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Propeller speed reduction units were the norm by World War 2. By 1939, most engines turned faster than was efficient for propellers, so they needed to be geared-down to reduce propeller tip speed. IOW If you try to turn a propeller at a tip speed of more than Mach 0.8 it produces far more noise than thrust. So you need to slow the propeller tip speed below supersonic to convert horsepower into thrust.Since a PSRU can be built with a hollow shaft, it allows you to run a blast tube up the middle. Blast tubes were manufactured for guns ranging from 8 mm to 37 mm (Bell P-39 Airacobra). 8mm was sufficient for damaging WW1 airplanes made of wood and fabric, but 20 mm or 30 mm was needed to destroy WW2 airplanes.Since a PSRU allows you to offset the propeller shaft from the crankshaft, it allows you to install a gun firing through the center of the propeller (motor-kanon in German). Starting during WW1 Hispano-Suiza laid auto-cannons between the cylinder banks (V8, liquid-cooled engine) and fired it through a propeller shaft that was raised above the engine's crankshaft.
Propeller speed reduction units were the norm by World War 2. By 1939, most engines turned faster than was efficient for propellers, so they needed to be geared-down to reduce propeller tip speed. IOW If you try to turn a propeller at a tip speed of more than Mach 0.8 it produces far more noise than thrust. So you need to slow the propeller tip speed below supersonic to convert horsepower into thrust.
Since a PSRU can be built with a hollow shaft, it allows you to run a blast tube up the middle. Blast tubes were manufactured for guns ranging from 8 mm to 37 mm (Bell P-39 Airacobra). 8mm was sufficient for damaging WW1 airplanes made of wood and fabric, but 20 mm or 30 mm was needed to destroy WW2 airplanes.
Since a PSRU allows you to offset the propeller shaft from the crankshaft, it allows you to install a gun firing through the center of the propeller (motor-kanon in German). Starting during WW1 Hispano-Suiza laid auto-cannons between the cylinder banks (V8, liquid-cooled engine) and fired it through a propeller shaft that was raised above the engine's crankshaft.
