On Sunday 24 April 2011 15:00:41 Pavel Cueto wrote:
> Hello to all
>
> I'm developing with a friend the PZL M18B "Dromader" in JSBSim, doing this
> first with aeromatic generated data; and now, we are mixing them with real
> aircraft data from very reliable sources. However, the engine behavior
> seems very different in numbers compared with what we expect from it:
>
> Engine: ASz-62IRM18
> Max.RPM: 2200 / Min.RPM: 550
> Constant speed propeller
>
> - The reduction gear ratio of the real engine is 0.687, but writting that
> value the engine has too low RPM, that it even can't start. Trying to
> "invent" a calculation for the ratio, i got a new 1.45 gear reduction
> ratio, but that number still being unaccurate.
0.687 is from the engine to the propeller. To specify from the propeller to
the engine use 1/0.687 or 1.456
> In example, according to
> engine manual, if RPM are 1900 with a fine propeller pitch, changing the
> prop pitch to coarse will reduce the RPM to 1500; but in the model RPMs are
> reduced even to 1000 RPM. I guess that FDM needs a ratio number wich are
> different to originals, true?
No, set the minrpm of the propeller to 1500*0.687 or 1031. This means the
propeller will not try to slow the engine below that point.
> - The max.RPM of the engine are 2200 RPM. But the model just reach circa
> 1600 RPM with full throttle and fine propeller.
Your engine power is too low to turn the propeller at the specified speed. No
boost as noted below.
> - The min.RPM of the engine are 550 RPM. But with min.throttle and coarse
> propeller it reach 790 RPM.
Your engine power is too high at idle to let the engine slow.
> - These values of RPM change and increase when the model starts to move
> (!).
Yes, there are two effects:
1 - wind through the propeller lowers the load and allows the RPM to increase.
2 - ram air into the engine raises manifold pressure (and power) slightly
> It's possible to get RPM over 2000 only when flying, but the min.RPM
> turns then too much strong to try sucessfull landings.
>
> How can i fix each point?
>
> For better understanding, i'll add the values of my xml files:
>
>
> ENGINE
> ------
> <piston_engine name="ASz-62IRM18">
> <minmp unit="INHG"> 24.21 </minmp>
minmp controls how low you can idle the engine. Our engine model likes really
low values for this, and 24.21 inHg is wide open throttle for an unboosted
engine at 6000 ft asl.
> <maxmp unit="INHG"> 41.34 </maxmp>
Speaking of unboosted, the model will no longer create fake boost by putting
in a large manifold pressure, a supercharger must be specified.
> <displacement unit="IN3"> 1823 </displacement>
> <bore unit="IN"> 6.10 </bore>
> <stroke unit="IN"> 6.87 </stroke>
> <cylinders> 9 </cylinders>
> <compression-ratio> 6.4 </compression-ratio>
> <maxhp unit="HP"> 980 </maxhp>
> <cycles> 4 </cycles>
> <idlerpm> 550 </idlerpm>
> <maxrpm> 2200 </maxrpm>
> <maxthrottle> 1 </maxthrottle>
> <minthrottle> 0 </minthrottle>
min and max throttle don't do anything anymore
> <bsfc unit="LBS/HP*HR"> 0.671 </bsfc>
<volumetric-efficiency> 1.38 </volumetric-efficiency>
ve is a measure of how much air mass moves through the engine.
> </piston_engine>
>
>
> PROPELLER
> ---------
> <propeller name="AW-2-30">
> <ixx> 14.7823 </ixx>
> <diameter unit="IN">129.92 </diameter>
> <numblades> 4 </numblades>
> <gearratio> 1.45 </gearratio>
> <minpitch> 17 </minpitch>
> <maxpitch> 32 </maxpitch>
You didn't include the tables, so I can't comment here too much
but the min and max pitch must be in the range. The propeller I'm
attaching has tables running from -10 to 90 degrees. You can use min
pitch to control when the engine reaches 2200 rpm vs manifold pressure
and max pitch to control when the propeller is no longer able to keep the
engine speed down to 1500 rpm.
> <minrpm> 380 </minrpm>
380 translates to the idle rpm of 550. You don't want to do that
set it to 1031 so the propeller won't load the engine too much.
> <maxrpm> 1510 </maxrpm>
2200 * 0.687 is 1511 so maxrpm is good.
> <constspeed> 1 </constspeed>
> <sense> 1 </sense>
> <table name="C_THRUST" type="internal">
> (Here is the table in the XMLs)
> </table>
> <table name="C_POWER" type="internal">
> (Here is the table in the XMLs)
> </table>
> </propeller>
>
>
> Well, I was needed to ask here, because the documentation available in
> JSBSim manual and/or wiki was insufficient to do it for myself. I'll be
> very glad to get any info to work.
>
> Thanks
>
> ---
> PD: What is
> "ixx"
Ixx is the rotational moment of inertia for the propeller/engine combination.
Small numbers accelerate faster but bounce around more. Large numbers
accelerate slower and regulate smoother.
> "Spark fail drop",
Spark fail drop is the amount of power you get for single magneto operation.
> "Volumetric efficiency"
VE controls how much air goes through the engine at a given RPM. You can use
it to fine tune fuel flow at a given manifold pressure/rpm setting.
Its partner, BSFC, is the amount of power the engine produces per unit of fuel
consumed. Use it to tune the power produced.
Both are on the property tree under /fdm/jsbsim/propulsion/engine/ so you can
control them at run-time.
> and "Dynamic (and static) fmep"?
Useless debugging properties. Please ignore them.
> How can i to adjust the cooling factor to avoid engine overheating?
The latest code has two properties that can be set in the xml file. Cooling
factor can also be controlled at runtime via property tree
under /fdm/jsbsim/propulsion/engine/cooling-factor.
cylinder-head-mass controls how fast the engine heats up and cools off. So if
you have a '5-minute' limit on a power setting you can
adjust this value so the engine just starts to overheat at
the end of the given time frame.
cooling-factor controls how much 'air' flows over the engine to cool it.
Raising the value makes the engine run cooler. This can be
used to simulate cowl flaps, for example.
Hope this helps,
Ron
<?xml version="1.0"?>
<!--
Reference: http://en.wikipedia.org/wiki/PZL_ASz-62
-->
<piston_engine name="ASz-62IRM18">
<minmp unit="INHG"> 10.0 </minmp>
<maxmp unit="INHG"> 28.5 </maxmp>
<displacement unit="IN3">1823 </displacement>
<bore unit="IN"> 6.10 </bore>
<stroke unit="IN"> 6.87 </stroke>
<cylinders> 9 </cylinders>
<compression-ratio> 6.4 </compression-ratio>
<maxhp unit="HP"> 980 </maxhp>
<cycles> 4 </cycles>
<idlerpm> 550 </idlerpm>
<maxrpm> 2200 </maxrpm>
<maxthrottle> 1 </maxthrottle>
<minthrottle> 0 </minthrottle>
<bsfc unit="LBS/HP*HR"> 0.77 </bsfc>
<volumetric-efficiency> 1.38 </volumetric-efficiency>
<!-- Boost numbers estimated -->
<numboostspeeds> 1.0 </numboostspeeds>
<boostoverride> 0 </boostoverride>
<ratedboost1 unit="PSI"> 6.3 </ratedboost1>
<ratedpower1 unit="HP"> 980.0 </ratedpower1>
<ratedrpm1> 2200.0 </ratedrpm1>
<ratedaltitude1 unit="FT"> 0.0 </ratedaltitude1>
</piston_engine>
<?xml version="1.0"?>
<!-- Generated by Aero-Matic v 1.1
Inputs:
horsepower: 1000.0
pitch: variable
max engine rpm: 2200
prop diameter (ft): 10.826666666667
Outputs:
max prop rpm: 1733.04
gear ratio: 1.27
Cp0: 0.064522
Ct0: 0.150336
static thrust (lbs): 4097.96
-->
<propeller name="AW-2-30">
<ixx> 44.94 </ixx>
<diameter unit="IN"> 130. </diameter>
<numblades> 4 </numblades>
<gearratio> 1.456 </gearratio>
<p_factor> 10.26 </p_factor>
<minpitch> 12 </minpitch>
<maxpitch> 45 </maxpitch>
<minrpm> 1031 </minrpm>
<maxrpm> 1733 </maxrpm>
<!-- thrust coefficient as a function of advance ratio and blade angle -->
<table name="C_THRUST" type="internal">
<tableData>
-10 0 15 25 35 45 55 65 90
-0.2 -0.0734 0.0413 0.1503 0.1842 0.2030 0.2142 0.1974 0.1691 0.0000
0.0 -0.1090 0.0000 0.1503 0.1842 0.2030 0.2162 0.2021 0.1691 0.0000
0.2 -0.1222 -0.0376 0.1297 0.1804 0.2001 0.2162 0.2021 0.1691 0.0000
0.4 -0.1222 -0.0873 0.0977 0.1786 0.1963 0.2142 0.2021 0.1691 0.0000
0.6 -0.1222 -0.1222 0.0517 0.1607 0.1879 0.2087 0.1992 0.1691 0.0000
0.8 -0.1222 -0.1222 0.0029 0.1203 0.1824 0.2012 0.1992 0.1691 0.0000
1.0 -0.1222 -0.1222 -0.0489 0.0734 0.1748 0.1908 0.1974 0.1691 0.0000
1.2 -0.1222 -0.1222 -0.1006 0.0226 0.1437 0.1842 0.1974 0.1691 0.0000
1.4 -0.1222 -0.1222 -0.1222 -0.0329 0.1034 0.1813 0.1936 0.1691 0.0000
1.6 -0.1222 -0.1222 -0.1222 -0.0836 0.0564 0.1748 0.1899 0.1691 0.0000
1.8 -0.1222 -0.1222 -0.1222 -0.1222 0.0095 0.1503 0.1842 0.1691 0.0000
2.0 -0.1222 -0.1222 -0.1222 -0.1222 -0.0376 0.1174 0.1834 0.1691 0.0000
2.2 -0.1222 -0.1222 -0.1222 -0.1222 -0.0846 0.0846 0.1804 0.1691 0.0000
2.4 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 0.0451 0.1473 0.1691 0.0000
2.6 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 0.0057 0.0932 0.1503 0.0000
2.8 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 -0.0338 0.0610 0.1222 0.0000
3.0 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 -0.0734 0.0320 0.0940 0.0000
3.2 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 -0.1128 0.0029 0.0658 0.0000
3.4 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 -0.0263 0.0376 0.0000
3.6 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 -0.0555 0.0095 0.0000
3.8 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 -0.0846 -0.0188 0.0000
4.0 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 -0.1137 -0.0471 0.0000
6.0 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 -0.1222 0.0000
</tableData>
</table>
<!-- power coefficient as a function of advance ratio and blade angle -->
<table name="C_POWER" type="internal">
<tableData>
-10 0 15 25 35 45 55 65 90
-0.2 0.0108 0.0215 0.0753 0.1710 0.2949 0.4194 0.4839 0.5355 0.5355
0.0 0.0430 0.0108 0.0645 0.1594 0.2820 0.4194 0.4859 0.5355 0.5355
0.2 0.0613 0.0172 0.0624 0.1484 0.2697 0.4194 0.4859 0.5355 0.5355
0.4 0.0826 0.0376 0.0537 0.1368 0.2562 0.4194 0.4859 0.5355 0.5355
0.6 0.1013 0.0570 0.0355 0.1271 0.2400 0.4110 0.4839 0.5355 0.5355
0.8 0.1194 0.0763 0.0108 0.1078 0.2258 0.3923 0.4839 0.5355 0.5355
1.0 0.1374 0.0948 0.0108 0.0755 0.2129 0.3723 0.4820 0.5355 0.5355
1.2 0.1561 0.0758 -0.0355 0.0290 0.1884 0.3568 0.4788 0.5355 0.5355
1.4 0.1742 0.1310 -0.0536 -0.0215 0.1452 0.3516 0.4729 0.5355 0.5355
1.6 0.1923 0.1497 -0.0626 -0.0645 0.0916 0.3420 0.4626 0.5162 0.5355
1.8 0.2110 0.1678 -0.0645 -0.1078 0.0269 0.3033 0.4484 0.5052 0.5355
2.0 0.2291 0.1858 -0.0826 -0.1503 -0.0323 0.2581 0.4271 0.4949 0.5355
2.2 0.2471 0.2045 -0.1013 -0.1936 -0.0968 0.2097 0.4142 0.4729 0.5355
2.4 0.2658 0.2226 -0.1194 -0.2368 -0.1613 0.1497 0.4020 0.4626 0.5355
2.6 0.2839 0.2407 -0.1374 -0.2794 -0.2045 0.0626 0.3923 0.4465 0.5355
2.8 0.3020 0.2594 -0.1561 -0.3226 -0.2452 -0.0213 0.3839 0.4407 0.5355
3.0 0.3207 0.2774 -0.1742 -0.3658 -0.2903 -0.0968 0.3710 0.4407 0.5355
3.2 0.3387 0.2955 -0.1923 -0.4084 -0.3336 -0.1723 0.3471 0.4304 0.5355
3.4 0.3568 0.3142 -0.2110 -0.4517 -0.3762 -0.2471 0.2691 0.4194 0.5355
3.6 0.3755 0.3323 -0.2291 -0.4949 -0.4194 -0.3226 0.1890 0.4084 0.5355
3.8 0.3936 0.3504 -0.2471 -0.5355 -0.4626 -0.3981 0.1052 0.3955 0.5355
4.0 0.4117 0.3691 -0.2658 -0.5355 -0.5355 -0.4729 0.0213 0.3658 0.5355
6.0 0.5355 0.5355 -0.5355 -0.5355 -0.5355 -0.5355 -0.5355 -0.3226 0.5355
</tableData>
</table>
<!-- thrust effects of helical tip Mach -->
<table name="CT_MACH" type="internal">
<tableData>
0.85 1.0
1.05 0.8
</tableData>
</table>
<!-- power-required effects of helical tip Mach -->
<table name="CP_MACH" type="internal">
<tableData>
0.85 1.0
1.05 1.8
2.00 1.4
</tableData>
</table>
</propeller>
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