Bug 174 on 12lbs boost. Review please.

[Osprey]

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.

[Glider]

looks good to me

[41Sqn_Banks]

- IIRC ABC stands for "automatic boost control" and not the cut-out.

- Spitfire II had "gate control" for fixed take-off boost (it gave a fixed throttle valve setting) and "boost control cut-out" like the Spitfire I, see this thread: http://forum.1cpublishing.eu/showthread.php?t=31319

[fruitbat]

I think 1C will be as stubborn about this as they were about the FW190 bar in il2.

Could you add any indications of the duration for which running at 12lbs / 12.5 lbs was allowed? They would have to model engine damage if the time was exceeded and is essential for any correct FM/DM

[Glider]

The official limit was 5 mins but there was nothing to stop pilots going for a lot longer than that. Dowding issued a memo to all pilots that warned them of overusing the boost. In some of the combat reports you sometimes see words along the line of switched guns to fire, lowered the seat and pulled the plug.

[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.

[bongodriver]

something is not quite right about a 9lbs boost being ok for climb for 30 minutes when climbing is the more stressfull in terms of cooling etc and only having a 5 min limit in level flight with more cooling, I don't see why 9lbs boost and 2,850 rpm couldnt be maintained in level flight for at least 30 mins.

[Kurfürst]

Originally Posted by bongodriver

something is not quite right about a 9lbs boost being ok for climb for 30 minutes when climbing is the more stressfull in terms of cooling etc and only having a 5 min limit in level flight with more cooling, I don't see why 9lbs boost and 2,850 rpm couldnt be maintained in level flight for at least 30 mins.

I don't see a reason either. These names for the ratings are somewhat arbitrary, naturally there's nothing preventing you from using 9lbs boost and 2,850 rpm for level flight, apart from temperatures and increasing engine wear. The names like 'combat' and 'climb' were somewhat arbitrary.

[bongodriver]

apart from temperatures and increasing engine wear

which logic dictates are lesser in terms of level flight.........it has occurred to me that those limitations seem more geared toward RPM as opposed to boost, if 9lbs boost can be used for 30 mins then tchnically it is not limiting, usually a 5 min limit is the true factor, this is still a factorisation that applies today even in jets, TO thrust is a 5 min limit, anything below that is a MCT (max continuous thrust), I susspect the 30 min limit is really a limit in terms of oil cooling for the prop govenor system.