Engine Performance Tuning Tutorial

[Ivan]

Here by popular demand is a short Tutorial on how to tune the Engine Output of a CFS Aircraft.

The Straight Line Performance of an Aeroplane is very dependent on the amount of Thrust (and therefore Engine Power) that is available at each altitude. Fortunately, this is not terribly difficult to do for Combat Flight Simulator using a simple AIR file editor. The only other necessary tools would be a means of measuring the effects of our changes to the AIR file variables.
The best set of measuring tools that I have seen are the gauges included in Jerry Beckwith's Test Panel which can be found here:

http://mudpond.org/gauge_index.html

For those who might be wondering, I will not make any claims that this method is a perfect solution. There are limitations to what CFS can represent. I also do not know enough to be able to generate my own AIR file Propeller (511 & 512) Records, so they will be "borrowed" from other stock aircraft.

One of the greatest limitations of CFS is that it cannot represent multi-speed superchargers. Aircraft can only have a single speed supercharger which means that we will be trying to make our power curve best fit the aircraft's actual power curve. Typically this means that the aircraft will have a smooth power curve plotted against altitude rather than a sawtooth curve that dips at the supercharger shift points.

The Subject Aircraft will be a Focke-Wulf FW 190D-9. Since I haven't built such a model yet, My FW 190A has agreed to be the stand-in for now. Most of the specifications are quite similar between the two aircraft and I happen to like the look of it.

- Ivan.

[Ivan]

The FW 190D series used the Junkers JuMo (Junkers Motor) 213.
The main production variant, the FW 190D-9 used the JuMo 213A-1.
Later variants used the JuMo 213E and JuMo 213F which had slightly greater power and significantly higher critical altitudes as well as the ability to mount a co-axial cannon.

The following data is required to use this method of engine tuning:

Cylinder Count
Displacement per Cylinder in Cubic Inches
Compression Ratio
Crankshaft to Propeller Reduction Gear Ratio
Maximum RPM
Propeller Diameter

Take-Off or Sea Level Power Rating
Manifold Pressure used to achieve this power level

War Emergency Power Rating
Manifold Pressure used to achieve this power level
WEP Type

Critical Altitude
Additional Power @ altitude data points are also useful.

Propeller Pitch Range is not absolutely necessary but very helpful.

[hubbabubba]

(...) I also do not know enough to be able to generate my own AIR file Propeller (511 & 512) Records, so they will be "borrowed" from other stock aircraft.(...)
- Ivan.

Sorry to interrupt...
You can change Records 511 and 512 using AirUpdate Utility. Also from Jerry Beckwith (HERE)

It will "dump" the AIR file data in a TXT format that you can edit, tables included, and recompile.

The download is HERE.

Back to you Ivan.

P.S.- And it is FREE!!!

[Ivan]

Hello Hubbabubba,

I have also written my own programs to pull out specific records and I can translate them to anything I want or even into a CSV so I can load into a spreadsheet. Changing every value in the table is no problem either.

The issue is that I don't know what the graphs should look like for a given engine power curve.

Thanks for the suggestion.
- Ivan.

[Ivan]

Specifications for the JuMo 213A-1 are the following:

Cylinders: 12
Bore: 150 mm
Stroke: 165 mm
Compression: 6.5
Reduction: 0.417 <---- This was hard to find. I found it in Janes Fighting Aircraft of WW II.
Maximum RPM: 3250
Propeller Diameter: 3.300 meters

Performance (varies a bit depending on the source)
1750 PS (1726 HP) or
1755 PS or
1776 HP at Take-Off (3250 RPM)
Throttle setting should be 1.8 ATA

WEP at sea level produces 2050 HP to 2240 HP depending on the source you believe
Throttle setting should be 2.02 ATA

Critical altitude is around 6500 meters altitude.
Aircraft critical altitude (presumably with ram-effect) is around 7000 meters.

Power at altitude varies all over the place depending on your source but appears to be at least 1600 HP at 18,000 feet.

If anyone has more reliable information, please respond along a listing of the source.

Thanks.
- Ivan.

[Ivan]

My understanding of the Propeller Power Coefficient is that it is a number representing the power absorbing ability of the propeller. It represents how hard the propeller is to spin. In the case of a constant speed or variable pitch propeller as typically found in a WW2 fighter, it is a representation of how hard the propeller is to spin at each pitch setting as plotted against its advance ratio.

This is the means by which the game selects the correct pitch setting to use based on engine power and how fast the aeroplane is moving. Strictly speaking, it isn't necessary to get this right to get the proper engine power but it does influence how the power is used.

The Power Coefficient will vary depending on air density, but I chose to use an altitude of 500 feet to gather my data.

Since I don't know how to generate my own graphs, I will copy the closest match from one of the stock aircraft. The values for the stock aircraft are shown in the attached spreadsheet.

[Ivan]

Note that the specifications for the JuMo 213A did not include the propeller pitch settings.
I could not find that data so will be using the values for the stock FW 190A which seem fairly reasonable:
(23-65 Degrees). In any case, although the attached spreadsheet lists them, they are grayed out because they are not actually used.

Although the prior post doesn't show it, the power output of the engines of the FW 190A-8 and FW 190D-9 were very similar at low altitudes. The better engine power at altitude, slightly better streamlining and lighter weight is what I believe made the difference.

Note that the Power Coefficient from the spreadsheet attached is 0.24. This is a measurement of how hard the engine with its reduction gearing can turn a propeller. Note that this calculation completely ignores the number of blades and the profile (activity factor) of the blades.

None of the stock aircraft go quite that high. The two closest are the Me 109G (0.1721) and the FW 190A (0.1684). I will probably start with Records 511 and 512 from the Me 109G and do a final test with the FW 190A. Perhaps it is a good idea to start with the FW 190A propeller because of its similarity to the one on the FW 190D.

Although the choice influences flight performance to some extent, it does not affect engine power.

The AIR File records may be copied from one file to another using AirEd.

Next comes the first "Flight Test"..... (which will not be tonight).

Good Night.
- Ivan.

[No Dice]

Thank you Ivan for this thread, although some is above my Intel level, it is very interesting to follow.

Thumbs up dude,

Dave