Horizontal Axis Fig. 2

Relative Wind

With knowledge of this forward propelling phenomena it is easy to see that Cierva would decide to anchor one end of his airfoil(s) and when they were propelled forward, they would rotate.

In figure 3 it is assumed that the values of lift ar drag, (measured in pounds) are drawn to the same scale. It can be seen that the "lift" line crosses the vertical axis of the airfoil. The value of the lift line ahead of the axis is a propulsive force. In figure 4, the angle of attack of the airfoil is increased which increases the drag. Lift is increased also, but not at the same rate as the drag. In this figure you will see that the "lift" line does not cross the vertical axis and no autorotative force is produced. In an actual situation,

very soon after the rotor was put in this angle of attack and if no power was being applied to the rotor, the rotation would stop.

Persons can usually understand that as long as the autogiro is in level flight, that the angle of attack will remain within the limits for producing an autotative force. Many never could, however, imagine how the angle of attack of the rotor blades could be kept at a low value when the autogiro was descending vertically, and the air is flowing straight up at the rotor.

Figure 5 shows thai while the autogiro and the rotor system are descending vertically, usually at about 10 miles per hour, the rotor tip speed is about 200 miles an hour, and because it is in vertical descent, the air speed at the tip in any point around the circumferance of the rotor is the same and there is no "advancing" blade which meets a greater air speed for part of its circumference or a "retreating" blade which meets a wind blowing towards its trailing edge, because it is not in forward flight. It will be seen from figure 5 that a resolution of all the winds will show a resulting wind from a low enough angle to permit autorotation to continue.

Early in Cierva's experiments he discovered to his dismay that his rotocraft rolled over on their sides as they began to leave the ground.

At this time, Cierva was solving another problem. He was concerned with the bending stresses on the rotor blades as the lift increased

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