ROTOR CONTRIBUTIONS TO STATIC AND DYNAMIC STABILITY. Changes to blade lift
The basic aerodynamics of rotor blades are similar to a conventional wing. At a given radial location the lift generated will depend on the flow velocity and incidence. The actual flow velocity and incidence will arise from the interaction of the rotational speed of the rotor, the inflow velocity, the airspeed, rate of climb or descent of the vehicle and the pitch setting of the blades. The variation of lift with changes in airspeed, vertical speed and blade pitch are best discussed by example. For simplicity this discussion will be restricted to the advancing and retreating blade tip (r = R and ^ = 90° and 270°). Consider a hovering rotor that is subjected to the following:
at = 90° and a reduction at = 270°, will result. This will change the AOA in a similar sense resulting in more lift being generated at ^ = 90° and less at ^ = 270°.
Now consider the same three disturbances applied to a rotor in trimmed forward flight with the cyclic pitch arranged to equalize the lift produced around the azimuth.
(1) Disturbance along the longitudinal axis. Suppose the rotor is subjected to a disturbance equivalent to the rotor developing a forward airspeed increment. The effect of the disturbance is similar to that observed in the hover: more lift is generated on the advancing side and less on the retreating side.
(2) Disturbance along the vertical axis. If the rotor develops a sink rate the inflow velocity component is reduced and the AOA consequently increased. Since the rotor is now in a combination of pure vertical flight and forward flight the increase in lift is not equally distributed around the azimuth due to the advancing/retreating effect.
(3) Change in blade pich. As before suppose the rotor is disturbed such that the swash plate is moved instantly and the rotor attitude is initially unchanged. If the swash plate is tilted nose-up an increase in blade pitch at ^ = 90°, and a reduction at ^ = 270°, will result. This will change the AOA in a similar sense resulting in more lift being generated at ^ = 90° and less at ^ = 270°. The resulting change in the pitch attitude of the rotor is discussed below.
Calculations can be made to illustrate precisely how changes in airspeed, rate of climb or descent and pitch affect the lift generated on a blade. A rotor of 5.5 m radius, rotating at 35 rad/s and with a lift curve slope of0.1/deg was used to produce the data contained in Tables 4.6 and 4.7.
The changes in lift described in these tables will cause the rotor blades to move about their flapping hinge, or bend within their flapping flexural element. An increase in lift will cause the blade to flap upwards and vice-versa. A rotor can be considered as a heavily damped system operating at (or close to) its natural frequency and therefore there will be approximately a 90° phase lag between input and output. The net result is that if an increase in lift reaches its maximum value at the point of maximum tangential velocity the blade will reach the point of maximum upwards flapping over the nose, that is approximately 90° later.
Table 4.6 Variations in lift:hover.
Table 4.7 Variations in lift: 120 kts.