Arggh, I have just read through the SpitII pilot's notes in more detail and think I must review my idea of how the boost control evolved from Spit I, to modified Spit I and finally to Spit II.
The Spit I originally had boost controlled by the single datum controller at +6 1/4 psi, this has some rather odd effects for the pilot in that sometimes you would have direct throttle control, sometimes the throttle lever would appear to ignore you if the boost controller was manipulating the throttle valve to maintain rated boost. Makes station keeping rather difficult! With 100 octane the boost controller cutout was cleverly modified so that instead of getting direct throttle control after pulling the red lever, you got a new boost setpoint of +12psi for combat use.
Originally I considered the Spit II worked the same way as the modified I, with the further addition of a throttle gate (an alternative "take off" override that gives extra boost at sea level but will decay quickely to the controlled value as you climbed).
But reading through the Spit II notes it seems clear to me that the system is different. The boost controller is now the variable datum type with a maximum rated value of +9psi. Although the throttle handle still feels like a throttle to the pilot, it is actually one step removed from the real throttle valve. The boost controller actually sets the throttle valve to maintain the boost corresponding to the handle position as you climb, a very intuitive system. The "throttle" handle is actually now a "boost" handle.
So to introduce some speculation: a pilot is flying a Spit II in the combat area at +7 psi boost (handle is not at maximum), and sees a 109 in threatening (or vulnerable!) position. To get +9psi boost he only needs to maximise the handle quickly. The extra gated throttle portion actually has no further effect except at very low altitude. But at any altitude if he goes through the gate, he gets tactile feedback that he has accessed the highest boost allowed.
But this leaves an extra point...what does the red cutout do now? It is still there in the notes, and appears to have reverted to it's original purpose...a true boost cutout (sealed, presumably with a wire) that disables all boost control as per the original MkI configuration. If the pilot pulls this at full throttle, he will get whatever maximum supercharger output is possible, which will likely be counterproductive for combat.
To me this seems logical, but I would value other opinions. The original MkI modification is an inspired seat of the pants "kludge", making the boost cutout into a control for going between two boost setpoints. With the subsequent Spit II on 100 octane and variable datum boost control, it makes sense to have a more designed system where the throttle controls boost up to the combat emergency level (+9psi), with the extra feature of the gate and "take off boost" to +12.5 psi.
Once the Spit II is approved for +12 psi during combat, it is a simple matter to alter the variable datum controller to give +12psi at full throttle handle application instead of +9psi. But I don't have a technical reference. If I am correct, the red tab is now untouched in combat, unlike in the modified Spit I. However, it could also be that the boost cutout is modified as per the MkI, and gives the +12psi when pulled.
Overall this system also seems to fit with this quote from Mike William's website:
Quote:
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Combat reports, official documents and literature of the period are replete with accounts of pilots using +12 lbs. emergency power, Hurricane as well as Spitfire, expressed variously as; breaking the wire, pulling the plug, pulling the tit, pushing the throttle through the gate, boost overide, boost cut-out, Emergency, Emergency power, etc.
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A Hurricane pilot of course gets to pull to plug or the tit.
A Spit I pilot would operate the modified boost cutout, breaking it's wire
A Spit II pilot would advance his throttle through the gate (although at any significant height, to the beginning of the gate would have the same effect). He leaves the red tab alone.
Cheers, camber