1.  12/08/2006, 08:00 AM Originally Posted by aprasad the treadmill matches the speed of the airplane: What is the reference frame in which the speed is being measured? The most common interpretation: the reference frame is attached to the ground => Plane will take off In that case, yes. The treadmill will then roll backwards at half the speed of the airplane relative to the treadmill and thus be the same as the speed of the airplane relative to the ground (in the opposite direction): velocity of the plane relative to the ground = -(velocity of the treadmill) velocity of the plane relative to the treadmill surface = 2 x velocity of the plane relative to ground. If the plane can drive at twice the normal takeoff speed, it can take off. “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
2.  12/08/2006, 08:05 AM Originally Posted by clulup That is true, but for gaining speed on the treadmill it makes no difference that the plane is powered by jet engines and not via the wheels. Acceleration works just as well with wheels powered like those of a car - at least until takeoff, then the wheels would be no good anymore and the plane would come down again. That's your fundamental mistake. The fact that the wheels aren't powered DOES make a difference. A huge difference. In a car, it would work the way you describe, because the treadmill offsets the power being applied to the wheels. With the wheels freely spinning (on the plane), the treadmill offsets nothing. Why do you think the physics of motion suddenly changes, dramatically, when the plane's tires are 1/1 millionth of an inch off the ground? The tires are a red herring--they have nothing to do with the physics of the plane's motion. Bob Meyer I'm out of my mind. But feel free to leave a message.
4.  12/08/2006, 08:39 AM Originally Posted by meyerweb Why do you think the physics of motion suddenly changes, dramatically, when the plane's tires are 1/1 millionth of an inch off the ground? Of course it makes a tremendous difference whether the wheels touch the ground or not. Lift your car so that the wheels are 1 millionth of an inch off ground. I'm sure you will notice the difference. Neverthelass, as long as the wheels touch the ground and there is friction, a car can drive on a treadmill without any problem. Of course the car would come down again immediately after takeoff (assuming it has wings) because it slows down again, but still... “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
6.  12/08/2006, 09:20 AM Originally Posted by clulup In that case, yes. The treadmill will then roll backwards at half the speed of the airplane relative to the treadmill and thus be the same as the speed of the airplane relative to the ground (in the opposite direction): velocity of the plane relative to the ground = -(velocity of the treadmill) velocity of the plane relative to the treadmill surface = 2 x velocity of the plane relative to ground. If the plane can drive at twice the normal takeoff speed, it can take off. This illustrates perfectly why there was such debate at airliners.net over this problem. The person who posted it there either worded it wrong or found it that way; someone else found the original problem (conveyor matches airplane's speed--not wheel speed--relative to Earth, in the opposite direction), and there wasn't much debate on that at all. Assume frictionless wheels and tires rated for infinite speed, and it doesn't matter what the belt does; it can run at any speed in either direction, and the plane will just get up and go as normal. Add friction, and that becomes the only factor. (Another question they debated at length is whether or not a huge airplane filled with birds would weigh less if the birds were flying around inside or sitting...) "Yeah, he can talk. It's gettin' him to shut up that's the trick!" -Shrek
7.  12/08/2006, 09:46 AM Originally Posted by aprasad 1. Will not happen because the thrust from the engines will move the plane forward relative to the ground.. no matter what the treadmill top surface does. So, picture the plane's turbine engines powering up and the plane rumbling down this loooong treadmill. This will happen. What the motors on the treadmill do: stay still, move the top surface rearward to match the plane speed, move in a random way, whatever.. will affect the rpm of the tires, which are free-rolling. If it is easier, replace the tires by a frictionless ski-sliders (line snow-planes have) sliding on the treadmill. Now do you have any doubts whether the plane will take off? "replace the tires by a frictionless ski-sliders"... Tires of planes and cars and ski-sliders are by no means frictionless. If they were, you would hardly use any gas when driving steadily on a road. Nobody said the tires in the original problem are friction-less. Besides, do you doubt a man running on a treadmill rolling at the same speed in the opposite direction stands still relative to the fitness studio? Why would the plane be different? In case you forgot: a plane running on the runway does NOT fly or float above the runway. It is tons of metal resting on wheels with friction. In case (1) above, it stands still relative to the air like the runner on a fitness studio treadmill, so there is zero lift and no takeoff. In case (2) above, it can move relative to the air and may take off if it can roll fast enough. “Reality is that which, when you stop believing in it, doesn't go away.” (Philip K. ****)
8.  12/08/2006, 09:55 AM Because the the force of propulsion in cars and the man (running) comes from the friction at the road/treadmill surface. The plane does not. That's why ski-planes can take off on ice. A plane with it's engines gunning (the way it normally does while taking off) will NEVER stand still relative to the ground, regardless of any super treadmill doing whatever the treadmill operator desires. What you describe is true for cars, not airplanes. Do you agree that the two cases (cars vs airplanes) are different from each other when it comes to operating on treadmills. Last edited by aprasad; 12/08/2006 at 10:22 AM. -- Aloke Cingular GSM Software:Treo650-1.17-CNG Firmware:01.51 Hardware:A
9.  12/08/2006, 11:18 AM Basically a plane blows itself forward by ejecting air. So when the plane takes of at 10% trottle it would do speed X, the treadmill would respond by going -X but since there is nothing (appart from friction of the wheels) stopping the plane going forward the plane would go at X MPH. If the pilot puts the trottle to 100% the plane would reach an airspeed of 10X which should be enough for liftoff (assuming it doesnt have a gailforce tailwind) (ex)VisorCentral Discussion Moderator Do files get embarrassed when they get unzipped?