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Travellator: will the plane take off or not?
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Hello. We are having a huge argument about the following topic: If there was a Travellator and whatever you placed on there it would match speed. So running at 10mph would make the thing go at 10mph so you would be standing still. A car at 80mph would make the thing move at 80mph so the car would appear still. Put a plane on there for arguements sake a relatively small jet like a Boeing 737, would the plane be able to move/take off? I reckon the plane would not move and so wouldn't take off. Ideas? Full topic here.
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Re: Travellator: will the plane take off or not?
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on 2006-05-23 10:41:57 |
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the answer is, the plane would take off, but it would use slightly more fuel. this is because the plane would begin with some backwards velocity relative to the air. also, the friction due to the tires would be greater due to the travellator. the reason a plane would behave rather normal, opposed to a person/car which would be slowed down, is that a plane moves against air as it's main impulse, whereas a person/car uses friction to convey impulse. also worth mentioning: if the tires on the plane had an odometer, they would register a longer runway than usual. 
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Re: Travellator: will the plane take off or not?
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Aren't you just talking about a treadmill? Anyway, if it matched the speed of the plane, then the plane would stay in one place. If the plane was not moving, then there would be no air moving across the plane's wings, thus making it impossible for it to take flight.
Wise Man says: "Take a dog off its leash and it will wander."
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Re: Travellator: will the plane take off or not?
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If I understand you correctly, Gendou, you are saying that the engine thrust does not affect the wheels at all. Is that what you are saying? *confused*
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Re: Travellator: will the plane take off or not?
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If the plane was stationary relative to the air, then no air would cross the plane's wings, something that is absolutely neccessary for it to take off.
Wise Man says: "Take a dog off its leash and it will wander."
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Re: Travellator: will the plane take off or not?
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on 2006-05-25 17:10:57 (edited 2006-05-25 17:31:37)
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Here is what a Travelator is: http://en.wikipedia.org/wiki/Moving_sidewalk @engineer: you are a moron! read what i posted! jesus christ. planes do not take off because they run fast with legs or wheels or anything else HOOKED TO THE GROUND. planes push against air. wether your on the earth, moving walkway, a vat of jello, or any other surface on earth, the air is just the same! time=0: plane is moving to the left relative the ground, pushed by the travelator via the force of friction acting on the wheels as they rest on the surface of the travelator's surface. time=1: plane's engines turn on, and begin sucking air. time=2: the plane begins to roll forward on the travelator, slowly. time=3: the plane is moving so fast relative the air (even faster relative the travelator!) that the lift on the wings pulls the plane off the ground.
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Re: Travellator: will the plane take off or not?
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The travellator is stationary, the plane is on top of the travellator. The only movement of air, is the air that is moving through the jet engines and blasting out behind them. No air is crossing over the wings. When a plane takes off, the jet engines are required to move the enormous object. Once the plane reaches the appropriate speed, and a vacuum is established at the base of the wings (due to air moving across the whole of the wing), the plane is able to take flight. @Gendou: Runways are a neccessary part of aeronautics, otherwise aircraft carriers could launch Beoing jets with the use of a giant treadmill.
Wise Man says: "Take a dog off its leash and it will wander."
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Re: Travellator: will the plane take off or not?
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on 2006-05-25 20:52:22 (edited 2006-05-25 20:54:29)
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the propulsion of the jet will make the plane move forward relative to the ground, weather it is touching the ground, a moving walkway, or jello, or if the plane is lying on your face. the reason runways are neccessary is because they save power. do vertical-lifting rockets and helecopters use runways? no. in other words, the plane will reach the "appropriate speed" relative the surrounding air, regardless of what is happening to its wheels. that is, unless the wheels are "hooked" to something. the friction of a wheel resting on the ground is negligable compared to JET ENGINES!
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Re: Travellator: will the plane take off or not?
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I should have said "aviation" not "aeronautics." So helecopters and verical lifting rockets are excluded. They use vertical lift though, a plane can't. And I do realize that the surface on which a plane starts does not matter, that is why planes can take off from water (another runway), but that is not the issue here. FOR A PLANE TO FLY, IT NEEDS AIR FLOW AC... Actually, I believe we are looking at this from different perspectives. You are focussing on the travellator, I am focussing on the air. Anyway, the travellator would match the ground speed of the plane and the plane would be immotile relative to the air around it. There would be no air movement across the wing. A vacuum could therefore not be established, and the plane would not take flight. When a wing is in flight, there is a vaccuum at the base of the wing. This vacuum sucks air across the wing, which hods it in the air. Helecopters use propellers; a totally different subject. Rockets use fuel directly; also a different subject. The jet engines do not move air across the wing, they accellerate the body of the plane. The faster the movement (relative to the air) the stronger the vacuum
Wise Man says: "Take a dog off its leash and it will wander."
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Re: Travellator: will the plane take off or not?
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on 2006-05-26 15:27:40 |
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i see where your thinking has gone wrong: the travellator would match the ground speed of the plane and the plane would be immotile relative to the air This is simply not true! the travelator would move in the negative x direction as the plane moves in the positive x direction, but the plane would still move relative to the air, because the impulse is on the air, not on the ground. Cars move using an impulse delivered to the ground, jet engines use a VERY VERY STRONG impulse applied to the air. The entire existence of the travelator is irrelevant to the experiment! All it's going to do is spin the wheels one way or the other.
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Re: Travellator: will the plane take off or not?
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I'm sorry Gendou, I just can't explain my point of view well enough. I asked my sister and she agreed with me. She is the smartest person I know, so that is good enough for me.
Wise Man says: "Take a dog off its leash and it will wander."
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Re: Travellator: will the plane take off or not?
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It appears that I'm going to have to disagree with Gendou here. What's happening with the world. The things that engineer is pointing out is that, the plane is not moving in reference with the surrounding environment. This is what counts here, Gendou you are saying that the plane will move relative to the air. I have to say this would be incorrect, ok the jet engines will be causing movement of the surrounding air but this is what give the force to move the plane forward. Not enough to cause the air around the wings to move a rate to give any lift. I will have to give some examples for you to think of in this situation. What would happen if you placed an object on the plane? In flight this object would be blown of the plane due to the forces from the air passing the plane. This is unlikely to occur in this case. Secondly. Say this travellator is then same length of the plane. Now if you were stood in front of the plane would the plane hit you or would you fill forces large enough to pull you in the direction of the plane |
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Re: Travellator: will the plane take off or not?
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on 2006-05-27 14:10:04 (edited 2006-05-27 14:24:48)
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THE PLANE WILL MOVE FORWARD RELATIVE TO THE SURROUNDING AIR. Why the hell not?! Just because its resting on a surface, moving relative to the ground, you think the jet engines will not create forward suction? Suction is a FORCE. the FORCE of suction will be far greater than the drag of the wheels, as the plane begins to move forward (and the wheels begin to spin, triggering the travelator to move backwards). OF COURSE if the plane was NOT moving relative to the air, no lift would occur. That was made clear in the first post! Think about what will happen when the plane turns its engines on. First, it will lurch forward, pushed by the force of suction through the engines. As it does so, the wheels roll forward. The travelator measures the lurch forward, and adjusts by rolling in the opposite direction. Friction adheres the plane to the travelator, at the time, and the plane lurches backwards. Later, the engines are at full-power. The plane is thrust forwards with incredible force. As it is thrust forwards with this great force, it accelerates rapidly. In response, the travelator accelerates the opposite way, matching the speed. Eventually, you have an impulse forwards that overcomes the friction keeping the wheels on the ground, and the plane will slide forwards, overcoming the force of friction keeping the wheels on the ground. The spinning of the wheels will not be able to keep delivering the energy to the travelator! It will be far to great. Of course, at first, the energy output of the engines will be absorbed. I argue that this will not happen to a jet when its engines are full-power. Another way to picture this is if the jet was not pushed by the force of suction caused by its engines, but rather pulled by a wire attached to the nose. (same force vectors, no effect on the scenario). Put a model plane on a treadmill, like an exercise one. Pull the plane with a thread attached to the nose. Say, for the sake of argument, the treadmill was very very sensitive and adjusted with negligible internal friction. Pull the model plane forwards gently, and the treadmill would roll backwards in response to the wheels rolling forwards. Pull it forwards fast, and you would yank it right off the treadmill, sending the treadmill into a frenzy!
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Re: Travellator: will the plane take off or not?
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So let me see here, just how am I to explain this? Lets start by looking at how a plane achives leaving the ground under normal conditions. The plane is stationary in place on the runway ready to takeoff. The jet engines are turned on and start off at full power. (to save complications) This will give an almost constant acceleation - various resistances - but we shall neglect. And assume constant acceleation The engine produce a force causing the plane to move forward, the rolling of the wheels allow the plane to accelerate at X m/s^2. so after 1 second the plane will be moving X m/s and if there is no wind will have the air moving at -X m/s relative to the plane The plane will need an air speed of Y around the wings to create enough down force to counteract the force of gravity and thus lift off the ground. Lets say the Y = 10X thus after the engine creating the same force for 10 seconds the air surrounding the plane will be moving at a speed of 10X with relation to the plane. During this whole process the air that is moving through the engines will be moving at relatively a constant speed say Z. Apply this to our travelator. Again the plane starts of stationary. The engines are turned on, again say straight to full power, this in turn again produces a force and thus an acceleration. This time however instead of the wheels turning and the plane moving (due the the ground being fixed in place) the wheels turn and the energy is transfered into the travelator. Thus the accelation takes place directly into the travelator. When we look at how the air around the plane behaves, we see that the instance the engines are turned on the air velocities will differ in the same way no matter if the plane is on a travelator or not. After this intial change the air patterns stablise as the force from the engines are being transfered from the plane to travelator. So after the same 10 seconds the air around the plane will still be moving at the same rate as after 1 second or 10^10 seconds (if the travelator and the wheels can lasts the constant force and not break) Again the air through the engines will have be moving at the constant speed of Z Thus at no point will the plane have air moving around it at a velocity to create the lift to have it take off. I do have to ask though why you think that friction is keeping the plane on the ground? - What the friction does it keep the wheels and the travelator moving at the same speeds in oppisite directions. Once the force of friction is over come the plane will move forward but not at a speed that could cause it to take off. Then the wheels will slow down and friction would grip again. This process would repeat until you stop it or something breaks. |
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Re: Travellator: will the plane take off or not?
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on 2006-05-28 13:36:21 (edited 2006-05-28 13:52:47)
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the first part of your argument is very well laid out. i like it a lot, and agree completely. what i meant about friction is that it is the force mediating transfer of energy from the engines suction to the travelator. no mater how perfect a machine the travelator is, the physical wheels on the plane will not be capable of transferring the enormous output of the engines. they will either slip or tear to pieces, thus breaking the perfect flow of energy from the plane's engines to the travelator device. the plane will then lurch forward, and continue to do so, until it reaches high velocity relative the air and take off. Here are some good points made in the original thread: The point is that the speed of the treadmill has no effect on the speed on the plane (ignoring the friction that the engines can easily overcome), since the wheels on the plane are not driven. The difference here is that the propulsion of the first two examples is the transfer of movement through the ground(travelator) as the examples use forward (or reverse) driven force through the ground,This moves the 'belt' because the driving force is through the contact with the belt. As the planes engines fire up, the plane will move forward. Its wheels will not roll as there is no need to, the belt will move instead. When (if) the plane reaches the end of the belt, its wheels will make contact with the runway, and start to turn.
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Re: Travellator: will the plane take off or not?
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Thanks for your comment on my argument. I must say I do respect your knowledge on physics, which makes it hard to understand how you are reaching the conclusions that you have. Now lets look at these three points you are pointing out. They all seem to looking at one aspect - which the wheels of the plane will not roll, and instead slide across the surface of the travelator. Although you say that there is no reason for them to roll, there is and it a simple one: The combination of gravity and friction. If you look at the design of wheels, they are made so that you use less force when trying to move an object in a direction. For in most cases it will take more energy to slide an object across a surface than roll it. When we go back to the plane (stated as a Boeing 737) which I assume are under normal conditions (atmospheric pressure and temperature with gravity of 9.81 m/s^2) The force of gravity will cause the plane to have a weight and unless the travelator has a low coefficient of friction (like ice) then it will as stated above take more energy to make the wheels slide rather than roll. The way that the force is applied to the plane will not make a difference on this. The difference between the driving force being from the wheels or not comes when friction is overcome %u2013 if through the wheels they will just spin and the plane will not move in from another source then the wheels stay relatively motionless and the plane slides Thus if the plane can slide across the travelator, then yes the travelator will not have an affect the speed of the plane and will assume the same situation as if you are having a plane taking off water, or a frozen surface. Oh and to the point that the wheels are unable to transfer the energy if the engines. Well they are as that what they are designed to do and do this when a plane takes off under normal conditions. There will be a point at which the wheels will not be able to take the strain and will break as hinted in my previous post. |
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Re: Travellator: will the plane take off or not?
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on 2006-05-30 11:46:47 |
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it sounds to me like we now agree on everything except the setup. i do not believe that the wheels will be capable of transfering the output of jet engines.if it was a little ol' propeller plane, however, maybe the wheels would be able to sustain the energy transfer. it would be fun to see such a plain grounded by such a simple thing as a treadmill! still, i would not expect the energy transfer from the engines to the wheels to the treadmill to be 100% effective. some energy will be lost as heat, and some as forward "slipping". increase the thrust, and you will see an increase in slipping. i stand by my theory that a jet engine would still generate enough forward motion to lift the plane. something for the myth bysters perhaps?!
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Re: Travellator: will the plane take off or not?
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on 2006-05-30 11:54:16 (edited 2006-05-30 12:08:57)
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wait this is all retarded. if you walk on a treadmil, you stay put. if you stand on the ground and push a toy plane, it will roll freely, and even drag the treadmill FORWARDS with it! (i tried it in the gym with a kleenex box) put another way, what if you blew air in the face of someone on the treadmill. they would be blown off! the plane would take off with more ease than if it were a dirt road!!! EDIT: another though experiment! what if you are walking on a treadmill underwater. you swim forwards (and a little bit down, so you dont float off) with your arms, you will be able to walk forwards easily! EDIT AGAIN: I just re-read the original post and it says the travelator MATCHES SPEED with the wheels by definition. This means it MUST be powered, so the problem is no different that a plane ancored to the ground in one spot by it's wheel frame! In that case, the wheel frame may or may not break...but the point is clear: it's not supposed to take off.... So i guess the fianal word is: it wont take off unless the wheels break off :P
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Re: Travellator: will the plane take off or not?
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Upon reading this topic, I was reminded of an interesting article written by Cecil Adams that I've read detailing the very same problem. In short, the plane will take off because the propellers or jet turbines will shove the air backwards and propel the plane forward. The wheels have no bearing on its actual movement, but rather as a means for reducing friction during takeoff, land, braking, etc. If you're interested in the article in its entirety, here's the link: http://www.straightdope.com/columns/060203.html I hope this helps. |