I have built 7 or 8 Flite Test RC models and designed and built a 53" high wing foam board model I still fly from time to time. Here are a few observations: 1) foam board single surface flying surfaces (stab/elevator, fin/rudder) are prone to warping, 2) even the folded over wings can warp on you, 3) I think that for control line use, models over 36" span would need balsa/plywood doublers inside the fuselage along with bulkheads inside the fuselage to have enough strength for a normal pull test, 4) to have a fuselage mounted landing gear a plywood former would be required, and birch blocks in the wing for wing mounted landing gear, 5) if not protected by oil based polyurethane, epoxy, rustoleum, etc. the paper of Dollar Tree foam board will unglue itself from the foam with only modest exposure to water (much less fuel). 6) the "waterproof" foam board Flite Test sells (and only sells in $50.00 batches) is more accurately merely water resistant if not further treated/painted, 7) Flite Test "waterproof" foamboard must be scuffed with 220 grit for paint to adhere,
foam board only comes in one thickness so you are often using a piece of foam board that is not the ideal thickness for the job at hand, 9) if I wanted a foam board CL model to last more than a handful of flights I would use an electric motor,never an IC motor.
I love building RC models with foam board. They build fast, and often repair with only packing tape. I have a Flite Test Tiny Trainer that is 3 yrs old and has over 150 flights on it. But having built with foam board for 3 yrs and being familiar with its limitations, I have never been tempted to spend any time or effort making a control line foam board model.
Just my 2 cents worth.
Joe Ed Pederson
Cuba, Missouri