Main rotor blades obey the same rules as any aerofoil, with special shaping on modern machines to suit peculiar requirements (such as the Bell 407). Otherwise, they are generally symmetrical in cross section, to restrict movement of the Centre of Pressure.
The pitch angle is that between the blade's chord line and the spin axis of the main blades, and the plane of rotation, which is parallel to it (usually above). The plane of rotation contains the centres of mass of the rotating blades (the axis of rotation is a vertical element, which the blades rotate around – it is at right angles to the plane of rotation, and not necessarily in line with the rotor mast. The difference between them is the flapping angle). The pitch angle is varied with the collective and cyclic controls (see Airframes & Engines, next), and is not the same as the angle of attack, between the chord and the relative airflow.
Rotor Profile Drag comes from rotor blades at zero pitch, occurring purely because the blades are rotating. Air flowing through the disc at positive angles of attack suffers from induced drag, which is highest in the hover.
The downwards motion of air through the blades is called the induced flow.
As the helicopter moves, the blades will move above and below the plane of rotation, in a process called flapping. A flapping hinge allows this movement to happen, to cope with different angles of attack around the disc, and equalise the lift around it. They are needed when you have more than two blades, which would use a teetering head, and work like a seesaw for the same effect, where the two blades will flap as a unit.
When the helicopter moves forward, the blade going forward will develop more lift because of the added speed (from the helicopter's forward movement and that of the blade), so it will fly higher. As it does so, the angle of attack reduces because of the change in relative airflow.
On the other side, the blade going backwards will generate a lot less lift because of its reduced speed, in some areas producing a reverse effect, which will cause the blade tip to stall if it gets large enough - on a Bell 206 at 100 kts, the non-lift producing area of the retreating blade is about 25%. This will make it fly down to increase its angle of attack, to create more lift (needing more forward cyclic to compensate).
Disymmetry of lift, therefore, is the difference in lift between the advancing and retreating blades, compensated for by flapping, which, unfortunately, causes the centre of mass of the blades to move, making them speed up or slow down relative to each other. Limited movement horizontally is provided with dragging hinges -dragging is the movement of a rotor blade forward or backward in its mounting. However, such hinges are only found in articulated heads (when a blade is ahead of its normal position, it is leading, and when behind, it is lagging). Semi-articulated heads (as with the AStar) may have a flexible coupling that allows fore-and-aft movement, but with no flapping hinges – instead, the blades flex when compensating for lift. The pitch angle of the blades is changed by feathering, i.e. allowing them to rotate around their axes.