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Engine Performance Tuning Tutorial
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.
JuMo 213A-1 Specifications
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.
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Propeller Power Coefficient - Part 1
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.
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FW 190D Propeller Power Coefficient
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.
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FW 190D Basic Engine Installation
I finally got around to installing the JuMo 213A in the FW 190D basic AIR File and took it for a test flight. Attached is the updated AIR file.
Here is what was changed: (All unstated data has been specified in prior posts.)
Record 500 - Reciprocating Engine Specifications
Propeller Pitch Max
Propeller Pitch Min
Propeller Diameter
Record 505 - CFS Reciprocating Engine
Displacement per Cylinder
Compression Ratio
Cylinder Count <---- Unchanged from 12
Maximum RPM
Maximum Horsepower <---- Changed to 1600. I believe this only affects sound effects.
Negative G Effects <---- Becomes "None". This is a fuel injected engine.
Engine Type <---- Remains "Water Cooled".
Supercharger <---- Remains "Yes".
Max Manifold Pressure <---- 1.8 ATA equivalent in Inches of Mercury.
Supercharger Boost Gain <---- THIS WILL NEED TUNED LATER!!!! Unchanged for Now.
Critical Altitude <---- 6600 Meter equivalent in feet.
WEP Type <---- Becomes MW.
WEP Manifold Pressure Increase <---- Becomes difference between 1.80 ATA and 2.02 ATA.
No Mixture Control <---- Copied from Stock FW 190A: "KommandoGerat" controlled everything.
No Panel Magneto Switch <---- Copied from Stock FW 190A.
Record 508 - Engine Torque versus RPM
Maximum RPM changed from 3000 to 3250.
Record 509 - Engine Per Cylinder Friction Loss
Maximum RPM changed from 3000 to 3250
Record 510 - Propeller Parameters
Propeller Diameter
Propeller Pitch Max
Propeller Pitch Min
Reduction Gear Ratio <---- Changed to 1/(0.417).
Record 511 - Propeller Efficiency
Copied from Stock Me 109G using AirEd.
Record 512 - Propeller Power Coefficient
Copied from Stock Me 109G using AirEd.
Attached for explanation of parameters is a copy of the FDE Control File I use.
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First Flight Test of the Jumo 213A
The First Flight Test was either very good or very bad depending on your point of view:
Without changes to anything other than what was specified in prior posts, the engine power at an altitude of 500 feet was almost exactly what we would want. The maximum speed was also pretty much exactly what we were trying for.
The Sea Level (Take-Off) Power is somewhere between 1726 HP and 1776 HP depending on which source you believe. Kurt Tank's Flight test of a standard D-9 showed 586 kph (360 mph) at 300 meters altitude.
See Attached Screenshot of performance values from my version of Jerry Beckwith's excellent test panel. One thing to note from this screenshot is that the propeller pitch is a couple degrees higher than I would have wanted. I expect a bit of overspeeding at higher altitudes with less dense air.
Good or Bad - Depending on Outlook
Values that match exactly what you are looking for are good if the goal is something to be used in a project. For a tutorial, this is bad because there isn't an opportunity to demonstrate how to adjust parameters to get what you want.
For the purposes of this tutorial, we will pretend that we are not trying to reproduce the mid-production Jumo 213A. Early versions of this engine typically produce about 100 HP to 150 HP LESS than advertised, so we will attempt to detune this fine running 1740 HP engine down to around 1620 HP without changing the manifold pressure limits. Depending on how quickly I get bored, we may also try to simulate a "Blueprinted" engine of 1800 HP or so.
At this point, some folks probably have figured out what is coming next. Adjusting the engine power is actually quite easy:
To INCREASE Engine Power,
Increase Torque in Record 508 -OR-
Decrease Friction in Record 509
(or some combination of the two)
To DECREASE Engine Power,
Decrease Torque in Record 508 -OR-
Increase Friction in Record 509.
Note that the effects are not the same.
Although the engine output at Sea Level (and up to critical altitude) will be the same with either method, the engine power will fall off faster if Torque and Friction are both higher for the same power output.
Next comes a lot of Modify and Test cycles....
(Lather, Rinse, Repeat....)
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The Different Ways of Tuning
Attached are two screenshots showing the results of Reducing Torque and of Increasing Friction.
Stock Torque multiplier is 0.56. It was reduced to 0.53 to get 1620 HP (Actually 1621 HP).
It took two tries to get here.
Stock Friction number is 68 I believe. To get power down to 1620 HP needed a Friction number of around 84.3. It took about 10 tries to get here because I wanted a power reading equal to the other method. The difference is only 1 HP so I believe that is sufficient. You can reach any level of precision as long as you have sufficient time to test.
Note that we have dropped around 7 mph by losing 120 horsepower.
- Ivan.
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High Altitude Comparision
Attached are two screenshots showing the power output of these two engines at altitude.
Note that the stock supercharger values are much too high and are providing too much power at this altitude. This isn't surprising because we haven't discussed tuning superchargers yet.
Nothing is different between them other than Records 508 and 509, but the power output is noticeably different. Power difference isn't very much yet (only 33 HP), but then again, we do not appear to be anywhere near the ceiling of this aircraft even at 40,000 feet.
It should also be obvious that we can continue to raise both the Torque and the Friction in a balanced manner to reduce high altitude power even more.
- Ivan.
Tuning Medium Altitude Performance
The maximum speed for the FW 190D was achieved at 6600 meters or 21650 feet.
On the graph in the last post, engine output looks to be 1440 PS.
The conversion factor is 1.01387 so this is equivalent to 1420 HP.
In the AIR file, the variable to adjust for tuning critical altitude can be found here
Record 505: Supercharge Boost Gain.
Since we started with the stock P51D, it is still 5.36.
A quick flight test gives the following results:
00500 feet ==> 52.3 inch MP 1722 HP
17500 feet ==> 52.3 inch MP 2020 HP
20000 feet ==> 52.3 inch MP 2071 HP
22500 feet ==> 52.3 inch MP 2123 HP
25000 feet ==> 52.3 inch MP 2177 HP
27500 feet ==> 52.0 inch MP 2219 HP
Only the two rows in BOLD are really relevant.
In fact, a single test at 21650 feet would have told us quickly that we needed to adjust.
Changing the Supercharger Boost Gain to
2.68 ==> 1163 HP which is obviously way too low.
3.30 ==> 1549 HP which is closer
3.20 ==> 1487 HP
3.10 ==> 1424 HP which is pretty close.
(There is no point in getting any closer at this stage.)
A quick speed run at this altitude gives us 421 mph which is way too low.....