MAC - offset in FSUIPC

[saabpilot]

Hi Pete,

I am curious how you calculate MAC % in FSUIPC.

When I have a specific load in an OPENSKY B738 TopCat gives me MAC% = 24.27 which is correct and the FSUIPC offset 2EF8 gives 39.7%.

The 39.7% is clearly not the correct MAC% as the aircraft would then be totally out of envelope.

The fuel and payloads made by TopCat are within the standard B737 loading envelope

( 2EF8 8 CG percent, as a double (FLOAT64). This is the position of the actual CoG as a fraction (%/100) of MAC )

Clearly an error somewhere - either the MAC% calculation vs. Opensky B738 or something else ??

I hope you can give some light on this.

Best from Stockholm,

Bjorn

[Pete Dowson]

I am curious how you calculate MAC % in FSUIPC.

I don't.

When I have a specific load in an OPENSKY B738 TopCat gives me MAC% = 24.27 which is correct and the FSUIPC offset 2EF8 gives 39.7%.

Blame FS then, or its modelling. The value is read from SimConnect varable "CG PERCENT" defined in the SDK as "Longitudinal CG position as a percent of reference chord". The description in the offsets list was provided by others. I don't understand that stuff. I think it was also called CG_PERCENT in FS9 BTW)

Pete

[saabpilot]

Hi again Pete,

That answered my question so thanks for the fast answer.

MS (FS) has made a weird internal (airfile) MAC% calculation, probably it did not matter at the time of FS9 birth :razz:

Now when you are able to "load" FS aircrafts by "stations" it does since you will try to keep as close to 25% MAC for your TOW as possible in a B737 model.

The below example is just to show how the MAC% numbers affects the envelope (on the right side).

( DOW = Dry Operating Weight, ZFW = Zero Fuel Weight and TOW is Takeoff Weight )

If out of the envelope - the a/c cannot fly.

Bjorn

[Pete Dowson]

If out of the envelope - the a/c cannot fly.

Yes, so best not to take FS's MAC computation too seriously. I use the one computed by Project Magenta in its CDU.

Regards

Pete

[herve_sors]

MAC calculation and position of current CoG (in %MAC from leading edge, the one that is reported by FSUIPC from FS) is complicated by the fact FS defines 2 MAC's, one for aerodynamical calculations, the other for gauge display. They may fit or no. However both can be precisely calculated from aircraft geometrical data (as defined in the aircraft.cfg) and weight distribution. When such "external" calculations are performed, resulting values perfectly fit what FS reports as CG %MAC. May be you could have a look to the excellent paper written by Yves Guillaume that undermine the foundations of how FS is doing that (and that have been implemented in my latest AFSD version)

http://library.avsim.net/download.php?DLID=170811

If you need some more details regarding practical calculations let me know by mail

Hope it will help

Hervé