16 minutes ago, Murmur said:

I don't think the position of the engine in Plane-Maker has any influence on the vertical CG. Infact, Plane-Maker has an explicit entry for the vertical CG position (in the "weight and balance" screen), so I assume that in order to calculate the vertical position of the CG, XP only uses that entry + the position and weight of the fuel in the tanks.

I never said anything about the vertical CG.

 

16 minutes ago, Murmur said:

So, changing the position of the engine in Plane-Maker should have no effect whatsoever on the aircraft being top heavy or not.

If anything, the position of the engine could be used by X-Plane to estimate the radius of gyration, which doesn't affect the aircraft being top heavy or not, but might just change its rotational inertia around the 3 axes.

Lower rotational inertia by a shortened moment arm from the center of rotation is exactly what this modification is supposed to achieve. Instead of separating engine and thrust point, one could also manually calculate and set the desired the RoG, but who wants to do that...

Edited October 22, 2025Oct 22 by Bjoern

1 hour ago, Bjoern said:

Yes, they are, as I've pulled them straight from Plane Maker.

In PM Standar / Engine Specs / Locations, we have to entries for each "long arm", "lat arm", ... 

Which ones do you change to (1.0 long, -0.7 vert) and which to via thrust offset (-5.77 long, 1.0 vert) ?

30 minutes ago, Bjoern said:

I never said anything about the vertical CG.

You wrote: "they wear their engine mass [...] at the rotor hub, which makes them top heavy", and "with all that mass on top, it handles like an egg on an inverted spoon", so I assumed you were talking about the vertical position of the CG.

Anyway, I think X-Plane dynamically calculates the radii of gyrations during runtime (when their values are left blank in Plane-Maker), so probably the difference in radii of gyrations caused by the difference of engine position could be analyzed looking inside a FM cycle dump.

1 hour ago, jcomm said:

In PM Standar / Engine Specs / Locations, we have to entries for each "long arm", "lat arm", ... 

Which ones do you change to (1.0 long, -0.7 vert) and which to via thrust offset (-5.77 long, 1.0 vert) ?

Here's some highlighting:

Quote

If the engine is moved to its proper location (1.0 long, -0.7 vert) and the rotor back to its proper position via thrust offset (-5.77 long, 1.0 vert),[...]

 

 

58 minutes ago, Murmur said:

You wrote: "they wear their engine mass [...] at the rotor hub, which makes them top heavy", and "with all that mass on top, it handles like an egg on an inverted spoon", so I assumed you were talking about the vertical position of the CG.

Okay, I see your line of thinking. I should've made it clearer that it wasn't about the CG.

 

58 minutes ago, Murmur said:

Anyway, I think X-Plane dynamically calculates the radii of gyrations during runtime (when their values are left blank in Plane-Maker), so probably the difference in radii of gyrations caused by the difference of engine position could be analyzed looking inside a FM cycle dump.

Yes, RoG calculation is dynamic and a change in engine position does have an effect on the values in a FM cycle dump.

Edited October 22, 2025Oct 22 by Bjoern

This all reminds me of a feature recently added by ASOBO to their FDM in FS 2024, called something like "inertia tensor matrix" (*) - something that X-Plane has been doing since ever...

That means a lot, like the way it's always different to feel how aircraft of all types behave in XP compared to the previous MSFS versions... specially the cart feel of most aircraft in MSFS 2020 and 2024, which now in models that use that tensor matrix, including choppers, can feel a lot better.

I tried the new C172 in the SU4 betas and it's a whole lot different in feel, reminding me a lot of the XP C172...

Regarding Plane Maker, it's been a good while since I last oppened it, I have to understand the meaning of the two columns of values next to the various long / lat /... arm entries. Should be for 2 engines right? but the R22 has only one engine?

2 hours ago, jcomm said:

This all reminds me of a feature recently added by ASOBO to their FDM in FS 2024, called something like "inertia tensor matrix" (*) - something that X-Plane has been doing since ever...

The inertia tensor is radii of gyration in another form. To be precise, the diagonal terms in the inertia tensor directly correlate with the radii of gyration. The inertia tensor is more versatile though, since it allows to also specify the cross inertia terms, which I think are still lacking in XP (but they can usually be neglected, being order of magnitudes smaller).

I find it very strange though that MSFS2020 lacked some way to specify moments of inertia. Even FSX and predecessors modeled them.

 

Edited October 23, 2025Oct 23 by Murmur

4 hours ago, Murmur said:

The inertia tensor is radii of gyration in another form. To be precise, the diagonal terms in the inertia tensor directly correlate with the radii of gyration. The inertia tensor is more versatile though, since it allows to also specify the cross inertia terms, which I think are still lacking in XP (but they can usually be neglected, being order of magnitudes smaller).

I find it very strange though that MSFS2020 lacked some way to specify moments of inertia. Even FSX and predecessors modeled them.

 

Great post, and yes indeed the cross-inertia moments, or cross-moments of inertia, aren't probably taken into consideration when Radii of Gyration are used insteads of tensor matrices, so, well, at least in this aspect the solution ASOBO developed and is already using in two of the default aircraft in FS 2024 is more detailed / complete than X-Plane's present approach.

Edited October 23, 2025Oct 23 by jcomm

So they finally get a +1 on one aspect of the flight model. For very very asymmetric airframe geometries being rotated very quickly, at least. Good job asobo! Doesn't help the realism of the forces acting on that fuselage though 😐

Edited October 23, 2025Oct 23 by blingthinger

1 hour ago, blingthinger said:

Doesn't help the realism of the forces acting on that fuselage though

IIRC that was recently added…?

10 hours ago, jcomm said:

Regarding Plane Maker, it's been a good while since I last oppened it, I have to understand the meaning of the two columns of values next to the various long / lat /... arm entries. Should be for 2 engines right? but the R22 has only one engine?

Ahem. It's a helicopter, so it has two columns because of what? Exactly, two rotors.

And you really should enable "invisible parts" ("Expert" --> "Invisible Parts" --> Activate "Show Parts and Objects") that display all flight model related items in PM, so you can monitor what exactly changes if you alter some position related values.

17 minutes ago, Bjoern said:

Ahem. It's a helicopter, so it has two columns because of what? Exactly, two rotors.

And you really should enable "invisible parts" ("Expert" --> "Invisible Parts" --> Activate "Show Parts and Objects") that display all flight model related items in PM, so you can monitor what exactly changes if you alter some position related values.

Yeah! That was ovious 🙂

Now fixed, but I used different values for the default R22:

long arm 1.00

vert arm 0.38

thrust-point long offset  -4.75

thrust-point vert offset   -1.0

1 hour ago, UrgentSiesta said:

IIRC that was recently added…?

As far as I know, Asobo takes the forces on the fuselage into consideration. I recall seeing it demonstrated on one of the DEV streams where the CFD was being discussed. There are fuselage parameters that can be configured via flight_model.cfg.

On the xplane side, I read this post from Austin a while back. https://developer.x-plane.com/2018/05/better-fuselages-through-science/

Edited October 23, 2025Oct 23 by brinx

26 minutes ago, jcomm said:

Yeah! That was ovious 🙂

Now fixed, but I used different values for the default R22:

long arm 1.00

vert arm 0.38

thrust-point long offset  -4.75

thrust-point vert offset   -1.0

Numbers can vary, depending where you think the most mass of the engine and all its ancilliary equipment sits, as long as the "invisible parts" rotor sits where its 3D object counterpart does in the end.

I'm curious what you think about the change in handling.

(And if you get some inexplicable, high frequency bouncing on some surfaces like I do.)

1 hour ago, Bjoern said:

Numbers can vary, depending where you think the most mass of the engine and all its ancilliary equipment sits, as long as the "invisible parts" rotor sits where its 3D object counterpart does in the end.

I'm curious what you think about the change in handling.

(And if you get some inexplicable, high frequency bouncing on some surfaces like I do.)

@Bjoern, I will have to test it more thouroughly, and will report back.

This is how it looks:

Link to 3 items

There are 2 orange spheres. I guess the one at the bottom corresponds to the "tail rotor engine", but it doesn't appear to move when I change the "arm" values(?)

5 hours ago, blingthinger said:

So they finally get a +1 on one aspect of the flight model. For very very asymmetric airframe geometries being rotated very quickly, at least. Good job asobo! Doesn't help the realism of the forces acting on that fuselage though 😐

@blingthinger, it's always good to have progress in any platform we use. It's not a competition, for me it's an independently commulative growth in features, accuracy, playability, and that's what matters independently of the sim...

 

Edited October 23, 2025Oct 23 by jcomm

4 hours ago, UrgentSiesta said:

IIRC that was recently added…?

My bad there. I should have used "airframe" twice in that post instead of fuselage. They did significantly improve the fuselage but these gyration values represent the sum total of the overall airframe and that was the context I was referring to. The fuselage by itself is indeed better than it was before but the blending needed to merge those resulting forces with the "wings", etc to then boil back down to the 1D tables is where dynamic fidelity is lost.

 

2 hours ago, jcomm said:

It's not a competition

Agreed there's no real "competing" with simzilla. When it comes to the flight model though, I do like to keep score!

Edited October 23, 2025Oct 23 by blingthinger