Conceptual Design of a High-Speed Variable Configuration Compound Helicopter

In response to the 31th AHS Student Design Competition Request for Proposal, one of the Politecnico di Milano entrants teams in graduate category submitted the co-X HERMES project. The RFP, titled X-VTOL, was inspired to DARPA 2013 program called VTOL X-PLANE, soliciting proposal on the design of a VTOL experimental aircraft capable to increase top speed without sacrificing range, payload and the ability to operate in hover. The co-X HERMES is a fully compounded manned helicopter with coaxial rotor and pushing propeller unit mounted in tail, adopting dedicated design features in order to go beyond the limitations of this architecture and fulfill the RFP requirements. The aircraft works without thrust compounding from hover to about 140 kt, then the transition maneuver is performed and vehicle switch from helicopter to airplane modality to attain higher speed. The change of configuration involves: the main wing span, the rotors diameter and their angular velocity. In this paper the vehicle conceptual design process will be described, providing details about the most critical systems: variable rotor diameter, variable wing span and transmission. The performance evaluation will be presented, demonstrating the RFP requirements fulfillment, together with the results of conducted rotor multibody analysis and vehicle static stability analysis. Moreover preliminary considerations about the transition maneuver, the flight control system and the emergency operations will be discussed.