Whirl Flutter Predictions for Distributed Propulsion Tiltrotor Congurations

A family of generic hingeless hub distributed propulsion tiltrotor configurations was modeled in RCAS and studied for whirl flutter along with a baseline conventional configuration for comparison. A sensitivity study was carried out using the baseline model to provide a fundamental understanding. The objective was to examine whirl flutter instability for a range of design variables to provide guidelines to designers. Wing torsion and rotor ap stiffness significantly impacted the instability speed. Reducing wing beam stiffness interestingly stabilized the system due to lower participation of wing torsion motion in the beam mode. Various distributed propulsion tiltrotor configurations were analyzed. Replacing the large rotor at the wing tip with two small rotors of half the disk area (same disk loading) greatly stabilized the system and pushed the instability speed by more than 100%. Placing these rotors at the wing tip in a coaxial conflguration also improved stability, but not as much as distributing them over the wing. It appears that distributed propulsion tiltrotor conflgurations can be expected to exhibit better stability characteristics than traditional tiltrotor aircraft; however, analysis for a real aircraft is necessary.