Black 6 Thu 19th Apr update re Spit II

[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.

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.

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. 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 ?). 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.

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.

[IvanK]

Some relevant bits to this discussion on ABC mechanics on Merlin II/III from this manual:







Further complication arises since reference to both Fixed Datum and Variable Datum Boost control is made. I think its safe to assume in the case of the Spit II it would be the late variable datum type. In either case with ABC out you are only going to get +12lbs thanks to the Bleed holes (Mod Merlin 154)

A little more on the gate issue from the 2nd addition of the AP2095:

So the rocking past the gate (for Take off) is in effect disabling the ABC however Mod 154) still ensures that only +12 to +12.5Lbs will be the max obtained.
There is discussion in the Merlin Manual about the Boost controller requiring an initial throttle advance of +0.5Lbs to get it in operation. Maybe this goes some way to explain the 12Lbs v 12.5lbs diff ?



My head now hurts

[41Sqn_Banks]

Originally Posted by IvanK

Further complication arises since reference to both Fixed Datum and Variable Datum Boost control is made. I think its safe to assume in the case of the Spit II it would be the late variable datum type. In either case with ABC out you are only going to get +12lbs thanks to the Bleed holes (Mod Merlin 154)

AP1590B Paragraph 239:



[/quote]A little more on the gate issue from the 2nd addition of the AP2095:
http://i40.photobucket.com/albums/e2...noverboost.jpg
So the rocking past the gate (for Take off) is in effect disabling the ABC however Mod 154) still ensures that only +12 to +12.5Lbs will be the max obtained.
There is discussion in the Merlin Manual about the Boost controller requiring an initial throttle advance of +0.5Lbs to get it in operation. Maybe this goes some way to explain the 12Lbs v 12.5lbs diff ?
[/QUOTE]

IIRC in later Merlins the Boost Control remained active for "Emergency boost" by adjusting the boost control to give higher boost. IIRC Should be explained in Pilot's Notes General 2nd Edition.

[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?