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Intelligent muscle flexing at 350 bar

In spite of all the electronics, the days of hydraulics in aircraft construction are far from over. In commercial aircraft, hydraulic systems handle tasks ranging from landing gear actuation to flight control. For example, so-called electro hydrostatic actuators are used in the Super Airbus A380.
And now for some information on the current state of hydraulics in aircraft engineering: In the year 2006, Liebherr Aerospace added electro hydrostatic drives to the conventional electrohydraulics in the A380. A distinctive feature here is the fact that an electric power connection now suffices, instead of being supplied through hydraulic lines. The control surfaces are activated directly on the spot by means of a combination of power electronics, electric motor and hydraulic pump that causes the fluid to move a hydraulic cylinder. This new technology enabled the elimination of a complete hydraulic system, including reservoir, pumps, lines, etc, from the A380.

Safety is one of the paramount requirements that hydraulic drives and control systems for aircraft must satisfy. The big challenge for system and component manufacturers is to ensure an extremely low failure probability. For example, the permissible failure probability for the vertical rudder system stands at one in a billion. In order to meet these demands, the probabilities of failure of the involved elements, such as energy supply, signal supply and actuator, all have to be taken into account.

Overall, this calls for the use of three independent systems for the energy supply (hydraulic or electric), signal supply (computer systems) and actuating elements in order to achieve the stipulated reliability in the rudder positioning drive already mentioned.

But how to supply three independent hydraulic systems in a commercial aircraft that has only two engines, as is common today? Take the Boeing 777 as an example: Each engine supplies one hydraulic system through a hydraulic pump (and an additional electrically driven motor pump). Boeing came up with an idea for providing the third hydraulic system with energy. The American aircraft manufacturer’s solution involves literally “bleeding” both engines and an additional supply unit, the so-called auxiliary power unit, by withdrawing so-called “bleed air”. A pneumatic system uses the bleed air to drive two pneumatically-operated hydraulic pumps.

But what happens if all engines fail, for example, if there is no more fuel? A look at the A380 and its four engines can help. Each of the eight pumps on the four engines normally supplies the so-called green or yellow system with energy. In the extreme case of no fuel, the so-called ram air turbine deploys. This propeller, which is driven by the airstream, generates electricity for the two electric systems. These two systems are capable of supplying all important functions, because the A380 is capable of flying (or, in this case, gliding) completely electrically, meaning without any hydraulics at all, although with reduced performance.

A word about hydraulic system technology: Servo-valves driven by the flight control computer regulate the position of the actuators, which then activate the control surfaces. Position sensors in the actuator report the current position to the flight control computer, and consequently close the control path. The systems are designed with multiple redundancy. If one actuator in the system fails, the actuator is switched into a restraining state by means of a value, while the other actuators working in parallel handle the control tasks with no reduction in function. Safe function stands or falls with the hydraulic actuators, which are characterized by the fact that they block extremely rarely.

Liebherr Aerospace in Lindenberg has developed electro hydrostatic actuators (EHAs) for the A380. An electric motor-pump unit with the associated highly complex power electronics takes the place of a servo-valve. The motor’s rotational speed is a linear function of the volumetric flow rate of the pump that determines the piston’s position.

Possible leaks are seen as a disadvantage, because, over the course of a long service life, these could drain the oil reservoir and consequently cause the drive to fail. As an alternative, Liebherr uses EHAs with a backup system (EBHA), which gives the option of running with the servo-valve or with the motor-pump unit. Although this is a very complex device, it displays a considerably lower risk of failure due to leaks. The system operates very reliably in the A380.

But the experts are also implementing electromechanical solutions. Two examples: An interesting development in drive technology is now in the offing for small, unmanned aircraft, for example, which are not equipped with hydraulic systems. Liebherr has developed a purely electromechanical activation system for the unmanned Barracuda aircraft by EADS. The safety requirements for this aircraft are lower than usual as there is no pilot.

A different electromechanical solution was implemented for the A310’s fueling system: The retractable fuel hose mounted on the aircraft’s tail is aerodynamically controlled using two control surfaces. An electromechanical actuator is responsible for the control. Two 4.2-kilowatt motors control the actively regulated system. In the event of a complete failure, the fuel hose is mechanically anchored to the structure’s underside with a cable.

A core problem with electric motor systems is that they can become blocked. The linear movement of the electromechanical system is roughly 100 times less reliable than that of a hydraulic drive. The possible wear of the mechanical construction, which can lead to play and wobble, must also be considered. A number of research projects have been launched in order to eliminate such problems. Engineers are attempting to detect impending failures ahead of time, for example, with the help of structure-borne sound analyses of the device. All in all, there is a lot to be said for hydraulics in aircraft. For example, hydraulically operated surfaces have been in use in transport aircraft for over 50 years. Although aircraft such as the A380 and A350 XWB feature a control architecture with a higher share of electronic systems today, these systems are not a substitute for the hydraulics. To date, not a single hydraulic actuator has been eliminated. There are no current figures indicating the degree to which the targets in weight savings and cost reductions have been reached by omitting a hydraulic system in the A380. But it is acknowledged that the electronics are gaining capabilities in terms of power density and weight. As a fully developed idea, hydraulics can no longer offer such performance leaps.

The increase in operating pressure in the A380 from 207 to 350 bar is the last crucial step forward to be discussed. But as a result, aluminum can no longer be considered as a material for valve blocks, and steel or titanium must be used instead. This means that the advantage of a more compact construction cannot be converted into a weight advantage due to the increased pressure. Nevertheless: Wherever there is a need for cost efficient and reliable solutions dealing with high loads and linear movements, there is just no way of getting around the utilization of hydraulics.

- Nikolaus Fecht -

German Summary
Aller Elektronik zum Trotz: Die Zeiten der Hydraulik im Flugzeugbau sind noch lange nicht vorbei. Ihre Aufgaben reichen in Verkehrsflugzeugen von der Betätigung der Fahrwerke bis hin zur Flugsteuerung. Im Super-Airbus A 380 beispielsweise kommen so genannte elektrohydrostatische Aktuatoren zum Einsatz. Der deutschsprachige Beitrag ist nachzulesen auf www.aerotec-online.com

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