Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: VooDoo on February 10, 2004, 11:39:52 PM
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In the pilot manuals of SpitV and SpitII this power setting described as "take-off", but SpitfireI pilots in BoB used it as combat. May be somebody have any info about max speed/climb figures on this power setting SpitfireV or SpitfireII ?
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::elbows Slack::
:D just kidding, but seriously, he can tell you whatever you want to know.
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Didnt get it. My question is too complicated ? The answer is too secret ? Everebody know that SpitV has xxx mph at +12lbs ? Nobody understand that Im talking about ?
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There were no Spit V's in the BoB. Spit II's,however.
I should have these figures somewhere,- will look them up for you.:)
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"but SpitfireI pilots in BoB used it as combat" - clear ;) ?
will look them up for you
Thank you :).
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Your information about the Spit II is incorrect. See
this page (http://www.fourthfightergroup.com/eagles/spit2pn.jpg) from the Spit II Pilot's Notes. +12 was a 5 minute combat rating. Spit V's were only limited to +12 takeoff for a short time, and were eventually cleared for +18 combat. Top speeds/climb varies among the data sets and is dependant primarily on aircraft configuration and fit and finish.
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Originally posted by mw
Your information about the Spit II is incorrect. See
this page (http://www.fourthfightergroup.com/eagles/spit2pn.jpg) from the Spit II Pilot's Notes. +12 was a 5 minute combat rating. Spit V's were only limited to +12 takeoff for a short time, and were eventually cleared for +18 combat. Top speeds/climb varies among the data sets and is dependant primarily on aircraft configuration and fit and finish.
No, my information IS correct. Its just not the only information exist :).
(http://www.sunday.ru/f101/spit2.jpg)
The main question is about speed and climb of SpitV and SpitII at +12 boost. Not how this boost was named ;). But any info about clearing of a SpitV for using boost higher than +12 are always appreciated. If you know exact date - share it ;). Im interested mainly in 1941 produced and used aircrafts (SpitV) and mid-late 1940 (SpitII).
Conditions of the tested aircrafts are third question. Im interested in standart production aircrafts (VB and VC) with standart engines (Merlin 45) and standart armament (2x20+4x7).
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As for Spitfire Performance Testing site - Ive digged it through already ;).
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August 1942 Air Tactics department Air Ministry issued a document for Spitfire pilots regarding survivability with the 190s around
Part of it mentions:
"Spitfires are now modified to give +16 emergency boost. It must be impressed upon pilots that this gives a great increase in speed under 21,500 feet and 18,250 feet for Merlin 46 and 45 respectively, and if used for combat only, there is no risk of engine failure"
another quote from it, "You can fly +16 boost and 3000 RPM without any danger of the engine blowing up, BUT, your consumption will be 150 gallons an hour."
Spit V Manual I have says +9 boost and 2850 RPM gave the maximum rate of climb. Nothing about +12 boost though.
No specific speeds that I can find though, So I don't think this really answers your question :(
Dan/Slack
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Hi Voodoo,
>But any info about clearing of a SpitV for using boost higher than +12 are always appreciated.
That first step was an increase to +16 lbs/sqin that came along with the introduction of the Fw 190A.
According to Alfred Price' "A Spitfire - The Complete Fighting History", the Air Ministry issued a memorandum in August 1942 stating "Spitfires are now being modified to give +16 emergency boost." I'd say that dates it pretty well.
With regard to the Spitfire speed at +12 lbs/sq in, the easiest way to get an answer is to estimate it graphically.
Plot N.3171 speed from
http://www.fourthfightergroup.com/eagles/spit1.html
on graph paper. Then have a look at the full curve above full throttle height (18900 ft) and draw an extension in a smooth shape to 10900 ft.
+6.25 lbs/sqin has a full throttle height of 18900 ft, and +12 lbs/sqin is 8000 ft below that according to the Merlin III power chart here:
http://www.fourthfightergroup.com/eagles/merlin3curve.jpg
Note that absolute heights differ due to the ram effect changing the pressure.
Next, draw a line parallel to the straight line denoting +6.25 lbs/sqin low altitude speed from the freshly extended +12 lbs/10900 ft line down to sea level.
That's a quick, simple and reasonably accurate method of estimating a speed change resulting from a boost pressure change :-)
From just staring at the numbers, I'd say this will give you something like 362 mph @ 10900 ft and 311 mph @ sea level.
Regards,
Henning (HoHun)
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Not to sound too newbish, but what kind of acceleration difference are we talking at 12 - 16 lb's?
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Not to sound too newbish, but what the hell is everyone arguing about?
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Hello HoHun !
(http://www.fourthfightergroup.com/eagles/spit1speed4.jpg)
It looks pretty similar to that you are talking about. Does it means that SpitV speed will be just 10 mph faster than SpitII ? 321 mph at sea level at +12 boost ?
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Originally posted by Guppy35
Spit V Manual I have says +9 boost and 2850 RPM gave the maximum rate of climb. Nothing about +12 boost though.
Dan/Slack
Yes it says :). But Im pretty sure that climb at +16 boost will be a way faster than at +9 and 2850 RPM. So as at +12. The only problem - it will not work for an hour ;). +12 lbs and 3000 RPM is a MAX TAKE-OFF as said in PM.
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Recommended to the max of 5 or 10 minutes, overboosting never the less was sometimes done for much longer, sometimes 30 minutes without ruining the engine. In such cases the engines were usually overhauled.
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VooDoo, I'm puzzled by your insistence that +12 lbs was used for takeoff only. See this page from the Spit V manual.
(http://www.fourthfightergroup.com/eagles/spit5pn.jpg)
I have a hunch you've already seen this :confused:
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 (http://www.fourthfightergroup.com/eagles/merlin45.jpg) 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.
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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 ?
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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.
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Comparing the powers of the early Merlins is interesting. I hope this chart illuminates rather than confuses.
(http://www.fourthfightergroup.com/eagles/merlinengcurves.jpg)
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.
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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.
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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) ?
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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 ;).
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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
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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
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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".
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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
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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
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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 :D. 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 ?
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So, say this would be put in. What would it do to gameplay?
How would it affect Spitfires vs. Other planes?
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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 (http://www.fourthfightergroup.com/eagles/w3228.html) :)
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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.
So the main purpose of the SpitV+Merlin45 was to enchance performance at heights above 10.000 feets.
In the real world that came in handy
Against what ?
I think you'd find those specialized Merlin low alt engines like the 45m and 50M to be quite fun arena planes. See here
Was there but dont know production numbers of these engines. Were them widely used ? And where ? In England ? FW190 interceptors ? Or in Med ? Or just both of it :) ?
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Originally posted by 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
Keep in mind that there were many extarnal modifications, ie. the Spit I you mentioned was tested w/o the additional armor plates, windscreen, which came with a 9mph speed decrease (IIRC), and as such a reduction in rammed FTH. I don`t know if the Spit had any special inlet design, to me it looks like a simply hole on the fusalage, but I would hardly think it would have anything to do with it. Anyway, if you have any info on the details of the "really excellent inlet design", please share.
Looking on the Bf 109 G-2 and G-6, for example, the additional drag of the G-6 slowed it down by -25 km/h at FTH compared to the G-2, and interacted with the decrease of FTH of 400m (1300 ft). You may notice the exactly similiar decrease in case of MkV W.3134, with and w/o snowguards fitted (370.5mph/20.1k and 365/18.8k). The loss of FTH decreases power available at altitude, which decreases speed, which again decreases the power and so on until it balances out.
So, IMHO it`s simple a case that the Merlin III (and possibly the others) curves were done using later Spits with more extra drag from new equipment, such as filters, armored windscreen etc, and comparing their FTH with the earlier, cleaner ones makes little sense. Looking at MW`s rammed power chart for Merlin III, the FTH is at around 17.5k-18k ft or so. This agrees very well with the measured FTH of Spit I (Merlin III) of N.3171, at 18.9k ft. I dont know what equipment the plane exactly had, the difference is not great, and easily explainable by extra drag from newer equipment or difference with indidvidual planes. As an example for the latter, the well known JL 165, the same plane in different test was found the have a FTH of 14 400 and 13 800. Same plane, just different tests!
In any case, the point is that MW`s chart shows dynamic powers in max level speed flight, and this should be taken into account when compared to, say, a static chart. Some might come to false conclusions about high altitude performance while being busy comparing apples and oranges. ;)
Voodoo : The acceleation figures were calculated by greg shaw - he posted in this thread, ask him about the details of the calcs - , based on available max SL speeds at given powers and weight. Thus they are based only indirectly on German testings, but should reflect the relative merits very well. After all, it`s not hard to see that 2000 HP will accelerate a 3362 kg plane better than 1840 HP at 3859 kg heavy one, especially if the latter needs more power to obtain the same speed.
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a kinda newbish question, why does the performence curve become the same for the different boost settings in a certien altitude? (merlin III about 16k, merlin XII 14k, merlin45 17k)
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The acceleation figures were calculated by greg shaw - he posted in this thread, ask him about the details of the calcs
OK. Gwshaw = Greg Show ? If so, Greg, could you repost calculation here ? Or just input data ?
After all, it`s not hard to see that 2000 HP will accelerate a 3362 kg plane better than 1840 HP at 3859 kg heavy one, especially if the latter needs more power to obtain the same speed.
Who is 1840 hp :) ? Spit14 :) ? Oh no... Not again... As for calculations - 5-10% in the propeller efficiency makes big difference. And 2000/1840 - its nonrammed figures and wo exhaust thrust - so it make almost no sense for calculating real in flight acceleration. That is why Im looking for the real test data - calculations are not so good ;).
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I'll see if I can dig through stuff and find the original posting from the Air War board. Unfortunately I don't keep very good archives of my own posts.
I still have the spreadsheet to do all the performance comparisons and could do them over again, if I find the time. Probably more accurately this time, I have better power data for some aircraft. Russian aircraft will probably be slightly worse, the Spitfires slightly better, most US and Germans stay about the same. The Fw 190A-8 will be lower, and the F4U and F6F will be significantly better.
I did the calculations at 250 mph @ SL to eliminate ram as a major concern. None of the planes should need ram to achieve full MAP, and all will lose power proportionally due to ram heating of the intake air. The Spitfires were figured using ram power figures, take ram out and it should gain 50+ hp and accelerate slightly better.
Exhaust thrust isn't much help at 250 mph, another reason why I did the calculations at that speed, although I may start at 200 mph next time. HP is 50% more effective than exhaust thrust at that speed, the same old thrust (lbs) = 375/250 * hp. Turn that around and 100 lbs exhaust thrust is only equal to 66 hp.
So a 1800 hp engine with 150 lbs of exhaust thrust is getting the following:
2700 lbs thrust from engine * .8 (propellor efficiency) = 2160 lbs thrust
150 lbs thrust from exhaust, about 7% propellor thrust.
That will certainly help, but since I'm not taking it into account on any of them the effect should be fairly neutral.
I'm using 80% prop efficiency, most should be right in that range at 250 mph.
The figures aren't absolutes, but should be pretty close relative to each other.
Greg Shaw
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Originally posted by gwshaw
None of the planes should need ram to achieve full MAP, and all will lose power proportionally due to ram heating of the intake air.
Greg Shaw
Just to take the discussion on a sidetrack, I wonder if anybody has a solid answer for why planes with fixed gear ratio loose power below FTH as speed/ram increases ? My guess is the air is already well compressed by the time it reaches the air intake, and since the supercharger just tries to keep it`s speed constant, it find it harder to further compress already dense air. In the process it does extra work, which goes into waste (I would guess they get rid of the extra pressure with a wastegate or something to avoid overboosting). Any thoughts?
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Power decrease below FTH is due to the temperature increase at lower altitudes.
sgrt(abs temp fth / abs temp new altitude) * hp @ FTH.
Example:
1500 hp @ 5000 ft
abs temp @ 5000 ft - 278.46
SL abst temp - 288.36
sqrt (278.46/288.36)
sqrt (.9656) = .9826
1500 x .9826 = 1475 hp @ SL at same manifold pressure.
Ram does the same thing, unfortunately I haven't figured out how to get the temp increase from ram to make the adjustment.
Greg Shaw
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thanks for your answer greg.
theres only 1 problem, i have no idea what does all the initials mean (FTH, sgrt, abs, fth, abst, SL, sqrt, Ram ) :o
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I still have the spreadsheet to do all the performance comparisons and could do them over again, if I find the time. Probably more accurately this time, I have better power data for some aircraft. Russian aircraft will probably be slightly worse, the Spitfires slightly better, most US and Germans stay about the same. The Fw 190A-8 will be lower, and the F4U and F6F will be significantly better.
Im looking forward... Changes looks same as I thought. But Ive found some data on P-51: 3.85 ft/s2 WEP S/L from 250 mph, and 3.23 ft/s2 MIL 10-15K feet from 250 mph.
although I may start at 200 mph next time
No need. 250mph is close to 0,7Vmax and so its comparable to russian tests.
And one question about meaning of the calculated acceleration. Is it "instant acceleration" at 250mph speed or average for 250mph-Vmax ? Average for 0-250mph ?
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Originally posted by gwshaw
Power decrease below FTH is due to the temperature increase at lower altitudes.
Ram does the same thing, unfortunately I haven't figured out how to get the temp increase from ram to make the adjustment.
In addition to the temperature difference, there is also the greater pressure difference between intake and exhaust.
The formula to calculate the temp increase from ram is simply (V/100)^2. The input velocity is in MPH and the output is in degrees C or K.
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Thanks Pyro,
There is increased backpressure at lower altitudes that will need more power to push the exhaust out. Didn't think about that.
I'll put that formula in my list and start trying to compute ram power values for some engines.
Just to make sure I'm following your right.
For 250 mph TAS
(250/100) ^ 2
2.5 ^2 = 6.25 C
So @ SL that would raise temp to 288.36 + 6.25 = 294.61
sqrt(288.36/294.61) = 98.93 * 1475 = 1460 hp rammed.
Greg Shaw
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Hi Flyboy,
>a kinda newbish question, why does the performence curve become the same for the different boost settings in a certien altitude?
Good question, not at all newbish :-)
The reason is that above the so-called "full throttle height" ("FTH", also called "critical altitude"), the supercharger can't generate the desired boost.
At sea level, the Merlin 45's supercharger can generate about +26 lbs/sqin boost pressure. That's much more than the engine can handle, so it has to be throttled back.
At 12000 ft, the Merlin 45 generates +16 lbs/sqin, exactly what the engine can handle. We can open the throttle fully - thus we call it "full throttle height".
Above 12000 ft, the boost provided by the supercharger drops off because the ambient air gets thinner and thinner. At 14000 ft, it's down to +12 lbs/sqin. At 18000 ft, it's down to +9 lbs/sqin. The higher you get the further it drops.
As low-altitude power depends on the strength of the engine, you can get more power from the same engine by making its components stronger. (There's a limit of course - internal combustion requires certain pressures and temperatures to work.)
However, above full throttle height, it doesn't matter whether your engine has a robust crankshaft, strong rods and pistons that don't melt at high temperature. Its power is inevitably limited by the amount of oxygen in the air provided by its supercharger.
To get more power there, you need to do something about your supercharger. You can either spin it faster by increasing engine rpm or by installing a two-speed gearbox, or you can mount a bigger supercharger to feed more air to the engine at high altitude.
If you leave the supercharger as it is, the high-altitude part of the power curve won't change at all even if you improve your engine so that it can run at higher boost pressures. That's why the performance curves coincede for the different boost settings coincede in the graph above.
Regards,
Henning (HoHun)
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Originally posted by gwshaw
There is increased backpressure at lower altitudes that will need more power to push the exhaust out. Didn't think about that.
I imagine that accounts for some power loss too, but what I was really getting at is that your charge weight goes up as the pressure differential between your intake and exhaust increases. The lower the pressure of the residual exhaust gas, the more charge is drawn into the cylinder provided that the intake pressure remains constant.
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Originally posted by VooDoo
Im looking forward... Changes looks same as I thought. But Ive found some data on P-51: 3.85 ft/s2 WEP S/L from 250 mph, and 3.23 ft/s2 MIL 10-15K feet from 250 mph.
Sounds about right, I don't know weight that is at, and don't remember what weight I was using for my figures.
although I may start at 200 mph next time
No need. 250mph is close to 0,7Vmax and so its comparable to russian tests.
And one question about meaning of the calculated acceleration. Is it "instant acceleration" at 250mph speed or average for 250mph-Vmax ? Average for 0-250mph ?
It is instant acceleration, ie fps. So if you are going 250 mph (250 * 1.47 = 367.5 fps), and accelerate at say 5 fps. After 1 second you would be up to 372.5 fps (253.4 mph). Then you can recalc again using the new speed, which will change the hp - thrust relationship, and both drag values. So the next second will have a slightly different acceleration value. You can continue that up until you reach vMax, the finer the time granulation the more accurate it will be.
Greg Shaw
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gwshaw,
After looking through quite many sources I still can't find any support for your claim that the RR rated engines with some sort of fixed RAM effect or something. Actually my impression is that ratings by RR are without RAM.
As an example the Merlin 45 (XLV) in the mw's chart does about 1200hp at 16000ft with +9lbs boost (3000rpm I assume). And in the real world speed tests rammed FTH were around 18800-20800ft depending on conditions (the lowest value is with snowguard), in the climb tests the AA.878 did still about 17000ft at about 200mph TAS with 3000rpm +9lbs.
As for comparison the V-1710-81 (according to "Vee's for Victory) did 1200hp with 47" MAP and 3000rpm at about 14000ft without RAM (chart p. 271) and in the real world speed test in the P-51A (p. 157) could keep this MAP up to about a bit over 15000ft at around 390mph TAS.
gripen
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Sounds about right, I don't know weight that is at, and don't remember what weight I was using for my figures.
That is why I want to look at them ;). So Im waiting till you'll have enough time...
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thanks alot henning :) really cleared the issue for me
btw, is the supercharger a part of the engine or a completely different part?
for example, can i load varios kinds of superchargers to the same engine\ plane ?
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A description of the Merlin
http://www.unlimitedexcitement.com/Pride%20of%20Pay%20n%20Pak/Rolls-Royce%20Merlin%20V-1650%20Engine.htm
and some pics
http://www.cjbalm.com/auto-aero/aitem60.htm
The opposite end from the prop shaft is the supercharger case with the carb below.
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An interesting chart (Source: Mercedes-Benz AG, Archives, Stuttgart, Germany) on the DB605 engines
http://w1.1861.telia.com/~u186104874/db605.htm#appraisal
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Speaking of acceleration and power, does not wing loading (and therefore A of A) influence initial acceleration from the stall towards a certain level?
That is, low wingloading will sooner reduce the aircrafts angle of attack, but as lift also creates induced drag, at higher speeds the lift becomes a negative factor, since it is creating lift that no longer is necessary?
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Originally posted by Angus
Speaking of acceleration and power, does not wing loading (and therefore A of A) influence initial acceleration from the stall towards a certain level?
That is, low wingloading will sooner reduce the aircrafts angle of attack, but as lift also creates induced drag, at higher speeds the lift becomes a negative factor, since it is creating lift that no longer is necessary?
Yes, wing loading and aspect ratio both effect the drag curve, and indirectly effect acceleration.
The drag curve is kind of U or V shaped. At low speeds there is a lot of induced drag due to the wing being at a high AOA. It falls off pretty quickly with speed, and the lower the wing loading and higher the aspect ratio the quicker it falls off.That causes the high left side of the V. After the lift curves cross the induced drag quickly becomes a very minimal part of the total drag.
Profile or pressure drag (I can never remember the correct terminology) increases with the square of the speed. So at low speeds it is minimal, but at high speeds it makes up probably 90%+ of total drag. That is the high right hand side of the V.
If you graph both curves they will cross at some speed, typically 150-180 mph IAS for WW II fighters. That speed is the optimum speed for that aircraft. That is theoretically where the best climb and best cruise would be, although propellor efficiency, control/stability and cooling limitations often require higher climb speeds.
Hope that helped answer your question.
Greg Shaw
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Lovely! Excellent!
So, initial acceleration is in favour by low wingloaded aircraft, but the true speedmonsters are aircraft with higher wingloading, or more correctly, less lift, which corresponds nicely with top speeds pr. hp of ww2 fighters. More or less I guess.....
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Hi Flyboy,
>btw, is the supercharger a part of the engine or a completely different part?
>for example, can i load varios kinds of superchargers to the same engine\ plane ?
The supercharger is mounted neatly to the engine.
(At the back in the case of the Merlin, on the left side in the case of the DB601/605. That's why the Merlin could easily accomodate a large two-stage supercharger, but never an engine cannon. It's also the reason why the Messerschmitts all have the supercharger air intake asymmetrically on the left side.)
As the supercharger's performance has to be closely matched to the engine's characteristics, there's usually only one supercharger to be used for each engine variant, and if you want a different supercharger, you have to create a new engine variant. (In fact, the main difference between engine variants often is the supercharger :-)
Regards,
Henning (HoHun)
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Hi Angus,
>So, initial acceleration is in favour by low wingloaded aircraft, but the true speedmonsters are aircraft with higher wingloading, or more correctly, less lift, which corresponds nicely with top speeds pr. hp of ww2 fighters. More or less I guess.....
Actually, acceleration is mathematically tied to climb rate.
The aircraft with the superior climb rate at a certain speed will out-accelerate the one with the inferior climb rate at the same speed.
Of course, climb rates are given at best climb speed, which varies from plane to plane, so using published climb rate figures is not perfect for a comparison.
It also gives only one data point for comparison. At high speed, the aircraft that accelerated worse at low speed might actually accelerate away from its rival.
So at sea level and starting at best climb speed, a Spitfire XIV at +18 lbs/sqin and 8500 lbs with a climb rate of 23.9 m/s will probably out-accelerate a Me 109K-4 at 1.8 ata and 3400 kg by a very small margin because the latter only climbs at 23.3 m/s.
Assuming the Me 109K-4 has a higher top speed at sea level for the sake of the example, there'll be a speed between best climb speed and top speed where both aircraft accelerate equally well, and above that speed, the Me 109K-4 will out-accelerate the Spitfire XIV.
(Of course, at high speed acceleration is much less rapid anyway, and so the difference will be much smaller in absolute terms, too.)
Regards,
Henning (HoHun)
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Originally posted by Flyboy
thanks alot henning :) really cleared the issue for me
btw, is the supercharger a part of the engine or a completely different part?
[/b]
Yes, it is either a part of the engine or a separate part, depending on the engine.
In the Merlin it was more or less bolted on to the back of the engine, which made it easier for Rolls-Royce to change it around. The two-stage engines just stuck another stage on back to back with the original with a small intercooler between them.
On the Allison it was more integrated with the engine, and when they went to two stage setups it was an aux blower completely separate from the main engine.
The DB and Junkers inlines stuck them on the sides, and like the RR setup they could be changed around without effecting the main engine assembly.
for example, can i load varios kinds of superchargers to the same engine\ plane ?
[/b]
To some extent. Most started off with a single-speed/single-stage blower, went to a single-stage/two-speed blower for better altitude performance without harming low altitude performance. Then went to two-stage/two-speed or variable speed setups to further improve altitude performance.
The DB engines had a single-stage/variable speed blower that was probably better than any other single-stage engine. Later models with the enlarged compressor were pretty competitive with the two-stage Merlins and Allisons. Mostly because the German engines operated at much lower manifold pressures and didn't need to compress the charge air as much to make power. So they didn't need the intercooling and aftercooling systems the Allied engines required.
Greg Shaw
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HoHun:
"It also gives only one data point for comparison. At high speed, the aircraft that accelerated worse at low speed might actually accelerate away from its rival. "
Exactly then! ?
Take a Spit I and a 109E (given the blessed same torque/(power) to a race where they open up side-by-side at 80 knots.
The 109 will be flying at a higher angle of attack, slots even deployed.
They open up, Spit will gain the lead. It will keep so for quite a bit, then eventually at some certain high speed the 109 will be faster. They might be level at 300 mph or so?
Yes?
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Hi Angus,
>They open up, Spit will gain the lead. It will keep so for quite a bit, then eventually at some certain high speed the 109 will be faster. They might be level at 300 mph or so?
Staying with the Spitfire XIV/Me 109K-4 example, that's about what would happen.
In a drag race, the plane with the better low-speed acceleration would soon get a lead with regard to speed, so that you're actually comparing two aircraft at different speeds most of the time.
That makes your example a bit tricky :-)
Regards,
Henning (HoHun)
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Greg, can you calculate approx. speed at given (variable) powers at altitude, if the several historical speeds and their associated powers are given ?
I wonder this because I want to make range diagram where cruise speed is presented vs. fuel consumption and thus would give a range@speed diagram, in which the best cruise speed with the largest range/endurance can be found. Quite complex, but would be interesting, I think my simple empirical formula is not accurate enough for that task (10 km/h error would give already 20-50 km range error or more for planes with large fuel capacity).
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gwshaw,
As for additional information, "Vee's for Victory" by D. Whitney is a pretty good source and highly recommened for anyone interested on the V-1710. Following part (p. 325) explains pretty well why the single stage Merlins were superior altitude engines if compared to the single stage V-1710:
"Reviewing Table 15-3 , it is not immediately clear why the single stage Merlin could develop both more power and more manifild pressure, and achieve higher critical altitudes than the V-1710. The combinations of rotor diameter and supercharger drive step-up ratios gave almost identical tip speeds to boyhy engines, meaning that the supercharger pressure ratios should be quite similar. Yet there was a difference in the outlet or resulting manifold pressure produced by the superchargers. This means that it must have been the pressure at the supercharger inlet, just downstream from the wide open throttle and ahead of the supercharger impeller, where the differences were occuring. An investigation of the differences in the flow areas at the single-stage supercharger inlets shows that the Allison area was about 25 percent less than that of the Merlin. When flowing the quantity of air needed to match the Merlin XX, the Allison would have had over 1 inHg of extra pressure loss, not including any losses upstream of the throttle."
In addition Whitney quotes a Wright field report which confirms his conclusions. This also explains why Spitfire Vs with +16 and +18 lbs boost had unlogical FTHs on climb; inlet size of the Spitfire V was too small (not designed such MAPs) therefore at low speeds these did not reach unrammed FTH (the Spitfire IX results are logical in this respect).
gripen
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Hi Gripen,
>This also explains why Spitfire Vs with +16 and +18 lbs boost had unlogical FTHs on climb; inlet size of the Spitfire V was too small (not designed such MAPs) therefore at low speeds these did not reach unrammed FTH (the Spitfire IX results are logical in this respect).
Hm, I'm not sure I understand that. Do you mean the Spitfire V didn't reach rated power at full throttle height?
Regards,
Henning (HoHun)
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Gripen,
I have read the same thing, and I did mention the larger carb on the Merlin. But I don't see that being responsible for 4250 ft or 4550 ft of additional alitude vs the F3R.
Another data point on the Merlin II/III: the Fairey Battle had a full throttle height of 13,000 ft @ 241 mph or 15,000 ft @ 250 mph depending on what source I look at. That is 1250 or 3250 ft below the rated altitude for the engine.
Since you have "Vees", take a look at the Merlin/Allison comparisons on page 322 and 329.
Compare the Merlin XX and the V-1650-1 on page 322. Yes, the 1650-1 had a smaller carb, but the Merlin XX figures are considerably more MAP, at a much higher altitude, but lower power.
Same blower setup, but note the different figures:
Merlin XX low - 1260 hp @ 12,250 ft & 58.6 in Hg
V-1650-1 low - 1385 hp @ 8000 ft & 54.3 in Hg
Merlin XX high - 1175 hp @ 21,000 ft & 62.6 in Hg
V-1650-1 high - 1250 hp @ 15,600 ft & 54.3 in Hg
The same manifold pressure will always put out more power the higher the altitude due to the lower temps. Higher MAP & higher altitude should be putting out much HIGHER power than the Packard, not lower.
Something on the order 1520 hp @ 12,250 ft in low blower after taking both temperature and increased MAP into account. Hi blower should be somewhere around 1470 hp @ 21,000 ft, not 1175.
Packard figures are flaky, note the next page gives values of 1235 hp @ 12,000 ft and 1130 hp @ 19,000. Unfortunately it doesn't give MAP, it just says mil power.
Next, take a look at page 329 and compare the Merlin 61 and the V-1650-3.
First off, the Merlin 61 figures look too high for +15 psi (60.5 in Hg) boost. They look more like +18 psi (66.6 in Hg).
Despite having a smaller blower diameter, 11.5 in vs 12.0 in, the Merlin 61 has a slightly higher low blower rated altitude than the V-1650-3, 12,000 ft vs 11,800 ft. That may just be the difference between 60.5 in Hg and 61 in Hg, I haven't crunched the numbers. But again, it puts out 40 less hp at 12,000 ft than the V-1650-3 puts out at 11,800 ft.
More importantly, look at high blower for both engines. In this case the Merlin 61 puts out 40 more hp, 1000 ft higher despite having both a smaller blower diameter and a lower blower gear ratio, 8.03:1 vs 8.095:1.
As I said earlier, I'm not sure what is going on with the RR engines, but they are frustratingly inconsistent compared to the US engines that I am sure are unrammed. I have been doing some simple calculations on them and ram seems to be the most consistent explanation.
Greg Shaw
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HoHun,
Check the AA.878 test results. In the climb test (3000rpm) you can see that at the unrammed +9 lbs FTH (16000ft) tested plane could still do +9,8 lbs (FTH being about 17000ft with RAM, speed around 200mph TAS) and in the speed test it could do +12,7 lbs at 16k (FTH being 19900ft with RAM, speed about 360mph). These all are logical ie with RAM the FTHs were higher than unrammed FTH for +9lbs.
Then check +16 lbs FTHs. In the speed test the result is again logical ie rammed FTH is about 2000ft (or over 3000ft) higher than unrammed FTH 11000ft (or 9250ft depending on source). But in the climb test the rammed FTH is only 8800ft ie 2200 (or 500ft) lower than unrammed FTH. Note that in the case of the W.3228 (Merlin 50M +18 lbs) reached FTHs in climb and speed tests are higher than unrammed FTH (2750ft).
gshaw,
There can be many reasons why the FTHs tend to vary in the tests (filters, ice guards, poorly designed intake, bad example in the test, measuring errors, maybe too small intake for given MAP and so on). But if you look for example those A&AEE results in mw's pages you can see that in the vast majority of the cases the FTHs are far higher than unrammed FTH. And if you compare rammed FTHs between Mustang I (V-1710) and various Spitfires (Merlin) in those tests, you can again see that there really is notable difference in altitude performance and ability utilize RAM air; that is exactly the phenomena Whitney writes about. Of course you can pick up just one or two tests which happen to support you but that is not a very constructive way to analyze engines.
As usual there are errors in the books and those values for the merlin XX in the p.322 are wrong. The Merlin XX was rated by RR (source RR spec sheet):
1485hp/3000rpm/ 6000ft/+14 lbs (M.S.)
1490hp/3000rpm/12500ft/+16lbs (F.S.)
The Merlin 61 was never qualified for the +18 lbs boost (some tests were made) and overall it was a strange animal among two stage Merlins. Probably the efficiency of the Merlin 61 just peaked around the FTH while the V-1650-3 was a real high altitude engine which worked well above FTH. The Merlin Mustangs usually reached top speed well above FTH.
gripen
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Just to add something to the V-1710. A very detailed german technical report about the C-15 often mentions that the production of this engine resembles to the car industry. That means, for production output and not for weight or other special requirements of an high performance aircraft engine.
Many parts, after being forged or casted, were left this way without fine finishing.
For example in the diffusor part of the charger, the air guiding channels were barely worked on (left rough).
Furthermore there spirale of the charger had a perpendicular connection to the outlet, which is also particular bad for an air flow.
The report mentions some data for th 1710-C15
1,345ata @ 4030m (military)
adiabatic pressure height H_ad: 6500m
charger wheel outer diameter: 160mm
v charger (outerdiameter): 331m/sek
adiabatic efficiency: q = (H_ad * g)/(v^2) = 0.582
niklas
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Question about Spitfire Performance Testing site. Does it just down for now or Mike decided to move it to another place or he decided to shut it down from public ?
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Hi VooDoo: I've had some security problems that need to be resolved. Its down until I sort things out.
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In what aircraft were these Allison engines used?
After all, wasn't the P38 the only Allison powered aircraft that performed well at altitudes over 15K?
Would really love to know more of the Allisons really.
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Originally posted by Angus
In what aircraft were these Allison engines used?
After all, wasn't the P38 the only Allison powered aircraft that performed well at altitudes over 15K?
Would really love to know more of the Allisons really.
The C15 was primarily used in the early P-40 variants. It was never really fully developed, since the E/F series was the main emphasis. A little over 2500 built between Feb '40 and Aug '41. It remained in production long after the much superior F3R entered production in Jan '41, probably for UK use.
The P-38 had a separate turbo-supercharger that effectively provided SL+ pressure at the carburetor inlet up to the turbo's critical altitude, about 25,000 ft.
So the P-38 engines only had to provide boost over SL+ pressure. ie if the desired pressure was 45 in Hg they only had to provide a pressure ratio of about 1.5:1 regardless of altitude. Non turbo-supercharged engines had to provide a pressure ratio of about 2.7:1 to provide 45 in Hg at 15,000 ft, or about 4.1:1 to get 45 in Hg @ 25,000 ft.
Greg Shaw
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Originally posted by niklas
1,345ata @ 4030m (military)
adiabatic pressure height H_ad: 6500m
charger wheel outer diameter: 160mm
v charger (outerdiameter): 331m/sek
adiabatic efficiency: q = (H_ad * g)/(v^2) = 0.582
niklas
Interesting, both the 1.345 ata @ 4030 m and the 6500 m adiabatic pressure height work out to a bit over 2.2:1 pressure ratio, pretty damn close to what I have calculated from "Vees for Victory."
Adiabatic efficiency formula is great, I'll have to add that to my collection.
q = (6500 * 9.8066)/(331 ^ 2) = .5818
I'll have to start doing some comparisons on other engines.
Greg Shaw
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You guys might find this stuff interesting ;) head on over to http://naca.larc.nasa.gov/ and search for "supercharger".