Black 6 Thu 19th Apr update re Spit II

[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.

[camber]

Hi Gentlemen

Originally Posted by IvanK

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.

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

Originally Posted by 41Sqn_Banks

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.

That makes sense

Originally Posted by 41Sqn_Banks

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.

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

[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.