I'm finally continuing with the Groundhog evolution. I re-read Jon Robbins Groundhog 16 article. It seems that he was most concerned with boost drag, and minimizing frontal area, while the real advantage of swing wings is that they have very low load during boost. Of course ideally you always want to boost high and establish a high enough sink rate that you keep thermals from stealing you model, while still winning. In practical terms you want to optimize over all three flight phases, boost, transition, and glide to maximize duration and then work with or around thermals. The Groundhog 16 was flown with a A5-2S, and has a very generous wing area. The large wing area is good because it is generally timed until the timers loose sight of it, and you prefer that timers lose sight of competitors models sooner. Sizing is one of the most important aspects of scaling the Groundhog to different motor classes. Through experience, I have found 1.5 " chord to be best for B motors, 1" for A (A3-2t) and 1/2A motors, 3/4" did not work well for 1/4A Motors, 2" for C motors, and 2.5" for D motors. I have not had occasion to fly E or higher R/G, but The swing wing should be best suited to higher boost speeds.
The first evolution is to get rid of that heavy wire and piston, and use a simple burn string release. At the same time I moved the elastic out from the pivot to pull forward, much like on the Hot Turkey. If you do not constrain the wing tips, the wings will flutter on boost and often shred. I push a straight pin into the bottom of the fuselage, just ahead of the tail surfaces. Pull the pin out and cut it to a shorter length. Push it back in the pin hole and and use pliers to drive it in until just enough is remaining to catch the burn strings. I use a thin nylon cord of the type commonly used for parachute shroud lines. Cut a long piece of string and fix a a small Avery adhesive pad (the same material commonly used to secure shroud lines to parachutes) to one end of the string. Hook it around the pin and stick it tightly to the underside of the folded back wing at the trailing edge. run the string straight up the bottom of the fuselage, over a shallow notch cut in the front if the fuselage, loop it through the motor pod vent ports twice, forming an X in front of the motor, run it back over the notch and down the fuselage and around the pin in the pin from the opposite side the the other folded wing. Pull the string tight, there will be some elasticity in the nylon chord, and tape it to the other wing just like the first one, and trim off the excess. This has proven to be enough to keep the wings from fluttering on boost, and I have never had deployment failure, aside from catos and human error (forgetting to attach the elastic).
The second evolution is using an undercambered airfoil. This will give about 15% more duration performance in glide with no boost penalty, due to the swung back wings. The undercambered wing will have more pitch down moment that will require more more trim down force from the stabilizer, or moving the CG aft. At this point it may be helpful to look at some AMA towline gliders. You can find designs with undercambereed lifting stabilizers, and far aft CGs. We could calculate the neutral point and establish aft CG stability limit, or find the condition that minimized trim drag. However, the R/G still has to contend with transition, so I recommend moving the CG just a little aft, but not as far as possible. You can pick an airfoil used by a successful towline glider flying a similar Reynolds Number, or you can pick one out of a catalog, or even design one. I'm not very particular about airfoils, I just use what seems about right.
The third evolution is just materials. For the wings, I use thicker C grain contest balsa, but I sand most of that material away, but save your lightest wood for other projects. I cover the inner portion of the wing with tissue, and on large models I'll add an second layer of tissue on the inner most portion of the wing. The Spruce Fuselage on the B and smaller sized Groundhogs is fine, but is too heavy when scaled up. For a 2.5" chord model, I used hard balsa laminated with carbon fiber reinforcement. It will not have the impact strength of Spruce, but it will be stiffer and lighter. Covering the stab with tissue is also a good idea.
The fourth evolution is simply a hodgepodge of improvements such as adding a pop up stab DT, more attention to streamlining the pivots, and making the wings/ pivots repairable by using replacing the pivot axles with nylon screws and bushings, and bit of plywood reinforcement.