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Elevator effects
Posted: Mon Jul 13, 2020 6:01 pm
by bomber
Here's the additional lift caused by elevator deflection
Code: Select all
<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...
Code: Select all
<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
Re: Elevator effects
Posted: Mon Jul 13, 2020 6:19 pm
by IAHM-COL
may I ask what aircraft are those tables coming from?
Re: Elevator effects
Posted: Mon Jul 13, 2020 6:52 pm
by 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.
Re: Elevator effects
Posted: Mon Jul 13, 2020 7:01 pm
by IAHM-COL
I think most of the c172p is about eye-candy. humble opinion.
Re: Elevator effects
Posted: Mon Jul 13, 2020 8:21 pm
by Richard
bomber wrote: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.
Re: Elevator effects
Posted: Mon Jul 13, 2020 8:28 pm
by Richard
bomber wrote: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.
Re: Elevator effects
Posted: Mon Jul 13, 2020 9:03 pm
by 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.
Re: Elevator effects
Posted: Mon Jul 13, 2020 9:23 pm
by Richard
bomber wrote: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)
Re: Elevator effects
Posted: Mon Jul 13, 2020 10:20 pm
by bomber
Well it's a bit complicated... yet if I was to query just one thing....
Code: Select all
<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.
Re: Elevator effects
Posted: Mon Jul 13, 2020 10:38 pm
by IAHM-COL
bomber wrote:due to high flamefest risk,
Hehe