Geometric or aerodynamic chord?

[Hugh Blackburn]

Hi there

Under Wing --> Plan, is the chord reported here the geometric or aerodynamic mean chord?  I think (and hope) it's the latter, since for the wing I have, the value reported is larger than the geometric mean chord.

thanks

Hugh

[Rob McDonald]

None of the above.

It is the average of the specified chords -- those at the wing breaks.

When you increase/decrease it, all the chords are changed in
proportion to the change you make to the 'total' chord.

Rob

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[Hugh Blackburn]

Coming back to this question - I believe that MAC used to be computed under the Comp Geom tool, but that output no longer seems to appear.  Is the MAC computed/available anywhere in OpenVSP?  It seems a shame to have to compute it using another tool, since the info is all there.

Thanks - I'm really enjoying the enhancements recently introduced to vspaero.

Hugh

[Nick Brake]

I don't believe there is a calculation of the MAC in VSP currently your best bet is to run DegenGeom and calculate the MAC integral external to VSP.

This would be a nice calculation to have though.

I'm glad you are enjoying the new vspaero integration there is more exciting work coming as well.  

n

[Rob McDonald]

Hugh,

We used to do an embarrassingly simple estimate of the MAC and AC for
a single trapezoidal wing in isolation (no tail included). It was
simply the sort of geometric construction from the textbooks.

Given the prevalence of multi-section wings and the expectation that
most aircraft have multiple lifting surfaces (at least a tail), I
found that this capability introduced more confusion than it was
worth.

Quite frankly, the MAC concept in general is a fairly dangerous one.
People often attempt to place the MAC on the wing (matching
leading/trailing edges) as a way of determining the position of the
AC. Since the position of the AC depends on the aerodynamics (twist,
tail, camber distribution, etc) a purely planform-based geometric
construction will always be wrong.

Running VSPAERO at two angles of attack will quickly allow you to
calculate the location of the aerodynamic center for a complex
configuration. Choose your reference chord length based on whatever
you like. This should provide everything you need.

Rob


On Wed, Sep 21, 2016 at 9:01 PM, Hugh Blackburn

[Hugh Blackburn]

Thanks for your response, Rob.  

(I know how to use vspaero to find the neutral point. :) I also love being able to use Sweep to find the trimmed AoA for a given CG location - something I realised after hearing Nick Brake state that in passing during one of the Workshop videos.)  The reason I wanted MAC is so I (and students) could set the static margin based on that measure, as it's what is typically stated in texts. Right now we use Mason's code to compute MAC on the side.  It just seems irrational to go outside OpenVSP to get that.

Hugh