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I honestly don't know why this problem causes such confusion. To me it's very simple.
-Lift is provided ONLY by air moving past the wings. It doesn't matter if the plane is moving through still air, or if air is blowing by.
-Engines ONLY provide lateral thrust. It doesn't matter how much force they provide, if there is no relative motion through the air, there is no lift.
Engines DO NOT make air flow around the wings. Modern aircraft engines are turbofans, meaning they have a combustion chamber, and a fan that pushes air through it. The fan moves air only to improve the efficiency of the fuel.
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United States24751 Posts
On February 21 2009 14:36 JeeJee wrote:Show nested quote +On February 21 2009 14:26 micronesia wrote:On February 21 2009 14:06 JeeJee wrote:
so if the engines make the airplane move relative to the air, not the ground (i'm having trouble with this sentence but i'll accept it as true for now), then why will the airplane fall during that clamp example earlier in the thread? as the engines are running at full capacity, the lift should be there, no? If the plane is clamped in place... the engines are still pulling air through them, but there is no air actually passing over/under the wings. The plane is stationary relative to the air. okay then i'm back to my original query. since the air and ground are stationary relative to each other, for the wings to be moving relative to air it must be moving relative to the ground as well, no? since the treadmill forbids that, how can there be lift? it seems that there is assumption that even with the treadmill, the plane is still moving forward relative to the ground. i thought the purpose of the treadmill was specifically to stop this from occurring? else, what the heck is the treadmill doing
A treadmill wouldn't be able to prevent a plane from moving relative to the ground unless you imposed very special conditions which were not mentioned in the OP.
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On February 21 2009 14:41 micronesia wrote:Show nested quote +On February 21 2009 14:36 JeeJee wrote:On February 21 2009 14:26 micronesia wrote:On February 21 2009 14:06 JeeJee wrote:
so if the engines make the airplane move relative to the air, not the ground (i'm having trouble with this sentence but i'll accept it as true for now), then why will the airplane fall during that clamp example earlier in the thread? as the engines are running at full capacity, the lift should be there, no? If the plane is clamped in place... the engines are still pulling air through them, but there is no air actually passing over/under the wings. The plane is stationary relative to the air. okay then i'm back to my original query. since the air and ground are stationary relative to each other, for the wings to be moving relative to air it must be moving relative to the ground as well, no? since the treadmill forbids that, how can there be lift? it seems that there is assumption that even with the treadmill, the plane is still moving forward relative to the ground. i thought the purpose of the treadmill was specifically to stop this from occurring? else, what the heck is the treadmill doing A treadmill wouldn't be able to prevent a plane from moving relative to the ground unless you imposed very special conditions which were not mentioned in the OP.
Here's a question:
Do the engines of an airplane cause forward movement, therefore moving the airplane relative to the air and in doing so, allowing an airflow to lift the plane?
However, if the treadmill is moving in the opposite direction then the airplane should not be able to move relative to the air. Problem is, you said that the engines DO allow the airplane to move relative to the air, even with a treadmill. How does that work? Doesn't the treadmill's movement bring the airplane back to where it was?
The question to answer here is: Does the airplane move relative to the earth/air? If so, how? If not, why not?
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On February 21 2009 14:51 BanZu wrote:Show nested quote +On February 21 2009 14:41 micronesia wrote:On February 21 2009 14:36 JeeJee wrote:On February 21 2009 14:26 micronesia wrote:On February 21 2009 14:06 JeeJee wrote:
so if the engines make the airplane move relative to the air, not the ground (i'm having trouble with this sentence but i'll accept it as true for now), then why will the airplane fall during that clamp example earlier in the thread? as the engines are running at full capacity, the lift should be there, no? If the plane is clamped in place... the engines are still pulling air through them, but there is no air actually passing over/under the wings. The plane is stationary relative to the air. okay then i'm back to my original query. since the air and ground are stationary relative to each other, for the wings to be moving relative to air it must be moving relative to the ground as well, no? since the treadmill forbids that, how can there be lift? it seems that there is assumption that even with the treadmill, the plane is still moving forward relative to the ground. i thought the purpose of the treadmill was specifically to stop this from occurring? else, what the heck is the treadmill doing A treadmill wouldn't be able to prevent a plane from moving relative to the ground unless you imposed very special conditions which were not mentioned in the OP. Here's a question: Do the engines of an airplane cause forward movement, therefore moving the airplane relative to the air and in doing so, allowing an airflow to lift the plane? However, if the treadmill is moving in the opposite direction then the airplane should not be able to move relative to the air. Problem is, you said that the engines DO allow the airplane to move relative to the air, even with a treadmill. How does that work? Doesn't the treadmill's movement bring the airplane back to where it was? The question to answer here is: Does the airplane move relative to the earth/air? If so, how? If not, why not?
Of course not, does a human on a treadmill move relative to the earth?
The invention of the treadmill is specifically to keep someone in place while running, otherwise you couldn't exactly watch a stationary TV while on a treadmill.
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On February 21 2009 14:59 inReacH wrote:Show nested quote +On February 21 2009 14:51 BanZu wrote:On February 21 2009 14:41 micronesia wrote:On February 21 2009 14:36 JeeJee wrote:On February 21 2009 14:26 micronesia wrote:On February 21 2009 14:06 JeeJee wrote:
so if the engines make the airplane move relative to the air, not the ground (i'm having trouble with this sentence but i'll accept it as true for now), then why will the airplane fall during that clamp example earlier in the thread? as the engines are running at full capacity, the lift should be there, no? If the plane is clamped in place... the engines are still pulling air through them, but there is no air actually passing over/under the wings. The plane is stationary relative to the air. okay then i'm back to my original query. since the air and ground are stationary relative to each other, for the wings to be moving relative to air it must be moving relative to the ground as well, no? since the treadmill forbids that, how can there be lift? it seems that there is assumption that even with the treadmill, the plane is still moving forward relative to the ground. i thought the purpose of the treadmill was specifically to stop this from occurring? else, what the heck is the treadmill doing A treadmill wouldn't be able to prevent a plane from moving relative to the ground unless you imposed very special conditions which were not mentioned in the OP. Here's a question: Do the engines of an airplane cause forward movement, therefore moving the airplane relative to the air and in doing so, allowing an airflow to lift the plane? However, if the treadmill is moving in the opposite direction then the airplane should not be able to move relative to the air. Problem is, you said that the engines DO allow the airplane to move relative to the air, even with a treadmill. How does that work? Doesn't the treadmill's movement bring the airplane back to where it was? The question to answer here is: Does the airplane move relative to the earth/air? If so, how? If not, why not? Of course not, does a human on a treadmill move relative to the earth?
That's exactly what I thought, but Micronesia said otherwise.
Someone's contribution:
"The speed doesnt matter. There is no lift, as there is no difference in airflow speed over the top and bottom of the wings since there shouldnt be any significant airflow at all.
An airplane wouldnt take off."
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hahaha i get it now
i finally understood it by reading this in detail: http://www.boingboing.net/2006/12/11/airplanetreadmill-pr.html
actually it's pretty funny
and yes it will take off, thanks micronesia :-)
edit: my main problem was assuming that the treadmill will make the airplane stationary relative to the ground
=(
edit2: although now that i think about it, i blame the question.
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On February 21 2009 15:04 JeeJee wrote:
edit: my main problem was assuming that the treadmill will make the airplane stationary relative to the ground
It does.
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On February 21 2009 15:11 inReacH wrote:Show nested quote +On February 21 2009 15:04 JeeJee wrote:
edit: my main problem was assuming that the treadmill will make the airplane stationary relative to the ground It does.
it doesn't though. not as specified anyway.
here's the explanation from the link where it finally clicked for me (along with the rollerblades on treadmill analogy earlier in the link)
The problem here, of course, is that the poster (and Neal) cannot disengage themselves from seeing the airplane as a car. The difference between a car and a grounded airplane is that a car uses its wheels to propel itself forward, and an airplane moves itself forward by moving air. They assume that the runway moving backwards would move the plane backwards. This is what would happen with a car (that is in gear), so why not for an airplane? Well, because an airplane’s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.
You can test this with a piece of paper and a matchbox car (which has free rolling wheels like an airplane… or like a car in neutral.) Place the paper on a table, and place the matchbox car on the paper. Take your hand, and hold the car still with a lightly placed finger on top of the car. At this point you are providing no forward thrust, and the “conveyor belt” is not moving. The car remains stationary. Now, continuing to hold the airplane with a lightly placed finger, and start to pull the paper out from under the car, in the backwards direction. According to Neal’s logic, the car should push back on your finger with the same force that you are exerting on the paper… but this is not what will happen. You will find that your lightly placed finger is not stressed to any noticeable extent. The paper will slide out, and the wheels will spin, but the car will not be propelled backwards. The reason for this is is that the rotation of the wheels is not related to the movement of the matchbox car except by the very small friction component of the axle, which your lightly placed finger can easily control.
So now we have established that movement of the surface beneath a free wheeling object does not exert a noticeable force on the object. Next, we’ll see what happens when the object is trying to move forward. Attach a string to the matchbox car. Place the car at one end of the paper, and use the string to start pulling the car forward with a steady force. As the car moves forward, start pulling the paper out from under the car, backwards. Do you feel increased resistance as you pull the string? Of course not. The wheels are free rolling! Spinning the wheels does not make the object move!
When an airplane takes off, there is one major forward force… the forward thrust. The main rearward force is air resistance. The turning of the wheels provides a small frictional force, but because the wheels are free-rolling, this friction is very small. Unless the wheels are locked, the friction is always going to be less than the thrust, which means that the overall force is still forward, and the plane will still move.
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On February 21 2009 12:12 BanZu wrote:
#1
An airplane is on a treadmill and they move in opposite directions. When the airplane's engines are running at full power the airplane's speed is the same as the treadmill's speed. Will the airplane be able to take off? Or will it simply stay in place?
Ok look, in the question, we can see that
Treadmill speed is = to airplane speed.
Micronesia is making the mistake of differentiating wheel speed with engine speed, as far the literal "airplanes speed", there is no difference.
More specifically, if the engines that drove the wheels were removed from the plane, but the speedometer left in place, and you then used the engines to propel the plane forward 200km/h(only according to the spedometer), THE PLANE WOULD STILL BE GOING 200 km/h(again only according to the speedometer) and the treadmill would compensate by upping its speed to match.
Thus, the plane never moves forward, it doesn't matter if the engines get that sucker up to what would 1000km/h, it's WOULD-BE speed is always compensated by the treadmill, as stated in the question.
"When the airplane's engines are running at full power the airplane's speed is the same as the treadmill's speed."
Look, 'airplanes engines', that means it is forever and consistantly being compensated.
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On February 21 2009 15:16 JeeJee wrote:Show nested quote +On February 21 2009 15:11 inReacH wrote:On February 21 2009 15:04 JeeJee wrote:
edit: my main problem was assuming that the treadmill will make the airplane stationary relative to the ground It does. it doesn't though. not as specified anyway. here's the explanation from the link where it finally clicked for me (along with the rollerblades on treadmill analogy earlier in the link) Show nested quote +The problem here, of course, is that the poster (and Neal) cannot disengage themselves from seeing the airplane as a car. The difference between a car and a grounded airplane is that a car uses its wheels to propel itself forward, and an airplane moves itself forward by moving air. They assume that the runway moving backwards would move the plane backwards. This is what would happen with a car (that is in gear), so why not for an airplane? Well, because an airplane’s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.
You can test this with a piece of paper and a matchbox car (which has free rolling wheels like an airplane… or like a car in neutral.) Place the paper on a table, and place the matchbox car on the paper. Take your hand, and hold the car still with a lightly placed finger on top of the car. At this point you are providing no forward thrust, and the “conveyor belt” is not moving. The car remains stationary. Now, continuing to hold the airplane with a lightly placed finger, and start to pull the paper out from under the car, in the backwards direction. According to Neal’s logic, the car should push back on your finger with the same force that you are exerting on the paper… but this is not what will happen. You will find that your lightly placed finger is not stressed to any noticeable extent. The paper will slide out, and the wheels will spin, but the car will not be propelled backwards. The reason for this is is that the rotation of the wheels is not related to the movement of the matchbox car except by the very small friction component of the axle, which your lightly placed finger can easily control.
So now we have established that movement of the surface beneath a free wheeling object does not exert a noticeable force on the object. Next, we’ll see what happens when the object is trying to move forward. Attach a string to the matchbox car. Place the car at one end of the paper, and use the string to start pulling the car forward with a steady force. As the car moves forward, start pulling the paper out from under the car, backwards. Do you feel increased resistance as you pull the string? Of course not. The wheels are free rolling! Spinning the wheels does not make the object move!
When an airplane takes off, there is one major forward force… the forward thrust. The main rearward force is air resistance. The turning of the wheels provides a small frictional force, but because the wheels are free-rolling, this friction is very small. Unless the wheels are locked, the friction is always going to be less than the thrust, which means that the overall force is still forward, and the plane will still move.
Unless the wheels are locked, the friction is always going to be less than the thrust, which means that the overall force is still forward, and the plane will still move.
Ah! I did not know an airplanes wheels were freerolling, interesting!
That does change everything, the treadmill would certainly not have enough influence over the plane to stop it from moving forward.
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United States24751 Posts
Uh okay is it clear now or I do I need to explain myself somehow? I'm pretty confident in my conclusions till this point.
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On February 21 2009 15:22 inReacH wrote:
More specifically, if the engines that drove the wheels were removed from the plane, but the speedometer left in place, and you then used the engines to propel the plane forward 200km/h(only according to the spedometer), THE PLANE WOULD STILL BE GOING 200 km/h(again only according to the speedometer) and the treadmill would compensate by upping its speed to match.
Thus, the plane never moves forward, it doesn't matter if the engines get that sucker up to what would 1000km/h, it's WOULD-BE speed is always compensated by the treadmill, as stated in the question.
Like most others who argue that the plane won't take off(for the airplane movement speed = conveyor belt speed version anyway, not the wheel rotation version), you are making the erroneous assumption that the force applied by the engines will be cancelled out by the wheels being placed on a treadmill moving backwards at an equal speed.
The wheels on a plane are free-rolling. They are just attached to provide support. The ground moving under them will only spin them faster, aside from the negligible backwards force on the airplane due to friction.
Imagine taking a paint roller, and then placing it on a quickly moving rotary belt or something. Is the belt's motion going to pull your arm along at full force? No. The roller is just going to spin, and you'll feel a slight tug due to the friction between the roller and the axle which eventually acts upon your arm. If you pulled this roller backwards while the belt moved forwards, you would be able to move it for any speed of the belt(within reason), because again the friction between the belt and the paint roller doesn't pull the whole roller along, it just spins the free-rolling part attached to the axle.
~Oops, didn't realise that that second post of yours was written by you. XD I guess I'll just leave this here for anybody who doesn't get it.
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Look let's just simplify things.
The first problem, what if the plane is not moving?
The plane would be moving backward at some speed, say 500mph.
So the opposite of micronesia said would be true, suppose the plane is not moving relative to the treadmill, and the treadmill moves backward at 500mph, the same as if the plane is in a 500mph backwind, and the plane is on flat ground.
Now, plane needs to move forward relative to air to generate a lift. However, the jet engie always move relative to air, so the question is not well defined. Thus, like micronesia was suggesting, instead of jet engine, we mount airplane on wheels, and the wheel "drives" the airplane forward.
So airplane driving down the runway at 500mph forward, wind blowing down the runway at 500mph forward. There is no relative speed between the plane and the wind. Plane cannot take off.
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okay i just read through the andwers in the whole blog section. I think the people argueing with the point of the free-rolling wheels are correct.
Now, let's change the question.
SUPPOSE wheels are NOT free-rolling, what will happen?
I suppose the plane can never move relative to the treadmill haha
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question 1 is a very common puzzle which actually changes yes or no depending on specifically how it is worded.
the question in the op is worded 'the treadmill matches the speed of the airplane', which is not specific so we are forced to assume the simplest scenario, the speed of the airplane when compared to the earth.
stationary plane, stationary treadmill. as the plane fires up its engines they grab on to the air and pull themselves forward*, in your mind you should liken this not to running on a treadmill but rather skating on a treadmill with rollerblades while pulling yourself forward using a rope attached to something in front of you. in this example your propulsion is not depending on the treadmill in the same way as the plane.
the plane moves forward at 50mph, the treamill moves backward at 50mph, the wheels on the plane spin at 100mph. the plane accelerates and takes off, to a person on the plane the treadmill is barely noticeable. in a real world test friction in the wheel bearings would actually transfer a tiny % of the treadmills speed to the plane fuselage, rather than all of it being lost in the free-spinning of the wheels, it would however not be enough to stop the plane from taking off.
when the question is worded 'the treadmill speeds up to keep the plane stationary' while intended to imply the same question it actually changes the result. we are forced to conclude the treadmill moves backwards many, many times faster than the plane 'would' move forward so that the thrust of the engines can no longer overcome the friction in the wheels.
the helicoptor question is actually different, as the force is applied directly to the rotorblades and will keep them stationary relative to both the ground and the air. a neutral observer will see the turntable and the helicoptor spinning, while the rotorblades remain stationary. the helicoptor will not take off.
pulling the aircrafts wheels backwards will not prevent it taking off, generally speaking.
spinning a helicoptor in a such a way as to cancel out the spinning of the rotorblades relative to the air around them will prevent it taking off.
*jet/rocket thrust is a result of pressure, pressure is applied in all directions inside the chamber except for backward where the pressure is exhausted, a net effect of pushing the engine forward and the exhaust gas backward.
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On February 21 2009 19:05 evanthebouncy! wrote:
okay i just read through the andwers in the whole blog section. I think the people argueing with the point of the free-rolling wheels are correct.
Now, let's change the question.
SUPPOSE wheels are NOT free-rolling, what will happen?
I suppose the plane can never move relative to the treadmill haha
well that would be a bit silly wouldnt? if the plane reached take-off speed by applying throttle to its wheels (aka a car with wings) as soon it left the ground it would no longer have any forward thrust and would lose speed due to wind resistance and drag among other things, causing it to very soon come back down. :p
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the comparison of the treadmill to the icy lake is a good way to picture it.
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EPT
