+12 boost

[Viper2000]

Originally Posted by ICDP

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.



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.

Originally Posted by ICDP

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.