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Re: JSBSim Piper J-3 Cub

Postby dany93 » Mon Feb 25, 2019 3:56 pm

Please, take me out of a doubt.

The Continental A-65-8.xml file contains
Code: Select all
 <maxrpm>                  2800.0      </maxrpm>

Which I cannot reach by doing tests with the 74" propeller. Hence my doubt and my search.

However, I have found 3 references saying that this engine spec is at 2300 RPM.
https://fr.scribd.com/doc/26043744/CONT ... GINE-SPECS
https://airandspace.si.edu/collection-o ... d-4-engine
http://www.neam.org/engine-collection-d ... inentala65

This would change everything when we check the RPMs.
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Re: JSBSim Piper J-3 Cub

Postby Knüppelrührer » Mon Feb 25, 2019 4:58 pm

When 2150 rpm is a typical cruise setting, 2300 max seems to be more realistic than 2800.
And isn't there a red line on the tachometer?
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Mon Feb 25, 2019 5:24 pm

It should be 2300.

The 90hp is set at 2800.0 and yet the IO320 (160 hp) is set to 2700.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Mon Feb 25, 2019 5:50 pm

The 65hp engine set at 2300 max rpm and removing the ct_factor brought this regime back into line somewhat. It may be a bit underpowered now. I can't reach the rated 450 ft/min climb rate nor the 87 mph max speed, I come in at about 300-350 climb rate at full power and 75 max speed.
But I totally notice a remarkable improvement on runway roll out, you now have to run a ways with no wind to get off the ground. I haven't tested the amphibious yet which will really be interesting.
I don't know anything about prop settings. Is there some changes to pitch or something else that would allow for fine tuning to squeeze out the 100 feet/min CR and 2 mph max speed were short on?
I know they have things such as power prop VS speed prop, do we have the ability to set props to these differences.
I wouldn't hesitate to add a couple different prop choices per engine, say a speed and power prop for each.
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Mon Feb 25, 2019 6:05 pm

When we have a propeller main features (diameter, CT and CP curves), the pitch angle (for a fixed pitch angle propeller) remains to be set. It can be long and tiresome.

A very convenient way that I use to do this is using a "variable pitch" propeller file. I added such a file in the c172p "Engines" folder.

1 - From the c172p, copy the prop_75in_2f_NACA_15-30deg.xml (done from the NACA curves for a two-blade propeller). Paste it in the J3Cub "Engines" folder.

2 - Change the diameter
Code: Select all
<diameter unit="IN"> 74.0 </diameter>

(and possibly the file name)

3 - In the FDM, add this propeller, replace the ususal one for this filename
Code: Select all
    <!-- <thruster file="CM7445 MCCauley"> -->
    <thruster file="prop_74in_2f_NACA_15-30deg">


4 - in the sim, you can set the blade angle from 15 to 30 deg in /fdm/jsbsim/propulsion/engine/blade-angle = .....

IMO, a good start is 18 to 20 deg.

18 deg gives a climbing at 2000 RPM, power-hp = 65 hp (best use of the engine power).
Unfortunately, the climbing rate (Gross Weight 1205 lbs) is about 700 RPM, greater than you think it should be.
However, I currently don't see a true fix to noticeably decrease this climbing rate, other than increasing the aircraft drag. The consumed power is due to the aircraft drag and the power needed for climbing at this vertical velocity. This power (plus some lost due to energy efficiency) corresponds to the one given by the engine. And this balance is only weakly (slowly) dependent on the propeller settings. The propeller settings just make that the engine is (or is not) used at its max power.

Or make a "bad" propeller !! :lol:
Last edited by dany93 on Mon Feb 25, 2019 10:35 pm, edited 1 time in total.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Mon Feb 25, 2019 6:24 pm

Thanks Dany, that's really helpful.

After doing some digging I found out the 2800 rpm is typical of the Continental 90. So I think that number must have accidentally been added to the 65. I did see a 2350 rpm rating on the Continental 65 hp engine.
Also there is the Lycoming...
Lycoming boosted the rpm and power output to 65 hp (48 kW) at 2550 rpm
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Re: JSBSim Piper J-3 Cub

Postby IceBreaker » Mon Feb 25, 2019 8:52 pm

I have exported the template in .obj format to Blender. All right, but I discovered some errors in the geometry of the model, in fact with Flightgear I immediately noticed some overlapping errors polygons and parts of the plane with sides and vertices that did not make some parts rounded.

here are the screenshots:

surface bug.
Image

Fast correction, to experiment in Flightgear, seems to work well now.
Image
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Mon Feb 25, 2019 9:00 pm

I still have some more fine tuning to do but I think we are fairly close to specs on all three engine/prop combinations. Far better that it was.
I tested both on land and water and now it takes some skill to take off and land on both. I'm much happier with the performance.
I pushed changes to the git repository only. I'll push to fgaddon after some more testing.
Anyone interested please test and feel free to give me feedback.
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Re: JSBSim Piper J-3 Cub

Postby Octal450 » Mon Feb 25, 2019 9:02 pm

Davtron803 is a clock btw @wlbragg, not an A/P controller: https://www.davtron.com/product-detail.php?M803-14
So not quite sure what you mean.

Also note: IT-AUTOFLIGHT GA is discontinued, I suggest replacing it with the much more realistic successor: http://wiki.flightgear.org/S-TEC_55X

I'll be happy to test for you, but keep in mind I've never flown a real J3. I can give you my opinions though.

Kind Regards,
Josh
Waste of time. Goodbye forever.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Mon Feb 25, 2019 9:14 pm

Wow, I look forward to seeing the model in detail. Are you going to put it in a public repository somewhere so we can have a good look at it? Eventually if it is to replace the existing model we will need to part out the objects for animation, of course.

You interested in doing the c172p, that is one model that could really stand to be retopologized?

Davtron803 is a clock btw @wlbragg, not an A/P controller:

Oops, so it is. I wasn't paying close attention when i added Stuarts changes to the FG1000 and I thought it required another AP system added to the set file. I guess I got confused with other changes added to the C182.
In my defense I am currently sick and running a fever. :lol:
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Re: JSBSim Piper J-3 Cub

Postby IceBreaker » Mon Feb 25, 2019 9:16 pm

The Piper J3 Cub is one of the most preferred aircraft, for simplicity and to enjoy flights in VFR (it is practically an ultralight disguised as a true airplane).

I'm testing, on the flight model (after your suggestion), and on some changes I made to the model (I also corrected the fuselage, the front side part had some sides that created angular areas.

I also control the cockpit, the area where the front glass resides was very angular and had many sides that I removed and reshaped (bridge function).

Do we want airplanes with a low-poly modeling? ok, but some models of FG are excessively low-poly :mrgreen:

Excuse me, I answered when you had already answered. Yes, I would like to publish the correct model on my github repository, obviously it will be available to be published on the official one if you like my job. I corrected other small things, I resized the parts to avoid strange openings between the various components, and I reshaped small areas that it was impossible to bring back to life.

The 172p? good question, I would like to improve the side angles of the cockpit because they are angularly anomalous, with Blender I can do beautiful things, the format .ac I begin to hate it deeply, I would give anything to use the .blend format in flighgear (and I do not think only I).
Last edited by IceBreaker on Mon Feb 25, 2019 9:23 pm, edited 1 time in total.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Mon Feb 25, 2019 9:19 pm

Also note: IT-AUTOFLIGHT GA is discontinued, I suggest replacing it with the much more realistic successor: http://wiki.flightgear.org/S-TEC_55X

I need to do some research on what after market AP people put into the J3 and what AP may have come stock on the PA-18, if any. I'd like to use equipment that has basis in RL when possible.

I'll be happy to test for you, but keep in mind I've never flown a real J3. I can give you my opinions though

Sure, your feedback would be welcome.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Mon Feb 25, 2019 9:20 pm

some changes I made to the model (I also corrected the fuselage, the front side part had some sides that created angular areas.

I also control the cockpit, the area where the front glass resides was very angular and had many sides that I removed and reshaped (bridge function).

Are you editing the original mesh or is this completely new mesh?
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Re: JSBSim Piper J-3 Cub

Postby IceBreaker » Mon Feb 25, 2019 9:24 pm

wlbragg sorry, we answered together, so I updated the post above (read also the second part), for anything I'm at your disposal, the j3 cub I love it!

Original Mesh correct, even if I actually hoped for someone who told me "make a new model using this as a reference", in my opinion it is the best thing, also because the piper j3 cub is modeled quite easily. If you want I can begin to shape fuselage, wings, and cockpit, then the more complex parts we see them together and I follow your advice ok?
Last edited by IceBreaker on Mon Feb 25, 2019 9:27 pm, edited 1 time in total.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Mon Feb 25, 2019 9:26 pm

Davtron803 is a clock btw

I see where I got confused (easy for me to do now days). Stuart did add some new AP code, it was the GFC700.
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