Copying existing plans is likely a copyright violation. You can do a 1-off for yourself, of course.
Eh, all that stuff that I trimmed out, and now I'm putting it back in piecemeal:
With electronics, my understanding is that you can copyright a
schematic but you cannot copyright a
circuit design. I'm not sure about whether you can copyright a board layout or a parts arrangement within a cabinet -- but I doubt it.
Plans are certainly a copyrightable piece of intellectual property. If they don't have a copyright notice on them then the law gets all smoky about who owns what -- so to be kind, if you're making a plan set that you intend to be used by whoever for whatever, you should put your name on it, and something like "released into the public domain". If you want to keep the rights, you should put "Copyright <so and so>" and the date, and maybe even terms like "for personal use only", "all rights reserved, mine! mine! mine!", etc.
Trademarking individual names and/or silhouettes comes in there somewhere, but I don't know the specifics. It's all out there on the web, though.
If you're seriously interested in producing work for the world to use, do a web search on "Creative Commons License" -- they have a whole slew of licenses that you can pick and choose from, that pretty let you choose your level of control. One thing, in particular, that a creative commons license will aid (but not guarantee) is to let you release your work into the world in such a way that anyone can build a plane from it, but so that some honkin' big toy company can't easily claim that they got there first and not only start producing your stuff without paying you, but take away your rights to let others build to your @#$% plans.
There's a whole slew of law about what you can and cannot restrict folks from doing -- copying some copyrighted work outright and selling it is definitely out, though, unless the copyright notice specifically allows this. Building a plane from it is definitely in. Making a Xerox of it is a gray area -- there's a slew of court decisions about what is fair use of copyrighted material, all of which are colored by whether or not the plans say "no copying allowed at all" vs. "copies made for personal use are allowed", or anything in between.
So if you weren't feeling particularly community spirited and you wanted to rip off a design, but you wanted to stay on the right side of the law while doing so (or perhaps you wanted to put the law to the left side of you), then you would at least need to draw up your own set of plans, with all the elements rearranged from the original, and you'd have to leave out the original author's name and the name of the kit company or magazine under which it appeared. You'd probably also want to start running for office, because you'd be morally equipped.
I take it as morally acceptable that if a kit company were totally gone without a trace, or if a designer was similarly gone without a trace, to go ahead and produce a kit with all due credit to the original designer/company all the while making it clear that you ain't them. But if you do so, be prepared to get a letter out of the blue that starts with "Hey! Whadda ya doing with our stuff!" and goes down hill from there, and be prepared to halt shipments of kits
right then and start your apologies/negotions if you do get that letter.