A few words about synchronizing gears.

Many people know that the combat aircraft is equipped with a special device that provides for a pilot

firing forward through the arc of a spinning propeller without the bullets striking the blades. And many

people think that this device renders the machinegun inoperative at the moment either blade is in line

with the machine gun muzzle. At the present time all fixed machine guns in RoF are working in this way:

You can shoot:

And you can’t do it:

However in reality, this is not so.

The first actual synchronizing gear was made by Anthony Fokker in the May of 1915. It consists of a cam

wheel attached to the propeller shaft and a linkage system that transmits the reciprocating movement

from the push rod resting on a cam to the trigger of the machine gun. The cam is adjusted so that it

pulls the rod when the blade is not opposite the muzzle of the gun. So this device effectively turned the

machine gun into the semi-automatic weapon. The pilot doesn’t press the trigger of the gun, but he just

engage the synchronized gear and the cam of this gear fired one shot for each turn of propeller if the

gun has time to recharge. The firing of the gun is controlled by a Bowden wire lever attached to the

control column of the airplane.

This synchronizer was called “Fokker gear” or “Stangensteuerung” and was installed on Fokker E.III and

some early German two-seaters in the summer of 1915:

After several months of defeats, the French have copied this device with minor modifications. This

synchronizer was called “Alkan-Hamy gear” by name of its developers: Junior lieutenant of the French

air service Alkan and naval engineer Hamy. This gear was installed on Nieuport 17 in the May of 1916:

The first British synchronized gear was put into production early in 1916. The “Vickers-Challenger gear”,

as it was named for the inventor: George Challenger, an engineer of the Vickers establishment, involved

a long oscillating rod to activate the Vickers trigger controlled by a cam and reduction gear

attached to the rotary engine pump spindle. This synchronizer was fitted to a Bristol Scout in

the January of 1916:

On a mission to England, lieutenant-commander Victor Dibovsky, an officer of the Imperial Russian

Navy suggested a synchronized gear of his own design, and Warrant Officer F. W. Scarff developed and

realized this gear which also consisted of came and rods and worked in much the same way as the

Fokker gear. Most of the Sopwith 1 ½ Strutters delivered to RNAS (Royal Naval Air Service) in 1916 was

fitted with the Scarff-Dibovsky gear:

Fokker gear worked satisfactorily on the aircrafts with rotative engines but it had troubles with

stationary engines due to the increased length of the rods. For this reason the new German fighter of

1916 year, Albatros D I, was equipped with its own synchronized gear which was named “Hedtke gear”

or “Hedtkesteuerung”:

All of these systems operate on the same principle: a cam mounted on the engine’s shaft presses the

machine gun’s trigger through a system of rods and generate thereby one firing impulse for each

rotation of propeller shaft.

From this it follows that a rate of fire strongly depends on engine speed.

Indeed, if the engine is stopped you can’t fire the gun! If you press the trigger on the control column

you just engage the synchronized gear and nothing happens then because all parts of the gear are not

on the run but you will able to shoot as soon as you start the engine.

How exactly the rate of fire depends on engine speed?

Suppose the claimed rate of fire of machine gun is 450 rpm (Spandau).

If the engine speeds is up to 450 revolutions per minute - the rate of fire is same as the engine rpm.

But the gun cannot exceed its own rate of fire! As soon as engine speed exceeded 450 rpm, the gun

would not have time to recharge and every second stroke of synchronizer rod would be blank.

Therefore, the rate of fire on 451 rpm dramatically dropped down to 225 rounds per minute and the

machine gun would fire only one shot at every other rotation of propeller.

Then, if we continued to increase rpm, the rate of fire would grow again up to 450 rounds per minute on

900 rpm and would drop off after that to 300 rounds per minute. The machine gun would fire only one

shot at every third rotation of propeller.

The dependence of rate of fire on rpm is shown there:

These graphs show that the rate of fire is highly dependent on motor speed. However, this dependence

can be significantly reduced by increasing the number of synchronizing pulses, but this is hindered by

the force of inertia of the system. So, further efforts of gunsmiths have been directed at reducing the

force of inertia. Their researches moved towards these areas:

1. Pulling is better than pushing.

If you pull on the rod, this rod can be made quite thin, and the force of inertia will be minimized. This

principle was realized in the British Sopwith-Kauper synchronized gear, invented by Australian motoring-

pioneer H. A. Kauper. This gear is the inverse of Fokker design: the firing impulse generates at each low

point of the cam instead at the lobe of the cam as a Fokker’s gear did. It was adopted in some Sopwith

machines in 1917.

2. Oscillating is better than pulling.

Marc Birkigt, ingenious French engineer and inventor of the engine “Hispano-Suiza” suggested the use

of the shaft, rotating alternately in clockwise and counterclockwise. The inertia of the oscillating motion

is smaller than the inertia of the reciprocating motion, so the device, which is based on this principle,

can operate at higher speeds. Birkigt gear was adopted by the French Air Force in September, 1916 and

was in use up to the World War II.

3. Move a cam into the gun.

In the end of 1916 Antony Fokker designed a new synchronized gear without any rods at all. His idea

was to move the cam directly on the gun. This gear consists of the flexible shaft connected with the end

of the engine camshaft through the disengaging clutch. The opposite end of this flexible shaft connected

with the cam of the trigger motor fitted to the machine gun. There were four balls in the case of the

trigger motor which functioned as automatic safety device and prevented accidental discharge during

engine start. This gear became standard on German air force since 1917.

4. Precision hydraulic system.

The Constantinesco gear, invented by Romanian engineer M. Constantinesco, became standard on

Royal Air Force in 1917. Its action is hydraulic. A pump attached to the engine transmits impulses to a

pipe-line filled with oil under pressure. A similar pump fitted to the gun responds to these impulses and

fires the gun at the desired moment, so that the bullet will pass between the blades of the propeller.

All of this advanced gears generated two firing impulses per propeller revolution, so the dependence of

the rate of fire on the engine speeds would be like this:

According to information received from the developers all of these features would be implemented in a

future update.

Wad