Ground-Cooling and Flight Tests of an Airplane Equipped with a Nose-Blower Engine Cowling
By David Biermann and L.I. Turner Jr.
Summary: "Flight and ground-cooling tests were conducted with a Northrop attack airplane (Air Corps designation A-17A) to determine the merits of a nose-blower engine cowling designed and built at the NACA Laboratory. The chief object of the tests was to determine the cooling characteristics of the nose blower, particularly for ground and low-speed operation. Of secondary interest was the effect on the speed or drag of the airplane.
The tests showed that the nose-blower cowling was definitely superior to the NACA cowling from the standpoint of ground cooling, since the engine was operated at full throttle for 15 minutes with the cylinder temperatures well below the recommended limit. Although there was slight decrease in speed with the nose blower for the particular installation tested, the results of the speed tests were really inconclusive as regards the possibilities of improved high-speed performance. The nose-blower cowling was definitely more powerful as a blower than it need have been for satisfactory ground cooling, and consequently the power absorbed was excessive."
During the late 1930s, an extensive amount of research was done investigating various methods to cool an aircraft engine. Traditionally, an air-cooled radial engine required a large open cowling to allow enough air to flow around the cylinder heads to keep the engine from overheating. The most significant problems were encountered during ground operation and low-speed flight. Preliminary laboratory tests were conducted in 1938 and 1939 and investigated wing-duct cooling systems as well as nose and side entrance blowers. Combinations of propellers, cowlings and spinners were also tested in a NACA wind tunnel.
"The nose-blower type ... was selected for the first experiment because it was the easiest to install, and, if it proved successful, it would have the greatest immediate application.
The chief object of these tests was to determine the cooling characteristics of the cowlings. It was realized that there was little chance for increasing the speed of this airplane by any improvements in the nose shape because the drag contributed by the NACA cowling is only a small part of the drag of the entire airplane."
The nose-blower was essentially a set of compressor blades mounted in a large propeller spinner. Air would be drawn into this compressor through a large diameter opening channeled through a pressure tube and exhausted over the engine cylinders. The air exited through traditional cowl flaps. The inlet 'hole' was initially designed with a 17.5 inch diameter to ensure the engine would be adequately cooled at all speeds and situations. Full power ground operation showed the nose-blower actually overcooled the engine. NACA engineers later reduced the inlet diameter to just under 13 inches, expecting the engine operating temperatures to rise. However, engine operation was only slightly effected. Because the compressed air expanded too rapidly when exhausted into the engine compartment, a redesign of the engine cowling would have been required to gain maximum efficiency.
The maximum speed with the nose-blower was only slightly reduced. At 10,000 feet with the engine at full power (2360 rpm) the nose-blower modified A-17A reached 198 mph while the original version was capable of 203 mph.
Although the nose-blower was a successful experiment, it was never used on a production aircraft equipped with a radial engine. However, NACA conducted an extensive amount of research during this period, investigating the cooling fins on the cylinders. Much of this research was used to improve radial engines.
TECHNICAL NOTES: Armament: Four fixed .30-cal. machine guns and one flexible .30-cal. Browning machine gun; four 100-lb. bombs externally mounted; 20 17-30-lb. bombs internally carried in four dispensers Engine: Pratt & Whitney R-1535-13 Twin Wasp Junior radial of 825 hp Maximum speed: 220 mph/191 knots Cruising speed: 189 mph/164 knots
Range: 732 statute miles/636 nautical miles Service ceiling: 19,400 ft.
Span: 47 ft. 8.7 in.
Length: 32 ft. 1.1 in.
Height: 12 ft. 0 in.
Weight: 7,543 lbs. gross takeoff weight Crew: Two (pilot and observer/rear gunner) Serial number: 36-184