Proportional bypass valve with dual variable orifice

An unbalance compensated bypass valve (14, 21, 23) for a gas turbine engine fuel supply system is responsive to the fuel pressure differential across a fuel metering valve (15) to open and close a bypass orifice (37, 39, 41; 89, 91, 93; 99, 101, 103). The bypass valve (14, 21, 23) selectively supplies high fuel pressure to a compensating cavity (45, 95, 120) providing a counterbalancing force against an annular area (47, 97, 124) of the bypass valve piston (34, 81, 83, 111, 112) to compensate for increased restorative forces of a helical reference spring (57, 112, 125) as that spring is collapsed as well as to compensate for other unbalance conditions within the valve (14, 21, 23). The compensating cavity may include a variable orifice inlet (37, 39, 41), (99, 101, 103), or (89, 91, 93) and a variable orifice outlet (105, 107, 109) or the inlet orifice may be variable while the outlet orifice (43, 96, 99) is fixed. The piston (34) or housing (100) may include additional sidewall openings (69, 70, 71) or (92, 94) which function during a 'shut-off' condition to divert all fuel flow from the metering valve (15) inlet back to a fuel pump (13) inlet. The piston (34) may also include further sidewall openings (73, 75) which provide a controlled fuel leakage from a bypass valve inlet (25) to the bypass valve pressure sensing port (29).