AS350 B3e course · Rotors and flight controls
The disc in forward flight
Dissymmetry of lift, flapping, and the retreating blade
The essentials
In forward flight the advancing blade meets faster air than the retreating one. On the Astar's clockwise rotor the advancing side is the left.
Blades flap to even out the lift. Accelerating through effective translational lift the rotor becomes more efficient. At high speed the retreating blade nears stall, which is what the never-exceed speed protects.
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Two sides, two airspeeds
In a still-air hover every blade meets the same airspeed. In forward flight the advancing blade adds the helicopter's speed to its own; the retreating blade loses it.
On a clockwise rotor the advancing side is the left.
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See the difference
Airspeed across the disc in forward flight, and the reverse-flow region. This diagram needs JavaScript.
Slide the airspeed up. Warmer colours are faster air. Watch the red circle near the mast on the retreating side: there, the air meets the blade from behind.
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Flapping evens it out
The advancing blade, meeting faster air, flaps up — which reduces its angle of attack. The retreating blade flaps down and gains angle of attack.
Lift evens out across the disc. This is flapping to equality, and it's what lets a rotor fly forward at all.
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Blowback
Flapping to equality has a side effect. The advancing blade is flapping up fastest on the left; with the quarter-turn lag, it is highest over the nose. The retreating blade is lowest over the tail. The disc tilts aft — blowback, or flapback.
Left alone, the aircraft would slow down. Forward cyclic holds the disc where you want it, and the faster you fly the more you need. This one is the same on both rotor directions: the mirrored left and right cancel out to the same fore-and-aft result.
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Coning
Each blade is pulled outward by centrifugal force and upward by its own lift. It settles where the two balance, so the disc is a shallow cone rather than flat.
More weight, more g, or lower rotor rpm means more coning: lift goes up, or the centrifugal force that holds the blades out goes down. Too much coning wastes lift and loads the blade roots — one reason low rotor rpm with a high load is so dangerous.
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Coriolis and lead-lag
When a blade flaps up, its centre of mass moves closer to the mast. Like a skater pulling in her arms, it tries to speed up — it leads. When it flaps down it lags. The handbook calls this the Coriolis effect.
On an articulated head the drag hinge allows it. On the Starflex, the spherical bearing lets the blade swing and the frequency adapter resists and damps it. That's why lead-lag is always present in forward flight, and why the adapters work constantly — not only when ground resonance threatens.
Inflow
A lifting rotor pulls air down through itself — induced flow. The more air it must accelerate downward, the less angle of attack each blade sees for a given pitch, and the more power the rotor needs.
In the hover with no wind, the rotor works in air it has already accelerated, recirculating at the tips. Wind or forward speed brings fresh air into the disc and reduces induced flow, at the front first. Translational lift and transverse flow are both consequences.
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Translational lift
In the hover the rotor works in its own recirculating downwash. As the aircraft accelerates, it moves into cleaner air and the rotor becomes more efficient. Somewhere around 16–24 kt the handbook's effective translational lift arrives: the same power gives noticeably more lift, and the aircraft wants to climb.
Coming the other way on approach, you lose it — and need more power just as you slow.
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Transverse flow, mirrored
Just before translational lift, the air through the rear of the disc has been accelerated more than the front, so blades there have less angle of attack. With the 90° phase lag, the disc tilts sideways and you feel a roll and vibration.
The handbook describes it for American rotors as a roll to the right. On the Astar it is mirrored: a roll to the left, held with a little right cyclic.
Why it's left on the Astar
Follow the quarter-turn. The weak lift is at the rear of the disc. A clockwise rotor, seen from above, carries a blade from the tail round to the left. A quarter-turn after the tail is the left side — so that's where the disc drops.
An anticlockwise rotor carries the blade from the tail to the right, which is why the handbook says right. Same aerodynamics, mirrored geometry. This is physics rather than an Astar test result; the size of the roll varies with weight, wind and technique.
Retreating blade stall, mirrored
On the Astar the retreating blade is on the right. When it stalls there, lift is lost on the right, and the effect appears a quarter-turn later — over the tail. So the nose pitches up, and the aircraft rolls towards the stalled side: to the right.
The handbook, written for anticlockwise rotors, says the roll is left. The pitch-up is the same; the roll is mirrored. The full chapter on retreating blade stall is in Part 8.
In fast forward flight in an AS350, which side is the retreating blade on?
The right. The rotor turns clockwise, so blades move forward along the left side (advancing) and aft along the right (retreating).
Where is the reverse-flow region?
Near the root on the retreating side, where the blade's own speed is less than the aircraft's — the air hits it from the trailing edge. It grows with airspeed.
You slow through about 20 kt on approach. What do you need?
More. Losing effective translational lift makes the rotor less efficient, so holding the approach takes more power.
Accelerating through transverse flow in an AS350, which way does it want to roll?
Left. Weak lift at the rear of the disc shows a quarter-turn later in the direction of rotation — the left on a clockwise rotor.
Retreating blade stall in an AS350 produces…
Nose up and roll right — towards the retreating side, which is the right on a clockwise rotor.
A blade flaps up. What does the Coriolis effect make it do?
Lead. Its centre of mass moves inward, so it speeds up — on the Starflex the frequency adapter resists and damps the motion.
Sources
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 2, Aerodynamics of Flight
- Starflex rotor · AOPA · 25 May 2011
- Clockwise-rotating helicopter differences · Helicopter Ground
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 11, Helicopter Emergencies and Hazards
Educational only. Not approved training material, and never a substitute for the Rotorcraft Flight Manual, an instructor, or your operator's procedures. Figures marked as manufacturer data or test results are not limitations. This content is a draft and has not yet been reviewed by an instructor.