John Keller, Editor in Chief
Just when you thought it was safe to start designing advanced aircraft without hydraulic fluid and its associated on-board plumbing systems, comes a pressing new need for other kinds of on-board aircraft fluids to deal with issues originally designed to make hydraulics a thing of the past.
For decades, aircraft designers have used hydraulic systems for important flight-critical tasks such as moving rudder, flaps, and ailerons. This technology, although long proven and understood, can be a big design and maintenance headache.
Ever-more-complex aircraft in years past needed ever-more intricate hydraulic systems to manipulate control surfaces and for other important jobs. Hydraulic systems often need miles of piping routed throughout the aircraft, as well as pumps and fluid reservoirs, to accomplish their tasks.
Miles of small liquid-carrying pipes can provide thousands of opportunities for leaks, outright breaks, clogs, and other kinds of system failures. Moreover, liquid and pipes are relatively heavy. Suffice it to say that aircraft designers for a long time have looked for ways to eliminate aircraft hydraulic systems and substitute something better.
Designers had thought they found their answer in advanced electric systems. Instead of hydraulics, they could use motors, magnets, actuators, and other electric components to accomplish the same tasks as hydraulics, except with lighter weight, enhanced reliability, and easy maintainability.
Government and industry researchers, operating under aegis of the so-called “All-Electric Aircraft” and “More-Electric Aircraft” programs, devised different electric alternatives to hydraulics. The U.S. F-22 and F-35 jet fighters are considered to be among the first examples of the “more-electric aircraft.”
Everything sounded good, until designers realized how many electronic components and subsystems they needed to build new generations of more-electric aircraft; it was sobering, as also was the large amount of electric power necessary to operate these new electric aircraft.
Increasing numbers of electric components translated into increasing amounts of heat that aircraft systems designers had to find ways to get rid of-somehow. The solution to the increasing heat problem, ironically, often tends to involve fluid.
That’s right. Instead of hydraulic fluid for systems that move control surfaces, aircraft designers now are faced with using on-board fluid systems to cool ever-hotter electric and electronic systems such as power supplies, computers, electro-optic sensors, lasers, and radar.
Pipes for fluids aboard the aircraft are not really going way; they’re just migrating to different parts of the aircraft. These fluids confront engineers and maintenance technicians with some of the same old problems-potential leaks, plumbing complexity, and the added weight of the cooling liquid itself.
Liquid spray and flow-through cooling systems may become standard equipment on advanced aircraft. The F-22-the first “more-electric aircraft”-uses liquid flow-through cooling for computer circuit boards. The F/A-18 fighter bomber, in addition, has liquid cooling for its active electronically scanned array (AESA) radar system. More liquid cooling is in use today, and undoubtedly is on the way.
Avionics designers will never tire of demanding ever-increasing system capability, no matter what the application. Experts insist on the latest computer hardware, the most advanced radar systems, the largest possible data storage, the best signal processing, graphics, and image-analysis systems.
Increasing system capability inevitably means more electric and electronic components and subsystems ... and more heat. The ability to remove waste heat from electric and electronic systems aircraft, in fact, has reached a critical stage today. Conventional heat-removal systems, such as conduction and forced-air cooling, often are no longer sufficient for the most modern avionics.
Designers also seek to use unused fuel in the aircraft’s tanks to circulate through hot areas, cool in heat exchangers, and return to the fuel tanks. Even this might not be enough for all electronic cooling demands.
Companies such as Meggitt Defense Systems Inc. in Irvine, Calif., and Parker Hannifin Corp. in Mentor, Ohio, are designing liquid flow-through systems that channel cooling fluid through tiny pipes to cool electronic printed circuit boards and chassis for commercial off-the-shelf (COTS) chips and boards.
Other companies, such as ISR Inc. in Liberty, Wash., are concentrating on liquid spray cooling, which literally sprays inert liquid over board-mounted components to reduce temperatures. Spray cooling involves a closed system that sprays liquid onto boards, where the liquid boils and evaporates on contact. The liquid then is condensed and recirculates into the spray system.
Certainly these kinds of approaches can be complex, heavy, difficult to maintain, and expensive, yet systems designers, today and in the near future, may not have many alternatives. Experts maintain that liquid spray and flow-through cooling systems aboard aircraft and other military platforms may only be sufficient for dealing with the latest electronics for the next five or so years.
After that, designers who want to use the latest, most powerful electronic systems may be faced with using even more exotic cooling methods, such as liquid immersion, or packing electronics in special cooling gels.
No matter what direction the industry heads, they are looking at complex liquid cooling systems for modern aircraft avionics.
The liquid for hydraulic actuators and control surfaces may be going away, but the liquid-and associated pipes, pumps, and reservoirs-is not.
Just when they thought it was safe to get rid of the hydraulics.
