Aircraft Propulsion Systems
Propulsion Systems covers the Brayton-cycle gas turbine, thrust generation, compressor stability and engine control. Expect questions on turbofan and turboprop matching, operating limits and FADEC indications.
Thrust and efficiency
Net thrust = m_dot(V_j − V_0) + (p_e − p_0)A_e. Bypass ratio is fan mass flow divided by core mass flow; high bypass improves propulsive efficiency at subsonic speed. N1 is fan or low-pressure spool speed; N2 is high-pressure spool speed. EGT is a limiting parameter, not a direct thrust indication.
Compressor stability
Surge is a global flow reversal; rotating stall is a local cell of stalled blades. Bleed valves, variable stator vanes and acceleration schedules keep the operating line clear of the surge line. A hot start is indicated by a rapid EGT rise; a hung start by low acceleration.
Worked example
Core engine mass flow 20 kg/s, exhaust velocity 500 m/s, flight speed 200 m/s, nozzle pressure balanced. Thrust = 20 × (500 − 200) = 6,000 N. If the same engine is fitted with a fan producing 100 kg/s at 300 m/s, fan thrust = 100 × (300 − 200) = 10,000 N, and total thrust is 16,000 N before pressure terms. This shows why bypass flow dominates subsonic thrust.
Traps
Mixing kg/s with lb/s, or using exhaust velocity as true airspeed, ruins the thrust answer. Do not assume a higher EGT means more thrust. Do not confuse N1 and N2 limits, and do not use gauge pressure in the pressure-thrust term without converting to absolute pressure.