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Originally Posted by IvanK
Greetings Banks. Good snippets in para IV excerpt especially.
These are however from the Generic or Pilots Notes General (AP2905) so arn't specific to the Spitfire installation. Though I do see your point there. I am trying to find these sections in my copy of the AP2095 (2nd addition dated April 1943) but so far cant... I guess thing s changed between the early and second versions.
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Yes the 1st Edition from June 1940 is much shorter and only deals with propeller and supercharger handling.
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Same goes for the discussion of excessive rich mixture for Take off boost its a generic statement. In short I believe it a discussion on Mixture use versus Boost but not an indication of a specific systems design.
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I agree. It doesn't support my statement very well. However a higher mixture might explain the difference between +12.5 "take off" and +12 "emergency".
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Reading the Spit II notes I see what you are saying about 2 systems. The manual states in the take off section that max boost can be obtained by pushing past the gate no mention is made of the Boost Cutout switch in this circumstance. So I agree with you here and I was incorrect. So the gate gives +9Lbs. Pushing past the gate on Takeoff will give you +12.5Lbs but with ABC still operative.
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Yes that's the way I see it. It is however stated that "take-off" boost also disables the boost control, so basically it seems to do the same thing as the cut-out only by a different "button".
I wonder why the "early" limitations give +12 for takeooff and the "later" limitations give +12.5 and if both disable the boost control why does one give +12 and the other +12.5.
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In Flight however as you said Boost cutout (ABC now not operative) gives you only +12Lbs ... so there is still something limiting you to +12Lbs .. (The mod driilled Bleed holes in the ABC I presume ?).
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Exactly, the drilled hole limits boost to +12 even if boost control is disabled.
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What would happen say at 2000Ft if you simply pushed full throttle past the gate +12.5Lbs or +12Lbs ? Why this diff between +12.5Lbs and +12lbs ... no idea. I need to do some more homework here.
I will delve into the Merlin manuals and the various early Spit maintenance manuals to see if there is anything more specific.
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Unfortunately I don't know and so far didn't find anything about it. Pilot's Notes General says the "take-off" boost is not effective above that altitude (I don't think this means above FTH as I would consider FTH as a "considerable altitude"). Note that other engines have the same limitations (e.g. DB601A 1 minute take-power was only allowed below 1-1.5 km).
My only explanation would be that a really rich mixture was used for take-off that wouldn't be possible above that altitude.
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Of note is the Spit II Merlin XII throttle system is different to the Spit MK I Merlin II/III system
What manual is the last jpg from the one with the line drawings ? This is dealing with a 2 speed supercharger running MS and FS gear (file name implies Merlin XX) , its not relevant to the Merlin XII or II or III as fitted to the Spitfire I and or II.
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Yes the Merlin III doesn't have a special take-off power and therefore no gate. The drawing is from "Pilot's Notes Merlin XX" (
http://www.flight-manuals.com/merl-xx-pn.html). As can be seen the Merlin XX has a similar system as the Merlin XII and as seen here:
http://www.wwiiaircraftperformance.o...s-10june40.jpg
http://www.wwiiaircraftperformance.o...xx-15nov40.jpg
the "take-off" boost (by gate) was cleared in June 1940 and "emergency boost" (by cut-out) was cleared in November 1940.