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Re: JSBSim Piper J-3 Cub

Postby Bomber » Mon Jul 31, 2017 3:35 pm

Thanks for the reply Dany, Richard

My concern now is that this topic against the wishes of the original author has become a theoretical flight modelling topic...

I'm quite happy to debate the merits of modelling an effect one way or the other, conventional or non but feel that this topic isn't the place.

My concern was that a certain effect wasn't modelled by the conventional approach presently coded.... if you or the planes guardian is happy that it is or that you can live without it, who am I to say otherwise.

If you are sure it's coded, I'd simply like to see how, so as to add to my own knowledge.

Regards

Simon.
Last edited by Bomber on Mon Jul 31, 2017 5:40 pm, edited 1 time in total.
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Mon Jul 31, 2017 3:53 pm

Bomber wrote in Mon Jul 31, 2017 3:35 pm:Thanks for the reply Dany

Dany or rather Richard ? :)
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Mon Jul 31, 2017 5:39 pm

Yes.....oops
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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Mon Jul 31, 2017 5:49 pm

My concern now is that this topic against the wishes of the original author has become a theoretical flight modelling topic...


Don't sweat it, write a PN to a moderator and ask that the off-topic posts are moved.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Mon Jul 31, 2017 6:05 pm

I don't believe that should happen as some good stuff had come out of it..... it's just that discussing the Ask-13 flight performance is going too far.
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Re: JSBSim Piper J-3 Cub

Postby wkitty42 » Mon Jul 31, 2017 6:40 pm

messages can be moved or copied... things like the ASK13 specific stuff can be popped into a new topic... no big deal, geez...
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Mon Jul 31, 2017 6:56 pm

My concern was that a certain effect wasn't modelled by the conventional approach presently coded...

At this point in time this would be my concern also, if it stands true.

As far as any discussion, if it gets a better FDM model for the Cub, I have no issues. I'm here to learn and create.
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Re: JSBSim Piper J-3 Cub

Postby Octal450 » Tue Aug 01, 2017 2:31 am

Hi Wayne
I make some flight tests. Still feels really stiff.

Kind Regards,
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Tue Aug 01, 2017 10:16 am

Richard wrote in Sun Jul 30, 2017 2:23 pm:Also needs to be considered are the derivatives, pitch damping due to q dot (CMq), change in pitch moment due to alpha dot. Last time I looked at one of your models you didn't have any derivatives

Bomber wrote in Sun Jul 30, 2017 6:02 pm:They are there, just that I have them effecting the Angle of Attack.

Richard wrote in Mon Jul 31, 2017 2:20 pm:Unfortunately that won't work and can lead to a twitchy model; pitch damping (due to pitch rate) needs to use the pitch rate (Q); without this the effects will be instantaneous. You need the aerodynamic derivatives.

Thanks Richard, interesting.

I've watched these Properties during pitch variations controlled by stick movements:
Code: Select all
    <function name="aero/moment/Pitch_damp">
       <description>Pitch moment due to pitch rate</description>
       <product>
           [....]
           <property>velocities/q-aero-rad_sec</property>
           [....]
       </product>
    </function>

and
Code: Select all
    <function name="aero/moment/Pitch_alphadot">
       <description>Pitch moment due to alpha rate</description>
       <product>
           [....]
           <property>aero/alphadot-rad_sec</property>
           [....]
       </product>
    </function>

velocities/q-aero-rad_sec is (almost) always at least 5 to 6 times greater than aero/alphadot-rad_sec (except at transients).
The AoA stabilizes quickly at a relatively small value, hence a small alphadot-rad_sec, even if the pitch movement is maintened.
Which would explain (IMO) why q-aero-rad_sec is the derivative to use for pitch damping.
Even in still air, these quantities are noticeably different.

About the pitch stiffness for the Cub I have in mind increasing this damping (by q-aero-rad_sec of course) but I don't dare to abuse. I don't know up to what values we can go for the factor. If someone has an idea... (or a different way?). I don't think that increasing the inertia is the way.
The Pitch_alphadot role is not clear for me (maybe of secondary importance for now). And there is also its effect in turbulences....
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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Tue Aug 01, 2017 12:08 pm

velocities/q-aero-rad_sec is (almost) always at least 5 to 6 times greater than aero/alphadot-rad_sec (except at transients).


It's a question of aerodynamical forces vs. inertia of the plane.

* qdot measures how quickly attitude changes given the moments
* alphadot measures how quickly flightpath adjusts to attitude

If you have a high kinetic energy and relatively low forces (extreme case, the Shuttle in hypersonic flight) the flightpath takes of order of minutes to change given attitude, so qdot and alphadot are near-identical. The Cub is pretty much on the other end of the spectrum with a very low inertia in the flightpath compared with the forces, so as you observed it adjusts near-instantaneously.
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Fri Aug 04, 2017 2:15 pm

Thanks Thorsten, clear.
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Fri Aug 04, 2017 2:18 pm

Elevator stiffness:

The "Pitch moment due to alpha" factor is currently unusually high, I think this is the cause why we need such an efficient elevator factor.

Decreasing "Pitch moment due to alpha" enables decreasing the "Pitch moment due to elevator", which becomes softer.

J3Cub.xml, lines about 1406 - 1443:
Code: Select all
    <function name="aero/moment/Pitch_alpha">
       <description>Pitch moment due to alpha</description>
       <product>
           <property>aero/qbar-psf</property>
           <property>metrics/Sw-sqft</property>
           <property>metrics/cbarw-ft</property>
           <property>aero/alpha-rad</property>
           <table>
                <independentVar lookup="row">aero/Re</independentVar>
                <tableData>     <!-- Du Y -->
                    1668183  -2.0327
                    3707224  -1.3432
                </tableData>
           </table>
           <value>0.3</value> <!-- 0.3 dany, tests, stiffness issue -->
       </product>
    </function>

    <function name="aero/moment/Pitch_elevator">
       <description>Pitch moment due to elevator</description>
       <product>
          <property>aero/function/qbar-induced-psf</property>
          <property>metrics/Sw-sqft</property>
          <property>metrics/cbarw-ft</property>
          <property>fcs/elevator-pos-rad</property>
          <!--value> -1.2004 </value-->   <!-- Du Y -->
          <!--value> -1.8 </value--> <!-- from dany93 -->
          <!-- + -0.10584 (5.88%) = -1.90584 -->
          <table>
                <independentVar>/sim/model/j3cub/pa-18</independentVar>
                <tableData>
                    0   -1.8
                    1   -1.9058
                </tableData>
          </table>
          <value>0.67</value> <!-- 0.67 dany, tests, stiffness issue -->
       </product>
    </function>

See the factors
<value>0.3</value>
<value>0.67</value>
at the bottom of the products.

These values are temporarily used for easier tuning in tests. They enable stall, reaching 18 - 20 deg AoA and 3 points touchdown. J3Cub 65 hp to PA18 160 hp tested.
Can you confirm and tell if you see no drawback?
If you find them acceptable, we can later merge them in their respective tables and delete the <value> lines.
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Re: JSBSim Piper J-3 Cub

Postby erik » Fri Aug 04, 2017 5:23 pm

I'm not a big fan of messing with the numbers just because they feel right.
Any change that the center of gravity is too far to the tail for instance?

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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Fri Aug 04, 2017 6:29 pm

Any change that the center of gravity is too far to the tail for instance?

Erik, are you implying that the center of gravity is to far to the tail in any configuration?

The elevator effectiveness needed to be changed in my opinion. It was not responsive enough. Maybe it was/is a COG issue?

Do you have a solution for that, or are you OK with the current solution?

With the elevator changes, the tightness of the aircraft becomes more obvious.

Dany, these latest changes do wonders, it does "feel" way better.

Erik your the originator of the FDM and I respect your decisions in all that is FDM with this FDM. So please help sort this out. What do you want to do?
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Re: JSBSim Piper J-3 Cub

Postby Richard » Fri Aug 04, 2017 7:47 pm

As Erik said just inventing numbers is never good; however if it is possible to adapt one from a similar aircraft, unless you can run something like OpenVSP.

I can provide tables from the Beagle Pup; and you can give it a bit of a tune up
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