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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Thu Aug 10, 2017 5:07 pm

I doubt the setting: Skip ADIRS Alignment, or Always show Sidestick position would screw up FGFS


So do I, but it's a matter of habit and setting a consistent design example.

See, I live in a rural area where traffic density is low. I doubt anything bad is going to happen if I do not look left and right before driving onto the 'main' road. Yet - that's still no good reason to do it. It's bad to acquire the habit of skipping the check, and it's even worse to set a bad example to others.

So let me ask you the other way - how do you know what settings are harmless to save that way? Can you be sure they all behave like you expect on other systems? I can offhand list a dozen examples where something worked perfectly fine for me and I never thought anything evil till it really malfunctioned for someone else - because he was using FG differently, or the graphics driver was different,... With time, there comes a certain humility in gauging one's potential to wreck havoc for others.

Anyway - please take it as a well-meant bit of advice from two people who have their share of experience in making bad mistakes - it's not a good idea.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Thu Aug 10, 2017 6:13 pm

Ok my first observation of this flight model havinf removed the engines and prop to see it's natural aerodynamics is that in a 10kft drop test it has no yaw and roll stability...it will go into a spiral dive...... you might want to think about fixing that.

It glides for want of a better word at an Alpha of -0.6deg, a Beta angle of -0.8 and a LIFT to DRAG of 12:1
(although that said I do have the door and window removed which adds extra drag, would like to stop that but this is a complex nasal nightmare)

Simon

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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Thu Aug 10, 2017 7:21 pm

@Bomber

Around line 1325 in j3cub.xml
Code: Select all
    <function name="aero/force/Drag_gear">
       <description>Drag due to gear</description>
         <product>
           <property>aero/qbar-psf</property>
           <property>metrics/Sw-sqft</property>
           <value> 0.0040 </value>
           <property>gear/drag-coef[1]</property> <!-- 36 Bush Wheels" -->
           <property>gear/drag-coef[2]</property> <!-- Floats -->
           <property>gear/drag-coef[3]</property> <!-- Amphibious -->
           <property>gear/drag-coef[4]</property> <!-- Ski -->
           <property>gear/drag-coef[5]</property> <!-- Ski Lite -->
           <property>gear/drag-coef[6]</property> <!-- Sprayer -->
           <property>gear/drag-coef[7]</property> <!-- Door Open/Off -->
           <property>gear/drag-coef[8]</property> <!-- Window Open/Off -->
         </product>
    </function>


You can remove any of the added drag, IE: drag-coef[7] and 8
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Thu Aug 10, 2017 8:34 pm

Now that you bring up the door and window being removed, I suppose I ought to reduce the "loss" of weight when removed, especially if I am going to add in drag.

Any idea what the door and window on a Cub weighs?
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Thu Aug 10, 2017 8:45 pm

Hm, not so easy to find. I got one at 11 lbs door, nothing for the window yet.
Last edited by wlbragg on Sat Aug 12, 2017 7:53 am, edited 1 time in total.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Thu Aug 10, 2017 9:06 pm

Well until I find something I'm guessing 21 lbs combined for one door and one window, done!
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Re: JSBSim Piper J-3 Cub

Postby wkitty42 » Sat Aug 12, 2017 2:12 am

it0uchpods wrote in Thu Aug 10, 2017 5:01 am:Yes, I always quit via the "X" on my operating system.

that 'X' is not available if you run in fullscreen mode... i just always hit ESC several times to close any dialogs i might have open... the last one is the "Exit" dialog where i just hit ENTER to select the "Ok" button and it closes properly... that 'X' is a killer and slams the sim shut rather than executing a controlled shutdown...

FWIW, of course ;)
"You get more air close to the ground," said Angalo. "I read that in a book. You get lots of air low down, and not much when you go up."
"Why not?" said Gurder.
"Dunno. It's frightened of heights, I guess."
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Re: JSBSim Piper J-3 Cub

Postby Octal450 » Sat Aug 12, 2017 5:23 am

What is "FWIW"???

For me, when I close FG several times during developing, I refer X to File Quit.

Also, I've never used a sim full screen.

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

Postby erik » Sat Aug 12, 2017 6:52 am

FWIW: For What It's Worth

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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sat Aug 12, 2017 11:55 am

This is the lift caused by the elevator deflection

Code: Select all
    <function name="aero/force/Lift_elevator">
       <description>Lift due to Elevator Deflection</description>
       <product>
           <property>aero/qbar-psf</property>
           <property>metrics/Sw-sqft</property>
           <property>fcs/elevator-pos-rad</property>
           <!-- value> 0.3274 </value-->   <!-- Du Y -->
         <!-- + 0.01925112 (5.88%) = 0.3466 -->
         <table>
            <independentVar>/sim/model/j3cub/pa-18</independentVar>
            <tableData>
               0   0.3274
               1   0.3467
            </tableData>
          </table>
       </product>
    </function>


This is the pitch caused by the elevator deflection

Code: Select all
    <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--> <!-- Du Y --><!--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>
       </product>
    </function>


But if we know the lift and the distance of the h-stab then we could just as easy say it's this.
Code: Select all
   <function name="aero/moment/Pitch_elevator">
      <product>
         <value> -1 </value>
         <property>metrics/lh-ft</property>
         <property>aero/force/Lift_elevator</property>
      </product>
   </function>


The values should work out to be the same, there's no magic here...it's just force x distance.

But this would be too simple, and obvious.

Simon
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sat Aug 12, 2017 3:20 pm

I've noticed the rudder and aileron trims are pre-set on this plane.

roll-trim-cmd-norm = 0.0025
yaw-trim-cmd-norm = -0.075

Having a spot of trouble locating where this is done, as I'd like to glide this plane without these values.

Any help appreciated.
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Re: JSBSim Piper J-3 Cub

Postby Alant » Sat Aug 12, 2017 4:02 pm

If JSBSim, is it the trim program which sets them to balance prop effects?
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Sat Aug 12, 2017 4:18 pm

I don't find either. Weird...
I never noticed that JSBSim iterations did that at start on the ground (??), it is usually in the -set.xml. A preset compromise, author's choice, for balance at a given engine RPM and airspeed.

Waiting for a better response, I think the easier is to manually set them in the props, controls/flight/aileron-trim = .... , rudder-trim = .... (set)
It works and the set values are kept for the flight session.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sat Aug 12, 2017 5:41 pm

Well I'm drop testing, without any engine or prop, solely to see the aerodynamics and I'm sure this is the first time I've seen this...

Anyway I've engaged the autopilot for level flight, seems to do the trick, but the aircraft should be able to be flown without any pre-set trims in my opinion.

So we have a glide Lift to Drag of 14.7 and an airspeed of 57.3kts and an Alpha of 0.26degs

Simon
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sat Aug 12, 2017 6:44 pm

Shoot me down if you want, but with very little dihedra and zero sweep your roll by beta is too aggressive, it's cancelling out any dutch roll that should exist.

Code: Select all
    <function name="aero/moment/Roll_beta">
       <description>Roll moment due to beta</description>
       <product>
           <property>aero/qbar-psf</property>
           <property>metrics/Sw-sqft</property>
           <property>metrics/bw-ft</property>
           <property>aero/beta-rad</property>
           <property>aero/function/kClge</property>
           <table>
            <independentVar lookup="row">aero/alpha-rad</independentVar>
            <independentVar lookup="column">aero/Re</independentVar>
            <tableData>         <!-- Du Y -->
                    1128067  1668183   3707224
             -0.035 -0.1024 -0.031363 -0.010147
              0.175 -1.0147 -0.310788 -0.100549
            </tableData>
          </table>
       </product>
    </function>
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