Hi Billski and Pepsi;
That question has certainly occurred to me too. It seems wasteful, but, it is also very common in homebuilts, especially replicas. I've seen oodles of examples.
I can't answer for everyone, but I'll try to answer for the Mustang replica that I am re-working.
First, I'm no engineer. I have an Aerospace Technology diploma, which qualifies me to work somewhat on aerospace stuff, but certainly doesn't make me capable of calculating anything related to airframe loads. I might have been qualified to do this, had I been a MUCH better student, 25 years ago at Ryerson (Toronto) but stress analysis was my worst subject, and the day after I graduated, I wouldn't have been qualified to design a true moncoque structure. For that matter, I'm not qualified to design a truss tube fuselage either, but my intentions are to use the work of others and "design" an aircraft with a weight and structure similar to others. However, I acknowledge your good point. And with changes I want, I'll still have to illustrate the strength of any structure on paper before building. God knows how I'm going to do that, because I sure don't.
Second, sheet metal forming can be truly difficult. I haven't used an English Wheel, have you? If you have, how'd it go? Making compound curves in aluminum, and ensuring that they are now capable of carrying loads would be WAAAY beyond my abilities.
Third, the truss tube just acts like a good place to hang things (other hardware) like bulkheads, control sticks, engine, tail, etc. It's like the skeleton. Just add meat and skin. I make it sound so easy, don't I?
Forth, like the Hawker Hurricane, a truss tube construction should be simpler for a one-off build. The Spitfire, a better aircraft in many respects (but not all) was much harder to build, and took more man-hours (and woman hours, sorry Rosie) than the Hurricane, even as production became "routine." So I think a space frame of tubes is more beginner-friendly for the homebuilder.
Fifth, riveting. I had hoped to stay away from solid rivets, for the most part, since this now takes additional equipment, at least some level of additional practice in riveting, two people ( a real disadvantage if you can't find a buck-bar volunteer), and makes real noise (neighbours would love me). I'd like to use Avex rivets, as these are one man items. The skin wouldn't really be load-carrying, so it just has to withstand the aerodynamic forces in flight. The wings are a different matter, I realize, and they might indeed be stressed skin. So I might have to go through these motions. I haven't dived into the whole wing-thing yet.
Lastly, weight. A distinct disadvantage to use a truss frame under the skin. You're right that a not-so-carefully designed structure would add weight and be inefficient, but, I will be without guns, armour plating, oxygen system, and the fuel to make it all the way to Berlin. I might want to take off and make it all the way to the breakfast meet at the nearest airport, or some sort of modest cross-country, but that's about it.
I can't speak for Titan, since I am not involved with the company, but I think I can see why they've gone this route and used small pieces of fuselage skin with simple 2-D curves, as opposed to fully wheel-formed 3-D curves. The average home builder has no skills for this work, or the equipment. So they approximate a curve by using a few pieces to make a chin cowl shape, a top nose shape, or a wing root fillet, rather than rely on real skill to form each piece. And, the more I see of the kit, the more respect I have for some of their design decisions, even though I'd like to see an aircraft that looks more like a D-model Mustang. Especially when they've gone to all that trouble to make an airplane kit for the masses. A few simple tweaks would even improve the replica look, especially in the V-stab area.
Thanks gents, for the points you've made. As always, feel free to point out flaws in my way of thinking. It all gets sifted and computed.
Cheers, Tom.
