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The three stub exhaust incorporated a hot air ducting system to heat the guns I think.I've got more questions regarding the "M" Series Merlins - 45M, 50M, and 55M. Firstly, I'm a bit confused by the performance. Most sources seem to quote the rated altitude as 2,750ft, but I can't find what they are basing this on. The only chart for any of these variants that I have found is for a 50M, and by that chart the rated altitude looks like it should be 5,900ft.
Am I understanding something wrong, or is there other data available?
My second question relates to exhaust stacks. Late Spitfire Mk.Vs often have 6-stub exhausts fitted instead of the more common 3-stub type. I have seen claims that this denotes an M series engine, or a 50 series engine, or that it wasn't specific to any particular variant. As my knowledge at this point is based on contradicting hearsay, I'm hoping to find something more definitive.
Thanks for any help!
Dan
What you're looking at in that chart is the performance in flight including Ram Recovery. A reaonably designed carburetor or supercharger air intake will convert a substantial portion of the dynamic pressure due to the aircraft's airspeed into static pressure entering the supercharger system.I've got more questions regarding the "M" Series Merlins - 45M, 50M, and 55M. Firstly, I'm a bit confused by the performance. Most sources seem to quote the rated altitude as 2,750ft, but I can't find what they are basing this on. The only chart for any of these variants that I have found is for a 50M, and by that chart the rated altitude looks like it should be 5,900ft.
Am I understanding something wrong, or is there other data available?
My second question relates to exhaust stacks. Late Spitfire Mk.Vs often have 6-stub exhausts fitted instead of the more common 3-stub type. I have seen claims that this denotes an M series engine, or a 50 series engine, or that it wasn't specific to any particular variant. As my knowledge at this point is based on contradicting hearsay, I'm hoping to find something more definitive.
Thanks for any help!
Dan
No, that's not what I'm talking about. There are four types of exhaust manifold that you see on the Mk.V. The early ones had three simple round outlet stubs per side. Then the fishtail exhausts you describe came into use - still with three stubs per side. A later variant of these included a duct routed to the guns to heat them. This boosted the existing gun-heating system which relied on hot air from the radiator. The duct is visible just aft of the exhaust manifolds. As far as I know, Merlin 45 and 46 engines used all of these three versions, depending on the date. The fourth version of the exhaust had six stubs per side instead of three. I just don't know if this 6-stub version was used on all Mk.V Merlin variants, only "M" series engines, or only 50 series engines.The three stub exhaust incorporated a hot air ducting system to heat the guns I think.
It was soon discovered that simple changes to the ejector exhausts from simply blowing out to the side to being directed back would increase speed. The exhausts evolved from round outlets to fishtail in appearance which also had the bonus of reducing exhaust glare during night flying. These changes resulted in harnessing the exhaust gases provided an additional 10mph or 70 horsepower. The exhausts alongside forward facing intake ducts were used to heat the guns in the wing which were prone to stoppages at altitude as a result of the colder temperature, and superior to the earlier heating from the engine coolant radiator.
What you're looking at in that chart is the performance in flight including Ram Recovery. A reaonably designed carburetor or supercharger air intake will convert a substantial portion of the dynamic pressure due to the aircraft's airspeed into static pressure entering the supercharger system.
Since the supercharger is increasing the pressure of the air entering it by factors mainly related to the supercharger's speed, it basically acts as if it's at a lower pressure altitude.
For a better indication of the static performance of the engine, let's look at the data for the same engine, at the same power setting, but a lower airspeed, let's take a look at the climb data -
Note that Indicated Air Speed in this case can be considered the speed at sea level that the True Airspeed at a particular set of Airspeed, Altitude, and Temperature conditions would be.
Where the Equivalent Airspeed is 170 mph at 3800', rather than the 335 mph at 5900' in the Maximum Speed chart, thus the pressure increase to the carburetor inlet is lower.
Ram recovery is significant at high speeds, and much less so when slow.
My only point or input to the discussion was that the exhaust manifolds on early Spitfires concerned gun heating. Regardless of engine type if they had individual ejector type exhausts gun heating wasnt involved. They were separate issues as far as I know.No, that's not what I'm talking about. There are four types of exhaust manifold that you see on the Mk.V. The early ones had three simple round outlet stubs per side. Then the fishtail exhausts you describe came into use - still with three stubs per side. A later variant of these included a duct routed to the guns to heat them. This boosted the existing gun-heating system which relied on hot air from the radiator. The duct is visible just aft of the exhaust manifolds. As far as I know, Merlin 45 and 46 engines used all of these three versions, depending on the date. The fourth version of the exhaust had six stubs per side instead of three. I just don't know if this 6-stub version was used on all Mk.V Merlin variants, only "M" series engines, or only 50 series engines.
So in the case of our Spitfire LF.Mk.V equipped with a Merlin 45M, the commonly quoted power output is 1,585 hp at 3,000 rpm and +18 psi boost, at 2,750 ft. If I am understanding correctly, the engine will produce +18 psi up to 2,750 ft even with the aircraft sitting still. Above that altitude, +18 psi can only be maintained with the aid of ram air, which the chart indicates is possible in level flight up to 5,900 ft assuming the aircraft is flying at its maximum level speed.
On Ram Recovery:No, that's not what I'm talking about. There are four types of exhaust manifold that you see on the Mk.V. The early ones had three simple round outlet stubs per side. Then the fishtail exhausts you describe came into use - still with three stubs per side. A later variant of these included a duct routed to the guns to heat them. This boosted the existing gun-heating system which relied on hot air from the radiator. The duct is visible just aft of the exhaust manifolds. As far as I know, Merlin 45 and 46 engines used all of these three versions, depending on the date. The fourth version of the exhaust had six stubs per side instead of three. I just don't know if this 6-stub version was used on all Mk.V Merlin variants, only "M" series engines, or only 50 series engines.
I am familiar with ram effects on full throttle height, but I think my terminology is confused. So are you saying that when an engine is rated as producing a specific amount of horsepower at a given altitude, rpm, and boost, this is a static, no-ram number? I guess I had been thinking that this was an aircraft-specific number which thus would also include the ram effect from the aircraft reaching its top speed. I take it I am wrong on that count? I kind of hope I am wrong, a static value would make more sense and be more useful.
So in the case of our Spitfire LF.Mk.V equipped with a Merlin 45M, the commonly quoted power output is 1,585 hp at 3,000 rpm and +18 psi boost, at 2,750 ft. If I am understanding correctly, the engine will produce +18 psi up to 2,750 ft even with the aircraft sitting still. Above that altitude, +18 psi can only be maintained with the aid of ram air, which the chart indicates is possible in level flight up to 5,900 ft assuming the aircraft is flying at its maximum level speed.
Hi Dan,
Quoting Lumsden, RR Merlin 45M, 50M and 55M, Maximum power was 1,585bhp, 3,000rpm, +18lb Boost at 3,000ft Altitude. This would be in standard conditions without Ram or other installation factors.
Cheers
Eng
Lumsden is wrong, critical altitude for 45M - 50M - 55M is 2750 ft, 10% lower.
From official Rolls-Royce documents (in Alec Harvey-Bailey 's "The Merlin in perspective, the combat years")...
Regards
AlainView attachment 653981
Hi again Dan,I've got more questions regarding the "M" Series Merlins - 45M, 50M, and 55M. Firstly, I'm a bit confused by the performance. Most sources seem to quote the rated altitude as 2,750ft, but I can't find what they are basing this on. The only chart for any of these variants that I have found is for a 50M, and by that chart the rated altitude looks like it should be 5,900ft.
Thanks for any help!
Dan
Hi Alain,
Yes, a good looking table of info on your post. Are you sure that was from "The Merlin in perspective"? It is not in mine.
Also, 3,000' and 2,750' are not much different in this context.
Cheers
Eng