Rotary-Wing Aerodynamics
Collective/cyclic control, dissymmetry of lift and flapping, precession, translational lift, ground effect and autorotation.
Rotary-wing aerodynamics, summary notes
- A helicopter rotor is a set of rotating aerofoils. Total rotor thrust is set by the COLLECTIVE (which changes the pitch of all blades together); the CYCLIC tilts the rotor disc to move the aircraft horizontally; the TAIL ROTOR provides anti-torque and yaw control against the main-rotor torque reaction.
- In forward flight the advancing blade meets more airflow than the retreating blade, DISSYMMETRY OF LIFT, which is equalised by blade FLAPPING: the advancing blade flaps up (reducing its angle of attack) and the retreating blade flaps down (increasing its AoA). Retreating-blade stall ultimately limits forward speed.
- Because the rotor is a large gyroscope, a control input takes effect about 90° later in the direction of rotation (gyroscopic precession), the control linkage is rigged to allow for this phase lag.
- Translational lift is the extra lift gained as the helicopter accelerates into undisturbed air; ground effect is the cushion near the surface that reduces the power required in the hover.
- After an engine failure the FREEWHEEL (sprag) unit disconnects the rotor from the engine and the helicopter enters AUTOROTATION, airflow upward through the descending rotor keeps it turning, storing energy for a controlled landing flare.
- ⚠ Exam trap: dissymmetry of lift is corrected by blade FLAPPING; control inputs act ~90° later due to gyroscopic precession; and autorotation relies on the FREEWHEEL unit disconnecting the dead engine so upflow drives the rotor.
An engine fails in the cruise. How does the rotor keep turning, and what component makes this possible?
The freewheel (overrunning/sprag) clutch automatically disengages the now-slower/stopped engine from the rotor drive, so the engine cannot drag the rotor down. The pilot lowers the collective and the helicopter descends; air flowing up through the disc drives the rotor (autorotation), maintaining rotor RPM and energy for a landing flare. Without the freewheel unit the seized/slow engine would stop the rotor.
Dissymmetry of lift & blade flapping
Autorotation & the blade regions
Rotary-wing aerodynamics concept map
Rotary-Wing Aerodynamics
Rotary-wing aerodynamics quiz
Rotary-Wing Aerodynamics, quiz
1. The collective control changes
The pitch of all rotor blades together (total thrust)The pitch of one blade onlyOnly the tail-rotor pitch2. The cyclic control
Tilts the rotor disc to move the helicopter horizontallyChanges engine RPMFeathers the tail rotor3. The tail rotor provides
Anti-torque and yaw controlMain liftForward thrust only4. Dissymmetry of lift in forward flight is corrected by
Blade flappingIncreasing engine RPMThe tail rotor5. The advancing blade flaps UP, which
Reduces its angle of attack (reducing its lift)Increases its angle of attackStops it rotating6. A control input takes effect about 90° later in the direction of rotation because of
Gyroscopic precessionGround effectTranslational lift7. After an engine failure the rotor keeps turning because the
Freewheel (sprag) unit disconnects the engine and the rotor autorotatesThe battery drives the rotor through an electric motor until the aircraft is on the groundThe tail rotor drives the main rotor back through the gearbox once the engine has stopped8. A helicopter's maximum forward speed is ultimately limited by
Retreating-blade stallTail-rotor failureFuel flow