Blast tubes and fabric gun port covers

[oldcrowcv63]

Can some one please explain to me the purpose of blast tubes placed over the barrels of wing mounted machine guns? Also, what was the protocol for using blast tubes instead of the fabric gun port covers. I assume fabric gun port covers were used for a combination of aerodynamic efficiency and preventing gun mechanism contamination by environmental debris (AKA dirt).

 

[Airframes]

The main reason for fabric gun port patches was to prevent 'damp cold' - moisture freezing, at altitude, in the barrels and working parts - and the ingress of dirt and dust.
Depending on the aircraft type, 'blast tubes' were part of the actual mounting of the weapon, intended to hold and align, whilst absorbing at least part of the blast/recoil effect. Muzzles exposed beyond the wing were covered in cardboard or 'plastic-type' caps, or sometimes fabric, for the same reasons as above.

 

[Shortround6]

A "blast tube" prevented muzzle blast from damaging the aircraft. Usually seen on cowl guns or fuselage guns. Perhaps seen on wing guns if the actual muzzle was behind the leading edge?

On P-47s the tubes were actually heat shields. Other countries may have had different reasons for different guns. The .50 cal Browning was recoil operated and the barrel moved a short distance with each shot. While a support for the muzzle may have helped ( a forward bearing ?) it wasn't really a "mount" in the sense of fasting the gin to the airframe. that would be done at the receiver.

 

[oldcrowcv63]

I am thinking specifically of the photos of P-40s and perhaps P-51s I have seen. Some photos show the three blast tubes protruding from the leading edge while others taken of the same a/c model show no indication of the presence of a gun muzzle other than the gun port in the leading edge. Evidently the gun muzzle is either recessed in or located right at the leading edge in these a/c.

Some photos show the ports covered with what appears to be a fabric patch and in addition 'movies' that appear to show the shredded fabric remnants of the patches fluttering with bursts of gas issuing from each muzzle as the guns are fired. I assume such photos were taken when the planes were mounted on their gun stands. My perception is that the guns can be fired with or without the blast tubes.

I have seen many photos of leading edges with some ports perforated and others not, and some muzzles with blast tubes installed and other guns without.

Here are some examples:

One is a 7th PS Andy Reynold's P-40E 'Stardust' #86 operated in Australia, spring 42. showing what appears to be three blast tubes

The second is 8th PS Clyde Barnett's P-40E 'Smiley' #55 also in Australia, spring 42. showing three patches, two appear to be fresh while the third appears a bit weathered.

 

[rednev]

a quick glance at adf-serials.com.au found the following among the p40 serials page .
a 29-408 p40n-1-cu 28th feb 44 port mainplane damaged by gun blast blowing up while straffin enemy positions
a 29 415 blew a blast tube during gunnery practise sent for repairs
a29423 29th september 43 damage was done to the port wing and ribs due to faulty ammo blowing the blast tube of of inner port gun landed without further damage ac sent for repairs
a 29 431 26th feb 45 blast tubes blown on practise gunnery range with f/o walter having to crash land the aircraft into the sea
lots more examples of aircraft receiving extensive damage from blown blast tubes .

I cant think of the top of my head but i have seen a photo of a p40 with a fairly well shreded wing that i assumed was from anti aircraft fire till i read the caption yup failed blast tube .

 

[stona]

There was a report in 1941 which examined the effects of faired and un-faired guns on the handling of the aircraft and performance (cooling etc) of the guns. It was called "An investigation of wing-gun fairings on a fighter type aircraft". It was carried out at Langley as a NACA report, originally published in October 1941.
It is an interesting read. I have a copy somewhere as it is listed in my database, but can't find it at the moment.
Cheers
Steve

 

[Wildcat]

How exactly did the blast tubes get blown? Was it a case of them coming loose and being hit by a fired round? I have often wondered as it seems like it was a fairly regular occurrence with RAAF P-40's.