Rotary-Wing Aerodynamics
Collective/cyclic control, dissymmetry of lift and flapping, precession, translational lift, ground effect and autorotation.
Rotary-wing aerodynamics — summary notes
Free- 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.
Rotary-wing aerodynamics concept map
FreeRotary-Wing Aerodynamics
Rotary-wing aerodynamics quiz
FreeRotary-Wing Aerodynamics — quiz
1. The collective control changes
2. The cyclic control
3. The tail rotor provides
4. Dissymmetry of lift in forward flight is corrected by
5. The advancing blade flaps UP, which
6. A control input takes effect about 90° later in the direction of rotation because of
7. After an engine failure the rotor keeps turning because the
8. A helicopter's maximum forward speed is ultimately limited by