Reference frame of aerodynamic stability derivatives

[Shenil Gobardhan]

Hi everyone, 

I have a question regarding the reference frame that OpenVSP calculates its stability derivatives in. More specifically, the aerodynamic derivatives relating to CD, CL, CS, CMl, CMm and CMn.

I would like to obtain these derivatives w.r.t a stability reference frame in which the x-axis points in the direction of the free stream velocity, y to the right wing and z downwards. The origin of this reference frame should be in the CG of the aircraft. 

Therefore I would like to know whether these coefficients are computed in the stability or aerodynamic reference frame and if this stability reference frame matches the one I described.

Thanks in advance for your reply.

With kind regards,

Shenil

[Brandon Litherland]

The stability derivative axes have been discussed in prior posts, more recently in detail here: https://groups.google.com/g/openvsp/c/Pb93zCFufJA/m/f5587bROBAAJ.

From a flight dynamics perspective, at least generally, the axes are as described as "forward, right, down" with XYZ wrt to the aircraft.  OpenVSP uses "aft, right, up" which you can see in the axis display in the corner.  From prior discussions on the forum, VSPAERO will use the body axes as reference or in some cases the "wind" axes (lift, drag, sideslip) per the linked post, but to my understanding does not convert those values to the forward/right/down convention.  

[Shenil Gobardhan]

Hi, 

Thanks a lot for the quick and elaborate reply!

With kind regards,

Shenil