Investigations on the Downwash Behind a Tapered Wing With Fuselage and Propeller Page: 28 of 52
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surfaces with parameters = const., cw = 0 and full
throttle may be read. .The related. ,value of any other
pair o' values was obtained from the polar diagrams or
from figures 7 and 10. In both tail arrangements, the-hor-
isontal tail surface does not move out of the slipstream
as a closer examination of the lateral axis downwash curves
disclosed (figs. 23, 29, 32, 34 to 37), so that at last
with the --- values of figure 62 the graphs of the rela-
tionship between aws and a could be .developed.
Figure 56 presents such a graph for the low-wing model"
with angular fuselage a-nd body-fixed tail.
Figure 67 shows ---= f(a). For comparison the stabil-
dM d a
ity values --- were plotted also for constant ch = 0 (fig.
68). This arrangement is termed "wind-fixed" as against the
normal "body-fixed" system. The graphs disclose the follow-
a) On a wind-fixed horizontal tail surface, the -
values are usually higher than on one that is body-fixed.
This result can be explained with the fact that with wind-
fixed arrangement the horizontal tail surface is far removed
from the vortex sheet and this distance increases with 'the
angle of attack.
b) For wind-fixed horizontal tail surface, all curves -
so long as cmax , is not"approached - lie above the refer-
ence curve; or in other words, there is in every case a sta-
bility gain relative to the reference model. On approaching
cmax the curve for "full throttle" drops below the reference
curve, but then very considerably.. On the body-fixed hori-
zontal tail surface on the other hand, the likewise very
steep, full-throttle curve meets the reference curve at a
much lower angle of attack (a = 6.80); then the stability
is deficient in the climbing range.
From this it becomes apparent that the stability of an
airplane can be improved if, in order to change the angle'of
attack, the wing is rotated.with respect to' the body" rather
N.A.C.A. Technical Memorandum.No. 876
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Muttray, H. Investigations on the Downwash Behind a Tapered Wing With Fuselage and Propeller, report, September 1938; (https://digital.library.unt.edu/ark:/67531/metadc63295/m1/28/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.