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.

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).

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.

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.

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

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.

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