Well, let's take a look how things started in aerial warfare, since the pioneers had to ask the same questions.
The first skirmishes between military planes were fought with pistols and carbines. Naturally, those proved to be quite ineffective, so the use of machineguns was considered.
There were different solutions to the problem of not damaging the propeller:
- one solution was "simply" putting the propeller behind the engine, called a pusher construction. This, however, made the planes less aerodynamic and was not an optimal solution. An example is the [Airco DH. 2] (https://en.wikipedia.org/wiki/Airco_DH.2)
- there could be a gunner with a mounted machinegun, who could aim around and shoot at other planes. A fair solution, but we obviously want our one-man fighers to be capable as well.
- the machinegun could simply be put outside of the propeller arc, so there would be no risk of hitting the blades. Options were putting them on the wings, placing them in an angle so they wouldn't hit the propeller or similar. Examples of this are the [S.E. 5] (https://en.wikipedia.org/wiki/Royal_Aircraft_Factory_S.E.5) or some versions of the [Sopwith Camel] (https://en.wikipedia.org/wiki/Sopwith_Camel). Problems were that the gun was either difficult to reload or difficult to aim with.
- the French started plating their propeller blades, so that bullets hitting the blades were simply redirected and didn't harm them. This solution was first used on the [Morane Saulnier L] (https://en.wikipedia.org/wiki/Morane-Saulnier_L). Edit: The plate would be in wedge form, thus redirecting the bullets that hit the propeller. The German term is "Ablenkbleche", lit. "deflection / redirection plating".
When such a plane fell into the hands of the German forces, the German high command wanted Anthony Fokker to copy the above solution. However, the penetration power of the German full-metal jacket bullets was higher, and thus simply plating the blades didn't help.
Fokkers' engineers, namely Lübbe, Heber und Leimberger, thus invented the synchronization gear (German Unterbrechungsgetriebe, lit. interruption gear), which meant that the machinegun would temporarily stop shooting when the propeller blades were passing in front of its muzzle.
The first plane equipped with this device was the [Fokker E.1] (https://en.wikipedia.org/wiki/Fokker_Eindecker_fighters), which was also known as the "scourge" to fighter pilots of the Entente, and which helped the German forces reach air superiority against their opponents for a period, as it had a much higher firepower.
Despite the advantages of this system, there were also some disadvantages:
- the system could malfunction, which would lead to a destroyed propeller.
- the firerate was lower, since the gun had to stop when the propeller blades were passing.
Nevertheless, the system was adapted by the other forces of the first world war, and still in use (although, naturally, in an improved version) in World War II fighter aircraft.
Sources (in German):
- Der Erste Weltkrieg (The First World War), H. P. Wilmott, 2009
- Militärtechnik des Ersten Weltkriegs (Military Technology of the First World War), Wolfgang Fleischer, 2014
- Jadgflugzeuge 1914 - 1918 (Fighter planes 1914 - 1918), Philip Makanna, 2006