Investigations on the Downwash Behind a Tapered Wing With Fuselage and Propeller Page: 14 of 52
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vortex is not even approximately in accord with the experi-
mental. This, then explains the peak of the curves in fig-
Figure 18 shows the same for the previously measured
elliptical wing (reference 1) at a = 8.6. Since "'the po-
sition of its rear stagnation point streamline in the plane
of symmetry had also been defined wakeb measurement), the
amount ACh of the reduction was thus known. It is Ach =
-0.24 according to figure 19 of reference 1. The peak or
maximum value of the recorded downwash curve of the normal
axis lies at ch = -0.24, thus affording almost perfect
agreement between peak position and height of vortex shot.
If the peak of tho theoretical curve for notrolled-up sheet
is made to coincide with the peak of recorded downwash
curve, then the shanks of tho curves also coincide again
approximately. The remaining discrepancies between the
theoretical and experimental values of figures 6 and 18 are
probably due to other than the different heights of marginal
and inside vortices, to the necessary but still not quite
exactly known correction factors for finite slipstream sec-
tion for normal axes. Besides, the experimental value is
higher as a result of the finite chord effect (fig. 16).
Prom the foregoing it is seen that comparatively good
agreement obtains between theoretical and experimental val-
ues if the lateral displacements of the free vortices are
neglected. The vertical displacements (sinking of vortex
sheet), on the contrary, may not be disregarded. (This
statement corrects the opposite assumption of the earlier
report (reference 1)). To be sure, this applies only to
the region of flow behind the inner part of a wing, that
is, in the range of the tail.
Longitudinal axes- This brings us to the discussion
of the downwash factors measured on the longitudinal axes.
Figure 19 presents the curves measured on longit dinal ,,.
axes through the coordinate origin in plane of symmetry
(dual-tube instrument) at different a,'along with the-
oretical curves for the substitute trailing vortex (k =
0.74) and an upperr and "lower" limit similar to the
original report (reference 1), Of course, the "upper" lim-
it is not meant to be interp'reted'as in "that report. There
the "upper" limit in accord with Helmbold''s report (refer-
ence 8) presented the values obtained with'nonrolled-up
vortox sheet and elliptical lift distribution, But now
the "upper" limit applies to those values which are obtained
N.A.O.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/m1/14/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.