The Helio AU-24 Stallion was an American armed gunship, counter-insurgency, and utility transport developed for the United States Air Force. A total of 20 were built during the Vietnam War, with most of the aircraft being later sold to the Khmer Air Force. The Helio HST-550 Stallion was an STOL utility aircraft developed in the United States in 1963, with the first prototype flying in July 1964. Initially conceived by the Helio Aircraft Company as a turboprop-powered variant of the Helio Courier, it eventually emerged as a completely new design of the same general configuration and much of its design was initiated from components used in the Helio H-500 Twin. It was a much larger aircraft than the Courier, and Helio soon discovered that it was too expensive for the market. The United States Air Force (USAF) however, emerged as a buyer for the design, purchasing the aircraft for the Credible Chase programme as the AU-24A. This was the gunship version of the Stallion, with a PT6A-27 680 shp (510 kW) turboprop, equipped with an M197 three-barrel 20x102mm rotary cannon mounted in the left cargo door. It also had five underwing and fuselage hardpoints. Of the 18 aircraft purchased by the USAF, fourteen or fifteen were eventually delivered to the Khmer Air Force (KAF) between January and November 1972 under the Foreign Military Sales program for use in border surveillance and counter-infiltration roles, where the threat of encountering anti-aircraft fire (other than small arms) was minimal. Variants H-550 – prototypes (2 built) AU-24A Stallion (H-550A) – production version (18 built) H-634 Twin Stallion – version with twin Allison 250 turboprops mounted on a beam across the nose of the aircraft (not built) H-1201T Twin Stallion – version with twin engines in underwing nacelles, retractable undercarriage, tiptanks, and underwing cargo pods (not built)
