Spitfire IX Supercharger

[cardboard_killer]

When using 150% octane fuel, at what altitude should the Spit IX change to stage 2?

 

Thanks,

CK

[VBF-12_Stick-95]

Based on nothing specific I usually change at 10,000ft.  Also, interested in knowing.

[Vortice]

Keep an eye on your boost pressure gauge. When it starts to drop without any changes to your throttle setting then it's time to engage stage 2.

[Toots_LeGuerre]

Spit IX superchargers in R/L were barometrically controlled, not manually controlled:

 

"The impellers were driven by a hydraulically operated two-speed gearbox.^[6] At low to medium altitudes, the supercharger was in Moderate Supercharger or M.S. gear (this referred to the gearing and thus the speed, at which the impellers were operating). Once the aircraft reached and climbed through a set critical altitude, (20,000 feet (6,100 m) for the Merlin 61 and 70 series) the power would start to drop as the atmospheric pressure (the density of air) dropped.^[7] As the critical altitude was passed a pressure-operated aneroid capsule operated the gearbox, which changed speed to Full Supercharger (F.S.) gear, which drove the impellers faster, thus compressing a greater volume of the air-fuel mixture."

 

from Wkipedia entry.

[ZachariasX]

57 minutes ago, vonNutz said:

Spit IX superchargers in R/L were barometrically controlled, not manually controlled:

When selecting 150 octane, you have to select manually. Both in game and R/L.

[cardboard_killer]

1 minute ago, ZachariasX said:

When selecting 150 octane, you have to select manually. Both in game and R/L.

 

I wonder how many people don't realize that. I forget what clued me in, probably the Pilot's Notes pdf document from this web page, and it just says "When using 150 octane fuel, the supercharger needs to be switched manually". I've been switching at 8,500 ft, like I do with the Tempest's supercharger.
 

[Barnacles]

4 hours ago, cardboard_killer said:

When using 150% octane fuel, at what altitude should the Spit IX change to stage 2?

 

Thanks,

CK

When the 1st gear isn't able to achieve the desired MP, plus a fudge factor of a thousand feet or so

[ZachariasX]

20 minutes ago, cardboard_killer said:

 

I wonder how many people don't realize that. I forget what clued me in, probably the Pilot's Notes pdf document from this web page, and it just says "When using 150 octane fuel, the supercharger needs to be switched manually". I've been switching at 8,500 ft, like I do with the Tempest's supercharger.
 

I quote from specs:

 

150 grade fuel

Allows +25 lb boost
Supercharger gear shift automat is disabled. Stages should be manually switched at 9000ft altitude.
Estimated speed increase at sea level: 29 km/h

 

8500 ft is good enough

[41Sqn_Skipper]

During climb (low RAM effect) it should actually be switched at 5,500 feet with 150 grade and 14,500 feet with 100 octane:

 

 

However in my opinion 5,500 feet could be a typo and actually mean 8,500 feet. The lower altitude doesn't fit the engine power graphs below and I don't think lower RAM effect during a climb would cause such a massive drop in altitude:

 

[Talon_]

Also remember this is only during WEP operation.

 

If climbing at +12lbs then no adjustment in supercharger change altitude is needed between fuel types and is around 16,000ft IIRC.

[ZachariasX]

On 6/22/2020 at 12:49 PM, 41Sqn_Skipper said:

However in my opinion 5,500 feet could be a typo and actually mean 8,500 feet.

I think @Talon_ makes an important point here.

 

It is engine shaft rpm that defines the amount of air the supercharger can provide. It is the MAP setting that defines how much air the engine actually consumes. This is a balance and you can see that in the lower sheet you posted. 18 inches boost is what the engine can do normally. The devs just took that rating for the critical altitude. If you are going at +25 inches, then you indeed need to change earlier. You go at +12 inches, your chart would bring you to ~16k ft to change gear as Talon said.

 

Thus, critical altitude is always at "it depends". That is why they felt the need to state that in the report. It would also be not smart to just state 5'500 ft as altitude to change gear, because most of the time, the pilot won't pull +25 inches. If he now made the switch at the lower altitude, he'd lose considerable mileage. In a plane badly constrained by its range, this is not a great procedure. Since the +25 inches are a mod, you don't change the normal engine spec sheet.

 

Take home message is: "If you are flying a supercharged aircraft at lower MAP, switch gears higher than stated and you gain mileage."

 

A corollary: "If low gear gets you the MAP you require, DO NOT switch to high gear."

 

Escorting Mustangs would conveniently fly on low gear at 20k ft along with the bombers. They gained a lot of range like that.

[41Sqn_Skipper]

Your airspeed also influences when you need to switch due to RAM effect. As a general rule: switch to FS if you can't set the desired boost on MS.

 

Ideally you switch a bit later (or lets say higher) as running the supercharger at FS will cost more engine power than the higher boost provides, e.g. if you want to set +18 and you switch to FS as soon as boost drops to +17.5 your net engine power is lower than if you would stay at MS with +16 boost. 

 

For economic flying you want to switch as late as possible, again because running the supercharger on FS costs more engine power than running MS. Of course you need to switch to FS if you can't maintain the necessary speed and altitude. But economic flying is usually not a consideration in the sim.