JSBSim propeller: Why is the idle prop rpm that low?

by **dg-505** » Sun Jan 01, 2017 3:21 pm

Hi

here's what I have:
1. A turboprop engine with an idle rpm of 19,500 (=52% n1), full power (100% n1) is 37,500 rpm.
2. The reduction ratio is ca. 15:1 which means that, according to my understanding, the propeller rpm is 15 times lower than the engine rpm.
=> at idle power, the rpm should be 19,500/15 = 1300 RPM

Instead, when power is at idle, the propeller rpm is ca. 114 ("fdm/jsbsim/propulsion/engine/propeller-rpm") which is definitely lower than the said reduction ratio of 15:1.

Now I wonder which parameters I have to adjust that the idle propeller rpm is at the correct value.

Thanks in advance
Jonathan

Engine file:

Code: Select all: <?xml version="1.0"?>  <turboprop_engine name="PT6A-27"> <milthrust unit="LBS"> 1388.8 </milthrust> <idlen1> 52.0 </idlen1> <maxn1> 101.5 </maxn1> <maxpower unit="HP"> 620.0 </maxpower> <psfc unit="LBS/HR/HP"> 0.74 </psfc> <n1idle_max_delay> 1.2 </n1idle_max_delay> <maxstartingtime> 0.1 </maxstartingtime> <startern1> 16.0 </startern1> <ielumaxtorque unit="FT*LB"> 1536.0 </ielumaxtorque> <itt_delay> 0.05 </itt_delay> <betarangeend> 64.0 </betarangeend> <reversemaxpower> 91.0 </reversemaxpower> <function name="EnginePowerVC"> <table> <description> Engine power, function of airspeed and pressure </description> <independentVar lookup="row">atmosphere/P-sl-psf</independentVar> <independentVar lookup="column">velocities/ve-kts</independentVar> <tableData> 0 50 100 150 200 250 503 0.357 0.380 0.400 0.425 0.457 0.486 1048 0.586 0.589 0.600 0.621 0.650 0.686 1328 0.707 0.721 0.731 0.757 0.786 0.821 1496 0.779 0.786 0.808 0.821 0.857 0.900 1684 0.850 0.857 0.874 0.900 0.943 0.979 1896 0.914 0.929 0.946 0.971 1 1.057 2135 1 1.011 1.029 1.043 1.083 1.150 2213 1.029 1.043 1.057 1.079 1.114 1.171 </tableData> </table> </function> <table name="EnginePowerRPM_N1" type="internal"> <description> Engine Power, function of RPM and N1 </description> <tableData> 0 5 60 86 94 95 96 97 98 99 100 101 0.0 0.0 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 803.5 0.0 0.4 29.2 166.9 292.1 317.1 350.5 383.8 417.2 442.2 484.0 517.4 1203.0 0.0 0.4 25.0 192.0 333.8 358.8 392.2 421.4 458.9 492.3 534.1 567.4 1604.8 0.0 0.4 4.2 200.3 367.2 396.4 425.6 458.9 496.5 525.7 567.4 600.8 2006.5 0.0 0.1 0.1 187.7 375.5 413.0 438.1 471.4 509.0 542.4 584.1 621.7 2207.4 0.0 0.1 0.1 171.1 367.2 400.5 429.7 467.3 504.9 538.2 579.9 620.0 </tableData> </table> <table name="ITT_N1" type="internal"> <description> Inter-Turbine Temperature ITT [deg C] depending on N1 and engine run (0=off / 1=running) </description> <tableData> 0 1 0 0 0 15 105.1 105.1 60 188.5 546.7 96 283.5 714.8 100 294.4 790.2 </tableData> </table> <table name="CombustionEfficiency_N1" type="internal"> <description>Dependency of fuel efficiency coefficient on N1 (and RPM)</description> <tableData> 90 0.1221 91.2 0.2834 92.2 0.5336 93.4 0.7188 94.1 0.7741 95.2 0.8471 96.5 0.9001 100 1 </tableData> </table> </turboprop_engine>

Propeller file:

Code: Select all: <?xml version="1.0"?>  <propeller version="1.01" name="HARTZELL-T10282H"> <ixx> 25.1034 </ixx> <diameter unit="IN"> 102 </diameter> <numblades> 3 </numblades> <gearratio> 15.0 </gearratio> <cp_factor> 3 </cp_factor> <ct_factor> 3 </ct_factor>  <minpitch> 0 </minpitch> <maxpitch> 87 </maxpitch> <minrpm> 1650 </minrpm> <maxrpm> 2110 </maxrpm> <constantspeed> 1 </constantspeed> <reversepitch> -15 </reversepitch>  <table name="C_THRUST" type="internal"> <tableData> -15 0 15 30 45 60 0.00 0.0492 0.1132 0.1773 0.2413 0.3054 0.3694 0.05 0.0431 0.1068 0.1704 0.2337 0.2967 0.3596 0.10 0.0371 0.1006 0.1638 0.2264 0.2886 0.3503 0.15 0.0311 0.0946 0.1574 0.2195 0.2808 0.3415 0.20 0.0253 0.0887 0.1513 0.2129 0.2735 0.3332 0.25 0.0194 0.0830 0.1454 0.2066 0.2666 0.3254 0.30 0.0136 0.0774 0.1397 0.2005 0.2600 0.3180 0.35 0.0079 0.0718 0.1342 0.1948 0.2537 0.3110 0.40 0.0021 0.0664 0.1288 0.1893 0.2478 0.3044 0.45 -0.0037 0.0610 0.1236 0.1840 0.2421 0.2981 0.50 -0.0095 0.0557 0.1185 0.1789 0.2368 0.2922 0.55 -0.0154 0.0505 0.1136 0.1740 0.2316 0.2866 0.60 -0.0212 0.0453 0.1088 0.1693 0.2268 0.2813 0.65 -0.0272 0.0401 0.1041 0.1648 0.2221 0.2762 0.70 -0.0331 0.0349 0.0994 0.1604 0.2177 0.2715 0.75 -0.0392 0.0298 0.0949 0.1561 0.2135 0.2670 0.80 -0.0453 0.0247 0.0904 0.1520 0.2094 0.2627 0.85 -0.0515 0.0195 0.0860 0.1481 0.2056 0.2586 0.90 -0.0578 0.0144 0.0817 0.1442 0.2019 0.2548 0.95 -0.0642 0.0092 0.0774 0.1404 0.1983 0.2511 1.00 -0.0706 0.0040 0.0731 0.1368 0.1949 0.2476 1.05 -0.0772 -0.0012 0.0689 0.1332 0.1917 0.2443 1.10 -0.0839 -0.0065 0.0647 0.1297 0.1885 0.2412 1.15 -0.0907 -0.0118 0.0605 0.1263 0.1855 0.2382 1.20 -0.0976 -0.0171 0.0564 0.1230 0.1826 0.2353 1.25 -0.1047 -0.0225 0.0522 0.1197 0.1798 0.2326 1.30 -0.1118 -0.0280 0.0481 0.1165 0.1771 0.2300 1.35 -0.1191 -0.0335 0.0440 0.1133 0.1745 0.2276 1.40 -0.1265 -0.0391 0.0398 0.1102 0.1720 0.2252 1.50 -0.1418 -0.0504 0.0315 0.1040 0.1671 0.2208 1.60 -0.1576 -0.0620 0.0232 0.0981 0.1626 0.2168 1.70 -0.1739 -0.0739 0.0148 0.0922 0.1583 0.2131 1.80 -0.1909 -0.0862 0.0062 0.0864 0.1542 0.2097 1.90 -0.2084 -0.0987 -0.0024 0.0806 0.1502 0.2065 2.00 -0.2266 -0.1116 -0.0112 0.0749 0.1465 0.2036 2.10 -0.2454 -0.1249 -0.0201 0.0692 0.1429 0.2009 2.20 -0.2648 -0.1386 -0.0291 0.0635 0.1393 0.1984 2.30 -0.2848 -0.1526 -0.0384 0.0578 0.1359 0.1960 2.40 -0.3055 -0.1670 -0.0478 0.0521 0.1326 0.1938 </tableData> </table>  <table name="C_POWER" type="internal"> <tableData> -15 0 15 30 45 60 0.00 0.0065 0.0313 0.0786 0.1483 0.2405 0.3553 0.05 0.0076 0.0339 0.0828 0.1541 0.2479 0.3643 0.10 0.0082 0.0361 0.0866 0.1596 0.2550 0.3730 0.15 0.0083 0.0379 0.0900 0.1647 0.2619 0.3816 0.20 0.0080 0.0392 0.0931 0.1695 0.2685 0.3900 0.25 0.0072 0.0402 0.0957 0.1740 0.2748 0.3983 0.30 0.0060 0.0407 0.0981 0.1782 0.2810 0.4066 0.35 0.0043 0.0408 0.1001 0.1821 0.2870 0.4147 0.40 0.0022 0.0405 0.1017 0.1858 0.2928 0.4227 0.45 -0.0004 0.0398 0.1031 0.1893 0.2985 0.4308 0.50 -0.0035 0.0387 0.1041 0.1925 0.3041 0.4388 0.55 -0.0070 0.0372 0.1047 0.1955 0.3095 0.4468 0.60 -0.0110 0.0353 0.1051 0.1982 0.3148 0.4547 0.65 -0.0156 0.0330 0.1051 0.2008 0.3200 0.4627 0.70 -0.0206 0.0303 0.1048 0.2031 0.3251 0.4708 0.75 -0.0261 0.0271 0.1042 0.2052 0.3301 0.4788 0.80 -0.0321 0.0236 0.1033 0.2071 0.3350 0.4869 0.85 -0.0386 0.0196 0.1021 0.2088 0.3398 0.4951 0.90 -0.0457 0.0152 0.1005 0.2103 0.3445 0.5033 0.95 -0.0533 0.0103 0.0986 0.2116 0.3492 0.5115 1.00 -0.0614 0.0050 0.0964 0.2126 0.3538 0.5198 1.05 -0.0701 -0.0007 0.0938 0.2135 0.3583 0.5282 1.10 -0.0794 -0.0069 0.0909 0.2141 0.3627 0.5367 1.15 -0.0892 -0.0136 0.0877 0.2146 0.3671 0.5452 1.20 -0.0996 -0.0207 0.0841 0.2148 0.3714 0.5538 1.25 -0.1106 -0.0282 0.0802 0.2148 0.3756 0.5625 1.30 -0.1221 -0.0363 0.0759 0.2146 0.3797 0.5712 1.35 -0.1343 -0.0449 0.0713 0.2142 0.3838 0.5800 1.40 -0.1471 -0.0539 0.0663 0.2135 0.3877 0.5889 1.50 -0.1746 -0.0735 0.0553 0.2116 0.3955 0.6070 1.60 -0.2046 -0.0950 0.0427 0.2087 0.4029 0.6253 1.70 -0.2372 -0.1187 0.0287 0.2049 0.4100 0.6439 1.80 -0.2726 -0.1445 0.0130 0.2001 0.4167 0.6628 1.90 -0.3106 -0.1725 -0.0042 0.1943 0.4230 0.6820 2.00 -0.3514 -0.2027 -0.0230 0.1875 0.4290 0.7014 2.10 -0.3950 -0.2351 -0.0435 0.1797 0.4346 0.7211 2.20 -0.4416 -0.2698 -0.0657 0.1708 0.4397 0.7410 2.30 -0.4910 -0.3069 -0.0896 0.1609 0.4444 0.7611 2.40 -0.5434 -0.3463 -0.1152 0.1498 0.4487 0.7815 </tableData> </table>  <table name="CT_MACH" type="internal"> <tableData> 0.85 1.0 1.05 0.8 </tableData> </table>  <table name="CP_MACH" type="internal"> <tableData> 0.85 1.0 1.05 1.8 2.00 1.4 </tableData> </table> </propeller>

by **erik** » Sun Jan 01, 2017 3:38 pm

What does the engine-rpm indicate?
I've just tried it with the pc7 and both the engine and propeller rpm are quite low, indicating the problem might be the engine idle rpm.

Erik

by **dg-505** » Sun Jan 01, 2017 3:58 pm

Well, engine-tpm is indeed just about 1991 but n1 is indeed at 52.
So how are n1 and engine-rpm related to each other?

by **erik** » Sun Jan 01, 2017 10:57 pm

To be honest I have no idea without diving into the code for that but I'm already busy with other projects.

Erik

by **AndersG** » Sun Jan 01, 2017 11:29 pm

dg-505 wrote in Sun Jan 01, 2017 3:58 pm:Well, engine-tpm is indeed just about 1991 but n1 is indeed at 52.
So how are n1 and engine-rpm related to each other?

IIRC the JSBSim turboprop model is of one particular type (not model :) of turboprop engine - there are several.
It might be that the modelled type is one where the power turbine (engine-tpm?) is separate from the gas generator (compressor + its turbine, n1?).

by **erik** » Mon Jan 02, 2017 10:14 am

That could well be. The question is how important is idle rpm?
When you push the throttle the engine spools up quite quickly towards operating rpm.

Erik

by **dg-505** » Mon Jan 02, 2017 1:45 pm

Hmm, I don't really know....

Until now I assumed that engine rpm and n1 are the same, with the difference that n1 is a percentage value and engine rpm is the related true speed of rotation.

In fact it seems that that these are two different values, however they are related to each other...

What I just want to know is how is the relationship between n1, engine-rpm and propeller-rpm.

by **erik** » Mon Jan 02, 2017 2:26 pm

It would be great to have it set properly but I have no experience with the code itself. Only with creating the appropriate tables for it.

Erik

by **Alant** » Mon Jan 02, 2017 6:41 pm

For this engine , which has a constant speed drive, there is no direct relation between N1 (sometimes called Ng) and propeller rpm.

This article may help http://hueyman.overblog.com/propellers-propellers

Alan

by **dg-505** » Wed Jan 04, 2017 5:11 pm

Thank you Alan, this article helped me quite alot!

Let's see if I understood it correctly:
There is no direct mechanical connection between betwenn the gas generator (ng) and the propeller turbine. The air first goes through the compressor and the gg. Then, after combustion it goes through tje propeller turbine and after that it exhausts.

Is that correct??

The gear ratio of 15:1 is not between gas generator and propeller, but between propeller turbine and the propeller itself. As this article states, 100% np are equal t0 33,000 rpm of th propeller turbine, which corresponds to 2,200 rpm of the propeller (33,000 / 15 = 2,200).

Now we need to know how the rpm is calculated. I tried to have a look at the JSBSim source code (the "FGPropeller" class defined in FGPropeller.cpp and FGPropeller.h). I'm far from understanding it completely but to me it seems that propeller rpm is calculated using the torque and the propeller inertia (line 283 of FGPropeller.cpp). After that the engine-rpm is calculated with propeller rpm * gear ratio (line 216 of FGPropeller.h).
If I read the code correctly, the engine rpm isn't calculated with n1, but with the propeller rpm.

Now, changing the only predefined value in the propeller rpm calculation (i.e. inertia) doesn't change anything of the idle rpm, but instead decreasing the inertia value in the propeller definition file (<ixx></ixx>) decreases the spool-up time, and increasing the value increases the spool-up time, but without changing the resulting rpm of the propeller.

So does anyone know what parameters I have to adjust to change the propeller rpm at a given power setting (as long as the prop lever is at the same position)?

Another thing I noticed: The engine doesn't have the capability to bring the propeller to 100% Np (2200 rpm), no matter of the prop setting. Even with a blade angle of 0° which means minimum drag, the propeller doesn't reach it's full rpm. On which parameters is this relationship based on?

Thanks

by **Alant** » Wed Jan 04, 2017 10:50 pm

I am slightgly familiar with the turbne engine code, but have never looked into the turboprop.
Perhaps it is as simple as playing with the xml engine and prop tables.
Failing that, as the prop rpm is only required to move the gauge pointer, you could make some nasal or xml code in your fdm which calculates a more realistic value dependant upon engine and ambient conditions.
A question on the JSBSim or Flightgear-developers mail list may get a better answer than this.

Alan

by **sanhozay** » Wed Jan 04, 2017 11:25 pm

dg-505 wrote in Wed Jan 04, 2017 5:11 pm:it seems that propeller rpm is calculated using the torque and the propeller inertia

Have you been able to find a specification that gives the weight of the propeller?

by **AndersG** » Wed Jan 04, 2017 11:31 pm

dg-505 wrote in Wed Jan 04, 2017 5:11 pm:So does anyone know what parameters I have to adjust to change the propeller rpm at a given power setting (as long as the prop lever is at the same position)?

Wouldn't that (propeller RPM) be a matter of the input mapping of the propeller speed control for a constant speed propeller? I.e. the power setting plays no role at all (for RPM) as long as the propeller pitch hasn't hit the fine or coarse stop.

I haven't made any aircraft with a constant speed propeller, but I think at least the initial adjustments can be made using the <minrpm>, <maxrpm>, <minpitch> and <maxpitch> elements in the propeller file.

by **sanhozay** » Thu Jan 05, 2017 12:00 am

In the process of modelling my Beagle Pup, (which is a piston aircraft, rather than a turboprop), I couldn't get anywhere near quoted values for idle and static RPM by tweaking engine parameters, especially minrpm and maxrpm. This was using Aeromatic++, which can only do so much with the propeller inputs it has.

With Bomber's help, I modelled the propeller(s) using JavaProp and calculated the inertia from the propeller weight. With that done, the RPM numbers dropped into place. With drag and engine parameters tuned, it matches performance and fuel consumption to within 5% at all the altitudes I have figures for (across two different aircraft models with different engines).

The discussion was here:

http://www.thejabberwocky.net/viewtopic ... s&start=40
http://www.thejabberwocky.net/viewtopic ... =40#p17282

Obviously a constant speed propeller is different and the turboprop engine is a different model, but I suspect that it will still be difficult to get quoted RPMs without working on the propeller model.

by **erik** » Thu Jan 05, 2017 10:04 am

That was also my idea, try to get a proper propeller configuration using, for instance, JavaProp.
Note that you probably want to set cp_factor and ct_factor to 1.0 again for that.

Erik

Board index ‹ FlightGear ‹ Development ‹ Aircraft ‹ Flight dynamics model

JSBSim propeller: Why is the idle prop rpm that low?

JSBSim propeller: Why is the idle prop rpm that low?

Re: JSBSim propeller: Why is the idle prop rpm that low?

Re: JSBSim propeller: Why is the idle prop rpm that low?

Re: JSBSim propeller: Why is the idle prop rpm that low?

Re: JSBSim propeller: Why is the idle prop rpm that low?

Re: JSBSim propeller: Why is the idle prop rpm that low?

Re: JSBSim propeller: Why is the idle prop rpm that low?

Re: JSBSim propeller: Why is the idle prop rpm that low?

Re: JSBSim propeller: Why is the idle prop rpm that low?

Re: JSBSim propeller: Why is the idle prop rpm that low?

Re: JSBSim propeller: Why is the idle prop rpm that low?

Re: JSBSim propeller: Why is the idle prop rpm that low?

Re: JSBSim propeller: Why is the idle prop rpm that low?

Re: JSBSim propeller: Why is the idle prop rpm that low?

Re: JSBSim propeller: Why is the idle prop rpm that low?

Who is online