Bug 174 on 12lbs boost. Review please.

[JtD]

+12 lbs boost was only usable for take off and at low altitudes, and it fell of rapidly, in contrast to Spitfire I pilot notes which notes its effective up to rated altitude.

Now how would they technically do that, make the boost of 12" drop off quickly instead of having it follow the outside pressure? What did the automatic boost control cut out do on the Spitfire II if not disable automatic boost control?

[Kurfürst]

Originally Posted by JtD

Now how would they technically do that, make the boost of 12" drop off quickly instead of having it follow the outside pressure? What did the automatic boost control cut out do on the Spitfire II if not disable automatic boost control?

As I understand, the cut out as its name suggest disabled to automatic boost control and gave direct control to the pilot over the boost. Maximum forward position on the throttle was set to open the cross section for +12 lbs at SL, and naturally this fell off with altitude; the pilot would have to open even further to compensate, which was however physically impossible - the throttle could not be pushed even further, it was already in the end position.

This graph suggest (see boost falling) that +12 fell back to +9 within about 2500 feet. This would also suggest that using the boost cutout was quite useless above that altitude, since it gave you the same performance as not using it at all.

http://www.spitfireperformance.com/p7280-speed.jpg

Come to think of it, it seems to work exactly the same as the 109E's 1-minute rating, altough that latter was automated.

[41Sqn_Banks]

Originally Posted by Kurfürst

As I understand, the cut out as its name suggest disabled to automatic boost control and gave direct control to the pilot over the boost. Maximum forward position on the throttle was set to open the cross section for +12 lbs at SL, and naturally this fell off with altitude; the pilot would have to open even further to compensate, which was however physically impossible - the throttle could not be pushed even further, it was already in the end position.

This graph suggest (see boost falling) that +12 fell back to +9 within about 2500 feet. This would also suggest that using the boost cutout was quite useless above that altitude, since it gave you the same performance as not using it at all.

http://www.spitfireperformance.com/p7280-speed.jpg

Come to think of it, it seems to work exactly the same as the 109E's 1-minute rating, altough that latter was automated.

The "gate control" set a fixed throttle valve position. Indeed this was not adjusted by decreasing atmospheric pressure and thus would fall off with altitude as you describe.

The "boost control cut-out" did not set a fixed throttle valve position. The position was directly controlled by the pilot but limited the opening of the valve that +12 boost was not exceeded. Thus when the throttle was fully forward (not in the gate position) the opening would increase with decreasing atmospheric pressure until it is fully open (at FTH).

This is explained here: http://www.enginehistory.org/Piston/...erlinABC.shtml

[camber]

Originally Posted by 41Sqn_Banks

The "gate control" set a fixed throttle valve position. Indeed this was not adjusted by decreasing atmospheric pressure and thus would fall off with altitude as you describe.

The "boost control cut-out" did not set a fixed throttle valve position. The position was directly controlled by the pilot but limited the opening of the valve that +12 boost was not exceeded. Thus when the throttle was fully forward (not in the gate position) the opening would increase with decreasing atmospheric pressure until it is fully open (at FTH).

This is explained here: http://www.enginehistory.org/Piston/...erlinABC.shtml

Exactly correct as I understand it from data posted in previous thread:

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

One thing about the Spit II throttle quadrant diagram posted by Banks in this thread is that it does not include the cable attached to the red tab going to the boost control cutout, without the cable it is easier to confuse it for some type of physical barrier (as the CloD devs did).

The Spit II is rather confusing with two separate systems (gate take off boost and boost override), especially as they give approx the same boost (+12psi) in quite different ways, from controls (throttle gate; red tab) that are about an inch away from each other. I think CloD should probably not model the take off boost system anyway, it is not really relevant for combat.

It also creates confusion that +9psi is the normal full throttle boost for the II (as +6 1/4 psi is for the Spit I), and does not require the boost override (which gives +12psi). CloD gets this rather wrong by modelling Spit II boost behaviour like a +6 1/4 psi Spit I with +9psi instead of +12 psi after boost override. On the bright side, the (post patch) Spit II sea level speed at +9psi is about right (even though the boost is acheived incorrectly), and the 109s are (unhistorically) slow enough to compensate for the lack of +12psi and give a most fun 109/Spit II matchup (at around 30kmh too slow for both ).

camber

[JtD]

Originally Posted by Kurfürst

This graph suggest (see boost falling) that +12 fell back to +9 within about 2500 feet. This would also suggest that using the boost cutout was quite useless above that altitude, since it gave you the same performance as not using it at all.

This graph shows that 12lbs boost would be available below about 15k feet, 2.5k feet below the full throttle altitude of 9lbs boost. That's how it works with outside pressure. And that's what being made possible with the abc cut out, see 41Sqn_Banks link.

[Kurfürst]

Originally Posted by 41Sqn_Banks

The "gate control" set a fixed throttle valve position. Indeed this was not adjusted by decreasing atmospheric pressure and thus would fall off with altitude as you describe.

The "boost control cut-out" did not set a fixed throttle valve position. The position was directly controlled by the pilot but limited the opening of the valve that +12 boost was not exceeded. Thus when the throttle was fully forward (not in the gate position) the opening would increase with decreasing atmospheric pressure until it is fully open (at FTH).

This is explained here: http://www.enginehistory.org/Piston/...erlinABC.shtml

The article decribes various systems used on the Merlin in general, not the Merlin XII in the summer of 1940, which the question at hand.

You have just posted the August 1940 manual of the Spitfire II, what engine limitations does it show? It would settle the matter quickly, wheter there was any change compared to the June/July manual, which clearly states +9 lbs for all out.

After all, this is what its all about. The fact alone that there's a boost control cutout doesn't give a single idea about the permissable boost, which as noted was set as +9 in the earlier manual. We need to know if this was changed or not.

It may well be a simple emergency override for manual boost control, as was its original function, i.e. a the pilot manually controlling boost depending on altitude, which may well allow him to overboost and damage the engine.

Originally Posted by 41Sqn_Banks

Both positions of the red painted lever are shown in the drawing. I painted red lines into the drawing to show that the lever doesn't obstacle the gate position in any of the two positions.

Hmm, you are probably right, at first the little notch seemed to be getting into the throttle's way, and it would be logical, given its location, but come to think of it, the notch is probably just to set the angle of the red lever itself against the t. quadrons when its not engaged.

Originally Posted by JtD

This graph shows that 12lbs boost would be available below about 15k feet, 2.5k feet below the full throttle altitude of 9lbs boost. That's how it works with outside pressure. And that's what being made possible with the abc cut out, see 41Sqn_Banks link.

IF the ABC cutout was set to maintain +12 lbs boost and not +9, that is. Currently it seems as per the July 1940 manual that the ABC cotout was maintain +9 lbs boost. But as the evidence stands, the ABC cutout was unlikely to have been modified until 1941, when +12 was cleared for 5-min all out level.

Yes theoretically its possible for the supercharger to maintain that boost, but certainly not with the gate control.

[JtD]

Originally Posted by Kurfürst

IF the ABC cutout was set to maintain +12 lbs boost and not +9, that is. Currently it seems as per the July 1940 manual that the ABC cotout was maintain +9 lbs boost. But as the evidence stands, the ABC cutout was unlikely to have been modified until 1941, when +12 was cleared for 5-min all out level.

Please show the evidence for a single Merlin engine ever being fielded with a boost control cutout modified for 9lbs boost. I may then consider this hypothesis of yours for an argument.

But IF we were talking hypotheticals, it's still totally absurd to assume that the boost control cut out has no effect if used under full throttle conditions. What a bunch of idiots the engineers must have been.

Curious to see evidence.

[41Sqn_Banks]

Originally Posted by Kurfürst

But as the evidence stands, the ABC cutout was unlikely to have been modified until 1941, when +12 was cleared for 5-min all out level.

You do have evidence for the clearance of +12 boost for 5 minute "all out" in 1941? This would obviously close the case.

[NZtyphoon]

Originally Posted by Kurfürst

It may well be a simple emergency override for manual boost control, as was its original function, i.e. a the pilot manually controlling boost depending on altitude, which may well allow him to overboost and damage the engine.

The Merlin was always equipped with an automatic boost control which prevented the engine being overboosted ie; with 100 Octane fuel it was potentially capable of reaching +17 lbs boost but the ABC limited it to +12 lbs for the Merlin II and III

Originally Posted by JtD

This graph shows that 12lbs boost would be available below about 15k feet, 2.5k feet below the full throttle altitude of 9lbs boost. That's how it works with outside pressure. And that's what being made possible with the abc cut out, see 41Sqn_Banks link.

The notes posted by Kwiatek show that for the later 100 octane planes where use of 12min for 5min instead of 3min was allowed the effectiveness of the boost is up to 10.5 kfeet

To my understanding one could go to 9 lbs without physical restriction on the throttle but had to push through the override (applying a small extra force?) in order to go to 12 lbs. Once engaged there was no more indication to the pilot through "feel" if the throttle was at 9 lbs or 9.5 lbs. Of course he would have to be carefull when using 9 lbs also.