Eric F. Prechtl

Design of a high efficiency, large stroke, electromechanical actuator

Large stroke, electromechanical actuator designs are considered. Special emphasis is placed on actuators designed to power a trailing edge servo-flap system for feedback control of helicopter rotor vibration, acoustics and aerodynamic performance. A survey was conducted comparing the advantages and disadvantages of a number of actuator designs. The major conclusions from this survey indicate that any successful actuator design will utilize a high bandwidth active material, produce large amplification of the active material stroke and incorporate a simple compressive pre-stress mechanism, while remaining efficient in a mass normalized sense. The mass efficiency, defined as the ratio of the specific work performed by the actuator to the specific energy available in the active material element, was used as a metric to rate the actuators considered in the survey. This metric is appropriate in aerospace applications where weight is critical. The most feasible discrete actuators are those where the active material reacts against an inert support frame housing. An upper bound on the mass efficiency of this type of actuator is shown to be a function of the ratio of active material to frame specific modulus. A new high efficiency discrete actuator, the X-frame actuator, is described. A prototype of this actuator was built and tested to confirm the predicted performance. The prototype demonstrates an output energy density of 14.6 ft lb/slug. It has a bandwidth of about 540 Hz when driving a nearly impedance-matched load.

Development of a piezoelectric servoflap for helicopter rotor control

A servoflap that uses a piezoelectric bender to deflect a trailing edge flap for use on a helicopter rotor blade was designed, built, and tested. This servoflap design is an improvement over a design developed previously at MIT. The design utilizes a new flexure mechanism to connect the piezoelectric bender to the control surface. The efficiency of the bender was improved by tapering its thickness with length. Also, the authority of the actuator was increased by implementing a nonlinear circuit to control the applied electric field, allowing a greater range of actuator voltages. Experiments were carried out on a bench test article to determine the frequency response of the actuator, as well as hinge moment and displacement capabilities. Flap deflections of or more were demonstrated while operating under no-load conditions at frequencies up to 100 Hz. The data indicate that, if properly scaled, the actuator will produce flap deflections greater than at the 90% span location on a full-scale helicopter. In addition, the first mode of the actuator was at frequency of the target model rotor. Proper inertial scaling of this actuator could raise this modal frequency to greater than on an operational helicopter, which is adequate for most rotor control purposes. A linear state space model of the actuator was derived. Comparisons of this model with the experimental data highlighted a number of mild nonlinearities in the actuators response. However, the agreement between the experiment and analysis indicate that the model is a valid tool for predicting actuator performance.

Design of a high-efficiency discrete servo-flap actuator for helicopter rotor control

Discrete trailing edge servo-flap actuator designs for use in rotor control applications are considered. A survey was conducted comparing the pros and cons of a number of feasible actuator designs. The major conclusions from this survey indicate that any successful actuator design will utilize a high bandwidth active material, produce large amplification of the active material stroke, and incorporate a simple compressive pre-stress mechanism while remaining efficient in a mass normalized sense. The mass efficiency, defined as the ratio of the specific work performed to the specific energy available, was used as a metric to rate the actuators considered in this survey. Thus, unnecessarily heavy actuators are penalized, which is appropriate when designing components operating under high centrifugal forces. The most feasible discrete actuators are those where the active material reacts against an inert support frame housing. An upper bound on the mass efficiency of this type of actuator is shown to be a function of the ratio of active material to frame specific moduli. A new high efficiency discrete actuator called the x- frame actuator, developed at MIT, and designed in accordance with the lessons learned from the actuator survey, is described. A prototype of this actuator, 150% of model scale, was built and tested on the bench top to confirm the predicted performance. The prototype demonstrates an output energy density of 14.6 ft-lb/slug. It also has a bandwidth of 543 Hz upon driving a nearly impedance matched load. This performance is shown to correspond to a mass efficiency between 18 and 31%.

Preliminary testing of a Mach-scaled active rotor blade with a trailing-edge servo flap

The results of preliminary tests on an active helicopter rotor blade are presented. The blade, a Mach-scaled model of a CH-47D helicopter blade, has a discrete piezoelectric actuator embedded within the spar that controls a trailing edge flap via a pushrod. Ultimately, the blade will be tested on a helicopter rotor hover stand at MIT. In this paper, we describe the tests performed prior to hover testing. First, the actuator was tested on the bench to determine its control authority and frequency response. Second, the actuator was tested on a shake table to simulate the out-of-plane accelerations that would be encountered in a full-scale helicopter in forward flight. Third, the actuator was embedded in the model blade, and its response to low-frequency sinusoidal actuation was obtained and compared to the bench test results. Finally, the frequency response of the actuator in the blade was determined using swept sine excitation. All test results indicate that the actuator should produce the desired level of control authority in the model-scale rotor.

X-Frame-actuator servo-flap acuation system for rotor control

A design is presented of a 1/6 Mach scaled CH-47D rotor blade incorporating a X-Frame discrete actuator for control of a trailing edge servo-flap. The second generation design of the X-Frame actuator is described focusing on the design changes made from the actuator prototype. The function of the components that restrain the actuator to the rotor blade and connect it to the servo-flap are described. The major challenge in placing a discrete actuator into a rotor blade is in allowing the required functionality in the aggressive acceleration environment of the blade. In particular, a new centrifugal flexure is used to restrain the actuator in the spanwise direction and special fittings are incorporated into the blades to allow the required actuator degrees of freedom while reacting the out of plane vibrational accelerations of the blade. Concentric steel rods are used to transfer actuator motion to the servo-flap and to eliminate the compliant blade fairing from the actuation load path. A slotted flap design was used to reduce the required hinge moments. The aerodynamic implications of using such a flap design are described. Furthermore, retention of the flap and the pre-stress of the actuator were accomplished by a steel wire centered on the flap rotational axis. The design of this part and its influence on choosing an optimum flap length is discussed. The manufacture of the composite rotor blades is described. The diversion of composite unidirectional plies to allow access to the actuator bay within the blade spar is described.