109g2 engine settings

[Tarmac]

I'm looking through a 109g2 field maintenence manual.  For one of the engine settings, it says Economy cruise, 2100 RPM; 5.7 km altitude, and then

Boost pressure
2%    Std. Ata.
1.00   +.02

I know these are manifold pressure settings, in standard atmospheres.  But I don't understand what they mean, and how they'd translate to AH throttle settings.  Can anyone shed some light on them?

Also, is it possible to figure out from these numbers (or from another source) what the economy settings for the G2 would be at say 5k feet and 0k feet (typical limping home with 1/8 tank of gas altitudes )?

Thanks in advance!

[HoHun]

Hi Tarmac,

>I know these are manifold pressure settings, in standard atmospheres.  

The boost is 1.0 ata with a permissible deviation of +0.02 ata.

I guess Aces High provides this in inches of mercury.

1" Hg = 3.386 kPa

1 ata = 98.07 kPa

so

1 ata = 28.96" Hg.

>Also, is it possible to figure out from these numbers (or from another source) what the economy settings for the G2 would be at say 5k feet and 0k feet (typical limping home with 1/8 tank of gas altitudes )?

2100 rpm/1.0 ata is the maximum power setting considered economic. Fuel usage would be 225 L/h according to the Motorenkarte. Power would be ca. 890 HP @ 5.7 km.

I'm not sure which cruise speed you'd get from that power setting, though.

Regards,

Henning (HoHun)

[gwshaw]

Originally posted by HoHun
Hi Tarmac,

>I know these are manifold pressure settings, in standard atmospheres.  

The boost is 1.0 ata with a permissible deviation of +0.02 ata.

I guess Aces High provides this in inches of mercury.

1" Hg = 3.386 kPa

1 ata = 98.07 kPa

so

1 ata = 28.96" Hg.

[/b]

Interesting, SL standard pressure is generally given as 29.92 in Hg. The Germans did use a slightly different standard atmosphere than the US and UK used, so 28.96 in Hg may be right.

>Also, is it possible to figure out from these numbers (or from another source) what the economy settings for the G2 would be at say 5k feet and 0k feet (typical limping home with 1/8 tank of gas altitudes )?

2100 rpm/1.0 ata is the maximum power setting considered economic. Fuel usage would be 225 L/h according to the Motorenkarte. Power would be ca. 890 HP @ 5.7 km.

I'm not sure which cruise speed you'd get from that power setting, though.

Plugged that into my calculator for the Bf 109G-2 and came up with 341 mph TAS @ 18,500 ft and 342 mph TAS @ 19,000 ft, both roughly 254 mph IAS.

That is an awfully high cruise speed. Looks like optimum lift/drag is down around 155-160 mph IAS, about 210-215 mph TAS. That should only require about 375 hp @ 80% prop efficiency.

Greg Shaw

[F4UDOA]

HoHun,

Is that max cruise for range?

With 106 gallons internal at roughly 50 gallons per hour at 1.0ATA that gives around 2 hours endurance at 340MPH gives a range of almost 680 miles.

Did the 109G2 have that range on internal fuel?

Tarmac,

How did you come up with the 340MPH cruise speed?

[Tarmac]

Originally posted by F4UDOA
Tarmac,

How did you come up with the 340MPH cruise speed?

gshaw came up with that.  I just open up the throttle and see how fast she goes.  

[HoHun]

Hi Greg,

>The Germans did use a slightly different standard atmosphere than the US and UK used, so 28.96 in Hg may be right.

To be accurate, they used the same standard atmosphere, but 1 at was one "technical" atmosphere, conveniently defined as the force exterted by 1 kg of mass on a 1 m^2 area at standard gravity.

>That is an awfully high cruise speed.

Well, it's just the engine's "maximum" economic power reported without consideration of the airframe. In a Me 109, cruise for best range probably will be slower, just as you pointed out.

Regards,

Henning (HoHun)

[HoHun]

Hi F4UDOA,

>Is that max cruise for range?

No, the power setting is determined by the engine alone. I assume "maximum economic cruise power" is defined by the highest power setting at which the engines operates near optimum specific fuel consumption up to rated altitude.

(Lower the rpm further, and full throttle height for economic power will drop. Increase the boost, and specific fuel consumption will rise above the minimum. Something like that.)

>Did the 109G2 have that range on internal fuel?

Probably not, as I made a typo up there!

The Motorenkarte reports the fuel consumption as 255 L/h, not 225 L/h as I wrote.

Regards,

Henning (HoHun)