Variation of autorotation conditions for the different blade elements
Answer 1. The different blade elements have different autorotation conditions. These conditions are determined by the geometric twist of the blade, i. e., by the magnitude of the blade element incidence angle and the magnitude of the angle of attack increment caused by the vertical rate of descent. The incidence angles are larger for the root elements than for the tip elements. Increase of the incidence angles leads to deceleration of the autorotation.
The angle of attack increment (Да = arc tg VjesA°r) depends only on r, which means that its magnitude is larger for the root elements. The larger Да, the more accelerated the autorotation will be. The effect of the angle of attack increment on autorotation is greater than the influence of geometrical twist, therefore, the root elements will have accelerated autorotation while the tip elements will have decelerated autorotation.
Answer 2. The autorotation conditions are different for the different blade elements. The autorotation conditions are determined by the tilt of the force vector AR, and this tilt, in turn, depends on the pitch of the given element. Consequently, the autorotation conditions are determined by the pitch of the given element. As a result of the geometric twist, each element has its own pitch. The pitch for the root elements is larger, and this means that for these elements the force vector AR is tilted aft more, and the autorotation is decelerated. The tip elements have lower pitch, therefore, they have accelerated autorotation.
Answer 3. The autorotation conditions of the different blade elements are determined by the geometric twist, circumferential velocity tor of the blade element, and the induced velocity. As a result of geometric twist, the root blade elements have more pitch and higher induced velocity, therefore, they will have a lower vertical flow velocity. As a result of the higher incidence angle and lower vertical velocity, there is a reduction of the angle of attack increment (Да = acr tg V^eg д/шг) » and, therefore, the force vector AR is tilted aft. The conclusion is that the autorotation of the root elements will be decelerated, while that of the tip elements is accelerated.