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#10
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cjcampbell wrote: Saw this question on "The Straight Dope" and I thought it was amusing. http://www.straightdope.com/columns/060203.html The question goes like this: "An airplane on a runway sits on a conveyer belt that moves in the opposite direction at exactly the speed that the airplane is moving forward. Does the airplane take off?" (Assuming the tires hold out, of course.) Cecil Adams (world's smartest human being) says that it will take off normally. I confess to having difficulty understanding the difference between a conveyer belt and a treadmill, although it seems important to some folks. Cecil does not seem to be arguing that prop wash or jet thrust are generating enough lift by providing downwash over the wings. He claims the airplane will take off normally -- that is, the airplane will move forward just as in a normal takeoff, developing airspeed as it goes. He also claims that this will not take any extra thrust. He likens the situation to someone on rollerblades on a treadmill who is holding onto a rope fixed to an object at the front of the treadmill. No matter how fast the treadmill is moving, it takes the same energy to pull yourself to the front of the treadmill as it would if the treadmill was stopped. The only difference is that the wheels would turn twice as fast. Cecil appears to be saying that the airplane cannot be prevented by the conveyer belt from moving forward, no matter how fast the conveyer belt moves, because the thrust is generated independently from the wheels. The airplane will accelerate and take off, using the same length of conveyer belt as it would an ordinary runway. The only difference is that the wheels will be spinning twice as fast at rotation as they would otherwise, thus the caveat "assuming the wheels hold out." I am not sure I understand why Cecil thinks this. If I understand him correctly, the drag of the treadmill against the wheels does not increase just because the treadmill is moving. Okay, I guess I can believe that. Still, it seems counterintuitive to me that if a plane is sitting on a conveyer that is moving backwards at exactly the same speed (I assume they mean groundspeed here) as the airplane is moving forward that the airplane will move forward at the same speed as if it was not on a conveyer at all. |
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