Helicopter aerodynamics: advancing blades & retreating blades

[Whitestar]

Why does a helicopter blades operates in an advancing direction while the other, the retreating blades travels backwards? That is, what would happen to the helicopter if the retreating blades were to travel forward like their advancing blades counterparts?

 

[Baluncore]

The blades are airfoils that generate lift. The blades are balanced and rotate together in the same direction. When hovering all blades have the same airspeed and lift.

When traveling forwards, the helicopter velocity is added to the blade rotation velocity on one side, while being subtracted from the blade velocity on the other side.

The airspeed and lift from the two sides is therefore different, so the blade angle of attack must be adjusted as they rotate to keep the helicopter stable.

 

[Filip Larsen]

I am not entirely sure what kind of answer you are looking for.

Assuming the helicopter in normal flight accelerates to increase forward airspeed, then at some airspeed the retreating blades may get into a stall[1] effectively preventing the helicopter to accelerate further in a controlled manner. There can also be issues and limits at airspeeds where the tip of the advancing blades gets close to Mach 1. Taken together these limits more or less excludes the blades from ever experiencing reverse airflow during normal flight, thus in my estimate making the situation unlikely to have been a target for detailed theoretical and practical studies.

That said, I can imagine that a (relatively low speed) reverse airflow situation can be established with a static or slow rotating rotor on a helicopter sitting on ground in a strong headwind, where the airflow over some of the blades indeed will be reverse. Depending on the blade airfoil profile this airflow could very well produce (a little) lift.

Symmetric airfoils obviously produce same lift at same pitch range for both forward and reverse airflow (with the pitch being reversed when the flow is reverse), but even high cambered airfoils have pitch ranges where they produce (some) lift when in reverse airflows. Of course, both fixed wings and rotor blades can easily have mechanical designs that makes them ill-suited for transferring any significant lift from reverse flow. Rotor blades, for instance, are typically designed to work optimally when in operational rotation and could very well be incapable of transferring any significant fraction of the full load when not rotating.

Or put in another way, I would not be surprised if the blades of most helicopters would break if you tie down the helicopter with a non-rotating rotor in a wind tunnel at normal flight airspeed and then pitched the blades up/down.

[1] https://en.wikipedia.org/wiki/Retreating_blade_stall.

 

[rcgldr]

Cambered air foils produced a downwards pitching torque, which would result in additional strain at the hub and twisting of the rotor blades, so most helicopters use symmetrical (or nearly so) rotors.

There would be reverse flow on the inner portions of the retreating blades (the portion next to the hub).

The cyclic changes the pitch of the blades as they revolve around the hub. Gyroscopic reaction to a torque is delayed 90° (relative to main rotor rotation), so that a roll torque results in a pitch reaction, and vise versa, so the input is advanced by 90° to compensate. My understanding is that the pilot has to compensate for increased speed with pitch down input.