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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Fri Nov 26, 2021 11:11 am

Now how do rich the mixture to get that AFR at sea level ? I found no way … went down to sea level with the 10000ft settings, got AFR 13, so increased the mixture to 1, but AFR is still hanging around at 11.3 …
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Fri Nov 26, 2021 1:18 pm

Got this:
Code: Select all
<?xml version="1.0"?>
<system name="engine">
    <channel name="Volumetric efficiency">
        <fcs_function name="Volumetric efficiency">
            <function>
                <table>
                   <independentVar lookup="row">atmosphere/density-altitude</independentVar>
                   <independentVar lookup="column">aero/qbar-psf</independentVar>
                   <tableData>
                                     0             130
                       130       0.8443   0.8443
                       428       0.8239   0.8239
                       4387     0.7038   0.7038
                       9188     0.6697   0.6697
                       13920   0.6373   0.6373
                       15864   0.6249   0.6249
                       17796   0.5899   0.5659
                       19706   0.5787   0.5428
                       21650   0.5648   0.5292
                       23672   0.6052   0.5666
                   </tableData>
                </table>
            </function>
            <output>propulsion/engine/volumetric-efficiency</output>
        </fcs_function>
    </channel>
   
    <channel name="Power">
        <fcs_function name="BSFC">
            <function>
                <table>
                    <independentVar lookup="row">atmosphere/density-altitude</independentVar>
                    <independentVar lookup="column">aero/qbar-psf</independentVar>
                    <tableData>
                                        0               130
                        130         0.4788     0.4805
                        428         0.4761     0.4776
                        4387       0.4464     0.4475
                        9188       0.4466     0.4475
                        13920     0.44655   0.4473
                        15864     0.4466     0.44728
                        17796     0.4126     0.4131
                        19796     0.3796     0.38
                        21650     0.3469     0.34724
                        23672     0.3464     0.34675
                    </tableData>
                </table>
            </function>
            <output>propulsion/engine/bsfc-lbs_hphr</output>
        </fcs_function>
    </channel>
</system>


But the 130 and 428 need to be rechecked once we find a solution to get 9.87 AFR at sealevel.


Also I've now found the reason why we are getting different qbars: in effect, we aren't. But, you went to 17000ft, set the wind to 200kt and read the qbar value. But at that altitude, the indicated airspeed (and with that the airflow influence on the propeller ! and the qbar) is much less - only 130 kt or something like that. The true airspeed is of course still at 200kt. But for the propeller we need an indicated airspeed of 200kt, so one has to increase the wind with increasing altitude
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Fri Nov 26, 2021 4:00 pm

This doesn't look right … with the McCauley prop on I got almost 310 HP at sea level and could cruise at 230 kts at 2000ft, where it's supposed to be 285 HP and ~170 kts.
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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Fri Nov 26, 2021 8:42 pm

The prop has to be designed correct for the engine power. They're a matched pair, so first we get the engine correct then match a prop to it, based on power and max speed.

You can't skip the steps required
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Fri Nov 26, 2021 8:54 pm

Nah, I didn't try to skip anything ! ;) I just had doubts that this will match. But if you say it will … Now what need I do next ?
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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Fri Nov 26, 2021 9:38 pm

Well it's my mistake I used the 310hp rather than the 285hp.

We need to understand the tables needed to keep the fuel consumption static at 0.045lbs/sec and power static at 285hp up to the critical alt.

I'll knock out the ve table.
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Fri Nov 26, 2021 9:41 pm

S&J wrote in Fri Nov 26, 2021 9:38 pm:We need to understand the tables needed to keep the fuel consumption static at 0.045lbs/sec and power static at 285hp up to the critical alt.

Yeah, that's just what I did, right ?
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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Sat Nov 27, 2021 12:32 pm

Ok I noticed you had VE being adjusted after critical alt, it should remain constant and fuel pps will reduce as the engine is continuously leaned with increase in alt.

Also BSFC remains constant after 6000ft so the table can be shorter.

Code: Select all
<?xml version="1.0"?>
<system name="engine">
    <channel name="Volumetric efficiency">
        <fcs_function name="Volumetric efficiency">
            <function>
                <table>
                   <independentVar lookup="row">atmosphere/density-altitude</independentVar>
                   <independentVar lookup="column">aero/qbar-psf</independentVar>
                   <tableData>
                                     0             130
                       130       0.8443   0.8443
                       428       0.8239   0.8239
                       4387     0.7038   0.7038
                       9188     0.6697   0.6697
                       13920   0.6373   0.6373
                       15864   0.6249   0.6249
                       17796   0.5899   0.5659
                       19706   0.5899   0.5428
                       21650   0.5899  0.5428
                       23672   0.5899  0.5428
                   </tableData>
                </table>
            </function>
            <output>propulsion/engine/volumetric-efficiency</output>
        </fcs_function>
    </channel>
   
    <channel name="Power">
        <fcs_function name="BSFC">
            <function>
                <table>
                    <independentVar lookup="row">atmosphere/density-altitude</independentVar>
                    <independentVar lookup="column">aero/qbar-psf</independentVar>
                    <tableData>
                                        0               130
                        130         0.4788     0.4805
                        428         0.4761     0.4776
                        4387       0.4464     0.4475
                        9188       0.4466     0.4475
                    </tableData>
                </table>
            </function>
            <output>propulsion/engine/bsfc-lbs_hphr</output>
        </fcs_function>
    </channel>
</system>
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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Sat Nov 27, 2021 12:41 pm

So

you've understood the changes to VE and created a table
You've understood the changes to BSFC at alts below 5000ft and created a table
You've understood that an AFR of 9.9 in jsbsim gives the best power

So you should have two set of tables VE and BSFC
and but not required if you want to adjust manually
a bit of code that automatically adjusts the mixture to maintain the AFR at 9.9

If you've that you've now the most accurate jsbsim turbo charged piston engine in Flightgear

Well done.. I know it's been a challenge

So if you want, you could now start a new topic to create the prop to go with it.
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Sat Nov 27, 2021 2:40 pm

S&J wrote in Sat Nov 27, 2021 12:41 pm:and but not required if you want to adjust manually
a bit of code that automatically adjusts the mixture to maintain the AFR at 9.9

Yep, I was planning to do that as part of a "virtual copilot" !
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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Sat Nov 27, 2021 2:56 pm

The P51 has a automatic mixture control, so it's not an unrealistic control.

You simply adjust the lever from lean to normal to rich and it automatically adjust the mixture to maintain this value with change in height.
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Sat Nov 27, 2021 3:05 pm

Ah … well, I don't think the Cessna 210 has such a feature, but I will look through the POH and see what I find !

I would like to keep the prop development in this thread - there is no need for a separate thread. When it's finished and working, I think I will compress this whole thread into a wiki Howto.


The throttle still has no effect between 60 and 100 % - what can we do about that ?
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Re: Cannot properly increase propeller pitch and engine thro

Postby S&J » Sat Nov 27, 2021 3:43 pm

The throttle is behaving correctly.

The throttle is adjusting the speed that the turbo charger is spinning at. At 60% speed it's spinning sufficient fast enough to produce the rated manifold pressure of 35.5. When you increase the throttle, the turbo charger increases speed and generates more pressure. However above 35.5 the waste gate opens and vents additional pressure to atmosphere, maintaining the 35.5 at the engine manifold.

At higher alts you'll notice the throttle does work past 60%
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Re: Cannot properly increase propeller pitch and engine thro

Postby TheEagle » Sat Nov 27, 2021 5:05 pm

Well, I doubt this works this way in reality - should be % throttle == % power. So how would I achieve that ?
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Re: Cannot properly increase propeller pitch and engine thro

Postby wkitty42 » Sat Nov 27, 2021 5:49 pm

TheEagle wrote in Sat Nov 27, 2021 5:05 pm:Well, I doubt this works this way in reality - should be % throttle == % power.

i don't think that applies to this type of engine... it would for a normally aspirated engine with a fixed pitch prop, though...
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