I don't think that the supercharger speed could be set from the cocpit. However the system could certainly be. Supercharger speed on the 601 was regulated by two oil pumps. The amount of oil in the sytem decided oil pressure, and thus the amount of slip, and so supercharger speed.
One pump supplied oil at a constant rate. That's essentially a fixed speed, that's why the DB power curve near SL is a straight line.
The second pump supplied oil at a variable rate. The rate was decided by a barometer (increasing with altitude). It did not operate at all until about 1.5 - 2 km, this is where the "smooth" power curve typical of DB engines begin.
If you switch this second pump off, you essentially have the low speed gear (Bodenlader, the Brits used MS gear for that term).
If you set this to deliver maximum amount of oil, the supercharger will run at max speed at all altitudes, it's essentially like a 2nd or high altitude gear (Hohenlader), exactly as on a two speed Merlin's FS gear. Naturally like the previous, it will develop less power at low altitude, hence lower speeds will be achieved (since running the s/c at full robs the engine of power, and there's a large loss power from increased charge temperatures).
A couple of later tests (i.e. Rechlin G-6 curves) show this was used sometimes, I believe because setting up the supercharger to minimum/maximum speed like that ensured that irregularities with the hydraulic coupling and supercharger speed did not interfere with accurate measurements, and SL speed and rated alt. speed could be easily measured. The interim was easy to interpolate, given the engine curves.
I pretty sure the same was followed by the Swiss and possible the "Handbook" test (which I believe may possibly replicate a rechlin trial's result). The US tests were quite clearly done this way, too.
Otherwise Quite simply no amount of drag or power difference between individual planes can explain no less than 40 kph top speed at low levels, while high altitude speed is pretty much the same in all tests. That alone rules out DRAG differences, since those would show up at all altitude, so it's NOT any perceived difference between the V15a 'prototype'
Propeller was the same as the serial type, it's clear from V15a papers.
That leaves POWER. To have such huge differences in speed, the power differences have to be present only at low altitudes, and has to amount to a massive scale, like 100-150 horsepower less. There was simply no such individual difference between production engines.
And here's the problem. V15a's engine was bench tested, which shatters any speculation about how the supercharger worked or not. They measured about 1015 PS in flight, which is the similar to the normal production output of the 601A series. That 1015 PS is the same power wheter it's a pre-production or production engine, or if Willy trained squirrels to drive the propeller.
There was, however, such huge difference between the high/low supercharger gears of the DB 601A.
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