Designing a hydraulic system for an aircraft involves some challenges that differ from industrial or mobile applications. Factors that must be addressed on an aircraft include pressure conditions (both internal and ambient), temperature extremes, weight, speed, materials, reliability, fluid compatibility, leaks, cost, noise, and redundancy. Fluids used on aircraft have a relatively flat viscosity vs. temperature curve. They must be fire resistant and remain fluid at -65 degree F. The newest fluid is MUL-H-87257 used in C135, E3, and U2 aircraft. Great emphasis is on maintenance schedules, filtration, and general maintenance of fluid conditions on aircraft. Extra costs of pumps develop from the following differences compared with an industrial pump: great attention must be paid to shoe bearing plates and plate faces, due to poor fluid qualities; aluminum is often used, due to weight considerations; the pump must be smaller and have a higher operating speed; it must withstand severe vibration, shock and g-forces. Since aircraft quantities are relatively small, modular, low-cost, mass-produced valves are not generally used. Pressurized reservoirs keep the charge inlet of pump pressurized to prevent cavitation. Reservoir size is dramatically reduced from industrial systems. Most aircraft have three or four redundant hydraulic systems, which are separate in many cases. Also noise can no longer being overlooked. In the F-22 Raptor, the 4000-psi hydraulic system plays a role in rudder control, landing gear, nose wheel steering, flight control surfaces, and in the weaponry. A Zero Leakage Hydraulics initiative was started by the Society of Automotive Engineers (SAE) Committee A-6, Aerospace Fluid Power, Actuation and Control Technologies, in 1997 with a symposium in Lake Tahoe, Nev. Among other results, revision of standards has been started.
The challenges of aircraft hydraulic design
Hydraulics and Pneumatics ; 51 , 7 ; 27-66
1998
5 Seiten, 4 Bilder
Article (Journal)
English
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