Bug 174 on 12lbs boost. Review please.

[Kurfürst]

Originally Posted by IvanK

Kurfurst you said:

"The Spit II runs on 100 octane by default, but its emergency limits are lower - 9 lbs vs 12 lbs - and is/was at low altitude. It is a bit better at higher altitudes though."

That is not IMO correct. In the case of the SpitII +9lbs basically became the full throttle setting (i.e. the equiv of 6.25Lbs in the MKI) 12Lbs was still available in two ways by way of the throttle gate for take off operations and by Boost Cut out for combat use.

This is a correct description IMHO for 1941. It is not a correct description for 1940.

It depends on what timeframe you are looking at.

The BoB era summer 1940 (unamended) manual of the Spitfire II clearly notes the limits being:

+12 lbs for take-off up to 1000 feet or 3 minutes.
+9 lbs for combat (5 minute limit)

This +12 lbs could be used near the deck of course, but its near useless since boost will immidiately start to fall with altitude (unlike the Spit / Hurri I's boost cutout, it does not lasts up to FTH).
Boost drop curves of Mk II trials suggest that even with the gate open, the boost will fall back from +12 at SL to normal combat rating of +9 lbs by 4000 feet altitude (ca. 1200 meters), obviously with the same performance.
Thus its somewhat similiar to the 109E/110C 1-minute takeoff boost - its effective up to 1-2000 meters only.



It's only later, amended manuals (presumably from 1941) that are clearing +12 lbs for combat, too.

The RAE standard climb tests are flown at +9Lbs Boost for instance.

RAE tests were flown +9 lbs boost and 2850 rpm, as noted in the Spitfire II manual this was the 30-min rating at the time. It did not give the same power and performance as +9 lbs/3000 rpm, of course.

In short our Spitfire II with its maximum +9 lbs rating and performance is correct and historically accurate for the BoB airframe. +12 lbs rating was not cleared for it for combat use during the BoB period.

Should 1C decide to introduce a post-BoB 1941 variant (doubtful), a +12 lbs version would be feasible, of course.