A quick comment on patents.
If you have a patent it IS NOT illegal for someone to copy the design and sell it themselves.
All the patent does is gives you the right to sue if someone does that.
If you're still thinking about a patent the timeline gets important. There is a limited amount of time (6 months, I think)from the first public demonstration until the design becomes "public domain" if the patent has not been applied for. No amount of dated research, dated plans, registered letters, etc. will get around this.
What constitutes a "public demonstration" is fuzzy. I don't think it's public just because someone can see it flying in your yard, unless this reveals critical design elements that you intend to patent (like blade shape for example).
As soon as you sell a unit it's definitely gone public and your countdown starts.
If you apply for the patent, make it as broad as you can and still get it issued. Very specific patents tend to do nothing more than provide the details of your invention to the public and give you a cool certificate to hang on the wall.
Hope this helps,
-Dan M