The Bf 109 was famously fitted with a nose cannon that fired right through the propeller spinner, but how does this well known, but often misunderstood system work? And why was it used?

As the most produced fighter ever, the Bf 109’s appearance is familiar even with those who have a minimal interest in aviation. One of its standout features is, of course, the large opening in the propeller spinner that housed the barrel of a powerful cannon.

The reasoning behind this interesting design choice is one of necessity and practicality.

The Nose Cannon

As mentioned, the Bf 109 is the most produced fighter in history, with over 34,000 units being built in total. As such, its armament varies greatly, from rather minimal on some variants, to heavy weapons packages specifically designed to bring down bombers on others.

But regardless of the type, all featured the centrally mounted nose cannon, firing through the propeller hub.

The requirements that led to the Bf 109 were published in 1933, with major German manufacturers competing against each other to produce an aircraft that most optimally fulfilled them.

From the start, Messerschmitt’s proposal, known then as P.1034, was to be armed with two 7.92 mm machine guns forward of the cockpit and above the engine. This was a hang-over from First World War-era designs, and the guns had to be synchronised to fire through the propeller.

This arrangement kept the guns out of the wings, bringing in the center of gravity and allowing the wings to be made thin and light for improved performance and reduced drag. In short, the Bf 109 was not originally intended to have any weapons mounted in the wings.

P.1034 was replacing biplanes that only flew around 200 mph with relatively light armaments, so this was deemed acceptable at the time.

However as the 1930s progressed, other aircraft around the world began mounting large assortments of weapons in the wings. The French Dewoitine D.520 for example, carried four machine guns in the wings, while the British Spitfire carried eight.

Seeing that their aircraft was going to be at a firepower disadvantage, Messerschmitt needed to add more guns to their Bf 109.

As the wings were designed without space for guns, a gun was instead placed in the engine, firing through a hollow propeller hub – this was known as a “motorkanone”. Initially this was a 20 mm, but some later variants carried a 30 mm nose cannon here. Plans for this had been considered early on, so this wasn’t too difficult to add.

Even more firepower was eventually needed, forcing Messerschmitt to make space in the wings for extra guns. There was only room for one gun in each wing though, and only with ammunition belts extending throughout the wings as there was no space for magazines.

The guns above the nose were also replaced with 13 mm machine guns in some variants too, totalling three weapons in the nose.

How did the Nose Cannon Work?

The concept of a nose cannon isn’t too hard to understand, but how do the mechanics actually work? How is there room for a large cannon inside an engine?

Well, the trick is that the gun isn’t actually inside the engine, it sits neatly between the engine’s cylinder banks.

The Bf 109 (for the most part) was powered by the DB 601 and its sibling, the DB 605. Both of these engines were V12s, meaning they have twelve cylinders arranged in a “vee” shape.

On each side of the vee is six cylinders, creating what is known as cylinder banks. Interestingly, the Bf 109’s engines were inverted, so the vee is upside down – an unusual feature for engines of this type.

This was done to improve the pilot’s visibility over the nose, but it also had the benefit of making maintenance easier – as much of the engine was accessible from the ground – and reduced the amount exhaust flames dazzled the pilot at night.

At this point you might be thinking: how does the cannon fire through the propeller if its located between the cylinder banks? Shouldn’t it be in line with the engine’s crankshaft?

This answer to this is simple: the Bf 109’s propeller was not in line with the crankshaft, it was located slightly lower thanks to a reduction gear system. This placed the propeller perfectly inline with the cannon.

The MG 151 20 mm cannon was mounted slightly behind the engine, with the barrel protruding into a blast tube fixed between the cylinder banks. The propeller hub was mounted around this, and connected to the crankshaft via the reduction gear.

Later, some models, such as the Bf 109G and Bf 109K, had a massive 30 mm cannon firing through the nose.

Benefits

Placing a weapon in this rather unorthodox position actually has a number of benefits.

First, the nose cannon is not likely to freeze and jam as it is continuously provided warmth by the engine. It also brings the aircraft’s center of gravity closer to the roll-axis, potentially allowing for a faster roll rate.

But the greatest advantage is with accuracy.

Wing-mounted guns do not fire straight forward, they fire slightly “inward”, so the streams of projectiles from each wing eventually converge at a predefined point.

This sets an effective range where the guns are most accurate.

Nose cannons are densely packed and all fire forward in a focused beam (a good example of this is on the P-38).

The Bf 109 gained all of these advantages by mounting one of its weapons between the cylinder banks, firing through the nose. Drag was reduced, accuracy was increased and the gun was kept at a working temperature.