Black 6 Thu 19th Apr update re Spit II

[camber]

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:

Combat reports, official documents and literature of the period are replete with accounts of pilots using +12 lbs. emergency power, Hurricane as well as Spitfire, expressed variously as; breaking the wire, pulling the plug, pulling the tit, pushing the throttle through the gate, boost overide, boost cut-out, Emergency, Emergency power, etc.

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

[41Sqn_Banks]

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.

[camber]

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

[41Sqn_Banks]

Originally Posted by camber

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.

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 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.

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.

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

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.

[41Sqn_Banks]

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

Gate Control
On gated engine a high take-off boost is obtained by going through the gate, the pressure obtained may be of the same nominal value as emergency boost, +12 lb, but the method of control is entirely different. With pilot's lever at gate, relay piston has moved almost fully rearward to control at Rated Boost - +9 lb. The action of going through the gate opens the throttles a positive predetermined amount, causing boost to rise. Regulator piston will then complete its rearward travel in an endeavour to hold boost down to 9 lb, but has insufficient range to do so and has no effect.

As through the gate is a fixed throttle position, boost obtained will vary in ratio to the day's atmospheric pressure, and will fall with altitude, the regulator having no control at all under these conditions, although it would maintain the boost when value has fallen to +9 lb. This condition should never arise, as "through the gate" should only be used to 1000 ft.

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.

[camber]

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

Originally Posted by 41Sqn_Banks

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.

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

[IvanK]

"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.

[41Sqn_Banks]

Originally Posted by camber

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

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.

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!

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.