This is the trainer I built for my son many years ago. Wing is two lengths of 1/2" sheet balsa butt glued together and sanded to what might laughingly be called a symmetrical section (mainly to reduce weight) and covered with doped on silk for strength. Fuselage was 1/2" balsa with 1/2" square hardwood engine mounts. We used a 40 engine but it'll take anything from around a 30-45. This is essentially identical to what I learned to fly with about 57 years ago using a 29 engine. I added flaps on this one though for a bit more wing area and it was quite comfortable doing round manoeurvres like loops, 8's and inverted.