Pitches up with tailwind on ground

[dsatgh]

When I'm on the ground with a tailwind somewhere above 10 kts, the plane pitches up until the tail hits the ground. I noticed a big difference in behavior to the c172p. I don't know if it's relevant, but when I look at the wind pitch moment in the properties, the c172p show much smaller values and reacts to elevator movements. The c182s has a much bigger value and doesn't react much to elevator movements.

[HHS81]

Did you check that the aircraft AND FlightGear version is the most recent?

I just tried, and yes, the aircraft is pitching up with winds more than 15ktn. The values should have been realistic once I set them, but may be altered some time ago. I'll check that

[hbeni]

Also look at the CoG: how was the plane loaded when this occurred? (Tanks, Passengers, Baggage)

[dsatgh]

I'm on master branch and put the CoG as much forward as possible. FG is next branch.

[HHS81]

Also look at the CoG: how was the plane loaded when this occurred? (Tanks, Passengers, Baggage)

I could reproduce it. The first impression is comparing different numbers, that it is not the elevator- there is something else.

[hbeni]

Any new insights into this? 🙏

[HHS81]

Any new insights into this? 🙏

Not yet, at least I can say it is not related with the climb rates issue. I do know that the C182 is sensible to wind on the ground and therefore it will need correct inputs of the yoke wheel in order not to roll over while taxiing, But here there inputs doesn`t have the needed effect. I have to try different things to rule certain things out.

[hbeni]

Interesting read, maybe related, on the Forums:

https://forum.flightgear.org/viewtopic.php?p=417356#p417356

It doesn't actually take much to tip a C172 or PA28 onto its tail; if you go to an airport after a heavy snowfall, you'll find several planes with their nosewheels in the air

and also that the fdm models gear is not spot on the 3D-model ones for the 172, something we should ceck here too

[HHS81]

Unless something had been changed on the fdm since I started the model, the fdm gears should be spot on. That was something I checked really early on the model.

[hbeni]

Had the mood to poke around with versions a bit to try to get a grip on when this was introduced.
All tests performed with:

• max forward CoG (pilot+copilot full weight, full tanks, otherwise no passengers or baggage) and elevator full down.
• Weather was set to 'manual' in Launcher and XXXX 012345Z 09010KT 9999 -SN OVC060CB SCT050TCU 20/M30 Q1005
• location was EGNX, runway 27 (threshold)

---

• FGFS 2020.3.19:
  ** with current master (f23bf4f) pitched up with 13kt; CoG=38.94.
  That seems to be pretty much the tipping point: more backward CoG pitches up and more wind also; moving the elevator to neutral also. ( 🥳 btw nice that it still loads on stable and is flyable, good to know that)
  ** version-1.2 tag (10f071f) from Jul 31 19:23:18 2016: sustained till 55kts CoG however was at 39.00 (so there is a small difference already)!

So that does in principal verify it is indeed not flightgear itself causing this, but FDM related.
I continued to test on next.

---

• FGFS next:
  ** current master same behaviour as in 2020.3.19.
  ** version-1.2 same behaviour as in 2020.3.19.
  ** version-1.4 has again the problematic behaviour (CoG=38.95), pitch up at 14kts.

---

So... I git bisected' from that on next and got:

7a5a0b6 is the first bad commit

Verified that by reverting it at masters Head (resolving the conflicts), and indeed the problem went away.
Pushed branch Issue_517-TailwindPitchUpfor investigation; its master with 7a5a0b6 reverted. ⚠️ "went away" does not mean "good" in this case ☝️; the plane wrongly sustains very heavy tailwind of 50 Kts with that reverted!

[HHS81]

@hbeni
So it was me who introduced the bug.
With the changing on the gear I also changed some values on the pitch coefficient- but can't remember why. I'll check that immediately, so we can revert the numbers on pitch coeffcients.

[hbeni]

That would be awesome;
I honestly have no clue what those numbers really mean, so I stopped trying to fix it...

[hbeni]

Can the pitch coefficient also introduce more lift?
Just asking again because of the climb numbers

[HHS81]

That would be awesome; I honestly have no clue what those numbers really mean, so I stopped trying to fix it...

I testet it and it works for me flawless. I guess I might have changed it, as there had been complaints about to be too sensitive on the elevator controls. But I remember some cockpit videos from real aircraft, which showed it needs only little input to takeoff

[HHS81]

Can the pitch coefficient also introduce more lift? Just asking again because of the climb numbers

No, and the climb numbers stayed the same with. I might have some time on the weekend, to finally correct this issue.

[HHS81]

@hbeni I will push/ pull request do tomorrow or saturday, as I need to update my authentification.

[HHS81]

The pitch up with tailwind is also a problem on the c172p (winking to @wlbragg ) and was there introduced with the ommit 3680740 while changing the adverse yaw effect.

[hbeni]

See also: c172p-team/c172p#1489
There are also linked forum topics

[ysopflying]

The formula without taking propwash into account and using a simple sin-expression can certainly be improved.

[ysopflying]

current FGAddon stable has:

            <function name="aero/coefficient/Cmalpha">
                <description>Pitch_moment_due_to_alpha</description>
                <product>
                    <property>aero/qbar-psf</property>
                    <property>metrics/Sw-sqft</property>
                    <property>metrics/cbarw-ft</property>
                    <sin>
                        <property>aero/alpha-rad</property>
                    </sin>
                    <value>-1.8</value> 

The sin-expression is not present on current github:

                    <property>metrics/cbarw-ft</property>

                        <property>aero/alpha-rad</property>

                    <value>-1.65</value> 

If there is a high negative alpha due to tailwind, a high pitch up angle occurs.

[hbeni]

Thank you for investigating further!
And what is the correct fix for the Situation?

My programmers mind just wants to add clamping to high negative alpha due to tailwind… which is certainly an ugly, if not evil, hack.

[TheFGFSEagle]

The non-ugly fix would be to use the first code, instead of the second in Ysops last post ! ;)

[hbeni]

current FGAddon stable has

Looks like it was commit b8e32b7 removing the <sin>

And Heiko's PR #561 also addresses this, but in a slightly different way

[ysopflying]

From a simple NACA0012 (using factors for 3D, different thickness, neglecting fuselage, main wing.....):

Alpha wing	cmm
-180	-0.24
-175.5	0
-171.305	0.23
…	…
174.659	-0.43
176.575	-0.39
180.305	-0.23

Actually it would need propwash effect. To be done at a later stage. Excluded are all other angles, as there are different versions floating around in different commits. Alpha -10-25 might be subject to a separate issue.

[hbeni]

Thanks, but where does this need to go (code wise)?

[ysopflying]

will be completed to a full commit after having a look at a few things

[ysopflying]

see #590
see #589

[hbeni]

Thank you, will test and report back!