I'm submitting a ... <li cl

Indeed, thank you for the excellent bug report <a class="user-mention notranslate" dat

Closing this issue as the bug has been fixed in PR <a class="issue-link js-issue-link"

The PR <a class="issue-link js-issue-link" data-error-text="Failed to load title" data

Wrong calculation of vPQRdot in FGAccelerations class about jsbsim HOT 4 CLOSED

mathieuflament commented on June 16, 2024

Wrong calculation of vPQRdot in FGAccelerations class

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Comments (4)

jonsberndt commented on June 16, 2024 2

Hi, Mathieu,

Thanks for this detailed report. JSBSim is where it is today in part due to continuous feedback from the larger community.

This issue got my attention - it's a very interesting issue. I was initially surprised to read of it partially due to the fact that JSBSim has been involved in comparisons against a number of other high-fidelity simulations and fared very well. But, as you pointed out, "this error has a limited effect on trajectory computations" so that could be why it has gone unnoticed if it is indeed an error.

I'll take a closer look at this myself (this one is going to need some quiet time and a few cups of coffee ;-), and I'm sure that Bertrand and Sean and others may also take a close look. I just wanted to post a note of appreciation for the thoughtful report.

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bcoconni commented on June 16, 2024 2

Indeed, thank you for the excellent bug report @mathieuflament 👍 Clear and thorough.

I agree with your analysis, and while investigating the code history, I discovered that the error was introduced almost 13 years ago by myself via commit fdf7cf7. However as you mentioned the side effects are very limited as vPQRdot is used in just a few places, namely:

jsbsim/src/input_output/FGOutputTextFile.cpp

Line 280 in d2d8f61

 outstream << (radtodeg*Accelerations->GetPQRdot()).Dump(delimeter) << delimeter; 

which writes the value of vPQRdot to a CSV file.

jsbsim/src/initialization/FGTrim.cpp

Line 436 in d2d8f61

FGColumnVector3 moment = MassBalance->GetJ() * Accelerations->GetPQRdot()

where vPQRdot is used to determine the position at which the aircraft will sit on the ground at the simulation start.

jsbsim/src/models/FGAccelerations.cpp

Lines 271 to 273 in d2d8f61

 FGColumnVector3 wdot = vPQRdot; 

 if (dt > 0.) // Zeroes out the relative movement between the aircraft and the ground 

 wdot += (in.vPQR - in.Tec2b * in.TerrainAngularVel) / dt;

where vPQRdot is used to compute the friction forces between the landing gears and the ground. Most likely the sign error has a small effect on this computation as the norm of in.vOmegaPlanet is very small (≈7.3e-5 rad/s).

This is most likely why the error remained unnoticed for such a long time. Anyway the bug must be fixed so you could perhaps submit a pull request (referencing this issue) to fix it ?

Thanks.

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bcoconni commented on June 16, 2024 1

Closing this issue as the bug has been fixed in PR #1036.

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bcoconni commented on June 16, 2024

The PR #1036 that fixes the bug reported in this issue has been merged in master. I guess that this issue can now be closed ?

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Wrong calculation of vPQRdot in FGAccelerations class about jsbsim HOT 4 CLOSED

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	FGColumnVector3 wdot = vPQRdot;
	if (dt > 0.) // Zeroes out the relative movement between the aircraft and the ground
	wdot += (in.vPQR - in.Tec2b * in.TerrainAngularVel) / dt;