Flight Dynamics and Control of a Helicopter with a Cycloidal Rotor as Torque/Thrust Compounding System

This study investigates the integration of a cycloidal tail rotor as a replacement for the conventional tail rotor in helicopter configurations, focusing on its dual functionality as both an antitorque mechanism and an auxiliary propulsion system. Unlike traditional tail rotors, the cycloidal tail rotor can direct thrust in any direction perpendicular to its axis of rotation, enabling enhanced maneuverability, including the ability to hover at arbitrary pitch attitudes, assist in forward flight, and execute rapid acceleration and deceleration. The cycloidal tail rotor was modeled for flight simulation and incorporated into a generic multi-rotor/wing flight dynamics code to simulate two configurations of a utility helicopter similar to an H-60: one with a conventional tail rotor and the other with a cycloidal tail rotor. A design optimization study defined the geometry of the cycloidal tail rotor, followed by trim, performance, and stability analyses. Results showed that the cycloidal tail rotor configuration exhibited similar trim characteristics to the conventional tail rotor, with minor differences in control inputs and power consumption. The cycloidal tail rotor provided additional propulsive force in forward flight, reducing the pitch-forward requirement and offloading the main rotor, leading to slightly higher power consumption in hover but reduced power requirements at high speeds. Additionally, the cycloidal rotor enabled trimming at arbitrary pitch attitudes. Stability and frequency response analyses revealed minimal differences in flight dynamics, suggesting the feasibility of integrating a cycloidal tail rotor without significantly altering stability or handling qualities. Dynamic inversion control laws adopting pseudo-inverse control allocation demonstrated the ability to reallocate control effort between the main rotor and the cycloidal rotor, with the cycloidal tail rotor acting as a pusher propeller. This reduced the main rotor workload and allowed for a less pronounced nose-down/up pitch attitude during aggressive acceleration/deceleration maneuvers.