Chapter Objective: Upon completion of this chapter, you will have a working knowledge of the theory of operation and the maintenance requirements for rotary-wing (helicopter) aircraft.

The helicopter has become a vital part of naval aviation. The helicopter, known also as a rotary-wing aircraft, has many military applications. It has antisubmarine warfare (ASW) and search and rescue functions, as well as minesweeping and amphibious warfare functions. The advantages of the helicopter over conventional aircraft are that lift and control are relatively independent of forward speed. A helicopter can fly forward, backward, sideways, or remain in stationary flight above the ground (hover). Helicopters do not require runways for takeoffs or landings. The decks of small ships or open fields provide an adequate landing area.

Learning Objective: Recognize the princi-ples of aerodynamics peculiar to the flight of rotary-wing aircraft.

The same basic aerodynamic principles apply to rotary-wing aircraft as fixed-wing aircraft. The main difference between the two types of aircraft is in the way lift occurs. The fixed-wing aircraft gets its lift from a fixed airfoil surface. The helicopter gets lift from rotating airfoils called rotor blades. The word helicopter comes from Greek words meaning rotating wing. A helicopter uses one or more engine-driven rotors, from which it gets lift and propulsion.

The main rotor of a helicopter consists of two or more rotor blades. The airfoils of a helicopter are perfectly symmetrical. This means that the upper and lower surfaces are alike. This fact is one of the major differences between a fixed-wing aircrafts airfoil and the helicopters airfoil. The airfoil on a fixed-wing aircraft has a greater camber on the upper surface than on the lower surface. The helicopters airfoil camber is the same on both surfaces. See figure 10-1. Helicopters have symmetrical airfoils because the center of pressure across its surface should not move. On the fixed-wing airfoil, the center of pressure moves fore and aft, along the chord line. The center of pressure changes with changes in the angle of attack. If this type of airfoil was on a rotary-wing aircraft, it would cause the rotor blades to jump around uncontrollably. With the symmetrical airfoil, this undesirable effect does not exist. The airfoil, when rotated, travels smoothly through the air. Rotor lift can be explained by either of two theories. The first theory uses Newtons law of momentum. Lift results from accelerating a mass of air downward. This action is similar to jet thrust, which develops by accelerating a mass of air out the exhaust. The second theory is the blade element theory. The airflow over an airfoil section (blade element) of the rotor blade acts the same as it does on a fixed-wing aircraft. The simple momentum theory determines only the lift characteristic, while the blade element theory gives both lift and drag characteristics. This theory gives us a more complete picture of all the forces acting on a rotor blade.

Lift changes by increasing the angle of attack or pitch of the rotor blades. This action produces enough lift to raise the helicopter off the ground and keep it in the air. On a helicopter, when the rotor is turning and the blades are at zero angle of attack, no

Figure 10-1.Center of pressure.

lift is developed. This feature provides the pilot with complete control of the lift developed by the rotor blades.

One assumption made is that the lift depends upon the entire area of the rotor disc. The rotor disc area is the area of the circle, the radius of which is equal to the length of the rotor blade. Engineers determined that the lift of a rotor is in proportion to the square of the length of the rotor blades. The desirability of large rotor disc areas is readily apparent. However, the greater the rotor disc area, the greater (he drag, which results in the need for greater power requirements.