Flat-felling gives such a nice finish to tailored items, but if you try it out on a seam with a bit of a concave curve happening it can end up looking messy and not much of an improvement to the looks of the garment after all!
I wondered whether running a line of gathering stitched along the seam allowance might help to solve the problem of turning under that excess fabric around the curve and help it sit nice and flat. So I tried it out on the sleeve cap seam allowance of the next shirt to roll out of the atelier, Craig's birthday shirt, and it worked like a charm... I did take some pictures to illustrate but unfortunately a couple seem to have been lost in a recent computer clean-up :(( so for the missing pictures I shall attempt to explain as best I can... :D
The sleeve cap has been attached flat to the body of the shirt, and the sleeve and side seams of the shirt are still unstitched. I want to flat-fell the seam allowance along that top curve of the sleeve cap. The sleeve cap seam has been pressed up towards the body of the shirt...
Lift up the top layer of the seam allowance and snip the underneath allowance to just less than half its width, just as you would with a straight flat-felled seam (tutorial for that here)...
Now run a single line of gathering stitch along the wider upper curved seam allowance, at the halfwidth line.
Now turn under the raw edge along the line of gathering stitches, treating the line of stitching as the fold line. If the curve is not very pronounced, the gathering may only need to be pulled in just a little... Distribute and ease out the gathering as needed to achieve a flat a finish as possible.
Press and pin in place.
(missing picture here, sorry) I topstitched the seam allowance down from the inside of the shirt; taking great care to keep the topstitching a perfectly even distance from the seam stitching, that is using the seam stitching as the guide to gauging the width of topstitching. If you forget this and aim to topstitch using the folded edge as your guide, it may end up looking a little uneven on the right side (because not even the most careful of us is Perfect at folding under and pressing an exact width fold), and this you want to avoid. After all, the appearance on the outside of the garment is what counts!
To illustrate; here is the underarm at the 4-corner point where the sleeve seam meets the side seam: Below; inside the armhole, that flat-felled edging has a few little bumps along that folded edge because of the allowance not turning under completely even-width so doesn't appear super perfect..
but below; the outside does. So I'm OK with that...
(another missing-in-action shot, so below is an "after" shot) Now I stitched the sleeve and side seam in one go; and flat felled this seam allowance.
Because this seam here is a convex curve the seam allowance has less fabric along the raw edge than the seam, (rather than more fabric as in for a concave curve) and so it folds under more easily. However because there is less fabric, you need to stretch out the seam allowance a little when flat-felling it down. In my experience this is a lot easier to achieve than dealing with the excess fabric in a concave curve, but very rigid fabric may either need a bit of a snip around very tight curves, or alternatively a HongKong finish may be more suitable.
(Hmmm, diverted a little into mathematics territory there, but I hope that helped to illustrate the point!)
I don't know I would bother with flat-felling the side-and-sleeve seam of a long sleeved shirt,; sewing neatly up the inside of a long tube is probably high up the list of things that are disproportionately difficult with consideration to actually how much it really improves the look of the shirt... but I've found sewing up the inside of a short-sleeved shirt to be OK and not too much of a hassle.