fin alignment problem

I need to mark where to put 3 fins on my 38mm motor tube, i am assembling the entire fin can before sliding it into the tube. The problem is I glued both the forward and aft centering rings on.

Thanks for the help ahead of time.

also, the airframe tube is 4 inches in diameter. If that matters.

If you have the slots in the airframe, you could insert the fin can and mark through the slots.

Otherwise, you could wrap a piece of paper around the motor tube, mark the overlap (the MT's circumference), remove the paper, measue it's length and put marks at the 1/3 and 2/3 points of the length.

Just a couple of late night suggestions.

You should be alright. Assuming the fins are reasonably straight, you could just partially insert the fin can and mark where the fins intersect with the airframe. Then extend a line with an angle bracket from those points further up the airframe. Use this as a guide to cut slots as far forward as you want your fins to sit. Epoxy placed forward of the fin slots (for the forward CR) and a bit on the rear CR (as you slide the fin can in) should be all you need to hold it in place. After the fin can is slid in, wrap the aft end of the airframe tightly with tape to hold it in place until the epoxy fully sets. Then fill any gaps/grooves from the slots at the aft end with epoxy or your favorite filler.

1-slide the motor mount into the airframe.

2-tack the fins in placewith a drop of c.a.

3-remove the fin can and finish gluing them.

4-post pics