Open-Loop Hover Testing of a Smart Rotor Model

Abstract

Thedevelopmentand open-loop hovertesting ofa smartrotormodelwithtrailing-edgee apsforindividualblade control of helicopter vibration are presented. First, the University of Maryland Advanced Rotorcraft Code was used to sizethetrailing-edgee apandto determinethee ap-dee ectionrequirementsforvibration suppression in the wind tunnel. Next an analytic model for the coupled actuator-e ap-rotor system was used to develop a multilayer piezoelectric bender cone guration that was capable of meeting the e ap-dee ection requirements. Based on this study, a matched set of six smart rotor blades were fabricated in-house. The four-bladed smart rotor model was tested in the open-loop mode in hover using a Bell-412 Mach-scaled hub. Flap dee ections of §4 to §6 deg were recorded in the 1 ‐5/rev frequency range at the model operating speed of 1800 rpm. The e ap dee ection increased to §23 deg at 8/rev because of actuator resonant amplie cation. Rotor collective pitch was found to have negligible impact on actuator performance. The maximum control effectiveness was observed close to the blade e atwise bending and torsion natural frequencies. For 3/rev actuator excitation oscillatory thrust levels of up to §10 lb (45 N) (60% steady rotor blade thrust at 6-deg collective ) were recorded, thereby demonstrating the open-loop control authority of the actuator-e ap system. I. Introduction T HEhelicopterspendsa largeportionofitsoperation in forward e ight, resulting in an aerodynamic asymmetry between the advancingandretreatingsidesoftherotordisk.Asaresultofthis,the rotor e owe eld is extremely complex and can include transonic e ow ontheadvancingbladetip,dynamicstallontheretreatingsideofthe disk, highly yawed and reversed e ows, and blade-wake interactions with tip vortices from preceding blades. This highly complex and nonsteady aerodynamic environment and the dynamic response of the long and e exible rotor blades result in large vibratory forces, which are e ltered through the hub to the fuselage. Typically for an N-bladed rotor, the dominant N i1, N and N C1/rev blade loads are transmitted to the fuselage as an N/rev forcing. Reduction in vibration and noise levels of the helicopter yield benee ts in terms of improved passenger comfort, reduced crew fatigue, improved community acceptance, and increased fatigue life of structuralcomponents. Currently, passive isolators and absorbers are routinely used to reduce vibration. However these devices cause signie cant weight penalties (up to 3% of gross weight ) and rapidly degrade in performance away from the tuned e ight condition. In contrast, active vibration reduction schemes offer the promise of achieving reliable vibration reduction over a wide range of operating conditions with lower weight penalty than conventional passive methods. There are two types of rotating frame active control systems: higher harmonic control (HHC) and individual blade control (IBC). HHC involves excitation of the swashplate at N/rev. The major disadvantage of HHC is the high actuation power required to pitch the rotor blades particularly in extreme e ight conditions and large weight penalties associated with the hydraulic actuators. Another important limitation is that HHC is limited to N/rev excitation of the swashplate. However, for performance improvement, dynamic stall alleviation, acoustic control, and alleviation of gusts and ma