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.

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.

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.

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. Newtons 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 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

Figure 10-2.Torque reaction.

by horizontal flight or by the wind when the helicopter is hovering. When hovering in a no-wind condition, the speed of the relative wind in relation to the rotor is the same. However, the speed reduces at points closer to the rotor hub, as shown in figure 10-3. When the helicopter moves into forward flight, the relative wind moving over each blade becomes a combination of the rotor speed and the forward movement. The advancing blade is then the combined speed of the blade speed and helicopter speed. While on the opposite side, the retreating blade speed is the blade speed minus the speed of the helicopter. For example, figure 10-4 shows a helicopter moving forward at 100 mph. The advancing blade has a tip speed of 350 mph plus the helicopter speed of 100 mph, or 450 mph. The retreating blade has a tip speed of 350 mph minus the helicopters speed of 100 mph, or 250 mph. Hovering over one spot in a 20 mph headwind is the same as flying forward at a speed of 20 mph.

During forward flight or hovering in a wind, the lift over the advancing blade half of the rotor disc is greater than the retreating half. This greater lift would cause the helicopter to roll unless something equalized the lift. One method of equalizing the lift is through blade flapping.