lift is developed. This feature provides the pilot with complete control of the lift developed by the rotor blades. ROTOR AREA 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 the drag, which results in the need for greater power requirements. ^.-. / PITCH OF ROTOR BLADES If the rotor is operated at zero pitch (flat pitch), no lift will develop. When the pitch increases, the lifting force increases until the angle of attack reaches the stalling angle. To even out the lift distribution along the length of the rotor blade, it is common practice to twist the blade. With the twist, a smaller angle of attack results at the tip than at the hub. SMOOTHNESS OF ROTOR BLADES Tests have shown that the lift of a helicopter increases by polishing the rotor blades to a mirrorlike surface. By making the rotor blades as smooth as possible, the parasite drag reduces. Dirt, grease, or abrasions on the rotor blades cause increased drag, which decreases the lifting power of the helicopter. DENSITY ALTITUDE In formulas for lift and drag, the density of the air is an important factor. The mass or density of the air reacting in a downward direction causes the lift that supports the helicopter. ‘Density is dependent on two factors. One factor is altitude, since density varies from a maximum at sea level to a minimum at high altitude. The other factor is atmospheric changes. Because of the atmospheric changes in temperature, pressure, or humidity, density of the air may be different, even at the same altitude. TORQUE Although torque is not unique to helicopters, it does present some special problems. As the rotor turns in one direction, the fuselage rotates in the opposite direction. Newton’s third law of motion (every action has an equal and opposite reaction) applies. This tendency for the fuselage to rotate is known as the torque effect. Since the torque effect on the fuselage is a direct result of engine power, any change in power changes the torque. The greater the engine power, the greater the torque. There is no torque when the rotary-wing head is not engaged or when the engine is not operating. The usual method of counteracting torque in a single main rotor is by a tail (antitorque) rotor. This auxiliary rotor mounts vertically, or near vertical, on the outer portion of the tail boom. The tail rotor and its controls serve as a means to counteract torque, and it provides a means to control directional heading. See figure 10-2. DISSYMMETRY OF LIFT Dissymmetry of lift  is the difference  in lift existing between the advancing blade half of the disc and the retreating blade half. The disc area is the area swept by the rotating blades. Dissymmetry is created . . . . . . *.**..*:.., *.*,......*, . *.*.*.*.*.*. ..*...m.*.*. ..:.:.:.:.:. *.-**.'.*,- S.'.... ..,. DiRECTiON :.‘A’.‘.’ . v.*.*.*. .‘.‘.‘.‘. OF TORQUE ot . . ...‘. :.>:.:. TAIL ROTOR THRUST -.*.*2.. ‘A*,‘, ‘.‘A*,~ ‘........ TO COMPENSATE .:.:.:.: FOR TORQUE Figure lo-2.-Torque reaction. 10-2