Manifold Boost

[gecko]

Got a question for the FM folks. I'm trying to do two representations of the Spitfire I, one fueled with 87 octane and one with 100 octane fuel. From stats I found, the engine could be boosted for an additional 6 psi manifold pressure on 87 octane and 12 on 100 octane. I've looked at the engine section in the aircraft.cfg and don't really understand the relationship between all the MP and emergency boost values. Any help would be appreciated. Thanks.

Daniel

[greycap.raf]

max_design_mp = 48.2 - Full throttle boost
min_design_mp = 9 - Idle boost
emergency_boost_mp_offset = 3.75 - The amount of extra boost on emergency power

When the engine is idling you have 9 in/hg of boost, when you firewall the throttle you get 48.2 in/hg up to the critical altitude and when you hit emergency power you get an extra 3.75 in/hg over the full throttle boost. You can probably work out the rest because I'm too far in the twilight zone after a day of work to explain it any better.

[gecko]

Thanks Rene! So all of these numbers would be in addition to the 29.92 inHg standard day atmospheric pressure (ambient pressure) that always exists? Doing some quick conversions, that puts a +12 psi boost at 24.4 inHg in addition to the 48.2. That's a huge difference from what's there! Is there any way to verify the 48.2 number?

Also, what this this value: emergency_boost_gain_offset = 0.256517886292617? Seems to be a multiplier, but which value does it affect?

Thanks for your help.

Daniel

[greycap.raf]

No, in/hg doesn't work like that, anything below 29.92 in/hg is vacuum, more or less.

But I can verify that an extra 12 lb/sq.inch is insane, I believe that your specs have a misunderstanding in them and they confuse "normal" boost with extra boost. Normally the full boost pressure on a Spitfire Mk I was +6.25 lb/sq.inch, emergency power being originally +9 lb/sq.inch and later +12 lb/sq.inch so the actual increase was 5.75 lb/sq.inch at most - around 11.7 hg/in, certainly believable.

Those example figures I originally used weren't taken from a Spitfire, to clarify a bit. If my calculations are anywhere near what they should be I'd say that 42.25 in/hg roughly equals 6.25 lb/sq.inch and the full emergency boost would result in around 54 in/hg. They actually match the difference in the previous paragraph so I must have got something right.

The emergency_boost_gain_offset controls the critical altitude of the emergency boost. It enables tricks such as what I did with the Bf 109G series by Nigel for which I made the flight model tweaks. There are some models with GM-1 injection that in real life retained full power to great altitudes - I used zero boost offset but a lot of boost gain offset and got emergency power that did absolutely nothing below the supercharger critical altitude but raised boost back to low altitude figures when used at high altitudes. It was a work around that did exactly what I wanted it to and so did the other one. Particularly the G-10 and K-4 models use MW50 injection that gives extra power below critical altitude but does nothing above, setting the boost gain offset to zero nailed it. Extra boost fades off when climbing to critical altitude and engaging the injection doesn't change a thing above it.

Yep, I've done this a bit too much...

[gecko]

So then, assuming 6.25 psi (42.65 inhg) at full throttle and 12 psi (54.35 inhg) at WEP, emergency boost mp offset should be 11.7 inhg or 6.11 inhg for 9 psi, if I've got this correct. Forgive the repeat of everything you just said, just wanted to try calculating it myself.

So gain offset values greater than or equal to 1 would seem to increase the critical altitude when WEP is engaged. What about values less than 1? 0 of course would provide no power increase when WEP is engaged above critical altitude. Is there a more or less scientific way to adjust this value or is it just your best guess?

[greycap.raf]

So then, assuming 6.25 psi (42.65 inhg) at full throttle and 12 psi (54.35 inhg) at WEP, emergency boost mp offset should be 11.7 inhg or 6.11 inhg for 9 psi, if I've got this correct. Forgive the repeat of everything you just said, just wanted to try calculating it myself.

Other than that you somehow have a larger difference between 6.25 and 9 than 9 and 12 (I suspect some slight brain hiccup or me misunderstanding something) that's exactly how it goes.

So gain offset values greater than or equal to 1 would seem to increase the critical altitude when WEP is engaged. What about values less than 1? 0 of course would provide no power increase when WEP is engaged above critical altitude. Is there a more or less scientific way to adjust this value or is it just your best guess?

The value doesn't have to be larger than 1 to get a higher critical altitude for WEP, much less will do. I used 1.2 for the GM-1 system and the difference is nearly 7000 feet, 0.8 will already get you a couple of thousand feet above the supercharger altitude on most engines. I have absolutely no idea what the value is based on or how it is calculated, it has something to do with the boost values and critical altitude but my knowledge doesn't go any further. The only way to get it right is to monitor the results somehow (AirWrench in read only mode does a great job) and go with trial and error.

Negative values produce interesting results. You can dial the WEP critical altitude to be very low but as a side effect you'll get boost loss with WEP engaged well below the normal critical altitude, to the extent that the engine produces more power at full throttle than with WEP. I know because I once tried to make a Merlin 55M before knowing how to do it properly and failed quite spectacularly. :

[gecko]

I think I punched in some wrong numbers on the calculator for that one.

Where would one get information about max manifold pressure (without WEP) for other engines or other models of the merlin for example? Can it be derived from other specifications? I know later Merlins using special fuel could be boosted to 25psi, but I can't find anything about how much of that was part of the emergency boost.

I have a feeling there are inaccuracies in many of my other aircraft as well, so I'd like to be able to work through and fix as many as possible.

[greycap.raf]

http://www.spitfireperformance.com/

A lot of boost pressures and other data, measured, not estimated.

There's very little data available on those "rocket fuel" Merlins but seeing that they originally ran 61" (using in/hg despite discussing British engines as it's the value CFS3 also uses) of full throttle boost and 67" of WEP boost, 75" to 80" of WEP boost sounds like too large of a leap to me. I once read somewhere, no idea where or when, that the "base boost" was raised to the old WEP value of 67" when equipping with 150 grade fuel. Maybe true, maybe not but it would make sense to me.

Having said all that, CFS3 has an issue with boost and power. If you spec the engine to have X amount of power on Y amount of full throttle boost and then put in Z amount of WEP boost you'll end up with too much WEP power. It seems that CFS3 doesn't model the greatly increasing efficiency losses with increased boost pressures so you'll have to run less WEP boost than in real life to get realistic power figures. Getting reliable power figures is a story in itself, when making the aforementioned Bf 109G series I had quite a hard time trying to find the maximum power of a DB605D with MW50 injection - some sources said 2000 bhp, some said 2200 bhp, and a lot of things in between. Some sources gave percentual power increases which matched neither of those figures seeing that the full throttle output was 1800 bhp. In the end I managed to make pretty much everything fall in place with 2000 bhp so that's what I used but the emergency boost was nowhere near the 1.98 ata used in real life because it would have thrown the power figure off by a large margin.

[gecko]

That's a great source, has all the info I need for Merlin-powered aircraft anyway. Thanks for your help!

[Rami]

Greycap.raf,

Thank you for explaining this...I always struggled with how to interpret boost when trying to correct the performance of aircraft which use "boost" info or "ata." I happened across this on a Google search and just wanted to say thanks.

Cheers,