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Alec246 avatar image
Alec246 asked Alec246 answered

Moments of Inertia Switched?

Doing some research into how Moments of Inertia affects the FlightModel in the MSFS2020 Aircraft, I stumbled onto a weird case of confuse Moment of Inertia Attribution.

In the Flight_Model.cfg it is stated:

"

empty_weight_pitch_MOI = 60000 ; Empty pitch moment of inertia, Jxx (SLUG SQ FEET)
empty_weight_roll_MOI = 13760 ; Empty roll moment of inertia, Jzz (SLUG SQ FEET)
empty_weight_yaw_MOI = 30090 ; Empty yaw moment of inertia, Jyy (SLUG SQ FEET)

"

But this is not a correct mathematical convention, at least to the material I Have? They state that

Ixx = Moment of Inertia for Roll

Iyy = Moment of Inertia for Pitch

Izz = Moment of Inertia for Yaw


I Wonder if this is just a simple typo, or somethng that can be affecting internally other areas of the sim? Worth checking anyway!


Regards!

flightmodel
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jcomm avatar image
jcomm answered

Well spotted Alexis !

Let's see what they say...

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boufogre avatar image
boufogre answered

Hey,

I don't know if that helps, but in the Flight Model Physics page, there is a mention about the axes and referentials used by MSFS.


Here is a screenshot that might answer your question:

screenshot-20210704-154327-firefox-nightly.jpg


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jcomm avatar image
jcomm answered jcomm edited

It's a bit confusing since, from the original MS FS SDK documentation one can get the following:

Axis

Longitudinal X Positive Forward

Lateral Y Positive to the Right

Vertical Z Positive Down i9n FS9/X

which agrees with the8old conventional Ixx, Iyy, Izz notation and moments...

Here's an interesting (old) post by Tom Goodrick on MOI estimates for MFS aicraft:

Calculating MOI's | BGA Forums (proboards.com)

where Tom advocates that in general and for most aircraft designs roll moi < pitch moi < yaw moi

The above example shows that the relation is not valid for that model, but I really don't know if it can be justified. Anyway the yaw moi is lower...


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Alec246 avatar image
Alec246 answered

Thank you all for the very good insights on the subject!

This page on the SDK saying MSFS doesn't follow the usual convention might explain why there are all swapped. I wonder if that was the case also in previous FSs, or something that was implemented in Asobo's Hands.

Very good Link Jose! For sure, Yaw is always bigger on all planes I have seen so far. Disregard my first example, as that 60.000 in Pitch MOI is just me trying to break the system :D



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