) and to stop.

-Skip

 

[jason]

Maybe because I'm on my fifth day of a cold and I'm spending much of my time trying to stay asleep, taking drugs and crapping every five minutes.

If anyone wants to take on this cold, I'll concede the conveyor argument. I really don't give a flip any longer.

If anyone is that angry with me such they have to start calling me names (that person knows who they are), they can take a flying fornication through a rolling donut.

Come on, Kenny, that's kind of a cop out. You've made 101 posts in the last 120 hours (5 full days). 14 posts in this thread alone in the last 24 hours. You're obviously feeling up for discussion...it just seems as if you're not acknowledging any of the points being made to counter your arguments. There have been many posts made in this thread that have had a lot of thought put into them and you haven't refuted any of them.

Nobody is angry...it's just seems as if you're not wanting to work through the exercise as much as you're wanting to be right.

 

[gkainz]

ok, so before I thought it wouldn't take off, I thought it would ... but that was before I thought about it at all, so the more I think about it the more I don't know ...

initially I thought it was simply the prop dragging the wing thru the air ... then SOMEBODY posted the floats scenario and now I'm all confused. Would you take off in the same relative position on the ground, essentially no ground (conveyor) roll? Then why didn't the geniuses put a conveyor belt on a carrier instead of a catapult? Dang, I'm so confused ... where's Bernoulli when you need him?

So, wheels and belt be damned, the prop provides thrust thru the air mass and relative air speed is relative air speed, regardless of what the belt makes the wheels do?

 

[Greebo]

So, wheels and belt be damned, the prop provides thrust thru the air mass and relative air speed is relative air speed, regardless of what the belt makes the wheels do?

DING DING DING DING DING DING DING!

You may keep your pilots certificate.

 

[KennyFlys]

I don't care.

 

[Greebo]

Keep it on topic and not about the posters, please.

 

[SkyHog]

Keep it on topic and not about the posters, please.

Or about crap....that's just my request though

 

[tonycondon]

wow its amazing to see this argument brought back up, and jesse didnt even have to kick start the thread about it!

 

[mikea]

ok, so before I thought it wouldn't take off, I thought it would ... but that was before I thought about it at all, so the more I think about it the more I don't know ...

initially I thought it was simply the prop dragging the wing thru the air ... then SOMEBODY posted the floats scenario and now I'm all confused. Would you take off in the same relative position on the ground, essentially no ground (conveyor) roll? Then why didn't the geniuses put a conveyor belt on a carrier instead of a catapult? Dang, I'm so confused ... where's Bernoulli when you need him?

So, wheels and belt be damned, the prop provides thrust thru the air mass and relative air speed is relative air speed, regardless of what the belt makes the wheels do?

We got one!

To answer the previous question, there can be motion of the plane in respect to a fixed point on the earth, but that really a side effect. That motion is going to be the airspeed less the headwind.

I've been along the runway at OSH and watched a J3 practically hover into the air because there was a stong headwind down the runway. If it needs 40 MPH airpseed to rotate and climb and you have a 40MPH headwind, there would be no relative motion to an observer along the runway...but that's a side issue.

Think relative wind, which equals airspeed. That's all that is needed to fly.

Note that the speed of the ground (runway/conveyor belt/river/lake/snow/ice) has no effect.

 

[Greebo]

More food for thought:

The earth's circumference is 24,900 miles.

So at the equator, the earth is moving at 1037.5 miles an hour. The rotation is from west to east.

The earth is a BIG conveyor belt, folks. The way some of you think about this problem, planes facing west shouldn't be able to take off...

 

[ScottM]

wow its amazing to see this argument brought back up, and jesse didnt even have to kick start the thread about it!

How do we know that ken is not Jesse's alter ego??

 

[KennyFlys]

I don't care.

 

[Greg Bockelman]

Which goes back to my question regarding relative speed.

I am afraid to ask.

 

[Greebo]

Which goes back to my question regarding relative speed.

It means it doesn't matter.

ALL that matters is airspeed.

There really is no real physical way for a conveyor belt to affect enough force on the airplane to stop the airplane from moving forward *relative to the air*.

Honest.

I mean it.

Tie a rope to a tree. Stand on a treadmill facing the tree. Turn on the treadmill REALLY really fast, and hold onto the rope and pull forward. You'll fall down.

Now do it wearing roller skates. You'll be pulled forward by your force on the rope.

The propeller is your arms and the wind is the rope.

Get it?

 

[gkainz]

I ain't buyin' it ...

Where's my floats?

 

[Everskyward]

The tires need no traction on the runway to take off. They need traction to steer (until the airspeed makes the rudder effective) and to stop.

Exactly. One day last winter we took off on a snow-packed runway. After the flight one of the passengers commented on how impressed he was that the airplane could have such good acceleration straight ahead on snow without fishtailing and spinning wheels. I explained that the thrust comes from the engines and the airplane is not driven forward by its wheels like in a car. I think this is a common misconception by people who are only familiar with ground vehicles. At least on takeoff, the wheels are only there to hold the airplane off the ground and to steer until the rudder becomes effective. Whenever the brakes are in use as in after a landing or an aborted takeoff it becomes another matter entirely and the airplanes acts more like a ground vehicle.

 

[flyingcheesehead]

initially I thought it was simply the prop dragging the wing thru the air ... then SOMEBODY posted the floats scenario and now I'm all confused.

Sorry, that was me. I was saying that the floatplane pointing up a 40 knot river scenario was a bad analogy to the conveyor scenario because the floatplane probably wouldn't take off. The plane on the conveyor will.

The difference gets right to the root of the problem. There is an extreme amount of hydrodynamic drag on the floats in the water, WAY WAY more than rolling resistance of wheels. That's why the very same airplane just barely staggers into the air on floats (on STILL water thankyouverymuch) but leaps of the runway when on skis or wheels. Does the difference make sense now? (happy to explain myself further if not.)

Would you take off in the same relative position on the ground, essentially no ground (conveyor) roll? Then why didn't the geniuses put a conveyor belt on a carrier instead of a catapult? Dang, I'm so confused ... where's Bernoulli when you need him?

No, it'll need roughly the same ground roll as on a runway. Just slightly more, actually, due to the slightly increased rolling resistance from the wheels spinning twice as fast, but the difference is probably almost negligible.

So, wheels and belt be damned, the prop provides thrust thru the air mass and relative air speed is relative air speed, regardless of what the belt makes the wheels do?

You got it. Now, do you understand the floatplane scenario too? Sorry for the confusion.

 

[flyingcheesehead]

Come on, Kenny, that's kind of a cop out. You've made 101 posts in the last 120 hours (5 full days). 14 posts in this thread alone in the last 24 hours. You're obviously feeling up for discussion...it just seems as if you're not acknowledging any of the points being made to counter your arguments. There have been many posts made in this thread that have had a lot of thought put into them and you haven't refuted any of them.

Nobody is angry...it's just seems as if you're not wanting to work through the exercise as much as you're wanting to be right.

*comment better aimed at Kenny's instructors deleted*
*Spin Zone fodder removed*