12.1 Theory of flight — rotary wing aerodynamics
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Scope and exam weight
Sub-module 12.1 is examined at level 1 for A3/A4 and level 2 for B1.3/B1.4. The syllabus lists: terminology; gyroscopic precession; torque reaction and directional control; dissymmetry of lift and blade tip stall; translating tendency; Coriolis effect; vortex ring state, power settling and overpitching; autorotation; ground effect. The EASA Module 12 paper has 100 questions (A) or 128 questions (B1); 12.1 typically supplies 6 (A) to 9 (B1) of them.
Terminology you must know
| Term | Meaning |
|---|---|
| Rotor disc (disk) | Area swept by the rotating blades |
| Tip-path plane | Plane in which the blade tips travel; total rotor thrust acts roughly perpendicular to it |
| Pitch angle / angle of incidence | Angle between blade chord line and the plane of rotation — set mechanically by the controls |
| Angle of attack | Angle between chord line and the resultant relative wind — aerodynamic, not set directly |
| Induced flow (downwash) | Air drawn down through the disc; it reduces the effective angle of attack |
| Coning | Upward bending of the blades from the balance of lift and centrifugal force |
| Feathering | Rotation of a blade about its spanwise (pitch-change) axis |
| Flapping | Up/down movement of a blade about a flapping (or teetering) hinge |
| Lead/lag (hunting, dragging) | Fore-and-aft movement of a blade in the plane of rotation |
| Solidity | Ratio of total blade area to disc area |
The three rotor types follow from which hinges are fitted:
| Rotor head | Flapping | Lead/lag | Feathering | Typical blades |
|---|---|---|---|---|
| Fully articulated | Individual flap hinges | Drag (lead/lag) hinges with dampers | Yes | 3 or more |
| Semi-rigid (teetering) | Blades flap as a unit about a teeter hinge | No (absorbed by blade bending) | Yes | 2 |
| Rigid / hingeless / bearingless | Flexing of blade/hub elements | Flexing | Yes (bearings or elastomeric flexure) | Any |
Torque reaction and directional control
Newton's third law: if the engine turns the main rotor counter-clockwise (viewed from above), the fuselage tries to turn clockwise. On a single-main-rotor helicopter this is opposed by tail rotor thrust controlled by the pedals. Alternatives are a fenestron (ducted fan), NOTAR (circulation control on the tail boom plus a direct jet thruster using fan air) and tandem or coaxial contra-rotating rotors, which cancel torque between the two rotors.
Key fact: More collective pitch = more power = more torque, so more anti-torque pedal is needed whenever collective is raised.
Gyroscopic precession
A spinning rotor behaves like a gyroscope: a force applied to it produces its maximum effect 90° later in the direction of rotation. Control inputs are therefore applied to the blade about 90° before the point where maximum disc tilt is wanted. In practice, the swashplate–pitch-link geometry builds this phase lag in, so moving the cyclic forward tilts the disc forward.
Dissymmetry of lift
In forward flight the advancing blade meets the air at rotor tip speed plus aircraft speed; the retreating blade meets it at tip speed minus aircraft speed. Without correction the advancing side would produce much more lift and the aircraft would roll.
Correction is by blade flapping (plus cyclic feathering):
- The advancing blade flaps up; its induced flow increases and its angle of attack falls.
- The retreating blade flaps down; its angle of attack rises.
- Lift across the disc is equalised. The resulting rearward tilt of the disc is called blowback (flapback) and must be countered with forward cyclic as speed increases.
Retreating blade stall (blade tip stall)
As forward speed increases the retreating blade must operate at an ever-higher angle of attack. Eventually its tip region stalls — this is the main limit on helicopter forward speed (Vne). The FAA describes the stall starting near the tip of the retreating blade and spreading inboard.
| Contributing factors | Typical symptoms |
|---|---|
| High forward speed, high gross weight, high density altitude, low rotor rpm, turbulence, steep or abrupt turns | Low-frequency vibration, nose pitch-up, roll towards the retreating-blade side |
Recovery: reduce collective, reduce forward speed / aft cyclic gently, increase rotor rpm to normal, reduce severity of manoeuvre.
Translating tendency (drift)
In the hover the tail rotor thrust pushes the whole helicopter sideways in the direction of tail-rotor thrust (to the right for a counter-clockwise main rotor). Corrections used by designers:
- Mast tilted slightly to the left (rigging of the main rotor mast/transmission);
- Cyclic rigging so that the disc is tilted slightly left with the cyclic centred;
- Flight control system that automatically adds a small amount of left cyclic.
Because the tail rotor is normally above or below the main-rotor torque plane, the helicopter usually hovers slightly left skid low.
Coriolis effect (conservation of angular momentum)
When a blade flaps up its centre of mass moves closer to the shaft, so it speeds up (accelerates in the plane of rotation); when it flaps down it slows down — like a figure skater pulling in the arms. On articulated rotors this is accommodated by the lead/lag (drag) hinge with a damper. On a two-blade teetering rotor the blades are underslung so that the centre of mass stays nearly at a constant distance from the shaft, which greatly reduces the effect.
Vortex ring state (settling with power) and overpitching
Vortex ring state is a condition in which the helicopter descends into its own downwash. The FAA quotes the conditions as:
- a vertical or nearly vertical descent at a rate greater than the normal induced-flow rate;
- airspeed below effective translational lift;
- 20 % to 100 % of available engine power being used.
Air recirculates around the tips (a doughnut-shaped vortex); raising the collective only makes the descent worse. A fully developed state gives uncommanded pitch and roll oscillations, little collective authority and descent rates that may approach 6,000 ft/min. Recovery is to fly out of the disturbed air by gaining airspeed (forward cyclic) and, height permitting, reducing collective — follow the technique in the rotorcraft flight manual.
Overpitching is pulling more collective than the available power can sustain: drag on the blades rises, rotor rpm decays, and lift is lost — a vicious circle that ends in loss of height or a heavy landing.
Effective translational lift (ETL)
Once forward speed exceeds roughly 16–24 knots the rotor moves out of its own recirculating downwash into undisturbed air. Induced flow falls, efficiency rises and the aircraft climbs without a power increase. Entry to ETL is felt as a brief vibration and a tendency to pitch up and roll right (counter-clockwise rotor).
Ground effect
When hovering within about one rotor diameter of the surface, the ground restricts the downwash and reduces the wingtip-type vortices, so induced flow and induced drag are lower and less power is needed.
| Hover type | Power required |
|---|---|
| IGE (in ground effect) — within about one rotor diameter, over a flat firm surface | Lower |
| OGE (out of ground effect) | Higher — the limiting case for performance charts |
Ground effect is reduced over long grass, water or sloping ground.
Autorotation
Autorotation is flight with the main rotor driven only by the upward airflow through the disc, the engine being disconnected by the freewheeling unit (sprag clutch). The disc is divided radially into:
| Region | Approximate position | What happens |
|---|---|---|
| Stall region | Inner ~25 % of radius | Blade above stall angle — drag only |
| Driving (autorotative) region | ~25 % to 70 % of radius | Total aerodynamic force tilts forward of the axis of rotation and accelerates the rotor |
| Driven (propeller) region | Outer ~30 % | Total aerodynamic force tilts aft — tends to slow the rotor |
Rotor rpm stabilises when the driving and driven forces balance. Collective lowered = more driving region = rpm rises; collective raised = rpm falls. The flare near the ground converts forward speed and rotor energy into a soft touchdown.
Key fact: In autorotation the tail rotor is still driven through the main transmission, so directional control is retained even though the engine has stopped.
Other hazards often examined
- Loss of tail rotor effectiveness (LTE): uncommanded, rapid yaw towards the advancing blade caused by aerodynamic interaction (wind from certain azimuths, weathercocking, vortex-ring of the tail rotor) — not a mechanical failure.
- Dynamic rollover: rolling over a skid or wheel that is in contact with the ground, beyond the critical angle.
- Mast bumping (teetering rotors): the hub strikes the mast in low-g conditions.
- Ground resonance: see 12.3 — a mechanical, not aerodynamic, problem.
Exam traps
- Pitch angle is set by the controls; angle of attack is not the same thing.
- Precession acts 90° after the input in the direction of rotation.
- Flapping corrects dissymmetry of lift; the drag hinge accommodates Coriolis.
- Vortex ring state needs low airspeed and power applied and descent.