Hover Test of a Mach-Scale Rotor Model with Active Blade Tips

Abstract

The smart active blade tip (SABT) rotor incorporates blade tips that can be independently pitched with respect to the main blade. The blade tips are used as on-blade control surfaces for vibration reduction. The experimental testing focussed on a small scale four-bladed rotor on a bearingless Bell-412 hub. The fabricated Mach-scale active-tip rotor has a diameter of 1.524 m, a blade chord of 76.2 mm and 10% span active tips. The blade tips are driven by a novel piezo-induced bendingtorsion coupled actuator beam, located spanwise in the hollow mid-cell of the main rotor blade. In 2000 rpm hover (tip Mach 0.47), at 2 deg collective, and for an actuation of 150 V rms , the measured blade tip deflection (pitch angle relative to the main blade) at the first four rotor harmonics is between ±2.1 and ±2.9 deg, increasing to ±5.7 deg at 5/rev with resonant amplification. These blade tip deflections generate respective oscillatory rotor thrust coefficients between 0.9 · 10 −3 and 1.6 · 10 −3 at the first four rotor harmonics, increasing to 2.2 · 10 −3 at 5/rev (single blade tip actuation). The corresponding thrust authority is 19, 13, 23, 19 and 33% at the first 5 rotor harmonics (normalized with respect to steady thrust at nominal 8 deg collective, 378 N). The analytic model predicts the 150 V rms relative blade tip pitch angle and associated change in rotor thrust within 12% at the first five rotor harmonics (except at 4/rev where the model underpredicts more). The active blade tip rotor was also used as a testbed for the evaluation of an adaptive neural-network based control algorithm. Effective reduction of 1/rev hover imbalance hub loads was demonstrated. In addition, the controller was used to generate desired multi-frequency control loads on the hub, based on artificial signal injection into the vibration measurement. The research program demonstrates the technical feasibility of the active blade tip concept for vibration reduction and warrants further investigation in terms of closed loop forward flight tests in the wind tunnel and full scale design studies.

The preliminary design of the SHARCS rotor blade

“…This was because the smaller mass of the blade for a Lock number of 5 would not allow for sufficient weight of the actuation system and other components. This decision was consistent with those by Bernhard and Chopra (2002), who had also discussed the construction of a Mach scaled rotor blade with active blade tips. However, in the design presented by those authors a Lock number of only 2.55 was achieved.…”

Active rotor control for helicopters: individual blade control and swashplateless rotor designs

“…The deflecting ends of the actuators are embedded in flexible filler to retain a smooth contour of the airfoil. A further variant is the active pitching tip [13,14]. It may be viewed as a flap with a flap chord to airfoil chord ratio of one.…”

System Identification Technique for Active Helicopter Rotors

“…Various mechanisms have been suggested for active materials application to the helicopter vibration reduction (Giurgiutiu, 2000). From the feasibility tests, potential capabilities about helicopter vibration reduction were substantiated for a number of mechanisms proposed (Rodgers and Hagood, 1998;Cesnik et al, 1999;Prechtl and Hall, 2000;Bernhard and Chopra, 2002). These achievements encourage to design and implement an appropriate closed-loop controller for helicopter vibratory load reduction.…”

“…However, to improve the controller design through preliminary evaluation of closed-loop system stability, further system identification results are also required in the vicinity of the target frequency. Recently, transfer function relationship for some prototypes of active rotor blade was identified experimentally over a certain frequency range by sine-sweep actuation (Rodgers and Hagood, 1998;Prechtl and Hall, 2000;Bernhard and Chopra, 2002). All those tests and the corresponding identification were conducted in hover condition.…”