I'm about to update bug #174 on the Merlin 100 octane issue. I have re-written the bug for Artist as requested and would like a review. Kurfurst and Crumpp will be ignored, I am interested in ratification from one or more from Glider/Banks/NZTyphoon/Robo/lane. If this is ok I'll post the update, if there is anything I can add such as a good graph or table or test please post. Thanks.
Description
Presently if the Automatic Boost Control (ABC) is pulled on Spitfires and Hurricanes and full throttle applied there is no increase in boost above 6.25lbs even when the ABC is enabled. This results in reduced and inaccurate performance of RAF fighter aircraft.
During the BoB all frontline fighters had been modified to use 100 octane fuel in their Merlin II and III engines which allowed the engine to achieve 12lbs boost under strict limits. The Merlin XII engine fitted to the Spitfire II was designed to use this fuel from the factory. The approval and introduction of these changes gave the RAF fighters a performance boost in top speed, acceleration and rate of climb up to the Full Throttle Height (FTH) of about 18,000ft. At sea level a Spitfire was about 30mph faster when the ABC was enabled.
Merlin II and III (fitted to Spitfire Ia and Hurricane Ia)
The Merlin engines have a mechanical supercharger and can deliver up to 20lbs of boost at S.L. with the throttle valve fully open using either 87 or 100 octane fuel however this would cause serious engine damage by pre-detonation in the cylinders. Because of this the Merlin II and III have a boost controller fitted which limits the boost to only 6.25lbs. With modification to use 100 octane fuel the pilot could achieve 12lbs boost by pulling the Automatic Boost Control increasing the ‘safe’ power that the engine could produce. When the pilot applied the ABC on the Spitfire a thin wire was broken to indicate to the ground crew that ABC had been used and to make necessary checks. The Hurricane had a valve which was pulled (“pulling the tit”)
Merlin XII (fitted to Spitfire IIa)
The Merlin XII did not require modification in order to use 100 octane fuel. It had a slightly different throttle system and used a ‘gate’ on the throttle control. This allowed the pilot to achieve 12.5lbs boost on takeoff and is allowed up to 1000ft when he moved the throttle past the gate. Up to the gate 9lbs boost was achievable up to the FTH of 17000ft and later clearance was granted to use 12lbs boost (see pilot notes) There are lots of combat reports supporting this.
Here are the engine power ratings for given boost vs altitude
http://www.spitfireperformance.com/merlin3curve.jpg
Spitfire Ia prop tests using 6.25lbs boost and 87 octane
http://www.spitfireperformance.com/spitfire-I.html
It contains information on the clearance and usage of 100 octane fuel including this graph for the Spitfire Ia using 12lbs boost
http://www.spitfireperformance.com/s...-rae-12lbs.jpg
Here is the performance test for the Hurricane Ia using 100 octane fuel and 12lbs boost
http://www.wwiiaircraftperformance.o...rricane-I.html
Here is the performance for the Spitfire IIa
http://www.spitfireperformance.com/spitfire-II.html
Note that the boost for the tests is 8.8lbs, that is up to the ‘gate’ and this falls away from 17,500ft, the FTH.
Pilot notes for the Spitfire IIa indicate the limitations
http://www.spitfireperformance.com/spit2pnfs3.jpg
This report is evidence for the approval of 12lbs/100 octane prior to the Battle of Britain start
http://www.wwiiaircraftperformance.org/ap1590b.jpg
This is supported by lots of evidence of its use in combat reports (supplied by others in this bug report)
These reports not only supply data on the speed per altitude but also rate of climb and acceleration, plus some information of spin characteristics, dive ability and rate of turn. This should all be used to improve the current flight models for RAF types.