Altitude and manifold drop

[321BAR]

I understand why this happens, but why does it raise gradually back to normal on ALL of the planes after hitting the 15k mark... anyone know why???

[Widewing]

Aircraft with two stage superchargers demonstrate this. This is due to the supercharger shifting gears, providing increased boost. Each stage effectively has its own critical altitude. Aircraft with turbochargers (P-38s and P-47s) generate max MAP from sea level up to critical altitude, which is usually quite high (25k for P-38s, 30k for P-47s) Single stage superchargers provide max boost up to critical altitude, above which power falls off (no second stage).

Two stage supercharger:


Turbocharger:


Single Stage supercharger:



My regards,

Widewing

[trotter]

Great post Widewing, thanks.

[Charge]

And this is the fluid coupled single stage:



Two stage, three speed supercharger:



The drop in manifold pressure can be big in planes with a single stage, two speed superchager:



I think there was a plan to put a two stage three speed unit in Tempest but I'm not sure if it ever realized.

-C+

[Mike Williams]

With respect to the Me 109 curve shown above, data from flight trials conducted by Messerschmitt’s Flight Test Department suggests that the performance of the ME 109 equipped with a DB engine with hydraulic coupled supercharger might more accurately be depicted as follows:

Geschwindigkeit der Me 109 G 1 Grundausführung, Leistungszusammenstellung Me 109 G
Leistungskurven, Leistungen Me 109 G mit DB 605 AS., Versuchs-Bericht Nr. 109 20 L43
Horizontalgeschwindigkeit, Leistungsmessung Me 109 G-6 Tropen mit DB 605 A, Versuchs-Bericht  Nr. 109 10 L 43

[Charge]

The reason for speed reduction for 109, when the coupling kicks in, can be in strange coupling problem visible in some data which can be seen as an abrupt drop in manifold pressure. Also the air temperature might cause a further reduction as the oil (which is used also for coupling) requires more cooling causing increased cooling drag.

However, looking at the dates of conducted tests (no date in first picture) it seems that colder air may cause more drop in "slip" area -maybe the coupling does not slip sufficiently thus requiring throttling to prevent over pressure -but that should be visible also in manifold pressure, which it doesn't.

If the manifold pressure does not drop there really cannot be other explanation other than increased drag which is not shown in graphs in other way than reduced speed. One possibility could be increased cooling drag or the drag caused by the charger intake if the airflow from charger is throttled for some reason.

http://kurfurst.allaboutwarfare.com/Performance_tests/109G_MT215/109G2_MT215_en.html

In Finnish 109 test there is a notch in manifold pressure but not a visible reduction in speed. Even if the speeds are corrected to same temperature so that test data would be comparable, the effects of temperature on automated engine management still remain different because of different temperatures during the test. Temperature during Finnish test was around 0 Celsius at ground level which could be around the same as in G5 test conducted in Germany in January -but there is no speed reduction in Finnish test, but a significant notch in German test around 4km of height.

Your bet?

-C+

[BaldEagl]

In a related question, I've been noticing that every plane in the set is 2100 RPM recommended at max cruise.  Surely that can't be realistic.

[Lusche]

In a related question, I've been noticing that every plane in the set is 2100 RPM recommended at max cruise. 

Almost

FW 190D-9 and TA 152 are 2400 RPM

[Stoney]

Max cruise settings in-game are a little dicey anyway, as they aren't really the maximum range settings.  Since maximum range is affected by altitude, speed, and specific fuel consumption, those values on the E6B are more of a "sort-of".  Every aircraft has a different "max cruise" at each altitude.  For example, a fully loaded B-17 cannot even maintain altitude at 20,000 ft + using the max cruise setting on the E6B.  So, obviously, its max cruise setting at that altitude is much higher.

For aircraft like the P-38 and P-47, your best range RPM are more like 1700-1800 RPM with MP settings in the upper 20's and lower 30's respectively.

[gripen]

The reason for speed reduction for 109, when the coupling kicks in, can be in strange coupling problem visible in some data which can be seen as an abrupt drop in manifold pressure. Also the air temperature might cause a further reduction as the oil (which is used also for coupling) requires more cooling causing increased cooling drag.

The second oil pump of the coupling kicks in when the aneroid control opens the valve and directs gradually the flow from second pump to the coupling according to pressure. This has not much to do with temperature because the aneroid works according to pressure. Some test data shows that aneroid opens the valve too late above the first FTH where the constant flow from the first pump  is no more enough to maintain manifold pressure causing therefore slight drop of MAP around altitude where the second oil pump kicks. Example can be seen here:

http://www.wwiiaircraftperformance.org/me109/14026pg7.jpg

However, looking at the dates of conducted tests (no date in first picture) it seems that colder air may cause more drop in "slip" area -maybe the coupling does not slip sufficiently thus requiring throttling to prevent over pressure -but that should be visible also in manifold pressure, which it doesn't.

This part does not make sense at all; manifold pressure drops only when the speed of the supercharger is too low for the wanted pressure. In other words manifold pressure drops due to too large slip or even the smallest slip is too much (above FTH).

If the manifold pressure does not drop there really cannot be other explanation other than increased drag which is not shown in graphs in other way than reduced speed. One possibility could be increased cooling drag or the drag caused by the charger intake if the airflow from charger is throttled for some reason.

The phenomena is simply caused by operation of the aneroid which is based on outside pressure instead MAP. The best way to understand the issue is to look difference between the "Ladedruck" and "Gebläsedruck", the difference follows closely the output curve characters of the engine.

In Finnish 109 test there is a notch in manifold pressure but not a visible reduction in speed.

In the MT-215 data the MAP varies slightly ( plus/minus 0,01ata) below the second FTH but this is normal variation of aneroid control.

[Charge]

"The second oil pump of the coupling kicks in when the aneroid control opens the valve and directs gradually the flow from second pump to the coupling according to pressure."

What "second oil pump"?

"The phenomena is simply caused by operation of the aneroid which is based on outside pressure instead MAP. The best way to understand the issue is to look difference between the "Ladedruck" and "Gebläsedruck", the difference follows closely the output curve characters of the engine."

Aneroid causes the aircraft to slow down? The point is: "what makes the aircraft slow down if the manifold pressure stays constant"?

-C+

[gripen]

What "second oil pump"?

There is two oil pumps for the hydraulic coupling in the DB 605A. The first one pumps constant flow to the coupling all the time, the second one pumps all the time as well but the flow is not directed to the coupling (it's used for cooling instead) until the first FTH when the aneroid opens the valve gradually, according to pressure, and increased flow start to reduce the slip of the coupling. At second FTH entire flow from the second pump is directed to the coupling and slip is at minimum level. Please check the manual of the DB 605A.

Aneroid causes the aircraft to slow down?

The phenomena happens when the aneroid opens the valve too late or too early. Generally there is no slow down but noticeable change in the speed curve unless the the aneroid opens the valve far too late or far too early. Given that the operation of the valve depends on outside pressure instead MAP, it rarely works at optimal point.

[Boozeman]

Talking about the DB605As supercharger setup, does anyone know why the C.205s Fiat build "DB605" has a distinctively different characteristic? It resambles more like the ones on the DB601-powered planes, at least in the speed vs. alt graph. On the climbrate vs. alt chart however, it looks pretty much like a German 605 though...

     

[gripen]

There is documentation stating that the DB 605s built in Italy developed slightly lower output than German made. However, the power curve characters should be pretty much the same and at least I don't see large differences between the AH C.205 and the G-6 characters.

[Boozeman]

I have made 2 speedchart overlays of the 205 and 109G2 to illustrate what i mean:
Note that both planes have the same performance at sealevel and critical altiude but in between the 109 makes significantly more power - thus a different supercharger "setup". However, and this is intresting, according to the E6B and the gauges both engines run on the very same setiings re. RPM and boost, at all altitudes.