Hi all. I’m working on a model glider wing mold to be CNC milled and am trying to figure out the best way to create my wing shape. In the past I’ve used both 2-rail sweeps and lofts to create wings for this purpose, but as I’ve been thinking about it more (perhaps thinking about it too much…) I’m wondering if I’m distorting my airfoils inadvertently.
The wings I’m modeling have elliptical planforms (i.e. the leading edge and trailing edge shapes are elliptical). I have multiple different airfoil shapes that are located at intervals (“stations”) across the wingspan (5 total airfoils at 7 stations). These airfoils need to be maintained very accurately at the scale I’m working in order to perform properly. The airfoil shape at any point between the defined stations should be a linear interpolation of the two foils on either side of it. For example, the foil midway between station 1 and station 2 should be a 50/50 blend of the station 1 foil and the station 2 foil. At 1/4 of the way between station 1 and station 2, it should be a 25/75 blend of the station 1 and station 2 foils, etc.
It is less critical for me to follow the exact elliptical planform (wing outline) shape than it is to ensure that the airfoils are accurate across the entire wing. However, it is desirable for me to be able to control the planform exactly.
If I do a loft of the foils (placed at the right locations, with the right rotational relationship to each other) I believe that I will get an accurate representation of mid-station airfoils, but I don’t have direct control over the planform outline (I’m not certain about the mid-station airfoil accuracy statement - any clarification on that would be appreciated). If I do a 2-rail sweep, I have direct control over both the airfoils at the defined stations and of the planform outline, but I’m not sure what process is used to obtain the shape of the sections between the defined airfoil stations, particularly in situations where the planform is not planar (often the rear of the wing rotates up near the tip to create washout). My gut tells me that it’s possible the mid-station foils could be getting scaled/blended in a non-2D fashion that could change the airfoil shape. It’s also possible that a 2-rail sweep performs linear blends of the defined section shapes and then subsequently scales and rotates those shapes as required by the rails, in which case it should create exactly the surface I’m looking for.
Any commentary on how 2-rail sweep creates surfaces, or from others who do shape-critical lofts/sweeps is most appreciated. Please feel free to ask clarifying questions, and my apologies if this is hard to follow. I can post an example file if desired. BTW I’ve just signed up for this forum and am quite impressed - it looks like a very helpful community.
Thanks very much,