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Re: JSBSim Piper J-3 Cub

Postby wkitty42 » Sat Aug 12, 2017 9:00 pm

it0uchpods wrote in Sat Aug 12, 2017 5:23 am:For me, when I close FG several times during developing, I refer X to File Quit.

F10->File->Quit is a good and proper exit as is using ESC to get to the exit dialog and selecting the [OK] button... i, and i'm guessing others, understood your using the 'X' comment to mean the 'X' in the top right corner where the maximize and minimize buttons are when running in non-fullscreen mode...

it0uchpods wrote in Sat Aug 12, 2017 5:23 am:Also, I've never used a sim full screen.

you should try it... it is beautiful without those frames and their decorations... easy to do via SHIFT-F10 IIRC... it toggles fullscreen mode on and off... i use ALT-TAB to switch to other tasks on the same desktop and can also switch to one of eight other desktops but that's something my OS provides as long as CTRL-F1-8 aren't eaten by something else ;)
"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 erik » Sat Aug 12, 2017 9:19 pm

Keep in mind that the low Reynolds numbers (aero/Re) are there to initiate and produce stall behavior. Which is what happens with the drop test.

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Re: JSBSim Piper J-3 Cub

Postby dany93 » Sat Aug 12, 2017 10:30 pm

Bomber wrote in 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.

For an aircraft with high wings (the wings aerodynamic center higher than the CG), the wing dihedra is not the only criteria. It can have a roll moment due to beta even with very little dihedra. Even with zero dihedra if we take the lateral drag due to wings and the turbulences by the fuselage on the downwind wing.
Do you know how to calculate the roll moment due to beta?
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sat Aug 12, 2017 11:03 pm

Yes there is some effect caused by the fuselage on the down wind wing..

But we're talking roll here..... and the majority effect of roll is caused by moment produced at the outer parts of the wing which are clear of the fuselage effect.

The effect at present is too great.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sat Aug 12, 2017 11:05 pm

Bump.

You guys can't even work out the moment caused by a force at a known distance so its beyond me why you think i should answer a question of can i work out the roll caused by beta.

Bomber wrote in 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 Alant » Sat Aug 12, 2017 11:22 pm

Simon

What is the fight about this time?

Every time that you get confused you attack everyone. Aerodynamics IS a complicated subject, with many confusing definitions. Definitions from one company, country or era are not always the same. Definitions posted on the internet are, without verification, about as reliable as those from a friend in the pub.

You post something which you say is wrong, but do not say exactly what the problem is. Who can be expected to read every line of one of your posts to spot the one line that is giving you a problem?

Please try to be a bit more helpful in explaining your problem instead of throwing abuse at everyone who tries to help.

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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Sun Aug 13, 2017 5:47 am

The values should work out to be the same, there's no magic here...


At least for the expressions posted it's not obvious that they work out the same (the lift expression used aero/qbar-psf, the first moment expression aero/function/qbar-induced-psf for instance - unless they're defined to be the same at all times which would render the second redundant, the expressions are generally different).

Also, the length and area references need to work out with the numbers in the table which isn't obvious. So there's some degree of 'magic' here.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Aug 13, 2017 9:36 am

Alant wrote in Sat Aug 12, 2017 11:22 pm:Simon

What is the fight about this time?

You post something which you say is wrong, but do not say exactly what the problem is. Who can be expected to read every line of one of your posts to spot the one line that is giving you a problem?

Please try to be a bit more helpful in explaining your problem instead of throwing abuse at everyone who tries to help.

Alan


Alan it's only a fight if people want it to be one.

The only thing I've said yesterday was wrong is the roll due to beta... what I've said is wrong with it is that it's too aggressive. I've posted up the relevant code that's responsible in the hope that the author or someone who knows flight modelling would consider the possibility that I'm right.

Being told other factors may be involved in roll other than dihedral for high mounted wings is correct but as I said there's no sweep on these wings... also I've explained that rolling moment is more a feature of armature than inner wing force.

Once all the extraneous code has been removed the basic FDM flys very well.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Aug 13, 2017 9:45 am

Thorsten, the fist code is the lift.... the second code is the moment caused by the lift.

The point I'm making is that the 3rd code uses the lift value of the first code and the known distance to the tail to produce a moment value... a very simple equation.

Codes 2 and 3 should produce the same value.....

Here again I was hoping the author or someone in the know would look at it.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Aug 13, 2017 9:57 am

wlbragg wrote in Tue Aug 08, 2017 6:10 pm:
Would it be OK if I modeled the h-stab and provide you with a switch such that you could swap between multiple methods whilst in flight so as to be able to compare effects ?

Of course you can, it'll be an interesting test case.



Wlbragg, I've written the code for this test...

Pm me your email address and I'll send it to you and you can distribute it to the Cub team.

Regards

Simon.
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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Sun Aug 13, 2017 11:21 am

The point I'm making is that the 3rd code uses the lift value of the first code and the known distance to the tail to produce a moment value... a very simple equation.

Codes 2 and 3 should produce the same value.....


The point is trivially true, but codes 2 and 3 do not obviously produce the same value for the reasons I wrote.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Aug 13, 2017 12:05 pm

You know it's easy to say that I'm being critical and disrespecting the work of the authors of this fdm....

I'm genuinely not, as these guys have attempted something that's very difficult, with very little actual data.
They're also open to an idea and a review of their work. So I have a lot of respect for someone who's able to put their work out there with the expectation that people look at it...

If I'm disrespecting anyone it's those people that say "I can't flight model but I can test your plane for you if you want"... because these things I'm pointing out can easily be seen in the properties, so I've got very little time for these types of people.
I'm also disrespecting those that say..."it's very complicated" as an excuse for not challenging simple things... We have the luxury of pressing the pause button and looking at the property values and understanding just what is going on at the simple level that builds up into a complex result. So I have little time for these people that hide behind this statement in attempt to make me look small and themselves big as they understand it's complicated.

I'm looking at simple things in the flight model... a force being generated, it's distance from the rotation point and thus the moment it should exert.

Lets imagine I'm flying this cub at a straight glide and I apply left rudder... I wait a few seconds and press pause and look at the properties.

rudder-pos-deg = 17.8
beta-deg = 18.9

So the rudder is 1.1 degs out of the relative wind....make of that what you want.

The side force that this rudder is producing is +46.9lbs, as you'd expect pushing the tail to the right
The roll cause by this rudder force is 236lbsft, as you'd expect rolling the plane to the right.
Simple maths tells me it's armature is therefore 236/46 = 5ft above the centre of rotation....make of that what you want.

And the roll caused by beta is -1591lbsft, as you'd expect rolling the plane to the left.

What do we understand of roll by beta ? basically it's a difference in the moment values from one side of the centre of rotation to the other.
If we think about the fuselage it's below the centre of rotation so any side force pushing left will rotate the plane to the right
We got a bit of V-stab and that's pushing the tail to the left, rolling the plane to the left in direct opposition to the rudder.
So we're just left with the wings... these have no sweep so the effective beta on each wing is identical, so no difference in lift generated by this effect. We have a very small dihedral (0.5 degs) which with a wind hitting it from the right will mean that the right wing see's a greater angle of attack than the left. The alpha-deg = 2 so any differential in AoA is in the region of +-sin beta dihedral. = 0.162degs

what remains now is the wind shadowing by the fuselage on the inner left wing.. So what do we know...
the weight of the plane is 1132lbs so roughly speaking each side of the plane generates 1132/2 = 566lbs
if we divide the wing by 8 section each section produces 556/8 = 70.75lbs
if the wing span is 35.25ft each wing is roughly 35.25/2 = 17.62 and the first sections armature is roughly 17.62/8 = 2.2ft and the second 4.4ft etc
So the sections generate moments of
70.75*2.2 = 155.65lbsft
70.75*4.4= 311.3lbsft
70.75*6.6= 466.95lbsft
70.75*8.8= 622.6lbsft
70.75*11= 778.25lbsft
70.75*13.2= 933.9lbsft
70.75*15.4= 1089.55lbsft
70.75*17.6= 1245.2lbsft

total = 5603.4lbsft

interestingly as you know each wing generates 566lbs.... the actual armature center of lift for the wing is approx 5603.4/566 = 9.9ft.... just a bit of fun.

So hopefully you can see that even if you remove the inner wing section from the moment calcs you're not going to end up with a massive moment differential between each wing.... remove the outer section and it's a different story.

Simon

(waiting for the "it's a lot more complicated than that" retort)
Last edited by Bomber on Sun Aug 13, 2017 12:20 pm, edited 1 time in total.
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Aug 13, 2017 12:10 pm

Thorsten wrote in Sun Aug 13, 2017 11:21 am:
The point I'm making is that the 3rd code uses the lift value of the first code and the known distance to the tail to produce a moment value... a very simple equation.

Codes 2 and 3 should produce the same value.....


The point is trivially true, but codes 2 and 3 do not obviously produce the same value for the reasons I wrote.


I've pointed out I don't have any engine or prop on this plane so the value for qbar and induced qbar are the same..

under those circumstances should the values be the same ?
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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Sun Aug 13, 2017 2:07 pm

I'm also disrespecting those that say..."it's very complicated" as an excuse for not challenging simple things...


I on the other hand tend to be wary of people offering simple answers to complex problems - in my experience, that almost never works out, be it in politics, science or engineering.

I'm looking at simple things in the flight model... a force being generated, it's distance from the rotation point and thus the moment it should exert.


Except of course you're already falling for the cartoon here, because in reality no single "force arrow" gets generated at some distance, there's a whole force field generated across every surface point.

You're already making multiple assumptions to come to this idealized picture - which you are by and large completely unaware of.

If we think about the fuselage it's below the centre of rotation so any side force pushing left will rotate the plane to the right


It's almost like you wrote this to illustrate my point above :D

I frankly don't believe that the z-coordinate of the CoG of the plane is located right at the top of the fuselage. To the degree that it's not and it will rotate around a lower point, you can push it from the side above or below that coordinate - of even above and below, and dependent on how the strength of the pushing arranges, you can have any sign of the rolling moment and any net side force.

It's a question of how the details of the airstream perturbed by the wing etc. work out on the fuselage.

what remains now is the wind shadowing by the fuselage on the inner left wing.. So what do we know...


In a world in which air basically works like ray optics in light, elements cast a sharp shadow and nearby sections of the wing don't influence each other the argument is sound - the question is - how realistic is that world?

Note for instance that this is a subsonic plane, so perturbations can propagate upstream - the forward parts of the plane do know that something else is coming behind for that reason. If you like, in air you can cast your shadows forward and backward.

Essentially you're arguing:

1) Assume my model of independent airfoils, wing sections and ray optics aerodynamics is true
2) My model doesn't give me the results of your model
3) Therefore your model must have a problem

But unfortunately the argument collapses at 1) - what if your picture isn't true? There's just the rub - there's no evidence that things really work out how you reason because to be able to apply the reasoning you need to drastically simplify aerodynamics. So maybe it's your model which has the problem.

I don't know how good it is for the Piper (I do know it'd suck for the Shuttle) - but neither do you. Yet we get plenty of definite 'this can't be' or 'this is wrong' from you - while we've seen since the beginning of this discussion two cases of real data which your reasoning completely failed to explain (prompting you to the somewhat funny reply that the data can't be true).
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Aug 13, 2017 3:33 pm

Real Data ?

I must have missed it in all the gossip and possibilities of perturbation and shadowing.
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