Supermarine Walrus with Oerlikon gun?

[Tony Williams]

I have been sent the following statement:

"In 1940 a Walrus was fitted with a forward-firing Oerlikon 20 mm cannon, intended as a counter-measure against German E-boats."

Does anyone know anything about this? In particular, are there any illustrations showing this installation?

 

[CostasTT]

I have been sent the following statement:

"In 1940 a Walrus was fitted with a forward-firing Oerlikon 20 mm cannon, intended as a counter-measure against German E-boats."

Does anyone know anything about this? In particular, are there any illustrations showing this installation?

Your post prompted a quick search and the results turned out to be interesting, to say the least. The airwar.ru site mentions a drum-fed Hispano in a fixed forward-firing installation, replacing the bow turret and the copilot's position. This seems to corroborate the wiki entry where it's mentioned that the muzzle flash tended to blind the pilot when the cannon was fired.
Hikoki Publications' The Secret Years Flight Testing at Boscombe Down 1939-1945 complicates matters, however. Although the serial number given in p.236 (L2271) agrees with the Airwar.ru entry, the entry leads to entirely different conclusions, as it says that "The normal single 0.303 in gun in the rear fuselage was replaced by a single 20mm gun in L2271; the report of the trial in August 1940 is missing".
P.S.: The way I first imagined the installation after reading your post was similar to the 20mm installation on He 59B-2s used for anti-shipping duties in the Spanish Civil War, as shown below:

 

[Tony Williams]

Thank you for this - the plot thickens!

There seem to be three areas of uncertainty:

1. Was the gun in a nose or dorsal position?

2. If in the nose, was it in a fixed or flexible mounting?

3. Was the cannon an Oerlikon or a Hispano?

I favour a fixed mounting in the nose, because the gun was massively heavy and would have unbalanced the plane if in a flexible mounting (heavier than fixed) which also required a gunner.

On the third of these questions: the Hispano made a better aircraft gun than the Oerlikon, being lighter and faster-firing, but in 1940 it was still being developed, whereas the Oerlikon was in naval service. Since the two guns fired different ammunition (and used different drum magazines), it would have made sense to use the Oerlikon as ammunition would have been readily available.

On the other hand, the two guns looked very similar and observers often confused them - there is a well-known photo of a Hurricane with an early cannon installation of two Hispanos fitted underwing, but these are often wrongly identified as Oerlikons.

 

[Schneiderman]

Tony,

From Air Enthusiast 17.

 

[Tony Williams]

Excellent - thank you very much!

So I was right about the mounting, but wrong about the gun (assuming that the drawing is accurate) because what is shown is a Hispano, not an Oerlikon.

 

[cluttonfred]

Wonderfull stuff, thanks guys, and an interesting case of parallel evolution since the Japanese, facing a similar challenge in dealing with American PT boats, came up with a very similar solution in the form of flexible 20mm gun firing through the belly of the Aichi E13A "Jake" reconnaissance floatplane. While the Jake was much more powerful, faster and longer-ranged than the Walrus, their gross weights were almost the same at 8,800 and 8,050 lbs respectively. It would be great to see the design studies that led to the choice of a fixed vs. flexible installation on the Walrus or the results of the (operational?) tests that nixed the idea in the end, if either exist.

 

[Schneiderman]

Tony,
The titles of the various Walrus drawings held by the RAF Museum that presumably refer to this installation just say cannon and do not specify which type. Profile 224 on the Walrus, published in 1971, also says Oerlikon, so maybe this is the original source of the error.

If you want to get order a copy of the original Supermarine drawings from Hendon I think that drawing no. 23662 4H is probably the overview of the installation.

 

[CostasTT]

Schneiderman, thanks for the information and the illustration. It explains a lot and finally clears the mystery.

 

[Tony Williams]

It would be great to see the design studies that led to the choice of a fixed vs. flexible installation on the Walrus or the results of the (operational?) tests that nixed the idea in the end, if either exist.

A Hispano cannon weighed around 60 kg, a full 60-round drum magazine 25 kg. So even if the plane carried just one magazine (probable - not easy for the pilot to change the bulky and heavy magazines while also flying the aircraft) that would be an all-up weight of 85 kg, plus probably some structural strengthening needed to allow the plane to withstand the recoil and vibration - say 100 kg in round figures.

That compares with the usual load of a front gunner (say 80 kg inc clothing) and a .303 VGO gun (9 kg) on a Scarff flexible mounting (c.10 kg). A full 97-round magazine weighed c.5 kg (not sure how many carried - say 4 adding up to 20 kg). So in total some 120 kg - roughly comparable.

The problem comes when you mount the Hispano in a flexible mounting. Your guess is as good as mine for the weight of that, but it would be a lot more than a fixed mounting and it would need to be much bigger and stronger than a Scarff for the VGO, so say 50 kg. Plus the gunner, still 80 kg. Giving an all-up weight of around 230 kg. Then add 25 kg for each extra magazine. I suspect that the Shagbat would have had serious balance problems with all of that extra weight in the nose.

 

[CostasTT]

If the installation were to be adopted for operational use, I'd certainly expect a crew member to act as a loader and change shell drums in flight. The Walrus has ample room and sufficient carrying capacity for extra ammo and a three man crew (pilot, cannon loader and dorsal gunner).

 

[Tony Williams]

Maybe, but from the look of the overhead drawing the gun magazine seems to be right next to the pilot, so not so easy to get at in flight.

 

[cluttonfred]

For all the reason Tony mentions, a flexible 20mm mounting in the Walrus nose or dorsal positions does not sound like it would have been a great idea. I also expect designers and crews alike would have been loath to cut a hatch in the hull bottom for a downward-firing mounting in the style of the Jake.

Still it does seem like heavy flexible gun firing out the port side window of the crew compartment and so located near the center of gravity could have worked for broadside strafing runs with all three positons blazing or even gunship-style orbiting attacks if anyone thought of that back in the day.

The Oerlikon does seem like the only likely 20mm option in 1940, but the 15mm BESA would have been available, though almost as heavy as the Oerlikon. Even the lighter .50 cal Browning M2 or the weaker Vickers .5 Mk.3 would have been a big improvement on .303 for engaging E-boats.

 

[CostasTT]

The original 20x72mm Oerlikon FF F weighing in at 24kg (about the weight of 2 Lewis guns) might have worked for a flexible installation. If a Vickers gun were to be used, the 12.7x120mm Class D round would have been sufficiently powerful for the task.