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Tuesday, May 1, 2001

Power-By-Wire

Will the Joint Strike Fighter be controlled in flight by an electro-hydrostatic system? A Lockheed Martin-lead team hopes it will. The Boeing team has another idea.

James W. Ramsey

A new electro-hydrostatic flight control and power system, intended for the Joint Strike Fighter (JSF), promises to revolutionize fly-by-wire technology. Calling it "power-by-wire," the Lockheed Martin JSF team developed the system and flew it on a U.S. Air Force F-16 as part of the Joint Strike Fighter Integrated Subsystem Technology (J/IST) program. The electric actuator design actually originated with the U.S. Air Force Research Laboratory.

Lockheed Martin believes the system will be as significant in fighter risk reduction as the F-16 was when it became the first U.S. production fighter aircraft to be equipped with fly-by-wire flight controls, in the 1970s.

"This is the first of a new generation of flight control technologies," says Dennis Eicke, J/IST program manager for Lockheed Martin Aeronautics Co. "We were successful in the development, integration and test [of the system]."

Lockheed Martin incorporated the power-by-wire system into its X-35 Preferred Weapon System Concept (PWSC) design proposal for the JSF program’s engineering and manufacturing development (EMD) phase submitted in February. The information has been shared with JSF competitor Boeing. Although the Boeing JSF team is offering a different flight control actuation system on its X-32 design (see sidebar), the JSF program office could mandate the Lockheed Martin system for production fighters regardless of which team wins the competition.

Reducing Weight and Cost

The new flight control subsystems package includes a power-on-demand electrical system and electric actuation (rather than the traditional, large, heavy and difficult-to-maintain hydraulic actuation) of flight control surfaces. Lockheed Martin maintains that government studies show the new approach could deliver up to a 5% reduction in procurement costs, 3% in lower life-cycle costs, and a 6% decrease in gross takeoff weight.

"We found that the emergency and secondary power system is much simpler and cleaner if you go with the electric approach," says Eicke. "It enables a much smaller and simpler airplane."

He adds that it can reduce aircraft weight by as much as 700 pounds (315 kg), but not because the system itself is lighter. "We’re taking weight out of the hydraulic system, weight out of the secondary power system, and out of the thermal management system, because we’re not generating as much heat.

"But we’re offsetting it with a relatively more complex actuation system," Eicke adds. "If you don’t do the trade studies at the air vehicle level, you don’t see the benefits.

"We also are simplifying maintenance on the aircraft. If you have to replace an actuator, you don’t have to bleed the fluid systems and break the hydraulic connections, which can cause environmental concerns."

In Supersonic Flight

The J/IST program was intended to reduce the risk of certain new technologies chosen as candidates for the JSF program. The Air Force’s Advanced Fighter Technology Integration (AFTI) F-16 demonstrator aircraft made its first flight with the power-by-wire subsystems package last Oct. 24. Since then, flight control effectiveness has been demonstrated during mach 1.3 supersonic flight. The aircraft performed flying quality maneuvers while supersonic, including 5-g turns, pitch, roll and yaw "doublets" and sideslips. The tests also included low-altitude strike missions and the chance for the Lockheed Martin team to study the actuator and generator subsystems’ thermal behavior.

Throughout integration and ground testing, "we struggled in a couple of areas," Eicke admits. "But once we started flight test, the actuation system, as well as the power generation system, performed really well."

Incidentally, the AFTI F-16, which has demonstrated advanced technologies through 10 different test programs since it first flew in 1978, completed its most recent tests and has been retired to the Air Force Museum in Dayton, Ohio.

Parker Hannifin’s Control Systems Division, in Irvine, Calif., and Hamilton Sundstrand Aerospace, in Rockford, Ill., are the two principal suppliers who worked with Lockheed Martin to develop the power-by-wire system. Parker Hannifin provides the flight control actuation system and Hamilton Sundstrand the (dual, electrically isolated) 270-volt DC electrical power generation system. The J/IST team also includes Honeywell, Lucas Aerospace, Eagle-Picher and BAE Systems.

Program Evolution

The power-by-wire system’s development began at Air Force Research Laboratories in the early 1980s. Prior to 1995, an industry group worked with the government to define key technology items for the JSF program that needed to be matured to support the EMD phase, in which the production version of the aircraft is developed.

In 1995, the JSF program office awarded three contracts–one to Lockheed Martin, one to McDonnell Douglas (now part of Boeing) and the other to Boeing–to work together to mature key aspects of this integrated subsystems technology. Lockheed Martin worked on component development and system integration for five years, in preparation to demonstrate the power-by-wire technology last year by installing the system in AFTI F-16. "To be honest, the bulk of the work was done by our subs–Parker Hannifin and Hamilton Sundstrand," he admits. "We integrated these systems into the AFTI F-16, which was a fly-by-wire system. We had to make some changes to the flight control system to interface with this new actuation system, modify the emergency system and electrical architecture to incorporate the new power system, and provide an uninterruptible power supply to the actuation system. And we conducted the flight demonstration flight tests."

How does the new electro-hydrostatic system differ from a conventional fly-by-wire flight control system found on most new fighters, on the C-17 military airlifter, and on the Boeing 777 and Airbus commercial transports?

Dual Level

Looking at the conventional F-16, there are two levels to the fly-by-wire-system, Eicke explains. One is the top level, or "control" layer, which is triple or quadruple redundant. It operates on 28 volts DC and sends out analog command signals to each of the flight control actuators. This signal drives the control valves (located with the actuators) that schedule the fluid from the high-pressure pump to position the control surface.

The other level is called the "power side." On the conventional F-16, dual engine-driven 3,000-psi hydraulic systems provide fluid power to each primary control surface actuator. The hydraulic system provides the muscle that allows the actuator to drive the control surface to the desired position.

In the new electro-hydrostatic design for the JSF, "We kept the (electrical) control system relatively the same...We have a flight control computer that interfaces with the new actuation system. We’ve yanked out the fluid system, and put in a new 270-volt DC electrical power system. We have two channels of power that provide the muscle to each actuator," Eicke explains.

The new electrical system provides power to five dual power electronics units, one for each actuator. The power electronics unit regulates the power to drive dual motor/pumps, essentially two independent hydraulic systems self-contained in each actuator. The separate control layer provides the signal to the actuator power electronics to "tell" the actuator which way to go. The dual motor/pumps convert the DC voltage electrical power into hydraulic power, allowing the piston on the actuator to move the control surface.

Taking the Heat

In Lockheed Martin’s PWSC for the JSF, the new integrated electrical system also provides electrical power to avionics and other systems. Going to an electrical distribution and electrical actuation system simplifies the secondary power and thermal management systems, "and that’s where we really get our savings," Eicke claims, "because we’re getting some redundancy and efficiency improvements by minimizing or reducing heat dissipation into the aircraft."

With current high-speed fighter thermal management systems, such as in the F-16, energy generated from the dual hydraulic systems is dissipated into the fuel system. While the F-16 actuators operate at 3,000 psi, the new electric actuator operates from 300 to 3,000 psi, providing power on demand.

"When flying straight and level, you are really not generating that much energy," Eicke points out. "The electrical power requirement is small. In our demonstrations, we were operating at about 500 watts per actuator.

"With the previous hydraulic power system, you were dumping power into the actuator whether you were using it or not," he continues. "All that energy becomes heat–thousands of BTUs–and gets dumped into the fuel system.

"With the new (electro-hydrostatic) actuation systems, if you are not generating any heat, you don’t have to dissipate it. It’s a much cleaner secondary power system. We don’t have to provide a secondary and an emergency source of hydraulic power," says Eicke.

It frees us–gives us more thermal margin, which we can apply to other systems on the aircraft, or we can reduce our thermal management system weight and volume to compensate for that, making our aircraft smaller, lighter and cheaper."

Boeing’s System

Boeing did not use electro-hydrostatic actuators (EHAs) on its X-32 Joint Strike Fighter (JSF) concept demonstrator aircraft, and did not include them in its Preferred Weapon System Concept (PWSC) proposal. In Boeing’s flight control system, primary flight control surfaces are powered by hydraulic actuators driven by redundant, centralized 5,000-psi hydraulic systems. The company maintains studies have shown that modern 5,000-psi hydraulics systems compare better than EHAs in terms of cost, weight and supportability.

Boeing shared in results of the J/IST studies and "used J/IST methodologies extensively to develop the subsystems architecture for its proposed JSF concept," according to a Boeing spokesman. "J/IST pursued vehicle-level benefits through the logical integration of aircraft subsystems. This approach was applied to our proposed concept and used to develop our advanced pneumatically-integrated secondary power system architecture."

Boeing feels "it is very unlikely" that the U.S. Defense Department would dictate use of the electro-hydrostatic actuator system, "as they have worked closely with us as our actuation concept has evolved." The JSF program office would not comment on Boeing’s or Lockheed Martin’s proposed flight control systems because the next phase of the program is currently in source selection.

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