Bug 174 on 12lbs boost. Review please.

[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