Another take on superchargers

[tomo pauk]

Recently, a member of this board posted a video about superchargers for aircraft engines. It is supposed to be a 1st part of the series of videos on that topic.
Some claims might raise an eyebrow.
link

 

[wuzak]

Have to watch that later - at work with no sound.

 

[Schweik]

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.

 

[Kryten]

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.

It's basically a viscous coupling, reduce the oil volume and it causes the impellor to slip as it requires the full quantity of oil to make a 1 to 1 speed with the input shaft.

 

[wuzak]

This Flight article describes the operation of the fluid coupling:

http://kurfurst.org/Engine/DB60x/files/Flight_16April1942_DB601N_Engine.pdf

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.

 

[tomo pauk]

...
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.

I reckon that speed ratios were infitely variable between the two extremes.

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.

There was throttle plate on DB 601/603/605 series of engines, throttle body being located 'after' the supercharger. For the 601N, it can be seen here.
The DB 603L and 605L discarded with butterfly throttle and got the swirl throttle instead, it was located just 'before' the S/C 1st stage.

 

[mad_max]

Instead of me trying to describe how it works here's a good article on the engine.

http://www.wwiiaircraftperformance.org/me109/DB-601A-Flight-7Nov40.pdf

 

[MIflyer]

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.

The P-63's auxiliary 1st stage supercharger also used a hydromatic drive.

 

[tomo pauk]

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!

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.

*as noted by mad_max, the -3 was indeed hi-alt, the -7 mid-alt Packard Merlin

 

[Mike Williams]

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.

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. Since barometric instruments are notoriously imprecise, the effect on a squadron climbing for altitude at high power settings meant that twelve superchargers cut in at slightly different times. The effect of the formation with these widely fluctuating power settings was like shuffling a pack of cards and trying to keep the same suit together. To overcome this discrepancy, we decided to climb well above the automatic setting utilizing the manual override. On a radio order from the leader we flipped the 'auto' switch and cut the superchargers in together. We were then able to contend with the foe at high altitudes, but it was prudent to keep the knowledge of our capability from him as long as possible. Bearing in mind the efficiency of the enemy's 'Y' service, we devised a code for 2nd supercharger engagement. The formation leader's instruction for simultaneous action by other pilots was 'Up you pipe; 'Up your' being the warning, and 'Pipe' being the executive command to cut in. The stratagem had the desired effect and 'Pipe' sent the squadron soaring aloft like a pack of homesick angles."

Also: S/L Robert Oxsbring, DFC of 72 Squadron wrote of his combat of 1 March 1943: "One 109 climbed away east of the melee and I followed it with my wingman, Red Hunter. As it gained height at full bore we were just about at maximum firing range, but not noticeably catching up; Red fired a long burst without effect and ran out of ammunition. We hung on until around 19,000 feet the supercharger cut in sending me close underneath the 109's tail. I let fly and white glycol streamed from the engine as the canopy peeled away and the pilot baled out. He dropped to my left and I saw his parachute blossom. A second later he fell out of the harness and careered earthward leaving a flapping canopy behind."

 

[mad_max]

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.

Here I thought the V-1650-3 was the high alt. Packard Merlin (basically equivalent to the Merlin 61) and the -7 was the mid alt. version. Not to get picky though.

 

[tomo pauk]

Here I thought the V-1650-3 was the high alt. Packard Merlin (basically equivalent to the Merlin 61) and the -7 was the mid alt. version. Not to get picky though.

My bad. You are of course right that -3 was the hi-alt, while -7 was 'mid-alt'.

 

[MIflyer]

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.

Mike: I have heard the story about Mustang formations getting ragged when the aneroid devices cut at slightly different altitudes, but that is the first I have heard of anyone deliberately switching over to manual high speed to prevent that from occurring. The regular P-51 high/low supercharger switch was spring loaded so you could not switch it manually to high speed except momentarily for test purposes. I guess the Spitfire's was not? I'll have to check the manual. But some Mustang units (on Iwo, for example) installed toggle switches that were not spring loaded and allowed manual operation (to the consternation of the Packard tech reps) so maybe the Brits did as well.

A review of P-51 manuals shows that postwar the high/low speed switch over point went from an aneroid device to a ram air sensor. I would guess that if the airplane was at low speed at lower altitude it allowed the high speed supercharger to kick in until higher speed allowed ram pressure to increase, after which it automatically kicked off. Or at least I think that is how it would work, but that is only educated engineering speculation on my part.

When the PI was attacked a P-40E pilot jumped in his airplane and shoved the throttle as far it would go and tried to build up some altitude and airspeed. Then he was shocked to see the manifold pressure was only something like 10 inches. Figuring his engine was failing, he despaired, but then gradually realized the gauge had gone all the way past 60 inches and started another circuit. Allison got V-1710's up above 80 in MP on the test stand, the limiting factor not being the engine but the bolts that held it to the test stand.

 

[mad_max]

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.

Just my understanding and I could be wrong.

This explains what I'm inferring about above.

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).

 

[tomo pauk]

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.
...

A tidbit on nomeclature: both S/C stages on Spitfires with 2-stage engines (and similarly equipped P-51s and Mosquitoes) were allways in function. It was 2nd gear that got activated (and 1st gear de-activated) past set altitude, so both stages were turning faster than at higher altitudes. Both stages' impellers were on same shaft, same system was applied on Jumo 213E/F (3-speed gearing), DB 603L and DB 605L (variable speed S/C), and of course on 2-stage Griffons (with 2-speed gearing, post-war with 3-speed gearing). American-designed 2-stage superchargers were mostly if not allways with each stage (each impeller) driven by separate shaft.

 

[Mike Williams]

"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...

Betriebs und wartungsvorschrift zum Mercedes Benz Flugmotor, DB 601 A u. B., Ausgabe C, October 1940

Abb.3 Leistungsschaubild D.(Luft)T.3601 A u. B., DB 601 A u. B Motoren-Handbuch, Mai 1942

Rolls Royce Merlin engines- Max power level flight

 

[Kryten]

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?

 

[Shortround6]

"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...

Well, there is flat within 5-10% and then there are the peaks and valleys in some of the Merlin charts

 

[Mike Williams]

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?

Hello Kryten, you make a good point. I'm uncertain if I have (A.&A.E.E./Res/170). I'll look around.

 

[tomo pauk]

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.
...

I'm not sure that thre is 'we' on the forum, but rather 'me', 'you', 'him'?
Merlin Mustang was not outfitted with Merlin 61, but, for example the P-51D with V-1650-7, an equivalent of the Merlin 63. The -7 was making more than 1500 Hp @ 67 in Hg at 15000 ft. 67 in Hg = full throttle for 130 grade fuel. More than 67 in Hg is dangerous to push unless one has fuel that is much better than 130 grade.
At what altitude the Japanese want to fight is not important, same as it was not important at what altitude the Fw 190As wanted to fight - flight planers and pilots of Merlin Mustangs knew too well that their best altitude is above 20000 ft, and acted accordingly. One that flies at high altitude has advantage, in this case that will be the Mustangs. Dive, fire, zoom-climb, repeat. Yes, decidedly unfair fight.