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: 16 of 171
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MACA TN 3790
The effective angles of downwash E, calculated from the force data
by means of the equation
Gatal on" Ctail offcconstant
E = c +t it - (3)
are presented in figure 24. The variation of e is affected by a number
of factors, some of which have opposing effects, and the relative impor-
tance of each is difficult to ascertain from the data available. The
data in figures 23 and 24 indicate that the variation of C with a at
constant Tc and the variation of a with Tc at constant am are greatly
dependent on the location of the tail relative to the slipstream. Also
very important in its effect on E is the location of the tail in the
field of downwash from the wing itself. Over most of the angle-of-attack
range, an increase of Tc increased the lift on the wing (see fig. 16)
which by itself would increase the downwash and also move downward the
point of maximum downwash. However, it can be seen in figure 24 that
there is a general reduction in the effect of increasing Tc on e for
those instances where the tail is in the slipstream (see fig. 23).
Comparison of pitching-moment increments.- The relative magnitude
of the various pitching-moment-coefficient increments due to the effects
of power and an indication of the effects on static longitudinal stability
are shown in figure 25 (flaps up) and figure 26 (inboard flaps deflected).
In these figures only, the pitching-moment coefficients have been referred
to a new moment center which is more representative of the vertical height
of the center of gravity for the assumed full-scale airplane. The longi-
tudinal location of this assumed center of gravity is maintained at the
quarter point of the mean aerodynamic chord but its vertical location is
lowered 0.10a (see fig. l(a)). The effect of this change of moment center
is to nearly eliminate the shaft thrust contribution to pitching moments
without materially changing any of the other increments. From figures 25
and 26 it may be observed that the propeller normal force contributed a
general increase in slope of the pitching-moment-coefficient curve, even
at zero thrust, and the effect was, as might be expected) essentially
independent of changes of flap configuration or tail height. For con-
stant Tc the slipstream on the wing contributed an increase in moment,
but no general change in slope of the pitching-moment-coefficient curve.
The tail contribution as a function of angle of attack was extremely
variable compared to the other components. This was, of course, due to
the variation in tail lift as the tail moved into or out of the slipstream.
Changes of tail height and deflection of flaps strongly influenced the
pitching moment contributed by the tail.
In figures 27 and 28 similar data are presented with the inboard and
the outboard propellers operating independently (that is, with one pro-
peller removed). These data show that the inboard propeller caused most
_ ~_~__. -- -- ICLC cl ---c~c cc e~c~ -- - -- - -- "
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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/m1/16/: accessed May 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.