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kerosene31

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Question for you....Your sitting at the end of a runway, getting ready to take off. But the runway is actually a very large treadmill, and this treadmill could match the forward speed of your aircraft exactly, but running in the opposite direction. Could the airplane takeoff?I'm inclined to think it would take off, but others have arguments.Don

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>Question for you....>>Your sitting at the end of a runway, getting ready to take>off. But the runway is actually a very large treadmill, and>this treadmill could match the forward speed of your aircraft>exactly, but running in the opposite direction. Could the>airplane takeoff?>>I'm inclined to think it would take off, but others have>arguments.>>Don After a month or longer of debating this on several other forums, I'll go for the takeoff.The answer also has much to do with exact wording, as versions have varied slightly.

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I would say it couldn't. The wheels of the plane may be moving fast but becuause the runway is moving equally fast (but in the opposite direction) the net result is the plane has no forward momentum. With no momentum there is no air flow over the wings and therefore no lift.

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>>I would say it couldn't.>>The wheels of the plane may be moving fast but becuause the runway is moving equally fast (but in the opposite direction) the net result is >>the plane has no forward momentum.Please describe the mechanism by which a treadmill halts the forward progress of an airplane.You may be tempted to respond, "it stops it by going in the opposite direction" but before you say that, stop for a minute and THINK about what you're saying. How does a treadmill, no matter how fast it's moving, stop an airplane? Think about it.Here is another, similar, physics problem for you: there is a butterfly sitting on a set of railroad tracks 20 meters in front of a locomotive. If the locomotive starts moving, the butterfly will flap its wings. The faster the locomotive goes, the faster the butterfly will flap.Will the train go anywhere?See, a butterfly is to a locomotive as a treadmill is to an airplane. That is to say, neither have any effect. If you disagree, then please describe for us the mechanism by which a treadmill halts the forward progress of an airplane.

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It would if the tires didn't shred before hand! This treadmill would simulate taking off with a 65kts+ tailwind!!!

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If it's matching your speed exactly, but in the opposite direct as you say, no, it could not. There'd be no airflow over the wings.----------------------------------------------------------------John MorganReal World: KGEG, UND Aerospace Spokane Satillite, Private ASEL 141.2 hrs, 314 landings, 46 inst. apprs.Virtual: MSFS 2004"There is a feeling about an airport that no other piece of ground can have. No matter what the name of the country on whose land it lies, an airport is a place you can see and touch that leads to a reality that can only be thought and felt." - The Bridge Across Forever: A Love Story by Richard Bach

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I was thinking about aircraft carriers,and relative winds.In any case,watch it!!,this may be a trap question, Mr. Ladamson is an Ole FOX. Soon he will pounce on us and show what a bunch of dummies we are.He will do it with pure surgical logic,Stick and Rudder, and with Dr. Einsteins Theory of Relativity thrown in.Then he will sit back and laugh at us. A word to the wise!! Better wish him "A MERRY CHRISTMAS" now,while I can!! VIN:-eek

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>Please describe the mechanism by which a treadmill halts the>forward progress of an airplane.>Have you never seen or used one of those treadmills that people like to use to run on? Do you ever see them having any forward momentum?Remember, it is not the momentum of the aircraft in relation to the surface it is on but it is its momentum in relation to the surrounding air.

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It's plain and simple. If there is no airflow over the wings, there is no lift being created. With no lift created, there is no takeoff. Who cares what the wheel speed is. If they don't blow up and burn by the time they reach 200 knots, the airplane is still not going anywhere through the air.Proof...the formula for Lift is: L=CL[(rho x V^2)/295] x S or the coefficient of lift times rho (density of the air) times the airstream velocity squared, all of that divided by 295, with that all multiplied by the planform area of the wing. The key here is the airstream velocity. If the engines are producing thrust, they propel the aircraft through the air. If the treadmill is spinning backwards at the same speed at which the airplane will move forward for a given thrust output, the airstream velocity of the airplane will be ZERO...the forward motion of the airplane is cancelled out by the backwards turning motion of the treadmill. You ain't goin anywhere kid!

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Hi Victor I was going that route also.But, put an unsecured plane on top of a truck,and drive to its lift off speed.The airspeed indicator will show the speed thru the air,or the air going over the wings.The venturi,or pitot tube will show that.Not unlike towing a glider,its the relative wind.Were the plane facing tailfirst into the wind,it will act as a tail wind.Next time in an airplane notice the airspeed indicator ,if the wind is blowing on its nose,it will show it.You can do that on the Sim also.Like an eagle lifting off in a stiff wind the plane will also,lift off at its lift off speed is reached. VIN

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I was in the "won't take off camp" for quite a while. I was proven wrong, and had my own doubts when I tied a model airplane to my treadmill with rubber bands to see if they'd stretch. As I increased the speed, they hardly stretched at all. After that, I figured that forward thrust would easily offset and beat any reverse treadmill speed.As of now, this problem was discussed on a pilots flight board, then to a magazine. In the end, the wheels just rotated faster, but the plane can take off.L.Adamson

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>> Have you never seen or used one of those treadmills that people like to use to run on?Completely irrelevant to the problem at hand. A person running is pushing off the treadmill. That's how you move forward over the ground, you push off the ground. Sure, if the ground moves in the opposite direction you aren't really going anywhere.Is an airplane pushing off the runway? Is an airplane using it's wheels to push itself forward?No, it's not. Not at all. The wheels of an airplane are like the wheels of a skateboard. They don't do anything but support the aircraft's weight.I told you - stop and think about this. It's an easy problem. Just think about it. By what mechanism does a treadmill stop an airplane from moving forward? The airplane is using PROPELLERS, not its wheels, to move forward. How does the motion of the treadmill affect the airplane? Please explain it to me.

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In this scenario the aircraft would have thrust but no drag so theoretically the treadmill speed would keep increasing to destruction. Depending on design, ie. if enough potential energy could be stored in the spinning wheels, and then the treadmill instantly stopped the aircraft would be launched and take off , burning rubber at the same time as the wheels would be accellerating the aircraft. A similar principle to the plethora of small friction drive toy cars that can be bought in toy shops.Left to its own devices, ie no outside interference there is no way the aircraft would take off.Roger

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L.Adamson said: >As I increased the speed, they hardly stretched at>all. After that, I figured that forward thrust would easily>offset and beat any reverse treadmill speed.But the question here is not whether the airplane will move backwards (airplanes don't do that, only those god awful helos), which would cause the rubber bands to stretch. The point is that the rubber bands don't become any less tense as the speed of the treadmill increases. This shows that there is no forward motion of the aircraft relative to the air, and thus no lift/flight.

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>>> Have you never seen or used one of those treadmills that>people like to use to run on?>>Completely irrelevant to the problem at hand. A person>running is pushing off the treadmill. That's how you move>forward over the ground, you push off the ground. Sure, if>the ground moves in the opposite direction you aren't really>going anywhere.>>Is an airplane pushing off the runway? Is an airplane using>it's wheels to push itself forward?>Yes, the aircraft IS pushing the runway.Using a traditional runway the runway pushes back just as hard causing the aircraft to move along the runway.On a treadmill the runway doesn't push back so the aircraft remains stationary.>I told you - stop and think about this. It's an easy problem.> Just think about it. By what mechanism does a treadmill stop>an airplane from moving forward? The airplane is using>PROPELLERS, not its wheels, to move forward. How does the>motion of the treadmill affect the airplane? Please explain>it to me.>>The aircraft uses the props (or jets, rockets, whatever) to provide momentum to the wheels which cause the aircraft to exert a force upon the ground which in turn causes the aircraft to move along that ground.Due to the design of the wing this produces lift which at some point is enough to get the aircraft off the ground and into the air at which point those same engines do the same thing with the air around them.In your world an aircraft could take off from a frictionless surface, and could also land on one easily as it wouldn't need the surface to slow down.

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>The aircraft uses the props (or jets, rockets, whatever) to>provide momentum to the wheels which cause the aircraft to>exert a force upon the ground which in turn causes the>aircraft to move along that ground."The aircraft uses the props (or jets, rockets, whatever) to provide momentum"ok, good so far."to the wheels"bzzz. wrong."which cause the aircraft to exert a force upon the ground"wrong. No force is exerted upon the ground except the weight of the aircraft. The wheels just support the aircraft. Unlike a car, the wheels have no part in moving it forward."which in turn causes the aircraft to move along that ground."an aircraft moves along the ground for the same reason it moves through the air* because of its engine, not because of its wheels. The wheels have nothing at all to do with this.It is true, a car on a conveyor belt could be held motionless by the turning of the conveyor. A person on a treadmill could also be held motionless. In both cases, the reason the person or the car doesn't move is because a person or a car attempts to move by pushing on the ground. This is not true of an aircraft, and as a result, the motion of the conveyor has a negligible effect on an aircraft.*note that I didn't say, "for the same reason it lifts off the ground" I'm not talking about the lift from the wings, I'm talking about thrust.

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I completely agree with you - I am a physicist (MS) and absolutely see nothing that will stop the takeoff (provided the tires can take increased rotation). The aircraft doesn't need the ground (or wheels) to propel itself forward - it needs the Newtonian 3-rd law to propel itself relative to the inertial platform- the earth.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

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>If it's matching your speed exactly, but in the opposite>direct as you say, no, it could not. There'd be no airflow>over the wings.Its is a falacy. The aircraft is not a car and its power is not delivered to tires. If aircraft engines were spinning the wheels in order to gain speed - yes, the takeoff in this case would be imposssible. But the jet engines use exhaust to take advantage of Netwon's 3-rd law and due to preservation of momentum the aircraft starts moving forward - regardless what is happening with the tires. Of course the tires will be spinning like crazy in this case and assuming they won't burst - the take off will happen normally.It is easy to modify this experiment slightly and see why this is the case. Replace tires with snow skis, replace concrete with snowy/icy treadmill and think again ...Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

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I'm shocked that this is even considered problematic. To think that a such threadmill could prevent the aircrfat from gaining any forward momentum, you would have to totally lack any sense of physics and understanding of the principles of how the the engines propels the aircraft forward to get an airflow over the wings that makes it fly.Now, if this threadmill somehow could move the air around the aircraft in addition to the ground a takeoff on the spot would be possible.-

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>point is that the rubber bands don't become any less tense as>the speed of the treadmill increases. This shows that there>is no forward motion of the aircraft relative to the air, and>thus no lift/flight.Mistake in reasoning. The rubber bands will keep the same tension - regardless of the speed of the treadmill - once the rolling friction is overcome it makes no difference if the treadmill moves with 5 mph or with 500 mph. So really the moving treadmill creates no significant force that prevents the aircraft to accelerate and take off. The treadmill and its friction on the tires is the same obstacle for the aircraft as the stationary runway.Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

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>I'm shocked that this is even considered problematic. To>think that a such threadmill could prevent the aircrfat from>gaining any forward momentum, you would have to totally lack>any sense of physics Tha sad part is that few people have any concept of physics regardless how elementary it might be ...Michael J.http://www.precisionmanuals.com/images/forum/pmdg_744F.jpghttp://www.hifisim.com/images/asv_beta_member.jpg

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>Tha sad part is that few people have any concept of physics>regardless how elementary it might be ...>Yes, probably right. And I'm also a physicist myself, so perhaps I am a little blind about the difficulties.-

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Bah, I knew my mind was goin', now there's the proof. Not that I was ever good at these sorta puzzles. Hehe----------------------------------------------------------------John MorganReal World: KGEG, UND Aerospace Spokane Satillite, Private ASEL 141.2 hrs, 314 landings, 46 inst. apprs.Virtual: MSFS 2004"There is a feeling about an airport that no other piece of ground can have. No matter what the name of the country on whose land it lies, an airport is a place you can see and touch that leads to a reality that can only be thought and felt." - The Bridge Across Forever: A Love Story by Richard Bach

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The movement relative to the ground provides the airflow which gives lift.If the aircraft is held stationary by the runway being a treadmill (as in this example) that lift is never generated, ergo the aircraft will never take off.That's where your logic is flawed, you don't take into account that the aircraft never gets enough speed to provide airflow over the wing and therefore never generates the required lift for takeoff.It may get forward momentum, but no forward speed.

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