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: 33 of 171
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NACA TN 3790
Effects of the Operating Propellers on the
As discussed previously, the factors which determine the static longi-
tudinal stability of a propeller-driven airplane are the stability with
the propellers removed, the direct propeller forces normal to and along
the thrust axis, and the effects of the propeller slipstream on the flow
on the wing and at the horizontal tail. Figures 58 and 59 show for sev-
eral Mach numbers these various effects of the operating propellers on
tail-on and tail-off static longitudinal stability at zero thrust, at a
comparatively high constant thrust coefficient, and at the conditions of
constant horsepower shown in figure 9. The data presented were obtained
by adding pitching-moment increments, referred to the center of gravity,
due to propeller thrust and normal force (from the propeller calibration
data) to the propellers-off pitching-moment data. This total was then
subtracted from the power-on pitching moments to ascertain approximately
the slipstream effects. For both constant thrust and constant power, the
various effects of the operating propellers on the pitching-moment char-
acteristics of the model were small.
Figure 60 presents, for a Mach number of 0.80 and a constant thrust
coefficient of 0.04, a comparison of the predicted and measured varia-
tions with angle of attack of the incremental pitching-moment coefficient
due to propeller normal force. The measured variations of increments of
pitching-moment coefficient with angle of attack due to propeller thrust
and propeller slipstream on the wing and tail are also shown. The effect
of propeller normal force on the pitching moment was calculated by a method
based on the oscillating aerodynamic forces associated with blades rotat-
ing in an inclined flow field. The predicted pitching-moment increments
due to the propeller normal force are in good agreement with the measured
effects. The small discrepancy at the lower angles of attack is believed
due to lift stemming from the asymmetry of the nacelle forebody. The the-
oretical computations did not account for any lift contribution due to the
The effects of propeller slipstream on the pitching-moment character-
istics of the wing and tail could not be predicted to any acceptable degree
of accuracy with existing methods. It is believed that the combination
of the effects of wing sweepback, of viscous separation, of propeller slip-
stream rotation, and of wing-nacelle interference makes the estimation of
slipstream effects on the pitching-moment characteristics of the wing and
tail virtually impossible for the present model.
Figure 61 shows the variation with Mach number of the various effects
of the operating propellers on the pitching-moment-curve slopes A(d.C/dDL).
The data are presented for a representative lift coefficient for level
flight (CL = 0.40) and for constant thrust coefficient and constant simu-
lated horsepower. The effects of slipstream on the horizontal tail were
assumed to be the differences between tail-on and tail-off slipstream
effects. The effect of propeller normal force varied with Mach number
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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/m1/33/: accessed May 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.