Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: HoHun on August 09, 2004, 06:47:56 PM
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Hi everyone,
I'm looking for something definite on the boost level (and rpm) specified for the early Spitfire V.
I'm convinced that it was +12 lbs/sqin @ 3000 rpm, but I can't prove it wasn't (as suggested by someone else) +9 lbs/sqin at either 3000 or 2850 rpm.
The copy of the Pilot's Notes I've seen are of no help as they are referring to the "late" situation with +16/+18 lbs/sqin cleared.
I've also found Alfred Price quoting an Air Ministry memorandum from August 1942 pointing out that the conversion of Spitfire V aircraft for +16 lbs/sqin was in progress then, but no mention of the previous boost limit is made.
Do you know of any documentation proving (or disproving) my point about the +12 lbs/sqin @ 3000 rpm?
Thanks in advance! :-)
Regards,
Henning (HoHun)
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MiloMoron will be here in a flash to tell you it ran on +25 lbs and 9000 rpm and had a grater range than the P-51.
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I'm convinced that it was +12 lbs/sqin @ 3000 rpm, but I can't prove it wasn't (as suggested by someone else) +9 lbs/sqin at either 3000 or 2850 rpm.
I've seen somewhere, though I can't put my finger on exactly where at the moment, that initial ratings were 3000 rpm 9lbs wep, 2850 rpm 9 lbs 30 minutes, 3000 rpm 12 lbs takeoff.
I'm pretty sure those were the figures to begin with.
I've also found Alfred Price quoting an Air Ministry memorandum from August 1942 pointing out that the conversion of Spitfire V aircraft for +16 lbs/sqin was in progress then, but no mention of the previous boost limit is made.
The limits before 16 lbs was approved were definately 3000 rpm 12 lbs, as noted in the trials against Faber's 190. I don't know when the change from 9 lbs to 12 lbs was approved, though.
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Originally posted by GScholz
MiloMoron will be here in a flash to tell you it ran on +25 lbs and 9000 rpm and had a grater range than the P-51.
LOL, what a first post in what would have been an intersting thread. Congratulations for being your usual ultra melon self, Col Klink's lap dog.
btw, a grater is something you use to grate cheese with.:)
..........
Henning, Nashwan or Mike Williams would be the best source.
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Ah, thank you for pointing that out. I strive to improve my spelling. :)
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Hi Nashwan,
>I've seen somewhere, though I can't put my finger on exactly where at the moment, that initial ratings were 3000 rpm 9lbs wep, 2850 rpm 9 lbs 30 minutes, 3000 rpm 12 lbs takeoff.
I figured that as with the Spitfire II, take-off figures would be cleared as WEP.
>The limits before 16 lbs was approved were definately 3000 rpm 12 lbs, as noted in the trials against Faber's 190.
That's interesting! Do you know where I could find these? Though Price' Spitfire book features the Spitfire Vb vs. Fw 190A-3 tactical trial, no boost pressure is given there.
I've found:
http://www.fourthfightergroup.com/eagles/aa878.html
This report describes that prior to +16 lbs/sqin, the combat rating was indeed +12 lbs/sqin.
However, it doesn't confirm that it was used from service introduction of the Spitfire V on.
Regards,
Henning (HoHun)
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I figured that as with the Spitfire II, take-off figures would be cleared as WEP.
It does seem strange they went from 12 lbs on the Merlin III and XII to 9 lbs on the Merlin 45. The info I've seen might have been from before service introduction. It might simply have omitted the 12 lbs rating, as the issue of wep didn't seem clearly defined by the RAF at the time.
If you do find a definite date when it was cleared, I'd be very interested.
>The limits before 16 lbs was approved were definately 3000 rpm 12 lbs, as noted in the trials against Faber's 190.
That's interesting! Do you know where I could find these? Though Price' Spitfire book features the Spitfire Vb vs. Fw 190A-3 tactical trial, no boost pressure is given there.
Ring's site has details of the tactical trials of Faber's 190:
http://prodocs.netfirms.com/
Section D page 8 of the "Various Fw 190 test reports" has the ratings used for the test.
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HoHun,
Look at this one on 4th Fighter Group:
http://www.fourthfightergroup.com/eagles/x4922.html
I has 3000 rpm +9 psi as the highest rating
This one as well:
http://www.fourthfightergroup.com/eagles/w3134.html
Greg Shaw
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"It does seem strange they went from 12 lbs on the Merlin III and XII to 9 lbs on the Merlin 45."
Did the displacement change from III & XII to 45?
If it increased so I'd imagine it run on lower boost.
-C+
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The Merlin III/XII is simply not the same engine series as the 45s. It`s quite believable that they had different max. allowed boosts cleared for them at the same time.
As the two early Spit V trials show, in the first year or so, the 45s appear to be limited to +9/3000rpm for level flight, and +9/2850rpm for climbs.
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The displacement didn't change between the XII and 45, indeed I don't think it changed between the Merlin I and Merlin 145.
The Merlin 45 was essentially a Merlin XII with a different supercharger.
The problem with Isegrim's assertion is that he's basing it on tests of the Spit V, which were only carried out at 9 lbs, then at 16 lbs.
However, the Merlin XII in the Spit II was rated at 9 lbs, and tested at such, even though 12 lbs boost was definately authorised.
Early in the war, the Spitfire didn't really have a WEP setting, the max boost was 6.25 lbs for cruise or combat. With the introduction of 100 octane fuel, they allowed 12 lbs boost, but only in "emergencies". They did some tests of what improvements could be expected, but 12 lbs wasn't considered a "normal" rating.
WEP was not really standardised until later on.
The other thing that contradicts what Isegrim says is that there isn't a test for 12 lbs on the Spit V at all, yet it was definately authorised at some time.
In the test of 16 lbs WEP, they say that the prior limit was 12 lbs, and in the comparison tests against Faber's 190, they ran the Spit V at 12 lbs.
We know for a fact that 12 lbs was authorised at some time, yet no tests seem to exist for it, which points to it being authorised from the start. After all, when 16 lbs was authorised, they tested it.
the 45s appear to be limited to +9/3000rpm for level flight, and +9/2850rpm for climbs.
9 lbs 3000 rpm was a 5 minute rating, 9lbs 2850 rpm was a climb rating, not a max allowed for climbing, ie it was allowed for 30 mins or 1 hour, and compares to the German designation "climb and combat power"
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The problem with Isegrim's assertion is that he's basing it on tests of the Spit V, which were only carried out at 9 lbs, then at 16 lbs.
You claim no +12lbs tests exist for the MkV? On what basis? Oh, it isn`t on the 4th FG website, I see, now THAT proves it. :D
Originally posted by Nashwan
However, the Merlin XII in the Spit II was rated at 9 lbs, and tested at such, even though 12 lbs boost was definately authorised.
Well let`s apply the same "definiete" logic to the DB 601N and E, since we know the 601N was rated at 1.35ata, we can definietely say the 601E, a different engine, was cleared for 1.35ata, too. After all, 1.4ata was already authorized on the 601A.
The typical silly strech-as-far-as-you-can story from Nashwan.
The other thing that contradicts what Isegrim says is that there isn't a test for 12 lbs on the Spit V at all, yet it was definately authorised at some time.
Contradicts? With what? With that :
"what Isegrim says is that there isn't a test for 12 lbs on the Spit V at all"
Well since Isegrim says that not, it`s only a classic strawman argument typical from Nashwan (not surprising), then we can only conclude Nashwan contradicts with himself (well that isn`t surprising either)
No +12lbs test at all? Maybe not on the 4th FG website, but that hardly proves anything. 4th FG hasn`t got the paper that shows no Spits had any armor fitted at the start of the Battle of France, yet we know such documents exist, and that`s the truth. 4th FG also shows figures for MkXIVs at +25lbs, a rating we all know was never cleared for them in service, because there are documents that show that, even though 4th FG website doesn`t show that either, for some odd reason. :D
We know for a fact that 12 lbs was authorised at some time, yet no tests seem to exist for it, which points to it being authorised from the start. After all, when 16 lbs was authorised, they tested it.
:lol That`s a classic one. :rofl
No proof is proof enough for Nashwan when it comes to Spitfires.
9 lbs 3000 rpm was a 5 minute rating, 9lbs 2850 rpm was a climb rating, not a max allowed for climbing, ie it was allowed for 30 mins or 1 hour, and compares to the German designation "climb and combat power" [/B]
Source?
Apperatnly +9lbs and 3000 RPM could not be used below 22k ft in a climb at all.
http://www.fourthfightergroup.com/eagles/ab320.html
See the switch of RPM : "R.P.M. changed from 2850 to 3000 at 22,000 ft. "
The document states these are maximum climb figures, so I guess they were done at max. available settings.
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Hi Greg,
>I has 3000 rpm +9 psi as the highest rating
Thanks!
Unfortunately, I have to suspect that the tested power levels were not necessarily identical to the permitted operational power levels, with differences possible in both ways.
(For example, we know that the Spitfire I/II were cleared to +12 lbs/sqin combat boost, but the tests only show the original +9 lbs/sqin rating.)
I think the manuals would be a good guide if we had the contemporary issues. Of course, most manuals were carefully amended to keep them up to date, so today we mostly see latest developments fixed in print.
Regards,
Henning (HoHun)
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Hi Nashwan,
>We know for a fact that 12 lbs was authorised at some time, yet no tests seem to exist for it, which points to it being authorised from the start.
That's illogical, captain! ;-)
I'd agree that +12 lbs/sqin very likely was cleared from the beginning, but in my opinion, the absence of +12 lbs/sqin tests would point towards the opposite. (Due to the Spitfire II analogy, I don't consider that important.)
>9 lbs 3000 rpm was a 5 minute rating, 9lbs 2850 rpm was a climb rating, not a max allowed for climbing, ie it was allowed for 30 mins or 1 hour, and compares to the German designation "climb and combat power"
Hm, though I think you're right, that's another can of worms. We have
+9 lbs/2850 rpm (30 min)
+9 lbs/3000 rpm (in my opinion 30 min wartime concession, in your opinion 5 min)
+12 lbs/2850 rpm (could be deducted from comments on 3000 rpm only to be used at altitude)
+12 lbs/3000 rpm (5 min)
To make matters worse, some Spitfire tests refer to the highest setting as "flat-out level power", implying that it could (or should) not be used in a climb.
I'd tend to think that both 3000 rpm settings were the wartime Climb & Combat/Emergency power settings respectively and applicable in climbs as well as in level flight, but I have to admit that there's not enough data for absolute certainty :-(
Regards,
Henning (HoHun)
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That's illogical, captain! ;-)
It might well be wrong, but my logic is that when they increased the rating from 12 lbs to 16 lbs, they tested it.
I haven't seen any tests of the Spit V at 12 lbs. Nor the Spit II.
They already knew how the Merlin III performed at 12 lbs, they simply based the Merlin XII figures on that, and, I think, did the same with the Merlin 45 as well.
If they had restricted 12 lbs on the Spit V to begin with, I think they'd have done tests to check it was okay when they finally allowed 12 lbs. Spitfire the History makes no mention of them.
If they allowed 12 lbs from the begining, there'd be no more need to test it than on the Spit II.
Hm, though I think you're right, that's another can of worms. We have
+9 lbs/2850 rpm (30 min)
+9 lbs/3000 rpm (in my opinion 30 min wartime concession, in your opinion 5 min)
+12 lbs/2850 rpm (could be deducted from comments on 3000 rpm only to be used at altitude)
+12 lbs/3000 rpm (5 min)
I think 3000 rpm was always rated for 5 mins, apart from on the Spit V in high alt climbs (and the same is true of the 109K4 climb chart, iirc, with a revs increase at 8,000m)
So, I doubt 12 lbs 2850 rpm was ever a valid rating on the Spit V.
You claim no +12lbs tests exist for the MkV? On what basis? Oh, it isn`t on the 4th FG website, I see, now THAT proves it
There's no mention in Spitfire the History either, that I can see.
Plenty of description of tests with modified ailerons, drop tanks, different exhausts, different temperatures, elevator bob weights, etc.
Of course, that doesn't mean such tests weren't done.
Well let`s apply the same "definiete" logic to the DB 601N and E, since we know the 601N was rated at 1.35ata, we can definietely say the 601E, a different engine, was cleared for 1.35ata, too. After all, 1.4ata was already authorized on the 601A.
The typical silly strech-as-far-as-you-can story from Nashwan.
We know the Merlin XII was authorised for 12 lbs boost because the manual says so.
Go to http://www.fourthfightergroup.com/eagles/spit2.html
and scroll down to the bottom. It clearly says combat limit 12 lbs.
Perhaps it's not such a silly stretch after all?
Contradicts? With what?
You claimed that because there was no test at 12 lbs, it was not authorised.
We know it was, yet there are no tests.
Abscence of a 12 lbs rating on a particular test cannot be proof it was not allowed, even at that time. They may simply have tested at normal rated power, not wep. ie not used the "emergency" rating.
4th FG also shows figures for MkXIVs at +25lbs, a rating we all know was never cleared for them in service, because there are documents that show that,
Are there? I've never seen them. I've seen documents saying it is not yet cleared, but will be in the future. If you've found anything new in the meantime, I'd be glad to see it.
No proof is proof enough for Nashwan when it comes to Spitfires.
I didn't say it was proof, I said it points to.
We know 12lbs was allowed, and tested, in the Spit I. We know it was allowed in the Spit II, yet the performance test of the Spit II, carried out on the 27th September 1940, when 12 lbs boost was in widespread use, didn't include a trial at 12 lbs boost.
If they banned the use of 12 lbs in the Spit V, then I'd have thought they'd have wanted to test it before finally allowing it.
It's nothing like proof, as I said it points to it.
We do know, for certain, that the abscence of the use of 12 lbs on the Spit II test doesn't mean the Spit II couldn't use 12 lbs, though.
Source?
The Spit V manual.
See the switch of RPM : "R.P.M. changed from 2850 to 3000 at 22,000 ft. "
See the Bf 109 k4 climb chart at 1.8 ata. There is a line showing a revs switch at 8,000m, and the "time to climb" line, which is at climb and combat power, reflects that change above 8,000m.
I believe we had a big argument about that before you finally accepted it :)
I'm not an engineer, so I don't know why rev increases were permitted during high altitude climbs, but it does seem to have had a valid reason on some 109s and Spits.
The document states these are maximum climb figures, so I guess they were done at max. available settings.
Note the test of AA878, where climbs were done at combat rating (16 lbs, 3000 rpm) and "Normal" rating, 9 lbs 2850 rpm.
http://www.fourthfightergroup.com/eagles/aa878.html
The Spit V manual I have says
Maximum climbing limit 2850 rpm +9 lbs
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Originally posted by Nashwan
We know the Merlin XII was authorised for 12 lbs boost because the manual says so.
Go to http://www.fourthfightergroup.com/eagles/spit2.html
and scroll down to the bottom. It clearly says combat limit 12 lbs.
Perhaps it's not such a silly stretch after all?
What`s the date of the manual?
I still don`t get what the MkII / Merlin XII has to do with the MkV/Merlin 45.
Kinda like saying just because the MkV was cleared to +16lbs in late 1942, then so was the MkIXs,too.
We know however at that time all MkIX were 'derated';) to +15lbs.
Another example, we know the DB 605D could run for Kampfleistung at 1.45ata, so can we say the DB 605A could run at 1.45ata kampfleistung as well, even though we have no reference to it?
You claimed that because there was no test at 12 lbs, it was not authorised.
We know it was, yet there are no tests.
Oh-oh, sorry I didn`t. I even pointed that one out. Get off that train. I simply say as there no indication of +12 lbs boost until 1942 AT ALL, it was probably not used initially on a new engine type, which is rather untested at the time.
Abscence of a 12 lbs rating on a particular test cannot be proof it was not allowed, even at that time. They may simply have tested at normal rated power, not wep. ie not used the "emergency" rating.
Expect of course if that test is explicitely about the maximum performance of the plane available to it at the moment. That is reached with maximum powers, u know.
Tell me, Nashwan, if you would like to know what is the maximum speed and climb ability of a plane, on what would you test it, +9 or +12, hypothesizing both are cleared at the time...?
They tested it on +9 in all tests I have seen from `41, the first mentioning of +12 is from an mid-1942 AFDU test vs. the 190A.
Re XIV at +25
Are there? I've never seen them. I've seen documents saying it is not yet cleared, but will be in the future. If you've found anything new in the meantime, I'd be glad to see it.
Well then it appears you and Mike are the only partisans of a lost cause. I thought you have seen the light in the meantime, but it appears I was overly optimistic. Even Neil says +25lbs wasn`t cleared for the Griffon 65 series. From the doc I have seen on that, +21 is enough for 366mph at SL (in fact a bit worser than I expected and calculated).
Moreover, the Mk21 tested in mid-45 on Mike site uses the same engine as the MkXIV (the only difference being reduction gear), and the test say the relevant limitation of the engine is still +21lbs, provide 150 grade is available.
BTW, it also appears that +25lbs 'tests' on the Spit XIV were really a short run at a single altitude, with the rest of the curve calculated from it, as the engine had problems during the tests running on that boost.
We do know, for certain, that the abscence of the use of 12 lbs on the Spit II test doesn't mean the Spit II couldn't use 12 lbs, though.
Though it points towards it wasn`t used at the same time as MkIs. Or just the test plane didn`t have the modifications for it, remember, the RAF did not fully convert to 100 grade fuel until November 1940.
What we have is a test report of a Spit II tested somewhere after May 1940, and an undated Spit II manual that could be anywhere between 1940 and 1945.
See the Bf 109 k4 climb chart at 1.8 ata. There is a line showing a revs switch at 8,000m, and the "time to climb" line, which is at climb and combat power, reflects that change above 8,000m.
I believe we had a big argument about that before you finally accepted it :)
I'm not an engineer, so I don't know why rev increases were permitted during high altitude climbs, but it does seem to have had a valid reason on some 109s and Spits.
There`s a simply reason to that, the supercharger speed is directly linked to engine speed, and the 603G supercharger probably couldn`t keep up the 1.45ata boost for Kampfleistung above 8000 without increasing the speed by 200rpm to 2800. I doubt this would be speciality on the 109K only, unless it happened automatically (to which I didn`t find any reference) possible all the others could do the same trick to maintain their 1.3ata to higher altitudes by simply pushing the throttle forward.
Ie. http://www.x-plane.org/users/isegrim/109/198_drehzahl.jpg
Notice the DB reference and the date of the report BTW. ;)
Note the test of AA878, where climbs were done at combat rating (16 lbs, 3000 rpm) and "Normal" rating, 9 lbs 2850 rpm.
http://www.fourthfightergroup.com/eagles/aa878.html
[/B]
Yep, in mid-42. We are speaking of mid41, combat ratings could have been easily raised during a year.
The Spit V manual I have says
Maximum climbing limit 2850 rpm +9 lbs [/B]
Hmm, and it`s date, early 1944 or 1943 I presume with increased ratings, supposed to prove in 1941 it was already used ?
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Hi everyone,
To sum it up:
+16 lbs/sqin is documented as in introduction in August 1942
+12 lbs/sqin is documented as being used before August 1942
+9 lbs/sqin was a documented setting, but the documents are not clear that it was ever considered maximum power
The documentation we have doesn't allow any more specific conclusions that these :-(
Regards,
Henning (HoHun)
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100% agree HoHun. That`s the maximum we can conlculde ithout the proper documentation.
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Well, 12 is what I heard.
That would be applyed by pushing the throttle through the gate, right?
Note, that although there may be time limits listed, especially in the manual, that was just on text. There was nothing stopping the pilot from using max boost untill either the mission ended or the engine seized up.
RR had a very tight limit compared to what their engines would actually take.
An overdosis of "panic boost" would by requirements lead to a total overhaul of the engine.
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Then IMO the AH2 SpitV should be dragged down back to +12 boost.
And a new sub-variant of +16 boost with other features such as clipped wings, should be introduced.
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Osprey's book "Spitfire Mk V Aces" by Alfred Price has an appendice on page 87, listing both the Merlin 45 and Merlin 46 as being 16 lb boost engines.
"Spitfire VB W3134 tested May 1941
RR Merlin 45 EMERGENCY COMBAT POWER (WEP in Aces High)1515 bhp at 11,000 ft at +16 lbs boost
Max speed 371 mph
This was a first production Spitfire V with combat gear"
"Spitfire VA (tropicalised) X4922 tested early 1942
RR Merlin 46 EMERGENCY COMBAT POWER (WEP In Aces High)1415 bhp at 14,000 ft at +16 lbs boost
Max speed 363 mph"
Im paraphrasing of course.
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So it's Osprey vs Hohun's summary?
...
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Hi Kweassa,
>So it's Osprey vs Hohun's summary?
No, actually it's Osprey vs Air Ministry.
The August 1942 date is from an Air Ministry memorandum.
You could also say it's Price vs. Price since I found the Air Ministry memorandum in his "Spitfire - The Complete Fighting History" :-)
But seriously - maybe we can make an educated guess in the +9/+12 lbs/sqin issue by calculating mean effective pressure of the Merlin XII (cleared for +12 lbs/sqin) and the Merlin 45 of the Spitfire V?
If the Merlin 45 had the same compression ratio (I haven't cheked if it has), wouldn't that be a good indication it could run at the same boost level as the Merlin XII?
Regards,
Henning (HoHun)
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Missed this thread completely.
Don't forget the intro of the Merlin 50M with the cropped supercharger in 1943 that allowed for +18 boost.
Performance was much better up til about 12K when it fell off rapidly but it was done for the low alt capability to help it cope with the 190s better.
In August 42 the Spit Vs with Merlin 45s and 46s were approved for the +16 boost
As mentioned only +9 boost is mentioned in the Spit V manual but that is in reference to climb rates. Nothing on wep or 'pushing it through the gate" so to speak
AS for in AH, The Spit V is definately a better performer now. I prefer it to the IX.
I don't know how you determine which is the way to go in the MA as it's competing against latewar stuff.
If it was confined to fighting within the 1942 and before timeframe it would have to be the lesser performance but as it's essentially fighting in 1945 in the MA, I'm glad it's got more pep.
I only wish the IX had been tweaked to give it the latewar performance not stuck with the 1942 performance.
Dan/Slackl
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Originally posted by HoHun
Hi everyone,
I'm looking for something definite on the boost level (and rpm) specified for the early Spitfire V.
I'm convinced that it was +12 lbs/sqin @ 3000 rpm, but I can't prove it wasn't (as suggested by someone else) +9 lbs/sqin at either 3000 or 2850 rpm.
Hi HoHun
I checked the chronology of the Merlin engine for you and these are the quoted take-off ratings showing only the major changes in take-off boost values:
Merlin Mk II 3000rpm +6.25 psi boost
Merlin Mk X 3000rpm +10 psi boost
Merlin Mk XII 3000rpm +12 psi boost
Merlin Mk 22 3000rpm +14 psi boost
Merlin Mk 35 3000rpm +12 psi boost
Merlin Mk 47 3000rpm +12 psi boost
Merlin Mk 66 3000rpm +25 psi boost
Even the early versions of the Spitfire MkV had versions of the Merlin engine able to employ +12 psi boost for take-off. Every Spitfire MkV produced is listed, and the engine types are easy to confirm, I scanned the lists for you, and it appears that you are correct!
Badboy
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Hi Dan,
>Performance was much better up til about 12K when it fell off rapidly but it was done for the low alt capability to help it cope with the 190s better.
I agree, from the performance graph I'd say the Merlin 50M Spitfire was aimed right between the Fw 190's eyes ;-) It seems they weren't loved anyway - if I remember correctly, Clostermann describes his mixed feelings escorting Hurricanes in a "cropped" Spitfire. Of course, it could be that pilots just preferred "high and fast" over "low and fast" ;-)
>As mentioned only +9 boost is mentioned in the Spit V manual but that is in reference to climb rates. Nothing on wep or 'pushing it through the gate" so to speak
Hm. The manuals I know have all the amendments and thus describe the late-war status. I would be interesting to see the old status, but unfortunately they had no Wikis back then or it would be easy :-)
>AS for in AH, The Spit V is definately a better performer now. I prefer it to the IX.
Roger, it's often forgotten the Spitfire V was a great deal lighter than the IX! :-)
>I only wish the IX had been tweaked to give it the latewar performance not stuck with the 1942 performance.
For a late-war arena, late-war performance should be the way to go! (Even better: The server should tell the front-end dynamically what maximum boost to allow :-)
Regards,
Henning (HoHun)
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Hi Badboy,
>I checked the chronology of the Merlin engine for you and these are the quoted take-off ratings showing only the major changes in take-off boost values:
Thanks! :-) Is there a date to go along with the boost settings?
>Even the early versions of the Spitfire MkV had versions of the Merlin engine able to employ +12 psi boost for take-off. Every Spitfire MkV produced is listed, and the engine types are easy to confirm
Does that mean it is definite proof? I imagine engines of one version could be cleared for different boost levels, as evident in the upgrade to +16 lbs/sqin later.
My reasoning was that if the Merlin XII was good for +12 lbs/sqin and the Merlin 45 had the same bore, stroke and compression, we can be almost sure that it was good for +12 lbs/sqin, too. I don't have data detailed enough to check that, though.
Regards,
Henning (HoHun)
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I think the whole issue is rather hard to nail down. I have 3 books that give the Merlin 45 a +16 lb boost capacity...but no exact dates in some of them.
The 2 questions that I have not seen answered definitively are: #1 Was +12 lbs the Merlin 45s "max allowable" before 1942 or was it in fact +16? #2 Did pilots push it to +16 anyways in combat prior to 1942? I have seen nothing that clears that up.
Here is an interesting quote from Yellow Series "Supermarine Spitfire V" page 81 describing the Merlin 45 engine:
"Takeoff 3,000 rpm +12 lbs
Climb 2850 rpm +9 lbs
Cruise 2650 rpm +7 lbs
Combat 3000 rpm +16 lbs
In normal conditions a barometric control unit limited the maximum pressure according to the alt. In emergency this could be switched off to give maximum boost (for a short period only, to avoid engine failure). This was done by pressing a button on the throttle lever (the action known to pilots as pressing the tit). Standard boost lever position in combat was 12 lbs."
...This would seem to indicate +12 lbs was "standard" max combat power?, and +16 lbs was "emergency boost" for the Merlin 45. Other sources claim 3-5 min max for that power setting.
Finally I will say this, because it was "sanctioned officialy" (whatever that means) at +16 at some date, does not mean it was not capable, or not used, prior to that. I cant say for certain though. My understanding is that all the Merlin 45s ran on 100 octane, and required no modification to run at +16 lbs with the boost regulator shut off.
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Hi Everyone,
According to various sources (Jane's etc) there were no changes in internal physical dimensions of the Merlins at all, although I admit I'm not sure of the very late versions (>70 series).
They all shared the same dimensions:
Bore 5.40 inches
Stroke 6.00 inches
Capacity 1649 cu inches
Compression 6.0 to 1
All changes to the engine seem to only be concentrated on improving the supercharging or strengthening components/construction to allow it to survive greater supercharging.
There were probably other subtle changes but I'm interested in those that affect performance and output.
The Merlins were much more closely related than the DB 601/605 series of engines which changed Comp Ratios and Max RPM frequently as well as supercharger capacity.
I understand however that there were reasons why one series would be cleared for a boost while another was not. This was usually due to a detail improvement in the construction.
As far as I am aware the only difference between the XII and the 45 was the capacity of the supercharger and an improvement in its effeciency.
I know we are all after documented proof but my moneys on 12lb boost under combat conditions, up to the change to +16.
>In normal conditions a barometric control unit limited the maximum pressure according to the alt. In emergency this could be switched off to give maximum boost (for a short period only, to avoid engine failure). This was done by pressing a button on the throttle lever (the action known to pilots as pressing the tit). Standard boost lever position in combat was 12 lbs."
This is correct, however the boost override had to be 'set' to give a maximum boost pressure so that at low altitude the supercharger could not deliver a pressure greater than the authorised amount. There is a document many will have seen describing the details of the changes necessary to the boost control cut out on the Merlin III so that the max pressure could be increased from +6.25 lb, but not exceed the new limit of 12lb above atmosperic. It is also my impression that the British tended to do most things 'by the book'.
Regards
BPNZ