Collection of zero-lift drag data on bodies of revolution from free-flight investigations Page: 14 of 374

                                
NACA TN 4201

base diameter to maximum diameter is much less important at the higher
values of /d   can be noted in figure 15(a), and is shown more graph-
ically by the shaded area on the lower figure which shows the limits
of configurations whose drags lie within about 10 percent of the minimum.
The range of optimum conical angles indicated (3.5 to 6.50) is of the
same order (5O to 70) as that used for some time by ballisticians for
the drag reduction of bullets.
Total body drag.- If the minimum afterbody drags at each value of
1/d  are taken, the resulting plot (fig. 14) may be said to represent a
near minimum possible afterbody pressure drag for M = 1.2. A similar
curve is presented for the nose drag and was obtained by fairing through
the blunt nose values from configurations 1 to 7, through the minimum
/d = 3 nose drag(r' = x,1/2     (fig. 12)) and through the M = 1.4
values for the higher values of  1/d   (fig. 11). These curves are pre-
sented to give some practical boundaries, admittedly empirical and rough,
to the minimum drag problem.
If the nose and afterbody minimum drags are added for bodies with
their maximum diameter at their midpoints, the solid curve on figure 15
is obtained. If the same drags are added with care taken to position
the maximum diameter at the most favorable position the dashed curve
is obtained.  (This position moves rapidly rearward from  x/Z = 0.55
for  1/d = 7  to  x/Z = 1  for  1/d = 3  for the near minimum curves of
figure 14; however, such values are extremely susceptible to small
changes in level in either of the nose or afterbody drag curves and must
only be considered as indicative of the trend.) Also, the drag rises
A(CD = CDtotal - CDfriction - CDfin pressure) for the smooth bodies of
this report are plotted at the fineness ratio representing the sum of
their nose and afterbody fineness ratios. Most of the bodies at low
values of  Z/d  actually had cylindrical center sections and thus their
interference drags were low. This must be kept in mind when the use of
either of the empirical curves as minimum drag boundaries is contemplated.
As an instance of this, compare the pressure drags of models 84 and 85
which are identical in shape (r' = x'1/2), and fineness ratio of nose
and afterbody, and differ only in the cylindrical center section of
model 85. The higher pressure drag of model 84 must be attributed to
interference of the nose on the afterbody. This interference drag seems
high in comparison with the drag produced by the interaction of nose and
afterbodies of the parabolic bodies of figure 6 which are indicated to
be of the order of model 85 (and essentially zero) by a breakdown of
their drags into component parts and a comparison of the pressure com-
ponents with second-order theoretical calculations (ref. 29). It seems
reasonable to assume that at total fineness ratios below 6, the effect
of nose induced pressures on afterbody drag and perhaps more significantly

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