Weird value for h_b-mac

[princekamoro]

I noticed that when the plane is on the runway, h_b-mac is about 0.054. That would correspond to a wing height of 0.054 x 35.8' wingspan = 1.9' above the ground, which seems much too low for a high wing aircraft. Might the model be calculating too much ground effect because of this?

(I've noticed the same on the 182S as well)

(Edit: fixed typo 0.54 x 35.8' -> 0.054)

[dany93]

Hi @princekamoro

h_b-mac is not the MAC height in ft, it is (from https://wiki.flightgear.org/Aero_input_and_outputs)

Altitude of MAC (Mean aerodynamic chord) divided by wing span: /fdm/jsbsim/aero/h_b-mac-ft

(the variable name is confusing)

With the c172p on the ground I read h_b-mac-ft = 0.102

Which gives MAC height about 35.8 * 0.102 = 3.56 ft.

[dany93]

@princekamoro Where did you find this "h_b-mac" variable name or property and this 0.054 value?

[princekamoro]

I don't know if it's the most up to date plane, it's the one that came with the data files for FG 2020.3.19.

Being a high wing (EDIT: and dihedral) aircraft, I would assume MAC should be higher than the CG, something almost 6 ft or so above the ground, which would be an h_b-mac somewhere in the ballpark of ~0.16 ish.

[dany93]

In this repository, we work with a 2020.4 FG version. I have (not the last but...) a recent c172p version, which has been changed. Particularly for its X and Z coordinates (aerorop, CG locations). Our differences from 2020.3 should come from this.

Being a high wing (EDIT: and dihedral) aircraft, I would assume MAC should be higher than the CG, something almost 6 ft or so above the ground

I think that you are right. I don't know where this MAC height is taken from, or where it is given or how it is calculated. I find no value in the FDM.