Analysis and testing of a Mach-scaled helicopter rotor in hover with piezoelectric bender-actuated trailing-edge flaps

This paper presents an analytic model and validation tests of Froude and Mach scaled rotors featuring piezoelectric bender actuated trailing-edge flaps for active vibration suppression. A finite element structural formulation in conjunction with time domain unsteady aerodynamics is used to develop the rotor blade and bender-flap coupled response in hover. The analysis accounts for the aerodynamic, centrifugal, inertial and frictional loads acting on the coupled bender-flap-rotor system in hover. To investigate the feasibility of piezo-bender actuation and validate the analytic model, a 6 foot diameter, two-bladed Froude scaled rotor with piezo-bender actuation is tested on the hover stand. Flap deflections of +/- 4 to +/- 8 degrees, for 1 to 5 /rev bender excitation were achieved at the Froude scaled operating speed of 900 RPM. The trailing-edge flap activation resulted in a 10% variation in the rotor thrust levels at 6 degrees collective pitch. The analytic model shows good correlation with experimental flap deflections and oscillatory hubloads for different rotor speeds and collective settings. Based on the analytic model, two Mach scaled rotor blades with piezo-bender actuation are designed and fabricated. To achieve the desired flap performance at Mach scale, an 8-layered, tapered bender is fabricated. The bender performance is further improved by selectively applying large electric fields in the direction of polarization for individual piezoceramic elements. A radial bearing is incorporated to reduce frictional loads at the blade-flap interface. Preliminary testing of the Mach scaled blades at 900 RPM in a vacuum chamber revealed negligible degradation in flap performance because of centrifugal and frictional loading. The analysis predicts that flap deflections of +/- 5 to +/- 10 degrees for 1 to 4 /rev bender excitation can be achieved at the Mach scaled operating speed of 2100 RPM. Future work will involve hover and wind tunnel testing at Mach scaled operating speeds.