the
intake air. This allowed the engine to run on its own lubricating oil
when the throttle was in idle position. As a result the engine idled too
fast, thereby causing either excessive taxiing speeds or rapid brake
wear. This inability to idle slowly also caused high landing speeds
since the propeller did not turn slowly enough to act as an airbrake.
Figure 1 shows the first model. Note that the tubular air intakes on top
of the cylinders have no valves. Figure 32 shows a later model. Note the
butterfly valves in the U-shaped air intakes. Here they are shown fully
opened. When the throttle was placed in idle position these valves
automatically closed and prevented air from flowing past them. Air could
then only enter from the back of the intakes. Since less air could flow
into the cylinders, the force of their explosions was reduced, which, in
turn, lowered the idling revolutions per minute. Figure 28 shows a
cylinder from a more advanced model. Note the circular opening between
the air intake and the intake/exhaust housing. A barrel type of valve
fitted into this opening. One of these valves can be seen just below and
to the left of the cylinder. When the throttle was placed in idle
position this valve rotated to a position which cut off almost all of
the airflow into its cylinder. This increased the vacuum formed toward
the end of the intake stroke, thereby causing more resistance, which
reduced the idling rpm to that of a gasoline engine.[16]

[Illustration: Figure 32.--Front left view of engine from Packard Motor
Car Co. 50-hour test, 1930, showing butterfly valve type venturi
throttles. (Smithsonian photo A48325E.)]

[Illustration: Figure 33.--Front left view of engine from U.S. Navy
test, 1931, showing spiral oil cooler. (Smithsonian photo A48324A.)]

Crankcase: It was strengthened by having external ribs added. Note the
contrast between the first engine, figure 2, and a later model, figure
32.

Oil Cooler: The drum-shaped honeycombed cooler was replaced by a spiral
pipe type located between the engine cowl and the crankcase. Figure 3
shows an example of the former type of cooler located at the top of the
engine between two of the cylinders. Figure 33 illustrates the latter
type located between the cowling and the crankcase.

Cylinder Fastening: Early models had their cylinders strapped and bolted
to the crankcase. Later ones had them only strapped. Figure 2 shows a
bolt-fastened clamp between two of the