Dissymmetry of lift

Dissymmetry of lift refers to unequal lift generation on opposite sides of a rotor disc in single-rotor helicopters and autogyros during forward flight. This phenomenon occurs because advancing blades (moving in the same direction as the helicopter) experience higher airspeeds than retreating blades (moving opposite to the helicopter's direction). As a result, advancing blades generate more lift than retreating blades. In hovering flight, all rotor blades have equal airspeed and thus produce equal lift. However, during forward motion, the advancing blade tip speed relative to the air increases (rω + v), while the retreating blade tip speed decreases (rω - v). Since lift is proportional to the square of airspeed, the advancing blade generates significantly more lift than the retreating blade. This imbalance causes a pitch-backward force rather than a roll. At high speeds, the retreating blade may stall due to insufficient airspeed, leading to an uncontrollable roll and pitch-up. Additionally, advancing blades may reach supersonic speeds at very high forward velocities, further limiting maximum safe speed (never-exceed speed, VNE). To counteract dissymmetry of lift, rotor blades are designed to flap: advancing blades tilt upward, reducing their angle of attack and lift, while retreating blades tilt downward, increasing their angle of attack and lift. Cyclic feathering also adjusts blade angles during rotation to balance lift. Coaxial rotors (rotating in opposite directions) cancel each other's lift imbalances, but tandem rotor systems still experience dissymmetry and rely on automatic feathering systems to maintain stability at higher speeds. In summary, dissymmetry of lift ...