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: 37 of 171
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3ANACA TN 3790
5. To avoid large longitudinal stability variation with lift coef-
ficient, the slipstream should not impinge on the tail. It is indicated
that one way to accomplish this with the configuration tested is by mov-
ing the propellers outward about 0.1 of the wing semispan. This modifica-
tion would also make the effect of propeller slipstream on the wing more
stabilizing and reduce the destabilizing effects of the propeller normal
forces. Calculations indicate great improvement of the longitudinal
stability characteristics both with flaps up and flaps down. The lateral
control required to offset the increase in rolling moment associated with
loss of the outboard propeller is estimated to be 15 percent more than for
the original configuration and the directional control, 20 percent more.
6. Other design changes tending to prevent the slipstream from
striking the tail and which do not affect the lateral and directional
control problem are reduction of the tail span and raising the horizontal
tail. The experimental results indicate that if the tail is placed high
enough to avoid the slipstream, the effect of power on the tail contri-
bution to stability will be destabilizing. This indicates that for the
configuration tested, some outward shift of the propellers would still be
required to produce satisfactory longitudinal stability characteristics.
7. Propulsive efficiencies for the complete configuration were
approximately equal to the efficiency of the isolated propeller if, in
calculating propulsive efficiency, the propellers were credited with the
lift they produced.
For the high-speed conditions, that is, for Mach numbers of 0.60 to
0.90, the following conclusions were indicated:
1. The over-all effects of operating propellers on the longitu-
dinal characteristics at high subsonic speeds were not large when com-
pared to the effects of operating propellers at low speeds. The pro-
pellers operating at constant thrust coefficients generally resulted in
a reduction in the longitudinal stability. Increasing the propeller
thrust coefficient while maintaining a constant Mach number increased
both the longitudinal stability and the tritmme lift coefficient.
2. Operation of the propellers at constant thrust coefficient
increased the wing lift-curve slope but had little effect on the varia-
tion of lift-curve slope with Mach number.
3* Operation of the propellers had little effect on the Mach num-
ber for longitudinal force divergence at a constant lift coefficient
but resulted in a decrease in the rate of change of longitudinal force
coefficient with Mach number at supercritical speeds. This effect
increased with increasing propeller thrust coefficient and with increas-
ing lift coefficient.
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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/m1/37/: accessed May 27, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.