Higher Harmonic Control: A Historical Perspective

Higher harmonic control (HHC) is an approach for achieving reduced helicopter vibration by controlling the vibratory rotor airloads in such a way that the fuselage excitation is minimized. This paper is a historical look at how a program aimed at helicopter vibration reduction started as an outgrowth of fixed wing flutter suppression at NASA Langley Research Center, proved the HHC concept on aeroelastically scaled wind tunnel models and went on to demonstrate viability in full-scale flight testing on the OH-6A helicopter in 1982. Following the OH-6A flight tests, the helicopter research community was stimulated to prove the effectiveness of HHC on different configurations through analysis, wind tunnel tests, and flight tests. All of these investigations have shown HHC to be effective in reducing vibration to levels not attainable with conventional vibration control methods and without any detrimental side effects. HHC development has progressed to the point that the technology provides one more option to address the ever-present vibration problem in helicopters. The literature demonstrates that helicopter ride quality equivalent to that of fixed wing aircraft is attainable with application of HHC.

Time-Domain Adaptive Higher Harmonic Control for Rejection of Sinusoidal Disturbances

This paper presents two new time-domain feedback controllers that reject sinusoidal disturbances with known frequencies acting on an asymptotically stable linear time-invariant (LTI) system. The first controller is time-domain higher harmonic control (TD-HHC), which is effective for uncertain LTI systems. The second controller is time-domain adaptive higher harmonic control (TD-AHHC), which is effective for completely unknown LTI systems. TD-HHC requires an estimate of the control-to-performance transfer function evaluated at the disturbance frequencies. In contrast, TD-AHHC does not require any information regarding the LTI system. We analyze the stability and closed-loop performance of TD-HHC and TD-AHHC. For both TD-HHC and TD-AHHC, we show that the controller asymptotically rejects the disturbance. We present numerical simulations comparing TD-HHC and TD-AHHC with frequency-domain higher harmonic control (FD-HHC), which is an existing frequency-domain controller for rejection of sinusoidal disturbances. We also present results from acoustic disturbance rejection experiments, which demonstrate the practical effectiveness of both TD-HHC and TD-AHHC.

Investigation of Linear Higher Harmonic Control Algorithm for Rotorcraft Vibration Reduction

A linear quadratic Gaussian controller for active vibratory loads reduction in helicopters is proposed based on a revisited higher harmonic control input by active trailing-edge flaps. Conventional individual blade control input is redefined using N-1/rev inter-blade phase lead, N/rev collective, and N+1/rev inter-blade phase lag signals where 1/rev frequency modulation originate from the multi-blade coordinate transform. A Mach-scaled flap blade is designed and analyzed by the multi-body dynamics analysis DYMORE. A linear time-invariant representation is identified from N/rev envelopes of the input and output responses obtained by DYMORE analysis. A MATLAB/Simulink closed-loop control simulation is designed using the identified state-space realization. The N/rev vibratory loads are reduced up to 52% with flap deflections and the linear control results match well with the nonlinear responses obtained from DYMORE. Furthermore, the multi-variable closed-loop stability estimated by the loop transfer functions using disk margin analysis reveals sufficient gain and phase margins.

Data Transfer Schemes in Rotorcraft Fluid-Structure Interaction Predictions

For a CFD (computation fluid dynamics)/CSD (computational structural dynamics) coupling, appropriate data exchange strategy is required for the successful operation of the coupling computation, due to fundamental differences between CFD and CSD analyses. This study aims at evaluating various data transfer schemes of a loose CFD/CSD coupling algorithm to validate the higher harmonic control aeroacoustic rotor test (HART) data in descending flight. Three different data transfer methods in relation to the time domain airloads are considered. The first (method 1) uses random data selection matched with the timewise resolution of the CSD analysis whereas the last (method 2) adopts a harmonic filter to the original signals in CFD and CSD analyses. The second (method 3) is a mixture of the two methods. All methods lead to convergent solutions after a few cycles of coupling iterations are marched. The final converged solutions for each of the data transfer methods are correlated with the measured HART data. It is found that both method 1 and method 2 exhibit nearly identical results on airloads and blade motions leading to excellent correlations with the measured data while the agreement is less satisfactory with method 3. The reason of the discrepancy is identified and discussed illustrating CFD-/CSD-coupled aeromechanics predictions.

A comparative assessment of vibration control capabilities of a L-shaped Gurney flap

ABSTRACTThis work presents the capabilities of a novel L-shaped trailing-edge Gurney flap as a device for vibration reduction. The primary effect of this L-tab is represented by a modification of the reference aerofoil mean line shape through by two counter-rotating vortical structures created at the trailing edge. The comparison of the aerodynamic loads generated by the novel L-tab Gurney flap and a classical trailing-edge flap allows to estimate the ranges of reduced frequency where the L-tab is expected to perform better than a trailing edge flap and vice versa. Linear aerostructural models for a typical section representative of a helicopter blade equipped with a partial-span L-tab or a trailing-edge flap are built, and a higher harmonic control algorithm is applied. Performance are compared between the two devices to reduce separately the N/rev harmonics of the blade root rotating frame vertical force, flapping and feathering moments. The attainment of similar results with classical trailing-edge device is a further confirmation of the potential feasibility of this novel L-tab as an effective alternative means for vibration reduction on rotor blades.

On-Blade Control of Rotor Vibration, Noise, and Performance: Just Around the Corner? The 33rd Alexander Nikolsky Honorary Lecture

A concise review of the active control approaches for vibration reduction in rotorcraft is presented. Next, the evolution and status of higher harmonic control and pitch link actuated individual blade control is presented since these serve as the foundations of on blade control. Despite the success of these approaches, demonstrated by both full-scale wind tunnel and flight tests, higher harmonic control and pitch link actuated individual blade control have not managed to earn their way onto a production helicopter. An alternative, on blade control, is defined as a special implementation of individual blade control, where the control surfaces are located on the rotating blade and each blade has its own controller. A concise description of four on blade devices: (1) the actively controlled flap, (2) the active twist rotor, (3) the active tip rotor, and the (4) deployable Gurney flap, or microflap, is presented. An outline of an aeroelastic response modeling capability used to simulate active vibration and noise reduction using flaps or microflaps is presented. The simulation is a thread that links the various parts of the paper. Next, selected results from simulations and scale wind tunnel model tests on active flaps are used to provide insight on the operational and modeling aspects of these systems. Full-scale wind tunnel and flight tests are presented as culmination of the research effort invested in active flap rotors. Then, the evolution and application of the active twist rotor, and deployable Gurney flaps, or microflaps, is presented. The paper concludes with lessons learned and speculation about the potential implementation of on blade control on production rotorcraft.