Do you think it would be possible to order the full size Vans aircraft plans and build from scratch? Such as the Zodiac range?
Yes it was done in the RV3 / RV4 / RV6 days. Van gave you engineering blue prints. Not any more. You do not get blueprints, only builders plans. Could it be reverse engineered with a kit in hand. Yes. Why?
The one or two RV's that were in part made from raw materials, about 30 yrs ago, one of which featured in Van's RVator newsletter (the news letter was discontinued long ago). Not sure if they finished the project, but they bought raw materials and pounded out wing ribs, formed parts.
Even if you fabricated all the sheet metal parts, the highly technical parts, gear legs, engine mount, fiberglass parts, plexi canopy, on and on, i.e., finishing kit, you would buy. Wing spars use to be sheet metal with straps. You could buy materials and make a spar (at expense and time) without fancy machining. New RV's have special extrusions machined and anodized. Could people do this? I guess if you can buy that extrusion and have the ability to machine long spar caps.
Keep in mind forming ribs requires using aluminum in the HT in "oh" condition (soft heat treat) and heat treated after to bring it back to full strength. Kind of specialized stuff but it can be done. Van has tooling to stamp them out with great precision.
There were people who made Van knockoffs such as the Harmon Rocket and Team Rocket. Team Rocket took a RV4 and widened and stretched it, hanging a (I)O540 off the nose. Many parts were the same between RV4 and the ROCKETS, so people bought partial kits from Van's, deleted a bunch of parts that Team Rocket provided. Van I recall stopped selling to people doing that, so the TEAM ROCKET folks just started making the parts (using Van's blueprints).
I don't know how old you are but plans + raw material HOME BUILT planes have been around since the 30's. Way before my day. The 1930 Pietenpol Air Camper, featured in Popular Mechanics were planes built. Almost all homebuilt were plans built until 1970's. They are still around. You can buy plans and material kits from Aircraft Spruce. Many of these planes are wood, steel tube, fabric construction. Whitman Tailwind and many plans built planes are very good, but required more labor and welding skills. I read in the last year or two of people building plans + raw material planes like the Whitman Tailwind. It came out great and is a high performance plane. It actually gives RV's a run for the money but they are also not very forgiving, high wing loading and spectacular stall behavior. You tailwind pilots correct me if I am wrong. Not saying unsafe, just not the same as RV's.
1970's Rutan and the Vari Ez and Long Ez, made cutting foam with hot wire and fiberglass over the foam. Some parts kits available. Not a true prefabricated kit but half way between plans and full kit plane.
Then Van's in the 1970's, set a new standard at least for sheet metal kits. Not only in quality, compactness, prefabrication of kit, but great handling and performing plane for a bargain price. Van's early kits were more work than standard RV kits today, it was still a step above any metal plane kit at the time. The contemporary planes to the RV kit planes, were the Mustang II and Thorp T18. They were part plans built and some parts kits. Van was far more complete, but did not have pre-punched holes. I built an RV6. I know about laying out and drilling holes. The PRE PUNCH is far better.
If you built your own RV kit from raw materials you would have to do a lot of work. Somethings like gear legs, the average person can NOT make. You could weld up your own engine mount, but by the time you bought the dynafocal mount, tubes, cut and welded it... well just buy it. Again blue prints or CAD drawings are not published. If you weld up engine mount you need to make a JIG... for one engine mount that makes zero sense. Same for molds to make cowling, wingtips, and plexi glass molds, to make one is not worth the effort.
Some early Kit planes like the Christian Eagle set a high bench mark on kit completeness. It is a tandem aerobatic bi-plane. They produced a great product but were expensive. They even provided the box cutter on the outside to open the box. The fuselage was pre-welded. Many of the kit planes of the day featured "rag and tube" construction.
KITS where you got not only materials but all the parts pretty much ready to install, were NOT the norm until they 80's. Then kit planes with prefabricated parts and assemblies exploded in the 80's. Glasair, Lancair in the 1980's set the kit world on fire. Many followed. It would be hard to scratch build any of these as they required tooling, molds. Of course by the 1990's Van's upped the game with pre-punched kits.
If you want to raw material plan build a plane it would not be an RV. If you are worried about cost or availability, the KIT is the cheapest part is the build. Engine, Prop, Avionics, Paint (if you hire a pro) will be big ticket items. If you want a discount may be look for a kit bought and never started.
Frankly the cheapest way is buy an RV already flying. If you go with a fixed pitch RV4 VFR panel with a paint job that looks good at 50 feet, you are likely to get a good deal. If you want a tricked out all the bells and whistles show winner RV14, better have deep pockets. It will still might be cheaper than building it yourself. However BUILDING IS AWESOME... at least for me. Some people like building. However I would NEVER EVER do a plans built RAW material project. I admire the people who do.