David G. Zimcik

Active vibration-suppression systems applied to twin-tail buffeting

Buffeting is an aeroelastic phenomenon that plagues high performance aircraft, especially those with twin vertical tails. Unsteady cortices emanate form wing/fuselage leading edge extensions when these aircraft maneuver at high angles of attack. These aircraft are designed such that the vortices shed while maneuvering at high angels of attack and improve the lift-to-drag ratio of the aircraft. With proper placement and sizing of the vertical tails, this improvement may be maintained without adverse effects to the tails. However, there are tail locations and angels of attack where these vortices burst and immerse the vertical tails in their wake inducing severe structural vibrations. The resulting buffet loads and severe vertical tail response because an airframe life and maintenance concern as life cycle costs increased. Several passive methods have been investigated to reduce the buffeting of these vertical tails with limited success. As demonstrated through analyses, wind-tunnel investigations, and full-scale ground tests, active control system offer a promising solution to alleviate buffet induced strain and increase the fatigue life of vertical tails. A collaborative research project including the US, Canada, and Australia is in place to demonstrate active buffet load alleviation systems on military aircraft. The present paper provides details on this collaborative project and other research efforts to reduce the buffeting response of vertical tails in fighter aircraft.

Feedforward piezoelectric structural control : An application to aircraft cabin noise reduction

The use of adaptive feedforward control within the active structural acoustic control framework was applied to the problem of propeller-induced noise and vibration reduction in the passenger cabin of the Bombardier (de Havilland) Dash-8 aircraft. Piezoceramic elements were used for structural actuation, and either vibration or acoustic sensing was employed. Actuators comprised of segmented piezoelectric elements were designed with the objective of reducing the noise and vibration levels at the propeller blade passage frequency (BPF) and the e rst harmonic.Theactuatordesignobjectivewassuppressionoftheoperating dee ectionshapes (ODS)ofthefuselageat the various frequencies by the judicious placement of piezoelectric elements. Using an actuator and sensor design optimized for the BPF together with vibration error sensing, the controller was successful in reducing interior noise in addition to vibration. Further improvement in noise reduction was obtained when acoustic error sensing was employed. Similar optimized designs for actuator and sensors were also found to exist at other frequencies, providing good noise and vibration attenuation. Furthermore, this strategy was successfully applied to noise reduction at two operating frequencies, where suppression of the ODSs at both the BPF and 2 £ BPF was the objective.

Development of a maximum energy extraction control for the smart spring

Most active vibration suppression approaches have attempted to suppress structural vibrations through the use of active material actuators, such as piezoceramic, that are incorporated into a structure to act directly against vibratory loads. These approaches require the actuators to simultaneously supply significant force and deflection to effectively suppress vibration. Unfortunately, successful implementation of these approaches has been hindered by the electromechanical limitations of piezoceramic actuators due to high power requirements in active vibration control applications. The Smart Spring concept is a unique approach that is designed to actively control combinations of dynamic impedance characteristics of a structure, such as the stiffness, damping, and effective mass to suppress vibration. The Smart Spring does not use actuators to perform work directly against excitation loads, but rather adaptively varies the effective structural impedance properties. Therefore, the piezoceramic actuators in the Smart Spring are not required to simultaneously produce large forces and deflections. Thus, the concept requires considerably less power because it enables active vibration control in an indirect manner. This study demonstrates the ability of the Smart Spring to control dynamic impedance characteristics of a structure through numerical simulations and experimental investigations. In addition, the development of a feedback control system is demonstrated. According to the control strategy, the impedance characteristics of the Smart Spring are continuously changing in order to maximize the extraction of the mechanical energy of the system.

Optimization of Piezoelectric Actuator Configuration on a Flexible Fin for Vibration Control using Genetic Algorithms

This study presents a novel approach to optimizing the configuration of piezoelectric actuators for vibration control of a flexible aircraft fin. The fitness (cost) function for optimization using a genetic algorithm is derived directly from the frequency response function (FRF) obtained from a finite element model of the fin. In comparison to existing approaches, this method allows optimization on much more complex geometries where the derivation of an analytical fitness function is prohibitive or impossible. This technique is applied to two optimization problems for vibration control of the fin. First, the position of a single actuator is optimized anywhere within a judiciously pre-determined area of the fin using a genetic algorithm for polynomial surface fitting of the FRF in order to obtain a continuous fitness function. Next, the configuration of a pre-determined number of up to 48 separate actuators is optimized within the same area. The optimization approach is verified against experimental results obtained from a set of 12 actuators fixed to an experimental model of the fin.

Next Generation Active Buffet Suppression System

Buffeting is an aeroelastic phenomenon that is common to high performance aircraft, especially those with twin vertical tails like the F/A-18, at high angles of attack. These loads result in significant random stresses, which may cause fatigue damage leading to restricted capabilities and availability of the aircraft. This paper describes an international collaborative research activity among Australia, Canada and the United States involving the use of active structural control to alleviate the damaging structural response to these loads. The research program is being co-ordinated by the Air Force Research Laboratory (AFRL) and is being conducted under the auspices of The Technical Cooperative Program (TTCP). This truly unique collaborative program has been developed to enable each participating country to contribute resources toward a program that coalesces a broad range of technical knowledge and expertise into a single investigation. This collaborative program is directed toward a full-scale test of an F/A-18 empennage, which is an extension of an earlier initial test. The current program aims at applying advanced directional piezoactuators, the aircraft rudder, switch mode amplifiers and advanced control strategies on a full-scale structure to demonstrate the enhanced performance and capability of the advanced active BLA control system in preparation for a flight test demonstration.

Active Cabin Noise and Vibration Control for Turboprop Aircraft Using Multiple Piezoelectric Actuators

The Active Structural Acoustic Control (ASAC) technique was applied to reduce propeller-induced noise and vibration in the passenger cabin of the deHavilland Dash-8 turboprop aircraft. Piezoceramic elements were used for structural actuation while velocity feedback of the fuselage was achieved through the use of accelerometers. Three actuators comprised of segmented piezoelectric elements were designed with the objective of reducing the noise and vibration levels at the propeller Blade Passage Frequency (BPF). Twelve accelerometers were grouped to effectively form three sensors. Second order classical compensators were designed for each of the three dominant control loops in order to achieve high gain at the BPF, rendering the closed-loop system insensitive to disturbances at the control frequency. The propeller acoustic field on the port side of the aircraft was simulated in a laboratory using a speaker-ring consisting of four loudspeakers. The control system was tested using this acoustic field, producing noise attenuation as high as 28 dB in the interior, and fuselage vibration reduction as high as 16 dB.

Development of Adaptive Seat Mounts for Helicopter Aircrew Body Vibration Reduction

Helicopter aircrew are exposed to high levels of vibration and noise during flight. This paper presents the investigation of adaptive seat mount approaches to reducing vibration on the helicopter seat. A flight test on a helicopter with typical pilot configurations showed that the vibration spectra on the pilot’s helmet not only included the dominant N/rev harmonic peaks of the rotor speed, but also consisted of a low-frequency resonant peak in the frequency range of human abdominal and spine resonant frequencies. Long-term exposure to this vibration may lead to occupational health issues such as damage to the pilot’s spine and neck. In order to address this issue, a novel adaptive seat mount concept was developed to mitigate the vibration levels transmitted to the aircrew. As a proof-of-concept demonstration, a miniature modal shaker was installed between the cabin floor and the seat bottom as an adaptive mount that provided the actuation authority. The objective was to reduce the vertical vibration transmitted to the aircrew helmet in order to decrease aircrew neck and spine injuries that are caused by the transmitted vibration. Extensive closed-loop control tests have been conducted on a full-scale helicopter seat and a mannequin with varying physical properties. A 10,000 lb(f) mechanical shaker was used to provide representative helicopter vibration profiles to the seat. Significant vibration reductions on the N/rev vibration peaks were achieved1 the low-frequency resonant peak was also suppressed simultaneously.

Smart Spring Impedance Control Algorithm for Helicopter Blade Harmonic Vibration Suppression

In this paper, an adaptive impedance control algorithm is developed for the Smart Spring to suppress helicopter rotor vibration through individual blade control. The harmonic frequencies of the blade response were estimated using parametric methods, which were used to synthesize the reference signal. Multiple equivalent notches were formed at corresponding frequencies to selectively suppress the blade vibration modes. To improve the adaptability of the algorithm, an on-line control path identification method is presented. The algorithm was implemented on a MATLAB xPC platform using the hardware-in-the-loop concept. Closed-loop experiments, conducted using a fixed helicopter blade section in both shaker and wind tunnel tests, have achieved significant vibration suppressions. Experimental results demonstrate that the algorithm, which is able to select the blade vibration modes and track the variations in vibration due to changes in flight condition, is promising for helicopter individual blade control applications.

Active Control of Aircraft Cabin Noise Using Collocated Structural Actuators and Sensors

This paper describes preliminary laboratory experiments conducted on a turboprop aircraft fuselage to reduce propeller-induced tonal cabin noise and vibration. Piezoelectric elements were grouped to construct a long one-dimensional array of actuators bonded to the fuselage in the main sound transmission path at the propeller footprint. Strain gauges and accelerometers were used as alternative sensor devices and were distributed along the actuator in a collocated arrangement. The array of actuators and sensors was designed to work in unison, generating a smart closed-loop array of control elements that possess wave-number filtering properties for the less critical acoustic modes of the cabin. The control system was tested in the laboratory using a simplified propeller pressure loading distribution. Promising results were obtained, as the closed-loop control system proved to be unconditionally stable and capable of significantly attenuating the fuselage vibration in the transmission path at the critical blade passage frequencies. Moreover, although only one array of control elements was used, interior noise reduction was also observed during the tests, proving the merit of the concept.

Smart spring: a novel adaptive impedance control approach for active vibration suppression applications

Most active vibration suppression approaches have attempted to suppress structural vibration by incorporating active material actuators, such as piezoceramic, within the structure to act directly against vibratory loads. These approaches require the piezoceramic actuators to generate significant force and deflection simultaneously to effectively suppress vibration. Unfortunately, successful implementation of these approaches has been hindered by the limited displacement capabilities of piezoceramic actuators. The Smart Spring concept is an unique approach to actively control combinations of dynamic impedance characteristics of a structure, such as the stiffness, damping, and effective mass to suppress vibration in an indirect manner. The piezoceramic actuators employed in the Smart Spring concept are not used to directly counteract excitation loads but rather adaptively vary the effective impedance properties of the structure. Therefore, the piezoceramic actuators in the Smart Spring are not required to produce large forces and deflections simultaneously. This paper demonstrates the ability of the Smart Spring concept to control dynamic impedance characteristics of a structure through numerical simulations and experimental investigations. Mechanical shaker tests using the proof-of-concept hardware verified the controllability of the impedance properties using the Smart Spring device and its ability to suppress vibration. More importantly, the tests conducted in a wind tunnel demonstrated the performance of the Smart Spring under highly varying unsteady excitation conditions. These tests confirmed that the Smart Spring system is able to actively suppress vibration through adaptive control of structural impedance properties.