THE HIGH SPEED
- Chapter 5: High-speed Cowlings,
Air Inlets and Outlets, and Internal-Flow Systems
- HIGH-SPEED COWLINGS
-  The NACA cowling
was an obvious subject for research in the new 8-foot high-speed
tunnel in 1936. It was the center of much attention in the NACA
program and the pressure distributions obtained in the PRT by
Theodorsen's group revealed local velocities as high as twice the
flight speed for the blunter shapes. Using Jacob's criterion, we
estimated that the critical speeds would be as low as 300 mph at
altitude, well within the performance spectrum of pursuit aircraft
of that period (ref. 175). R. G. Robinson planned a high-speed program which
would start by testing five of the cowlings used in Theodorsen's
program and then proceed to develop improved less-blunt shapes. I
was fortunate to be project engineer on the cowling investigation,
my first substantial project assignment.
- During design of the cowl models I had
noticed one of the then-new  DC-3's parked on
the Army flight line. It had a very blunt cowl shape and a large
fixed exit opening. I picked up some straightedges and clamps from
our shop and walked over to the flight line where I found a
sergeant servicing the DC-3. He located a ladder and helped me set
up my equipment to obtain accurate profile ordinates at several
stations on the DC-3 cowls. They turned out to be intermediate to
two of the blunter Theodorsen cowls and were therefore not
included in our test program. A couple of years later, however, we
used them in our DC-3 test project (ref. 31). This is a small illustration of the direct
informality with which things were done in those days.
- Tests of the blunt cowlings confirmed our
low critical-speed estimates,
and showed prohibitive drag
increases beyond the critical speeds (ref. 173 and fig. 37). They also provided the live demonstration for the
1937 Engineering Conference previously described (page 26).
- The next phase of the cowling work
provided my first experience at Edisonian tailoring of an
aerodynamic body in an effort to obtain a particular pressure
distribution. The basic difficulty in subsonic flow...
- FIGURE 37.-High-speed cowling test setup
in the 8-Foot High-Speed Tunnel, 1937.
-  is that a change
in local shape and pressure also causes usually unwanted changes
elsewhere on the body. After testing a given profile, I would lay
out a speculative change, carry the model back to our machine
shop, and often help the machinist in laying out a new nose
profile template. When the new shape had been tested and the
pressure distribution studied, the process was repeated until
eventually we obtained the lowest suction pressure peak possible
within the dimensional limits established for the cowling study.
These limits were set by the specified diameter of the inlet
opening and the distance between the cowl lip and the plane of the
engine face which were typical of advanced engines. The resulting
high-speed shape, designated cowling "C," had a critical Mach
number of 0.64 or 480 mph at sea level, about 170 mph faster than
the blunt cowls. The "C" cowling also had somewhat lower
drag at subcritical speeds than the blunt shapes and it
found use on a wide variety of piston-engine aircraft, including
several where the high critical speed was not required.