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: 20 of 171
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NACA TN 3790
at lift coefficients well below the maximum. Comparison of these data
with the curves of lt(qt/q) vs. a (fig. 34) indicates that dJdCL
became more negative because the tail entered the slipstream. Moving
the flaps from the inboard to the outboard location moved the effective
center of pressure of wing sections affected by them out along the span,
which not only produced more negative pitching moments at a given CL
and Tc (apparent in fig. 41 in spite of the change of tail incidence) but
also, at a given CL, reduced the change of pitching moment with increas-
ing Tc. The latter effect can be explained on the basis of the data in
figure 21 which show that the pitching-moment increment due to slipstream
on the wing with outboard flaps became more negative with increasing Tc;
whereas, with inboard flaps, it became more positive. Moving the flaps
outboard also caused a large reduction in effective downwash e at all
thrust coefficients (as may be seen from fig. 24). This effect in com-
bination with the more negative pitching moments from the wing caused the
large negative tail incidence required to trim the model at moderate lift
Effect of single-propeller operation.- The data obtained with the
inboard and the outboard propellers operating independently are of con-
siderable interest, not only because they help to explain the large
effects of operating propellers on the model as tested, but because they
can be used as the basis for estimating the effects of configuration
changes such as moving the nacelles to other spanwise positions.
In figure 42 the pitching-moment characteristics of the model with
the tail off and both propellers operating are compared with similar data
with the inboard and outboard propellers operating independently. Data
are presented for the model with the flaps up and with the inboard flaps
deflected. The translation of the pitching-moment curves with increas-
ing Tc, evident in all of these data, is primarily the result of positive
pitching moments contributed by the propeller thrust. (As may be seen
from fig. 19, this increment of pitching-moment coefficient was essen-
tially independent of angle of attack at a given thrust coefficient.)
The data of figure 42 for the case of only the inboard propeller operating
show an increase in dCm/dCL with increasing Tc. This effect was caused
by the contributions of propeller normal force and slipstream effect on
the wing (see figs. 18 and 20). With outboard propeller only, the slope
of the pitching-moment curves decreased with increasing Tc. In this
case the portion of the wing affected by the slipstream lies behind the
moment center. Consequently, the moment due to slipstream effect on the
wing opposed the moment created by the outboard propeller normal force,
the latter moment being of considerably less magnitude than that from
the inboard propeller because of the more rearward location of the pro-
peller disc (see figs. 18 and 20). The changes in slope of the pitching-
moment curves caused by inboard and outboard propellers appear to be
_ _~_ - - ~ CIC~L ~~-~- ~ CC - -- - i
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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/m1/20/: accessed May 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.