The Helio HST-550 Stallion was an STOL utility aircraft developed in the United States in 1963. The prototype flew in July 1964. Initially conceived 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 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 a General Electric XM-197 three-barrel 20 mm Gatling gun mounted in the left cargo door. It also had five underwing and fuselage hardpoints.
Of the 18 aircraft purchased, fourteen or fifteen were eventually sold to the Khmer National Air Force.
- H-550 - prototypes (2 built)
- AU-24A (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)
Data from Jane's All The World's Aircraft 1976-77
- Crew: One pilot
- Capacity: 9 passengers
- Length: 39 ft 7 in (12.07 m)
- Wingspan: 41 ft 0 in (12.50 m)
- Height: 9 ft 3 in (2.81 m)
- Wing area: 242 ft2 (22.48 m2)
- Empty weight: 2,860 lb (1,287 kg)
- Gross weight: 5,100 lb (2,313 kg)
- Powerplant: 1 × Pratt & Whitney Canada PT6A-27, 680 hp (507 kW)
- Maximum speed: 216 mph (348 km/h)
- Cruise speed: 160 mph (257 km/h)
- Range: 641 miles (1,031 km)
- Service ceiling: 25,000 ft (7,620 m)
- Rate of climb: 2,200 ft/min (11 m/s)
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- Taylor 1976, pp. 301–302.
- Econ cruise
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