Black 6 Thu 19th Apr update re Spit II
 

http://www.spitfireperformance.com/spitfire-II.html

I pressume the new Spit II speeds that that Black 6 mentioned in the Apr 19th update will be in accordance with the speed tests contained in the attached link.

If I read the data correctly, the Spit II should make 294 mph ASI at 1,000 ft and approx 319 mph (+25 mph) when emergency power boost cut-out is operated; or have I got it all wrong?

As I understand it, the emergency power boost cut-out gave approx 25 to 30 mph extra if operated below the full throttle height of 12,800 ft.

Thoughts?

Talisman

I'm not sure if the emergency boost was cleared for the Merlin XII at that time, e.g. for the Merlin XX it was cleared in November 1940, see http://www.wwiiaircraftperformance.o...xx-15nov40.jpg.

It was cleared.

I got original manual for SPitfire MK II from July 1940 where it is emergency boost +12 boost allowed for take off or for 3 minutes.

There is a link at the bottom of the trial data web page to the pilot manual (1940) which shows 12lbs boost available. It is in very small print.

Talisman

"Take-off" boost is not the same as "emergency" boost. See Pilot's Note General (1st Edition, June 1941):

http://forum.1cpublishing.eu/attachm...2&d=1326733398

The description exactly applies to the Spitfire II. There was a "gate" for "take-off" boost and a "cut-out" for emergency power.

From July 1940


http://i42.tinypic.com/34j38t4.jpg


and another manual (probably later)

http://www.spitfireperformance.com/spit2pnfs3.jpg

I'm sure you are referring to this page.
Note the header where it reads "Amended in Vol. I by A.L. 31 and P.N. by A.L./L". This means the content of the page was changed at a later date.
Judging from the date of the previous A.L. 30, which was issued December 1943, I would guess it is from early 1944 if not later.

The page without amendments looks like this: http://forum.1cpublishing.eu/attachm...9&d=1332086948

Note the "List of Contents" page from June 1940. Para 1 of Section 2 didn't contain "Engine data" but "Introductory notes".

http://forum.1cpublishing.eu/attachm...7&d=1332086792
http://forum.1cpublishing.eu/attachm...8&d=1332086871

See this post for more details: http://forum.1cpublishing.eu/showpos...&postcount=677

Of course this doesn't mean that +12 "Emergency boost" was cleared not before early 1944. I would guess it was cleared in November 1940 at the same time as the Merlin XX.

Talisman, pm me your email addy.

Pstyle

Thanks for posting. Wikipedia says:
Merlin XII (RM 3S)
1,150 hp (860 kW); fitted with Coffman engine starter; first version to use 70/30% water/glycol coolant rather than 100% glycol. Reinforced construction, able to use constant boost pressure of up to +12 psi using 100 octane fuel; Used in Spitfire Mk.II.[76] First production Merlin XII, 2 September 1939.[18]

First production Merlin XII Sep 1939 and trial data from the link is dated May 1940, so I would hope RAF pilots were provided with an emergency boost capability, and other improvements, by the time the first Spit Mk II sqn was fully operational later in the year, or they would have had less capability than the Spit MkI. Amendments to RAF Air Pubs show a vertical line in the margin to mark the place the amended information has been inserted on the pages that are effected; I do not see any vertical lines on the page for the latest amendment. Still perhaps someone will be able to clarify the point you raise. Perhaps some combat reports from Spit MkII sqns will help if any of them mention operating the boost cut-out.

Talisman

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.

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
http://i40.photobucket.com/albums/e2...oostcutout.jpg
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:
http://i40.photobucket.com/albums/e2.../Spitifuel.jpg

http://i40.photobucket.com/albums/e2.../Spit1quad.jpg

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.

Hello Ivan, I strongly disagree.

http://forum.1cpublishing.eu/attachm...2&d=1326733398

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":

http://forum.1cpublishing.eu/attachm...8&d=1334956606


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.


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?

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.

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

http://i40.photobucket.com/albums/e2...AP1590bcvr.jpg

http://i40.photobucket.com/albums/e2...BCMerlinII.jpg

http://i40.photobucket.com/albums/e2...CMerlinII2.jpg

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:
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 ?



My head now hurts :)

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

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".

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.

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

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.

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.

AP1590B Paragraph 239:
http://forum.1cpublishing.eu/attachm...8&d=1334998230


[/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.

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.

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

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

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.

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.

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

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,

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.

Agreed, but i'm already resigned to seeing our Mk I spits on 6 1/4 boost and 87 octane fuel performance unfortunately.

Hi Emil, it is Fruitbat said, a bit of extra info:

The supercharger can deliver about +20psi at sea level with throttle valve fully open, so the boost controller quietly (without moving the pilot handle!) moves the throttle valve to give +12psi if the red cutout is on. When altitude has increased to where the supercharger maximum output is +12 psi, the boost controller cannot control any more (both the pilot's throttle handle and actual throttle valve are at 100%), and the boost will drop from then on.

This will happen a bit earlier than if the boost controller was on +6 1/4 boost, so the full throttle height is lower for +12psi than +6 1/4 psi. It is still >15000 feet though.

The gate system is different though because it gives a set throttle valve position...it will give the higher boost at takeoff, then decay quite rapidly to the lower controlled value (and stay there) as you climb. I think the system should act exactly the same if you are through the gate or not, above a few 1000 feet.

camber

High Banks,

Just to clarify, are you suggesting that the RAF introduced a new MkII version of the Spitfire in the middle of the battle that did not have emergency operational power for combat and thus was slower in combat than the Spit MkI with operational emergency boost engaged?

The Spit MkII test data states the following vs the Spit MkI (N.3171):

4.0 Level Speeds.

.......The top speed of this aeroplane is the same as that of N.3171 but is reached at 17,600 feet, 1400 feet lower than the Mk.I Spitfire. Consequent upon this and the increase in power of the Merlin XII over the Merlin III below full throttle height the aeroplane is about 6 - 7 miles per hour faster at heights less than 17,000 feet and about 4 - 8 m.p.h. slower at heights above 20,000 feet. It should be noted that though the boost pressure on the Merlin XII is +9 lb. per sq.inch as against +6 1/4 lb. per sq.inch on the Merlin III there is little difference in the engine power at heights of 16,000 feet and above.

So, if the Spit MkI is 6 to 7 mph slower at heights less than 17,000 ft, then to my mind the data indicates that the level speed of the Spit MkI at 1,000 ft was 287 mph ( Spit II, 294 mph at 1,000 ft minus 7 = 287). If the Spit I pilot then engages operational emergency boost, he then gets an extra 25 to 30 mph, giving a speed of 312 mph (287 + 25) at 1,000 feet on 100 Octane fuel.

So, with the Spit MkI at 312 mph on emergency boost at 1,000 feet and the Spit Mk II at 294 mph without emergency boost, it is slower in combat than the Spit MkI unless the Spit MkII has emergency boost available. Or am I missing something? Surely the Spit MkII had emergencey power operational boost available to provide and extra 25 to 30 mph for operational emergency, just like the Spit MkI.

For me, the test data shows how the Spit MkI can be bench-marked against the Spit MkII in terms of performance for the CloD dev team. What do you think?

The fact that the CloD sim provides a Spit MkI that only makes approx 240 mph level speed at 1,000 feet, rather than the historic record of approx 287 mph (312 mph with emergency boost on 100 Octane fuel) is very disappointing from a historical accuracy perspective.

Also, surely the boost dial on the Spit Mk II in the CloD sim should read over 8lbs.

Talisman

There could be a reason for the difference; read #1219. I know that there is a set of Spitfire II notes on Scribd; these show that there have been no amendments and it is possible that the operational limits for the Merlin XII were originally set at +9 lbs boost, later pushed to +12 1/2 lbs. (Paragraph 4 of the attached document does state that the Merlin XII was cleared for this boost.) It is possible that on operational units an amendment slip specifying +12 1/2 lbs was issued with the notes.

There is a small possibility of printer error, for example on p3 it describes the Spitfire II as being "powered by a Merlin III..."

Correction: This particular set of notes does incorporate some amendments, on top of page 6 and 19 "Amended by A.L.No.6" - interesting; this indicates they could have been republished later than July 1940.

Yes that's my conclusion from the available data.

Definitely it was physically available and the amended pages confirm it. However we don't know since when the pilots where authorized to use the emergency boost with a Merlin XII. The pages from the Pilot's Notes Spitfire II from the 1940 time-frame that I've seen so far do not authorize the use.

I would be glad if someone could provide a proof since when +12 boost was authorized for emergency conditions and not only take-off.
In addition we don't know what happened if the pilot used the +12 take-off boost for example during combat (engine damage, no boost increase, nothing ... we don't know).

Compare it with the Hurricane II with Merlin XX which was introduces at about the same time (August-September 1490) and had +12 take-off boost authorized since introduction and emergency boost was approved shortly later in November 1940.

Now that's a interesting find. The page without amendments only give +12 boost for take-off, but the 1939 documents states +12.5 boost for take-off.

However it's still clear from the June 1940 "List of content" that the page that contains the +12 emergency boost was not contained at that date and was added later and at that date only the page without +12 emergency boost was contained.

IMHO the easiest would be to get the combat reports of the "units concerned" (pun intended ;) ) to find one that proofs the use of +12 in a Spitfire II or search in the National Archives for a similar doc like the one that clears the use of +12 emergency boost for Merlin XX.

On Spitfire Is the pilot did have to push the throttle lever through a thin wire, which was replaced by the ground crew. If for some reason the pilot failed to notify the ground crew that +12 Lbs boost had been used - a note had to be made in the engine's log book - the broken wire was probably a good way of showing the engine had been subjected to overboost.

With regard to the pilot notes, RAF Air Publication (AP) amendments are well known for being behind the times as far as up-to-date common practise at the front line is concerned. It may be many months and over a year or so before an AP is fully updated to reflect what is actually taking place on the ground and in the air.

A fighting force does not wait for the AP to be updated before taking actions that are operationally required to provide an advantage in combat (AP amendments are not a priority when fighting a war). In the mean time, RAF personnel may be informed and corporate knowledge developed by other means of authorised advanced information contained in a variety of communication methods, such as signals, memos, letters, advanced information leaflets (destroyed once formal amendment leaflet is incorporated into the AP), briefings and local training.

The date an Amendment Leaflet is issued is not an indication of when the subject practise was first authorised or carried out. Moreover, the vast number of aircraft AP copies in existence would not have all been amended with updated changes on the same date; different copies of the same AP held across the RAF at squadron and flight level will have different dates recorded on the amendment leaflet record for the incorporation of the amendment. Also, it is not unusual for amendment leaflets to go missing in transit and for a unit to receive an amendment leaflet out of sequence, or for the AP to have a number of missing amendment leaflets; no system is perfect.

As for the pilot in the air in a life and death situation over his own territory, given a situation where extra boost is available, I do not believe that he is going to wait for authorisation to use extra boost if it enables him to win a fight, survive another day or save the life of another pilot or people on the ground. After all, pilots were known to ram enemy aircraft and such authorisation will not be found in AP pilot notes (tongue in cheek, LOL).

Lane has posted the proof that a Spitfire II used emergency boost on 21 August 1940:
http://forum.1cpublishing.eu/showpos...postcount=1441

That emergency boost is +9 lbs I believe. Its pity the documents do not specify.

And thats just it, thats just your 'belief'.

You know how the mechanism works though, don't you.....

Its a tad bite more than that. The BoB era Spitfire II manual clearly states that the maximum boost allowed in flight is +9 lbs.

Just like the Spitfire I manual states +6.25 as normal maximum boost (better known as "All out" boost by RAF terminology).

The "cut-out control ... allows the normal maximum boost to be exeeded for special purposes ("emergencies")."

http://forum.1cpublishing.eu/attachm...1&d=1335714873

Pilot's Notes General, 1st Edition June 1941.

I don't understand why you guys bother, devs don't.
This is what it looks like we'r getting next patch, Hurricane,Spitfire on 87 octane. Maybe a patch another year it will get 100 octane?
Or is it to avoid english planes used in Moskva map?

Edit; Note the spacing, 400-600, not 400 - 500 ,the difference is huge.

If the overboost came with the correct penalties (high probability of engine damage when overused) for Spit and 109 the problem wouldn't be one.
If everybody was forced to conserve his engine and to keep it well in the green, the peak power would heavily loose in importance, as it would only be advisable to use in emergency or in a chase with air superiority.
Even then, as further away from home as less advisable would be the use of WEP.
Regretfully CoD is still missing a lot in the CEM-part to make this workable.
To give the power now, as it is would give all advantages to the spits, turn and speed, very unrealistic.
Imo, of course

I found this the most interesting from the OP's link.

It was only slightly faster than that Spit I below 17k and slightly slower above 20k if I've read that correctly.

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:

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

Hello camber,

Here is the description of unmodified variable and fixed datum boost control for Merlin II and III engines:
http://forum.1cpublishing.eu/showpos...62&postcount=8
http://forum.1cpublishing.eu/showpos...66&postcount=9
http://forum.1cpublishing.eu/showpos...9&postcount=10

In Short: All Merlin III and majority of Merlin II had variable datum type boost control. It's save to assume that a typical Spitfire I had a variable datum type boost control (except of a really early one with fixed propeller).

The cut-out disables the boost control and throttle valve is controlled directly, however the modification contains a hole that prevents the boost to raise above +12. In case of a malfunction of the boost control the cut-out could still be used for it's original purpose even if modified.

The Spitfire II had both, a take-off gate and a boost control cut-out. Take-off gate was used for take-off boost only, the boost control cut-out for emergency (=combat) boost. "Pilot's Notes General" is very specific at that point, see:

http://forum.1cpublishing.eu/showpos...00&postcount=5

So in combat with the 109 the pilot wouldn't go "through the gate" but use the cut-out.

Note that for later Merlins (my guess would be from 60 series onward) the boost control was not disabled for "emergency boost". The "cut-out" set the boost control to maintain a higher value at each throttle setting.

Hi Banks,

I was hoping you would pop up.

Point taken on variable datum controller.

Rereading the Spit II manual AP1565B I still think that rated boost (highest boost without cutout pulled and not using the gate) is +9psi, and hence a 100 octane only condition.

I admit I am on less certain ground with the hypothesis that the +12psi (when approved) was not delivered for the Spit II via the cutout as per Spit I, but by making it available in normal throttle operation. AP1565B states that Spit II boost cutout is emergency use only (and does not give a boost pressure) which muddies the water a bit. I guess it could be "emergency" combat use or "emergency" boost controller failure (original purpose of the MkI cutout).

However the pilot's notes general link you posted suggest that the red tab WAS used in the Spit II for combat boost. Of course the system could just as easily be set up either way in theory.

The bit about the gate during combat is pure speculation based on the original hypothesis :) If the highest useable boost is available on the normal throttle handle traverse and there is a gate, pilots would tend to go through the gate to ensure they have the maximum available, even if their height meant that the post gate throttle traverse had no effect. Subsequently they would refer to "going through the gate" to get combat boost, which does crop up in combat reports.

Cheers, camber

Yes. AP1565B states for the actions after takeoff that the throttle should be move back to the rated +9. The 87 octane limits were not part of the initial manual and were added later. It's save to assume that the pilot had to maintain these lower limits himself in case 87 octane was used.

I don't have any reference that shows a difference between take-off and cut-out. Both disabled the boost control. Maybe the take-off didn't gave control over the throttle valve but simply opened it completely so the boost jumped directly from +9 to +12 and the cut-out gave direct control over throttle valve to regulate the boost also between +9 and +12.

I'm wonder as well why some pilots called it "going through the gate". If my theory above is correct the pilots could use both methods if they simply wanted maximum boost but had to used the cut-out if they wanted +10 or +11 boost.

Or maybe this is simply referring to earlier engines and it had became a common sentence that was used even if it was technically wrong.

I've found a description of the "Gate Control":

http://www.enginehistory.org/Piston/...erlinABC.shtml

According to this description the boost obtained by the gate control may vary depending on atmospheric pressure, this would explain why the maximum take-off boost is given as +12.5 in the later manual, as this would possibly be achieved on days with high atmospheric pressure.

I think it is evident that the RAF enjoyed "home advantage" in the BoB in terms of managing worn engines, factory output, replacement aircraft availablility, short logistics chain and pilots flying over their own country. Therefore, I do not view worn engines as such a big deal as far as the CloD sim is concerened. However, as I understand it, the main penalty for the pilot in the first instance was a huge increase in fuel consumption if emergency boost was activated, so this must be included in the flight model for 100 Octane Spit and Hurri me thinks.

Talisman

It may have not posed a problem to handle it for the RAF but I do think that they did not change it after every flight. So it should be modelled as one day we may end up with spits flying in the pacific.

http://atouchfarvetched.files.wordpr...ntpanic_11.jpg


Much better to have a massive advantage to blue in speed, climb, roll, firepower and dive right?

That limitation wasn't meant for the long view, exceeding the 1', 5' or 30' limit should bring the immediate high risk of engine trouble and a increasing loss of power, as it was in real life, with temperatures rising over the limits and all, for all planes, blue or red.
The damage to longevity was done when still flying by the book and using overboost.

Btw, the Luftwaffe did change a 109 engine in the field in 15 minutes, the RAF took a day, iirc.

RAF took 2 days and LW 30 seconds. In the snow. Uphill. ;)

I thought the RAF were still replacing engines in Spitfires and Hurricanes? There's a race on to be the first one done. It's the main reason there are so few left flying today actually.

Hi Guys,

Fwiw, a couple of Spitfire II combat reports from 74 Squadron noting use of boost cut out & throttle through the gate in combat.

Sgt. N. Morrison, Spitfire II, 74 Squadron, 2 November 1940
http://www.wwiiaircraftperformance.o...son-2nov40.jpg

P/O H. M. Stephen, Spitfire II, 74 Squadron, 30 November 1940
http://www.wwiiaircraftperformance.o...en-30nov40.jpg

Excellent! Two references which really clarify the Spit II boost operation:

This is a great explanation of the "take off boost" gate. Because the gate represents not an automatically controlled boost level, but an ability to reach a given throttle valve opening without boost control, this override throttle valve opening % must be calibrated for a typical atmospheric pressure at sea level (to give +12.5). So the boost level of +12.5 psi is approximate, under sea level it would get higher :)

Lane:

Those are very useful combat reports. Even though it contradicts my idea that the +12psi was delivered through main throttle operation :( Clearly the Spit II red tab IS used in combat, logically it was to deliver +12psi as per Banks post (although this is not stated explicitly).

Although the gate can only give any benefit at low level, the second report re-inforces my idea that pilots might use it "just in case" at any level. In this case the pilot pulled his boost tab, gated the throttle at maximum rpm, and felt re-assured that he was getting maximum performance!

Very interesting that a Spit pilot would refer to pushing the red boost override tab as "pressing the tit" (second report). Although the Hurricane boost cutout is much more, err, breast like I guess it was just too enjoyable a reference for Spit pilots to not take up too. It starts to explain how those terms get so jumbled up in combat reports.

Lastly, the second report suggests that he can only just keep up with his leader using all boost enhancements available in his MkII. What is blue leader flying if Blue 2 is writing "performance notes on the MkII" and suggesting he had to go all out to keep up? A MkI +12psi? This seems rather intriguing to me!

Cheers, camber

"Lastly, the second report suggests that he can only just keep up with his leader using all boost enhancements available in his MkII. What is blue leader flying if Blue 2 is writing "performance notes on the MkII" and suggesting he had to go all out to keep up? A MkI +12psi? This seems rather intriguing to me!"

I guess the lead was maxed out as well. The convention is the lead would in normal circumstances give the wingman a "bit to play with" by setting a slightly reduced power setting. In a combat tail chase I guess the formation protocols were overlooked.

"Pressing the Tit" is a pretty common (for 40's and 50's" bit of Brit slang for pushing or pressing any button or catch.

My interpretation is that it is calibrated to +12 but on some days would rise to +12.5 and on other only to +11. So there is a 0.5 safety mentioned in the manual so the pilot doesn't panic if boost is higher than +12 on one day.

We should have a closer took to the mechanic of the boost control to understand what happens if gate and cut-out are activate at the same time. I'd suspect that the gate has the upper hand.

Hi Gentlemen

That is a more logical explanation. Perhaps that part of the report is a subtle dig at blue leader :)

That makes sense

I am only going from http://www.enginehistory.org/Piston/...erlinABC.shtml, but my model of the function is as follows:

For the sake of argument I will state that at sea level, +12.5 psi boost is given at 60% throttle valve opening (real percentage unknown). The rated boost setpoint is +9psi, +12psi emergency with red tab pulled.

The pilot takes off with pilot throttle through the gate. The boost controller piston senses that boost is too high (>9psi) and is against "boost too high" rearward stop. But the gated throttle extension pulls the throttle valve to 60% open giving +12.5 psi.

As plane climbs initially, boost remains >9psi so the boost controller piston remains stuck on the "boost too high" stop. If the pilot throttle is kept gated, the resulting 60% throttle valve opening corresponds to less and less boost with height, and finally boost decreases until it is under +9psi.

At that point the boost controller piston has a job to do. It moves away from the "boost too high" stop, opens the throttle valve further (i.e. > 60%) and then keeps increasing it with height to maintain +9psi boost. At 100% throttle valve opening it has reached FTH for +9psi, the boost controller piston is at the "boost too low" stop and boost starts decreasing with more altitude.

At a "combat" height where the boost controller already has throttle valve opening >60% while maintaining +9psi (i.e above the effective take off boost height), what would happen if a pilot seeking more power suddenly goes through the gate?

Effectively nothing, as boost would remain at +9psi. But actually, boost would momentarily rise (how much depending on mechanical dynamic constraints on the control system). The boost rise would be momentary because the boost controller piston would react by moving closer to the "boost too high" stop in order to override the gate input and return the boost to +9psi. The only difference afterwards is that inside the boost controller, the piston is at a different position than before.

If the red tab is pulled, the same applies but the +9psi is instead +12psi.

So my take is generally the boost controller wins, not the gate. The documentation states the take off boost is ineffective over low altitude, but does not warn of adverse consequences if it is used higher. So I still think it is just ineffective and going through the gate is fine in combat (but make sure you pull the red tab, that IS important!).

Cheers, camber

Hi camber,

I don't know what the benefit would be of operating the boost cut-out above full throttle height. I wouldn't think there would be any benefit, but this is far from being the only instance where I've heard of it - beats me.

Blue I in this instance was also operating a Spitfire II. The notable thing, I think, is that the engagement opened with the Spitfires climbing from 29,000 ft. up to 34,000 ft. to attack the Me 109s. Blue 1 was certainly aggressive and confident in his abilities as well as the capabilities of his aircraft as he initiated combat, disadvantaged both in numbers and altitude. I could well imagine Blue I giving the Spitfire everything it had, with Blue 2 doing his best to keep up. Pretty good team work it seems to me and nicely done.

F/Lt. J. C. Mungo-Park, Spitfire II, 74 Squadron, 30 November 1940
http://www.wwiiaircraftperformance.o...rk-30nov40.jpg

As an aside, I've encountered a number of Battle of Britain accounts where the pilots would engage the boost cut-out as part of a routine preparation for the possibility of combat, for example:

Geoffrey Wellum, 92 Squadron:

Things are starting to get rough. Automatically I have followed my self-imposed drill that I always do at times like this. Reflector sight on; gun button to fire; airscrew pitch to 2,650 revs; better response. Press the emergency boost override. Lower my seat a notch and strap tight. Ok men, I’m all set. Let battle commence.

P/O David Crook, 609 Squadron :

It was now obviously a matter of moments only before we were in the thick of it. I turned my trigger on to 'Fire', increased the engine revs. to 3000 r.p.m. by slipping the constant speed control fully forward, and 'pulled the plug', i.e. pushed the small handle on the throttle quadrant that cuts out the automatic boost control thus allowing one to use emergency power.

Bob Doe, 234 Squadron

Once we were in the vicinity of the enemy, I would 'pull the plug', which was the release so that we could get extra boost, but I wouldn't use it, and would start my search.

Tom Neil, 249 Squadron

The familiar stomach-clenching tension and surging wave of excitement. Another check around. Gun-sight on? Gun-button to 'Fire'? Plug pulled and 2,850 revs?

K. W. McKenzie, 501 Squadron

Climbing to gain some height we 'bustered' with the sun behind us and spotted the formations. With the tit pulled for absolute full power we broke formation to attack, sights 'on', guns to 'fire', harness tight, attacking individually.