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.
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.
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:
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:[ul][]Stating the problem unambiguously, then []Applying knowledge of simple physics.[/ul]
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.
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.
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?
Good. Then you aren’t the party to whom I addressed that comment and we have no disagreement.
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,
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.
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?
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.)
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.
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.
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!
‘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.
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.
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…
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.
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!
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.
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…
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
treadmillSpeed = treadmillSpeed + airplaneSpeed
where
airplaneSpeed > 0
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.