I guess you are right, I had read it went into a flat spin, but found this, way too many details:
Eyewitness Carolyn Pearl told local news station KCBA-TV that she saw a puff and heard a "popping" sound before the crash. The plane "kind of went up a little bit and absolutely straight down, not spiraling, just absolutely straight down," she said. "I thought it was doing some kind of acrobatic move, or something, and then realized it wasn't."
https://www.check-six.com/Crash_Sites/John-Denver-N555JD.htm
He made a lot of mistakes, not checking the oil or gas levels.
As far as using model plane tech to build a plane to fly in, that is probably nuts.
There are many failure points,
real planes go thru a lot of testing, to get FAA certified.
If you look at how many real planes crash and kill people, it is absurd. There have been at least half a dozen
small jet crashes in the last year. One guy flew his family up from Columbus to Cleveland to see a Cavs game, on the way
back crashed in Lake Erie, I have not heard that they found the reason. Another one slammed into an apt bldg in Akron,
some guy had just left to buy Hot Pockets, the plane plowed thru his apt. Cessna. I hate flying anyway, in a car you can just stop the engine and get out!
The foam boy could have the wing break off, power system die, lose control, or have the landing gear cause a nose over.
Boom. Broken spine. I think I will stick to elevators and cars with airbags!
Oh, and the kids at Cuyahoga Airport not to long ago. 4 college students, in a Cessna prop. Took off, but were over loaded, heavy football players. Pilot could have gone over a road and landed in a golf course, but instead tried a hairpin turn to go back to the airport, he was not climbing. So it just crashed. All dead. Parents suing the flight school saying they did not train the pilot right.