Hydraulics shrink in size but not in role

By Michael Jermann, assistant editor

For decades, people have been predicting the downfall of hydraulic power and trying to move to an all-electric form of operation. In 1994, Raymond Ross, then president and chief operating officer for Milacron Inc., said the migration from hydraulics to all-electric drives was a natural advancement and would be an accelerating and irreversible trend.

But no one seems to have told the aerospace industry about this trend. Hydraulic systems continue to be an important part of aircraft design because of their speed, reliability and power density.

It’s true that hydraulic systems in the aerospace industry are changing. Gone are the days of commercial aircraft using a centralized hydraulic system with all its tubing stretching through out the aircraft. Nowadays, electro-hydraulic actuators have replaced the centralized system with more localized systems that are controlled by electrical input, rather than mechanical linkages.

These smaller, self-contained hydraulic systems help to reduce the overall weight of the aircraft, increase efficiency and reduce emissions. An added advantage over older hydraulic systems is that they only draw power when in use and maintain pressure even when the pump is inactive. The technology has been proven in the F-35 Joint Strike Fighter and numerous commercial aircraft from Airbus and Boeing.

The reason why advanced aircraft like the F-35 continue to use an old technology such as hydraulics is because the technology is fast, strong and reliable.

“Until that electric system offers some real advantage from a technical engineering and customer operating standpoint, there would have to be some real innovation to make any manufacturer want to pursue it,” said Jeffrey Johannsmeyer, project engineer for Cessna.

“Electrically actuated systems need to be developed with the same level of reliability and safety as hydraulic systems,” Avraham Ardman, chief systems engineer for Bombardier, said. “They must be compatible with the requirements needed to control the aircraft in normal and abnormal conditions.”

All-electric actuators do not currently have the power density or speed needed to operate an aircraft’s flight surfaces effectively. If a pilot finds himself heading toward a thunderstorm, he expects a powerful and immediate response from the aircraft the moment he inputs a command.

“It’s just difficult to imagine an electric motor that can give you that kind of power density with a really instantaneous response over a long distance,” said Robert Keller, technical manager with Freudenberg-NOK.

“You’re scooting along in that Embraer regional jet at Mach 0.86—if you had to manually move those surfaces, you’d have to be a Bulgarian weight lifter,” Keller said. “They just don’t move at those speeds, so you need that hydraulic power density to move those flight controls.”

To be fair, not all hydraulic systems within an aircraft are safe from the move toward electrical actuation. Certain systems that do not necessarily require rapid movement are more susceptible to being replaced. The brakes on an aircraft’s landing gear are one example. Because the rotor and stator don’t need to move nearly as much as the primary flight surfaces, an electric motor is sufficient in applying the needed force and speed of movement.

Jamming is a real problem for these electromechanical actuators, which is another reason why they are unlikely to take the place of an EHA in control of a flight surface. A jammed brake or landing gear can be bad, but a jammed aileron or rudder would be disastrous.

Now you might think that if EMA’s aren’t strong enough for larger aircraft, certainly they would be better suited for smaller aircraft such as private jets or single engine aircraft. But there’s a snag to that logic. In any aircraft, it is important to have multiple redundant systems, whether they are extra actuators, wires or cables. In an aircraft where all systems are electrically controlled, you need multiple batteries or generators, just in case the aircraft were to lose power and thus lose control.

These redundancies are large and would take up a lot of space within the aircraft—as well as add a lot of extra weight. It is for this reason that many smaller aircraft continue to use old-fashioned mechanical linkages.

“Most of our airplanes today are muscle powered,” Johannsmeyer said. “There are cables and pulleys connected to the controls in the cockpit and its pilot muscle and mechanical advantage that flies the airplane.”

According to Johannsmeyer, the Citation X, a long-range business jet, is the only aircraft currently produced by Cessna that uses hydraulic actuators to move the primary flight surfaces. This is because the aircraft is too large and too fast for mechanical control systems to be effective.