Retreating blade stall

Why the retreating side runs out first

Each blade meets air at its rotational speed plus or minus the aircraft's forward speed. On the retreating side the forward speed is subtracted, so the blade moves through the air more slowly and has to fly at a higher angle of attack to carry its share.

Near the root, part of the blade meets air from behind — the reverse-flow region — and makes no useful lift. The outer part has to make up for it. As speed rises, the outer retreating blade is pushed towards its critical angle, and that's where the stall begins.

Sources 1, 2

What brings it closer, and why

Everything that raises the retreating blade's angle of attack:

• airspeed — less air speed over the retreating blade
• high gross weight — more lift needed from every blade
• low rotor rpm — slower blades everywhere
• high density altitude — thinner air, more angle for the same lift
• steep or abrupt turns and pull-ups — g multiplies the lift required
• turbulence — gusts add angle of attack suddenly

They stack, which is why the stall speed isn't one number.

Sources 1, 2

How it develops

It starts small. The tip of the retreating blade stalls first, briefly, once each revolution, and the pilot feels vibration. Push further and the stalled area grows inward.

As the retreating side loses lift, the disc tilts and the aircraft pitches up and rolls towards that side. A pilot who notices the vibration and eases off rarely sees the rest.

Source 1

Why pitch-up and roll

Stall on the retreating side reduces lift there. With the rotor's 90° phase lag, the effect appears a quarter-turn later — at the tail — so the disc tilts back and the nose pitches up. The handbook adds a roll towards the retreating side: usually left for American rotors.

Mirrored, in the Astar that's a pitch-up and roll to the right.

Sources 1, 2

Recovering

The cure is less demand on the retreating blade. The handbook says the first step is usually less collective, then deal with the cause: roll out of the turn, restore low rotor rpm. It warns that forward cyclic deepens the stall by raising the retreating blade's angle of attack — and that aft cyclic does too.

Your RFM and instructor set the technique.

Source 1

The logic behind the table

Vne isn't only a structural number. In thinner air every blade needs more angle of attack for the same lift, so the retreating blade reaches its limit at a lower airspeed. Add mass and the same is true again.

That's why the RFM gives Vne against altitude and temperature, and why other configurations — doors off, external load, hydraulics off — can carry their own limits. The figures are your aircraft's RFM's, and modifications can change them.

Sources 1, 2, 3