+12 boost

[reflected]

Right now we seem to have 1939 Messerschmitts, Spitfires, and Hurricanes.

Apart fro mthe lack of Rotol Constant speed prop on Spitfire MKIa-s, there's something else: 100 octane fuel and +12 boost:

Trials were successfully carried out in October 1939 to increase the power of the Spitfire's Merlin II and III engines by raising the manifold pressure to +12 lbs./sq.in. 26 Air Ministry A.P.1590B/J.2-W. dated 20 March 1940 gives official notice that "The emergency use of higher boost pressures up to +12 lb./sq. in. is now permitted for short periods by operation of the modified boost control cut-out". 27 Also during February and March 1940 Spitfire and Hurricane Squadrons were converting their aircraft over to 100 octane fuel, which made possible an increase in engine power by raising the boost to +12 lb/sq.in.. 28a 28b 28c 28d 28e 28f 28g 28h 28i 28j 28k 28l 28m 28n 28o 28p 28q 28r 28s Combat reports show that +12 lb boost was used by the Spitfire (and Hurricane) squadrons during their first combats with the Me 109 E in May 1940 while covering the Dunkirk evacuation. 29 30 Hurricane Squadrons based in France during May of 1940 were also employing +12 lbs/sq.in. boost in combat. 31 31b


Apparently this was used even during Spring 1940, when the RAF still used de havilland props.

[Sauf]

Not again? Wasn't the last umpteeth page thread enough! :rolleyes:



http://forum.1cpublishing.eu/showthread.php?t=19754

[ICDP]

This has been gone over a few times Reflected and I disagree, we do not have 1939 planes, in some cases they are much worse. Though the Spitfire Mk IIa is giving +12 lbs boost speeds even with +9 lbs boost showing on the guages.

Another link where some of the community are testing the AC performance.

http://forum.1cpublishing.eu/showthread.php?t=20196

[Peril]

http://forum.1cpublishing.eu/showthread.php?t=20110

[reflected]

Sorry, I must have missed the threads, and haven't read any official feedback on the issue. So, is it being looked at?

[ICDP]

It is easy to miss the pertinent threads among the others. I believe the devs are looking into high alt FM/CEM issues and would hope they are going to fix these issues also.

[Blackdog_kt]

Just as a side note, i don't think the instruments were calibrated for the extra amount of power, at least not early on in the war.

Boost cut-out probably started as an over-ride in case the automatic boost control failed, then pilots discovered it could be used to over-boost the engine. As such, it went from a backup system that was supposed to be used to maintain boost in normal operating range in case of malfunction, to a war emergency type system and instrumentation had some catching up to do. Viper2000 explains it in much more detail, possibly in the threads you guys linked.

As for the 100 octane issue and boost values, i agree it should be corrected but if the CEM is also realistic it will be of minimal tactical value. I mean, what is a minute or less (depending on the engine's temperature at the start of WEP usage) of running +12 psi boost before it gets damaged in the grand scheme of things during a sortie? If people think we'll be able to cruise around at those values they will be sorely disappointed.

Such power settings are more like what you would use when you see a 109's cannons "blinking" in your direction to command extra power for a maximum G evasive maneuver or to accelerate better, ie as a backup for when you've already put yourself in a place that you've probably lost the fight. It certainly doesn't represent any kind of sustainable power setting if it blows the engine to bits within minutes, it's just an "oh sh*t, let's get out of here" helper :grin:

[Kwiatek]

It wouldnt be minimal effect Blackdog when using higher boost. At low alt it would be huge difference beacuse it give 300 HP more to engine for MErlin III. It could be used for 3-5 minutes and it make big difference in speed and climb. It has no effect at higher alts thought.


For 109 E-3 emergency power 1.45 Ata ( 1.4 Ata for 601A) was allowed only for 1 mminut so it was very short time to use, also 1.35 Ata was also only allowed for 5 mintues - so its mean that both 1.45 Ata and 1.35 in 109 was seriously restricted by time - much more then in Merlin. Of course it was only for low to medium alts. At higher alts such boost pressure wasnt possible to achive due to lack of supercharger power.

[*Buzzsaw*]

This has been gone over a few times Reflected and I disagree, we do not have 1939 planes, in some cases they are much worse. Though the Spitfire Mk IIa is giving +12 lbs boost speeds even with +9 lbs boost showing on the guages.

Another link where some of the community are testing the AC performance.

http://forum.1cpublishing.eu/showthread.php?t=20196

Wrong. The Spit II has lower performance than an aircraft at +8.8 boost:

http://www.spitfireperformance.com/p7280speed.gif

Image courtesy of Mike Williams Spitifire Performance site:

http://www.spitfireperformance.com/spittest.html

[Blackdog_kt]

It wouldnt be minimal effect Blackdog when using higher boost. At low alt it would be huge difference beacuse it give 300 HP more to engine for MErlin III. It could be used for 3-5 minutes and it make big difference in speed and climb. It has no effect at higher alts thought.


For 109 E-3 emergency power 1.45 Ata ( 1.4 Ata for 601A) was allowed only for 1 mminut so it was very short time to use, also 1.35 Ata was also only allowed for 5 mintues - so its mean that both 1.45 Ata and 1.35 in 109 was seriously restricted by time - much more then in Merlin. Of course it was only for low to medium alts. At higher alts such boost pressure wasnt possible to achive due to lack of supercharger power.

All i'm saying is that it's not IL2:1946 anymore where we can run with 110%+WEP by opening radiators every 5 minutes and lowering prop pitch because the overheat is directly tied to RPM. In the new sim there are limits and as such, WEP really means emergency power and can't be used all day long like it was in IL2:1946.

Sure, 12lb boost could save your virtual life a few times. It's just not something you can run for the entire mission if you fly at full difficulty and that's good, because that's the way it worked in reality as well.

I would still like it to get fixed and accurate according to historical data, i just don't expect any massive advantage for being able to run it for a couple of minutes during a 60 minute sortie.

[Kwiatek]

Wrong. The Spit II has lower performance than an aircraft at +8.8 boost:

http://www.spitfireperformance.com/p7280speed.gif

Image courtesy of Mike Williams Spitifire Performance site:

http://www.spitfireperformance.com/spittest.html

If you give more focus on both test - Spitfire MK1 and MKII you will note thath MK II was equimpment with aditional pilot armour ( was more heavy) and MK1 has only armoured windscreen ( without pilot armour). Thats why at higher alts above 18 000 ft where boost preasure for both engines - Merlin III and Merlin XII was similar Spitfire MK1 with lower weight was faster. At lower alts MKII at nominal power ( +9lbs) was faster then MKI ( + 6 1/2 lbs) but at emergency power +12 lbs MK1 could be little bit better ( expecially in climb rate).

[ICDP]

Wrong. The Spit II has lower performance than an aircraft at +8.8 boost:

http://www.spitfireperformance.com/p7280speed.gif

Image courtesy of Mike Williams Spitifire Performance site:

http://www.spitfireperformance.com/spittest.html

I posted over 20 days ago speed tests for the Mk IIa in CoD that showed it matches a +12 boost Spitfire. I even asked you directly if you had tested it yourself in the sim, I ask once again, have you tested it yourself in CoD? The chart you posted shows 290 for SL top speed for a +9 lb boost Mk IIa, the Mk IIa in CoD gets 315 at SL.

Here, once again is the Mk IIa Spitfire CoD speeds compared to real Spitfire speeds.

http://i17.photobucket.com/albums/b58/ICDP/CoDSpitfireMkIIaSpeeds.jpg

As can be seen the Mk IIa speeds are spot on when compared to a +12 lbs boost Spitfire Mk Ia. The speeds are around 20-25mph faster than the 9 Lbs boost Mk II a speeds which would put them about right for 12 lbs boost. Also at the full throttle height of around 18,000 ft the boost there should be no speed difference between 9 lbs or 12 lbs boost.

The Mk IIa in CoD (at least in speed for altitude) is definately a 12 lbs boost version. The guage in CoD is broken.

[TomcatViP]

Excellent post ! Thx

As someone as alrdy said (BlckDog ?) it shld be reasonable to admit that the gauge were not modified as soon as fuel grade was uprated to the 100 .

[Kurfurst]

I even asked you directly if you had tested it yourself in the sim, I ask once again, have you tested it yourself in CoD?

How could he, he doesn't even have CoD... ;)

[*Buzzsaw*]

I posted over 20 days ago speed tests for the Mk IIa in CoD that showed it matches a +12 boost Spitfire. I even asked you directly if you had tested it yourself in the sim, I ask once again, have you tested it yourself in CoD? The chart you posted shows 290 for SL top speed for a +9 lb boost Mk IIa, the Mk IIa in CoD gets 315 at SL.

Here, once again is the Mk IIa Spitfire CoD speeds compared to real Spitfire speeds.

http://i17.photobucket.com/albums/b58/ICDP/CoDSpitfireMkIIaSpeeds.jpg

As can be seen the Mk IIa speeds are spot on when compared to a +12 lbs boost Spitfire Mk Ia. The speeds are around 20-25mph faster than the 9 Lbs boost Mk II a speeds which would put them about right for 12 lbs boost. Also at the full throttle height of around 18,000 ft the boost there should be no speed difference between 9 lbs or 12 lbs boost.

The Mk IIa in CoD (at least in speed for altitude) is definately a 12 lbs boost version. The guage in CoD is broken.

Thanks for your comment, please link the test you did, I have not seen it.

[Viper2000]

I posted over 20 days ago speed tests for the Mk IIa in CoD that showed it matches a +12 boost Spitfire. I even asked you directly if you had tested it yourself in the sim, I ask once again, have you tested it yourself in CoD? The chart you posted shows 290 for SL top speed for a +9 lb boost Mk IIa, the Mk IIa in CoD gets 315 at SL.

Here, once again is the Mk IIa Spitfire CoD speeds compared to real Spitfire speeds.

http://i17.photobucket.com/albums/b58/ICDP/CoDSpitfireMkIIaSpeeds.jpg

As can be seen the Mk IIa speeds are spot on when compared to a +12 lbs boost Spitfire Mk Ia. The speeds are around 20-25mph faster than the 9 Lbs boost Mk II a speeds which would put them about right for 12 lbs boost. Also at the full throttle height of around 18,000 ft the boost there should be no speed difference between 9 lbs or 12 lbs boost. This is not the case.

More boost means more power.

FTH for some boost pressure P is the height at which this boost is delivered with a wide open throttle.

As you climb with the throttle wide open, whatever height you're at is the FTH for whatever boost you've got.

What you mean is that above the FTH for +12 psi boost, being allowed to use +12 is academic, because the supercharger can't deliver it.

The Mk IIa in CoD (at least in speed for altitude) is definately a 12 lbs boost version. The guage in CoD is broken.

The Spitfire I has a Merlin III; supercharger gear ratio 8.588
The Spitfire II has a Merlin XII; supercharger gear ratio 9.089

The Merlin III at +12 psi delivers 1310 bhp at 9000'.
The Merlin XII at +12 delivers 1280 bhp at 10500'.

AFAIK both figures exclude ram. In any case, the two aeroplanes are not identical.

The last time I tested the Spitfire II, I got the distinct impression that it was faster than it should be.

I think that operating the boost cutout really does give about +8 boost; if you look at the instruments in the no-cockpit view then you'll see that they read up to +12, but only show +8.

I think that we actually have the R.M.2.S. rating; +8¼ psi boost takeoff power, which gives 1000 bhp at 3000 rpm at sea level, but with +12 power levels. This means that we have too much power at altitude, and the brief testing I have conduced at 18000' gave me a top speed in the region of 380 mph... The alternative explanation would be reduced airframe drag.

Anyway, R.M.2.S. was an interim rating for the Merlin III prior to the introduction of the +12 psi combat rating; the Merlin XII was a somewhat different animal designed for +12 boost from the start and would deliver about 1165 bhp at sea level and +12 psi, and takeoff power of 1175 bhp at +12½ psi boost.

It seems to me that more testing is required, both with CEM on and off, especially given that the sim has been quite extensively patched in the last 20 days...

It is also important to remember that whatever results we get from the sim need to be corrected to standard conditions before they can be directly compared with historical test reports.

[ICDP]

This is not the case.

More boost means more power.

My knowledge on these matters doesn't approcah yours, all I know is that despite the different engines and boost levels, above a certain altitude meant the planes were similar in speeds. Thanks for the explanation Viper.

The last time I tested the Spitfire II, I got the distinct impression that it was faster than it should be.

Where you doing the tests with CEM off? I ask beacuse when I tested without CEM I was able to get 388mph at FTH with the Spitfire IIa in CoD.

I think that operating the boost cutout really does give about +8 boost; if you look at the instruments in the no-cockpit view then you'll see that they read up to +12, but only show +8.

I noticed this as well, I mentioned it on one of the other threads. Here is the post I made on this.

http://forum.1cpublishing.eu/showpost.php?p=258348&postcount=12

I think that we actually have the R.M.2.S. rating; +8¼ psi boost takeoff power, which gives 1000 bhp at 3000 rpm at sea level, but with +12 power levels. This means that we have too much power at altitude, and the brief testing I have conduced at 18000' gave me a top speed in the region of 380 mph... The alternative explanation would be reduced airframe drag.

Anyway, R.M.2.S. was an interim rating for the Merlin III prior to the introduction of the +12 psi combat rating; the Merlin XII was a somewhat different animal designed for +12 boost from the start and would deliver about 1165 bhp at sea level and +12 psi, and takeoff power of 1175 bhp at +12½ psi boost.

It seems to me that more testing is required, both with CEM on and off, especially given that the sim has been quite extensively patched in the last 20 days...

It is also important to remember that whatever results we get from the sim need to be corrected to standard conditions before they can be directly compared with historical test reports.

Absolutely agree, the Spitfire Mk IIa shows +8 lb boost and gets speeds similar to a +12 lb boost version. I stopped testing when I realised the high altitude FM or CEM was broken. I feel that further tests are needed once the devs fix the previously mentioned issue.

[TomcatViP]

More boost does not mean more power : 0+0x12 = 0 :rolleyes:

Temp might be the limiting factor to consider (tht's where MW50 (with water) was so important for German eng with their poor quality materials). Early war Merlins shld be weaker than their late counterparts. Hence drawing a comparisons with later Merlins even at the same boost level is risky

~S!

[Peril]

All i'm saying is that it's not IL2:1946 anymore where we can run with 110%+WEP by opening radiators every 5 minutes and lowering prop pitch because the overheat is directly tied to RPM. In the new sim there are limits and as such, WEP really means emergency power and can't be used all day long like it was in IL2:1946.

Sure, 12lb boost could save your virtual life a few times. It's just not something you can run for the entire mission if you fly at full difficulty and that's good, because that's the way it worked in reality as well.

I would still like it to get fixed and accurate according to historical data, I just don't expect any massive advantage for being able to run it for a couple of minutes during a 60 minute sortie.

We tried hard in TW to run this same policy with WEP as well, but the DM did not have the fidelity at that time to pull it off. If you can get the engines to overheat in "required" time periods, if the cooling behaves as you would need I still think this is a viable option to restricting overuse. Although 'not' strictly realistic, it would still be a good way (best way?) to enforce realism in the use of WEP/Overboost.

Historically a lot of these limits were not strictly set because of blowing a motor, or overheating ie. within 1 or 5 mins. Most of the time engine life was also a serious consideration factored into these time limits. So, if you could track a CoD pilots time in the air, in all his flights in that plane type, over how ever long he survives in that plane, then you could really trigger a 'realistic' blown engine from the 'consistent' over use of WEP.

IMHO the direct link to heating would be a good/simple way to enforce the time limits short of anything better, but it's not strictly realistic.....



On a side note the below may be of interest on this topic of over boost:

Allisons in P40s were very resilient to over use and abuse of over boost, RAAF pilots did it routinely and they did not seem to suffer from this practice.

See the below document:

http://www.raafwarbirds.org.au/targetvraaf/p40_archive/pdfs/Allison%201710-39%20abuse.pdf

Those interested in P40 Data may be interested in my online collection here.

http://www.raafwarbirds.org.au/targetvraaf/p40_archive/p40_archive.htm

[Viper2000]

More boost does not mean more power : 0+0x12 = 0 :rolleyes:

Temp might be the limiting factor to consider (tht's where MW50 (with water) was so important for German eng with their poor quality materials). Early war Merlins shld be weaker than their late counterparts. Hence drawing a comparisons with later Merlins even at the same boost level is risky

~S!

Firstly, I don't understand your logic at all.

A Merlin III with the R.M.2.S. rating could achieve +6¼ psi boost at 15500', which would deliver about 1000 bhp.

A Merlin XII with an improved supercharger intake elbow and AVT40 carburettor could achieve +9 psi boost at 15750', which would deliver 1150 bhp.

More boost = more power.

Alternatively, compare the takeoff power of Merlin III engines at different ratings.

The R.M.1.S. rating was +6¼ psi boost, 3000 rpm, giving 880 bhp at sea level.
The R.M.2.S. rating was +8¼ psi boost, 3000 rpm, giving 1000 bhp at sea level.

The engines were physically identical, with the extra power allowed by the use of 100 octane fuel.

More boost = more power. Indeed, you can see that an extra +2 psi is worth about 120 bhp, whilst an extra +2.75 psi is worth about 150 bhp.

To a first order approximation, you can see that running a first generation Merlin at +21 psi absolute gives about 1000 bhp unthrottled if the supercharger gear ratio is about 8.5.

So in round numbers, that would be roughly 50 bhp for every extra psi boost.

Now, this is very rough and ready stuff, but it's quite a good first order guess; a Merlin 66 on 150 grade fuel gives a little over 2000 bhp in MS gear unthrottled at +25 psi boost.

1000*(25+15)/20 ~ 2000.

Basically what we're saying here is that the amount of power produced by the engine is proportional to its air consumption, which is limited by the physical size of its intake & exhaust valves.

Supercharging increases the air consumption by increasing the charge density in the intake manifold.

P*V = roh*R*T

So actually the error associated with drawing a comparison between early and late Merlins is smaller than you might perhaps expect.

Thermodynamically, the piston engine at the heart of the Merlin doesn't change much after the ramp head combustion chamber was discarded c.1938. The vast majority of its power development came from improvements to the supercharger. Valve timing was only changed for the prototype R.M.17.S.M engine.

Mechanically there were considerable changes devoted to improving life at high power, but they didn't greatly impact upon the thermodynamics of the machine.

Indeed, although the Griffon is mechanically very different from the Merlin, if you take the Merlin model developed by Hooker et al and plug in Griffon numbers then you'll find that the agreement is impressive, because thermodynamically they're extremely similar machines.

///

ICDP,

My high speed testing of the Spitfire II was indeed conducted with CEM disabled. I was actually trying to tackle the prop pitch change controversy at the time, rather than to investigate the performance of the aeroplane itself. But I was struck by just how much faster the Spitfire II was than the Spitfire I, and I suspect that what's happened is that 1c have done the same thing with the early Merlin that they did with the later Merlin when the Mustang III was introduced to IL2, namely increased the boost without changing the FTH appropriately.

However, it's very difficult to be sure at this stage given the various bugs and our lack of knowledge of the atmosphere model etc.

[*Buzzsaw*]

Where you doing the tests with CEM off? I ask beacuse when I tested without CEM I was able to get 388mph at FTH with the Spitfire IIa in CoD.

Testing with CEM off is useless and irrelevant. Were all your tests done that way?

Tests should be done with full CEM and full fuel loads from a takeoff as close to sea level as possible.

I would ask you again to link me to your tests, with the details of exactly what the parameters and conditions were.

[Kurfurst]

Temp might be the limiting factor to consider (tht's where MW50 (with water) was so important for German eng with their poor quality materials).

MW-50 had nothing to do with the allaged 'poor quality' of materials used in German engines, nor does it have to do anything with the cooling of the engine itself - something that ordinary radiators could already take care well. The primary use of MW-50 was step in an as a quasi intercooler of combustion temperatures and raising allowable boost without detonation with low octane fuels, although it was not that much of an issue with high octane C-3. Generic cooling qualities were a secondary, but very useful and effective feature of water injection (MW-50).

[Peril]

Actually, depending on if the aircraft parameters are coded as aircraft min/maxes or as a function of the operation of the engine/airframe, this MIGHT be a relevant test. Someone should test both CEM off and on, and see what the results are.

I know your point, but without more information on the internal system, who's to say CEM being off is wrong?

Agreed here, if IL2 programming is in line with CEM being the best tuned outcome performance wise, it's actually 'better' to test this way as you're getting the best result possible with all factors perfectly tuned. CEM may just be a simple 'disabling' of the inherent automatic controls, that's my feeling. There seems no point in making Auto CEM performance any different to manual CEM.

My preferred way to test the 'best performance curve' is away from any influence from poor pilot manual CEM and/or engine DM/heat effects.

Of course drag items like cooling flaps would need to be set as per the data you are using.

Wish we could be 'told' how this Auto/Manual CEM functions, that would define it for testing.

[ICDP]

Testing with CEM off is useless and irrelevant. Were all your tests done that way?

Tests should be done with full CEM and full fuel loads from a takeoff as close to sea level as possible.

I would ask you again to link me to your tests, with the details of exactly what the parameters and conditions were.

My tests were done with CEM on. I did a CEM off test to check how the AI would handle prop pitch, that is when I noticed the serious discrepancy.

I created a simple mission in the FMB, one aircraft over the channel at 500m. No wind or turbulence set in the weather. I know it is hardly scientific but I tested all of the fighters and only the Spitfire Mk IIa, Bf110 and Hurricane CSP (Rotol) achieved their rated speeds. So if there is a problem with the atmospheric conditions in the test it should effect all aircraft the same.

[TomcatViP]

Hi Vip,

My quick formula sketched above was to show by the absurd that you can't boost anytime anywhere anyhow. In my example if P=0 (outside air pressure) there is no compression at all.

Secondly the compressor is actioned by the engine and such take a non-negligible power out of the engine for the compression of the intake air.

The higher you climb, the more the air need to be compressed but as the air is less dense the at a cte fuel/air mixture, the less fuel would be injected lowering the subsequent power available for the mechanical compression of the air.

This is why as you know Compressor are rated for a given alt.

Now let's get back to the higher octane fuel. Let says that the M engine (this is an example) give 8lb at 15000. As the engine need to be fed by more air at any given altitude to outsource the power of the higher graded fuel, at 15000 the compressor would hve run earlier (e-g at a lower alt) it's full potential.

Hence you max boost would hve been at 3000ft lower hence the max power wld hve been available at a denser air hence your max speed hve some chance to be less spectacular than with a slide rule (the ^3 effect of power/drag as you hve rclled us earlier - where drag means everything and not only external body drag)

SO it's more balanced that saying that if I hve got 100hp more out of an engine powering a giving airframe I would hve a top speed increased by 100/initial power * top speed of airframe

Ok Ok most of this you hev taken it into account. But what I mean is that only by changing the fuel, the SPit won't hve automatically a better top speed. Many thing hve to be adapted like the carb inlet (what you hev mentioned) or the pulley, the size of the compressor wheel (the volumetric ratio) etc... It is doubtfoul that BoB Spitfire where converted to the final standard due to historical events.

Yes the 100 wld hve had a better accel and this is confirmed by the combat report man can read (I am thinking abt that top leading ace that was shot down racing deck with spit in in 6 sure enough that he was safe in his faster 109 - need to dig out his name)

Material ?

to put it short : the amount of E of a system [du] equate somewhat the amount of caloric E (Q in joules) and the amount of work [dW] (1st thermodynamic low - :confused: damn always forgot the right nbrs )

If you put in more internal U you also got more Q to get a slight amount of extra W (as it's easier for the E to be drawn out as extra Q than to be converted in more W). And don't forget that the more Q you had in any piping (I mean a closed syst) the more T is raising hence the more the viscous effect you hve to deal with (air then drag (see above))

Take for example the compound series of eng from the Germans with for example two 601 mated together on the same shaft . Did the output power was increased by a factor of two... no !

This was a silly example as well but it was meant to show by the extreme my purpose.

Here is what I mean : higher grade means higher strain on the eng (and I mean direct mechanical works too) with the consequence of a lower eng life. Yes engineer knew in 1940 how to pull out 2600Hp out of a merlin ... but for how long ? Would it fit any fighters pilot expectation ?

To drive :grin: a comparaison with cars : I can pull 1000HP out of a Nissan Primera but would I out speed a Porshe ? (Plsd post it on Youtube :grin:)

Sry this is a rather long post

~S

[Viper2000]

More boost = more power

You might not be able to get more boost (i.e. because you're already running WOT at max rpm). But if you could get more boost then it would result in more power.

If you want to get technical then the chain was something like

IHP>SHP>BHP

IHP would just be a function of boost & rpm
SHP is the actual mechanical power produced by the piston engine itself
BHP is the power available to do useful work after the power required to drive the supercharger has been subtracted.

As a general rule, if you improve the fuel available and use this improvement to increase the maximum boost of an engine, the maximum TAS that the aeroplane can achieve doesn't change much. What happens is that the minimum altitude at which it can attain that maximum TAS decreases to the new FTH associated with the increased maximum boost.

So you basically drop a vertical line on the TAS/altitude diagram from the old FTH to the new FTH. This approach makes all manner of implicit assumptions and is therefore somewhat quick & dirty. But it's generally quite close to the truth for WWII fighters.

Obviously, the IAS at the new FTH will be considerably higher.

The extra power will also dramatically improve acceleration and rate of climb at or below the new FTH.

The increased ROC was one of the main attractions of 100 octane fuel for fighters during the Battle because it reduced the amount of warning time required to make a successful interception.

If I put a bigger supercharger onto the engine but maintain constant boost, then SHP stays basically the same (it probably goes down a little due to increased compressor delivery temperature, but piston engines aren't generally explicitly T4 limited like gas turbines, so this is a second order effect).

FTH obviously goes up.

BHP at FTH goes down, because of the increased power required to drive the supercharger.

However, neglecting transonic dragrise and assuming reasonable compressor efficiency, the max TAS will always go up, because the increased FTH means that the density of the air that the aeroplane is trying to move out of its way has decreased.

power required = 0.5*roh*v^3*Cd

So long as roh decreases faster than BHP, you're winning.

Eventually, if you take the process to the extreme, you find that at some very high FTH, the BHP available is only just sufficient to sustain the aircraft in flight at its maximum endurance speed; at this point you need to start looking to the airframer for further improvements. Additionally, low altitude performance will be extremely marginal due to throttling losses, but this may be improved simply adopting a multi-speed supercharger drive system of some sort (or possibly by using VIGVs in the supercharger if that's your bag).

You also find that simply bolting a bigger supercharger onto the engine stops working well once the pressure ratio required exceeds 3-4ish for a single centrifugal stage due to the fact that the tipspeed required becomes mechanically challenging. It's therefore expedient to use multiple stages.

[TomcatViP]

MW-50 had nothing to do with the allaged 'poor quality' of materials used in German engines, nor does it have to do anything with the cooling of the engine itself - something that ordinary radiators could already take care well. The primary use of MW-50 was step in an as a quasi intercooler of combustion temperatures and raising allowable boost without detonation with low octane fuels, although it was not that much of an issue with high octane C-3. Generic cooling qualities were a secondary, but very useful and effective feature of water injection (MW-50).

If Germans pilot had the ability to choose I'm not sure they would hve taken the pain to carry heavy water tanks up to 5/6K to boost their eng.

The C3 optimized eng was a real relief on that point.

Regarding the quality of the materials used in german engines, I would only point out the respective wet weight of Allison, DB605 and Jumo engines. Pls remind that late vers of all those 3 had nearly the same output power.

It remind me the engine mount designed for the Allison powered D9 flying today (warbird). The goal of the engineer was to add weight to get back the right balance (usually you work with a contrary objective).

~S!

Note : sry to dig out tht one. :-p