"M" series Merlins

[gecko]

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.

http://www.spitfireperformance.com/w3228speed.jpg

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

 

[pbehn]

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.

http://www.spitfireperformance.com/w3228speed.jpg

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

The three stub exhaust incorporated a hot air ducting system to heat the guns I think.

The Spitfire Society - Technical - Spitfre | The Spitfire Society

,

www.spitfiresociety.org

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.

 

[PStickney]

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.

http://www.spitfireperformance.com/w3228speed.jpg

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.
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.

http://www.spitfireperformance.com/w3228climb.jpg

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.

 

[gecko]

The three stub exhaust incorporated a hot air ducting system to heat the guns I think.

The Spitfire Society - Technical - Spitfre | The Spitfire Society

,

www.spitfiresociety.org

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.

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.

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.

http://www.spitfireperformance.com/w3228climb.jpg

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.

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.

 

[pbehn]

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.

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.

 

[Shortround6]

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.

that is the way it works. For confirmation you can look at the Hurricane and Spitfires with Merlin III engines. Same engine but the Spitfire could hold a given pressure, like 6lbs, to a higher altitude than the Hurricane when flying at max level speed.

P-39 usually has a lower FTH than a P-40 using the same level of development engine due to the behind the canopy air intake.

 

[ThomasP]

Some(most?) of the USAAF aircraft pilot' manuals include engine SEFCs (Specific Engine Flight Charts) which often give separate critical altitudes for the static engine and for the specific aircraft they are being used in. These are listed as "WITH RAM" and "NO RAM". Example below:

Note though, that there are many errors in the charts, particularly the early-war - and a few have the "NO RAM" values listed in the "WITH RAM" columns, or vice-versa.

 

[ThomasP]

I do not know if this is any help, but from the end of war Merlin manual 'bible':

and

 

[gecko]

Thanks, the information about static FTH is extremely helpful! For the exhaust, does that source happen to mention which engines used the multi-ejector non-flame damping type?

 

[ThomasP]

No, but if you Google "Spitfire Mk V (or VII or IX) exhaust manifold" under images you should get a few photos of Spitfires with that type. I know some of the Mk VII and Mk IX used them, plus I think some of the Griffon engined models used them also.

 

[K5083]

I surmise that the bolt spacing didn't change at least after the first merlins, so you could fit any exhaust. The M series being pretty late-war and daytime use only would get the six-stub because that was what was available and best performing for LF Vs and Seafires.

 

[gecko]

I think I tend to agree. As I'm looking around it looks like it is a certainty that some M series used the 6-stub exhaust, and also a certainty that it wasn't all of them - at least some of the 45M's used the 3-stub fishtails. The more difficult questions seem to be whether ONLY M series or ONLY 50 series engines used the 6-stubs (both seemed like possibilities) or if a Merlin 45 or 46 might have them - there was enough overlap time for it to be possible. I have since found that the 6-stub exhausts were initially specified for tropical aircraft (as opposed to specific engines) and later made their way to UK-based MK.Vs as well. This leads me to believe that the presence of a 6-stub exhaust is not necessarily an indicator that a given engine is either an M series or a 50 series. Would be nice to find something definitive, but that's the theory I'm working with for now. Thanks you your help everyone!

 

[ThomasP]

Did a quick look through all the Spitfire and Seafire photos that I have, and the only ones I found with the multi-ejector non-flame damping manifolds were:

Spitifre Mk VII
Spitifre Mk VIII
Spitifre Mk IX
Spitifre Mk XII

Seafire Mk IIC
Seafire Mk III
Seafire L Mk III

These are the Seafires I found with the type.

Seafire Mk IIC LR691
Seafire Mk III BL818 (I think) landing on Furious
Seafire L Mk III NF521 in air

And an entire deckload of 809 Squadron Seafires (Mk IIs I think) on HMS Stalker passing thru the Suez Canal heading for the PTO to join the BPF.

I also found some immediate post-war Seafire Mk IIIs with this type of manifold. One photo was of Seafire Mk III PR265 in French Naval service, the other was FAA Mk III PR129 landing on HMS Theseus in 1946.

If you look up the airframes by SN you can probably find out which engines they had.

 

[gecko]

There were definitely Mk.Vs with 6-stub exhausts. Good idea on the engine lookup, though I know the service histories are often incomplete or contain errors.

JK314 - M50A

Spitfire MkVcTrop RAF 352Sqn O JK314 Yugoslavia 1945 01

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JL168 - M45

Spitfire MkVcTrop RAF 352Sqn P JL168 Yugoslavia Mar 1945 01

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JK808 - M46

Spitfire MkVcTrop RAF 352Sqn B JK808 Yugoslavia Mar 1945 01

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MH592 - M55

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MA346 - M50

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Couldn't find a photo, but Johnnie Johnson's BL415 is always depicted with 6-stub exhausts and had an M45M.

EP654 - M46

Spitfires at RAF 110 Maintenance Unit Brindisi Italy 1943 IWM CNA3285

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Unkown

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Spitfire MkVb USN VCS 7 4Z Robert Doyle Normandy 1944 01

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Spitfire MkVb USN VCS 7 4X William Denton Normandy 1944 01

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Spitfire MkVb RAF 326Sqn GCII.7 V Mediterranean 1944 01

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Most Seafire Mk.IIIs were using M series engines. So assuming the above serial/engine combinations are correct, we've got pretty much the whole range of engine fitted to the Mk.V shown with 6-stub exhausts.

 

[PStickney]

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.

On Ram Recovery:
You have to be very careful when you're comparing engine performance numbers, particularly Critical Altitude. There isn't much consistency, even with primary sources such as Pilot's Operating Handbooks. Your Spitfire/Merlin 45M example is accurate, since you have a good no-ram value for critical altitude, supported by the flight test data. That's not always the case. Note also that since ram recovery is dependent on true airspeed, the air pressure entering the supercharger system will vary, of course.
Also - not all engine configurations have ramming intakes, or ramming intakes under all conditions - In the case of the F6F Hellcat, when the auxiliary stage is in Neutral (Main stage only) the main stage air is drawn from within the engine cowling - only the Aux Stage gets ram air, so there's no ram recovery in neutral blower. The essentially identical (In performance terms) engine in the F4U Corsair draws ram air under all conditions.

 

[Engineman]

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

 

[Bretoal2]

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

Alain

 

[Engineman]

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 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

 

[Engineman]

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.

http://www.spitfireperformance.com/w3228speed.jpg

Thanks for any help!

Dan

Hi again Dan,
I suspect that your chart for W3228 is showing the effect of ram on the FTH.
Cheers

Eng

 

[Bretoal2]

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

I confirm. My copy is the fourth edition, 1995, "reprinted with minor adjustments in 2003". ISBN 1 872922 06 6.

Yes, 2750 isn't much different from 3000, but that's almost a 10% difference. I think Lumsden mixed up the rev numbers, and altitude.