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Old 23rd Mar 2012, 04:41   #1 (permalink)
 
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Merlin Siamese Exhaust Stubs

Can anyone tell me why the early Merlins had Siamese exhaust stubs (Spitfires 1 to V) and then the later ones (Spitfire IX, etc.) had individual stubs on each cylinder?

Was the change related to the improved superchargers?
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Old 24th Mar 2012, 04:23   #2 (permalink)

 
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IFT, the following is just supposition on my part.

Rolls Royce recognised the importance of utilising the jet thrust available from the exhaust as a means of increasing performance. Broadly speaking, the exhaust thrust contributed some 10% of the engines output (have figures of test carried out by Hawker on a Hurricane Mk II with a Merlin XX at various altitudes).

During the Spitfire trials a number of different exhaust set ups were tried - stub, streamlined, ejector, streamlined ejector and ducted ejector. The ejector types were trialled with five different sized nozzles, with 3" being considered the best all round. The prototype Spitfire was able to increase its full power speed at 20,000 feet from 334 mph with stub exhausts to 347 mph with the ejector type.

The first Spitfire to have the individual stacks was the Mk VII, and this came about with the first use of a two speed, two stage supercharged Merlin 61. (NB - I should add that experiments with exhaust configurations seem to be many. A Mk VI was fitted with individual stacks experimentally to reduce glare during night flying, as was a totally enclosed system). The Merlin 61 was designed to be a high altitude engine, with a full throttle height in FS gear of 23,500 feet, nigh double that of previous versions. I'm guessing that the individual stubs on the 61 were optimised for some particular sweet spot, the previous ejector in use would not have been optimised for the higher gas flow emanating from the exhaust.
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Old 24th Mar 2012, 06:11   #3 (permalink)
 
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Thanks, Brian.

That reads like "very well informed" supposition!
Simon
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Old 24th Mar 2012, 11:01   #4 (permalink)
 
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I42, I`d concur with Brian`s observations,especially in regards to the Hurricane experiments,as it is documented in a book by the R-R Heritage Trust,`Performance of a supercharged aero engine`.Quite a fascinating book where the theory,developed on a single cylinder test engine was the basis proven by the full flight test results...
Possibly also relates today to the exhaust `aerodynamics seen on F1 Race cars..
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Old 24th Mar 2012, 14:17   #5 (permalink)
 
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I don't know about the Spits engines exhaust but the Rolls Royce Merlin Mk. 102 which had straight-out exhausts (versus the shrouded exhausts of the Mk 85) were exceedingly painful to the ears of crews of the Lincoln Mk 31 - Long Nose. With a take off power of +12 boost and 3000 RPM increasing to +18 in an emergency, and the inboard engine exhausts barely 15 feet from the cockpit, it was almost impossible to talk on the oxy mask intercomm system until power was reduced to climb boost of +9. Extra speed attained through these exhausts? I don't recall it.
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Old 25th Mar 2012, 01:17   #6 (permalink)

 
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What you needed was the crossover exhaust BOAC ended up fitting on the Argonauts Centaurus. Done to give the suffering passengers a quieter ride, with a side benefit of increased performance.
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Old 25th Mar 2012, 02:12   #7 (permalink)
 
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Didn't the Argonauts have Merlins ? Another bonus with the crossover ducts was to hide the purple exhaust fingers coming out of the stacks and scaring the overwing passengers to death ! The old lags at British Midland once told me they spent most of the down time welding up the cracks . . .
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Old 25th Mar 2012, 02:49   #8 (permalink)
 
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Merlin exhausts

Trans Canada Airlines engineers developed the cross over exhaust for Merlin engines on the Canadair North Star. Adopted by other operators as well.

Canadair North Star - Wikipedia, the free encyclopedia
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Old 25th Mar 2012, 04:30   #9 (permalink)
 
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sycamore,

Thanks. I'll have to look for that book. A good book that has a lot of Merlin information, particularly concerning superchargers, is Stanley Hooker's autobiography "Not Much of an Engineer".

There is a lovely anecdote in it, which I am paraphrasing from memory, where Stanley Hooker told Lord Hives that the full-throttle height of a new prototype supercharger would be about 30,000 feet. After a flight test, Lord Hives passes over a piece of paper with one number on it - 29,000+ feet.
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Old 25th Mar 2012, 05:45   #10 (permalink)
 
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Quote:
There is a lovely anecdote in it, which I am paraphrasing from memory, where Stanley Hooker told Lord Hives that the full-throttle height of a new prototype supercharger would be about 30,000 feet. After a flight test, Lord Hives passes over a piece of paper with one number on it - 29,000+ feet.
I think it was 29,900' - I just finished reading the book a couple of weeks ago. Fantastic read I highly recommend it.
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Old 25th Mar 2012, 21:58   #11 (permalink)

 
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The crossover system was first developed by Rolls Royce. TCA subsequently developed their own independently.

1952 | 1449 | Flight Archive
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Old 25th Mar 2012, 22:24   #12 (permalink)
 
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Thanks Brian--I stand corrected on TCA being 'the first'.
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