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Helicopters are used extensively by the Departments of the Army, the Navy and the Air Force for such purposes as liaison, sea and land rescue, as aerial ambulances, for reconnaissance, and for the rapid movement of troops, supplies and equipment to critical areas. Commercial uses for helicopters include activities such as prospecting, crop dusting, aerial inspection and rescue, and short haul mail and passenger service. Helicopters are basically of two types: the single-rotor helicopter (Fig. 1) which has one main rotor mounted on the top of the fuselage, and the twin-rotor helicopter with two main rotors. The latter may be arranged mounted side by side (Fig. 2), coaxially one above the other on shafts that turn one inside the other (Fig. 3), or one behind the other on each end of the fuselage (Fig. 4). In the single-rotor helicopter, a small rotor is usually mounted on the tail assembly at right angles to the main rotor to counteract the torque and to facilitate control. In twin-rotor helicopters, the main rotors turn in opposite directions for the same reason. The diameter of main rotor systems ranges from a minimum of 13 ft for the Navy XRON-1. one-man helicopter to 82 ft for the Air Force YH-16 Transporter. In flight, the main rotor blades rotate at a wing tip speed approaching that of the speed of sound. The blades constantly change in pitch as they progress from a forward to a rear position and are subject to high bending and torsional loads with changes in speed and direction of flight. Because of the severe stresses imposed, early attempts to build all-metal main rotor blades by conventional means, such as bolting, riveting, and spot-welding, were abandoned because blades built in this fashion proved to have inadequate fatigue strength, aerodynamic smoothness, and durability.
Stevens, Jay M.
Materials Engineer, Bureau of Aeronautics, Washington, D. C.