An unpressurised aeronautical generator (2), is provided which includes a two stage oil pump (6) comprising a centrifugal pump (8) and a gerotor pump (26). The centrifugal punp draws oil through openings (18) in a manifold, pressurises it and supplies it to the gerotor pump. The gerotor pump then pumps the pressurised oil throughout the generator. A small gerotor pump is used, which is still sufficient to operate at low pressures and high altitudes.