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High pressure, low weight

High pressure, low weight

Toulouse, France -Overall weight and fuel efficiency are important to an aircraft's performance, so Airbus engineers looked past the 3,000-psi systems typically used in large aircraft and designed the first-ever commercial 5,000-psi hydraulic system for use in the A380 aircraft. The new high-pressure system provides the fluid power needed for controlling the aircraft's flaps, rudder, and other control surfaces while reducing the overall weight of the hefty 555-passenger aircraft by 2,500 lb.

Instead of the standard 3,000 psi hydarulic systems found on many large commercial aircraft, the new Airbus A380 will use a 5,000 psi hydraulic system for moving the rudders, flaps, and other flight-control surfaces.

"Any saving in the basic weight of an aircraft is directly translated into an increase in its revenue payload capability," says Airbus A380 Program Engineer Derek Davies. The problem he and other engineers encountered when designing the A380 hydraulic system is that the equipment, if designed to operate at the traditional 3,000 psi, would be too big. "For example, the diameter of the hydraulic inlet hose line for the A380 at 5,000 psi is two inches," says John Halat, the director of research and new product development at Eaton Corporation's aerospace operations. "It would be much bigger and bulkier, up to three inches, using a conventional 3,000 psi system," says Halat. Bigger hoses require bigger fittings and mounting brackets, which also add weight and bulk.

Typical applications of hydraulic power on commercial and military aircraft include rudders, wing flaps, engines, and landing gear.

Halat explains that the weight reduction on the A380 is a direct result of how the hydraulic power is converted into motion by the electrohydraulic actuators that control rudders, ailerons, flaps, and other flight surfaces. "Consider, for example, a single actuator controlling the rudder movement," explains Halat. "This actuator needs a specified amount of mechanical power for moving the flight control surface at a given rate and against a given force." The A380 has upper and lower rudders that are sized for a stall load of 22.5 tons each. The mechanical power required by the actuator is the product of the force needed to move the surface times the velocity of the flying control surface.

The hydraulic system provides the equivalent amount of power to the actuator in the form of fluid power. The hydraulic pump, which is usually mounted on the engine gearbox, extracts mechanical power from the engine and converts it to hydraulic power for use in the system.

As the pump supplies hydraulic flow to the system, the pressure developed is the resistance to the flow in the system. The hydraulic power available in the system is the product of the pressure times the flow rate. This power flows through the fluid tubing and hoses to the actuator. The available fluid power equals the pressure times the flow. With a fixed power requirement, these two variables vary inversely with each other. So, if the pressure goes up, the flow rate comes down.

For the A380, Eaton supplies Airbus with the hydraulic hose, fittings, and engine-driven hydraulic pumps like the one shown here.

"Designing the system to operate at 5,000 psi allows a 40% reduction in the flow rate compared to a 3,000-psi system built to the scale needed for the A380," says Halat. The jumbo jet contains a large number of actuators, many to drive its large number of control surfaces. The pump and fluid delivery system must meet the maximum flow rate of multiple actuators functioning at one time. In a 3,000-psi system for the A380, the flow rate would require a 70-gpm pump. However, the 5,000-psi system reduces the necessary flow to only 42 gpm. Airbus achieved a 20% weight reduction by going to the higher pressure and using a smaller size displacement pump with lower flow rate.

The overall system component weight reduction attributed to the 5,000-psi hydraulic system is approximately 2,500 lb. While the volume of power generation and motion control components is also smaller, the majority of the volume and weight saving occurs in the 250 to 500 fittings and the hydraulic lines which, in some cases, run the length of the aircraft.

Trade-offs resulting from the move to 5,000-psi hydraulics include materials used for the sealing of cylinder blocks, pistons, and other pump components. Elastomeric sealing is now required in places that normally allow metal-to-metal sealing.

Eaton has a track record in 5,000-psi hydraulic applications. The military currently uses the company's 5,000-psi hydraulic pumps, fittings, and hose in the F-18, V-22, and other aircraft.

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