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: 26 of 171
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NACA TN 3790 25
Reduction of Adverse Effects of Propellers on
The longitudinal characteristics of the subject model demonstrate
some of the undesirable effects of propeller operation which should be
suppressed or eliminated. There is, of course, a need for theoretical
methods of calculating these effects of operating propellers on the longi-
tudinal stability. However, the results of attempts to calculate power
effects for this model entirely by means of existing theory have been dis-
couraging. Such calculations may be considered in three parts which treat
separately the effects due to direct propeller forces normal to and along
the thrust axis, those due to slipstream action on the wing and nacelles,
and those due to slipstream action on the flow at the tail. Obviously,
the pitching moment due to propeller thrust can be calculated accurately.
It has been found that the propeller normal force, and therefore the pitch-
ing moment due to it, can be calculated with fair accuracy for the isolated
propeller using a method based on the oscillating aerodynamic forces asso-
ciated with blades rotating in an inclined flow field. However, since a
sizable portion of the measured normal force was attributable to slipstream
effect on the forward portion of the nacelle, correlation between experi-
ment and theory was not very satisfactory. Actually, an attempt was made
to predict the normal force due to slipstream on the nacelle, but the
agreement with experiment was not good.
An attempt was made to calculate the pitching moments arising from
slipstream effects on the wing by consideration of the lift increments
on the portions of the wing immersed in the slipstream. The calculations
followed the method of reference 9 in which the propeller is regarded as
an actuator disc (no rotation in the slipstream). Lift due to slipstream
on the nacelles was neglected. The total lift increment due to propeller
slipstream effects was predicted with adequate accuracy but the pitching-
moment increment, which depends on the center of pressure of the lift
increments on portions of the wing behind each propeller, was not pre-
dicted satisfactorily. The latter result is not surprising in view of
the experimental pressure-distribution results presented in reference 10
which show a large effect of slipstream rotation on the distriubtion of
incremental lift due to slipstream on the wing. Some of the discrepancy
was, of course. due to neglect of slipstream effects on the nacelles.
Finally, with regard to prediction of the pitching-moment contribu-
tion of the tail, a strictly theoretical approach seems quite impractical
for configurations such as considered herein where the tail passes into
and out of the slipstream with changing angle of attack. Such predictions
would require not only satisfactory estimates of the dynamic pressure and
of the flow angles in the slipstream, but equally as important and prob-
ably more difficult, satisfactory estimates of the location of the slip-
stream relative to the tail. On the other hand, the longitudinal
stability changes associated with slipstream effects on the tail have
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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/m1/26/: accessed May 26, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.