[Bruce Dennis]

Originally Posted by Bruce Dennis

... 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.

Placing all possible weight along the centre line, gathered as close to the CG as possible, is an axiom of attaining maximum manoeuvrability and minimising loads under +/- G forces. A lighter spar in response to the weight of non structural components is a poor trade-off.

You are of course right about the NA P51 wing, it was a laminar type wing. As to whether the wing's armament was 'heavy' or not I suppose depends on whether you are comparing it with the other wing armament known to be in use at the time it was designed.