The increase in pressure from 3000 psi to 5000 psi allows the necessary power to be transmitted with smaller piping and hydraulic components. That in itself reduces the aircraft's weight by about 1 metric ton. Military aircraft have been using such high-pressure systems for years, and that evolution to 5000 psi has stood up well to qualification testing. Trials with existing hydraulic fluids and components have shown that the fluid does not degrade under higher pressures.
Airbus selected Eaton to provide the 5000-psi hydraulic power generation for the A380. The system consists of eight Vickers PV3-300-31 engine-driven hydraulic pumps, along with four AC motorpumps and the associated electronic control and protection systems. The pumps are pressure-compensated, variable displacement type, and deliver 42 gpm at 3775 rpm. Their displacement is 47 ml/rev.
Two unique aspects set this pump apart. First is the disengagement clutch, a feature not currently found on any commercial or military aircraft pump. In cases where a pump is malfunctioning while the plane is on the ground, the clutch can segregate that pump from the system. This allows the plane to be dispatched using the remaining seven pumps (seven pumps are needed for normal flight operations; the remaining pump is redundant, for safety). Typically, in the event of a grounded aircraft, the airlines would have to accommodate more than 500 passengers - which could be very costly to the airline. In flight, the pump can be detached in case of failure, so it won't contaminate the rest of the system. Explains Phil Galloway, engineering manager for Eaton, "The pump can't re-engage except on the ground, via manual means."
The other important feature of the PV3-300-31 is its extremely low noise and pressure pulsation. Pressure in a given 5000-psi system might vary from 4900 to 5100 psi, due to normal pressure pulsations created from the pump. On aircraft, the variation has typically been 10% or less, improving to 5% with more recent models. In order to improve reliability and minimize fatigue throughout the aircraft's fluid distribution lines, Airbus asked for a pump with ±1% pulsation.
Eaton responded with a pump model that features a built-in attenuator and an 11-piston rotating group - meeting the Airbus goal and resulting in one of the lowest noise pumps in the market.
Galloway explains that making the switch to 5000 psi isn't as easy as pulling different parts off the shelf - there are some serious pump design issues at higher pressures, including:
- material selection,
- stresses,
- deflections,
- pressure time velocity factors,
- pressure balances,
- efficiency, and
- pressure pulsations.
"Above 3000 psi, using aluminum as a pressure vessel becomes risky," says Galloway. "5000 psi systems almost exclusively rely on steel or titanium. To minimize stresses, the sizes of passages can be reduced. Computational fluid dynamics is also used to minimize pressure drop in critical areas."
With higher system pressures, "internal" pump leakage naturally increases - which decreases volumetric efficiency. But, because of improved oil film thickness and resulting reduced sliding friction, torsional efficiency tends to increase.
Pulsations can be minimized with the use of pulsation attenuators, essentially fluid volumes added to the outlet line of the hydraulic pump. The size of the attenuator is a function of the size of the pump and the amount of attenuation desired. The drawback to large attenuators is that the weight can be significant, as a high pressure volume is required.
Inside the plane of the future
