Propellers
Domain 14.4 examines how a propeller converts shaft power into thrust, how blade angle is controlled, and how feathering, reversing and vibration control are achieved. Expect pitch and advance-ratio arithmetic alongside governor logic questions.
Pitch and blade geometry
Blade angle is measured between the blade chord and the plane of rotation. Geometric pitch is the distance the propeller would advance in one revolution with no slip; effective pitch is the real advance; slip = geometric − effective. Blades are twisted because tangential speed rises with radius, which keeps the local angle of attack roughly constant along the span.
Advance ratio and efficiency
J = V/(nD), with V in m/s, n in rev/s and D in metres. Propeller efficiency η = thrust power ÷ shaft power = (T × V)/P. Efficiency peaks near the design advance ratio, so a fixed-pitch propeller must compromise: fine pitch favours take-off and climb, coarse pitch favours cruise. Beyond the useful J range the blades stall or the propeller windmills and produces drag rather than thrust.
Constant-speed control
Governor flyweights sense rpm against a spring; if rpm falls below the setting, the pilot valve ports oil to reduce blade angle (finer), unloading the engine so it accelerates back to set rpm. Feathering drives the blades to about 90° to stop windmilling, using oil pressure or a feathering pump with spring or nitrogen backup. Reverse uses the beta range below flight idle and is blocked in flight by the power lever and interlocks. Synchrophasing holds a fixed blade phase relationship between engines to cut noise and vibration.
Worked example
V = 72 m/s, D = 2 m, n = 40 rev/s (2 400 rpm). J = 72/(40 × 2) = 0.9. With 2 000 N thrust and 180 kW shaft power, η = (2 000 × 72)/180 000 = 0.80, i.e. 80%. If rpm dropped to 30 rev/s at the same speed, J = 1.2 and, at unchanged blade angle, the angle of attack rises toward the stall region, so efficiency falls and vibration increases.
Traps
Blade angle and pitch are related but not identical - pitch is a helix distance. Finer means less load and higher rpm. Feathering is not reversing. Governor failure toward coarse or fine has opposite rpm effects, so read which way the oil is ported. Slip is geometric minus effective pitch, not the reverse.