Weight, lift, thrust, and drag are the four forces acting on a helicopter. The cyclic for directional
control, the collective pitch for altitude control, and the antitorque pedals to compensate for torque are
the three main controls used in a helicopter.
Torque is an inherent problem with single-main-rotor helicopters. Gyroscopic precession occurs
approximately 90 in the direction of rotation from the point where the force is applied. Dissymmetry of
lift is the difference in lift that exists between the advancing and retreating half of the rotor disc.
Settling with power can occur when the main rotor system is using from 20 to 100 percent of the
available engine power, and the horizontal velocity is under 10 knots. At a hover, the rotor system
requires a great volume of air upon which to work. This air must be pulled from the surrounding air
mass. This is a costly maneuver that takes a great amount of power.
Ground effect is improved performance when hovering near the ground at a height of no more than
approximately one-half the main rotor diameter. Translational lift is achieved at approximately 18
knots, and the rotor system receives enough free, undisturbed air to improve performance. At the instant
translational lift is in effect and the hovering air-supply pattern is broken, dissymmetry of lift is created.
Autorotation is a means of safely landing a helicopter after engine failure or other emergencies. A
helicopter transmission is designed to allow the main rotor to turn freely in its original direction if the