10.6 Propeller Systems, Governors, and Inspection

Blade Angle, Angle of Attack, and the Governor

A propeller is a rotating airfoil that converts engine torque into thrust. Two angles must be distinguished. Blade angle (pitch) is the angle between the blade chord and the plane of rotation, measured at a reference station (usually 75% radius); it is set by the hub/governor. Angle of attack (AOA) is the angle between the chord and the relative wind, which is the resultant of rotational velocity and forward (flight) velocity.

Because forward speed changes the relative wind, AOA changes with airspeed even when blade angle is fixed - the reason a fixed-pitch propeller is efficient only near one airspeed/RPM combination and lets RPM rise and fall directly with power and airspeed. A constant-speed propeller varies blade angle to hold a selected RPM: low pitch (fine) for takeoff/acceleration, high pitch (coarse) for cruise efficiency.

The governor is the heart of a constant-speed system. Flyweights spun by the engine sense RPM and tilt against a speeder spring whose tension the pilot sets with the propeller control. A pilot valve attached to the flyweights ports **engine oil - boosted by a governor gear pump to roughly 200-300 psi - ** to or from the propeller hub piston:

• On-speed: flyweight force balances spring force, the pilot valve is centered, oil is trapped, and blade angle holds.
• Overspeed: RPM rises, flyweights tip out, the pilot valve rises and ports oil to increase blade angle (coarser pitch), which absorbs more power and brings RPM back down.
• Underspeed: RPM falls, flyweights drop in, the pilot valve releases oil and a counterweight/spring (or oil, depending on design) moves blades finer to restore RPM.

Twisting Forces, Feathering, Reverse, and Inspection

Four forces act on a turning blade. Centrifugal force tries to throw the blade out of the hub (the largest steady load). Thrust bending force bends blades forward as the aircraft is pulled along. Torque (air-resistance) bending force bends blades opposite the direction of rotation.

Two twisting moments fight over pitch: the centrifugal twisting moment (CTM) comes from the blade's mass trying to align with the plane of rotation and tends to rotate the blade toward low (fine) pitch - it is by far the stronger; the aerodynamic twisting moment (ATM) arises because the airfoil's center of pressure is forward of the pitch-change axis and tends to rotate the blade toward high (coarse) pitch.

Designers exploit these: CTM is often used to help drive blades to low pitch, and counterweights plus ATM (and oil) drive them to high pitch or feather.

Feathering turns the blades to about 90 degrees to the plane of rotation so a dead engine's propeller stops windmilling and presents minimum drag; feathering systems typically use a spring and counterweights to feather on loss of governor oil pressure, with an unfeathering accumulator or oil to recover. Reverse/beta moves the blades to a negative angle to produce reverse thrust for ground deceleration; beta and reverse must be rigged with low-pitch stops and locks because unintended movement below the flight-idle stop in flight could be catastrophic.

Propeller inspection is fundamentally fatigue prevention. A small nick, gouge, crack, corrosion pit, or erosion is a stress riser where a fatigue crack can start under the high cyclic loads a blade sees. Filing/blending of metal blades must stay within approved dimensional limits because removing material changes strength and balance; composite blades add delamination, erosion, moisture intrusion, lightning, and bond-failure concerns, and wood blades add moisture and glue-joint concerns.

Blade track (tip path within tolerance) and dynamic balance are checked, and any propeller strike is an airworthiness event requiring engine teardown/inspection per approved data.

Governor troubleshooting logic: if RPM rises above the selected value, the blades are not increasing pitch enough - suspect governor fault, low oil pressure to the hub, internal leakage, control rigging, blade friction, or an engine power change. If RPM is low or sluggish, blades may be too coarse or not returning fine, or the engine may not be making power. Compare manifold pressure/torque, RPM, oil pressure, and control position before condemning the propeller - a good engine can overspeed if the prop fails to coarsen, and a good prop turns slowly if the engine cannot make torque.

Two final inspection facts are commonly tested. Blade track is measured by rotating the propeller by hand and comparing each tip's path against a fixed reference; tips out of track beyond the allowed difference (often a fraction of an inch) indicate a bent blade or a loose hub and cause vibration. Dynamic balance is checked with an analyzer that reads vibration amplitude and phase, then adds small weights to the spinner bulkhead to reduce it.

A propeller strike - any contact with the ground or an object while turning - requires a sudden-stoppage / propeller-strike inspection of the engine per the manufacturer's data, because the shock can crack the crankshaft or gear train even if the blades look intact.