Basic Regimes of Operation

The operating conditions of the main rotor or its working regime are defined as the position of the main rotor relative to the air stream. Depending

on the position, two basic working regimes are considered, those of axial and translational flow.

The axial flow regime is the term used for the operating condition of the main rotor where the axis of the hub is parallel to the oncoming free stream flow. In the axial flow regime the free stream passes perpendicular to the plane of rotation of the main rotor hub (Figure 12a). This regime covers the hovering, vertical climb and vertical descent conditions of the helicopter main rotor. An important feature of the axial flow regime is that the loca­tion of the blade of the rotor, relative to the oncoming free stream, is not changed. Consequently, the aerodynamic forces on the blade as it moves around the circle are not changed.

The oblique flow regime is the term used for the operating conditions of the main rotor, where the airstream approaches the rotor in a direction not parallel to the axis of the hub. An important difference of this regime is that, as the blade moves around in a circle, it continuously changes its location relative to the flow approaching the rotor. As a consequence, the velocity of the flow at each element is changed and also the aerodynamic forces on the blade. The translational flow regime occurs in the horizontal flight of a helicopter and in flight inclined upwards and downwards.

From consideration of the operating conditions, one can see that the position of the main rotor in the airflow is important. This position is determined by the angle of attack of the main rotor.

The angle of attack of the main rotor is termed angle A, and it is formed by the plane of rotation of the hub and the flight velocity vector, or by the undisturbed flow approaching the rotor. The angle of attack is positive if the flow approaches the rotor from below (Figure 12b). If the flow approaches the rotor from above, the angle of attack is negative (Figure 12c). If the airflow approaches the rotor parallel to the plane of rotation of the hub, the angle of attack is zero (Figure 12d).

Figure 12. Operating regimes and angle of attack of the main rotor.

It is not difficult to observe the connection between the operating regime of the main rotor and the angle of attack:

In the axial flow regime, the angle of attack of the main rotor A = + 90°. In the oblique flow regime, А Ф – 90°.

If the angle of attack A = 0°, the operating regime of the main rotor is termed the planar flow regime.