Power Effects on Trim

If the shift in longitudinal control position while going from a full-power climb to steady autorotation at the same forward speed is excessive, pilots will complain. A typical minimum value written into the flying qualities specifications is 3 inches. The shift is primarily due to the change in aerodynamic pitching moments on the airframe—especially those contributed by the fuselage and the horizontal stabilizer as the angle of attack changes from one flight condition to the other. The calculations for the example helicopter in autorotation, level flight, and climb at

2,0 feet per minute—all at 115 knots—are given in Table 8.8.

For this helicopter, the change in Вx + ax required to trim the rotor aerodynamically is almost equal to the change in ax required to trim the airframe pitching moments so that practically no stick motion is needed when going from one trim condition to another. This is one benefit of having a small horizontal stabilizer. If it were large enough to produce positive angle of attack stability, the shift in control position would have been much greater.