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National Advisory Committee for Aeronautics, Technical Notes - Resistance of a Fifteen Centimeter Disk

The results of this test show that the dynamic scale has
very little effect on the drag coefficient of a disk over a
wide range of Reynolds number. A comparison of these results ‘
with those of tests made on a series of disks at Gettingen Uni—
versity furnishes a good check on the method of testing in this
tunnel.
A lS—centimeter disk was_tested in the variable density
wind tunnel of the National Advisory Committee for Aeronautics,
for the purpose of comparing results from this tunnel with those
of other tunnels. The drag coefficient was determined for a
series of tank pressures ranging from 1 to 20 atmospheres. The
results check very well with those of the Gettingen tests.
The disk used was made of steel, 15 cm (5.91 in.) in diame—
ter on the face. The edges were chamfered back, leaving a sharp
edge at the face. The method of support is shown in Fig. l. A_
threaded fitting which was brazed to the center of the disk, was
screwed on to a 3/8 in. (”.955 cm) rod about 11 in. long. This
rod was threaded into a 3/4 in. (1.905 cm) rod 7 in. long, which
in turn was mounted on the angle of attack bar carried on the
tunnel balance. For a description of the tunnel and the balance
mechanism see Reference 1. The support drag was measured, after
disconnecting the disk from the supporting spindle and suspend—
ing it from the tunnel well by means of 4 wires. Clearance
was left between it and the 3/8 in. (.953 cm) rod, so the only
forces transmitted to the balance were those on the supporting
rods. The net drag of the disk was found by subtracting this
support drag from the combined drag of the disk and support.
Table I gives the tank pressure, dynamic pressure, Reynolds
Number and drag coefficient of the disk for the points taken.
Fig. 8 shows graphically the variation of drag coefficient with
Reynolds Number. 0n the same scale are plotted points from
tests made at Gdttingen University,£see Reference 2.
The results show very good agreement between the two tun—
nels. They indicate that there is very little scale effect on
the drag coefficient of a disk, although there seems to be a
slight decrease in drag coefficient with increasing Reynolds
Number.