gun synchronization to fire through propeller arc

[kennethklee]

The WWII-era Luftwaffe seemed to favor gun synchronization through the propeller arc of its piston-engined fighters throughout the war, whereas the British and American fighters more often had their guns located in the wings outside of the arc, especially in the latter part of the war. Almost every single-engined Luftwaffe WWII fighter had at least two guns firing through the arc whether they were cowling-mounted or located in the wing roots. The Focke-Wulf Fw 190-series and Ta-152 series examplified the pinnacle of this technology, with most examples having two cowling-mounted machine guns (replaced by two 20mm cannon in the Ta 152C) and two wing root-mounted 20mm cannon all firing through the propeller arc. I can't recall another air force implementing this philosophy to this extent, especially with four guns (and up to four cannon) firing through the propeller arc.

I have not been able to find written references that discuss why the Luftwaffe favored this approach to gun armament in their fighter aircraft. I suspect that guns so mounted would have their firing rates decreased, and that there must have been outweighing advantages for this approach? I surmise that this type of gun mounting allowed centralization of the guns, which 1) increased the concentration of fire against targets; 2) improved the maneuverability and flight characteristics of the fighter aircraft; 3) reduced the frequency of gun jams. Are any of my suppositions correct, or were there other explanations for this trend? I also imagine gun synchronization was a tricky bit of technology and prone to malfunction, which would result in the propeller being damaged or shot off. Was this technology generally reliable?

I greatly appreciate any thoughts and ideas on this topic.

Thanks,
Kenneth

[Graham Boak]

The Russians did it too. The technology was entirely reliable, and was preferred because of the increased accuracy of the weapons and agility of the aircraft. Wings distort under flight loads, particularly in hard turning combat, which means that the guns would not necessarily be pointing in the same direction as the sight. The additional weight of the guns and ammunition increased the inertia of the aircraft in roll and yaw, whilst taking up space that could be used for fuel for extra range.

[kennethklee]

Originally Posted by Graham Boak

The Russians did it too. The technology was entirely reliable, and was preferred because of the increased accuracy of the weapons and agility of the aircraft. Wings distort under flight loads, particularly in hard turning combat, which means that the guns would not necessarily be pointing in the same direction as the sight. The additional weight of the guns and ammunition increased the inertia of the aircraft in roll and yaw, whilst taking up space that could be used for fuel for extra range.

Graham-

Thanks for the information-laden response, I understand the advantages of gun synchronization much better; I especially didn't consider that wing distortion under load could be created by internal wing guns and that it could be severe enough to distort their line of fire-->good stuff to learn and know.

I didn't mean to suggest no other WWII air forces used it; several other major nation combatants in WWII, including the Russians (as Graham pointed out), Italians, and Japanese, had 1-2 synchronized guns (excluding engine-mounted guns firing through the center of the airscrew spinner) in most of their fighters. However, I labeled the Luftwaffe as the WWII leader in this field because most of the Focke-Wulf fighter line (Fw 190A-series, Fw 190D-9, Ta 152C) successfully synchronized four guns (two cowling machine guns, two wing-root 20mm cannon; four 20 mm cannon in cowling and wing roots in the Ta 152C) through the same airscrew, which seems impressive to me--perhaps I'm just easy . I'm not aware of other fighters that carried four synchronized guns, although I could be wrong.

Kenneth

[Harri Pihl]

The center line battery installation offers theoretically ideal weapon placement due to concentrated firepower and minimized aiming problems. However, in practice the other shooting errors cause far larger variation than any gun location caused error.

[Graham Boak]

I suspect there were biplane fighters with 4 fuselage-mounted machine guns, although I can't think of one offhand. The Me 209-II had a similar wingroot mounting to the Fw series. The La 7 is nagging at me - there were versions with three cannon in the fuselage and others with either two or four. My books are temporarily out of reach.

Edit: the main advantage of fuselage mounting for the machine guns in the 1920s and 30s was the ease of clearing a jam. 1940s guns were more reliable, and multiple mountings were easier to design (and service) if in the wings. The increasing use of cannon meant fewer guns, but required greater space in the fuselage and wingroots which complicated the design. It was easier for Tank with a radial.

Possible a case could be made around the supposed German tendency of over-engineering for small benefit? However, Yakovlev was very vocal against putting guns in the wings, on agility grounds, so the size of the benefit can be argued.

[Kutscha]

The P-51/Mustang I had fuselage mounted weapons,two 0.50" M2s.

[Bruce Dennis]

The Fokker Eindekker, if I recall correctly, was experimentally fitted in 1916 with four guns firing through the propeller with an interrupter gear. The advantages didn’t justify the extra weight. Although I can’t access it at the moment, I recall an account of a similar installation on a Morane mono-plane.

Concerning the WW2 era installations, after clearing blockages was removed from the engineers priority list, 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.

Hope this helps,

Bruce

[Graham Boak]

Not quite. Strengthening would be needed fore-and-aft for recoil forces, but the main strength of a wing is required to carry the lift forces and these point upwards. Adding weights out along the wing permits a lighter spar structure by relieving this upward load. This is the principle behind pylon-mounted engines, although here they can also act as anti-flutter devices.

And to flog a tired horse, the Mustang wing was not truly laminar. Nor was its wing-mounted armament particularly heavy. The outstanding features of the Mustang were the integration of the radiator into the fuselage using expansion/contraction ducts (leaving space for significantly larger fuel capacity) and the very high standard of external finish achieved.

[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.

[Graham Boak]

Well, a lighter spar and hence lighter wing is an advantage not to be sneered at. I was merely pointing out that statements about strengthening wings was not as clear-cut as you suggested. As the original poster pointed out, by no means the majority of successful WW2 fighters had the masses concentrated such as you describe, practical disadvantages (such as fuselage size leading to excessive drag) being one counting against any gain in agility. Part of the trade-offs for all fighters.

As for armament, at the time the Mustang was designed the P-47 had eight 0.5 guns, the RAF standard for new fighters was 4x20mm cannon, the Zero and Fw 190 were flying. The first Mustang was the Mk.1, which had weaker armament (a mix of machine guns) than the majority of its contemporaries, largely cannon-armed. The following Mk.1A (P-51 no suffix) had 4x20mm, but this did not see mass production. The mass production P-51B/C had only four 0.5 machine guns, arguably the weakest of contemporary fighters other than the Ki 43 (although some of the Yaks could also be included, perhaps). As it was highly successful, heavy armament cannot have been a key factor.