Black 6 Thu 19th Apr update re Spit II

[41Sqn_Banks]

Originally Posted by Kwiatek

Not true see my post above with engine settings from Spit Mk II manual from July 1940 not November or later time.

The July 1940 page only allows +12 boost for "take-off". As said, "take-off" boost is not the same as "emergency boost". "Take-off" boost was only allowed below 1,000 feet or for 1 minute whichever applies first. It was activated by a gate at the throttle. Pilot's Notes General actually says it is not effective up to any considerable height.

"Emergency boost" was allowed for 5 minutes and was effective up to FTH and was activated by the boost control cut-out, but this is nowhere mentioned in the Spitfire II manual, except for the posted page which is dated after December 1943. Of course it doesn't mean it was introduced after December 1943, it only shows that it wasn't cleared in June 1940.

Please see the relevant pages from the Spitfire II manual.

[IvanK]

It is my belief that that the Boost cut out "lever/catch" actually prevents the the throttle from being pushed past the gate. So if you want +12lbs be it for take off or in flight you still need to operate the Boost Cut out catch, this uncovers the gate and allows further throttle movement forward. With the catch in place the the Gate limits the throttle movement so max boost is the rated value (+9Lbs for the Spit MKII and +6.25Lbs for the MKI).

The division of Take off use and in flight use in the engine limitations section of the notes does not imo amount to much other than stipulating the desired 1 min take off limitation.
In other words Take Off Boost and Emergency boost are two different sets of limitations but the mechanism is the same. .... i.e. activate boost cutout to enable +12Lbs to become the max limited Boost.

Jpg below is from Spit MKI pilots notes dated June 1940

As can be seen this indicates removal of the Boost Cutout catch allows further forward throttle movement to achieve up to +12Lbs boost.

looking in the MK I manual to the same fuel system schematic as Banks illustrated above shows no gate per see in the MKI quadrant. As does the Throttle drawing in the MKI notes:




So a change in quadrant design seems to have occurred between the MKI and MKII. This commensurate with the increase in rated boost to +9Lbs in the MKII (as against 6.25in the MKI). However I don't believe this makes any real difference how you get +12Lbs boost in either case Boost Cut out switch needs to rotated forward.

I surmise that the inclusion of the gate in the MKII simply provides the pilot an additional tactile feedback for Rated (+9lbs boost in the Spit II) throttle position. So that if he has the the Boost cutout catch rotated forward then the Gate provides a tactile stop to +9lbs rated position.
In combat the pilot might enter the fight with Boost Cutout switch already rotated forward ready to go, so he doesnt have think about finding and pushing the cutout switch forward in the heat of combat. Then he can easily get +12 by simply rocking the throttle past the gate and pushing it all the way forward. If not in dire straits then he knows pushing the throttle to the gate will give him rated +9lbs boost.

Getting back to limitations these are not Instant failure values but rather wear and tear values on the engine.

[41Sqn_Banks]

Hello Ivan, I strongly disagree.

The division of Take off use and in flight use in the engine limitations section of the notes does not imo amount to much other than stipulating the desired 1 min take off limitation.

It's clear that they are two different mechanisms. Both increase the boost (and deactivate the boost control) but the "take-off" boost additionally uses "excessively rich mixture":

As can be seen this indicates removal of the Boost Cutout catch allows further forward throttle movement to achieve up to +12Lbs boost.

The Spitfire I manual states that "and so open full throttle at the carburretor at any time". It's not speaking about the throttle lever but the throttle valve, which can't be opened at any time when the boost control is enabled.

In combat the pilot might enter the fight with Boost Cutout switch already rotated forward ready to go, so he doesnt have think about finding and pushing the cutout switch forward in the heat of combat. Then he can easily get +12 by simply rocking the throttle past the gate and pushing it all the way forward. If not in dire straits then he knows pushing the throttle to the gate will give him rated +9lbs boost.

As soon as the switch is rotated the boost control is deactivated and and at low altitudes almost any throttle lever setting will give +12 boost.

Also looking at the throttle control drawings it don't see how the switch would prevent the throttle from moving to the most forward position or through the gate.

If both things were the same, how was it possible that in the updated Spitfre II manual take-off boost is given as +12.5 and emergency as +12?

[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

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.

[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.

[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?

[camber]

Hi all,

This previous thread has a lot of hashing out of the mechanics of boost control in Merlins:

http://forum.1cpublishing.eu/showthread.php?t=29031

A quick summary of what I concluded from the discussion:

* The Merlin (e.g. III) has a mechanical supercharger, which can deliver about 20psi boost at sea level (predetonation, power loss and engine damage will result even if 100 octane fuel was used)

* The early boost controller limits boost to +6 1/4 psi, by altering the mechanical relationship between the pilot's throttle handle and the actual throttle plate just before the supercharger. The original red tab pulls a cable which alters air channels in the controller and gives full throttle control back to the pilot, in case of controller malfunction.

A careful pilot could pull the red cutout, get full throttle control and manually "sneak" the throttle up to true maximum power (i.e just before predetonation drops it again), at some risk to the engine.

* With 100 octane the boost control system was modified with extra drilled air channels that made the red tab not a true boost cutout, but instead increased the boost pressure setpoint to +12psi when activated.

There is a second method of boost override possible, the gate system which appears in the Spit II. Instead of a pressure setpoint increment, this gives a maximum throttle position. (E.g you can have 75% throttle even if the resulting boost is greater than the controller setpoint). So it is useful to give a set higher boost at sea level which will decay as altitude increases (i.e "take off boost")

There is no problem with having both systems at once, I did not understand the system well enough to work out effects of activating both at once!

camber