The Propellers subject is 5 to 10 percent of the Powerplant test. Most of its questions come from chapter 7 of FAA-H-8083-32B and test cause and effect: what changes when blade angle or oil pressure changes.
Efficiency and blade angle
- Propeller efficiency is thrust horsepower divided by brake horsepower: the engine supplies brake horsepower and the propeller converts it to thrust horsepower.
- With engine power held constant, a larger blade angle raises the angle of attack, lift and drag, so more horsepower is needed to turn the propeller and it slows down.
- Centrifugal twisting force is greater than aerodynamic twisting force and tends to force the blades toward a low blade angle.
- A ground-adjustable propeller operates as a fixed-pitch propeller; its pitch can be changed only when it is not turning.
The constant-speed governor
- The oil pressure for most hydraulic pitch-changing mechanisms comes from the engine lubricating system, boosted by a pump in the governor.
- The cockpit propeller control adjusts the tension on the governor speeder spring, which sets the rpm the governor holds.
- In an overspeed condition in a typical single-acting system, the flyweights move outward and raise the pilot valve, oil flows from the propeller piston, and the propeller increases pitch to slow the engine.
Feathering
On a constant-speed feathering propeller with flyweights and a feathering spring, if governor oil pressure drops to zero in flight the flyweights and spring push toward high pitch and the propeller feathers. This also happens if the engine loses oil pressure. A standpipe on the oil tank outlet leaves a reserve of oil so the propeller can still be feathered if the engine fails (FAA-H-8083-32B chapters 6 and 7).
Turboprops
In the beta range, below the flight idle power lever position, blade angle is not governed by the propeller governor but controlled by the power lever. Fixed-turbine turboprop engines are shut down with the propeller on the locks (ground idle) to prevent load on the engine during restart.
Inspection and repair
- Verify engine tachometer accuracy at 100-hour intervals or at annual inspection, whichever comes first, because an inaccurate tachometer can let the propeller turn faster than rated.
- A transverse crack of any size in an aluminum blade is cause for rejection.
- A dull or hollow sound when coin-tapping a composite blade suggests a debond or delamination.
- Do not use metal scribes to mark blade stations; scratches act as stress concentrators.
- Certificated mechanics may not perform major propeller repairs. They must be done by a certificated propeller repair station, the manufacturer, or an air carrier with the right rating.
- The small drain holes in the metal tipping of a wood propeller let moisture drain and must be kept open.
- Before turning a propeller by hand to check blade tracking, remove one spark plug from each cylinder to make it easier and safer to turn.
Practice FAA Aviation Mechanic Powerplant questions with explanations and timed mock tests.