Pitching moment and CM-alpha

[timjim333]

Hi, I'm running SU2 in fixed-CL mode. I noticed that in the forces breakdown file, it outputs approximate partial derivatives, which I thought was pretty neat. Does this mean that the value of Approx. Delta CMy / Delta CL is essentially the aircraft's CM-alpha? i.e., as long as it is negative, the aircraft is statically stable? (Although not necessarily trimmed.)

If I'm not mistaken, this value will change depending on the reference point though, is that correct? So if I set the ref point to 0, 0, 0, then decide where my CG is after the simulation, I would need to reset the reference points and run SU2 again. Or is there a quick way to 'move' the value to a new CG location?

Is CMy non-dimensionalised by the REF_LENGTH and REF_AREA input parameters so that CMy * REF_LENGTH * REF_AREA * dynamic pressure will recover the moment about the specified reference point?

Thanks very much! Another great release from the team.

[timjim333]

Just a quick additional question - based on the outputs, would it be possible to estimate the aerodynamic centre (or neutral point for tailed configurations)? Thanks.

[timjim333]

Having slept on it a bit, is Approx. Delta CMy / Delta CL essentially the negative of the static margin SM = -dCM/dCL?

Assuming cbar is the reference length and xcg is the REF_ORIGIN_MOMENT_X, it would then be possible to recover the neutral point as
SM = (xnp-xcg)/cbar

Would that be correct?