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propeller of the aeroplane. To obtain lift to offset the force of gravity, the propeller (rotor) is shifted from the vertical to a horizontal plane. To obtain forward motion the rotor is tipped forward. Since the rotor can be tipped in any direction, flight sidewards and backwards, as well as forward, is possible. The rotor thereby provides simultaneously the vertical lift to maintain altitude and the horizontal force to provide motion. Solution of the complex problems eluded experiments until many years after the relatively more simple aeroplane had been developed into a useful vehicle.

How does this merging of driving and lifting functions benefit the helicopter? Basically, it means that the force developed by the tuning rotor can, through proper control, be divided in an infinite number of ways-each division of which will produce a different direction or speed of motion. When, for example, all of the rotor's force is directed upward by keeping its rotor in a horizontal plane, the helicopter will either climb, descend, or hover, depending upon the amount of power the pilot elects to draw from the engine and supply to the rotor.

If, on the other hand, and again through proper control, the pilot tips the rotor out of a horizontal plane of rotation an immediate division of the rotor's upward force will take place. Much of it will continue to offset gravity, but a portion will act in the horizontal direction in which the rotor is tilted and result in motion in this direction. In addition, the pilot may elect to climb, descend, or maintain altitude while moving laterally by controlling the amount of engine power he supplies to the rotor.

Since the helicopter obeys the same laws of aerodynamics as the aeroplane, how is it possible to fly below some predetermined minimum speed? The answer to this question also rests with the merging of the lifting and propulsion functions in the rotor. By the application of the proper amount of power with the correct rotor tilt the pilot is able to approach a landing area at any angle and any speed he desires.

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