Open-loop hover and wind tunnel testing of Mach-scaled rotor with trailing-edge flaps

This paper presents the design, fabrication and open-loop testing of an active rotor system with trailing-edge flaps for individual blade control of helicopter vibration. First, the University of Maryland Advanced Rotorcraft Code (UMARC) was used to size the trailing-edge flap and to determine the flap deflection requirements for vibration suppression in the wind tunnel. Next an analytic model for the coupled actuator-flap-rotor system was used to design a multilayer piezoelectric bender configuration that was capable of meeting the flap deflection requirements. Based on the design study, a matched set of six Machscaled active rotor blades were fabricated in-house. The 4-bladed rotor model was tested in the open-loop mode in hover and then in the wind tunnel using a Bell-412 Mach-scaled hub. Flap deflections of ±4 to ±6 deg were recorded in the 1-5/rev frequency range at the Mach-scaled operating speed of 1800 RPM. The flap deflection increased to ±23 deg at 8/rev due to actuator resonant amplification. Rotor collective pitch and advance ratio were found to have negligible impact of actuator performance. The maximum control effectiveness was observed close to the blade flapbending and torsion natural frequencies. For 3/rev actuator excitation, oscillatory thrust levels of up to ±36 Ibs (60% steady rotor thrust at 6 deg collective) were recorded, thereby demonstrating the open-loop control authority of the actuator-flap at Mach-scale.