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Question for the FM experts: What does the chord value in the wing sections of the aircraft_data.ini mean? How is it used in the FM?

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how many octaves it can sing??


(sorry, please forgive me. I just HAD to do that!! goofy mood today! :smile:)


My guess might be related to CG and how the game figures Mean Aerodynamic Chord (but aren't Mac statement seperate in the wings and stabs sections???)

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I know the definition of chord. I wanted to know what the chord values in the aircraft_data.ini means. As far as I understand for the pitch moment calculation the ReferenceChord value is used.

TK wrote:


Pitching moments use reference chord (c), and yaw and roll moments use reference span (b). All coefficients listed in the aircraft data file should use the same reference area and distance regardless of which part. So inner wings, outer wings, h-tail, v-tail, etc, should all use same set of S, c, b (usually measured on the whole wing, Biplanes use S for both wings). and that set of reference geometry should be listed at near the top of the aircraft data file.

Therefore in my opinion the extra chord values don't make any sense. What am I missing here?

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I think it affects lift and wing high\low pressure values in the aerodynamics data..for different AOA etc

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The "Ymac=" entry for each wing listed just below the relevant "Chord=" entry appears to be the distance from the center line the given Wing, Outer Wing, or Stabilizer's Ymac is located. Maybe the ReferenceChord= is used as the base reference for the wing and the "Chord=" on each wing is the wing's chord at the distance the "Ymac=" is and/or with intact slats and flaps in retracted or extended position?

You might find this thread useful as well. Maybe it should get added to the Knowledge Base.


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Thanks for the link. Unfortunately "Chord=" is not described there.:sad:
I already searched my TW forum archives, but didn't found anything.

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This is from my SF Notes files:

mac = Mean Area Chord, Is used as Reference Chord=x.xx under the aircraft data section. It's also the area of the wing surface from where one would calculate the Xac tables from. Usually a range starting 25% of the distance along a longitudinal line drawn along the mac, aft from the leading edge (LE), and ending 60-70% of the way back from the LE along the chord.

BTW, calculate the the coordinates fore or aft of the centerline using an accurate three view for reference...not the 3D model.

Ymac = The mean area chord as represented along the "Y" axis, measured out to either side of the centerline of the aircraft(left=-value, right=+value).

And yes, the 'significant digits do matter.... if you're taking the trouble to do it, why not do it right? :biggrin:

BTW, Kreelin's Aeroconvert is under utilities, in our downloads section.

I'll rummage around through my FM files to see if I can find more for you.

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This might be helpful:

Moments are non-dimensionalized using length (either span for lateral coefficients like Clb, or chord for pitch coefficients). Moments due to force (Clb and Cnb) are divided by (span), and moment due to rate (Clp, Clr, Cnp, and Cnr), are divided by (span*span*0.5), I _think_...

So, equation wise, (where q=0.5*density*v*v)

[Roll moment due to sideslip] = q*S*(span)*(Clb)


[Roll moment due to roll rate] = q*S*0.5*(span)*(span)*(Clp)
[Roll moment due to yaw rate] = q*S*0.5*(span)*(span)*(Clr)

[Pitch moment due to pitch rate] = q*S*0.5*(chord)*(chord)*(Cmq)



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