XLEMAC in openVSP

[Violet Philip]

Hello everyone,

Does anyone know how to obtain the accurate XLEMAC (Leading Edge of the Mean Aerodynamic Chord) directly from OpenVSP?

I'm having trouble determining the correct CG location within the envelope. When I calculate XLEMAC manually, I end up with very high Static Margin values, even after trimming the aircraft with tail incidence.

This is for a B767-200ER model. I assigned masses to each component using BlankGeom.

Any guidance would be greatly appreciated. Thank you.

Violet

[Rob McDonald]

You probably don't want to find your CG in a bottoms up way.  There are so many parts to an aircraft that accounting for all of them to find the total mass and the CG location typically is not practical.

Instead, you should probably use other sources of information to set the CG -- or set it based on wherever it needs to be for the stability you need.  When designing a transport aircraft, the designer has great latitude to move the wing and also major masses around to get the CG where it needs to be.  It is not a passive exercise.

The MAC is a chord length that is determined entirely by geometry -- it really shouldn't be called the 'aerodynamic' chord (because no aerodynamic calculations were performed to find it).

Placing the MAC on the wing is not just a geometric calculation.  You often see a graphical construction (building a large trapezoid with the tip and root chord transposed, then drawing a big X) -- that is bollocks.

Instead, if you want to know where the aerodynamic center of a wing (or group of lifting surfaces), then you need to do an aerodynamic calculation.

Fortunately, VSPAERO will do this for you.  Just run in the static stability mode, then look in the *.stab file.  It should report the xac.  If it doesn't (maybe it got omitted in the recent update), it is easily calculated from dCM/dCL.  Worst case, run two angles of attack and you can figure it out.

Rob