Aircraft Propellers | Control and Operations (Part 3)

Hydromatic Propellers

Basic Operation Principles : The pitch changing mechanism of hydromatic propeller is a mechanical-hydraulic system in which hydraulic forces acting upon a piston are transformed into mechanical forces acting upon the blades.

Piston movement causes rotation of cam which incorporates a bevel gear (Hamilton Standard Propeller) . The oil forces which act upon the piston are controled by the governor

Single Acting Propeller: The governor directs its pump output against the inboard side of piston only, A single acting propeller uses a single acting governor. This type of propeller makes use of three forces during constant speed operation , the blades centrifugal twisting moment and this force tends at all times to move the blades toward low pitch , oil at engine pressure applied against the outboard side of the propeller piston and this force to supplement the centrifugal twisting moment toward the low pitch during constant speed operation., and oil from governor pressure applied against the inboard side of the piston . The oil pressure from governor was boosted from the engine oil supply by governor pump and the force is controlled by metering the high pressure oil to or draining it from the inboard side of the propeller piston which balances centrifugal twisting moment and oil at the engine pressure.

Double Acting Propeller: The governor directs its output either side of the piston as the operating condition required. Double acting propeller uses double acting governor. This type of propeller , the governor pump output oil is directed by the governor to either side of the propeller piston.

Principle Operation of Double Acting :           Overspeed Condition : When the engine speed increases above the r.p.m. for which the governor is set . Oil supply is boosted in pressure by thr engine driven propeller governor , is directed against the inboard side of the propeller piston. The piston and the attached rollers move outboard. As the piston moves outboard , cam and rollers move the propeller blades toward a higher angle , which inturn, decreases the engine r.p.m.          Underspeed Condition : When the engine speed drops below the r.p.m. for which the governor is set. Force at flyweight is decrease and permit speeder spring to lower pilot valve, thereby open the oil passage allow the oil from inboard side of piston to drain through the governor. As the oil from inboard side is drained , engine oil from engine flows through the propeller shaft into the outboard piston end. With the aid of blade centrifugal twisting moment, The engine oil from outboard moves the piston inboard. The piston motion is transmitted through the cam and rollers . Thus, the blades move to lower angle

The Feathering System

Feathering : For some basic model consists of a feathering pump, reservoir, a feathering time-delay switch, and a propeller feathering light. The propeller is feathered by moving the control in the cockpit against the low speed stop. This causes the pilot vave lift rod in the governor to hold the pilot valve in the decrease r.p.m. position regardless of the action of the governor flyweights. This causes the propeller blades to rotate through high pitch to the feathering position.

Some model is initiated by depressing the feathering button. This action, auxiliary pump, feather solinoid, which positions the feathering valve to tranfer oil to feathering the propeller. When the propeller has been fully feathered, oil pressure will buildup and operate a pressure cutout switch which will cause the auxiliary pump stop. Feathering may be also be accomplished by pulling the engine emergency shutdown handle or switch to the shutdown position.

Unfeathering : Some model is accomblished by holding the feathering buttn switch in the out position for about 2 second . This creates an artificial underspeed condition at the governor and causes high-pressure oil from the feathering pump to be directed to the rear of the propeller piston. As soon as the piston has moved inward a short distance, the blades will have sufficient angle to start rotation of the engine. When this occurs , the un-feathering switch can be released and the governor will resume control of the propeller.