109 prop pitch (rpm) and the supercharger

[Kurfürst]

Originally Posted by NZtyphoon

still it is correct that the hydraulic coupling of the DB supercharger led to problems with overheating that the pilots needed to control by constantly altering rpm - not maintaining it.

Nope, it's incorrect. Hydraulic couplings generate more heat to the oil during slip (friction generating heat), but it doesn't mean the oil was overheating, because

a) the fact that increased oil cooling requirements could be and were compensated by increasing oil cooling capacity (note the rather sizeable oil radiator on the 109)

b) the fact that at and above FTH the hydraulic coupling has minimum slip (in the order of about 3%) and therefore, the heat load is only marginally different from a fixed ratio mechanically geared supercharger. If there's no extra friction, there is no extra heat, simple as that.

This is evident from DB heat charts, i.e. the DB 605A lubricant heat transfer was 65 000 kcal/hour at sea level, when the hyd. supercharger was operating at maximum slip, but only 43 000 kcal, or roughly 2/3s at FTH, where the hyd. supercharger was operating at minimum slip.

Wrong; the pilots were needing to adjust rpm and pitch constantly to periodically rest the supercharger - any gain in speed was a by-product, not a tactic.

Bull on all accounts.. the supercharger itself does not "overheat", its just a piece of magnesium alloy and does not operate at extreme temperatures, neither does it have any sort of forced cooling.

Secondly, increasing revs by about 200 rpm _was_ a sanctioned tactic that increased the supercharger capacity and altitude output of the engine, as noted in the November 1940 LWHQ notice that has been already posted, and led to some noteworthy speed increase above rated altitude, as noted by the 109F manual. And if the speed increases, the pitch angle does need to be changed of course, just as at any rpm and at any altitude, when the speed increases.

All they did was manipulating the pitch to let rpm increase, and then - by when the rpm has increased - manipulating pitch to compensate for increase airspeed AND maintain increased rpm.