Aircraft General Knowledge — PPL(H)

Helicopter systems: rotor head, transmission, engine, governor, fuel system, flight instruments, and the systems unique to rotary-wing aircraft.

Exam Focus

Most Relevant To

  • Aircraft General Knowledge
  • Principles of Flight

Know This Cold

  • Main rotor head types: fully articulated, semi-rigid (teetering), rigid.
  • Tail rotor purpose — to counteract main rotor torque and provide directional control.
  • Governor and correlator function on piston helicopters.
  • Droop stop purpose: prevents blade droop when rotor is stopped or at low RPM.
  • Fuel system: R22 uses AVGAS 100LL. Fuel drain procedure before every flight.
  • RRPM (rotor RPM): the most safety-critical instrument on a helicopter.

Main Rotor Systems

The main rotor is the primary lift-generating surface on a helicopter. The rotor head design determines how the blades can move relative to the hub and how control inputs are transmitted.

  • Fully articulated: blades can flap, lead/lag, and feather independently. Used on medium and heavy helicopters.
  • Semi-rigid (teetering): blades are connected in pairs, teetering on a central hinge. Used on R22 and R44.
  • Rigid: blades are fixed to the hub; all movement is through blade flex. Used on some advanced types.
  • Collective pitch: changes the angle of attack of all blades simultaneously — controls altitude.
  • Cyclic pitch: changes blade pitch differentially as the rotor turns — controls direction of flight.
  • Anti-torque pedals (tail rotor pedals): control tail rotor pitch — provide yaw control.

Instructor Tip

The R22 uses a semi-rigid, teetering rotor. This means both blades are a single rigid unit that teeter on a central bolt. If one blade flaps up, the other flaps down. This is important for understanding mast bumping — a dangerous condition where the rotor contacts the mast.

Tail Rotor

The tail rotor serves two purposes: it counteracts the torque reaction of the main rotor (which would otherwise spin the fuselage in the opposite direction) and it provides directional control in the yaw axis. Tail rotor failure is one of the most serious emergencies in a helicopter.

  • Torque reaction: main rotor turns anti-clockwise (UK/European convention from above) — fuselage tends to yaw right.
  • Left pedal = increases tail rotor pitch = more anti-torque thrust = nose left.
  • Right pedal = decreases tail rotor pitch = less thrust = nose right (or tail left).
  • Tail rotor failure at cruise: may be landable with an autorotation approach.
  • Tail rotor failure in hover: requires immediate landing before control is lost.

Engine and Transmission

Most training helicopters use a piston engine (R22: Lycoming O-320; R44: Lycoming O-540). Unlike an aeroplane, the engine does not directly drive the rotor — there is a clutch and transmission system between them.

  • Centrifugal clutch: engages as engine RPM increases during start-up.
  • Freewheeling unit (sprag clutch): allows the rotor to continue spinning if the engine stops — essential for autorotation.
  • Governor (R44): automatically maintains a target RRPM by adjusting throttle.
  • Correlator (R22/R44): links collective to throttle — raises throttle as collective is raised, reducing pilot workload.
  • RRPM gauge: most critical instrument; low RRPM = insufficient lift and control.

Common Mistake

Carburettor icing affects piston helicopters exactly as it does aeroplanes. The same conditions apply: +10–20°C with high humidity, at low power. Application of carb heat on a helicopter causes RRPM to drop momentarily — expect this and do not raise collective to compensate.

Flight Instruments

The flight instrument suite on a training helicopter is similar to a fixed-wing aircraft. The key additions or differences are:

  • RRPM gauge (tachometer): rotor RPM — must stay within green arc.
  • Engine RPM gauge: on helicopters with a governor, this should track RRPM in normal ops.
  • Manifold pressure gauge: indicates power output on normally aspirated piston engines.
  • Slip ball: same as fixed-wing; a balanced helicopter shows the ball in the centre.
  • Pitot-static instruments (ASI, altimeter, VSI): same function as fixed-wing.