Aircraft Engines and Propellers
This domain tests reciprocating-engine cycles and components, turbine-engine sections and parameters, and propeller pitch control and efficiency. Expect questions on ignition, carburetion, cooling, turbocharging, engine indications and the forces produced by a propeller.
Reciprocating engines
A four-stroke cycle has induction, compression, power and exhaust strokes. The magneto is an independent ignition source and does not depend on the aircraft battery. Carburettor icing is most likely with high humidity at ambient temperatures roughly between -7°C and 21°C, and can occur even when air temperature is above freezing. Mixture must be leaned as altitude increases. Turbocharging raises intake manifold pressure by using exhaust energy.
Turbine engines
A turbojet or turbofan has compressor, combustor, turbine and exhaust sections. The compressor raises pressure, the combustor adds heat at relatively constant pressure, and the turbine extracts energy to drive the compressor. N1 is the low-pressure or fan spool speed; N2 is the high-pressure spool speed. EGT indicates turbine temperature and is limited to protect hot-section life. Thrust is controlled by fuel flow, and EPR compares turbine discharge pressure with compressor inlet pressure.
Propellers
A constant-speed propeller uses a governor to vary blade angle and keep RPM constant. Feathering aligns the blades with the relative airflow to reduce drag after engine failure; reverse pitch produces backward thrust. Geometric pitch is the theoretical forward distance per revolution; slip is the difference between theoretical and actual advance.
Worked example
A propeller has 80 in pitch and turns at 2,400 RPM. Theoretical speed = 80 × 2,400 × 60 / 63,360 = 181.8 mph. If true airspeed is 160 mph, slip = 181.8 - 160 = 21.8 mph, and efficiency = 160 / 181.8 ≈ 88%.
Traps
Do not treat manifold pressure and RPM as interchangeable. Carburettor heat can raise intake temperature and reduce mixture density. Exceeding EGT limits shortens turbine life even if thrust appears normal. Feathering is not the same as reverse pitch.