Shaft Tilted in Forward Flight

If the shaft is tilted laterally in forward flight, the effect is the same as it is in hover—the tip path plane follows the shaft, and the flapping with respect to the shaft remains unchanged. If, however, the shaft is tilted longitudinally, the nonuniformity of the velocity distribution produces a different situation.

Figure 7.5 illustrates this with a rotor that, for simplicity’s sake, starts from a condition of zero lift on the advancing and retreating blades. Following a sudden nose-up tilt, the immediate result is the same as in hovering: the advancing blade receives a increase in angle of attack and the retreating blade a decrease— producing an unbalanced lift that auses the tip path plane to flap nose up. In forward flight, however, when the blade flaps nose up until it is perpendicular to the shaft, the forces are not yet balanced. The unbalance is due to the forward flight velocity vector. The airflow coming at the rotor as a result of its forward speed modifies both the local angle of attack and the local velocity. Both blades have positive angles of attack, with the angle on the retreating blade actually being the greater, as shown in Figure 7.5c. The lift on the retreating blade, however, is less than on the advancing blade beause the lift is proportional to the product of the angle of attack and the square of the local velocity. This causes the rotor to flap past the perpendicular to the shaft to a more nose-up position where the forces are in balance, as shown in Figure 7.5d. The magnitude of the excessive flapping is approximately proportional to the square of the forward speed. The result is negative angle of attack stability, since the aft flapping generates a nose-up pitching moment about the helicopter’s center of gravity that tends to ause а further increase in the shaft angle of attack. It is an undesirable characteristic, but it an be compensated for rather easily with a horizontal stabilizer of reasonable size.

From this illustration it may be seen that increasing the angle of attack of the shaft also increases the rotor thrust, just as increasing the angle of attack of Л wing increases its lift.

Resultant Velocity Vector

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