# What determines the blade element autorotation conditions?

Answer 1. The blade element autorotation conditions are determined by the inclination of the elemental aerodynamic force relative to the main rotor hub axis. If the force is directed parallel to the hub axis, its projection on the plane of rotation equals zero, and the autorotation will be steady-state.

If the force vector AR is inclined forward relative to the hub axis by the angle y, the autorotation will be accelerated. If the force vector AR is inclined aft by the angle y, the autorotation will be decelerated. The inclina­tion of the vector AR depends on the blade element pitcn and angle of attack increment (Да = arc tg /u) . The lower the pitch and the larger Act, the larger the forward tilt of the vector AR.

Answer 2. The blade element autorotation conditions are determined by the aerodynamic efficiency К = Y/X. The higher the aerodynamic efficiency, the smaller the aerodynamic efficiency angle 0^, the larger the forward tilt of the force vector AR, and the higher the main rotor rpm will be. Since the

aerodynamic efficiency and the efficiency angle depend on the angle of attack, the autorotation conditions are determined by the angle of attack. At the optimal angle of attack, the force vector AR has the maximal forward tilt, therefore, the autorotative regime will be accelerated. At angles of attack greater than and less than the optimal value, the blade element autorotation will be decelerated.

Answer 3. The blade element autorotative conditions are determined by the tilt of the elemental resultant aerodynamic force R relative to the main rotor hub axis. If this force vector is tilted forward, the blade element will have accelerated autorotation; if it is tilted aft the autorotation will be decelerated. If the direction of AR is parallel to the hub axis, the autorota­tion is steady-state. The tilt of the force AR depends on the angle of attack increment Да formed as a result of the vertical rate of descent (Да = arc tg ^des^Ur0" .The higher the vertical rate of descent, the larger Да, the larger the tilt of the force AR, and the higher the main rotor rpm.