|Boeing Stearman PT-17|
The Stearman (Boeing) Model 75 is a biplane formerly used as a military trainer aircraft, of which at least 10,626 were built in the United States during the 1930s and 1940s. Stearman Aircraft became a subsidiary of Boeing in 1934. Widely known as the Stearman, Boeing Stearman or Kaydet, it served as a primary trainer for the United States Army Air Forces, the United States Navy (as the NS and N2S), and with the Royal Canadian Air Force as the Kaydet throughout World War II. After the conflict was over, thousands of surplus aircraft were sold on the civilian market. In the immediate postwar years they became popular as crop dusters, sports planes, and for aerobatic and wing walking use in air shows. - WikipediaThe engine is a "Continental R-670-5 7-cylinder air-cooled radial piston engine, 220 hp". Radial engines have an odd number of cylinders, it makes for evenly spaced power strokes. 9 cylinders is most common and is also easier to layout as 360 degrees is evenly divisible by 9. Seven cylinders is not as popular and one reason for that might be that nothing is evenly divisible by 7.