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Up to this point our discussion has centered on theory of lift and the shape of airfoils best suited to the rotating wings of a helicopter. The second phase of this chapter will deal with the functioning of the flight controls. In a sense, dividing the subject into these two categories parallels the development of the helicopter. The first phase, roughly up until the early 1920's, dealt largely with the problem of perfecting rotors that were efficient enough to permit flight. The next step, through the 1930's and into the 1940's, was the perfection of control systems and mechanisms that would provide the capability for real flight at a fair rate of speed.
Our examination of the pilot's control system will be limited to one helicopter configuration, surely the best example, the type which incorporates one main overhead rotor with a small tail rotor at the rear of the fuselage. This classic form, perfected by Igor I.Sikorsky in the United States between 1939 and 1941, remains the predominant type today, although there have been many successful aircraft with various other rotor configurations. It should be noted that the control systems described here are not limited to the single main rotor type but are employed in one form or another in most helicopters, no matter what the rotor configuration.
With the Sikorsky configuration, the small tail rotor is intended to offset the torque reaction mentioned in earlier chapters. Torque has no effect while the aircraft is sitting on the ground, since the ship is prevented from rotating by the weight on the landing gear. But once the ship is clear of the ground, the torque effect tries to take over. The practical effect of this as far as the pilot is concerned is simply that the tail has a tendency to swing in one direction. To prevent this, a small anti-torque rotor is mounted at the tail, driven by an extension shaft from the engine transmission. This tail rotor is really more like an airplane propeller, since it turns much faster than the main rotor. To keep the helicopter moving straight ahead without turning, the pitch of the tail rotor blades must be just enough exactly to counteract the force of the torque reaction. On most helicopters with a single main rotor, the pitch of the tail rotor is adjusted so that at cruise speed the aircraft is trimmed to fly a straight course.
Besides holding the tail end of the fuselage straight against the twisting force of torque, this rear rotor provides control for steering to the right or left, as does the rudder of an airplane. This is accomplished by the pilot's foot pedals, which control the pitch angle of the tail rotor blades: by pressing on the right pedal, the pitch is decreased and the tail swings to the left |
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