More efficient fluid power systems using variable displacement hydraulic motors

The approach and landing phase is dimensioning for today’s aircraft fluid power systems. In this flight phase, large hydraulic consumers (flaps/slats, landing gear) have to be operated while the available hydraulic power reaches it’s minimum due to the reduced engine speed. During most of the flight the installed resources exceed the hydraulic power requirements by far; resulting in a low overall-efficiency. This paper presents an approach to increase the efficiency of today’s fluid power system by using variable displacement hydraulic motors (VDMHs). Two applications will be introduced: the VDMH driven slat/flap power control unit (PCU) and the bi-directional hydraulic-electrical power conversion unit (HEPCU). The variable displacement PCU reduces the design loads for the fluid power system during take-off and landing. Compared to conventional PCUs used today, a flow reduction of about 50% is expected using the VDHM technique. The HEPCU transfers hydraulic into electrical power and vice versa depending on the current load and flight situation ("hydraulic-electrical power management"). The main benefit from this approach is down sizing the primary power sources (engine driven pumps and generators), a significant increase in reliability and a higher efficiency for both, the hydraulic and the electrical power generation system.