[Bruce Dennis]

The Fokker Eindekker, if I recall correctly, was experimentally fitted in 1916 with four guns firing through the propeller with an interrupter gear. The advantages didn’t justify the extra weight. Although I can’t access it at the moment, I recall an account of a similar installation on a Morane mono-plane.

Concerning the WW2 era installations, after clearing blockages was removed from the engineers priority list, I always believed that the motivation to mount the armament in the fuselage was to put the weight as near the centreline as possible. This is standard procedure for arranging any items on an aircraft. The Bell P-39 and it’s descendants were conceived to exploit this point, and Mitsubishi also considered it a vital aspect of the A5M and A6M concepts.

The rewards of this approach were clear, the first being that strength = weight in aircraft design. Any reinforcing of the wings to support the weight/bulk/recoil would mean an increase in specification of the main spar, plus a possible compromise in the shape of the wing to accommodate the weapons/ammunition around the spar. One of the reasons the NA P-51 wing was called a 'miracle' was the mixture of laminar flow shape wrapped around a high performance spar and heavy firepower.

Hope this helps,

Bruce