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STATIC STABILITY LIMIT, hs
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The critical C. G. position for zero elevator trim slope (i. e. for stability) can be found by setting (6.4,24) equal to zero. Recalling that Gmx = CL (h — hn), this yields
h — h,
where hs = hn ———— ——- (6.4,26)
0Lr + 2 CLe
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Depending on the sign of Gmy, hs may be greater or less than hn. In terms of hs, (6.4,24) can be rewritten as
(h — hs) is the “stability margin,” which may be greater or less than the static margin.
FLIGHT DETERMINATION OF h„ AND hs
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For the general case, (6.3,19) suggests that the measurement of hn requires the measurement of Gm and CL . Flight measurements of aerodynamic derivatives such as these can be made by dynamic techniques. However, in the simpler case when the complications presented by propulsive, compressibility, or aeroelastic effects are absent, then the relations implicit in Figs. 6.17 and 6.18 lead to a means of finding hn from the elevator trim curves. In that case all the coefficients of (6.4,13) are constants, and
Thus measurements of the slope of <5„ vs. GL^_m at various C. G. positions produce a curve like that of Fig. 6.20, in which the intercept on the h axis is the required N. P.
When speed effects are present, it is clear from (6.4,27) that a plot of (ddetrijJdV)t against h will determine hs as the point where the curve crosses the h axis.
