Well, I said that if I ever got another one I'd put the fins on backwards...
You inspired me to do some sim work. Grabbed an .ork from RocketReviews, after checking
@K'Tesh's index and finding no Hi-Flier XL there. No idea if this file is accurate enough to rely on for flight, but I'm just comparing changes, so it doesn't really matter. I changed the finish on the fins to smooth paint and rounded edges. Note the stability of the stock model with an E12-6 is barely within the 8-15% range.
Rather than just gluing the fins on the wrong side, I set it up so the correct side of the fin was glued to the BT, but cut the rear of the fin as if another fin, arranged as you have it, was used as a template to cut a new planform TE intersecting the original tip. Stability went from 8.63% to 15.3% and apogee increased from 1199 ft to 1289 ft. Watch the migration of the CG and CP.
I don't really love the shape of that visually, so I trimmed the fins a little differently, to something vaguely Black Brantish, being careful that it could be cut out of the original fin provided in the kit. Stability went to 15.6% and apogee gained another 12 ft.
Just in case someone objects to the fins hanging off the back of the airframe not being as robust for landings on hard ground, I pulled out my go-to of offset trapezoidal fins, the Little John. Scaled them to the BT-60 diameter, then 133 percent of that because 3 fins instead of 4. CP location is similar to stock, but the CG is forward due to the smaller mass of fins at the back. This sim doesn't appear to have paint in it, so a painted model would show an even greater effect. Stability was only 10.4%, still significantly better than the stock Hi-Flier XL design, but apogee gained another 75 ft.
Just for S&G, I made the body tube a single tube 380mm long, eliminating the coupler, and shifted the fins forward a few mm to more resemble the real Little John. It only gives up a hair to still have two body tubes and just cut them both to 190mm, or whatever combination you like. Added two pats of Estes nose weight clay (15g) to the tip of the NC. That's similar to the weight of an Eggfinder Mini or other small electronic device that one might put up there. Stability factor increased to 12.3% and apogee gained another 103 ft. Plus, the proportions look a lot better. Obviously, this is now a completely different rocket, but it's flying a heck of a lot higher than the Hi-Flier XL, with the same basic materials. You could totally buy a Hi-Flier kit and build this. You can even go as far as just leaving out one of the stock tubes. It does start to look a little Gobliny and will need some more mass in the nose for stability with an E12 in the tail.
So, yeah, the Hi-Flier fins kinda do suck.
