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Turning the Airplane | |
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The position of the airplane over time is called the flight path. This flight path can be changed primarily by varying the amount and direction of the lift and by increasing or decreasing the thrust. In steady, level flight, the lift force exactly balances the weight force. The pilot can increase the lift on the plane by moving a control surface such as the elevator to produce a nose-up motion or by increasing the thrust so the plane moves faster through the air. The lift now exceeds the weight! When this happens at a constant velocity, the airplane experiences a vertical acceleration upwards, and its altitude increases. If a line were drawn for the flight path, it would show a curved section for this altitude change. When the lift exceeds the weight, there is an effect on the pilot and crew. The ratio of the lift to the weight is called the load factor. When the lift is equal to the weight, as in level flight, the load factor is 1, and this is interpreted as 1 g. If the load factor were 2 (the lift is twice the weight), then the pilot and crew would be experiencing 2 g's. Most small aircraft and commercial aircraft are not designed to experience high load factors; they rarely exceed about 1.5 g maneuvers. Military fighters, however, are designed for many more g's for tighter turns and increased maneuverability. Obviously, these increased g's affect the pilot as well as the airplane; in fact, the latest generation of fighter airplanes can pull more g's than most human beings can withstand without being rendered unconscious. The computers that help control the airplane also prevent the pilot from attempting to exceed established g-limits. Furthermore, these pilots are trained to resist the g forces to continue to fly the plane. The direction of the lift force is perpendicular to the wing of the airplane. This means that it extends straight out from the wing. The pilot can change the wing angle with respect to the forward flight by using the ailerons to roll or bank the aircraft for a turn. This changes the direction of the lift force with respect to the weight force. It adds a lateral acceleration to the airplane, which causes the plane to turn. The flight path will show a curve as the plane turns. If the pilot banks the airplane, but doesn't change the speed or anything else, the plane will turn, but it will also lose altitude. When the lift changes direction, the amount of lift in the vertical direction, which is counterbalancing the weight, is less than the original lift at level flight. Therefore, since the vertical lift is less than the weight, the airplane will descend. In order to counteract that, the pilot will also give the nose a slight pitch up as he or she rolls the aircraft. This increases the total lift enough so that the vertical lift once again equals the weight and no altitude is lost. Just as for the pitch up motion, the ratio of the modified lift to the weight can be computed, and pilots can experience g forces on turns, too.
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