Quote:
Originally Posted by klem
(Post 415180)
The problem may be this: if we assume that the gauge reading of +6 1/4lbs is a true value, i.e. it truly represents the maximum manifold pressure, then there is something wrong in the conversion of the boost value to thrust and speed. I don't know how this is coded in CoD but its probably somewhere in the chain of formulas and values that convert throttle position to manifold pressure, manifold pressure to engine power output, rpm setting, propeller/thrust modelling, drag factors, atmospheric pressure, and probably other factors, to arrive at speed.
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It's hard without knowing how the devs have configured their flight model formulae, but it seems likely to be as you say. Of course coding in this way makes it hard to calibrate to exact speed vs altitude (or any other) profile, as max speed (i.e where accelleration = 0) is at the termination of a chain of calculations where changes along the chain (to constants or altering the formulae themselves) can have unpredictable effects, and it may come down to laborious trial and error getting the two curves to match. Of course just getting the speed vs altitude curve right does not mean sim fidelity, for example if the plane accellerates in a non historical way to get there.
But we can compare along the chain where there is historical data, and currently boost is kind of right but the final speeds are wrong.
There are some problems with boost though, last night looking at the offline with cockpit off (apologies, I'm sure similar data has cropped up in many other threads):
Spits and Hurris have a boost controller (red cutout "off") that gives +5 1/4 to +5 1/2 psi minus boost cutout at 3000rpm full throttle. Boost drops as soon as the throttle is retarded, so CoD model is closest to the "variable datum" type of boost controller for all RAF aircraft. This should be +6 1/4 psi though.
Oddly, CoD boost increases to about right (+6.2 psi) when rpm is decreased to the 2600-2800 rpm range. From my reading this is incorrect, the boost controller should not be rpm dependant.
The Spit II with boost cutout activated (sea level; full throttle) achieves +9psi at 2600 rpm, +8psi at 3000 rpm (again the rpm-boost quirk). So the CoD Spit II acts most like a 100 octane conversion where the cutout was modified for +9 instead of +12 psi. At 3000rpm, the engine makes some distressed noises/ vibration but does not fail. Dropping rpm to 2600 rpm and/or putting mixture to weak smooths the engine. At sea level, boost cutout on, rad half open, weak mixture, 2600 rpm and +9 psi boost I could maintain 320 mph (IAS and TAS) for a full sortie. That is some serious speed (512 kmh)
The CoD Spit Ia/Hurri act (in boost characteristics) like 100 octane conversions where someone drilled the wrong size holes in error and gave tiny increments in boost. You can see the boost cutout effect in Hurri at 2600 rpm if you turn it off and on and watch the boost gauge(+6.2 to +6.4 psi), alhough I could not detect a speed difference. So you can't tell if the virtual tanks have 87 or 100 octane in them, it doesn't matter because the boost never gets high enough.
But getting the boost behaviour exactly right doesn't help much if the model does not pass the next check..that the boost combined with other inputs ends up producing overall performance in line with historical norms...which is more of a problem.
I didn't realise the Spit II could sit on 320 mph. Had an online sortie on ATAG, I just zigzagged up and down the channel on the wavetops at 320 mph, and started bouncing people from below. Fun, but not very historical I guess :)
camber
