Bug 174 on 12lbs boost. Review please.

[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.