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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Thu Jan 10, 2019 5:21 am

Check out the recent retrofit of the FG1000 to the J3Cub and the PA-18.
Available in the development repository and fgaddon.

Thank you Stuart!

Image

With the shadow cube effect...
Image
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Re: JSBSim Piper J-3 Cub

Postby Knüppelrührer » Sat Feb 23, 2019 7:16 pm

This aircraft is really good fun! Thanks to everyone developping it!

Can it be, that the switching between different engine versions does not work?
94kts cruise at 2150rpm and 1200fpm climb rate is a bit too powerfull for 65hp from 1943.
When changing via aircraft options, there is not much difference.

The other item is, that I am really struggling to bleed of the energy. Making a 3-pointer is almost impossible within a reasonable runway length. Anybody with a similar experience or is it just missing technique?


FG2018.3.2, standard Piper J-3
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sat Feb 23, 2019 8:38 pm

Hum, you have pretty much confirmed my observations of the power of this aircraft. I am one of the people that have done a lot of the work on this aircraft but unfortunately I am really weak on the fundamentals of FDM development. Maybe we can get Erik or Dany to comment and help make some improvements to the FDM if necessary. I doubt that the different engines are not switching correctly but I suppose it is possible. I can take a look at that.

This brings up something I am a bit concerned about regarding another aircraft. I have a potential issue with an FDM that for no "apparent" reason is performing beyond its original power settings and don't really have any explanation as to why.

General question for any other developers reading this post, is it possible that some of the upgrades made to the core systems that have produced better general overall simulator performance over the last year or two could have caused FDM engine performance to show a noticeable increase, including any changes to jsbsim?
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sat Feb 23, 2019 8:47 pm

Knüppelrührer out of curiosity, have you tried the yasim version and if so, what do you think about the performance of that 65hp engine?
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sat Feb 23, 2019 8:51 pm

Hum, there may be a problem with the fgaddon version repository.
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Re: JSBSim Piper J-3 Cub

Postby Knüppelrührer » Sat Feb 23, 2019 9:22 pm

Thanks for confirming!

Tuning FDM would be interesting, but also no experience on my side. I can only contribute by test flying.

In Yasim with 53 lbs fuel, 180lbs pilot it gives at 100% power 72kts at 1700ft altitude.
Climbrate over 1000ft at 60mph=700ft/min.
Seems to be more in the correct ballpark.

picture below is in the 65hp JBSIm version:

Image
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Re: JSBSim Piper J-3 Cub

Postby erik » Sun Feb 24, 2019 10:12 am

I'm fairly sure the FDM itself is is decent shape but I was never really satisfied with the propeller table creating code of AeromatiC++.
So the first thing I would do is to see if you can get better thrust and power tables for the propeller.

Update: A quick fix would be to lower the ct_factor in the CM7445 MCCauley.xml configuration file in the Engine directory.

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Re: JSBSim Piper J-3 Cub

Postby Knüppelrührer » Sun Feb 24, 2019 2:05 pm

For example ct=1.2 does not seem to be too bad.
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Sun Feb 24, 2019 3:37 pm

I have stopped a long time ago of using the propeller tables from Aeromatic. I found several times that it was a calamity.
I use the NACA curves NACA Report 640 (1938)

Yes, the current CM7445 MCCauley.xml propeller file seems to have a serious issue.

Can you try merely with the copied c172p propeller file, which is in the J3Cub "Engines" directory?
prop_75in2f.xml.

With the 65 hp engine, Full thrust, I find (about)
- climb at 60 MPH (54 kt): 600 to 670 fpm, 1841 RPM, 52 hp.
- level flight: 2000 RPM, 55 hp, 100 MPH (88 kt). (Although full thrust is normally not practiced at level flight)

If you confirm, possibly to be refined (I think with smaller pitch angle), but more plausible than the current one.
Last edited by dany93 on Mon Feb 25, 2019 3:16 pm, edited 1 time in total.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sun Feb 24, 2019 4:51 pm

Thanks Erik and Dany.

@Knüppelrührer I posted the changes in the development git repository at
https://github.com/wlbragg/J3Cub

I think I need to find some more information on this propeller and the expected thrust. I feel like this needs more top end power and less idle power.
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Re: JSBSim Piper J-3 Cub

Postby Knüppelrührer » Sun Feb 24, 2019 5:20 pm

Thanks mates!

A bit of research:
https://www.risingup.com/planespecs/info/airplane464.shtml

The other number taken from a youtube video:
Cruise 79mph at 2150 rpm with two persons on board.

So using the C172 propeller might still be overpowered.

I was wondering about my landings being extremely difficult.
So glide ratio with the current model is measuered with engine off 1:9, which might be realistic.
With engine idle it is 1:12. So there might be too much thrust at idle, as already mentioned above.
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Sun Feb 24, 2019 7:07 pm

wlbragg wrote in Sun Feb 24, 2019 4:51 pm:I think I need to find some more information on this propeller and the expected thrust. I feel like this needs more top end power and less idle power.

The prop_75in2f.xml is the initial propeller file for the C172p 160 hp (probably by Megginson). Still used.
Its values are different from the NACA graphs (seems not far from proportional) and it works well... I wonder why.
I hope that the NACA curves will work, if we need them (they did well for the c172p with the 180 hp engine).

If the propeller pitch angle is decreased, the max reachable RPM will rise, and the max power at the same time.

Usually, the idle RPM can be set by <minmp unit="INHG"> in the engine file.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sun Feb 24, 2019 7:43 pm

I discovered something else and am going to try to do some extensive testing for myself.
The CM7445 MCCauley we've been using has some newer tags that I think may be part of the issue for the over power condition.

cp_factor = 1
ct_factor = 2

The cp_factor I assume is nil at 1?
However the ct_factor was set to 2. By commenting out the both of them The max speed fell right in line with the specs at 87 mph.

Leaving the ct_factor set to 2 increased the max speed to 115 mph.

The prop_75in2f was still a bit over powered for max speed at 92 mph

Changes to the CM7445 MCCauley and CM7445 MCCauley2 have been pushed to the git repository.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Mon Feb 25, 2019 3:20 am

I added the Davtron803 AP functionality to the J3Cub/PA-18 FG1000 variant. It appears to work well. It's really nice to have AP functionality built in to the FG1000. I left the ga-it-autopilot hooked into the GUI Autopilot menu so we still have a general aviation autopilot in all the variants. I suppose I should add the Davtron803 AP equipment to the panel of PA-18 version?
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Mon Feb 25, 2019 11:18 am

wlbragg wrote in Sun Feb 24, 2019 7:43 pm:I discovered something else
cp_factor = 1
ct_factor = 2

I hadn't seen that... :?

ct factor = 2 is obviously wrong: multiplying Ct by2 gives an energetic efficiency of up to 160 %.
Parameter signification, and the coefficients, calculations are well explained in JSBSim Thrusters
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