SpitfireV speed at +12lbs.

[mw]

VooDoo, I'm puzzled by your insistence that +12 lbs was used for takeoff only. See this page from the Spit V manual.


I have a hunch you've already seen this

There was little difference in SL powers between the Merlin XII and the Merlin 45.  SL speeds of the Spit II and V would therefore  be similar, given similar boost and configuration.  See this chart of a Spit V using +12 lbs/sq.in. boost.  As an interesting aside this chart, (together with its companion chart for climb) shows why the Spit III was dropped.  There was little difference in speed where it mattered, while climb of the III was inferior to that of a V with Merlin 45.

[VooDoo]

VooDoo, I'm puzzled by your insistence that +12 lbs was used for takeoff only.
Total misunderstanding. Im talking about speed and climb of Spits at +12 lbs. Im not talking about +12 lbs setting as only available for take off. Things are pretty simple: 1) I knew that Spits used +12 in combat (thx to Spitfire Performance Testing ). 2) I was unable to find any info about Spit performance at +12 boost. 3) I started this topic . Its only about +12 boost because data about over boost settings can be found at  the "Spitfire Performance Testing" . That is why I have little interest in it . Very straightforward topic . I think that it is better than start topic named "Why I was killed by Jug in the Spit" and get same info but at the 20-30th page of the totaly flamed conversation .

I have a hunch you've already seen this
Yes I have .

See this chart
Very Big Thanks. Now I have the answer . I cant see which line is SpitV but I can guess its the left one. 311 mph at surface level.

together with its companion chart for climb
May I hope that Ill see it as speed chart ?

There was little difference in speed where it mattered, while climb of the III was inferior to that of a V with Merlin 45.
Hmmmm... Just why ?

[mw]

My apologies VooDoo, I suppose I did misunderstand you.  Perhaps I took your writing too literally.   I'll try to shed more light on your question if I find more time than I have at present.   You are assuming the correct curve, ie. 311 mph @ SL.

[mw]

Comparing the powers of the early Merlins is interesting.  I hope this chart illuminates rather than confuses.



Sea level horsepower for the early Merlins at +12 lbs/sq.in. is as follows:

Merlin III
SL 1190

Merlin XII
SL 1170  (1175 at +12.5 for takeoff)

Merlin 45
SL 1185

Its easy to see that the SL speeds of the I, II and V would be very similar given similar configuration.  I’d actually expect the Spit I to be slightly the faster of the three at +12.  The Merlin XII and 45 had a supercharger gear ratio of 9.089 against the 8.588 of the III.  Its of interest to note that those Spit Is equipped with the DH CS prop were slightly faster than those with the Rotol.  A.&A.E.E. trials of Spit I R.6774 dated 30th July 1940 give speeds of 288 at SL and 355 at 17,800, using +6.25, with the DH CS prop,  while N.3171 with the Rotol achieved 282 at SL and 354 at 18,900.  Those Spit I, II, and V with the cannon would also be a tad slower than those with 8 .303s.  A source of significant drag on these early Spits was the externerally mounted bullet resistant windscreen.   Pre Bob Spits averaged in the mid 360 s for top speed prior to the changes that were necessary for battle.

[VO101_Isegrim]

Mike, you should note on the chart that these engine powers are for Dynamic output, ie. they include the ramming effect of the airplane flying at full speed, effectively increasing power output above the static FTH.

Static powers are appx. the same, but are achieved 2-3000 ft lower. Which means to get a comparable static power output, one has to slide the power curve to the left by 2-3000 ft.

[VooDoo]

Thanks for the charts. Such a large nomenclature of the engines and so little difference between them. Were they produced on the same plant ? And, Ive one more question - Merlin45M and 50M - what was the purpose ot this engines (mean planes which have it) ? Was it "immediate response" to FW190 threat ? Any paralles with soviet engine building (almost all watercooled engines were low-level ones) ?

[VooDoo]

engine powers are for Dynamic output
And its OK. Only figures with ram effect make sense. Im talking about flying planes not sitting in a labaratory .

[gwshaw]

The 45M and 50M meant they were the low altitude engines. Impellor diameter was clipped from 10.25 in to 9.5. Lowered the pressure ratio, lowering rated altitude dramatically. But it also decreased the power loss to run the blower, so the engine put out more power at the same MAP. But it couldn't sustain high MAP very high. I believe the difference between the 45 & 50 was the use of a pressure carb in the 50. The 45, 46 and 47 all had different gear ratios for the blower, changing the full throttle altitude and peak powers.

The problem with rammed figures is nobody else used them and it makes an apples-apples comparison difficult. You don't always know what the speed was, never know what the ram recovery % is, which makes it tough to work back to get static performance for comparisons. A lot of the difference between single stage Allisons and Merlins isn't that the Merlin was better, but that 350 mph ram is artificially increasing performance compared to the US engine which was rated without ram.

Give a Allison V-1710-33 (C15) 350 mph ram and a decent recovery % and mil ratings should be right in the same 16,000-16,500 ft and 1050-1100 hp range as the Merlin II/III.

Same thing with the later V-1710-81 (F20) in the P-40N/P-51A compared to the Merlin 4x series in the Spitfire V.

Greg Shaw

[gripen]

It's a bit difficult to say if those powers are (all) rammed. As HoHun noted N.3171 (Merlin III) had FTH 18900ft in speed trials (+6,25 lbs) and mw's chart gives about 16k (+6,25 lbs).  Spit IIA P.7280 (Merlin XII) had FTH 17550ft in the speed trials (+8,8 lbs so +9 would have been a bit above 17k) and mw*s chart gives a bit more than 14k (+9 lbs). Spitfire V W.3134 (Merlin 45) had FTH 20100ft in speed trials (18800 with snow guard) (+9 lbs) and mw's chart gives around 18k at same boost. Spitfire V AA.878 had FTH 13000ft at +16 lbs and 19900ft at +9 lbs in speed trials, mw's chart gives about 11k for +16 lbs. In climb test AA.878 had FTH just 8800ft at +16 lbs 3000rpm (AA.878 values are unlogical because in climb test +9 lbs 2850rpm it had fth 14900ft) so mw's +16 lbs values seem to contain RAM. So if there is RAM, it's seem to be something else than max speed effect.

Anyway, Daimler Benz used to give engine powers with about climb speed RAM effect. From DB 605A manual:

"Die Höhenleistungen sind abgestellt auf den Gesamtdruck (statisch + dynamisch) und ergeben sich ohne Berücksichtigung der Rückstoßenergie"

gripen

[VO101_Isegrim]

Originally posted by VooDoo
engine powers are for Dynamic output

And its OK. Only figures with ram effect make sense. Im talking about flying planes not sitting in a labaratory .

Yep, and it`s all OK as long as you want to use the data for aerodynamic calculations only and not for comparisons. Most other countries engine output data seem to give static or near-static power output vs. altitude, and as soon as you want to compare that to the rammed power outputs of Merlins above, you will get a false impression about their relative high altitude performance.. most if not all "official" British power curves I have seen refer to dynamic output, either at FT or at 400mph, which latter was not achievable by most planes at low levels (except in dive). That`s why I think the fact that these refer to rammed powers should be noted, surely in the case of the Merlin III paper from which MW took the values it is noted : "Curves at 3000 RPM and max boost in level flight".

[gripen]

Well, then I quess the Spitfire had really excellent inlet design because in every case in the flight tested critical altitude seems to be at least 2k higher than given in the power charts. Another possibility is that flying speed used for power curves is very low.

gripen

[gwshaw]

I'm undecided what exactly the Merlin II/III performance really was.

1) I think I have a chart around somewhere stating that the Merlin II/III performance is with 300 or 350 mph ram, probably 300 mph

2) Quick back of the envelope calculations show its performance as way out of line with the Allison's.
(figures in () are comparisons to C15 baseline)

Allison V-1710-33 (C15, P-40B/C)
blower tip speed - 1091 fps
pressure ratio - 2.15:1

Allison V-1710-39 (F3R, P-40D/E, P-51)
blower tip speed - 1094 fps (1.002)
pressure ratio - 2.20:1

Merlin II/III
blower tip speed - 1151 fps (1.05)
pressure ratio (+6.25 psi) - 2.65:1 (1.24)
pressure ratio (+16 psi) - 2.55:1 (1.19)

A 5% increase in blower tip speed should give somewhere around 10% (1.05 ^2 = 1.1025) increase in energy. Pressure ratio should scale pretty closely with energy. So given similar blower efficiency % the Merlin II/III should have a pressure ratio of about 2.35:1 to 2.45:1, not 2.65:1. Given that apparently Allison had higher blower % than the pre XX Merlins call it about 2.4:1.

Using the 2.4:1 pressure ratio gives 42.6 in Hg (+6.25 psi) and about 1025 hp up to approx 13,800 ft unrammed. That compares pretty closely to the C15 doing 1090 hp @ 13,700 ft on 38.7 in Hg and the F3R doing 1150 hp @ 12,000 ft on 42 in Hg.

I know the Merlin had a larger carburetor inlet area, that will help some. But I can't see it being good for 2500 ft difference vs the Allisons.

Another problem is that all Merlin II/III are rated the same, regardless of whether it is a 360 mph Spitfire I, a 330 mph Hurricane I, a 300 mph Defiant or a 250 mph Fairey Battle. I have a hard time believing they all had the same ram recovery %.

My guess is that the engine was rated at about 300 mph ram with some fixed ram recovery %. The Spitfire being roughly 20% faster than that should get considerably better FTH, 18,900 ft for N.3171. The Hurricane I figure I have is 330 mph @ 17,500 ft FTH. For the Defiant I the figure I found is 304 mph @ 17,000 ft, that could easily be explained by better ram recovery % for the Defiant than the RR baseline.

Greg Shaw

[VooDoo]

Yep, and it`s all OK as long as you want to use the data for aerodynamic calculations only and not for comparisons.
No. Not a Spitfire vs 109 flame again . Let this discussion to be more productive . F.e. In the "109 kill ratio" topic you've posted:

"I also found this in my archieves,

Acceleration, as calculated by Greg Shaw. At SL, at Full Throttle, from 250mph TAS:

in feet/seconds


109 K-4 : 6.85
La-7 : 6.58
190 D-9 : 6.05
109 G-14: 5.59
Spit XIV: 5.55
109 G-10: 5.50
Yak-9U : 5.27
190 A-8 : 4.97
Yak-3 : 4.80
P-38L-5 : 4.75
109 G-2 : 4.62
P-47 M-5: 4.49
SpitIXLF: 4.41
109 G-6 : 4.22
P-38J-25: 4.17
F-6F : 4.09
F-4U1D : 4.08
P-47D-25: 3.79
P-51D-25: 3.34

I checked the Mustang acceleration data with America`s Hundred thousend, and it`s matches very well."

I remember that topic on the AllAboutWarfare even saved it but I lost my HDD and all info on it . So maybe you can post all info from that discussion here or via email ? I want to check calculations. And I always wanted to know something about German tests of the linear acceleration. Absolutely no info about it. And how it compares with russian or american test ?

[lasersailor184]

So, say this would be put in.  What would it do to gameplay?

How would it affect Spitfires vs. Other planes?

[mw]

VooDoo:  what the Merlin 45 gave the Spit V over the Spit I was about 20 additional mph speed above 16k when both are running at +12 boost.  In the real world that came in handy   I think you'd find those specialized Merlin  low alt engines like the 45m and 50M to be quite fun arena planes.  See here