here some pictures from the Hürkus
Why does Hurkus have such a huge dorsal fin?
Yes, we know that the extra dorsal fin helps with spin recovery.
It looks like an after-thought.
The trend towards huge after-thought fins, strakes, etc. on trainers seems to have started with the Pilatus P-3, piston-pounding trainer introduced in 1953. Pilatus structural engineers argued for a short fuselage to save weight while stability and control engineers argued for a longer fuselage. Structural engineers won that debate, but shortly after it entered service, Swiss Air Force instructor pilots complained about poor spin recoveries. So Pilatus borrowed a Stanley Extractor ejection system and flew exhaustive spin tests. The solution was a huge extra ventral fin under the aft fuselage.
Later trainers: BAE Hawk, Beechcraft Musketeer (in Canadian Air Command service), Canadair Tutor, Cessna T-37 Tweety Bird, French Epsilon and all successive Pilatus trainers got a variety of extra fins, strakes, etc. Heck! Even deHavilland's 1930s vintage Tiger Moth needed extra horizontal strakes to meet RAF spin recovery standards.
Is it that difficult to design a fresh-sheet trainer for good spin characteristics?
Which tail configurations are best suited to good spin recovery?
I am thinking about long tail moment arms, generous control surfaces, horizontal tail NOT masking the rudder, squared aft fuselage, etc.