Flight Control Systems
The swashplate, collective/cyclic/pedal control, hydraulic servos, mixing and stability augmentation.
Flight control systems, summary notes
- The SWASHPLATE is the heart of helicopter control: a non-rotating (lower) plate and a rotating (upper) plate. The pilot's collective and cyclic move the non-rotating plate; the rotating plate turns with the shaft and drives the blade pitch links.
- COLLECTIVE raises or lowers the whole swashplate, changing all blades' pitch equally (total thrust). CYCLIC tilts the swashplate, so each blade's pitch changes cyclically once per revolution, tilting the rotor disc to move the aircraft (the input is applied ~90° ahead because of precession).
- The PEDALS change the tail-rotor blade pitch for anti-torque and yaw; a mixing unit combines collective, cyclic and pedal inputs mechanically before the servos.
- Powered helicopters use IRREVERSIBLE hydraulic servos (the pilot cannot feel or be back-driven by rotor loads); loss of hydraulic power makes the controls heavy and may limit manoeuvring, so dual/independent hydraulic systems are common.
- The helicopter is inherently unstable, so a STABILITY AUGMENTATION SYSTEM (SAS/SCAS) and/or autopilot continuously damps and stabilises it; rotor RPM is held by a governor coordinating collective and throttle.
- ⚠ Exam trap: the SWASHPLATE (stationary + rotating halves) converts fixed collective/cyclic inputs into rotating blade-pitch changes; cyclic changes each blade's pitch ONCE PER REVOLUTION to tilt the disc, and the input leads the desired result by ~90° (precession).
How does moving the cyclic forward cause the helicopter to pitch forward and accelerate?
Forward cyclic tilts the (non-rotating) swashplate, which cyclically varies each blade's pitch once per revolution. Because of gyroscopic precession the maximum pitch change is applied about 90° before the point where the disc should drop, so the rotor disc tilts forward. The tilted total-rotor-thrust vector now has a forward component, pulling the helicopter forward.
Flight controls concept map
Flight Control Systems
Flight control systems quiz
Flight Control Systems, quiz
1. The swashplate converts
Non-rotating collective/cyclic inputs into rotating blade-pitch changesFuel flow into thrustElectrical signals into hydraulic pressure2. Raising the collective moves the swashplate so that
All blades change pitch equallyEach blade changes pitch once per revOnly one blade changes3. Tilting the swashplate with cyclic changes each blade's pitch
Once per revolution, tilting the discEqually for all bladesNot at all4. The pedals control
Tail-rotor blade pitch (anti-torque/yaw)Main-rotor collective pitchEngine fuel flow5. Helicopter flight-control servos are described as 'irreversible', meaning
Rotor loads cannot back-drive the controls to the pilotThe pilot feels every rotor loadThey cannot be moved6. Because the helicopter is inherently unstable, it uses a
Stability augmentation system (SAS/SCAS) or autopilot to damp itFixed trim tabs only, set on the ground to balance the aircraft at its normal cruising conditionNo stabilisation at all, on the basis that the pilot's own control inputs damp the natural motion7. Loss of hydraulic power to the controls typically makes them
Heavy and possibly limited in movementLighter and easierUnaffected8. Forward cyclic input results in the disc tilting forward because, due to precession, maximum pitch change is applied
About 90° before the point where the disc should dropAt the point where the disc should drop180° later