Bug 174 on 12lbs boost. Review please.

[41Sqn_Banks]

@Ospey I'd like to rephrase some sentences in the initial post. The changes sentences are in bold.

Description
Presently if the boost control cut-out is enabled on Spitfires and Hurricanes and full throttle applied there is no increase in boost above 6.25lbs. 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 boost control cut-out was enabled.

Merlin II and III (fitted to Spitfire Ia and Hurricane Ia)
The Merlin engines have a mechanical supercharger and can deliver up 17lbs 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. A boost control-cut was fitted to disable the boost control and give the pilot direct control over the throttle valve. With modification to use 100 octane fuel the boost control cut-out was modified to limit the boost to +12 even if the boost control was disabled. The pilot subsequently could increase from +6.25lbs boost to +12lbs boost for emergency (= combat) purposes by enabling the boost control cut-out, which was the ‘safe’ power that the engine could produce when 100 octane was used. The use of emergency boost was limited to 5 minutes. The boost control cut-out was wired to indicate to the ground crew that it had been used and to make necessary checks. The Spitfire had a red lever at the front of the throttle control that was flipped and the Hurricane had a valve at the instrument panel 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 and had two systems to increase the boost above rated boost of +9lbs.

a. Boost control cut-out
The boost control limited the boost to the rated +9 lbs of the Merlin XII. The boost control cut-out was fitted to achieve emergency (= combat) power of +12lbs boost (see pilot notes) for 5 minutes up to full throttle height. The use of emergency boost is reported since 21 August 1940, 8 days after the Spitfire II entered service.

b. Gate control
In addition a gate control was fitted to achieve a special take-off boost. The gate control was enabled by pushing the throttle above the rated position through the gate. The gate control set the throttle valve to a predefined opening to achieve 12-12.5lbs boost. As the opening is not adjusted with the decreasing atmospheric pressure at altitude the boost will fall off very quickly. The take-off boost was limited to 1 minute and 1000ft altitude.

[Kurfürst]

Emergency / Combat boost was +9 on the Spitfire during the BoB, not +12.

[NZtyphoon]

Originally Posted by Kurfürst

Emergency / Combat boost was +9 on the Spitfire during the BoB, not +12.

Utter nonsense, of course

http://www.spitfireperformance.com/ap1590b.jpg

http://www.spitfireperformance.com/dowding.pdf

http://www.spitfireperformance.com/lane-26-5-40.jpg

http://www.spitfireperformance.com/webster-28-7-40.jpg

http://www.spitfireperformance.com/gribble-12lbs.jpg

http://www.spitfireperformance.com/elliott-9-9-40.jpg

http://www.spitfireperformance.com/McMullen-15oct40.jpg

[Kurfürst]

Correction: Spitfire II limitations for BoB were +9, not +12.

See:

[Robo.]

Originally Posted by Kurfürst

Correction: Spitfire II limitations for BoB were +9, not +12.

I am sorry but you're wrong. +12 was used in Spitfires Mk.II during BoB.

[Kurfürst]

Originally Posted by Robo.

I am sorry but you're wrong. +12 was used in Spitfires Mk.II during BoB.

And the source for that..?

[41Sqn_Banks]

The use is reported on:
- 21 August 1940 http://forum.1cpublishing.eu/showpos...postcount=1441
- 2 and 30 November 1940 http://forum.1cpublishing.eu/showpos...8&postcount=55

The page from the Pilot's Notes is from June 1940. Spitfire II entered service on 13 August 1940.

[Kurfürst]

Originally Posted by 41Sqn_Banks

The use is reported on:
- 21 August 1940 http://forum.1cpublishing.eu/showpos...postcount=1441

Only mentions "emergency boost", at 4000 feet, does not specify boost used/obtainable.

- 2 and 30 November 1940 http://forum.1cpublishing.eu/showpos...8&postcount=55

Only mentions "boost cut-out", at 4500 feet, does not specify boost used/obtainable.

The page from the Pilot's Notes is from June 1940. Spitfire II entered service on 13 August 1940.

And the point is?

+12 lbs boost was only usable for take off and at low altitudes, and it fell of rapidly, in contrast to Spitfire I pilot notes which notes its effective up to rated altitude. There was no 'two different' sytems on the Spitfire II during BoB.

See:

[41Sqn_Banks]

Originally Posted by Kurfürst

There was no 'two different' sytems on the Spitfire II during BoB.

AP 1565B, Vol I, Section 8 was issued in August 1940 and mentions the "two different" systems. "6. The quadrant plate has a ... gate for the throttle lever in the take-off position". And in paragraph 7 mentions that the "red-painted thumb lever can be pushed forward in emergency".

[JtD]

+12 lbs boost was only usable for take off and at low altitudes, and it fell of rapidly, in contrast to Spitfire I pilot notes which notes its effective up to rated altitude.

Now how would they technically do that, make the boost of 12" drop off quickly instead of having it follow the outside pressure? What did the automatic boost control cut out do on the Spitfire II if not disable automatic boost control?