Some rough studies of ultralights, an RC motor control, and some mil-aero design work.
Aeronautical design is fascinating to many engineers. It requires high performance as well as extreme reliability. Cost can take a back seat to getting the most from materials, processes and electronics.
I always learned something at the military contractors I worked for. This included ESL (Electromagnetic Spectrum Laboratories, later TRW) and Teledyne. Projects there included airborne reconnaissance systems in light airplanes and radar jamming pods for F-16 fighter jets.
Aircraft design is inspired by the freedom of flight. Its analogous to loving cars because they provide us freedom of mobility.
Fascination with kit-built aircraft inspired some designs for radical architectures that would be fun to at least build as RC models. Sadly, the cost of prototyping aircraft has kept these studies from progressing.
3D CAD (computer-aided design) served me well in the electronic and mechanical design of a TWT (traveling wave tube) radar jammer used on fighter jets. None of this work is classified.
An SBIR (small business innovation research) project involved a design that had extendable wings and a fan instead of a rudder.