Boosting Behavior

[Wedge]

Hi all,

I haven't been able to come on comms much at the moment due to my house being renevated, I mentioned it to Osprey, but probably should let other know too.

The plastering is going well and I might just be able to get set back up in time for next Sunday's campaign.

In the mean time I have been doing some performance tests.

If I take a Spitfire Mk.1a with a contant-speed propellor and 87 octane fuel, I take of and climb to 10,000 feet. I adjust the boost to 4.25 lbs and then dive back to sea level. On the deck my boost is reading 4.25 lbs. If I repeat the test, but engage the boost-cut after my climb what would you expect my boost to be back at sea level?

I believe this should be reading 4.25 lbs on the first test and 6.25 lbs on the second test.

My test in Cliffs resulting in both tests being 4.25 lbs.

So the question, is my expectation wrong or the in-game behaviour?

Discuss.

P.S Robo have you got that TIR profile?

[Robo]

You are absolutely right here Wedge - the ABC is completely wrong in the game, especially so in case of 87 octane fuel and Merlin III.

Aoh and mate, you should have the TIR profile in your email for about a month but in case you need it, find here:

https://dl.dropbox.com/u/10668862/Joystick/Robo..xml

It's for TIR5 used with ClipPro thingy

[Wedge]

Still no joy with the link to dropbox. Plus nothing in my legio mail.

[Robo]

I believe this should be reading 4.25 lbs on the first test and 6.25 lbs on the second test.

My test in Cliffs resulting in both tests being 4.25 lbs.

So the question, is my expectation wrong or the in-game behaviour?

Now just thinking about that again - 10000ft. is still below full throttle height so now I am not too sure what would happen. With the BC-O engaged you simply bypass the boost control (that keeps the boost at +6.25 PSI between S/L and FTH) and you get whatever the supercharger is capable of at given altitude. I know this would be slightly higher that rated boost, but I have no information as for how much exactly on the sea level and with 87 octane fuel. I know that with unmodified Merlin III and 100 octane fuel it would give you as much as +16 PSI boost.

This has been considered as too much and a modification has been carried out with a shaft drilled in with specific diameter to release the extra pressure and settle the raw boost at +12 PSI - which is the emergency boost cut out override on 100 octane Mk.I fighters.

Now for lower octane fuel - the rated boost was +6.25 PSI, the BCC took care of this pressure not to be exceeded. The Merlin III powered aircraft have been equipped with BCC (or ABC, automatic boost control) which was a device that could fail and therefore the pilot was able to bypass it or override it if necessary. Only later, this became a tool to obtain extra manifold pressure that became available with 100 octane spirit..

So with 87 octanes, you would probably could go slightly higher than +6.25 PSI but then you would have severe issues with detonation. If you bypass the ABC at 10000 feet at +4.25 PSI and dive to the s/l I suppose the manifold pressure would not rise to +6.25, it would probably stay around 4.25ish imho unless you tinker with the throttle lever.

With the throttle fully forward and ABC off, you would definitely ruin your engine (87 octanes that is).

[Wedge]

I knew their was no safe limit on the unmodified Merlin, around +16 lbs at sea level, but for some reason didn't apply it here in my experiment.

So I think the key here is that you need to not only activate the boost cut-out, but also than adjust the throttle to get manual boost control.

So if I changed the parameters of the test to be ~+16 lbs on the second test, this should be correct (engine life taken out of the equation).

Either way, we don't have manual override in CLOD at the moment. (Unlikely we ever will)

[Robo]

I knew their was no safe limit on the unmodified Merlin, around +16 lbs at sea level, but for some reason didn't apply it here in my experiment.

So I think the key here is that you need to not only activate the boost cut-out, but also than adjust the throttle to get manual boost control.

So if I changed the parameters of the test to be ~+16 lbs on the second test, this should be correct (engine life taken out of the equation).

Either way, we don't have manual override in CLOD at the moment. (Unlikely we ever will)

Oh that! Yes you're right there, we definitely don't get the complex Merlin III engine management. I would need to check how they modelled this in the a2a Spitfire, I never actually tried that to be honest. I tried with the P-40B though which had same ABC and it worked nicely. I strained the engine quite a bit when I was testing everything but I never did that with the Spit, I was always trying to keep the engine within limits.

In theory, the ~+16 lbs would be raw boost on the sea level (actual value depending on atmospheric conditions) with the throttle fully open just as you say. I believe the initial idea was that if you fly and the ABC fails or plays silly tricks (which was not completely uncommon, in which case you would get very little boost even with throttle lever fully forward or probably even more likely - much more boost than was good for the engine, without the ability to control it), at which point you could always bypass the ABC (BCC-O in British terms) and andjust the boost by moving the throttle to the position where 6.25 max boost would be obtained. You would need to fiddle with constantly and be very careful with it as the flight altitude changed.

But as for your 4.25 test at 10000ft, I am not sure for I don't know what the FTH with this raw boost would be with BCC-O engaged.... Interesting discussion though!