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  #101  
Old 12-13-2006, 11:25 AM
ethicalBob ethicalBob is offline
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Quote:
Originally Posted by Musicat
Anywhere in this discussion, to avoid misunderstanding, we must specify (1) what object we are talking about (plane, treadmill, ground, air) and (2) what the speed is relative to (plane, treadmill, ground, air). No assumptions should be made, or confusion will result.

So, "If the plane achieves this speed on the treadmill (which is standing still relative to the earth)...", WHAT is standing still? The treadmill or the plane? And it is standing still relative to WHAT? The air? The ground? The treadmill? WHAT is achieving this speed? Relative to WHAT? The air? The ground? The treadmill?

Using different assumptions will get you different answers.
This is why you were taught english in High School; if you know how to read a sentence properly, you would understand how to read what is written.

The subject of this sentence is obviously the plane - if you don't know why, you need to brush up on your knowledge of sentence structure. Once you understand the subject of the sentence, all supporting ideas take care of themselves.

There are no "assumptions" here, you just need to learn how to read properly.
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  #102  
Old 12-13-2006, 11:34 AM
Musicat Musicat is offline
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Quote:
Originally Posted by RJKUgly
The plane’s engines push directly on the plane. They push the same if the plane is on the ground or in the air, or on skis on the water. The wheels do nothing but spin, their only function is to reduce friction between the plane and the ground.
Absolutely. Some people don't seem to understand that the wheels on a plane don't cause it to move forward, backwards, faster or slower. They're just there to keep the plane from pancaking on the ground until it can support itself thru other means.

Consider this: a plane on a treadmill. At start, both the plane and the treadmill are stationary with respect to the ground. As the engines rev up, the plane begins to roll normally. Then what if the treadmill suddenly started to move backwards, then stopped, then moved forwards. What effect would the treadmill have on the plane?

Answer: nothing that would inhibit takeoff. The wheels would spin at different speeds and directions at different times, but they would have negligable effect on the forward motion of the plane relative to the air or the (non-treadmill) ground. Therefore, the plane would take off in about the same distance, relative to the ground, as if the treadmill were not there. And it would take off in about the same time (relative to my watch) as if the treadmill were not there, too.

Unlike a car, where the motive power travels thru the wheels to the ground, a plane is not powered thru the wheels. They're just there to keep the plane from pancaking on the ground until it can support itself thru other means.
  #103  
Old 12-13-2006, 11:46 AM
Musicat Musicat is offline
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Quote:
Originally Posted by ethicalBob
This is why you were taught english in High School; if you know how to read a sentence properly, you would understand how to read what is written.

The subject of this sentence is obviously the plane - if you don't know why, you need to brush up on your knowledge of sentence structure. Once you understand the subject of the sentence, all supporting ideas take care of themselves.

There are no "assumptions" here, you just need to learn how to read properly.
I do not see how your insults add to this discussion. But I will attempt to show you what I mean by a quote from your own post:
Quote:
If the plane achieves this speed on the treadmill (which is standing still relative to the earth)...
You specified "speed". Is that speed relative to ground, air, or treadmill? They could be different, and the problem takes on a different perspective depending on how you define it.

The antecedent to "which" is "treadmill", but I suspect you might have meant "plane". Before continuing, I would like to know, without question, what is being stated here.

The problem with assumptions is yours might not be the same as someone else's, whether they are geniuses or idiots. If we eliminate assumptions by stating the problem clearly, we will have fewer misunderstandings. I would say this discussion boils down to just two factors:
  • Stating the problem unambiguously, then
  • Applying knowledge of simple physics.
  #104  
Old 12-13-2006, 12:05 PM
JimOfAllTrades JimOfAllTrades is offline
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Quote:
Originally Posted by dublos
Now this one I really don't get. Take my water example above, add a third element.. a pontoon boat, with no engine, and it's attached to a cable/winch that's 2 miles up stream.

Is the force required (provided by the plane's engine, the winch pulling, the boat/s propeller) to move all three of these things 2 miles upstream the same? Because unless there is something that causes the boat to exert more force to move that distance than the other two options, then I'm still missing whatever factoid, point of view or advance physics formula you're using to say that the plane still moves independent of the ground before it gets lift.

Personally, I can see where the winch might actually require less, but I think that the plane, at best, exerts a level of force between that of the winched boat and the propeller in the water driven boat.
Maybe the thing that hasn’t been said explicitly here is that the amount of the resistance to the plane moving forward caused by the wheels is extremely small compared to simply overcoming the inertia and wind resistance of the plane itself.

As zut very carefully pointed out, by rolling the plane across a treadmill, the only increase in drag is however much extra friction is caused by the wheels having to turn more quickly. There is no additional wind resistance, no additional acceleration of the plane. And the additional resistance from the wheels turning faster is, from what I’ve seen talked about, very small.

So it takes practically no extra force to push the plane right over the treadmill and on down the runway, at least compared to the total force the engines are capable of in normal operation.

At least that’s my understanding for anything like real world airplanes and their wheels.

In your water example, I believe the resistance of the water on the bottom of the boat or pontoon plane would be higher percentage of the total resistance. In that case the speed of the current in the water would have more effect on the pontoon plane than the treadmill would on a wheeled plane. But I don’t know enough about it to say for sure, or even to guess how much more the drag might be.
  #105  
Old 12-13-2006, 12:05 PM
dublos dublos is offline
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Quote:
Originally Posted by Musicat
Some people don't seem to understand that the wheels on a plane don't cause it to move forward, backwards, faster or slower. They're just there to keep the plane from pancaking on the ground until it can support itself thru other means.
I'm sorry, but as one of the people who don't understand something about this whole process, I'm feeling somewhat talked down to by the way you said that. I know full well that the wheels of the plane do not have any motive connection at all to the engine.

Lets adjust your scenario.. The plane starts moving second, not first. It's stationary at one end of this mile long treadmill. The treadmill's ramping up to 1 mile per hour, now are you going to tell me that the wheels will start rolling and the plane will remain stationary relative to the ground, even without the plane's engine running, because the wheels aren't connected to the engine?

I'm going to assume the answer is no. So, I made this hypothetical treadmill nice and long for a reason.. lets start the plane's engines and start giving it some gas. At what speed/level of thrust/some other force I don't understand does the plane start ignoring the fact it's still on the treadmill and that treadmill is moving in the opposite direction? Immediately? I'm pretty sure we're going to have to reach a level of force equal to the force that would move it forward at 1 mile per hour just to get the plane stationary.

At what point does this element I don't understand enable the plane to move forward, even though we increase the treadmill's speed as soon as the plane is seen to move forward relative to the ground?

And most importantly of all.. can you please explain in little words what that force is and why. The plane's engine is pushing against air the whole time, so it's not simply the fact that the engine is not attached to the wheels. It is something beyond that because it doesn't happen immediately.
  #106  
Old 12-13-2006, 12:21 PM
dublos dublos is offline
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Quote:
Originally Posted by RJKUgly
Maybe the thing that hasn’t been said explicitly here is that the amount of the resistance to the plane moving forward caused by the wheels is extremely small compared to simply overcoming the inertia and wind resistance of the plane itself.

As zut very carefully pointed out, by rolling the plane across a treadmill, the only increase in drag is however much extra friction is caused by the wheels having to turn more quickly. There is no additional wind resistance, no additional acceleration of the plane. And the additional resistance from the wheels turning faster is, from what I’ve seen talked about, very small.
I'm trying to come up with another example that doesn't change "the element I don't understand yet" without knowing what that element is, so I apologize if this tangent is not quite on target.

Lets take the plane out of the equation, wings and lift don't actually have anything to do with what you're describing if we're talking about motive power being pushing against air rather than being transmitted through the wheels.

Lets put me on really good roller skates and put a really big fan on my back.

Rev that fan up to the point I'm moving forward.

Same motive force as the plane scenario, right?

Ok... so put me on to a treadmill, start the fan, start the treadmill.

Same thing?
  #107  
Old 12-13-2006, 01:27 PM
Musicat Musicat is offline
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Quote:
Originally Posted by dublos
I'm sorry, but as one of the people who don't understand something about this whole process, I'm feeling somewhat talked down to by the way you said that. I know full well that the wheels of the plane do not have any motive connection at all to the engine.
Good. Then you aren't the party to whom I addressed that comment and we have no disagreement.
Quote:
Lets adjust your scenario.. The plane starts moving second, not first. It's stationary at one end of this mile long treadmill. The treadmill's ramping up to 1 mile per hour, now are you going to tell me that the wheels will start rolling and the plane will remain stationary relative to the ground, even without the plane's engine running, because the wheels aren't connected to the engine?

I'm going to assume the answer is no.
That's where we part company and this seems to contradict what you said above. The answer is yes, the wheels will start rolling (yet) the plane will remain stationary relative to the ground. There is little or no friction in the wheel bearings, and a treadmill cannot exert force on any other part of the plane but the wheels.

If you don't understand this, you will not be able to continue with the thought process in this question. The motion of the wheels of the plane do not inhibit its forward movement nor add to it; they are not a significant factor at all.

Put the brakes on, and you have a different situation. With brakes, the plane will follow the treadmill as closely as possible, given inertia, etc. Without brakes, plane and treadmill are not closely coupled. Or, as you just said,
Quote:
Originally Posted by dublos
I know full well that the wheels of the plane do not have any motive connection at all to the engine.
Footnotes:

I will admit this is more theoretical than practical. IRL, a tiny bit of friction (since no bearing is frictionless) may cause the plane to move slightly (without brakes), but that must be conterbalanced by a slight air resistance, another minor counterforce.

And just to clear up another possible misconception, in all the above, the plane is sitting on the treadmill aligned with the direction of treadmill movement. If one or the other were rotated 90 degrees, the angle would have the same effect as applied brakes, and a treadmill would carry the plane along as much as possible.
  #108  
Old 12-13-2006, 01:36 PM
Musicat Musicat is offline
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Quote:
Originally Posted by dublos
Lets put me on really good roller skates and put a really big fan on my back.
Can we assume you mean the fan is mounted on your back, not mounted on the ground and pointed at your back? "Putting a fan on my back" is to me ambiguous, and we don't want to be that, do we?
Quote:

Rev that fan up to the point I'm moving forward.

Same motive force as the plane scenario, right?

Ok... so put me on to a treadmill, start the fan, start the treadmill.

Same thing?
Yes. Same thing. Now you've got it! Absent any friction in your skates, the treadmill does not transmit motion from the treadmill to your body. Given a sufficiently large fan, the motion of your body relative to the ground will be determined mostly by it.

The same caveats of my previous post apply, i.e., alignment direction, brakes (do skates have brakes?), negligable friction, ignore the extension cord to the fan, etc.

(If I misjudged your "fan on my back" line, we will have to start over, so that's critical.)
  #109  
Old 12-13-2006, 01:48 PM
Musicat Musicat is offline
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Quote:
Originally Posted by dublos
... lets start the plane's engines and start giving it some gas. At what speed/level of thrust/some other force I don't understand does the plane start ignoring the fact it's still on the treadmill and that treadmill is moving in the opposite direction? Immediately?
Yes. Immediately and always. The wheels of the plane do not have any motive connection at all to the engine or the plane. They are free-wheeling.

You must, must, must get away from the idea that primary motion is transferred thru wheels to/from the plane. In an auto, it is. In a plane, it is not.
  #110  
Old 12-13-2006, 02:21 PM
dublos dublos is offline
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Originally Posted by Musicat
(If I misjudged your "fan on my back" line, we will have to start over, so that's critical.)
You did not misjudge, i do indeed mean a fan mounted on my back (or more appropriately rigged to some elaborate backpack frame so there's actually air input that's not immediately blocked by my shirt.)

I still feel like I'm missing something here, but it may take someone with a treadmill, roller skates and a fan mounted on a backpack to prove it to me.

I cannot escape from two things that keep nagging at me. 1) I'm rather firmly planted on the ground, really good roller skates or not, and 2) the ground (well, the treadmill) is moving in the opposite direction I am trying to move, even if the air above the ground (treadmill) is not.

None the less, it is starting to make sense to me and overcome those two things nagging at me.
  #111  
Old 12-13-2006, 02:27 PM
ChrisEmerson ChrisEmerson is offline
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Quote:
Originally Posted by RJKUgly
Imagine it like this. The plane has no engine. Way down at the far end of the runway is a winch, with a two-mile long cable that comes all the way back and attaches to the nose of the plane.

The winch starts to pull the plan the plane forward at 5 mph relative to the ground.

At the one mile mark, the plane rolls up onto a treadmill that is running at a constant 5 mph backwards. What happens?

Does the plane stop relative to the ground? Of course not. The winch doesn’t care about the treadmill, it just keeps sucking that cable in at 5 mph relative to the ground. The only way the plane could stop is if the cable pulled the nose off.
Actually, the wheels will now be turning at 10mph, so the treadmill would match this, meaning the wheels are turning at 20mph, and so on. You would get an infinite speed theoretically, which would create quite a lot of friciton!

Quote:
Originally Posted by Musicat
There is little or no friction in the wheel bearings
'Little' is not the same as 'none'. If you ignore the friciton, then you would probably come to the conclusion that the wheel can't stop the plane moving, and you'd be right at normal speeds. At much higher speeds however, this friction is amplified and becomes rather significant.
  #112  
Old 12-13-2006, 02:30 PM
JimOfAllTrades JimOfAllTrades is offline
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Quote:
Originally Posted by dublos
Lets take the plane out of the equation, wings and lift don't actually have anything to do with what you're describing if we're talking about motive power being pushing against air rather than being transmitted through the wheels.

Lets put me on really good roller skates and put a really big fan on my back.

Rev that fan up to the point I'm moving forward.

Same motive force as the plane scenario, right?

Ok... so put me on to a treadmill, start the fan, start the treadmill.

Same thing?
I believe so, yes.

Unless the wheels on the skate have an unusually high resistance to turning, they should basically freewheel at whatever speed is needed to account for the fan pushing you forward relative to the ground, and the treadmill running backward relative to the ground.

So I think you would move forward relative to the ground at about the same speed on or off the treadmill.

Now that word “about” in the above paragraph can get sticky. There is some extra force required to overcome the wheels having to spin more quickly.

But my understanding is that the extra force needed to overcome the extra resistance is a *very* tiny fraction of the whole, especially when you are comparing the wheels spinning at X speed (rolling forward relative to the ground on the normal runway) to spinning at twice X speed (rolling forward relative to the ground on a treadmill going backward relative to the ground), as long as both speeds are within the capabilities of the wheel. Obviously if you spin the wheels so fast they explode or seize up, that changes things radically.

As a check on my sanity, I thought of this: Imagine you have a parasail along with the fan strapped to back and your roller skates. You are skimming along at 5 mph forward relative to ground, at an altitude of 12 inches off the ground, holding your legs up.

You come to a treadmill, which is going 5 mph backward relative to the ground.

Put your feet down so the roller skates contact the treadmill.

Do you come to a stop? I don’t believe so. You might slow down some, especially as the wheels accelerate up to whatever speed is needed. But once the wheel rotation speed is stable, they add almost no resistance to your movement. I think you’d just continue moving forward relative to the ground, just as if the treadmill wasn’t there.
  #113  
Old 12-13-2006, 02:40 PM
JimOfAllTrades JimOfAllTrades is offline
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Originally Posted by ChrisEmerson
Actually, the wheels will now be turning at 10mph, so the treadmill would match this, meaning the wheels are turning at 20mph, and so on. You would get an infinite speed theoretically, which would create quite a lot of friciton!
Actually, no.

Or at least, not always.

This is only true if you define “the speed of the plane” in a specific way, i.e. the speed at which the wheels are turning.

As has been said in many, many, many messages on this subject, the definitions you choose when analyzing this question determine how to solve the problem. And those definitions are not spelled out well in the normal way the question is stated.

In the scenarios I’ve been discussing, you’re comment does not apply, because I’ve been using the “speed of the plane” as “the speed of the body of the plane relative to the ground”, which gives a different outcome than the one you state.
  #114  
Old 12-13-2006, 02:46 PM
ChrisEmerson ChrisEmerson is offline
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Originally Posted by RJKUgly
Actually, no.

Or at least, not always.

This is only true if you define “the speed of the plane” in a specific way, i.e. the speed at which the wheels are turning.

As has been said in many, many, many messages on this subject, the definitions you choose when analyzing this question determine how to solve the problem. And those definitions are not spelled out well in the normal way the question is stated.

In the scenarios I’ve been discussing, you’re comment does not apply, because I’ve been using the “speed of the plane” as “the speed of the body of the plane relative to the ground”, which gives a different outcome than the one you state.
OK, serves me right for not reading the question I guess - I came from another discussion on the same topic where the question had the words 'tracks the speed of the wheels' - I guess you could interpret it that way then. In which case, I agree, the plane would take off. It is a far more interesting problem though when you consider the wheel speed instead...
  #115  
Old 12-13-2006, 03:12 PM
Saltire Saltire is offline
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Quote:
Originally Posted by ChrisEmerson
OK, serves me right for not reading the question I guess - I came from another discussion on the same topic where the question had the words 'tracks the speed of the wheels' - I guess you could interpret it that way then. In which case, I agree, the plane would take off. It is a far more interesting problem though when you consider the wheel speed instead...
Of course, 'tracks the speed of the wheels' is also ambiguous without further explication. If it means 'the speed of the wheel hubs' it is identical to 'the speed of the plane's fusilage.' You seem to mean it as 'the tangential speed of the wheel's point of contact with the runway or treadmill,' which leads to the infinite speed problem. I disagree that that's far more interesting, however.

Look back to the most-excellent post #66 on this thread. zut covers all the various interpretations of the problem in a clear and entertaining way.
  #116  
Old 12-13-2006, 03:41 PM
JimOfAllTrades JimOfAllTrades is offline
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Originally Posted by ChrisEmerson
OK, serves me right for not reading the question I guess - I came from another discussion on the same topic where the question had the words 'tracks the speed of the wheels' - I guess you could interpret it that way then. In which case, I agree, the plane would take off. It is a far more interesting problem though when you consider the wheel speed instead...
Absolutely, yours is a perfectly valid way to look at it, and I agree that it’s a more interesting way. Lots more fun talking about a feedback loop that bumps you up to a huge rotational speed almost instantly!
  #117  
Old 12-13-2006, 05:07 PM
Musicat Musicat is offline
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Originally Posted by dublos
I still feel like I'm missing something here, but it may take someone with a treadmill, roller skates and a fan mounted on a backpack to prove it to me.

I cannot escape from two things that keep nagging at me. 1) I'm rather firmly planted on the ground, really good roller skates or not, and 2) the ground (well, the treadmill) is moving in the opposite direction I am trying to move, even if the air above the ground (treadmill) is not.

None the less, it is starting to make sense to me and overcome those two things nagging at me.
I think you're getting the hang of it. But you might have some difficulty designing an experiment to prove it unless you reduce the problem to simple elements.

There are many things in our existance that don't make much sense at first and seem to violate our human observations, such as "why, if the earth is rotating, don't we fly off?" and the fact that a feather and a bowling ball fall at the same rate on earth in a vacuum. Another might be the innate feeling that a person wearing roller skates is not totally isolated from the ground, and indeed, if that were always true, you couldn't skate across the rink! The reason you can skate is partly due to the selective application of friction. You can turn your skates sideways and push off, for example.

Newton's laws tell us that we should not be able to detect movement in a non-accelerating car, for example. But if you are seated in a moving car, that would be hard to believe, right? Was Newton wrong? No, your senses are detecting other clues, such as sight, vibration, sound and the dashboard speedometer. If you could travel at 100 MPH without those clues, you wouldn't be able to tell that you are moving at all. You'd probably have to go into space to try this out, though.

Also, while your fanonbackpersononskates is an accurate analogy to the planeonarunway in theory, the difference between the two paired forces (fan vs. rollerskate friction, plane engines vs. tire friction) might influence a test. I suspect there might be only a little difference between the force applied by the fan and the resistance of the skates as compared to the diff between those big, powerful engines on a plane and the relatively little rolling resistance. Nevertheless, the principle is the same.

Looking at it another way, if you want to compute the rate of fall of a bowling ball in air, you could probably ignore air resistance, at least for a while. But air resistance would be a huge factor to a feather, and could not be ignored. See what I'm getting at?

When I took physics, we did experiments with dry ice, hocky puck-sized discs that enabled us to remove almost all the friction factors from the experiments. Perhaps having skates made of dry ice would be a good test! Can you envision what would happen to a puck sitting on a smooth-surfaced treadmill, with very litle friction between them when the treadmill started moving? Compare that to a standard hockey puck on a rough treadmill. No friction -- no motion imparted. Much friction -- much motion transferred. Some friction -- some motion transferred.
  #118  
Old 12-13-2006, 05:15 PM
hanguker hanguker is offline
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Hi everyone. As with most of you I've been tossing and turning trying to figure this one out. I've read Cecil's analysis about ten times. Now I'm not a "physics/math type" person but I really want to get this.

My basic premise is that the plane needs air to pass over its wings to generate lift. The engine provide thrust which pushes the plane forward. This part is important. I agree with Cecil that the plane will take off, BUT it will MOVE FORWARD to do so. It will not take off if it remains stationary (and it won't remain stationary because of the thrust). Therefore, you would need a conveyor belt that is as long as a conventional runway for the plane to take off. End result...very worn out wheel bearings and a pointless argument?

Is that right? and if so...what's the point of the discussion? The only cool or amazing thing that kept me reading was the "magical" possibility of a jet taking off from a stationary position.

Thanks and hope I don't come off sounding to dim...
  #119  
Old 12-13-2006, 05:43 PM
John W. Kennedy John W. Kennedy is offline
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Quote:
Originally Posted by hanguker
Is that right? and if so...what's the point of the discussion? The only cool or amazing thing that kept me reading was the "magical" possibility of a jet taking off from a stationary position.
The point (presumeably -- we don't actually know what clown first came up with this) is that a lot of people will get confused and answer that the plane won't move, because it's on a treadmill, which would be true for a car or a bicycle, but isn't true for an airplane, which does not have powered wheels. Unfortunately, the thing (as Cecil received it) is worded in such a way that one possible interpretation allows the plane not to take off (although a strict version of that reading requires that the treadmill be moving at infinite speed, since that is the only speed that satisfies the equation
Code:
treadmillSpeed = treadmillSpeed + airplaneSpeed
where
Code:
airplaneSpeed > 0
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  #120  
Old 12-13-2006, 07:46 PM
rch427 rch427 is offline
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The ONLY kind of aircraft that fly entirely because of their engines are VToL craft, such as helicopters. It doesn't matter how powerful an engine a winged airplane has or how much air it pushes, the fact is that without sufficient airflow around the wing's airfoil, there's no bernoulli effect, and consequently no lift.

Actually, I spoke too soon -- there is one plane that might work: the Custer Channelwing -- but that's because it uses a pair of props located within the wing's airfoil area to move air over lift-producing areas of the wings and generate the lift. The Custer is pretty close to VToL.

Every seen an airboat or swamp buggy? They're essentially a very shallow boat with a big prop on the back, pushing air to move it forward. Well, think of it as an airplane with no wings. No matter how fast that prop pushes, the swamp buggy is never going to fly off the water, and no matter how fast a wing-less airplane's engine pushes the air, it ain't gonna fly.

In the same respect, you put a glider in a wind tunnel and, once the airflow reaches a sufficient speed, it will "fly", despite not having an engine -- all it takes is sufficient "wind" flowing around the wings. Where does that wind come from in your treadmill experiment? Unless the air being pushed by the prop or jet engine is somehow ducted around to go in front of the wings, there will be no wind.

So, even if the hypothetical airplane's engine is pushing it at a wheel-speed of 500mph, and even if there's no friction (or the conveyor belt moves at whatever speed necessary to allow the plane to stay in place), without wings and wind coming from ahead of the aircraft and going around those wings, there will still only be negligible lift. No lift, no take-off.

It will NOT fly.
  #121  
Old 12-13-2006, 08:14 PM
John W. Kennedy John W. Kennedy is offline
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Quote:
Originally Posted by rch427
So, even if the hypothetical airplane's engine is pushing it at a wheel-speed of 500mph, and even if there's no friction (or the conveyor belt moves at whatever speed necessary to allow the plane to stay in place), without wings and wind coming from ahead of the aircraft and going around those wings, there will still only be negligible lift. No lift, no take-off.

It will NOT fly.
You still don't understand. The plane will move forward unless the treadmill is moving much, much, much faster than the takeoff speed if there is realistic friction, and will move forward no matter how fast the treadmill is going if you neglect the friction in the wheel axles.
  #122  
Old 12-13-2006, 08:35 PM
zut zut is offline
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Quote:
Originally Posted by dublos
I cannot escape from two things that keep nagging at me. 1) I'm rather firmly planted on the ground, really good roller skates or not, and 2) the ground (well, the treadmill) is moving in the opposite direction I am trying to move, even if the air above the ground (treadmill) is not.

None the less, it is starting to make sense to me and overcome those two things nagging at me.
I'm pretty much going to echo Musicat here. The problem is that, intuitively, it seems that moving at higher velocity requires higher force. It's intuitive because some losses (air resistance, for example) increase with speed, and because activities like running or skating just take more effort at higher speed.

However, force isn't really related to velocity, it's related to acceleration.
This particular problem (the plane on a treadmill, or the fan-guy on rollerskates) strips away some of the losses and the answer isn't intuitive any more. (And it doesn't help that there's *probably* some velocity dependence in the wheel friction, so there's a *little* truth to the intuition, only not so much as you might think...)

Quote:
Originally Posted by hanguker
My basic premise is that the plane needs air to pass over its wings to generate lift. The engine provide thrust which pushes the plane forward. This part is important. I agree with Cecil that the plane will take off, BUT it will MOVE FORWARD to do so. It will not take off if it remains stationary (and it won't remain stationary because of the thrust). Therefore, you would need a conveyor belt that is as long as a conventional runway for the plane to take off. End result...very worn out wheel bearings and a pointless argument?

Is that right? and if so...what's the point of the discussion? The only cool or amazing thing that kept me reading was the "magical" possibility of a jet taking off from a stationary position.
Sure, that's right. Or, rather, it could be right. The issue is that different folks make different assumptions about the original problem, and different assumptions lead to different conclusions. As long as you're clear about the wording of the problem (which varies!) and your assumptions, and proceed to a logical conclusion, you're right. See, for example, this post in this very thread for a rundown of different scenarios.

Quote:
Originally Posted by rch427
So, even if the hypothetical airplane's engine is pushing it at a wheel-speed of 500mph, and even if there's no friction (or the conveyor belt moves at whatever speed necessary to allow the plane to stay in place), without wings and wind coming from ahead of the aircraft and going around those wings, there will still only be negligible lift. No lift, no take-off.

It will NOT fly.
Sure. Or rather, "sure", if you're making the right assumptions up front. Everyone agrees that if the plane stays stationary with respect to the air, it will not take off. Most of the argument revolves around whether or not the plane really will be stationary. And the answer to that depends on the wording of the problem and the assumptions you make. See, for example, this post in this very thread for a rundown of different scenarios.
Quote:
Originally Posted by John W. Kennedy
You still don't understand. The plane will move forward unless the treadmill is moving much, much, much faster than the takeoff speed if there is realistic friction, and will move forward no matter how fast the treadmill is going if you neglect the friction in the wheel axles.
And inertia of the wheels, don't forget. If you neglect friction and inertia (which is not unreasonable, if you're gonna talk about ultra-fast treadmills), the plane will move forward no matter how fast the treadmill is going. With inertia, force coupling between the treadmill and plane is still possible.
  #123  
Old 12-13-2006, 10:24 PM
Xema Xema is offline
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Quote:
Originally Posted by zut
See, for example, this post in this very thread for a rundown of different scenarios.
We may be nearing the point where your rundown needs to become a "sticky," and is required reading for anyone who wishes to post.


Quote:
Everyone agrees that if the plane stays stationary with respect to the air, it will not take off.
This deserves a lot of emphasis. Many in the "does not take off" camp seem to feel that those in the other camp somehow believe a plane can fly without airspeed. Yet there's thus far no evidence that anyone actually holds that belief.
  #124  
Old 12-13-2006, 10:48 PM
Chronos Chronos is offline
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Quoth hanguker:
Quote:
My basic premise is that the plane needs air to pass over its wings to generate lift. The engine provide thrust which pushes the plane forward. This part is important. I agree with Cecil that the plane will take off, BUT it will MOVE FORWARD to do so. It will not take off if it remains stationary (and it won't remain stationary because of the thrust). Therefore, you would need a conveyor belt that is as long as a conventional runway for the plane to take off. End result...very worn out wheel bearings and a pointless argument?
Yes, exactly. The plane will move forward (relative to the air, the ground, and everything else other than the treadmill), and will lift off at just the same distance as it would on a runway. The difficulty is that many folks don't seem to grasp that, for some reason. Many people seem to think that the treadmill will stop the airplane from moving, even though there's no practical way for it to do so.
  #125  
Old 12-13-2006, 11:31 PM
thebristolkid thebristolkid is offline
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Physicists forgive me, I am but a simple designer. A few points:

1. This is 85% thought experiment, 15% physics question. The assumption here, as realistically implausible as it seems, is that the treadmill perfectly counteracts the thrust of the engines, and the plane has zero lateral movement relative to the Earth, and more importantly, the surrounding air.

2. I've read through this entire thread, plus the comments on Boing Boing, and only one person mentioned Bernoulli. If the plane's wings, relative to the surrounding air, has no movement in the appropriate direction, there is no airspeed, there is no lift generated, and there is no takeoff. End of story. Everyone wants to take about friction and groundspeed and thrust. When do we discuss wings and lift, which are, you know, the only reasons most planes are capable of taking off and maintaining flight (except for, of course VTOL aircraft)?

3. A better question would be this: put a stationary aircraft in a wind tunnel capable of generating infinite airspeed...does the plane take off?
  #126  
Old 12-14-2006, 12:12 AM
hanguker hanguker is offline
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In light of everything, I think a lot of people (including myself) would be satisfied if the question simple stated "...would the plane would MOVE FORWARD, generate lift, and take off?" Then it would be a resounding yes because of the thrust generated by the engine. However...

I think many, MANY people are attracted to this question because it seemingly purports that the treadmill will generate lift and the plane will rise from a stationary position once the treadmill/wheels reach take-off velocity...which is BS.

I'm kind of disappointed with the amount of time I put into reading and considering this flawed question now...who's with me?
  #127  
Old 12-14-2006, 12:18 AM
Princhester Princhester is offline
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Fer crying out loud. As others have said, I really think this thread (indeed every thread on this subject) needs to utilize the little known feature of vBulletin by which you can set it so you can only post if you've visited a special page which requires you to tick a box saying "I have read Zut's post #66 and still wish to post."

thebristolkid nobody is suggesting that the plane will take off without air movement over the wings. The argument is about whether the plane will move so as to create that air movement.

Quote:
3. A better question would be this: put a stationary aircraft in a wind tunnel capable of generating infinite airspeed...does the plane take off?
Better in the sense of "more obvious, less fun" perhaps.
  #128  
Old 12-14-2006, 06:31 AM
zut zut is offline
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Quote:
Originally Posted by Chronos
Many people seem to think that the treadmill will stop the airplane from moving, even though there's no practical way for it to do so.
Thought experiment, Chronos. There's nothing wrong with saying, "assuming the treadmill can achieve any velocity required, then what happens when..." No more than saying, "assuming we could build a spaceship that could approach the speed of light, then what happens when..."

In fact, if your measuring stick is practicality, you could test this airplane-on-a-treadmill thing relatively easily (relative compared to constructing a forever-accelerating spaceship) by tinkering with the airplane. Depending on what you wanted to show, you could construct a small-scale plane with enormous tires, and/or high friction wheel hubs, and/or underpowered engines, and/or substantial lift and/or whatever.
Quote:
Originally Posted by thebristolkid
If the plane's wings, relative to the surrounding air, has no movement in the appropriate direction, there is no airspeed, there is no lift generated, and there is no takeoff. End of story. Everyone wants to take about friction and groundspeed and thrust. When do we discuss wings and lift, which are, you know, the only reasons most planes are capable of taking off and maintaining flight (except for, of course VTOL aircraft)?
No one's discussing wings and lift because everyone knows airspeed is required for lift. People are talking about friction and groundspeed and thrust because the question boils down to: will the plane move forward? And the answer to that depends on the wording of the problem and the assumptions you make. See, for example, this post in this very thread for a rundown of different scenarios.
  #129  
Old 12-14-2006, 10:20 AM
yoyodyne yoyodyne is offline
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Quote:
Originally Posted by Musicat
The answer is yes, the wheels will start rolling (yet) the plane will remain stationary relative to the ground. There is little or no friction in the wheel bearings, and a treadmill cannot exert force on any other part of the plane but the wheels.

If you don't understand this, you will not be able to continue with the thought process in this question. The motion of the wheels of the plane do not inhibit its forward movement nor add to it; they are not a significant factor at all.
You are missing a factor here. Even if there is no friction in the wheel bearings, if the treadmill starts moving the plane will move with it, not just sit there stationary. The wheels have inertia. Getting them to spin requires adding rotational momentum from the treadmill, and this will couple with the axle to generate force on the plane in the direction of the treadmill motion.
  #130  
Old 12-14-2006, 10:59 AM
JimOfAllTrades JimOfAllTrades is offline
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Quote:
Originally Posted by yoyodyne
You are missing a factor here. Even if there is no friction in the wheel bearings, if the treadmill starts moving the plane will move with it, not just sit there stationary. The wheels have inertia. Getting them to spin requires adding rotational momentum from the treadmill, and this will couple with the axle to generate force on the plane in the direction of the treadmill motion.
Yeah, but I think the general consensus is that the force required to overcome the inertia of the wheels is very small compared to what the planes engines can generate.

Unless you accelerate the rotation speed of the wheels up to fantastic speeds in a very short amount of time, percentage wise there just isn’t enough extra inertia to matter.
  #131  
Old 12-14-2006, 11:23 AM
yoyodyne yoyodyne is offline
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Quote:
Originally Posted by RJKUgly
Yeah, but I think the general consensus is that the force required to overcome the inertia of the wheels is very small compared to what the planes engines can generate.

Unless you accelerate the rotation speed of the wheels up to fantastic speeds in a very short amount of time, percentage wise there just isn’t enough extra inertia to matter.
I was refuting Musicat's claim that if the plane's engines were off and the treadmill started slowly ramping up to 1 MPH, the plane would sit stationary with respect to the ground with the wheels turning.
  #132  
Old 12-14-2006, 11:42 AM
Zakalwe Zakalwe is offline
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Quote:
Originally Posted by Princhester
Fer crying out loud. As others have said, I really think this thread (indeed every thread on this subject) needs to utilize the little known feature of vBulletin by which you can set it so you can only post if you've visited a special page which requires you to tick a box saying "I have read Zut's post #66 and still wish to post."
Dude, that would be *awesome*.
  #133  
Old 12-14-2006, 12:13 PM
Contrapuntal Contrapuntal is offline
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To anyone's knowledge, has a Doper ever posted a concise summary of all the different assumptions inherent in the treadmill question as stated, as well as a review of said assumptions' necessary ramifications? Maybe someone who seems to have a grasp of things, oh, I don't know, say Zut for example, could do so in this very thread, and save us all from the relentless rehashings.
  #134  
Old 12-14-2006, 12:32 PM
yoyodyne yoyodyne is offline
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Quote:
Originally Posted by Contrapuntal
To anyone's knowledge, has a Doper ever posted a concise summary of all the different assumptions inherent in the treadmill question as stated, as well as a review of said assumptions' necessary ramifications? Maybe someone who seems to have a grasp of things, oh, I don't know, say Zut for example, could do so in this very thread, and save us all from the relentless rehashings.
Such a thing may not be possible. It would truly require a dizzying intellect.
  #135  
Old 12-14-2006, 12:37 PM
Contrapuntal Contrapuntal is offline
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Quote:
Originally Posted by yoyodyne
Such a thing may not be possible. It would truly require a dizzying intellect.
True dat. Nobody ever went broke underestimating the intelligence of the American public. Or something.

Besides, if Zut's so smart, why ain't he rich?
  #136  
Old 12-14-2006, 03:31 PM
DSYoungEsq DSYoungEsq is offline
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Quote:
Originally Posted by rch427
The ONLY kind of aircraft that fly entirely because of their engines are VToL craft, such as helicopters. It doesn't matter how powerful an engine a winged airplane has or how much air it pushes, the fact is that without sufficient airflow around the wing's airfoil, there's no bernoulli effect, and consequently no lift.

Actually, I spoke too soon -- there is one plane that might work: the Custer Channelwing -- but that's because it uses a pair of props located within the wing's airfoil area to move air over lift-producing areas of the wings and generate the lift. The Custer is pretty close to VToL.

Every seen an airboat or swamp buggy? They're essentially a very shallow boat with a big prop on the back, pushing air to move it forward. Well, think of it as an airplane with no wings. No matter how fast that prop pushes, the swamp buggy is never going to fly off the water, and no matter how fast a wing-less airplane's engine pushes the air, it ain't gonna fly.

In the same respect, you put a glider in a wind tunnel and, once the airflow reaches a sufficient speed, it will "fly", despite not having an engine -- all it takes is sufficient "wind" flowing around the wings. Where does that wind come from in your treadmill experiment? Unless the air being pushed by the prop or jet engine is somehow ducted around to go in front of the wings, there will be no wind.

So, even if the hypothetical airplane's engine is pushing it at a wheel-speed of 500mph, and even if there's no friction (or the conveyor belt moves at whatever speed necessary to allow the plane to stay in place), without wings and wind coming from ahead of the aircraft and going around those wings, there will still only be negligible lift. No lift, no take-off.

It will NOT fly.
The wind flow comes from the thrust of the engine acting upon the plane. The treadmill is totally irrelevant, except under one highly improbable condition that leads to the paradox of infinite treadmill speed.

When will people understand that the wheels are irrelevant to the concept of a plane moving? (not directed at the VERY limited number of people here who truly comprehend what happens if the wheels are forced to go super super fast...)
  #137  
Old 12-15-2006, 03:59 AM
Hairyman Hairyman is offline
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I'm beginning to come to the conclusion that the premise of the plane, its wheels and the treadmill is a flawed interaction.

Let us add the fairly non-controversial rider that the treadmill with the plane on it is sitting on the ground. We shall have as our reference for motion the ground. That is we shall discuss the motion of all other things with reference to the ground which at the beginning of the experiment is not moving relative to any other object in the scenario and does not itself accelerate or decelerate during the experiment.

The wheels of the plane are not driven, that is, they passively rotate as the plane taxis forward or backward.

The treadmill only moves to match the movement of wheels.

The wheels shall not turn if the plane does not advance with reference to the ground given that everything is at rest to begin with.

If the plane is to advance it must also do so relative to the treadmill, but we are told that the belt treadmill will move in an equal and opposite fashion to the wheels.

Those arguing that the plane will remain motionless relative to the ground must also argue that the plane never moves in the first place.

However, if the plane has not moved, then the wheels will not have moved, so the treadmill will not move.

Herein lies the problem of the interaction. The argument immediately becomes circular (as we have seen within the thread), not withstanding some excellent knowledge of Newtonian physics by several posters, because of the disconnect between the cause of the motion of the wheels and the wheel's motion.

We have struck a failure of logic.

You can re-run the whole thing with the wheels and the treadmill turning at some constant rate and the logic will still fail.
  #138  
Old 12-15-2006, 07:32 AM
zut zut is offline
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Quote:
Originally Posted by Hairyman
Those arguing that the plane will remain motionless relative to the ground must also argue that the plane never moves in the first place.

However, if the plane has not moved, then the wheels will not have moved, so the treadmill will not move.

Herein lies the problem of the interaction. The argument immediately becomes circular (as we have seen within the thread), not withstanding some excellent knowledge of Newtonian physics by several posters, because of the disconnect between the cause of the motion of the wheels and the wheel's motion.

We have struck a failure of logic.
I think that's reading a bit too much into the problem. There's no requirement that the treadmill match the wheel speed with zero error.

In a real engineering setting, you'd build a feedback circuit that would control the treadmill with some small dither. "Small enough" dither will give a good enough result. If you wanted less dither, you'd build a better feedback circuit and/or improve the smoothness of the input signal (wheel velocity in this case). And if we're postulating mile-long, high-powered, heat-resistant, infinitely-accelerating treadmills anyway, I don't see a problem with assuming a control circuit that matches speed to within whatever error band you desire.

These kinds of problems don't depend on zero-error physical parameters. For instance, suppose someone asked, "what happens when two cars of the same mass, going the same speed, crash head-on?" The answer of "the question exhibits failed logic, because two cars can never go exactly the same speed" is less than enlightening.

In addition, if you're actually worried about the treadmill perfectly matching the wheels, remember that the tires are made out of rubber. Rubber flexes. The tire distorts where it contacts the ground, and it shears under torque loading, and it slips and skids a bit at the *edges* of the contact patch, and the whole thing's rather messy if you're trying to exactly match the speed of the tire.
  #139  
Old 12-15-2006, 09:46 AM
Musicat Musicat is offline
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Quote:
Originally Posted by yoyodyne
Quote:
Originally Posted by Musicat
The answer is yes, the wheels will start rolling (yet) the plane will remain stationary relative to the ground. There is little or no friction in the wheel bearings, and a treadmill cannot exert force on any other part of the plane but the wheels.

If you don't understand this, you will not be able to continue with the thought process in this question. The motion of the wheels of the plane do not inhibit its forward movement nor add to it; they are not a significant factor at all.
You are missing a factor here. Even if there is no friction in the wheel bearings, if the treadmill starts moving the plane will move with it, not just sit there stationary. The wheels have inertia. Getting them to spin requires adding rotational momentum from the treadmill, and this will couple with the axle to generate force on the plane in the direction of the treadmill motion.
I will amend my statement to "...There is little friction in the wheel bearings..." (removing the "none"), as unless we have a frictionless bearing, there will always be some friction.

But not a lot. Absent the bearing friction, if the treadmill starts moving, the plane will not move with it. With friction, the plane will be slightly affected. We seem to agree that the bearing friction is very little compared to other forces at work in the overall problem.

To reiterate,
Quote:
Originally Posted by yoyodyne
Even if there is no friction in the wheel bearings, if the treadmill starts moving the plane will move with it, not just sit there stationary.
No, it won't. Why would it? What exactly is "couple with the axle"?
  #140  
Old 12-15-2006, 11:45 AM
zut zut is offline
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Quote:
Originally Posted by Musicat
But not a lot. Absent the bearing friction, if the treadmill starts moving, the plane will not move with it. With friction, the plane will be slightly affected. We seem to agree that the bearing friction is very little compared to other forces at work in the overall problem.

To reiterate,No, it won't. Why would it? What exactly is "couple with the axle"?
Yes it will. Even absent bearing friction, the wheels still have mass. If the treadmill changes speed (i.e., accelerates), and the tires don't slip, the wheels must also accelerate. The acceleration requires force (F=ma, right?), and the force is tranferred to the fuselage of the plane, which must also move. This is what Cecil calls "BR#2" in his second column on the subject.

He also give a thumbnail explanation, which has the advantage of being short and the disadvantage of glossing over some of the whys. I give a more substantial explanation in this post in an older thread. There's also a long, meandering argument about about how this works that takes up the last four pages or so of the same thread, if you have the stomach for it.
  #141  
Old 12-15-2006, 08:54 PM
EWC@BC EWC@BC is offline
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If you accept the version of the problem that, “the conveyor belt is (somehow) designed to exactly match the speed of the wheels,” the problem seems to be more a matter of logic than physics. The plane can’t “roll” forward faster than the conveyor belt if for no other reason than the problem doesn’t allow it. Therefore, the rules of the problem prevent the wheels from being used to reduce the friction of the plane against the conveyor until it takes off. The plane could however skid down the runway (until it takes off). This doesn’t violate the rules of the problem because the wheels aren’t rolling nor is the conveyor belt moving. To the extent the wheels do roll, the plane still has to complete its takeoff using a full skid (and the pilot would seem wise to apply the brakes in order to shut off the conveyor belt!). The question then is can the plane take off, in effect, with the wheels locked. The answer is, “I don’t know because the problem doesn’t specify anything about the plane (and it’s engines).” But I would guess that most jet airliners couldn’t.
  #142  
Old 12-15-2006, 10:16 PM
Xema Xema is offline
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Quote:
Originally Posted by EWC@BC
If you accept the version of the problem that, “the conveyor belt is (somehow) designed to exactly match the speed of the wheels,” the problem seems to be more a matter of logic than physics. The plane can’t “roll” forward faster than the conveyor belt if for no other reason than the problem doesn’t allow it.
Again, it depends on what interpretation you put on "speed of the wheels".

If (as many have) you choose to interpret this as the speed of the wheel hub (which is of course the same as the speed of the airplane - fuselage, wings etc.) then the problem does allow the plane to roll forward. It could, for example, be headed north at 80 knots while the treadmill trundles south at 80 knots and the wheels spin as they would at 160 knots on normal pavement.
  #143  
Old 12-16-2006, 07:16 AM
Sapo Sapo is offline
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did I just wake up in yesterday again?
  #144  
Old 12-16-2006, 11:11 AM
Musicat Musicat is offline
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Quote:
Originally Posted by zut
Yes it will. Even absent bearing friction, the wheels still have mass. If the treadmill changes speed (i.e., accelerates), and the tires don't slip, the wheels must also accelerate. The acceleration requires force (F=ma, right?), and the force is tranferred to the fuselage of the plane, which must also move. This is what Cecil calls "BR#2" in his second column on the subject.
Don't you think the horizontal force (treadmill->wheel) will be converted into rotational force? With no wheel bearing friction, that would be a pretty good conversion. I confess I don't know how to calculate how much this would be.

If the conversion efficiency is 100%, the plane won't move. If less, it would, at least initially and slightly.
  #145  
Old 12-16-2006, 11:39 AM
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ok, I just had a thought about this. Please excuse me if it has been brought up already.

I read the famous post with all the scenarios and understood it and found it excellent. I am just not sure how this idea pertains to those scenarios.

The problem states that the belt matches the speed of the wheels. The problem arises when we assume it to mean the speedometer speed of the wheel.

Now, in its most basic form, all this statements means is that there is no slippage between the rubber of the wheels and the tarmac.

The treadmill could be spinning at any speed and in any direction and the statement holds up and the plane just remains stationary. But then it is not the belt matching the speed of the wheels but the other way around, the wheels are matching the speed of the belt. Does this difference mean anything?

If the belt senses and matches the speed of the wheels, the only way the wheel can have any (rotational) speed (considering there is no engine transmission) is by moving forward. But we already established that cannot happen as the belt will not allow it. What is happening then to the thrust of the engine?

Please be patient with me and explain this to me without pointing out that it has been covered a thousand times. I really have no agenda here for either solution. Just trying to figure this out.
  #146  
Old 12-16-2006, 12:18 PM
EWC@BC EWC@BC is offline
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Quote:
Originally Posted by Xema
Again, it depends on what interpretation you put on "speed of the wheels".
Sorry I should have been more precise. Having read the log, I thought your interpretation of the problem was solved long ago. I was OBVIOUSLY refering to "the outer diameter surface velocity" which I thought ZUT and the rest of you all agreed was "the more interesting problem." I just thought all the analysis about hub wheel inertia and such, while brilliant, overlooked what the problem is really all about, which is to see whether you can discover what the real issue is (i.e. whether a jet has enough power to skid down the run way and take off with it's wheels locked).

The "insteresting" part of this problem is not really about physics. Who really cares whether a passanger jet can or can't so skid? Someone can just look that up in a book.
  #147  
Old 12-16-2006, 12:20 PM
zut zut is offline
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Quote:
Originally Posted by Musicat
Don't you think the horizontal force (treadmill->wheel) will be converted into rotational force? With no wheel bearing friction, that would be a pretty good conversion. I confess I don't know how to calculate how much this would be.
There's no "conversion" of force in the sense that you're thinking. There's no requirement that a certain proportion of a force be allocated to translational motion and another proportion be allocated to rotational motion. A single force can cause both translational and rotational motion, depending on how it's aligned with the center of mass. (As a side note, this is apparently a relatively difficult concept to grasp. At least one other regular poster has been militantly and snidely pro-ignorance on the subject, comparing the acceleration to a perpetual motion machine. Not everyone likes to pay attention, it seems.)

Anyway, let's look at the wheel. There's a couple ways of looking at this. First, look at the wheel alone, unattached from the airplane. If the treadmill moves beneath it, and there is no slip, there will be a force between the treadmill and wheel, right? That force will accelerate the wheel translationally, because the force is unopposed, and F = ma. The force also creates a torque around the wheel center of mass, so the force will also accelerate the wheel rotationally (because T = I a).

There's a more involved explanation in this post. Also, you can prove it to yourself experimentally. Take a small roll of tape (or something else small and round) and sit it on a piece of paper. If you pull the paper very slowly, the tape will just sit on the paper because the surfaces will compress slightly. If you pull the paper very quickly, the paper will slip out from underneath the tape because the coefficient of friction is too low. But if you pull it at medium speed, the tape will move forwards and roll backwards at the same time. Try it.

Now, when you attach the wheel back to the plane, clearly it can't translate without moving the plane, also. If you assume the plane stays in the same place (i.e., there is no translation), then the force between the treadmill and wheel must be transmitted to the plane--otherwise, there would be an unbalanced force, and the wheel would accelerate in translation, because F = ma.

So that force--the one between the treadmill and the wheel--is transmitted to the plane, and if the force is large enough it will cancel out the engine thrust.
  #148  
Old 12-16-2006, 12:46 PM
EWC@BC EWC@BC is offline
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ZUT, you are a giant... But you are also the devil! The problem is designed to see who does or doesn't get diverted down the very tangent of this log. And you are clearly being paid to facilitate that! You solved the physics behind this problem long ago, now you are just giving free physics lesson (which is the reason I came here and read the log in the first place). Thank you. But I can’t help but wonder if it’s not time for you to crack the next problem.
  #149  
Old 12-16-2006, 01:07 PM
Musicat Musicat is offline
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Zut, I appreciate your physics lesson, I really do, and I'm aware of the action in your experimental tape roll, but the inertial values there are quite a bit different.
Quote:
Originally Posted by zut
Now, when you attach the wheel back to the plane, clearly it can't translate without moving the plane, also. If you assume the plane stays in the same place (i.e., there is no translation), then the force between the treadmill and wheel must be transmitted to the plane--otherwise, there would be an unbalanced force, and the wheel would accelerate in translation, because F = ma.
If the plane were locked to the ground, all force initiated by the treadmill would have to be translated into rotary motion (or heat, I guess). Compared to the inertia of a wheel, the much greater mass of the plane would have an effect similar to the locked-down scenario, and the plane wouldn't move a wholeheckuva lot.
Quote:
So that force--the one between the treadmill and the wheel--is transmitted to the plane, and if the force is large enough it will cancel out the engine thrust.
Aye, but again, the relative value of engine thrust is large compared to the friction/wheel/inertial forces, unless you are proposing the treadmill accelerate to something like a million miles an hour in one second. I guess it is necessary to define reasonable ranges to really answer this thought experiment.
  #150  
Old 12-16-2006, 04:08 PM
zut zut is offline
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Quote:
Originally Posted by Sapo
I read the famous post with all the scenarios and understood it and found it excellent.
Heh. I'm pretty sure Princhester wasn't serious. Still, thanks for the compliment.

Quote:
Originally Posted by Sapo
If the belt senses and matches the speed of the wheels, the only way the wheel can have any (rotational) speed (considering there is no engine transmission) is by moving forward. But we already established that cannot happen as the belt will not allow it. What is happening then to the thrust of the engine?

Please be patient with me and explain this to me without pointing out that it has been covered a thousand times. I really have no agenda here for either solution. Just trying to figure this out.
If i'm understanding you correctly, you're thinking about this similar to the way Hairyman was a few days ago: for the treadmill to respond, the wheels must start moving. FOr the wheels to start moving, the plane must lurch forward. For the plate to lurch forward, the treadmill must not be matching the speed...and that violates the problem. Yes?

I think, in this case, you're reading the problem too strictly. In a real engineering setting, if you wanted to experimentally test this scenario, you'd build a feedback circuit that would control the treadmill with some small dither. In other words, the speeds would be matched to within some small amount. What's a "small amount"? I dunno. You tell me. How close do you need to match speeds to be "close enough"?

Also, remember that the tires are made out of rubber. Rubber flexes. The tire distorts where it contacts the ground, and it shears under torque loading, and it slips and skids a bit at the *edges* of the contact patch. That means that the meaning of "the speed of the wheels" isn't exactly defined, particularly when the engines just begin to light off.

Quote:
Originally Posted by EWC@BC
ZUT, you are a giant... But you are also the devil! The problem is designed to see who does or doesn't get diverted down the very tangent of this log. And you are clearly being paid to facilitate that! You solved the physics behind this problem long ago, now you are just giving free physics lesson (which is the reason I came here and read the log in the first place). Thank you. But I can’t help but wonder if it’s not time for you to crack the next problem.
Actually, this problem has been surprisingly educational for me. What happens when you explore every single possible aspect of a thought experiment? In detail? In the...half dozen..extensive threads on this problem, a number of people have pointed out things I didn't know.
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