So if I understood that properly, it sounds like he's saying that the supercharger on the DB 601 had variable speed based on how much oil was in the rotor housing, (or whatever you call it) rather than an actual gear ratio of some kind. Which is interesting.
I had thought there was a maximum and minimum speed ratio for the coupling, but the article says that it was infinitely variable.
Note that there is a minimum slippage of 2.5%, which means that it never is a 1:1 ratio and more power is lost through the coupling than for a set of gears.
However, because it is variable, when the engine is below rated altitude the supercharger turns at a slower speed than the fixed gear ratio, consuming less power, and doesn't need to be throttled, which costs more power.
According to Bill Gunston at low altitudes the DB's supercharger actually consumed more power than it added. I recall reading where a BF-109 pilot sneaked up behind a couple of P-38's in the Med at low altitude. Then the P-38's spotted him and poured on the coal. He said the results were astonishing, "They just disappeared!" It must have been like a UFO encounter, ZIP!On the other hand the "hydromatic drive" on the DB's supercharger conferred some advantages. The V-1650's two stage supercharger was set to switch over from low speed to high speed at 18,000 ft via an aneroid device. Thus, at that altitude the Mustang or Spitfire's engine was surging, requiring the pilot to pull back on the throttle to keep from overshooting his target and then losing power if he lost 500 ft or so. A canny 109 pilot could try to force combat at that altitude where his engine was not switching gears. Some Mustang pilots fighting over Japan had the momentary high speed supercharger test switch replaced with a regular non-spring loaded toggle switch so they could put in in high speed and leave it there.DB's supercharger certainly 'produced' much, much more power than it consumed at low level. Without supercharger, DB's engine would've been making less than 30 in Hg worth of manifold pressure, or under 1000 HP. Low level performance of Allied A/C was significantly boosted due to higher boost pressures allowed by use of hi-oct fuel, that Bf-109s were mostly without.
On the other hand the "hydromatic drive" on the DB's supercharger conferred some advantages. The V-1650's two stage supercharger was set to switch over from low speed to high speed at 18,000 ft via an aneroid device. Thus, at that altitude the Mustang or Spitfire's engine was surging, requiring the pilot to pull back on the throttle to keep from overshooting his target and then losing power if he lost 500 ft or so. A canny 109 pilot could try to force combat at that altitude where his engine was not switching gears. Some Mustang pilots fighting over Japan had the momentary high speed supercharger test switch replaced with a regular non-spring loaded toggle switch so they could put in in high speed and leave it there.
Leaving the S/C gearing in high speed for low-alt missions will provide A/C with less power, not more. There was a reason for 2-speed superchargers, after all.
A non-dumb pilot in a Merlin Mustang, or in a Spitfire with 2-stage engine will try and fly above 20000 ft, or under 15000 ft. It is much easier to dive down at enemy flying at it's best altitude, after all.
Let's also recall that V-1650-3 was a 'mid-alt' Merlin (that would've been 'high alt' for non-RR engines ), while RR themselves also manufactured 'low-alt', 'mid-alt' and 'hi-alt' Merlins themselves. The Bf 109 drivers, good as hey were, were still not able to look through aluminum cowling from half a mile and read whether the plate says 'Merlin 66', 'Merlin 70', or 'Merlin 66'. The Mk 66 making 1600 HP at 16000 ft, for example.
Throttle valves in the updraft carburetor throat were controlled by an automatic boost control through the throttle linkage to maintain the selected manifold pressure with changes in altitude. The valves were only partially open during ground and low-level operation to prevent overboosting of the engine. As air density decreases with increased altitude, the throttle valves were progressively opened in response to the reducing atmospheric pressure. This system provided full power within engine boost limitations up to the critical altitude of 26,000 feet (7,900 m).
I’m again reminded of Bobbie Oxspring’s experience flying Spitfire IXs with 72 squadron in North Africa during early 1943: “The second stage supercharger of the Merlin 61 had an automatic barometric control gauged to cut in at 19,000 feet.
Bearing in mind the efficiency of the enemy’s ‘Y’ service, we devised a code for 2nd supercharger engagement.
Also I believe the idea of having the toggle switch that could force the second stage to engage below what the bellows would call for was to prevent the shifting back and forth between 14-19k, thus preventing the clutches from over heating and the surging of power. If I remember right you could use the second stage above 14k/below 19k and you would get more boost than provided by the first stage. Of course below 14k more power would be consumed to turn the supercharger than power produced as only so much boost would be funneled to the engine.
Well, there is flat within 5-10% and then there are the peaks and valleys in some of the Merlin charts"The 601 engine will have almost a flat horse power line from take-off to rated altitude..." “By having a variable speed supercharger drive the Messerschmitt 109 with the Daimler-Benz engine is able to exceed the power output of a design like the Merlin which has a fixed speed supercharger.”
Interesting presentation but, hmmm...
Mike, do you have any info on - British Performance Reduction Methods for Modern Aircraft (A.&A.E.E./Res/170) used by UK aircraft testing facilities.
I have been reading performance tests on different aircraft and it seems the UK use completely different corrections to the Luftwaffe, a lot of which seems to be calculated or not corrected?
This seems to lead to quite considerable differences in results?
We are not talking about having the Merlin 61 supercharger in high speed at 1000 ft but rather at, say, 15,000 ft, which is where the Japanese wanted to fight. At 15,000 ft the Merlin Mustang was getting toward the upper end of its power curve. Manually engaging the high speed on the supercharger meant the pilot had to be more careful about advancing the throttle - but it gave him MORE throttle to advance.
Regardless of the "theory" that says it would produce less power, actual pilot reports said it worked.