Recent Developments of the Mechanism of the Hydraulic Variable-pitch Aircraft Propeller

Abstract

After briefly outlining the main features of the variable-pitch propeller, this paper proceeds to describe the development of the piston-engined hydraulically operated propeller as a brake, both in the air and on the ground. Examples are given of the magnitude of the braking effort of a propeller when windmilling under controlled conditions and when in reverse pitch under power. The advent of the gas turbine, originally intended as a means of jet propulsion, opened up a new field of application for the variable-pitch propeller and this application with its attendant problems and their solution is discussed. Three types of gas-turbine power plant, together with the appropriate propeller arrangements are reviewed. These are: (1) the direct-connected turbine; (2) the compound-compressor turbine; and (3) the free-propeller turbine. Each engine requires a plurality of controllable pitch stops peculiar to the engine type, and these are described. The necessity for powerful controlling forces in maintaining the blades at any given pitch angle is made clear by consideration of the centrifugal action of a blade element, and an example of the magnitude of these forces is given for a specific case. Brief reference is made to developments in blade bearings, followed by an illustrated description of a contra-rotating propeller.