Challenges of flight control in a compound helicopter

Piasecki Aircraft Corporation is flight demonstrating the X-49A compound helicopter to research the advantages of compound rotorcraft technology. Based on a modified SH-60F, the redundant control surfaces and lift and thrust devices of the X-49A provide increased performance, reduced vibration and fatigue loads, and the opportunity to expand the gross weight/speed envelope beyond that which conventional helicopters generally achieve. In terms of performance, the wing and 8-ft diameter Vectored Thrust Ducted Propeller (VTDP) offload the rotor in forward flight to achieve higher speeds or reduced power at the same speed as a conventional helicopter. Redundant control effectors (flaperons, elevator, propeller pitch) enable the aircraft to be trimmed for maximum performance, enhanced maneuvering, or minimum vibration and fatigue loads. This paper presents a summary of the first flight test phase, including data on load redistribution due to compounding. The simulation used for performance prediction and piloted tests is discussed. Initial results of an effort to integrate redundant flight controls with the intrinsic aeromechanical characteristics of the X-49A to allow the full benefits of compounding to be explored are also presented. The existing mechanical control system is augmented with a programmable flight control system to enable redundant control to be flexibly allocated and overcome inherent limitations of the mechanical controls. The flight control challenges associated with the complex aerodynamic interactions and mixing of redundant controls are discussed.