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You could use evDistance between two of the faces, but since fCuboid is axis-aligned you could skip the geometry creation and just check the difference of the x, y, or z components of the two vertices.

I've been using Onshape for a while now, I lecture in Aerodynamics at Northumbria University in Newcastle, UK and actively use Onshape as part of the modules I teach across different Engineering disciplines. Recently I've been working on a slightly larger project with students to design an open-wheeled F1 style car and quickly found that by using Feature Scripts, some of the more repetitive and complex geometries could be made in a much easier, more consistent way. So I thought I'd have a go at adding a generic aerofoil profile generator to Onshape. 

I spent a number of years designing F1 cars for real, working in the aerodynamics department at Williams F1 and the one thing common to the design of these vehicles is the number of wing elements you end up having to draw. In industry (certainly at Williams) we had a number of tools integrated into our workflow in order to help create aerodynamic wing profile. There are a number of important things a good tool profile should do:

• Relatively few parameters to control the shape
• Scale in a sensible way (from small winglets to rear wings)
• Allow positive and negative camber (the direction the aerofoil curves)
• Be flexible and easily extendable to multiple elements

The approach is to allow the user to draw a line representing the "chord" of the aerofoil. This defines the overall length and incidence angle of the profile as well as a plane (that the line should be coincident with) which defines the 2D extent that the profile will be "sketched" on, in the example I've created two lines arbitrarily on two planes offset from one another to create the "span" of the eventual wing:


Once the chord line and plane have been selected, the from the dialogue box a number of parameters are presented that can be changed to alter the shape of the aerofoil:

• Thickness ratio: This controls how thick the profile is relative to its overall chord length
• Thickness position ratio: Where the point of maximum thickness occurs relative to chord length (0 is at the leading edge...)
• Leading edge (LE) angle: Controls the amount of overall camber for the profile
• Trailing edge (TE) angle: Controls the amount of "curviness" at the trailing edge of the profile

The output of the custom feature script is a nice, tidy fill surface that can be quickly and easily lofted/extruded/swept into a wing. Here's one I made earlier:


Have a play around and feel free to post and comments/feedback/suggestions. Below is screenshot of one of the projects I'm currently using the profile generator for. All 26 of the profiles here are driven by the custom feature described above . I'm glad I found out about Custom Features and I'll definitely be using them more in the future, thanks Onshape!

If a query refers to multiple entities in the current context (e.g. faceQuery referring to 6 faces of a cuboid), you can iterate through all the entities it refers to by calling evaluateQuery.

evaulateQuery returns an array of specific queries for individual items. You can iterate through this array to evaluate the area of each face individually:

var faceQuery is Query = qCreatedBy(id + "cuboid1", EntityType.FACE);
var faces is array = evaluateQuery(context, faceQuery);
for (var i = 0; i < size(faces); i += 1)
{
    var area = evArea(context, {
            "entities" : faces[i]
    });
    println(area);
}