Blade Flapping Motion Restriction and. Flapping Compensator

The blade flapping motions are limited by the action of the centrifugal force moment. Moreover, the flapping motions themselves create aerodynamic limiting of these motions. The essence of the limiting amounts to the follow­ing. As the blade flaps upward (Figure 40a), the blade element angle of attack is reduced by – Да as a result of the vertical flapping velocity V^, which leads to reduction of the blade thrust and moment and, consequently, to more rapid restoration of equilibrium about the horizontal hinge. When the blade flaps downward (Figure 40b), the angle of attack increases, which leads to increase of the thrust and limitation of the downward flapping motion.

But the restriction of the flapping motions as a result of centrifugal forces and aerodynamic limiting is not sufficient. Therefore use is made of the so-called pitch control arm compensation or flapping compensator.

The essence of the flapping compensator lies in a special positioning of the blade pitch control elements. It was established earlier that the blade pitch (incidence angle ф) changes with rotation of the blade about its longitudinal axis. Blade rotation is accomplished with the aid of the axial hinge, on the body of which there is the "blade pitch horn" lever. The /57

vertical rod from the main rotor tilt control is connected to the blade horn arm. Connection of the cyclic control rod with the blade horn is accomplished by means of the pitch horn hinge.

If the tilt control rod moves upward, the blade incidence angle is increased (Figure 41a).

A<x

Blade element angle of attack change.

If the control rod moves downward, the blade incidence angle is reduced (Figure 41b). The location of the horn arm hinge relative to the main rotor hub horizontal hinge is of fundamental importance. It may be located on the axis of the horizontal hinge (Figure 41c) or it may be shifted relative to this axis by the distance a (Figure 41d).

In the first case, during flapping motion the horn rotates together with the blade about the horizontal hinge axis so that it does not hinder blade rotation. The blade chord displaces parallel to itself and the incidence angle remains constant.

In the second case, the shift of the horn hinge relative to the horizontal hinge axis leads to change of blade pitch during flapping motions. Thus, when the blade flaps upward the horn hinge, remaining stationary, holds back /58 the blade leading edge, i. e., it causes reduction of the pitch (Figure 4ld).

When the blade flaps downward its pitch increases. This sort of pitch change leads to limiting of the flapping motions. For example, when the blade moves upward the blade pitch is reduced and its thrust and moment are also reduced. Therefore, equilibrium is restored more rapidly and the flapping angle is reduced.

When the blade flaps downward, the increased pitch leads to increased thrust and the flapping angle is restricted. The effectiveness of the flapping compensator action depends on the flapping compensation coefficient K.

The compensation coefficient is the ratio of the distance a between the pitch horn hinge and the horizontal hinge axis to the distance b between the pitch horn hinge and the blade longitudinal axis (Figure 41d).

a

T*

The larger the compensation coefficient, the larger the blade pitch change with variation of the flapping angle and, consequently, the more the blade up and down flapping is restricted.

For most helicopters the compensation coefficient is about 0.5.

By increasing the compensation coefficient we can limit the increase of

the flapping angle for the advancing blade to a point where maximum flapping will not occur at the ip = 210° azimuth, as we have noted above, but rather at the ip % 160° azimuth. In this case, the minimal flapping angle of the retreat­ing blade will occur at the ip ^ 340° azimuth. With this change of the flap­ping angles, the main rotor cone axis will be deflected aft and in the direc­tion of the retreating blade, and the side force will be directed to the right.