Airfoil - LE vs TE trimming - Effect of camber

[Etienne Behar]

Hello!

    I consider an airfoil with camber, with a simplified case given in attachment. I observe that if I trim this airfoil's leading edge X-wise with relative value 0.7, or if I trim the same airfoil from the trailing edge, X-wise, with relative value 0.3, the two resulting airfoil do not have parallel trimmed faces, as illustrated below (blue and red meshes are not parallel locally at the trimmed point). Another way to put it, the two trimmed airfoils (LE-trimmed and TE-trimmed) are not falling on the exact same point of the camber line, despite the ratio 0.3/0.7 in their relative values. It seems contradictory with the expectation that the trim operates along and perpendicularly to the mean camber line, as stated here at 2:32. Visually, it looks like the LE trim is made perpendicular to he convex side of the airfoil, while the TE trim is perpendicular to the concave side; could that be the case? If so, is it on purpose for a specific reason? Can this behavior be changed by the user, from either the GUI or the APIs, in order to trim perpendicularly to and along the camber line, and get well aligned blue and red airfoils?

    Context: I am trying to create separate geometries for control surfaces, which could be rotated with a smooth transition from the fixed wing to the control surface, using round caps at the trim points (and an additional hinge geometry to pilot the rotation). Larger context: producing geometries for CFD runs, running over several deflection angles of the CSs, piloted thought python APIs. I am almost there and would be happy to share the script, but this aspect described above is blocking me still!

Thank you so much for your help!

Etienne.

[Rob McDonald]

I would have to dig into this behavior a bit more to figure out exactly what is going on.

OpenVSP does a search to find the desired point along the curve as a distance from the LE/TE.  I think it is then matching the equidistant points from the Upper and Lower surface.  So, in the case of a cambered foil, it is not surprising that the trim location is different when found from the LE vs. TE.

This is not something that can be changed by the user from the GUI or API.  If you wanted to dig into the source code, I might be able to direct you to where you should start poking around.  However, there are probably better ways.

Somewhat recently, OpenVSP got the ability to model deflected plain flaps (commonly used for control surfaces).  It is on the 'Modify' tab for lifting surfaces.

Will this meet your needs?

Rob