Emerging Technologies: Distributed Electro-Hydraulic Systems

Entrenched technologies are often thought to be flywheels of infinite mass: You can't change their speed or direction. Throw in a federal bureaucracy with absolute power over approval of a replacement technology and you have something that veteran engineers are very familiar with: A project with technical obstacles compounded by bureaucratic delays.

Distributed Versus Centralized Systems

Air loads on modern airliner flight surfaces and landing gear are high and mass is great so force multipliers - high pressure hydraulics - are the control systems of choice and have been for decades.

However, because of the weight and complexity of the engine-driven, centralized system, fluid power companies are working on an alternative: A squadron of electrically powered Hydraulic Power Packs (HPPS) located near the actuators they drive, but far away from the rotating components of the jet engines.

Operationally, this means that hydraulic systems become zonal, with long runs of vulnerable tubing eliminated along with many of the manifolds and significant quantities of hydraulic fluid. If access panels are intelligently co-located with the HPPS, maintenance time is reduced as well. Catastrophic failure modes are better controlled, system pressure losses are minimized, and a weight savings of around 15 percent is projected.

As an example of the current approach, the new Boeing 787 Dreamliner hydraulic system features a primary pump on each engine plus a third, slipstream driven RAT (Ram Air Turbine) to operate the actuators in an emergency. Separate, dedicated pumps cycle the landing gear. Filters, manifolds and flow directors are part of the system, and hydraulic fuses are employed to sense and seal problems in any of the three branches.

These standard systems work well, but are complex and hardware-intensive. And because there's so much high-pressure hydraulic tubing used, leaks are a frequent maintenance issue.

So it's no surprise that companies have been working on an alternative. "Adoption of zonal hydraulics will have a definite effect on the design of airplanes," says John Halat, chief engineer, Advanced Development at Eaton Aerospace Group. "Most of the major hydraulics suppliers have development activities in this area."

The hydraulic industry's trade organization, the National Fluid Power Assn., believes the technology has potential once specific uses (such as zonal systems for airliners) have been identified and the technical challenges overcome. As a consequence, it has been working with its members to develop a road map for success in this area.

Airliner Technical Challenges

Engineers are no doubt busy working on the technical challenges, which inevitably will involve some wrestling with trade-offs in the choices to be made.

Halat says he feels that one of the principle airliner challenges is to increase the capability of the current HPP electric motors by a factor of 10 without increasing the weight by anything near that amount. "Today's 5 to 10-kW HPP motors are sufficient to operate many of the flight control actuators, but it takes from 50 to 100 kW to cycle the landing gear," he says.

There may be another way to skin this cat, though. Dr. John Hansman, a professor of aeronautics at MIT where he teaches aircraft system design, suggests another approach would be to charge up accumulators and use the stored energy to overcome the power shortage. In Hansman's view, this makes the instantaneous power requirements much lower, with the same results.

Another challenge for engineers is coming up with a strategy to manage the reaction of hydraulic fluids to high altitudes. Jet airliner systems require fluid reservoirs to be pressurized, because at high altitudes hydraulic fluids outgas and foam. In a centralized system that's easy to manage, as there is only a few large reservoirs. But in a distributed system there is a large number of small reservoirs that will need pressurization.

"The physics is the same for the small HPPS, but if you pre-charge the reservoirs and you have good seals, the hydraulic fluid isn't likely to foam," says Hansman. The solution would require fitting all the HPPS with pressurized reservoirs and then monitoring the state of the air charge so that a pressure loss would be shown on the cockpit system status displays as an early indication of impending loss of a HPPS. No breakthrough technologies are required, but a well-executed design approach would be needed.

Another challenge is coping with the noise generated by multiple HPPS Flight Control Devices constantly cycling on and off. Halat and his group believe the best design approach is to locate the HPPS as far from the passengers as possible and acoustically shield those that have to be placed near the fuselage.

Hansman agrees. "Placement of the HPPS as far away from the seating areas as feasible is critical, so the short rise time noise of the HPPS turning on and off doesn't frighten the passengers," he says. He also agrees that although acoustic shielding adds weight, it may be unavoidable in some cases.

Halat and Hansman share an enthusiasm for the technology. They both think the use of HPPS would cut down on assembly time and total parts count, which the airframe manufacturers would love. But how much of a weight reduction will occur is up for debate.

Halat says he believes a 15 percent reduction in weight will result from eliminating many of the manifolds and the need to run redundant hydraulic tubing throughout the wing and tail structure or fore and aft in the fuselage.

Hansman, on the other hand, thinks some weight reduction may result but he's not sure how high the percentage might be. "It depends on the execution of the design. It certainly saves weight to run electrical cables instead of hydraulic tubing," says Hansman. "But if you have to use up portions of the weight reduction budget compensating for other problems - like the HPPS noise issues or the landing gear motor size - the 15 percent reduction probably won't be realized."

The FAA: Biggest Obstacle of All?

Given enough time and money, engineers are likely to solve all the technical challenges. But since a distributed hydraulic system has never been done before - at least not in airliners - convincing the FAA to certify it may prove to be the biggest challenge of all.

In order to be successful, fluid power companies and airliner manufacturers will have to work hand in hand. According to FAA airworthiness inspectors contacted by Design News, the provisions of the rules governing aircraft certification, (Federal Air Regulations, Part 21 and its subparts), indicate that an airframe manufacturer seeking an aircraft type certificate is ultimately responsible for policing the suitability of parts used in a new airliner.

The inspectors also point out that the parts manufacturers must adhere to the relevant provisions of the same FARs. Both hurdles have to be cleared.

FAA certification personnel demand that any new way of doing things is, at minimum, as safe as the technology it's replacing. It matters not whether the new idea might save lives. Until proven that it will not place more lives in danger than the old technology, it's a non-starter.

Computer simulations will help, but the fluid power companies and aircraft manufacturers must both demonstrate that they have rigid testing programs in place: programs that stress discovering potential failure modes, achieving repeatability of results, demonstrating adequate safety margins and use of redundancy where required. And the aircraft manufacturer has to demonstrate that the new system works in concert with the other aircraft systems and that it's at least as safe as the current centralized system.

But there's a good prospect of gold at the end of the rainbow. The requirements for certification are spelled out in the FAA's rules. If the fluid power companies and the aircraft manufacturers keep a copy of that guidance handy and keep the FAA advised of their actions every step of the way, there's no reason for this technology to remain characterized as "emerging." In a few years we could instead be calling it "Leading Edge."

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