The Spitfire Forums

Historic Aviation => Spitfire => Topic started by: Editor on September 15, 2009, 08:26:58 PM

Title: "Coffman starter" blister on Mk. IX
Post by: Editor on September 15, 2009, 08:26:58 PM
Remember the Coffman starter blister on the stbd side of the cowling on Spitfire Mk. II? The Mk. IX had a similar blister, but no Coffman starter, so what was the blister for? I remember reading somewhere about what it was, but sadly I forgot :(. Anyone?


Title: Re: "Coffman starter" blister on Mk. IX
Post by: NZTyphoon on September 16, 2009, 12:51:24 AM
I remeber reading somewhere that it covered a cabin blower driveshaft...hmmm... seems to be a source.  :D :D ;)

Title: Re: "Coffman starter" blister on Mk. IX
Post by: Antoni on September 16, 2009, 09:45:07 PM
The fairing is related to the Coffman starter but used in a different way. It was only necessary if the Spitfire was fitted with a Merlin 61 or 63A.

The Merlin 61 was developed as a high-altitude engine and the Spitfire variant fitted with it, that is the Mk VII, would have a pressurised cockpit. This required a cabin blower to feed air to the cockpit at higher pressure than outside. In order to save excessive design the blower drive was a modification of the Coffman starter gear but driven by the engine instead of cranking it. Later on an improved version of the Merlin 61, the Merlin 63, was fitted to Spitfire F.IXs. The Merlin 63 did not have the pressurising drive gear. There were two varieties of the Merlin 63, the Merlin 63 itself and the Merlin 63A. The Merlin 63’s crankcase did not have a Coffman casing but the Merlin 63A used the crankcase of the Merlin 64 (an improved variant for the pressurised MK VII) and this had the protruding casing but not the drive gear itself.

The bulge is found on every MK VI and Mk VII. Photographs show that it is usually found on Supermarine-built and Rolls Royce-converted F.IXs. It appears on virtually all Mk VIIIs (all built by Supermarine). It is also found on two-seat trainers converted by the factory after the war. This suggests that there was one standard type of cowling panel fitted regardless of the engine variant. CBAF-built Mk IXs did not feature this bulge as standard as it is only found on a few aircraft fitted with Merlin 61 or 63A engines.