I was reading through this thread in its entirety. I got hung up on the engine discussion and skipped to the end. LOL I think designing for any one particular engine is a mistake. Currently, I'm working through Raymer's book on a VW powered single-seater, but what made sense to me, and what I've kept in mind throughout my design, and what I think makes sense for a project such as this, is to design an aircraft that can meet your design within the larger portion of a range of variables. For instance, design a plane that meets Pt103 criteria with 95lb 18hp motor and also a 38hp 30lb motor. Then when people ask what motor to use, you can say, "Anything between 30 and 95lbs, 18-38hp." (This isn't a suggested range on my part, I'm just throwing out some approximate imaginary numbers here to use use for argument's sake). The only thing that needs to change is the engine mount, max weights, and maybe the location of a few accessories, or maybe the wing location by a couple of inches. Just account for it. If your design can account for both extremes, then the average joe schmoe is going to have an easy time finding an engine that will work... today, 5 years from now, 20 years from now... and that will add to the appeal and longevity of your design. Engine optons will change and evolve over time. Just set up a reasonable range, design to both ends of that range (assume the worst case scenario within any calculation), and trust that whoever builds it will be able to EASILY find something that fits within that range. In my case, I am designing something for a VW engine, but leaving enough space under the cowling for a turbocharged O-200. A Rotax 912/914 will also fit, as will a corvair motor. How do you manage to design for engines weighing 40 pounds more? Well, structurally, you design to the heaviest engine. Aerodynamically, you design to the biggest, lowest power engine. And you make sure to leave yourself some wiggle room. I weigh 160, but am designing for a 220lb pilot. If I wind up having to lop 30 pounds off that requirement, well, no big deal. I can make the change with a couple strokes of a keyboard rather than a reiteration of the design. If I need to switch from a battery in the nose, to one in the tail, that's doable too. Is it "optimized"? I guess not. But planes like these shouldn't be optimized to some crazy degree, they need some margin built into them to suit different builders' preferences. Foir a single-seater, I think you will find that the weight difference between a light and heavy engine can easily be accomodated by relocating the battery and moving the wing a very small amount, maybe an inch or two. Or moving the fuel cell. And if all else fails, you can move the wing an inch or two without royally screwing up you previous calculations. So, as a designer, forget the decision about which engine to use. Just focus on a reasonable range of options that may exist and account for their variances. With that said, I like the strongback design concept. What an awesome way to make a super-simple fuselage over the course of a weekend for cheap!!!!