Is less than 18 hp viable hp figure for an aeroplane ?

Burt Rutan has done it at 18 hp ( taken off ). Michael Colomban did the same with Cri Cri proto ( 2 x 9 hp ). http://www.museumofflight.org/aircraft/rutan-model-54-quickie

Whing Ding II flew at 10 hp but was open cockpit at 45 kilos..here is the lightest closed cockpit ac yet; http://www.scalesoaring.co.uk/VINTAGE/Documentation/Elfe_P1/Elfe_P1.html