In WWI several countries experimented with planes that had engines in the fuselage connected by shafts to propellers in the wings. This innovation has obvious aerodynamic advantages, so why was it forgotten?

by DroneDamageAmplifier
Bloodsquirrel

Engineer speaking, since no historians are piping up:

I can think of several serious drawbacks to that design.

First off, a shaft running through the wing means that you have to have a point where you're changing the direction of motion by 90 degrees. Two points, actually, since your engine probably needs to be aligned along the body of the airplane for space reasons. These are going to be major stress points, which adds an additional point of failure.

Second, the shaft, joints, and extra structural rigging is going to add weight to the airplane. Weight is, obviously, one of your primary design constraints when dealing with aircraft.

Third, you're going to run into space constraints. The fuselage needs to be large enough to fit the engine (which will need to be larger if it's doing the work of two or more engines that would have been on the wing), and whatever was there before needs to be moved somewhere else. Putting the engines on the wings also gives you good balance; the engines are heavy, and the wings are where the lift is coming from. Putting the engine in the fuselage might make it hard to put it exactly between the wings, and having it farther back might throw your balance off. Also, your shaft is taking up space now, which complicates doing something like putting fuel tanks in your wings.

Fourth, you now have more places where a stray bullet or piece of shrapnel from an AA cannon can kill your aircraft. Before they had to hit the engine. Now they could hit the engine, or the shaft, or the point where shaft connects to the rotor.

All in all, having the engine directly behind the propeller is the simplest design possible, which gives you a lot of benefits in the areas of reliability, weight, and space. Whatever aerodynamic advantages the design had likely weren't enough to offset the downsides of the more complicated design.

Plus, you already had the alternative design of putting the engine on the nose of the aircraft, which gives you all of the advantages with none of the downsides (as long as a single engine is enough to fly the airplane).