43 inches;
One of the biggest nails in the GA coffin was lack of innovation since the late 70s. No one built or tried anything different from the bog standard Piper or Cessna for 30 years. You wonder why a general lack of interest developed and the whole infrastructure died. Why buy a plane when its 50s tech, not particularly exciting, have to work at getting qualified and you have to keep up with rules and procedure changes. I can buy the latest Tesla now and it drives me by itself to my destination, and I can show it off. Show a plane to my business mates, yeah cool, didn't bob kill himself in one of those in the 80s? next, who wants drinks on my 2021 60 footer down the marina while we watch the game on the 200" LED in the stateroom?
I think a little engineering advice might help you.
1) In terms of the actual aerodynamic design of subsonic piston engined aircraft, the science is cut and dried, meaning the trade - offs are known. In other words, no one is likely to come up with an aerodynamic design that ticks all existing performance boxes and has twice the performance. Materials, Engines and avionics are a different matter. The changes in Avionics are amazing, Carbon fibre is here and there is steady incremental progress in engines. So in terms of pure technology, we are stuck with incremental improvements. I am told for example that the performance of a thirty year old Bonanza design is not too different from a Cirrus. The laws of physics don’t change much
2) The cost of certifying a new design is simply horrendous.
3) Some technological advances are not applicable to small piston engined aircraft, like variable valve timing etc. because the aircraft engine duty cycle is very different to an automobile. New battery technology, enabling greater performance and a better power to weight ratio and greater range would be a game changing stimulant for new designs.
Therefore the tendency is to leave things alone.