What if someone developed a radial engine with a motor-kanon?
My alternate history starts at Canadian Car and Foundry during the late 1930s. They are testing airplanes that include Wright R-1820m single-row, 9 cylinder radial engines. The synchronizer gear fails on a Grumman FF (CCF G-23) biplane fighter and its .5o caliber machine gun shoots off the propeller. Management is annoyed.
A light plane powered by a Pobjoy radial visits CCF at the critical moment.
A senior engineer muses "What if we install the machinegun to fire through the center of the prop shaft?"
CCF cooperates with Wright to develop an offset propeller speed reduction unit that displaces the prop shaft to the top of the engine. The entire engine has to be rotated 20 degrees to keep the prop shaft centered on the airframe. This also requires minor modifications to oil scavenging pipes in the lower cylinders. The prop hub now extends from the top edge of the engine cowling.
CCF builds and tests a G-23 with the modified engine/gun configuration and deems it a success. But it does not enter production because biplanes are obsolete by then.
When Gregor designs his FDB-1 biplane, his second prototype incorporates an R-1820 and the new PSRU. It still fails to sell to the RCAF.
A couple of years later, CCF gets a contract to build Grumman F4F Wildcats under license. Since they are destined for the Royal Navy Fleet Air Arm, they get Wright R-1820 engines. CCF modifies a single example with the new PSRU. This leads to a second Wildcat prototype with the new PSRU and sells a few to the RCAF.
CCF soon adapts a Hispano-Suiza 20 x 99 mm cannon (standard on later marks of Hurricane and Spitfire) to fire through the R-1820. They even experiment with a Browning M4, 37 X 145 cannon "borrowed" from a Bell P-39 Airacobra.
Since this concept is limited to single-row radial engines, development continues through WW2. Fortunately, the R-1820 had plenty of room for improvement. While pre-war R-1820-78 Cyclones only produced 700 horsepower, Cold War R-1820-82WA (Grumman S2F Tracker) variants produced 1,525 hp.
Meanwhile Bristol did similar experiments with Pegasus and Perseus engines.
Alfa-Romeo did similar experiments with Model 128 radial engine.
Nakajima did similar experiments with a Hikari radial engine.
For further development ... What was the most powerful, single-row radial engine?
