not to tryying to throw a wet towel on your project but an os 60 fp will not fly a 15 to 20 pound plane.maybe 10 pounds tops.i just drew up 1/6 scale at-9 plans with a 80.6 inch span and projected weight of 14-16 pounds using 2 saito 100 four strokes.wing area is about 914 square inches.plane has retracts and flaps.i put all the numbers into modelcalc and at 228 oz weight it says the design is good at 229 oz it says plane will glide like a rock.i took all the design numbers and inputted them into csm v10 flight simulator and flew it from weights of 14 to 20 pounds with different props.at 14 pounds it flew nice and could land dead stick fairly easy.could land with or without flaps.at 16 pounds loops were falling off at the top and landing were full flaps at about 1/3 power.at 20 pounds it neede 14/8 props to get into the air instead of the 15/6 and landings were full flaps at 3/4 power.stall speed went from about 32 mph to about 44 mph flaps down.at 20 pounds the glide was straight down.you may find that you are going to have to make compromize from full scale to a model that flys.you may have to go to a thick airfoil like 18 percent symetrical to make up for the lack of wing area.