Black 6 Thu 19th Apr update re Spit II

[41Sqn_Banks]

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.

Yes the 1st Edition from June 1940 is much shorter and only deals with propeller and supercharger handling.

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.

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".

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.

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.

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 ?).

Exactly, the drilled hole limits boost to +12 even if boost control is disabled.

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.

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.

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.

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.

[klem]

If I understand you correctly you are discussing the mechanical method of overriding the boost cutout on the Spitfire MkI. (When and where you would use +12lbs boost is going to be a choice of the pilot whether it be takeoff, combat or just extra performance).

Regarding the mechanics, does this help?

The June 1940 Spitfire Pilots Notes state:

Boost Cut_out EMERGENCY control:

36. If it is desired in an emergency to override the automatic boost control, this control can be cut-out by pushing forward the small red-painted lever (17) at the forward end of the throttle quadrant. The lever is sealed against use.

It does not mention a 'gate'. I believe the 'seal' was a thin wire which would be broken when the boost override was operated.

[Moggy]

Is that in case the pilot fail to report using the boost klem?

[klem]

Originally Posted by Moggy

Is that in case the pilot fail to report using the boost klem?

As a 'tell tale' the ground crew would know it had been used. I'm fairly sure I have read of that being the case but I would have to trawl through about 20 autobiographies

This page
http://www.spitfireperformance.com/spit1vrs109e.html
refers to 'breaking the wire' but I can't put my hand on definitive data.

[IvanK]

Klem the "Gate" it would appear only came in to use with the SPIT II. The Throttle Quadrant on the MKI didn't have a Gate.

Camber thanks for reminding us of that thread and your summary. Great reading.

[Moggy]

Originally Posted by klem

As a 'tell tale' the ground crew would know it had been used. I'm fairly sure I have read of that being the case but I would have to trawl through about 20 autobiographies

This page
http://www.spitfireperformance.com/spit1vrs109e.html
refers to 'breaking the wire' but I can't put my hand on definitive data.

No need for that klem, it makes perfect sense what you're saying.

[JG5_emil]

Ivan/Banks
"up to the rated altitude this will increase boost (about 12lb./sq.in. at sea level)."

It looks to me that 12 Lb boost is only possible at or close to sea level and that the amount of boost decreases with altitude, which makes sense to me. Is this correct or wrong?

[fruitbat]

Originally Posted by JG5_emil

Ivan/Banks
"up to the rated altitude this will increase boost (about 12lb./sq.in. at sea level)."

It looks to me that 12 Lb boost is only possible at or close to sea level and that the amount of boost decreases with altitude, which makes sense to me. Is this correct or wrong?

To quote 'The Spitfire Story' by Alfred Price,

Re 100 octane fuel and boost,

Although this gave no improvement at or above the 16,500 foot full throttle height of the merlin II and III engines fitted to spitfires, below that altitude the maximum boost pressure could be raised from 6 1/4 pounds to 12 pounds without causing detonation in the cylinders; the resultant extra power increased the maximum speed by a useful 25 mph at sea level and 34 mph at 10,000 feet.

[41Sqn_Banks]

[ATAG_Bliss]

It'll be interesting to see how the new FMs fly. They can show us all the graphs they want, but I still think it's a little premature to argue over the picture until we've experienced new FMs 1st hand.

Good bit of info though.