TM 1-1500-204-23-5c. FeatheringPropeller.A feathering propeller is acontrollable propeller having a mechanism to change thepitch to an angle so that forward aircraft motion producesa minimum windmilling effect on a power-off propeller.Feathering propellers must be used on multiengineaircraft to reduce propeller drag to a minimum underengine failure conditions.d. Reverse-PitchPropeller.The reverse-pitchpropeller is a controllable propeller in which the bladeangles can be changed to a negative value duringoperation. The purpose of the reversible-pitch feature isto produce a high negative thrust at low speed by usingengine power. Although reverse-pitch may be used inflight for steep descents, it is used principally as anaerodynamic brake to reduce ground roll after landing.2-5. Propeller Efficiency. A propeller is used with anengine to provide thrust. The engine supplies brakehorsepower through a rotating shaft. The propellerabsorbs the brake horsepower and converts it into thrusthorsepower. Propeller efficiency is the ratio of thrusthorsepower. Thrust horsepower usually constitutesapproximately 80 percent of the brake horsepower. Theother 20 percent is lost in friction and slippage.Controlling the blade angle of the propeller is the bestmethod of obtaining maximum propeller efficiency for allconditions encountered in flight.2-6. Engine Efficiency. Mechanical, thermal, andvolumetric efficiency are explained in the followingparagraphs.a. MechanicalEfficiency. The mechanicalefficiency of an engine is measured by the ratio of theshaft output or brake horsepower to the indicatedhorsepower or power developed in the cylinders. It canbe expressed by the following formula:Brake horsepowerMechanical efficiency = Indicated horsepowerFigure 2-1. Hartzell Pitch Change Mechanism2-4
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