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Re: JSBSim Piper J-3 Cub

Postby LesterBoffo » Sat Mar 02, 2019 7:21 pm

The early Cub came with the A40 L-head 40 hp engine, good practice to fly with an underpowered mill in this plane, since it's practically a powered glider
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Re: JSBSim Piper J-3 Cub

Postby IceBreaker » Sat Mar 02, 2019 8:17 pm

LesterBoffo wrote in Sat Mar 02, 2019 7:21 pm:The early Cub came with the A40 L-head 40 hp engine, good practice to fly with an underpowered mill in this plane, since it's practically a powered glider


the Piper J-3 Cub is beautiful for its simplicity.

Obviously the engine of the J-3 Cub will be 4 cylinders falling on the sides of the chimney. Now I have to be careful to make the different parts of the two floors so that they are the same as the real ones.

As soon as I find peace at work, I publish the 3d model with the progress achieved, so users can help me when I'm not there. Thank you all.

In the screenshots below I highlighted the supports, which are simple cylinders with the final flat part (scaled cube). Other details I made them through fusion of the mesh (attacks supports for the wings for example) the rest is all separate.

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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sat Mar 02, 2019 8:54 pm

@IceBreaker Looks really nice!

@dany93 beware I want to try to use the following engine/prop combinations (even if we need to adjust the prop files). I already made some changes to the two CM7445 MCCauley prop files, ie: CM7445 MCCauley and CM7445 MCCauley2.

The setups are as follows
65 hp engine with CM7445 MCCauley .xml prop.
90 hp enginw with CM7445 MCCauley2.xml prop.
Lycoming O-320-A1A reduced to 150 hp with the prop_75in2f-J3.xml prop.

Maybe you could start with these as the base for testing and changing?
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Sat Mar 02, 2019 10:12 pm

Where do the CM7445 curves come from?

Did you change the tables in github since 25 Feb 2019? (it seems that you didn't)
Last edited by dany93 on Sat Mar 02, 2019 10:21 pm, edited 3 times in total.
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Re: JSBSim Piper J-3 Cub

Postby legoboyvdlp » Sat Mar 02, 2019 10:13 pm

@icebreaker

Beware of publishing .blend files on git and asking users to work on it. That is a recipe for disaster since got does not work well with binary files. If two people work on it at the same time you will end up with many problems.

That model looks excellent though, nice work!
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sat Mar 02, 2019 10:45 pm

@dany93. I didn't change the tables, but I did change or add some of the parameters.
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Sat Mar 02, 2019 10:56 pm

OK.
But where do the CM7445 curves come from?
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sat Mar 02, 2019 11:35 pm

I'm pretty sure they come from Aeromatic. But I would like to know the details of how they were generated. If no real basis custom to that particular prop then I'm not opposed to changing the tables
Not do I want want to settle for the prop used on the c172p if it wasn't used on the Cub.
Just keep it in mind, but please feel free mix, match and edit wharever is needed.
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Re: JSBSim Piper J-3 Cub

Postby IceBreaker » Sat Mar 02, 2019 11:54 pm

wlbragg wrote in Sat Mar 02, 2019 11:35 pm:I'm pretty sure they come from Aeromatic. But I would like to know the details of how they were generated. If no real basis custom to that particular prop then I'm not opposed to changing the tables
Not do I want want to settle for the prop used on the c172p if it wasn't used on the Cub.
Just keep it in mind, but please feel free mix, match and edit wharever is needed.


the propeller in wood, because from what I read the j-3 cub had the wooden propeller, and personally I really like it. The propellers of the cub and supercub model differently. The supercub has a very similar helix to 172p, the one that requires more detail is the helix of the j-3 cub (frontal attacks and materials / textures used, in addition to its general shape).

I pay attention to recreate the real details based on photos and information that I find. Thank you all.
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Re: JSBSim Piper J-3 Cub

Postby erik » Sun Mar 03, 2019 12:25 pm

wlbragg wrote in Sat Mar 02, 2019 11:35 pm:I'm pretty sure they come from Aeromatic.

According to the header in the files they do: "Generated by Aero-Matic v 3.2.5"

Is see there's also an updated version of the CM7445 generated by Aero-Matic v 3.3.8 with different tables. I remember that I improved the tables during development but it would still be great to find some validated tables somewhere.

Update: I see the Ercoupe had the non Aeromatic generated tables for the McCauley 7557, maybe that's a good starting point.
Which seems to be taken from here (figure 2)

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Re: JSBSim Piper J-3 Cub

Postby dany93 » Sun Mar 03, 2019 3:22 pm

wlbragg wrote in Sat Mar 02, 2019 11:35 pm:I'm pretty sure they come from Aeromatic. But I would like to know the details of how they were generated.

Aeromatic, yes, but I would also like to know from which source Aeromatic takes them.
Anyway, it cannot work for every possible situations. Too simple. Too few input parameters. A glance in the NACA Report and you understand.
But as I've already written, I've stopped using the aeromatic propeller curves after one or two very wrong results.

If you tweak them please check that, at least, the energy efficiency is realistic. See the the NACA curves (e.g Fig.5). The energy efficiency reaches max values in the 80 and 87% range, obviously never more than 100% (which can happen if you increase CT via the ct factor or C_THRUST without increasing CP) .
And, doing this way, you have to insure or hope that you do not try to fix an upstream error by another error.
The energy efficiency is given by
Code: Select all
eta = J * CT / CP

with (JSBSim C_THRUST and C_POWER tables)
Code: Select all
J = V / (N*D)
CT = ct * C_THRUST
CP = cp * C_POWER

J is the first column in the prop tables.

The best is to plot these curves and compare to the real NACA measurements. "Bad" propellers (lower energy efficiency) can be acceptable.

The curves used for the c172p come from Megginson (160 hp) and the NACA Report (180 hp). These are not specialized to the C172 propeller because I don't know if we can find propeller curves for it (I couldn't). Merely, I use them because they generally give correct enough results (not only on C172). Take these curves as generic if you want, but still as based on wind tunnel measurements on a given type of real propeller.

You can also try the Erik improved curves from aeromatic or the McCauley 7557 given by Erik in the previous message and see if they match better to your expectations.
The McCauley 7557 from his reference may be not that far from the NACA Report with 18° pitch angle that I proposed in this message. The diameter (probably 75") is not an issue because it is taken into account in the J = V/ND calculation.

A reason to change the tables would be that they do not give the expected performances on the J3Cub. Neither the NACA based one, nor the aeromatic one, nor another one. Which needs pointing out the disagreements.
But, in this case the expected performances have to be defined (such as climb rate at which airspeed and RPM, cruise airspeed at what RPM, and more if possible).
And keep in mind that it would be tweaked values, not based on reliable or measured propeller data.
No guarantee, but with well defined expected performances I could try to tweak the tables to more or less match these performances. Not at all my preference, but...
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sun Mar 03, 2019 6:46 pm

Never easy is it. Thanks for that detailed explanation. I really do appreciate your enlightening me on the basics behind these things. Because of your obvious experience I am deferring to your suggestions.

Here is want I'd like to do. Keep "three" separate prop files and start each one based on the best data we have. Then we can start on the 65hp engine with the prop file we designate for that engine and tune it best we can.
I simply don't have enough background to really be of much help here other than the "general" feel of the aircraft and my expectations of how it handles with the different variants. But I want to start off on a sound foundation and your explanation of the power curves of these tables shows me I am way over my head for the moment.

@dany93 would you mind posting either here or as a PR in the git dev branch an initial prop setup of your best guess (using your preferred best data) for the three engines?
I realize we can probably use the same prop for multiple engines and if that is how you want to set it up initially then I am OK with that as well (but still in three files). I just to retain the three files so we can tweak the individual power plants separately if and when needed.

Then everyone or anyone that wants to (including myself) can start testing and providing feedback.

@dany93 do you have the time to do that, three initial prop base files, power plant will be 65hp, 90hp and 150hp?
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Sun Mar 03, 2019 11:03 pm

I think I can do that.

Prior to going too far, would the <thruster file="prop_74in_2f_NACA_15-30deg"> with its blade angle set at 18°, with the changes on the engine parameters as explained in this message be a good start? Can you test this?
If yes, I will do the first prop file (74") and engine setting for the 65 hp with these data.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Mon Mar 04, 2019 12:52 am

Yes, I'll test it tonight.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Mon Mar 04, 2019 2:25 am

OK, let me start by saying I'm not sure how I should be testing some of the performance aspects.
For example, the max speed rating is 87mph. Does that mean a NE rating or that is what the max the prop is capable of obtaining?

At a blade pitch less than 30 it appeared to be too fast? At 30 deg the aircraft only was capable of around 88mph.
However the climb rate could only be kept at between 200 to 300 ft/min.

At a pitch of 20 it topped out at a speed of 97 which might not be all that out of spec depending on what the "Top Speed" actually represents. It also was really close to being able to sustain a 450 ft/min climb rate.

At 18 the overall speed or climb rate difference was not really appreciable over that of 20, at least the way I was measuring it.

So performance wise based on only these two parameters it seems like a really good place to start.

I'm in the process of determining what the ceiling is with a pitch of 18 at a climb rate of 100 ft/min.

All this was starting with about 25 lbs of fuel and a 180 lbs pilot in the back seat, 65 hp., stock wheels.
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