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Re: JSBSim Piper J-3 Cub

Postby dany93 » Sun Feb 26, 2017 1:28 pm

wlbragg wrote in Sat Feb 25, 2017 6:14 pm:I adjusted the empty weight CG to 8.5" and what a difference it makes. I feel way more comfortable with this number and the handling and feel of the aircraft in all configurations. It may very well be the sweet spot.

I think it is not good because it is based on the AERORP at x=0 (leading edge). See below.
But you can get the same handling by shifting the AERORP and CG aft by the same quantity.
dany93 wrote:On SVN, I see that the AERORP is at x=0. Weird for the geometry and not compatible with CG at 12" aft ... (?)

wlbragg wrote in Sat Feb 25, 2017 6:14 pm:I don't quite understand this, could you please explain "Weird for the geometry" and "not compatible with CG at 12" aft ".

dany93 wrote:The AERORP location is a difficult subject. For a single wing, the aerodynamic center is about 25% the mean aerodynamic chord but for the entire aircraft it is a more or less aft.

"Weird for the geometry" because the AERORP should not be at the leading edge. As a first indication, 25% of MAC gives 63"/4 = 15.75" aft from the L.E..
Those data for the entire aircraft are very hard (if not impossible) to find. To start, I would take it between 25% and 30%, that is between 15.75" and 18.9".
With this AERORP aft from the Leading Edge it will be easier to move the empty CG more aft, in the (+10.6) to (+22.7) or (+8.5) to (+20.3) range.
More complicated, the AERORP changes with the AoA. At my knowledge not taken into account in FG, but this can be circumvented (see below).

Moreover (see my previous post) a general rule is that the CG must be reasonably before the AERORP (for stability).
To start, I would locate the empty CG at about 16.5" or 13.8" (if this one with empty wt 766.5 lbs can be trusted). Or this Weight and Balance doc for 1216 lbs empty.
Still, you should also check that, with these AERORP and empty CG locations, you do not get the CG aft from the AERORP with some acceptable loading configurations.
However, there is some margin for the CG even if placed aft from the AERORP because you wrote that 8.5" aft from the AERORP (with AERORP at the LE) was still pleasant to fly (in FG ! Dangerous IRL).

And you test it in flight.
As the CG location seems more reliable than the AERORP location, you can fiddle firstly with the AERORP. I dislike that cheating but as long as we have nothing else...
There is also some margin for the CG slightly before from my figures.
If the aircraft is permanently (but not excessively) nose heavy, it can be corrected by some (preset) elevator trim or, better, some "pitch moment at zero alpha" with zero trim.
Still better, an alpha-dependent "pitch moment due to alpha", which also comes to take into account the AERORP variation with AoA (not perfect, but that's simulation).

If 12 " is not compatible, would 8.5" also not really be compatible?
Are you basically saying that if the AERORP is at x=0 and 3D X-origin is at the datum, then the CG should be close to 0".
If the above is so, is there anything wrong with moving the empty weight to 8.5", so long as the aircraft feels correct and performs well (or better) and allows for the recommended CG load-outs?
I hope that I have responded above, those values are for the AERORP at X=0 (Leading Edge).

Those considerations for the AERORP location are mainly based on feeling (flight tests) by lack of documentation.
Most often, the empty CG for an aircraft can be found in the POH.

Generally, I try as much as possible to base my settings on published data. Easier for many like lift, drag, glide ratio. Not here.
When introducing a new feature, I also insist (at least try) to rely on physics or / and aerodynamics laws tu justify it. JSBSim is a wonderful tool for that.
Last edited by dany93 on Tue Feb 28, 2017 10:50 am, edited 2 times in total.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sun Feb 26, 2017 6:26 pm

@dany93, that was really helpful, thank you again.

After a quick look at the definition of AeroRP, I agree that intuitively that does not look like a reasonable position for AeroRP. Then, add in the standard acceptance of CG needing to be forward of AeroRP and the numbers we do have available for load outs, it does appear "Weird for the geometry and not compatible with CG at 12" aft".
I think "15.75" aft from the L.E." is a good place to start.

Even though I am not sure I want to go this in-depth, I saw this while education myself on all this.
More complicated, the AERORP changes with the AoA. At my knowledge not taken into account in FG

JSBSim provides a way to shift the aerodynamic reference point (AeroRP) forward (negative values) and aft (positive) in response to mach, pitch or other influences. Use the tag <aero_ref_pt_shift_x> in the <aerodynamics> section. The value this tag function returns is internally multiplied by the chord entered in the <metrics> section to get the final shift. You could use this by setting the AeroRP to the leading edge of the wing and use a 1D table indexed by mach starting at .25 chord and getting bigger as the mach number increases to simulate mach tuck.

Code: Select all
<aero_ref_pt_shift_x> 
    <function>
        {function contents}
    </function>
</aero_ref_pt_shift_x>



I don't know how or why Erik or Aeromatic came up with that number. I could really use some clarification on this.

The datum, in general, appears to be a really arbitrary point (in this case a fairly factual number from the manufacturer). Its importance being mostly for verification of or reference to other calculations.

So looking at it from that point of view I now can see what your getting at and I have a path forward for experimentation.

Hopefully, so I don't waist my time, Erik, could you shed some light on where and why the AeroRP is at 0" across the board?
Is there something else I am missing or not taking into consideration?

@Bomber, I'm still reading!
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Re: JSBSim Piper J-3 Cub

Postby Richard » Sun Feb 26, 2017 10:34 pm

from the link Simon posted (p18)
Image

This is probably enough to get the basic geometry. Usually the wheels are a guide to the position of the AeroRP. Often AeroRP is the CG for an empty craft. So I'd say it's certain that the aerorp will always be behind the wheel centre for a tail dragger, or in front of the main gear for a tricycle (otherwise it would tip forwards on the ground)

The AeroRP will almost certainly be within 11% to 35% MAC; this relates to the wing but from this you can figure this out in whatever the coordinate system you're using in the JSBSim model (i.e. it will be an offset).

If the wing isn't swept (much) then MAC is mean of tip chord, root chord; otherwise there is geometry required to calculate this.

Usually the AERORP will be the CG when the aerodynamic data was collected, whether in flight, or a wind tunnel.
Last edited by Richard on Mon Feb 27, 2017 9:53 am, edited 2 times in total.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Sun Feb 26, 2017 10:57 pm

Well, another days worth of testing and I think I have some numbers that work pretty well.
AeroRP = 18.9"
CG emptywt = 13.8"

This allows for almost all standard load-outs to fall within the (+10.6) to (+22.7) or (+8.5) to (+20.3) range and still be forward of the AeroRP. Only in the extreme bad management of loading does it pass aft (only an inch or 2 max.) of the AeroRP. That happens when you are almost empty of gas and have the rear seat occupied with the pilot and the aft of rear seat compartment loaded with weight close to or past the max weight.
All other time it is within range. The aircraft flies well and is actually much easier to roll out on the runway without wildly swinging the ass end around. It is easier to apply brakes without doing an endo.

I have been concerned about the 65hp engines lack of power and the overpower of the 90 and 160. I tuned them a bit using the volumetric-efficiency of the engine. I set the 65hp to .95, the 95hp to .8 and the 160hp to .75. I also added some substantial drag if you fly with the door or window open or off (I added a new GUI choice to remove door and window). So on the J3Cub 65hp you want to make sure you have the window and door closed and the carb heat off, especially when taking off.

Thanks Richard, I'll take a good look at this and see how much it might influence what I did today.

I'm going to push this and wait for any feedback.
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Sun Feb 26, 2017 11:20 pm

Richard wrote in Sun Feb 26, 2017 10:34 pm:Often AeroRP is the CG for an empty craft.

Often ? On several aircraft that I know (C172P, DR400) we have seats, luggage, behind the empty CG. This would make the CG behind the AeroRP with loading.
J3 Cub is an other example.
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Re: JSBSim Piper J-3 Cub

Postby Richard » Mon Feb 27, 2017 12:02 am

dany93 wrote in Sun Feb 26, 2017 11:20 pm:
Richard wrote in Sun Feb 26, 2017 10:34 pm:Often AeroRP is the CG for an empty craft.

Often ? On several aircraft that I know (C172P, DR400) we have seats, luggage,.


maybe I should have said sometimes instead of often.

The important point I was making is that AeroRP is simply the CG when the aerodynamics were measured or calculated and It will only affect the moments.
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Tue Feb 28, 2017 3:43 pm

@wlbragg,

You might find this Weight and Balance Calculator useful.

Among others, I see
Arm = 14.5598" at Empty weight 719.9 lbs.
C.G. range +10.6 to +22.7

Or this one (blank).

It comes from this Cub forum.
Last edited by dany93 on Tue Feb 28, 2017 4:05 pm, edited 2 times in total.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Tue Feb 28, 2017 3:48 pm

Nice, thank you @dany93
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Tue Feb 28, 2017 4:12 pm

@dany93, I can't seem to edit either sheet. They are locked in a way I can't figure out. I admit it's been awhile since I uses a spreadsheet, but I grew up on them when that is all we had.
Any idea?
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Re: JSBSim Piper J-3 Cub

Postby Johan G » Tue Feb 28, 2017 4:32 pm

wlbragg wrote in Tue Feb 28, 2017 4:12 pm:[...] I can't seem to edit either sheet. They are locked in a way I can't figure out.

The author made them read-only by accident and posted fixed ones a bit further down. You might want to read the entire thread, there are both some fun and insightful posts there. ;)
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Tue Feb 28, 2017 4:36 pm

What I've done:
- Open them from the link,
- Save as... (*.xls or *.ods, both worked).

Then I could write to change the values.
If I understand well, you can enter your values in the yellow boxes, others are calculated.
A protection of the calculated cells against an overwriting would be better. Otherwise, be careful.
Last edited by dany93 on Tue Feb 28, 2017 4:44 pm, edited 1 time in total.
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Re: JSBSim Piper J-3 Cub  

Postby wlbragg » Tue Feb 28, 2017 4:41 pm

Genius, thanks, that worked!

EDIT: I forget I am an a more secure OS with Linux VS Windows.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Tue Feb 28, 2017 8:55 pm

I have another set of calculations that I need to do. The flaps, they are borrowed from the c172p which only go to 30. The pa-18 manual explains the adjustment of the cable to the flaps as tight at 50deg +/- 2deg. So I need the calculations to 50deg flaps.

Now, I did some research on the subject, I read understand enough of it to know I don't want to be engineer or expert on the subject. I am not interested in the formulas or background on how to get there. I see that it is a very complex problem for us because of all the variability.

I want the PA-18 model to act as a close to reality as possible.
What I am hoping is that an experienced FDM person will give me a close approximation based on their knowledge of what these numbers should be. If that is not possible and you have to do the math, then I really hope I can get a kind soul to do it for me so I don't have to become the expert.

The figures I currently have are

Code: Select all
aero/coefficient/CLDf
Delta_lift_due_to_flap_deflection
        product
           aero/qbar-psf
           metrics/Sw-sqft
           aero/function/kCLge
           table
                independentVar-fcs/flap-pos-deg
                tableData
                    0.0000   0.0000
                    10.0000   0.2000
                    20.0000   0.3000
                    30.0000   0.3500
                    40.0000   ? my guess proposal 3750
                    50.0000   ? my guess proposal 3875

I found an article on lift due to flap deflection in general and it indicated for plain flaps numbers that looked like this
0.0000 0.0000
10.0000 0.3000
20.0000 0.5800
30.0000 0.7600
40.0000 0.8700
50.0000 0.9500
But I don't know, it appears too far off according to what the c172p is using.

Code: Select all
aero/coefficient/CDDf
Delta_drag_due_to_flap_deflection
        product
           aero/qbar-psf
           metrics/Sw-sqft
           aero/function/kCDge
           table
                independentVar-fcs/flap-pos-deg
                tableData
                      0.0000   0.0000
                      10.0000   0.0070
                      20.0000   0.0120
                      30.0000   0.0180
                      40.0000   ? my guess proposal .0220
                      50.0000   ? my guess proposal .0250

Code: Select all
aero/coefficient/Cmdf
Delta_pitching_moment_due_to_flap_deflection
        product
            aero/qbar-psf
            metrics/Sw-sqft
            metrics/cbarw-ft
            table
                independentVar-fcs/flap-pos-deg
                tableData
                    0.0000       0.0000
                    10.0000      -0.0654
                    20.0000      -0.0981
                    30.0000    -0.1140
                    40.0000      ? my guess proposal -0.1230
                    50.0000      ? my guess proposal -0.1290
Last edited by wlbragg on Wed Mar 01, 2017 3:21 pm, edited 1 time in total.
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Wed Mar 01, 2017 11:33 am

Hi wlbragg,

To say it right, that's the first time I'm using this article for flaps calculations and I'm not at all an aeronautics engineer. I've just discovered and tried to understand it.
Another criterion for the lift coefficient with flaps is the stall velocity with deflected flaps (assuming the stall velocity in clean configuration is good).

Of course, your 0.95 value is too high. It only take the k2 value.
Instead, you have to use the equation (8.4) with values:
- from Fig.8.11 for (DeltaCLmax)base,
- Fig. 8.12 for k1,
- Fig. 8.13 for k2.
I strongly think that another important correction should be something like the (flap length / wing length) ratio.
[EDIT] Confirmed. It comes to using the equation (8.6). The area ratio (SW,f / SW) as defined in Fig. 8.20 is the same as the length ratio in the case of our rectangular wings.

To test it, I've made a quick (probably approximate) calculation for the C172 (flaps 30°)
NACA 2412 profile : 12% max thickness ==> (DeltaCLmax)base = 0.9
18% - 20% wing chord ==> k1 = 0.9
30° max ==> k2 = 0.75
Length 0.41 x wing ==> x 0.41

Which gives 0.9 x 0.9 x 0.75 x 0.41 = 0.25 (at 30° deflection)

instead of 0.35 in the C172P table. I do not know which is better. If I remember well, the stall velocity for the C172P with flaps is correct (although not easy to accurately check).


Same calculation for the J3 Cub, with even more approximate start values:
USA 35B profile (I don't know, just seen this drawing from this thread response: 11% max thickness ==> (DeltaCLmax)base = 0.85
18% wing chord (?? I don't know, I let you see) ==> k1 = 0.9
50° max ==> k2 = 0.95
Length 0.41 x wing (?? I don't know, I let you see) ==> x 0.41

Which gives 0.85 x 0.9 x 0.95 x 0.41 = 0.30 (at 50° deflection)

I do not know which is better, either extrapolate the C172 values (as you did) or take these ones.
If you have the stall velocity with flaps it might help (33 kts from this link and written in the FDM).
sqrt(0.3775/0.30) = 1.12, gives 1/1.12 = 0.89 ==> -11% difference on stall speed.

From my calculations with CL Max = 1.4746 clean (your FDM), both would be good for a stall velocity of 33 - 34 kts at sea level with 1220 lbs full load. (please check !)
[EDIT] correction, see this post [/EDIT]
0.3875 gives about 33 kts <--- my preference
0.30 gives about 34 kts

Instead of 37 kts clean with 1220 lbs (calculated; if you can confirm, thanks).
Last edited by dany93 on Sat Mar 04, 2017 9:54 am, edited 2 times in total.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Wed Mar 01, 2017 8:34 pm

Thank you @dany93!

I don't know if I could have deciphered this data in a meaningful way without your help, I really appreciate it. I need to take some time to more thoroughly understand what your presenting. I'll go through it as you have and see if I can come to a consensus. I guess there is no other way to get realistic numbers without doing the work. :)

Thanks again for sharing your knowledge with me.
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