RR Griffon at low RPM

[ACW599]

Firstly, apologies if this isn't the best forum to ask the question. I mentioned on another thread that the BBMF's Spitfire PR XIX landed at Cosford yesterday on a weather diversion and made a lot of people very happy in the process. After the aircraft had taxyed past us and departed with the usual glorious audio accompaniment, several of us were subsequently wondering why the Griffon sounds almost as though it's only running on one bank of cylinders when running at low RPM. In fact at one stage it sounded for all the world like a rotary engine being 'blipped'.

I don't recall Shackletons sounding like that but another spectator observed that the Sea Fury's Centaurus sounds very similar ("like a bag of bolts" was the expression used) at low power. Is this a general characteristic of big piston engines, and if so why?

[CirrusF]

There is no inherent reason that a large multi-cylinder piston engine, even of high performance, should sound like that at idle.

I'd guess that on the Griffon the primitive (by modern standards) fuel injectors are optimised for high power settings, and at low power settings their metering is poor, resulting in random misfires and backfires.

[GOLF_BRAVO_ZULU]

another spectator observed that the Sea Fury's Centaurus sounds very similar ("like a bag of bolts" was the expression used)

I remember the Aer Turas C54 that used to be a regular visitor to Cambridge when I was there at flying school. That description sums up well the sound of that machine taxying. A total contrast to take-off and the music of 4 Twin Wasps in close formation.

Regarding the wonderful Griffon, though, my money is on CirrusF‘s suggestion.

[Fareastdriver]

There is another thread

P51

that covers this. Something to do with a different firing order.

[ACW599]

>I'd guess that on the Griffon the primitive (by modern standards) fuel injectors are optimised for high power settings<

I'd guess that the Griffon in the PR XIX has a Stromberg injection carburettor rather than "injectors". But sub-optimal mixture distribution at other than full power is probably the answer.

[nacluv]

At Waddo Airshow last year, the BBMF PRXIX did a beautiful formation display with one of CBY's Typhoons. Afterwards, coming in on the downwind leg on it's own, it sounded decidedly sick - popping and banging like mad, really rough.

At the time I put it down to having had to work bloody hard to keep up!

But I prefer CF's explanation.

[Jhieminga]

I seem to remember reading somewhere that the Griffon in a PR.XIX is an ex-Shackleton unit. As such it has a pretty high idle rpm which needed to be decreased to enable the Spitfire to taxi at a normal speed. Because of this the idle rpm of this Griffon is a compromise between keeping the engine running and keeping the revs low enough to enable a normal taxiing pace. At this specific rpm setting the engine isn't smooth.

[treadigraph]

Jhieminga, you may be thinking of the French Spitfire XIX which was rebuilt by Steve Hinton with a Shackleton Griffon 56 and contra prop unit as they were easier to come by than the normal Griffon 65, and also removed the torque problem which would have enabled Hinton to go after some brakes off to height records - had somebody else not got there first in a homebuilt!

Jacquard, the current owner, has now converted it back to a Griffon 65 this year and while it's a shame that an intriguing shape and sound has gone, the aeroplane is back as she should be.

Now... if the American owner of the airworthy Seafire 47 could be persuaded to bring it over here for a few weeks next year...

I love the sound Griffon makes - it does sound quite rough while throttled back, but at full snort that ripping snarl sounds fantastic! In the BoB film, you can distinctly hear the difference between one of the Merlin powered Spits and a Griffon during one of the scramble sequences.

[Jhieminga]

I wasn't actually, I looked it up, it was in Aircraft Illustrated from November 1999. The article is about the RR PR.XIX PS853:

...and a Griffon RG30 SM-S has been fitted in place of the original Griffon 65. The Griffon RG30 SM-S was modified from a Shackleton Griffon by changing the external gearbox, reshaping the supercharger and camshaft cover and deletion of the Shackleton engine's FS gear.

The engine in the Shackleton was required to run at an idle of around 1200 rpm; the preferred speed for the Spitfire is around 350 rpm. The idle speed on this installation has been reduced to a compromise figure of 500 rpm.

It could well be a Griffon 65 in the BBMF PR.XIX but the comment on the idle speeds for the Spitfire vs. the Shackleton may explain the different characteristics.

[treadigraph]

As ever one lives and learns... looking at the BBMF website PM631 has a Griffon 66 (well it says it was built with one but doesn't say what it has now) and PS915 has a modified Griffon 58.

Jacquard has definitely changed the engine, but I wonder if he's actually fitted a suitably modified 58 from a Shackleton?

No matter, they still sound great! But a sad reflection that I can't now remember what a Shackleton sounds like taxiing - the last few I saw arrived, displayed and flew off again.