Aerodynamic center - for symmetrical airfoils

[Carlos Xisto]

Hi Everyone,

Anyone here trying to make a simple prediction of the aerodynamic center (or neutral point) using the stability analysis or a simple angle sweep in vspaero. 

I have been trying to validate the method for a symmetrical  NACA00XX airfoil and are having a hard time hitting the c/4 location. 

My predictions point to about 0.23c for a NACA0012. This seems a bit off, going lower in thickness does not seem to improve. 

I wonder if this is caused by the CD0 correction applied in VSPaero? 

Thanks,

Carlos 

[Rob McDonald]

Are you in VLM -- in that case, thickness plays no part whatsoever.

If you are in Panel mode, thickness will influence the location of the AC.

Beyond that, you are looking at a 3D solution, why would you expect a perfect match for 2D thin airfoil theory?

Is your wing swept, tapered, twisted in any way?  If not, what is the aspect ratio?  You should get closer to the theoretical answer for higher AR -- but the 3D effects will always be there.

If you don't want 3D effects, then don't run a 3D code.

Rob

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[Carlos Xisto]

Hi Rob,

Thanks for your help here. I'm using VLM, I had the impression that a flatplate skin friction model calibrated with NACA0012 empirical data was available in VLM. 

This is basically a straight, planar wing  with very high aspect ratio (up to 100) to crank down the 3D effects. I was hopping that it would still be possible to match the theory and wind tunnel data. 

Thanks again. The effort you guys put into the forum is amazing.

Carlos

[Rob McDonald]

Yes, there is a simple parasite drag term -- but it won't change the AC or the SM.

The SM is the slope of the CM vs CL curve -- sm=dCm/dCl

There are a lot of complexities trying to drive VSPAERO to a 2D solution.  I would expect the results to be close, but not exact.

Similarly, we check some cases between VSPAERO and lifting line theory -- but lifting line theory has its own flaws and we do not expect them to match perfectly.

Rob

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[Carlos Xisto]

Ok,

So it is correct to the assume that the viscous correction will not afect the resultant force and pitching moment in vspaero (i.e. vspaero only relies on the inviscid  coefficients to calculate Cmy)? 

Thanks

Carlos

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[Rob McDonald]

The viscous contribution dues not change lift or moment. 

Rob

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