Elevator effects

[bomber]

Here's the additional lift caused by elevator deflection

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            <function name="aero/coefficient/CLDe">
                <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>0.4300</value>
                </product>
            </function>

Let's say I agree with this, and that it's convention to use the wing area to calculate the additional lift generated by the elevator.

the difference in area between the wing and tail is a ratio of 4.54.... so if we were to use the tail area the Cl value of 0.43 would have to rise to 1.922 to produce the equivalent force. That seems a rather large 'additional' Cl...

So now lets look at the drag for the elevator...

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            <function name="aero/coefficient/CDDe">
                <description>Drag_due_to_Elevator_Deflection</description>
                <product>
                    <property>aero/qbar-psf</property>
                    <property>metrics/Sw-sqft</property>
                    <property>fcs/mag-elevator-pos-rad</property>
                    <value>0.0000</value>
                </product>
            </function>

Ooops no drag effect of elevator deflection.... that's a serious Lift 2 Drag ratio.

I don't think this is correct.

Simon

[IAHM-COL]

may I ask what aircraft are those tables coming from?

[bomber]

C172... the plane we've all got, that comes with Flightgear.

Which considering FG holds this up as a topl quality JSBsim flight model..... could make Helijah's point about JSBsim and the modelers using it valid.

[IAHM-COL]

I think most of the c172p is about eye-candy. humble opinion.

[Richard]

Here's the additional lift caused by elevator deflection

Let's say I agree with this, and that it's convention to use the wing area to calculate the additional lift generated by the elevator.

the difference in area between the wing and tail is a ratio of 4.54.... so if we were to use the tail area the Cl value of 0.43 would have to rise to 1.922 to produce the equivalent force. That seems a rather large 'additional' Cl...

Using the same normalization factor in each axes makes sense.

For some reason this model denormalizes into forces for each coefficient when it would be easier to do something like:

Code: Select all

CDRAG = CFXB + CFXDAD*aileron + CFXDRD*rudder + CFXGEAR*gear + CFXFLAPS*flaps + CFXDGE + CFXMN + CFXDADMN*aileron + CFXDRDMN*rudder

Drag = qbar * Sw-sqft * CDRAG


The normalization process doesn't change the underlying values - and doesn't relate to each individual surface instead it relates to each axes.

[Richard]

C172... the plane we've all got, that comes with Flightgear.

Which considering FG holds this up as a topl quality JSBsim flight model

Possibly the model needs some review; maybe it has flaws in it- but then so do a lot of aero models. I have heard various folks complaining that the C172 is unrealistic but I have never really looked into it.

[bomber]

The point I hope I've got across is that I could continue to do this all night... either on this fdm or anyother anyone chooses.

This is a peer reviewed flight model, distributed by flightgear as something to look at when producing your own flight model.. All too often we read "take an existing plane and adapt it."

Now I'm quite happy to have my flight modeling methods dissected but out of respect I think 'vanilla flight modeling' ought to get it's own house in order.

These topics are things I could never have brought up in FG forum or on Discord due to high flamefest risk, brought about by fanbois and JSBsim zealots.

[Richard]

The point I hope I've got across is that I could continue to do this all night... either on this fdm or anyother anyone chooses.

You've made your point - the C172 could do with some work on its FDM.

If you want to review one of my FDM's feel free to either use the F-14, F-15 or my most recent OpenVSP models (SEPECAT Jaguar, Supermarine Swift)

• https://raw.githubusercontent.com/swamp ... ar-vsp.xml
  https://github.com/swampmo/Jaguar-Gr1/b ... 9-0258.pdf
• https://github.com/Zaretto/Supermarine- ... ft-vsp.xml
  https://github.com/Zaretto/Supermarine- ... 092020.pdf
• https://github.com/Zaretto/fg-aircraft/ ... /f-14a.xml
• https://github.com/Zaretto/fg-aircraft/ ... 5/f15a.xml

[bomber]

Well it's a bit complicated... yet if I was to query just one thing....

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    <function name="aero/c/CFZMN">
    <description>LIFT INCREMENT DUE TO MACH</description>
          <table>
            <independentVar lookup="row">velocities/mach</independentVar>
            <independentVar lookup="column">aero/alpha-deg</independentVar>
            <tableData>
                                -10            -5            -2          0.01             1             2             6             7            10   
                    0.2      0.01974      0.00856      0.00226     -0.00227     -0.00448     -0.00677     -0.01610     -0.01648     -0.02602   
                    0.4      0.01284      0.00541      0.00146     -0.00149     -0.00293     -0.00442     -0.01034     -0.01051     -0.01710   
                    0.6     -0.00001     -0.00038      0.00000      0.00000      0.00000      0.00000     -0.00001      0.00088     -0.00004   
                    0.8     -0.02009     -0.01022     -0.00247      0.00261      0.00501      0.00748      0.01713      0.02046      0.02814   
                   0.86     -0.02841     -0.01463     -0.00358      0.00375      0.00722      0.01073      0.02444      0.02888      0.03926   
                   0.88     -0.03189     -0.01641     -0.00403      0.00421      0.00811      0.01203      0.02728      0.03227      0.04334   
                   0.91     -0.03760     -0.01970     -0.00486      0.00504      0.00972      0.01434      0.03221      0.03816      0.05070   
                  0.955     -0.05329     -0.02812     -0.00715      0.00721      0.01408      0.02079      0.04575      0.05348      0.07221   
                   0.98     -0.08167     -0.04193     -0.01103      0.01061      0.02101      0.03128      0.06908      0.07942      0.10508   
                   0.99     -0.10970     -0.05500     -0.01503      0.01344      0.02717      0.04080      0.09217      0.10540      0.13807   
                    1.4     -0.08805     -0.04102     -0.00097      0.03025      0.04600      0.06197      0.12491      0.14060      0.17374   
            </tableData>
          </table>
    </function>

There doesn't seem to be any drop off at mach 1..... maybe it's the type of airfoil, I'm not too familiar with these fast jets, so I'm not saying it's wrong just that I'm not used to seeing this shape of curve.

[IAHM-COL]

due to high flamefest risk,

Hehe