THE AFT LIMIT
The most rearward allowable location of the CG is determined by considerations of longitudinal stability and control sensitivity. The behavior of the five principal control gradients are summarized in Fig. 3.10 for the case when the aerodynamic coefficients are independent of speed. From the handling qualities point of view, none of the gradients should be “reversed,” that is, they should have the signs associated with low values of h. When the controls are reversible, this requires that h < h’n. If the controls are irreversible, and if the artificial feel system is suitably designed, then the control force gradient dP/dV can be kept negative to values of h > h’n, and the rear limit can be somewhat farther back than with reversible controls. The magnitudes of the gradients are also important. If they are allowed to fall to very small values the vehicle will be too sensitive to the controls. When the coefficients do not depend on speed, as assumed for Fig. 3.10, the NP also gives the stability boundary (this is proved in Chap. 6), the vehicle becoming unstable for h > h’n with free controls or h> h„ with fixed controls. If the coefficients dependent on speed, for example, Cm = Cm(M), then the CG boundary for stability will be different and may be forward of the NP.
As noted in Chap. 1, it is possible to increase the inherent stability of a flight vehicle. Stability augmentation systems (SAS) are in widespread use on a variety of airplanes and rotorcraft. If such a system is added to the longitudinal controls of an airplane, it permits the use of more rearward CG positions than otherwise, but the risk of failure must be reckoned with, for then the airplane is reduced to its “inherent” stability, and would still need to be manageable by a human pilot.
THE FORWARD LIMIT
As the CG moves forward, the stability of the airplane increases, and larger control movements and forces are required to maneuver or change the trim. The forward CG limit is therefore based on control considerations and may be determined by any one of the following requirements:
1. The control force per g shall not exceed a specified value.
2. The control-force gradient at trim, ЭP/dV, shall not exceed a specified value.
3. The control force required to land, from trim at the approach speed, shall not exceed a specified value.
4. The elevator angle required to land shall not exceed maximum up elevator.
5. The elevator angle required to raise the nose-wheel off the ground at takeoff speed shall not exceed the maximum up elevator.