Azimuthal variation of the blade element autorotation conditions in helicopter gliding

Answer 1. During helicopter gliding in the main rotor autorotative regime, the flow conditions about the blade element change continuously. Therefore, the autorotation conditions will also change. The resultant velo­city (=к+ V sin ф) will increase continuously for the advancing blade. This leads to increase of the elementary force AR and to acceleration of the auto­rotation. The resultant velocity of each blade element of the retreating blade decreases and reaches the minimum (W = u – V) at the ф = 270° azimuth. Therefore, the force AR also decreases, and the autorotation will be decelera­ted. Each blade element becomes alternately "driving", then "driven". Most of the elements of the advancing blade will be "driving"; most of those of the retreating blade will be "driven".

Answer 2. During helicopter gliding, the autorotation conditions of each element depend on the blade azimuth. With change of the azimuth there is a change of the element resultant velocity (W=u+V sin ip). At the 90° azimuth this velocity reaches its maximal value, therefore, the angle of attack incre­ment is minimal (Да = arc tg V ^ sin 0/ior + V ^ cos 0) . The force vector AR tilts aft, and the autorotation will be decelerated.

At the 270° azimuth the element resultant velocity is minimal (Да = arc tg

V, sin 0/u – V, cos 0). The forward tilt of the force vector AR will be gl gl

maximal, and the autorotation will be accelerated. The conclusion is that during gliding the retreating blade creates a driving torque and "drives" the advancing blade, which develops a retarding torque.

Answer 3. The blade element characteristics during helicopter gliding are determined by two factors: azimuthal variation of the resultant flow

velocity over the blade element, and the presence of flapping motions and vertical flapping velocity.

At the 90° azimuth the resultant velocity is maximal; the vertical flapping velocity is also maximal and directed upward. The angle

V V

gl sin 0 – fl

u + V, cos 0
gl

is minimal; therefore, the force vector AR is tilted aft, and the autorotation will be decelerated.

At the 270° azimuth the resultant flow velocity is minimal, and the vertical flapping velocity is maximal, but directed downward. • The angle [4]

is maximal; therefore, the force vector AR is directed forward, and the blade element autorotation will be accelerated.

The conclusion is that the retreating blade develops a driving torque while the advancing blade develops a retarding torque, but the rotor autoro­tation will be steady-state.