Using the Complete Authority of Multiple Active Trailing-edge Flaps for Helicopter Vibration Control

Abstract

In the current work, the application and advantages of multiple active trailing-edge flaps for helicopter vibration control in forward flight are investigated. Small active flaps of this type can be actuated using smart materials. A comprehensive aeroelastic analysis is used to model a helicopter with multiple trailing-edge flaps. The objective function containing the hub vibratory loads and flap control effort is minimized by an optimal controller. The control effort is differentially weighted for different flaps, such that all trailing-edge flaps are exploited to their full authority and limited to ±2 degrees peak deflection. Numerical results are obtained for one, two and three flaps and their optimal locations identified. A reduction of 76% in the hub vibration level is achieved at an advance ratio (nondimensional forward speed) of 0.3 using three flaps. For the four-bladed, soft-inplane, hingeless rotor considered in this work, it is observed that the extent of hub vibration reduction is directly related to the effect of trailing-edge flap motion on the contribution of the second flapwise bending mode to blade motion. It is also shown that multiple flaps can effectively modify the participation of second flapwise bending mode at a much lower control power than is needed with single and dual flap configurations. Furthermore, multiple flaps are shown to allow redundancy in operation and are considerably more effective than single flaps in low speed transition flight regimes because of their capability in distributing the airloads.