6.1. Vertical autorotation
In the case of vertical autorotative descent (without forward speed) without wind, the forces that cause a rotation of the blades are similar for all blades, regardless of their azimuth position.
During vertical autorotation, the rotor disk is divided into three regions (as illustrated in Figure 16a ): driven region, driving region, and stall region. Figure 17 shows the blade sections that illustrate force vectors. Force vectors are different in each region, as the relative air velocity is lower near the root of the blade and increases continually toward its tip. The combination of the inflow up through the rotor with the relative air velocity creates different aerodynamic forces in each section along the blade.
In the driven region, illustrated in Figure 17 , the section aerodynamic force T acts behind the axis of rotation. This force has two projections: the drag force D and lift force L. In this region, the lift is offset by drag, and the result is a deceleration of the blade rotation. There are two sections of equilibrium on the blade—the first is between the driven area and the driving region, and the second is between the driving region and the stall region. At the equilibrium sections, the aerodynamic force T coincides with the axis of rotation. There are lift and drag forces, but neither acceleration nor deceleration is induced.
In the driving region, the blade produces the forces needed to rotate the blades during the autorotation. The aerodynamic force in the driving region is inclined slightly forward with respect to the axis of rotation. This inclination provides thrust that leads to an acceleration of the blade rotation. By controlling the length of the driving region, the pilot can adjust the autorotative rpm.
In the stall region, the rotor blade operates above its stall angle (maximum angle of attack), causing drag, which tends to slow rotation of the blade.
6.2. Autorotation in forward descend
Autorotative force in forward flight is produced in exactly the same scheme as when the helicopter is descending vertically in still air. However, because of the forward flight velocity there is a loss of axial symmetry in the induced velocity and angles of attack over the rotor disk. This tends to move the distribution of parts of the rotor disk that consume power and absorb power, as shown in Figure 16b . A small section near the root experiences a reversed flow; therefore, the size of the driven region on the retreating side is reduced [1].
Тема: Авторотация с положительным шагом "по вертолётному" не по модельному. (Прочитано 9710 раз)