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: 25 of 171
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NACA TN 3790
the negative values of d[slt(qt/q)]/da (fig. 35) combined with increasing
values of 0mtail off (fig. 38(a)).
With deflection of the inboard flaps, the slipstream was moved down-
ward. Reference to figures 33 and 36 reveals that in this case
nt(qt/q) increased with increasing lift coefficient and d[tnt(q/q)1/da
became positive as the tail moved into the slipstream, both trends tend-
ing to increase stability according to equation (10).
Considering next the stability curves in figure 45 for the model with
a tail height of 0.10 b/2, it may be noted that with flaps up, the sta-
bility increased with increasing lift coefficient (at constant thrust
coefficients other than zero). This increase in stability was due pri-
marily to the increase in jt(qt/q)(fig. 32) and the positive value of
d[jt(qt/q)]/d (fig. 35) combined with positive values of Cmtail off
(fig. 38). With inboard flaps deflected, the slipstream was deflected
downward so that the tail remained out of the slipstream over most of
the angle-of-attack range. Consequently, nt(qt/q) did not change with
lift coefficient and d[pt (q/q)]/dc approached zero. There was also a
sizeable reduction in 1-(de/da ) due to deflection of the flaps as may
be seen in figures 32 and 33. The result was a loss of stability due to
deflection of the inboard flaps (fig. 45, o0.o b/2).
The stability curves in figure 45 for the model with outboard flaps
may be interpreted in a manner similar to that outlined for the other
cases, noting that in this case, Cmta ff was negative according to
figure 40. It should be observed t W th the outboard flaps deflected,
a large negative angle of tail incidence (it to -14o) was required to
trim the model at high angles of attack.
Effects of vertical movement of the center of moments on the longi-
tudinal stability.- The effects on longitudinal stability of displacing
the center of moments, or center of gravity, a distance 0.,1 below the
original moment center (located at 5/4) are shown in figure 46 for the
case of flaps up. It is observed that with the low tail the effect of
lowering the center of gravity was to increase the longitudinal stability;
whereas, with the high tail, the effect was either much reduced or actu-
ally destabilizing. In both instances a change of tail incidence was
required to retrim about the new center of gravity. The influence of
tail height results from differences in the effect of tail load changes
on stability which, as may be seen in equation (10), are in turn depend-
ent upon the values of d [Lit(qt/q) ]/d.
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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/m1/25/: accessed May 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.