By F. G. Irving
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Additional info for An Introduction to the Longitudinal Static Stability of Low-Speed Aircraft
As mentioned in the previous chapter, F is frequently neglected. g. g. margin" and, as explained in Chapter 4, is equal to the stick-fixed static margin under the conditions of this analysis. Stick-free Stability In this case, the static margin is to be found subject to the condition that CH = 0 and ß is constant. From eqn. 35) the elevator hinge moment coefficient is CH = b1aT + b2r\ + b2ß. If CH = 0, , _ _ ( ί ϋ ^ ) . 9) This equation defines the angle at which the elevator will float freely for given values of αΓ and β.
Suppose now that a disturbance occurs so that the lift coefficient increases to that corresponding to point B. The disturbance might obviously be an increase of incidence due to, say, an up-gust. In FIG. 2. Pitching moment coefficient curve for a statically stable aeroplane. the most general study of static stability, the precise nature of the disturbance is of some importance. As explained in Appendix I, it is usual to consider a change of lift coefficient subject to the condition that eqn. 3) always applies.
G. margin. e. g. must be ahead of the neutral point. If h > hN the static margin is negative and the aeroplane will be statically unstable. So far, it has only been assumed that a change in CL is accompanied by a change in C L T , SO that dCLJdCL has a value. In order to consider static stability in more specific terms, this quantity must be considered in more detail. The Influence of the Tail lift-curve Slope From eqn. e. CL = CLwb + CLT^. e. dCLvvb/da) is denoted by a, this becomes CL = aa + C L T ^ .