THE PLT CONTACTED GROUND CONTROL & WAS CLEARED TO TAXI FROM A HANGAR ON THE SOUTH END OF THE ARPT TO A HANGAR ON THE NORTH END VIA TWY-A. TWY-A WAS LOCATED 400' WEST & PARALLEL TO RWY 17/35. THE PLT SAID HE WAS AIR TAXIING THE HELICOPTER JUST ABOVE THE SURFACE AT 3 TO 5 KTS WHEN AN AIR FORCE C-141 MADE A TOUCH-&-GO LANDING ON RWY 17. THE C-141 LIFTED OFF ABOUT 500 TO 1000 FT AHEAD OF THE HELICOPTER & WAS AT AN ESTD 50 TO 100 FT AGL WHEN IT PASSED ABEAM OF THE HELICOPTER'S PSN. SHORTLY THEREAFTER, THE HELICOPTER ENCTRD WAKE TURBULENCE FROM THE C-141. THE PLT TRIED TO COMPENSATE FOR PITCHING & ROLLING, BUT THE HELICOPTER TOUCHED DOWN HARD & WAS DAMAGED. THE WIND WAS RPRTD FROM 130 DEG AT 5 KTS. THE PLT WAS CERTIFICATED FOR FIXED WING ACFT & WAS ENDORSED TO AIR TAXI THIS HELICOPTER, MAKE HOVER TURNS & FLY IN GROUND EFFECT BELOW TRANSLATION SPEED (15 KTS). THE ATC HANDBOOK DID NOT SPECIFICALLY ADDRESS AIR TAXIING, BUT IT REQUIRED 3 MIN SEPN BHND A HVY JET DEPG OR MAKING A LOW/MISSED APCH ON THE SAME OR PARALLEL RWY W/I 2500'.