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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Sun Aug 06, 2017 7:28 pm

I'm simply asking if the shuttles wing has zero dihedral then why do its ailerons add side force with deflection.... where is rhis vector coming from ?


There's (as often) something wrong in the assumption underlying your question, but...

of course I could have found it


Please do it yourself this time then :D Forgive me if I'm not in the mood to explain something to be told later how you can all work it out yourself and just wanted to play a game (I linked the relevant resource, I trust you can read it).
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Sun Aug 06, 2017 7:51 pm

Heh you're quick to give your opinion about others work / opinion but in this simple question reticent.

Whatever man.
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Re: JSBSim Piper J-3 Cub

Postby Alant » Sun Aug 06, 2017 8:25 pm

Side force due aileron is NOT a simple question. You will not find it discussed in most aerodynamics text books.

Usually it is small and ignored.

One explanation may be that the shuttle has short wings and therefore the fuselage interaction with the modified airflow due to aileron deflection will be more important than for your average aircraft. If you are really interested there may be an explanation in the Shuttle aerodynamics reports.

Sideforce due to aileron is included in my TSR2 FDM. However the TSR2 has all moving tailerons, which are pivoted from the rear fuselage. The flow interaction with the fuselage and the tail fin is therefore quite large.

Estimation of sideforce due to aileron is with wind-tunnel or CFD as the subject too complicated for the methods used to estimate roll and yaw due to aileron.

What this has to do with the Cub I have no idea. It will be completely insignificant.

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Re: JSBSim Piper J-3 Cub

Postby Richard » Sun Aug 06, 2017 10:02 pm

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Re: JSBSim Piper J-3 Cub

Postby Thorsten » Mon Aug 07, 2017 5:09 am

Heh you're quick to give your opinion about others work / opinion but in this simple question reticent.


On the contrary, I've had years to get the measure of you, and I linked a document that addresses both your simple question and the complex follow-up you were aiming at. Alas - you didn't read it (which confirms you weren't so much interested in getting an explanation for something you don't understand but in something else).

You know, you could just read it and we'd be done here. :D
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Re: JSBSim Piper J-3 Cub

Postby erik » Mon Aug 07, 2017 7:24 am

dany93 wrote in Sun Aug 06, 2017 12:27 pm:@erik,
I have the impression of some misunderstanding, at first view I partly disagree.

To be clear:
- When I speak about 'lift' in this context, it is not the wing lift or the wings + tail. It's only the tail contribution (hstab or hstab+elevator)
- Usually, the CG is before the aerodynamic focus, making the aircraft wanting to pitch down without the tail effect. Hence, the tail lift is negative in steady flight to counteract.

Now, let's imagine two steady flight situations at low airspeed, high alpha:

1 - Usual one:
The aircraft tends to pitch down with the elevator at or close to neutral. The pilot holds the stick pulled back to get a high AoA. The elevator (up deflection) makes negative lift. The hstab, seeing an incident wind from below (high alpha), makes positive lift. The sum of both is still negative to keep the steady equilibrium, steady pitch up moment (counteract the nose down tendency).
- The elevator tends to make the aircraft pitching up, with its own area and deflection,
- the hstab tends to make the aircraft pitching down (because of alpha) with its own hsatb area only.

First of all this would be such an extreme situation that the usual math, used for instance by AeromatiC++, does not apply. Also I don't think a Cub could ever get into such a situation. since it is well beyond the stall before it happens.

Before that the usual lift theorem still applies: the airflow follows the curve of the airfoil (including lift enhancers like flaps, ailerons or, in this case, the elevator) until it reaches the end of the airfoil. So you have to take into account the complete length of the chord from tip to tail.

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Re: JSBSim Piper J-3 Cub

Postby dany93 » Mon Aug 07, 2017 9:42 am

erik wrote in Mon Aug 07, 2017 7:24 am:So you have to take into account the complete length of the chord from tip to tail.

I agree.
But with this (good) way of calculation we are in the complicated situation where the reference line orientation (chord from hstab leading edge to elevator trailing edge) changes with the elevator deflection.
This calculation for Cm_alpha and Cm_elevator is doable in a good software for Longitudinal Aerodynamic Derivatives & Control coefficients, but is it the case? (hard to know, I admit).

After looking at Yiongliang Du coefficients, I find there's a flaw (I suspect in my reasoning):
- My reasoning leads to Cm_alpha decreasing with full up elevator deflection (high AoA, low speed).
- Yiongliang Du Cm_alpha increases.
I don't know...

Do we have to admit that Yiongliang Du coefficients are right and that the stall AoA cannot be reached by the real Cub ? (unlike every aircraft I've ever seen, ultralights for me, and unlike some videos seem to show)
Or something else in the FDM? but I don't know what...
The empty CG location seems correct relative to the FDM axes (reference for seats, loads,.. locations) because the limits with loads (at least those that I tested) are correct, inside the weight and balance graphs.
Moving the AERORP forwards from initially 18.90" (which seems correct IMO) to 15.67" helps, with keeping your (= Du Y) initial Cm_alpha and Cm_elevator values. But it becomes located before the most aft CG (19" or 20"). Usually considered as bad for stability IRL. (this was only an investigation test, not something that I propose, although it seems to work in FG)
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Re: JSBSim Piper J-3 Cub

Postby erik » Mon Aug 07, 2017 10:42 am

dany93 wrote in Mon Aug 07, 2017 9:42 am:Do we have to admit that Yiongliang Du coefficients are right and that the stall AoA cannot be reached by the real Cub ?

But you can already both stall and (mildly) spin with my configuration.
In a default stall the wing just loses lift and as a result altitude in level flight.
For the spin you need to stall and move the rudder.

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Re: JSBSim Piper J-3 Cub

Postby dany93 » Mon Aug 07, 2017 3:53 pm

Spin is not the issue. Not implemented, and of secondary importance for now. However, yes, the small roll hesitation near stall is pleasant and realistic.

But I feel (!?) that the stall is very soft, rather a gentle ballooning near CL_Max (alpha ~ 13 -15 deg flaps up, 10 - 12 deg full flaps). The aircraft still flies with a CL ~ 1.5 or 1.55.
Moreover the 3-point touchdown is nearly impossible (it needs ~ 12 deg pitch-angle, which makes 18 - 20 deg for alpha-angle). At my knowledge, that's the best way to touchdown at the minimum airspeed with a tail wheeler. Although many pilots don't do it (more difficult) if the runway is long enough.

Also, the 'Lift due to alpha' table is probably for the wing angle of attack. As the 'wing_incidence' is 2 deg in the FDM, aero/alpha-wing-rad would be more appropriate
Code: Select all
    <function name="aero/force/Lift_alpha">
      <description>Lift due to alpha</description>
      <product>
          <property>aero/qbar-psf</property>
          <property>metrics/Sw-sqft</property>
          <property>aero/function/kCLge</property>
          <table>
            <independentVar lookup="row">aero/alpha-wing-rad</independentVar>

However, as alpha is smaller than alpha-wing-rad (by 2 deg), a 3-point touchdown should still be more difficult to achieve (if my reasoning is good, not tested).
Last edited by dany93 on Tue Aug 08, 2017 1:21 pm, edited 1 time in total.
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Re: JSBSim Piper J-3 Cub

Postby erik » Tue Aug 08, 2017 8:25 am

Just the other day I noticed the thesis has a lift table for the Cub at page 68 which is slightly different from mine (which was based on the USA-35B airfoil). You could try to match the Lift_alpha to the one in the thesis and see if it makes any difference.

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Re: JSBSim Piper J-3 Cub

Postby Bomber » Tue Aug 08, 2017 5:27 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 ?
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Re: JSBSim Piper J-3 Cub

Postby dany93 » Tue Aug 08, 2017 5:42 pm

The lift curves are very different (blue = Du Y Thesis).
Image

I've made tests with the PA18 160 hp at default load,
Pitch alpha = Du Y = erik,
Pitch elevator, below:
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--> <!-- 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 x (-1.8) = -1.206 ~ Erik ini, dany, tests, stiffness issue -->
       </product>
    </function>

factor 0.67 in order to get again the initial basic value from erik (-1.8 x 0.67 ~ -1.21 for J3Cub, or -1.9 x 0.67 ~ -1.27 for PA18).

In the Lift tables, I've changed aero/alpha-rad for aero/alpha-wing-rad in order to take the FDM <wing_incidence> 0.0349 rd ( 2deg) into account. (this is the wing angular shift from the fuselage axis).
This gives (both are kept, can be selected by un-commenting)
Code: Select all
    <function name="aero/force/Lift_alpha">
      <description>Lift due to alpha</description>
      <product>
          <property>aero/qbar-psf</property>
          <property>metrics/Sw-sqft</property>
          <property>aero/function/kCLge</property>
          <table>

            <!-- USA-35B -->
            <!-- <independentVar lookup="row">aero/alpha-wing-rad</independentVar>
            <independentVar lookup="column">aero/Re</independentVar>
            <tableData>         
                      1668183  3707224
             -1.5700   0.0000   0.0000
             -0.3491  -0.0085  -0.5085
             -0.2443  -0.5085  -0.8136
             -0.1745  -0.5085  -0.5085
             -0.0873   0.1017   0.1017
              0.0000   0.5339   0.5339
              0.0873   1.2204   1.2204
              0.1309   1.4746   1.4746
              0.1745   1.5000   1.6272
              0.2182   1.6201   1.7797
              0.2618   1.5645   1.8306
              0.3054   1.4272   1.6272
              0.3491   1.3138   1.4238
              1.5700   0.0000   0.0000
            </tableData> -->

            <!-- Du Y -->
           <independentVar lookup="row">aero/alpha-wing-rad</independentVar>
            <tableData>
                -0.0349     -0.0500
                -0.0175     0.0500
                0.0000      0.1250
                0.0349      0.3000
                0.0698      0.5000
                0.1047      0.6800
                0.1396      0.8800
                0.1745      1.1000
                0.2094      1.3000
                0.2443      1.4750
                0.2793      1.6200
                0.3142      1.7250
                0.3316      1.7750
                0.3491      1.8130
                0.3665      1.8000
                0.3840      1.7500
                0.4014      1.6200
                0.4189      1.1750
                0.4363      0.7500
            </tableData>
           
          </table>
          <table>
            <independentVar>aero/function/total-wing-damage</independentVar>
            <tableData>
                  0.0000   1
                  1.0000   0
            </tableData>
          </table>
      </product>
    </function>


1 - As I had changed for aero/alpha-wing-rad, I have made some tests with the initial aero/alpha-wing-rad lift table.
USA-35B
At stall, stick kept pulled, no flaps, the aircraft pinches down then balloons with
~ 37 kts airspeed,
alpha-deg 15
pitch-deg 7.5
vertical speed -500 to -600 ft/mn.
It is mild, but I could not make a 3-point touchdown (with or without flaps).

2 - Du Y CL table
balloons with:
35 kts,
-480 ft/mn
alpha-deg 14,
pitch-deg 7
jsbsim/aero/alpha-rad 0.249 (~ 14.3 deg)
jsbsim/aero/alpha-wing-rad 0.284

However, the wings are not at their stall angle (which is 20 deg).

Without flaps, the 3-point touch is doable but a bit delicate because the 'stall' is brutal.
airspeed 33 - 34 kts,
11 -14 deg pitch angle.
The stall occurs rather because of the decreasing airspeed (we are not at the CL Max).

With full flaps, a very soft near-3-point touch is possible (9 - 10 deg pitch) because the aircraft looses height very softly at the end, at grazing the ground with the stick full pulled back.

At 105 - 110 kt airspeed, the pitch attitude is close to zero (-1.5 deg) because of the CL curve at low angles.

With such different lift curves, I let you test, maybe choose or match the curves. The Du Y curve seems more pleasant but I do not know if the behavior is realistic, it is not the J3Cub wing profile.
The USA 35B CL is unusually high at low alphas. The Re = 1.668 E6 curve might be more "rounded". Airfoil Tools for Re = 1 E6.
The Du Y curve is more usual, however its CL-max angle cannot be reached.
Last edited by dany93 on Tue Aug 08, 2017 10:20 pm, edited 3 times in total.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » 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.

@dany93
I'll spend some time with this and see what I think.

Thanks to everyone for spending your valuable time on this!
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Re: JSBSim Piper J-3 Cub

Postby Bomber » Tue Aug 08, 2017 6:52 pm

In that case can I have a link to the plane so as we're all 100% working on the same version.

It's not me being funny just carefull.
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Re: JSBSim Piper J-3 Cub

Postby wlbragg » Tue Aug 08, 2017 7:07 pm

It's in fgaddon, no other exists except a version I use for IT-Autopilot updates.
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