Airplane on a conveyor belt?

[Joelski]

Psst, you're debating actual engineers.;)

So what? Matlock has to present a solid case. I think the mythbuster guys could do a better job of proving or disproving the theory than any of us, yes, I'm including myself. Just saying "because it can" ain't gonna get the job done.

[Joelski]

Your second post does not explain the mechanism by which the conveyor belt keeps the plane stationary. Neither do any of your other posts.

Newton's Second Law ... look into it. :thumbup:

Not enough data in the original question. You can apply laws all you want, but if you can't prove it, what you have is a qualified guess.

Edit: :)

[racer2c]

I like the Mythbusters show but they are hardly truly scientific. They come to conclusions based on trial and error in a non-laboratory environment. Not bad though for a couple of special effect artists.

What's a Matlock?:gomer:

[Joelski]

What's a Matlock?:gomer:

A person that proves something beyond a shadow of a doubt. ;)

[Ed_Severson]

So says the guy who thinks the wheels on the plane are there for drag reduction. :laugh:

There are some simple assumptions involved, sure. For example, the coefficients of friction between the wheel and the belt are similar to those between the wheel and a conventional runway. Rolling resistance for aircraft wheels is a similar order-of-magnitude effect to rolling resistance on a typical automobile. No wind velocity is given, therefore it is assumed to be zero.

If you can challenge those assumptions legitimately, go for it. Otherwise, you've got nothing. :)

Better yet, take the devilmaster challenge. Stationary plane, no thrust, no wheel rotation, no conveyor motion. Instantaneously, 50,000 pounds of thrust is generated by the engines. Does the plane move with respect to a ground-fixed frame of reference or not? Why or why not?

[Ankf00]

Please read and understand my second post.

I began ignoring your posts the second you began with your propellor vs. jet engine inanery. :gomer:

[Joelski]

If you can challenge those assumptions legitimately, go for it. Otherwise, you've got nothing. :)

http://www.speedtv.com/_assets/library/img/large/19365_tony.george.hs1.jpg

Whatever you say, boss.

Here's a brain-teaser for ya: If you tether a 747 to a pole with a 1,000' unbreakable rope made of kryptonite, and give that plane exactly 0 feet of rope to move forward, will the plane eventually float to an altitude of 1,000 feet?

[Ankf00]

So what? Matlock has to present a solid case. I think the mythbuster guys could do a better job of proving or disproving the theory than any of us, yes, I'm including myself. Just saying "because it can" ain't gonna get the job done.

MMMM, yeaaaaaah. I'm gonna have to go ahead and disagree with you there...

http://www.exitplayers.com/images/blumberg/bill-lumberg.gif

[Gangrel]

http://www.speedtv.com/_assets/library/img/large/19365_tony.george.hs1.jpg

Whatever you say, boss.

Here's a brain-teaser for ya: If you tether a 747 to a pole with a 1,000' unbreakable rope made of kryptonite, and give that plane exactly 0 feet of rope to move forward, will the plane eventually float to an altitude of 1,000 feet?

LMFAO That question demonstrates exactly how badly you are missing the point. Funny you should use the word "tether", since the key point in this whole brain teaser is that the plane isn't tethered to anything. :p

[Joelski]

Better yet, take the devilmaster challenge. Stationary plane, no thrust, no wheel rotation, no conveyor motion. Instantaneously, 50,000 pounds of thrust is generated by the engines. Does the plane move with respect to a ground-fixed frame of reference or not? Why or why not?

If you don't need no steenking wheels, let's take them off and let the plane rest on it's belly. Instantaneously, 50,000 pounds of thrust is generated by the engines. Does the plane move with respect to a ground-fixed frame of reference or not? Why or why not?

I'm saying a person with a walker could outpace a plane in the original scenario. Instead of eloquently calling me a dumbass, explain why my theory is incorrect. Why do engineers become so defensive around a little skepticism?

[Ed_Severson]

As seen on ESPN.com:

Philadelphia
Florida 7:05 PM ET

Pittsburgh 5
Chi. Cubs 3 Bot 5th

Joelski 0
Engineers 1 F

Cleveland
Chi. Sox 8:05 PM ET

[Joelski]

LMFAO That question demonstrates exactly how badly you are missing the point. Funny you should use the word "tether", since the key point in this whole brain teaser is that the plane isn't tethered to anything. :p

That was not an analogy. Sorry, but I'm not that stupid.:p

[Wheel-Nut]

http://www.kottke.org/06/02/plane-conveyor-belt

[Gangrel]

That was not an analogy. Sorry, but I'm not that stupid.:p

'Fraid I may have to beg to differ with ya' there... :D

Unless you live in a world where planes take off by building ground speed using a drive shaft attached to the wheels and only use their engines for propulsion once they are in the air, I don't see why we are continuing to have this debate.

Joelski, Sir Izaak Newton called. He wants you to stop mutilating his life's work. :p

[Ankf00]

Have you ever seen a waiter jerk a tablecloth off a table with all the dishes remaining in place? That's 1000x easier if the dishes had wheels.

v-g== velocity wrt ground
v-c== conveyor velocity

v-g == 0 == v-c
v-g incrementally increases, to do this the wheels must rotate, meaning v-c incremently increases to match, and the wheels increase in rotational velocity, w


The problem is that when the wheels have w!=0, not all of v-c translates to the strut supporting the wheel. Some of it is converted to w. And the greater the dv-c/dt, the more of v-c is translated into w.

Due to the plane's large mass, once v-c is great enough to overcome the bearing's static friction (which will happen anyway because to have any v-c you need to have v-g and this in turn requires w) v-c will be turning wheels but not affecting v-g in any meaningful way.

[Ed_Severson]

If you don't need no steenking wheels, let's take them off and let the plane rest on it's belly.

Nobody said you don't need the wheels. What I said is that they aren't there for drag reduction. In fact, they add to the drag. If you understood what drag was, you'd have picked up on that already.

Moratorium, please?


Instead of eloquently calling me a dumbass, explain why my theory is incorrect.

Done at length prior to your arrival and subsequent attempt at comic relief in this thread. I'm sure the folks who run this website will be happy to let you read it free of charge. :)

If you insist on having me go through it again for your benefit, then you can answer the question posed above, and I promise you, we'll get there eventually.


Why do engineers become so defensive around a little skepticism?

Nobody's defensive. I'm amused when people insert themselves into technical debates, start throwing around words like "momentum" and "drag" and "trajectory" that they clearly don't understand, and then try to lecture qualified professionals on the basics of powered flight, such as, for example, "you need air movement over the wings in sufficient quantity to overcome the effect of the plane's own mass."

O RLY? Thanks for that insight, professor! :tony: Is it OK if I turn in my AERO 101 homework early?

[Ankf00]

Also, I've gone quite a few posts without being an ass. I've turned a new leaf.





I want my cookie!!! Peanut butter please :)

[Gangrel]

Also, I've gone quite a few posts without being an ass. I've turned a new leaf.





I want my cookie!!! Peanut butter please :)

LMAO You get nothing and like it! :laugh:

Gotta' give Corky some of that same kool-aid you've been drinking, though...

[Ankf00]

http://cache.deadspin.com/sports/ozzieguillenkissingagain.jpg

Cookies are overrated anyways :gomer:

[Ed_Severson]

Dude, not cool! Nobody wants to have to look at the White Sox!

[Joelski]

'Fraid I may have to beg to differ with ya' there... :D

Unless you live in a world where planes take off by building ground speed using a drive shaft attached to the wheels and only use their engines for propulsion once they are in the air, I don't see why we are continuing to have this debate.

Joelski, Sir Izaak Newton called. He wants you to stop mutilating his life's work. :p

http://www.kottke.org/06/02/plane-conveyor-belt

Plenty of debate in that thread, but no solid proof. I must say that those people debated the question with much more civility....

I guess until someone can duplicate this scenario, we are all the same amount of stupid. :gomer: What would be even dumber, would be to continue arguing the point without proof, so I respectfully retire from the debate. Continue on, gentlemen. :)

[Ed_Severson]

Oh, c'mon ... this is all in good fun. In all seriousness, if you answer the question I posed, I promise you, we'll get there. You're going to have to take my word for it on some basics of vehicle dynamics, and (gulp!) make a couple of reasonable assumptions, but it's pretty easy to figure out if you take the right approach.

[Joelski]

I began ignoring your posts the second you began with your propellor vs. jet engine inanery. :gomer:

Please scan your AE degree and I'll be happy to shred and repost it for you. :D

[Ankf00]

Please scan your AE degree and I'll be happy to shred and repost it for you. :D

Scan your English BA and I'll be glad to take a dump on it :gomer:

[Ankf00]

Plenty of debate in that thread, but no solid proof. I must say that those people debated the question with much more civility....


You could have 10000 aggy discussing how the miracle of running water works with Bernoulli himself explaining pressure as a function of height & gravity, and they still wouldn't know the answer. :gomer:

[Insomniac]

You guys are still stuck in the rut of thinking the plane remains stationary while on the conveyor. It won't. The plane moves forward regardless of the conveyor.

I'm one of those people. I don't see how If I'm beside the plane at one end of the conveyer belt how it moves to the other end to take off.

[Insomniac]

Better yet, take the devilmaster challenge. Stationary plane, no thrust, no wheel rotation, no conveyor motion. Instantaneously, 50,000 pounds of thrust is generated by the engines. Does the plane move with respect to a ground-fixed frame of reference or not? Why or why not?

I'm going to assume ideal situation and assume all the thrust to move the plane is transferred to the wheels (that is, if the plane is moving the wheels are rotating, not sliding). It is moving. Because there is a force to propel it forward and the ground is not moving.

[Gangrel]

I'm one of those people. I don't see how If I'm beside the plane at one end of the conveyer belt how it moves to the other end to take off.

To put it in layman's math, in a car, the movement of the car is a function of the rotation of the wheels. With an airplane, the rotation of the wheels is a function of the movement of the plane. The movement of the plane is a function of the forces exerted on the atmosphere behind the plane, thus rendering the conveyor irrelevant.

[Gangrel]

I'm going to assume ideal situation and assume all the thrust to move the plane is transferred to the wheels (that is, if the plane is moving the wheels are rotating, not sliding). It is moving. Because there is a force to propel it forward and the ground is not moving.

Ahh...we're almost there, but not quite...go back to our problem. All the conveyor does is add energy in the form of speed to the rotation of the wheels. It does not detract from the engine's thrust against the atmosphere behind the plane. Therefore, unless the wheels are exerting sufficient friction on the conveyor to stop the plane from moving forward due to the thrust of the engines, the plane moves forward.

Let's go back to the rollerblades on the treadmill example again. Let's put you on rollerblades, on a long treadmill, think along the lines of a moving sidewalk at the airport. Now, put me on the ground walking next to but not on the conveyor belt. I put a hand on your back and walk, pushing you with me. Do you remain still or do you move forward?

That hand is the same as the thrust coming off the engines, pushing against the atmosphere behind the plane, which is not moving along with the conveyor.

[Lizzerd]

Dear Joelski and the other naysayers:

I'm going to try to explain why the plane will fly one more time. The plane is using the thrust of the engines (prop, jet, rocket, I don't' care) to propel it forward. This thrust is applied to the air, not the ground.

What is described in the original supposition, which is will the plane take off, describes, if I recall my Control Systems courses in college correctly, an open loop system. When the engines are fired up, and the plane moves forward (again, the thrust being applied to the air, not the wheels and the ground), the conveyor will begin moving backwards. As the plane moves (accelerates) with respect to the air, and hence the fixed ground, the conveyor, which senses the wheel speed and attempts to match it will, in theory, quickly reach infinity.

The key phrase above, it open loop control system.

[Lizzerd]

As I said in a much earlier thread, imagine you are on a treadmill. You run as fast as you can, but you get nowhere, because your propulsion is being supplied by your legs.

Now, imagine being on the same treadmill with a jet engine strapped to your back. Joelski, what do you think would happen if you lit up the engine? Assuming you can run at the speed of sound if you wanted, do you really think you could run fast enough to keep yourself from being blown through the walll by that engine on your back?

Maybe we could get you to try this out for us?

[RacinM3]

Maybe we could get you to try this out for us?

:laugh:

[RichK]

You could have 10000 aggy discussing how the miracle of running water works with Bernoulli himself explaining pressure as a function of height & gravity, and they still wouldn't know the answer. :gomer:

I am surprised, nay, SHOCKED, that a t.u. grad spelled Bernoulli correctly. :tony:

[Insomniac]

To put it in layman's math, in a car, the movement of the car is a function of the rotation of the wheels. With an airplane, the rotation of the wheels is a function of the movement of the plane. The movement of the plane is a function of the forces exerted on the atmosphere behind the plane, thus rendering the conveyor irrelevant.

I'm an engineer, and without doing the math/physics, I guess I have a hard time conceptulizing this.

As long as we can agree that for the plane to take off, it has to move from one end to another I'm on the same page as you guys.

I agree the movement of the plane is a result of that thrust on the air behind it. That is how it is propelled. But, the wheels are still on the ground. For the plane to move 5 feet forwards the wheel has to rotate that same distance. But if the surface below it is pulling it back 5 feet how is it getting anywhere?

[Ankf00]

I am surprised, nay, SHOCKED, that a t.u. grad spelled Bernoulli correctly. :tony:

:rofl: Between the knife hits, lines, and the gravity bongs, things get tend to get mixed up over in Hill Country, huh? :gomer:

[Insomniac]

Let's go back to the rollerblades on the treadmill example again. Let's put you on rollerblades, on a long treadmill, think along the lines of a moving sidewalk at the airport. Now, put me on the ground walking next to but not on the conveyor belt. I put a hand on your back and walk, pushing you with me. Do you remain still or do you move forward?

That hand is the same as the thrust coming off the engines, pushing against the atmosphere behind the plane, which is not moving along with the conveyor.

I move forward. But now I want to figure out what the difference is and why I think if I had a jet pack on I'd go nowhere. :)

[Ankf00]

But if the surface below it is pulling it back 5 feet how is it getting anywhere?

You're assuming that the displacement of the conveyor is translated 100% into displacement of the plane.


Think of the mass of the plane and the force required to move such mass vs. the force required to increase rotational velocity of the wheels alone.


Not all of the displacement of the conveyor will translate to the displacement of the plane I think. I think as the acceleration increases, the conveyor begins to affect the wheel rotation far more than plane's linear displacement.

[Insomniac]

Ohh I know why I'm still stuck on this. Given that I am at the airport on roller blades. If I was just standing there, I'd be going backwards. So, there is some amount of external (your hand) thrust (force) that can stop me from going backwards and keep me where I am, right? So would the force needed to keep me there increase if the belt got faster? I think the answer is no, it would be the same amount of force regardless of belt speed (assuming frictionless bearings). So I think at some point, you can no longer increase thrust or the plane will begin to move forward on the belt. Does this make sense?

[Ankf00]

^^^ exactly. Once it came to me that there's a certain threshold after which the wheels will be more or less free spinning, it made sense.

[Insomniac]

^^^ exactly. Once it came to me that there's a certain threshold after which the wheels will be more or less free spinning, it made sense.

Now that clouds the whole question. Can it apply to something with free spinning wheels. How do you match the speed of the airplane in the opposite direction? How is the speed of the airplane determined? What variables can go to infinity?

[Gangrel]

I'm an engineer, and without doing the math/physics, I guess I have a hard time conceptulizing this.

As long as we can agree that for the plane to take off, it has to move from one end to another I'm on the same page as you guys.

I agree the movement of the plane is a result of that thrust on the air behind it. That is how it is propelled. But, the wheels are still on the ground. For the plane to move 5 feet forwards the wheel has to rotate that same distance. But if the surface below it is pulling it back 5 feet how is it getting anywhere?

Because when the plane moves 5 feet forward, the wheels actually rotate 10 ft. 5 ft for the forward movement of the plane itself and 5 ft for the backward pull of the treadmill. Still doesn't change that the plane has moved 5 feet in relation to the earth. It has just moved 10 feet in relation to a specific point on the conveyor belt.

[Turn7]

Forget about how fast the wheels are turning.

The problem states that the conveyor will only move as fast as the plane.

If the plane is not moving as some of you can't get past, then the conveyor won't be moving at all. If the plane is stationary with the conveyor cancelling all thrust forward then the speed of the plane is ZERO. Therefore as stated in the problem the conveyor is going to match the speed of the plane. So if the conveyor is moving at 100mph, that means that the plane is making a forward progress of 100 mph.

Simple really.

[Ed_Severson]

I'm going to assume ideal situation and assume all the thrust to move the plane is transferred to the wheels (that is, if the plane is moving the wheels are rotating, not sliding). It is moving. Because there is a force to propel it forward and the ground is not moving.

Yeah, the plane in that case will move. It will move because the only way it won't move is if some force opposes the thrust precisely. Since nothing imposes any horizontal forces on the plane other than the gases leaving the jets and the conveyor, and we've already established that the gases leaving the jets have to be opposed, your candidates for providing the opposing force must come from the conveyor belt.

You may choose from:

a) Static friction -- valid only when the wheels are not moving with respect to the conveyor
b) Rolling resistance -- valid only when the wheels are rolling on the conveyor
c) Dynamic friction -- valid only when the wheels are moving with respect to the conveyor in translation

Since nothing is moving yet, rolling resistance and dynamic friction are out, leaving you with static friction as your only option. Now, assuming that the coefficient of static friction between the wheels and the conveyor is not wildly different than the coefficient of static friction between the wheels and a conventional runway (a very safe assumption, as I'm sure any trained person would agree), then you have a situation which is analagous to a plane on a conventional runway, which as I'm sure we would all agree, would be compelled to move forward by the application of the 50,000 pounds of thrust.

Anybody object so far?

[Ankf00]

^^^ What Ed said, consider the 5 external forces that act on the plane, what they are functions of, then consider the wheels & the external forces acting on them.



Forget about how fast the wheels are turning.

The problem states that the conveyor will only move as fast as the plane.

If the plane is not moving as some of you can't get past, then the conveyor won't be moving at all. If the plane is stationary with the conveyor cancelling all thrust forward then the speed of the plane is ZERO. Therefore as stated in the problem the conveyor is going to match the speed of the plane. So if the conveyor is moving at 100mph, that means that the plane is making a forward progress of 100 mph.

Simple really.

yes, simple, but you should know posting something "simple" like that in a forum full of engineers will cause nothing but headaches :D

[Insomniac]

Because when the plane moves 5 feet forward, the wheels actually rotate 10 ft. 5 ft for the forward movement of the plane itself and 5 ft for the backward pull of the treadmill. Still doesn't change that the plane has moved 5 feet in relation to the earth. It has just moved 10 feet in relation to a specific point on the conveyor belt.

No no. I was saying that in respect to the treadmill. To move 5 feet forward on the belt.

[RacinM3]

Man, it's so simple.

To only way to stop this plane from moving forward is to apply a force exactly equal to, but in the exact opposite direction, of the force being applied. Since the engine's thrust is being applied to the plane's fuselage, you would have to also apply that opposite force to the fuselage to keep the plane stationary.

A conveyor belt CANNOT apply that force to the fuselage, since the plane's wheels are free spinning. Thus, the conveyor belt cannot keep the plane from moving, and the plane will build up enough airspeed to create the lift required for takeoff.

If the plane has a 120 MPH takeoff speed, theoretically ONLY difference between a normal takeoff and a "conveyor belt" takeoff is the wheels will be spinning the equivalent RPMs of a 240 MPH takeoff speed.

[Gangrel]

Now that clouds the whole question. Can it apply to something with free spinning wheels. How do you match the speed of the airplane in the opposite direction? How is the speed of the airplane determined? What variables can go to infinity?

The only way to match the speed of the airplane such as to stop it from moving forward on the conveyor is to meet the plane with a headwind equal to the rearward thrust of the engines. But this would still allow the plane to take off, it just wouldn't move forward in a physical sense.

Now, if the goal is to keep the plane from taking off, meet it with a tailwind equal to the thrust exerted by the engines. Now the wings cannot generate lift. However, the wind itself combined with the thrust of the engines will most certainly push the plane forward.

What I am not sure of in that second case is whether the engines pushing against the tailwind would generate enough additional speed to allow the plane to take off....In other words, a plane that needs to take off at 100 mph being met by a 100 mph tailwind with the engines thrusting to go at 100 mph - would the plane travel at 200 mph, 100 mph faster than the tailwind, and still be able to take off?

[RacinM3]

What I am not sure of in that second case is whether the engines pushing against the tailwind would generate enough additional speed to allow the plane to take off....In other words, a plane that needs to take off at 100 mph being met by a 100 mph tailwind with the engines thrusting to go at 100 mph - would the plane travel at 200 mph, 100 mph faster than the tailwind, and still be able to take off?

Yes, it would fly, but not necessarily at 200 MPH....probably substantially slower (due to the mass of the plane acting against the tailwind), but more than the 100 MPH it would be at without the tailwind (ground speed). The thrust will still act against the tailwind, and move the plane forward. THe plane would also require a longer airstrip, most likely....thus the reason they like to take off into the wind.

This is why it takes less time to fly from LA to New York than from New York to LA (jet stream).

http://avstop.com/AC/FlightTraingHandbook/RelativeWind.html

[Insomniac]

Yeah, the plane in that case will move. It will move because the only way it won't move is if some force opposes the thrust precisely. Since nothing imposes any horizontal forces on the plane other than the gases leaving the jets and the conveyor, and we've already established that the gases leaving the jets have to be opposed, your candidates for providing the opposing force must come from the conveyor belt.

You may choose from:

a) Static friction -- valid only when the wheels are not moving with respect to the conveyor
b) Rolling resistance -- valid only when the wheels are rolling on the conveyor
c) Dynamic friction -- valid only when the wheels are moving with respect to the conveyor in translation

Since nothing is moving yet, rolling resistance and dynamic friction are out, leaving you with static friction as your only option. Now, assuming that the coefficient of static friction between the wheels and the conveyor is not wildly different than the coefficient of static friction between the wheels and a conventional runway (a very safe assumption, as I'm sure any trained person would agree), then you have a situation which is analagous to a plane on a conventional runway, which as I'm sure we would all agree, would be compelled to move forward by the application of the 50,000 pounds of thrust.

Anybody object so far?

Nah. The problem is the whole thing is changing. Once the thrust starts you have rolling resistance and dynamic friction. What I didn't realize that at some point, the amount of thrust will be enough to keep the plane in it's current position regardless of belt speed.

I made the assumption, which turned out to be poor that the conveyer can pull the plane back as much as it can try to go forward. At some point it will hit a wall where if the thrust goes up the belt can't stop the plane from going forward. The way I read the question, if the plane is trying to go forward at 100 MPH the belt is going backwards at 100 MPH and the plane will remain where it is. In reality, to keep it in place at the beginning, the speeds won't be the same and eventually it would overcome it if you increased the thrust. The problem is poorly worded (I think). I was trying to equate the "speed" of the plane to the amount of thrust. But, at some point, Thrust X will be the point where it's stationary and the Speed of the belt could be increasing while the thrust is constant. Just because the belt is moving at 100 MPH and the plane is staionary, does not mean the plane is moving at 100 MPH also.

[B3RACER1a]

LOL wow on the airplane thing still going! :laugh:

Anyways, on the boat question with the rock, the level will go down. Already been explained.

[nissan gtp]

we're not the only people thinking about this

Physics forum (http://forum.physorg.com/index.php?showtopic=2417)

read on, it's only 358 pages long, 5367 posts :tony:

[Rocketdoc]

Devilmaster; you are correct.

Your prize is a Toony George "Booble head"; sorry.

[coolhand]

what the hell happened here :\

[Robstar]

LOLZ... Don't bother... Look at the image & page 2... :)

[SteveH]

I think K-Mart has a sale of life this weekend, just in case any of you guys need one. ;) :D

[Robstar]

I might Lay-buy one... It's Friday afternoon !

I'll probably need it Monday... ;)

[MDDriver]

New guy here...but a pilot with 39 years and 27,000 hours of flying time. Military and commercial...Cessna 172 to KC10 (military version of the DC 10)

Lizzard (my buddy from IMS) sent me some emails about this discussion...

Bottom line...an airplane needs movement through the atmosphere (i.e. air) to fly...if it's on a treadmill and NOT moving across the earth's surface, it will NOT move "through the atmoshphere" hence it will NOT fly.

[Lizzerd]

New guy here...but a pilot with 39 years and 27,000 hours of flying time. Military and commercial...Cessna 172 to KC10 (military version of the DC 10)

Lizzard (my buddy from IMS) sent me some emails about this discussion...

Bottom line...an airplane needs movement through the atmosphere (i.e. air) to fly...if it's on a treadmill and NOT moving across the earth's surface, it will NOT move "through the atmoshphere" hence it will NOT fly.

And like I tried to explain in those emails, my good friend... NONSENSE!

The engines are working against the air, not the ground. The wheels are a passive component in the system. The conveyor could never keep up with the airspeed, because the plane is in constant acceleration.

Screw it... I'm gonna email Myth Busters.

[B3RACER1a]

Holy crap. Someone drew a freakin picture. What else do you need to be convinced? :rofl:

RECAP: The conveyor belt DOES NOT matter. The air is being accelerated through the engines. Since air has mass, there needs to be a force to move the air (F=ma), which is provided by the engines. This force has an equal an opposite reaction on the engines, which are connected to the fuselage. This then accelerates the plane forward. The only difference a conveyor belt moving the opposite way of the plane makes is the wheels are going to spin twice as fast. THATS IT.

:gomer:

[Lizzerd]

Holy crap. Someone drew a freakin picture. What else do you need to be convinced? :rofl:

RECAP: The conveyor belt DOES NOT matter. The air is being accelerated through the engines. Since air has mass, there needs to be a force to move the air (F=ma), which is provided by the engines. This force has an equal an opposite reaction on the engines, which are connected to the fuselage. This then accelerates the plane forward. The only difference a conveyor belt moving the opposite way of the plane makes is the wheels are going to spin twice as fast. THATS IT.

:gomer:

Once again, thank you.

[nrc]

What we need here is a good old fashioned force diagram. If the engine is producing 40,000 lbs of forward thrust, something has to generate 40,000 lbs of resistance to keep the plane from accellerating forward and taking off. There is no way that the conveyor belt can generate 40,000 lbs of resistance through free spinning wheels.

[B3RACER1a]

I'll draw one up on break at work tomorrow. :)

[RacinM3]

What we need here is a good old fashioned force diagram. If the engine is producing 40,000 lbs of forward thrust, something has to generate 40,000 lbs of resistance to keep the plane from accellerating forward and taking off. There is no way that the conveyor belt can generate 40,000 lbs of resistance through free spinning wheels.

Bnigo. Sad part is, my brother in law flies corporate jets for a living, and he can't grasp this concept.

Here's how you can test it at home: Take a matchbox car and a sheet of newspaper. Put your fingers in front of the matchbox car to keep it from moving, then slide the paper slowly out from under it, getting a feel for the resistance the car puts up. Now, do it again, only slide the paper out really fast. Feel a difference? I didn't think so.

The only way that plane doesn't take off is if a force equal and opposite to the engine's thrust is applied TO THE FUSELAGE.

[MDDriver]

And like I tried to explain in those emails, my good friend... NONSENSE!

The engines are working against the air, not the ground. The wheels are a passive component in the system. The conveyor could never keep up with the airspeed, because the plane is in constant acceleration.

Screw it... I'm gonna email Myth Busters.

OK, one step at a time.

1. To fly an airplane must MOVE through the air, the atmosphere, which is a part of the earth itself.

2. In order to MOVE through the air an airplane must FIRST accelerate, while still on the ground, i.e. earth, to a speed where the air flowing over the wings can generate enough lift to make the airplane fly.

3. How the airplane on the ground is accelerated is irrelevant. Be it a jet engine, a propeller (conventional engine or turboprop), or even an external source (catapult, tow rope from a motor vehicle) makes NO difference. It makes NO difference what the motive power of the plane is operating “against”. Be it jet thrust, prop thrust, a tow line, a catapult, or even motors in the airplanes wheels! The plane must MOVE across the earths surface from it’s starting point. If it does NOT, it will not fly as there will be NO air flowing across the wings.

OK so far…

4. If you prevent the airplane from MOVING from its starting spot you will keep it from flying. Preventing movement from the starting spot can be done in two ways. One…some sort of restraint. i.e. TIE it down to the earth. Or, TADA…move the earth beneath the planes wheels in the opposite direction, i.e. a treadmill. Yes it must be the “mother of all treadmills” capable of moving at the maximum speed the airplanes motive power (whatever that is, again irrelevant) can accelerate the airplane to. If the treadmill CAN NOT keep up with the acceleration of the airplane, then the airplane WILL MOVE. (This seems to be what you are hung up on…the airplane moving OFF of the treadmill) But in this theoretical scenario, that treadmill, the “mother of all treadmills” IS capable of matching the acceleration generated by the plane’s motive power, regardless of how much that is and regardless of how much acceleration the plane can attain. Further this “mother” has to be able to keep increasing it’s speed to match the acceleration of the plane. Practical ? No, probably not. But we’re talking a theoretical scenario here so practical limitations have to be ignored.

Your comment “The conveyor could never keep up with the airspeed,” negates the entire theoretical scenario and hence negates the idea of keeping the airplane static relative to it’s starting place on the face of the earth. What you are saying is that due to the inability to build a “mother of all treadmills” makes it possible for the plane to fly because it moves off of the treadmill. Again, the practicality of building such a device has to be ignored in the theoretical discussion.

Bottom line…IF the airplane does NOT move from it’s static starting point on the face of the earth, for what ever reason (TIED down, or TREADMILL) it will not have any air moving across the wings and will NOT fly as NO lift will be generated.

Trust me on this one…

FWIW even the "Myth Busters" aren't capable of building an in ground treadmill that would support, oh say a KC10 weighing 600,000 pounds with 160,000 pounds of thrust capable of accelerating that mass to 200 mph on the ground. Nor would the goobers in "Two Guys Garage"!

[Warlock!]

Funny that this is still goin' on... :D



Your comment “The conveyor could never keep up with the airspeed,” negates the entire theoretical scenario and hence negates the idea of keeping the airplane static relative to it’s starting place on the face of the earth.
No one is saying the airplane will be static. From the original post:


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).
So, it says nothing about keeping the plane static... just that the conveyor will be the same speed as the plane. The "static" part is being thrust into your mind because of the "exact opposite speed and direction" statement.

Look at it this way. If you're landing your plane at X mph and you land on a conveyor spinning at exactly X mph in the opposite direction, will your plane still roll forward when it touches down? Why?

[MDDriver]

Holy crap. Someone drew a freakin picture. What else do you need to be convinced? :rofl:

RECAP: The conveyor belt DOES NOT matter. The air is being accelerated through the engines. Since air has mass, there needs to be a force to move the air (F=ma), which is provided by the engines. This force has an equal an opposite reaction on the engines, which are connected to the fuselage. This then accelerates the plane forward. The only difference a conveyor belt moving the opposite way of the plane makes is the wheels are going to spin twice as fast. THATS IT.

:gomer:

First you assume jet engines. Suppose they are props? Or no engines at all, i.e. a glider which has to be towed? In the end the airplane (regardless of type) must MOVE from it's static starting point. If that point is constantly "moved" in a direction opposite to the forward movement (regardless of motive power) i.e. a "treadmill", then the airplane (again regardless of type) does NOT move from it's original static location, and will NOT "fly".

Using that glider and a tow rope. If the tow rope is pulled in by a winch, which then "moves" the glider forward, it will "fly". But suppose the winch itself is not fastened down and when it turns rather than pulling the glider, it merely winds itself up until it reaches the glider. In that case the glider has NOT moved and did NOT fly. The winch moving toward the glider is in essence the same as a treadmill moving to counter a plane, which has it's motive power attached, and keeping the plane in the same physical location, relative to the earth itself.

Like I said, in theory a treadmill will prevent movement from the static starting point. As a practical matter, building such a treadmill (unless you have a Formula One team budget) just isn't possible.

There is another practical matter that comes into play here. Aircraft tires have a "max speed". For example those on a KC10 (military version of the DC10) have a max ground speed limit of 205 knots. When one considers just how fast a treadmill would have those puppies spinning to counter the thrust of the engines, well you get the picture. But again, this is practical versus the theroretical issue which ignores such practical constraints.

[MDDriver]

Once again, thank you.


Hey Lizzerd, try this on for size.

You are standing flat footed on the ground, a paper airplane in your hand. You move your hand rapidly forward (action) and thanks to the fact the friction of your feed on the ground prevents the opposite force (reaction) from moving you backward. Hence you make the paper airplane "fly" because you impart a force that makes it "move" through the air.

But suppose, just suppose, there is nothing counter the reaction to your action of moving your arm forward, i.e. friction of your feet and your weight, and instead of that paper plane moving foward, you move backward, i.e. away from your original starting spot. That paper airplane will not move from where it started as the motive force (your arm) was negated by your moving backward. That plane will simply fall to the ground (unless you keep a grip on it) as you move backward in reaction to the action of your arm moving forward.

Theoretically possible? yes (just take gravity away and watch what happens) Practically possible? no (unless you can get away from gravity)

Or if you can't get away from gravity...you must remove your own weight, considerable as it may be my rotund friend! Woo Hoo!

[MDDriver]

Funny that this is still goin' on... :D


No one is saying the airplane will be static. From the original post:


So, it says nothing about keeping the plane static... just that the conveyor will be the same speed as the plane. The "static" part is being thrust into your mind because of the "exact opposite speed and direction" statement.

Look at it this way. If you're landing your plane at X mph and you land on a conveyor spinning at exactly X mph in the opposite direction, will your plane still roll forward when it touches down? Why?

OK, now things are changing a little from how I viewed the original scenario. (basically
from an email Lizzard sent me) I was working under the premise that the whole purpose of the conveyor (i.e. a treadmill) was to move in such a direction and speed to keep the airplane in the same physical starting spot, relative to the surface of the earth upon which it started. If in fact the airplane does move away from it's original starting spot due to being able to out accelerate the conveyor (i.e. treadmill) then it will fly. But only because the plane itself did MOVE away from it's original static position. It’s movement causing air to flow over the wings and generate the necessary lift to counter it’s weight and “off we go into the wild blue yonder!”

The landing scenario is different because now the airplane IS MOVING relative to the earth. It is FLYING due to it's movement through the earth's atmosphere, which is a part of the earth itself. An airplane takes off because it has enough speed to generate the requisite lift. It lands because you fly it back onto the ground. It still has speed and is generating lift at the time of touchdown. It is not static relative to the earth. Yes, you can “full stall” an airplane (i.e. no lift, no speed), but then the “landing” becomes a “crash”.

Funny thing about all this talk of conveyors and treadmills and static versus movement. Reminds me of a time long ago when I worked on the railroad during my summer break in college. I remember marveling at some of those old guys who could step off of or on to a moving train. In some cases they were at 20 mph and stepped off without breaking a leg or going tumbling down the tracks. In the other, the went from zero speed to the speed of that moving train, all with the greatest of ease. Had to do with that first step and how they positioned themselves. Amazing feat to this day. As for me? Well anything over maybe 5 mph and I just didn’t try!

[Gangrel]

4. If you prevent the airplane from MOVING from its starting spot you will keep it from flying. Preventing movement from the starting spot can be done in two ways. One…some sort of restraint. i.e. TIE it down to the earth. Or, TADA…move the earth beneath the planes wheels in the opposite direction, i.e. a treadmill. Yes it must be the “mother of all treadmills” capable of moving at the maximum speed the airplanes motive power (whatever that is, again irrelevant) can accelerate the airplane to. If the treadmill CAN NOT keep up with the acceleration of the airplane, then the airplane WILL MOVE. (This seems to be what you are hung up on…the airplane moving OFF of the treadmill) But in this theoretical scenario, that treadmill, the “mother of all treadmills” IS capable of matching the acceleration generated by the plane’s motive power, regardless of how much that is and regardless of how much acceleration the plane can attain. Further this “mother” has to be able to keep increasing it’s speed to match the acceleration of the plane. Practical ? No, probably not. But we’re talking a theoretical scenario here so practical limitations have to be ignored.

Even though I don't buy into the whole "mother of all treadmills" thing, allow me to put the kabosh on this one using the wording of the problem itself. The treadmill has a tracking system that matches the speed of the plane itself. If the plane is moving forward 10 mph, the treadmill is moving backward 10 mph, not at infinity speed to keep the plane from moving forward at 10 mph. A treadmill moving backward at 100 mph is not going to generate enough resistance on the wheels to keep the plane from moving forward at 100 mph.

Also, what we are "hung up on" has nothing to do with the plane moving off the treadmill. It has to do with the fact that moving the earth backward beneath the wheels is not going to prevent the engines from pushing against the air behind the wings, and therefore the treadmill will be rendered irrelevant in the equation.

[RacinM3]

Again.....


The only way that plane doesn't take off is if a force equal and opposite to the engine's thrust is applied TO THE FUSELAGE.

Instead of "take off" I maybe should have said "move forward"....but I assumed people would know "take off" means the plane IS moving forward. The physics of flight aren't that hard to understand.

[indyfan31]

Apparently, to some folks, they are.
7 pages. :shakehead

[Insomniac]

Even though I don't buy into the whole "mother of all treadmills" thing, allow me to put the kabosh on this one using the wording of the problem itself. The treadmill has a tracking system that matches the speed of the plane itself. If the plane is moving forward 10 mph, the treadmill is moving backward 10 mph, not at infinity speed to keep the plane from moving forward at 10 mph. A treadmill moving backward at 100 mph is not going to generate enough resistance on the wheels to keep the plane from moving forward at 100 mph.

How is the speed of the plane determined?

[Insomniac]

The physics of flight aren't that hard to understand.

Speaking for myself, the physics of flight are pretty simple. It's the belief that the plane would go nowhere given the scenario that made me believe it would not take off. Once I understood the plane would move forward I got it.

[Gangrel]

How is the speed of the plane determined?

Now THAT is a very interesting question...

Irrelevant, but interesting....

If speed is determined by the speed at which the wheels move forward, then the conveyor belt will go to infinity because the wheels will always spin faster than the conveyor, due to the force driving the plane's movement being exerted on the air behind the plane, not through the wheels to the ground.

If the speed is determined by how much force is exerted on the air behind the plane by the engines, the conveyor will go to twice the speed of the plane.

Either way, the plane moves forward and takes off. Unless example #1 is used, that is. In that case, as the wheels of the landing gear approach infinity speed, the bearings seize, the wheels fall off the gear, and the plane winds up on its belly, then goes backwards really fast. :D

[nrc]

I'd like to see mythbusters test this with their JATO rocket car. :D

[RacinM3]

Speaking for myself, the physics of flight are pretty simple. It's the belief that the plane would go nowhere given the scenario that made me believe it would not take off. Once I understood the plane would move forward I got it.

My comment wasn't directed at you...it's directed at those who feel they need to explain that an airplane needs air moving over it's wings to generate lift....uh, yeah, we know. If you're looking at that part of it, you're looking in the wrong direction re: the answer to the "riddle".

HAHA...8 pages!!! :)

[racer2c]

My comment wasn't directed at you...it's directed at those who feel they need to explain that an airplane needs air moving over it's wings to generate lift....uh, yeah, we know. If you're looking at that part of it, you're looking in the wrong direction re: the answer to the "riddle".

HAHA...8 pages!!! :)

8 pages of the same people repeating the same thing. :)

[RacinM3]

8 pages of the same people repeating the same thing. :)


And I keep expecting a different response. I believe that's the definition of insanity, no? ;)

[Turn7]

Why in the hell would anyone think the plane was staionary? It clearly states that the conveyor speed matches the planes speed. If the plane isn't making forward progress then the conveyor would not be moving.

There is no cancelling of forward movement because if that were the case then the conveyor would not be moving at all.


Where in the world are people getting the notion that the conveyor is cancelling the speed?

[G.]

"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?"


I say yes.That's what threw me. Substitute "wheel" speed for "plane" speed, and this topic goes 3 pages max.

[Gangrel]

That's what threw me. Substitute "wheel" speed for "plane" speed, and this topic goes 3 pages max.

If the word were changed to "wheel" the plane would still take off provided the wheels didn't combust.

What? No one saw the humor in my comment about the plane going backwards really fast? :D

[G.]

If the word were changed to "wheel" the plane would still take off provided the wheels didn't combust.Of course. If you read the riddle and assume that the plane is stationary, you get 8 pages.

What? No one saw the humor in my comment about the plane going backwards really fast?anyone got one of those .avi's showing a treadmill accident? :D

[Insomniac]

Either way, the plane moves forward and takes off. Unless example #1 is used, that is. In that case, as the wheels of the landing gear approach infinity speed, the bearings seize, the wheels fall off the gear, and the plane winds up on its belly, then goes backwards really fast. :D

http://www.youtube.com/watch?v=6AqkLMyJcvY
http://www.youtube.com/watch?v=BUPnxpsD05g

[indyfan31]

To this day, that treadmill-in-the-laundry-room commercial is one of the funniest things ever filmed. :rofl:

[nrc]

Mythbusters did an episode on this recently and it turned out exactly as I expected. :)

Don't want to spoil it for you.

[grungex]

Mythbusters did an episode on this recently and it turned out exactly as I expected. :)

Don't want to spoil it for you.

I'm not convinced their methodology accurately reproduced the original question, but it was interesting anyway. Funny about the pilot's view of what the result was going to be...