Some of you know that I'm working on the plans for Walter Umlands rendition of the Sterling Spitfire. It's my desire to change design features wherever possible to improve, and, or, remove excess weight, from the design.
This, of course is internal, and not affecting the external dimensions. We hope to arrive at a lighter. stronger, plane, that when finished will externally be the same as the original.
Excessive weight seems to be one of the problems to address, esp. with the relatively narrow wing, compared to todays designs. So, much of my effort at this time is to reduce, even if it's a small amount, structure weight, while maintaining enough strength to carry the load.
I have been told, and do believe that a stunt ship when crashed hard, should turn to dust. If it doesn't, you've over built it, and it's therefore too heavy.
I have a queston though, and would like your input.
It has to do with spar design. The usual practise, and, what I would normally use with the Spitfire wing, uses a pair of 1/4" sq. medium hard balsa spars, top and bottom, with possibly some vertical shear webbing between them. Literally thousands of planes have successfully been built with this style of spars.
Lately though I've gone to a different style that has worked very good so far, and comes out slightly lighter in weight. They also seem to be, and should be, from my studies, much stronger.
For a weight savings of 25%, I can use 3/8" X 1/8" medium hard balsa oriented with the flat to the outside of the airfoil. According to my studies, this puts more "section modulus' at the extremes, adding considerably to the spar strength.
I can also change from a simple vertical shear web, to a Warren Truss arrangement, which depending on the type and amount of glue used, could save an additional percentage of weight.
My problem is that I fear some builders may not understand the thinking behind this change, and consider it poor engineering at best, or modify and over build, believing that it's a better way to go.
I know that many builders re-engineer their kits, to reflect their own beliefs, style, or, because that's simply the way they have always done something. In most cases that's fine, and perhaps even needed.
Anyway, I'm posting this on both forums to get some feedback from others. Hopefully, I can put together a redesign that will accomplish our design goals, be understood, and not be subject to a lot of re-engineering by the builder.
What do you think?