BMatthews

I think it's simpler than that. I use the same DXF files and ran into similar problems. I think it has to do with how the DXF file is drawn and what TC THINKS you selected. Instead of "object trim" try using "split entities" at all the intersection points (draw all your spar notches and other cutouts slightly "long" at first) of the notches and the airfoil. Then select and delete the now separate bits that are in the way. Finish up the over length notch lines by node editing the end points and "V" snap them to the airfoil curve segment ends.

Node editing the airfoil may or may not work. I've seen it go both ways. Profili creates DXF files with MILLIONS of damn node points and it's always been too much for my TurboCAD's until I got up to Ver7. Even then it's not certain to work. My new Ver 9 should be much better but there's just way to many segments to work with. In any event I try to not do too many changes to these airfoil objects because odd things happen as you're finding.

Finally, if none of this works out or I want to do similar stuff I trace over the airfoil using the Bezier tool and "N" snap about 30 or 40 nodes per side down against the DXF shape. Finish the tracing with a "V" snap at the trailing edge and then add one more point in line but off the shape as a grab handle or tail. Now select the whole thing to grab BOTH the shapes and then hold the shift down and tag the "tail" you made on the new tracing to deselect the tracing. You can now move or delete the original DXF object. Node edit to remove the "tail" node and you've got a true copy to work with that has a few thousand fewer nodes to deal with and a LOT less problems.