Actuator research completeBy Alan BrownAerospace Projects Writer Dryden researchers recently concluded flight validation of three advanced aircraft control-surface actuators that promise improved safety and reliability and a significant reduction in weight and complexity over conventional hydraulic actuation systems for future advanced air and space vehicles. Actuators are devices that drive the movements of aircraft flight control surfaces such as ailerons, elevators, stabilators and rudders. Flight validation of the experimental actuators was conducted on Dryden's Systems Research Aircraft (SRA), a highly modified F-18 Hornet, under the Electrically Powered Actuation Design (EPAD) program. EPAD is a jointly funded development and evaluation effort of the Air Force Research Laboratory (AFRL) at Wright-Patterson Air Force Base, Ohio; the Naval Air Warfare Center Aircraft Division, Patuxent River, Md.; and NASA Dryden. The AFRL had overall management of the EPAD development, while NASA Dryden managed the flight evaluations. Hydraulic actuators power the flight control surfaces on all high-performance and medium to large civil and military aircraft. However, the complex hydraulic systems used to power the actuators are heavy, high-maintenance items that are costly in terms of efficiency and logistical support. According to Dryden's SRA project manager John Sharkey, the overall goal of EPAD was to establish the credibility of electric actuation as a primary method of control for flight-critical control surfaces on tactical military aircraft. Successful development of these alternative electrically powered or hybrid actuators will lead towards more-electric future aircraft designs that will reduce reliance on central hydraulic systems. "Besides potential savings in weight, costs, logistical and maintenance support for future aircraft, electrical systems promise diminished vulnerability in combat by eliminating hydraulic lines in the fuselage and wings. The power-by-wire arrangement will also reduce overall complexity and improve reliability," Sharkey said.
The pilots reported that the actuators functioned exactly the same as standard actuators, with no discernible difference in aircraft performance. Data analysis is continuing at Dryden. The first actuator to be evaluated was the so-called "Smart" Actuator, a hybrid device that was test-flown on Dryden's SRA F-18 in 1993-94. Developed by the Naval Air Warfare Center, the Smart Actuator incorporated a standard hydraulic actuator with self-contained control and monitoring electronics. Although still dependent on the central hydraulic system, the Smart Actuator replaced the large wire bundle which connects the F-18's flight control computer to the actuator with a simple fiber-optic cable pair, with a weight savings benefit. The second and third actuators flown were true power-by-wire devices, with no connection to the F-18's central hydraulic system. Both used similar electronics packages that were installed in the left wing a short distance from the actuator and in the F-18's fuselage, and both incorporated new-technology brushless 270-volt direct current motors. The first of these, the Electro-Hydrostatic Actuator (EHA), was evaluated on Dryden's SRA at various times from 1996 to early 1998. Its powerful motor drove a hydraulic pump which drew fluid from its own small reservoir to drive the aileron, independently of the central hydraulic system. A more powerful follow-on version of the EHA designed to drive the horizontal tails is scheduled for flight validation on a Dryden F-18 in the future. The Electro-Mechanical Actuator, or EMA, was the last experimental actuator to undergo flight validation, with its series of flights completed in late 1998. Like the EHA, the EMA received its commands from the flight control computer. The commands were then transmitted to two motors which drove a mechanical screw-type ram to move the aileron up or down. All three experimental actuators received their commands electronically based on the pilot's control-stick movements. For the EHA and EMA, an intermediate Power, Control and Monitoring Electronics (PCME) unit, an electronics interface box and a power conversion unit (PCU) assisted in processing the computer's commands, powering the actuators and monitoring their operation. A follow-on version of the EMA is being used on the prototype spaceflight-rated X-38 Crew Return Vehicle technology demonstrator, said Dryden systems engineer Stephen Jensen, who was principal investigator on the EMA. EMA actuators are also being developed for use on the X-33 reusable launch vehicle technology demonstrator. Electric actuators are also being considered for use in thrust-vector control systems on reusable launch vehicles and for primary flight control surfaces on one of the two Joint Strike Fighter candidates. "EMA actuators are particularly well-suited for space applications, where hydraulic seals tend to dry out and leak," Jensen explained.
Lockheed-Martin Control Systems, Johnson City, N.Y., designed and built the EHA and the PCME units. MPC Products Corp., Skokie, Ill., designed and built the EMA. Hydraulic Research Textron, Valencia, Calif., designed and built the Smart Actuator. Dynamic Controls Inc., Dayton, Ohio, designed and built the direct-current PCUs and the EHA/EMA electronics interface units. |
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Responsible NASA Official: John Childress For questions, contact: Dryden Web Group Page Curator:Mel Garcia Modified: January 12, 1999 |
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