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: 29 of 171
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NACA TN 3790
from the tail as the outboard propeller can produce a decrease in e
attests to the possible strength of the effect. This fact also points
up the possibility that a propeller situated between the present inboard
and outboard positions may give a large stabilizing effect of downwash
if the major portion of the tail area lies inboard of the wing sections
immersed in the slipstream and if the rotation of the propeller has the
same direction as those on the model. Since the magnitude of the effect
is uncertain, it is neglected in present estimates of stability with the
nacelles moved outward. However, if the tail is no longer subjected to
the high dynamic pressure of the slipstream and if the tail-off stability
can be improved, the stability contributed by the tail will be a smaller
part of the model stability than was the case with the existing model,
and the importance of the effect of power on downwash will be diminished.
On the basis of the foregoing considerations, the assumption will be
made that if the nacelles are moved to stations 0.35 b/2 and 0.60 b/2,
to a first approximation the effects of power on the tail contribution to
stability can be neglected, leaving only the pitching-moment contributions
of the direct propeller forces and the slipstream effect on the wing to
be considered. It will be assumed that the nacelles are moved outward
to stations 0.35 b/2 and 0.60 b/2, the longitudinal position of the
nacelles being established by maintaining the distance between the pro-
peller planes and the reference sweep line, and the vertical position of
the thrust line being established on the basis of linear variation with
spanwise position. The calculation of the pitching moments due to pro-
peller normal force and shaft thrust for the new nacelle locations was
made simply by changing the previous values in proportion to the changes
in the lengths of the moment arms. These data are presented in figures 49
The calculation of the new values of pitching-moment contribution of
the slipstream on the wing involved the use of the increments of lift and
pitching moment due to slipstream derived from the experimental data,
adjusted for changes in the areas of the wing immersed in the slipstream
and for changes in the moment arm resulting from outward movement of the
nacelles. It was assumed that for a given thrust coefficient the distri-
bution of incremental lift over each wing area in the slipstream was
unaltered by moving the propeller outward. The latter assumption implies
that for the case of flaps deflected, the flaps were moved outward with
the nacelles. The estimated pitching-moment contribution of the wing
derived on the above assumptions is presented in figure 51.
The estimated longitudinal stability of the model with nacelles moved
to stations 0.35 b/2 and 0.60 b/2 was calculated using the data in fig-
ures 49 to 51 and equation 10 (the tail lift was not neglected, however).
The slope of the pitching-moment curve, tail off and power off, was
assumed changed by movement of the nacelles and flaps. The factors
l-(e/d) and t( qt/q) were assumed equal to those measured with power
off, while d [t(qt/q) ]/d was taken as zero. The results of these
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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/m1/29/: accessed May 22, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.