Vorview: Calculating mean camber line for arbitrary airfoils
DanielG
Vorview calculates the mean camber line of arbitrary airfoils. I've
found lots of references/function calls/variable names that seem to be
related but no luck finding the actual algorithm yet... Any
suggestions?
Daniel Garmendia
Rob McDonald
I'm not sure that I'm much help. Most of that work is done in vorSlice.cpp
The hard part isn't finding mean camber surfaces of arbitrary airfoils
-- the tricky part is fuselages and engines and combinations of
components merged together.
Roughly speaking, Vorview starts with the XSec/Hermite *.hrm wireframe file.
Vorview looks for discontinuities in planform and places Key Slices
there. The user can also manually place key slices. It then
distributes the desired number of slices (which become spanwise
panels).
Vorview slices through the wireframe geometry creating a composite of
all components which intersect that slice. The camber points
(chordwise panels) are distributed along each slice. Vorview looks
for the upper and lower surface envelope of the slice to determine the
mean camber surface.
The routine for just finding the mean camber is here:
https://github.com/OpenVSP/OpenVSP/blob/master/src/vsp/vorSlice.cpp#L5961
However, the entire slicing process is more complicated than just this
bit of code(that is where the magic is).
I hope this helps,
Rob
DanielG
Thank you for helping me find that bit of code and for the rapid
response. I see now that it builds the mean camber line as the
average of the upper/lower z-locations for a given x-location.
This method doesn't find the true mean camber line (the definition I
use is in Theory of Wing Sections (Abbott)): It's the midway point
between the upper/lower curves where the distances are measured
perpendicular to the mean camber line. Here is a visualization of the
difference: http://en.wikipedia.org/wiki/File:Airfoil_thickness_definition.svg
So my follow-up question: Is the method used in VSP a good
approximation for all airfoils? How much does it matter? For my
study I will be using airfoils shaped like the N2A centerbody, where I
won't have analytical solutions for the mean camber line. Basically I
want to make sure I am sending accurate information to Vorlax.
Daniel
> Daniel,
>
> I'm not sure that I'm much help. Most of that work is done in vorSlice.cpp
>
> The hard part isn't finding mean camber surfaces of arbitrary airfoils
> -- the tricky part is fuselages and engines and combinations of
> components merged together.
>
> Roughly speaking, Vorview starts with the XSec/Hermite *.hrm wireframe file.
>
> Vorview looks for discontinuities in planform and places Key Slices
> there. The user can also manually place key slices. It then
> distributes the desired number of slices (which become spanwise
> panels).
>
> Vorview slices through the wireframe geometry creating a composite of
> all components which intersect that slice. The camber points
> (chordwise panels) are distributed along each slice. Vorview looks
> for the upper and lower surface envelope of the slice to determine the
> mean camber surface.
>
Rob McDonald
Glad to help out.
I thought that might be what you were after -- I almost included that
discussion in my response.
I think that when you consider the large number of gross
approximations which go into Vorlax, you won't be particularly worried
about whether the mean camber surface is what VSP uses or the 'true'
definition.
In my experience, Vorlax works amazingly well -- frankly, it works far
better than we have any right to expect.
Due diligence will require you to do some validation for your problem,
but I doubt you will be able to trace any error to the definition of
the mean camber surface.
Rob