Analysis of Wind-Tunnel Tests to a Mach Number of 0.90 of a Four-Engine Propeller-Driven Airplane Configuration Having a Wing With 40 Degrees of Sweepback and an Aspect Ratio of 10 Page: 23 of 171
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NAA TN 3790
It is evident from equation (10) that the magnitude of the pitching-
moment coefficient, tail off, can affect the tail contribution to stabil-
ity if d[nt(qt/q) ]/da is not negligible. Figures 32 to 37 show the
quantities making up the tail contribution as they are affected by power
changes while figures- 15 to 21 indicate the effects of power on the tail-
off components of stability. The nonlinearities in the variation of
(dCm/dCL)trtm with CL, shown in figure 1(b), are due largely to changes
in the tailload term.
The static margin, shown in figure 4(c), represents the maximum
distance the center of gravity may be moved rearward without making the
airplane unstable. It is normally the most convenient stability para-
meter where center-of-gravity travel is to be considered. It may be
noted from figures 44(b) and 4(c) that for this configuration there were
in some cases large differences between (dCm/dCL)trim and the static
margin. The differences can be explained by means of the following equa-
tions which describe the moment relationship between the existing center
of moments and a more rearward center of moments (indicated by a prime)
separated by the distance Ax:
Cm' = Cm+ C-- L (11)
dCm' dCm Ax
dC,- d (12)
dCL dCL c
It is understood that all derivatives are for constant tail incidence.
If the model were trimmed about the original center of moments, these
cm' Ax (13)
dCm dCm +Ax (P)
dCL = KdCL/trim +
Thus, Ax/s is the change in slope of the pitching-moment curve at
the original tail incidence. Now, an additional increment of slope may
occur when the mbdel is retrimmed since this involves a change in at
which, as may be seen from equation 4, can change the tail contribution
to the slope of the pitching-moment curve if d[Bt(qtl/q)]/d is not zero.
Neglecting tail lift, this increment may be expressed
(adCm' at - altC(s t)
4 - V (15)
\dCL aw da
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Edwards, George G.; Buell, Donald A.; Demele, Fred A. & Sutton, Fred B. Analysis of Wind-Tunnel Tests to a Mach Number of 0.90 of a Four-Engine Propeller-Driven Airplane Configuration Having a Wing With 40 Degrees of Sweepback and an Aspect Ratio of 10, report, September 1956; (https://digital.library.unt.edu/ark:/67531/metadc56014/m1/23/: accessed May 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.