In 1950, after trials with the XH-20 had been terminated, McDonnell engineers remained convinced that for certain specialized applications the intrinsic simplicity of this system would outweigh its uneconomical fuel consumption rate. Hence, after failing to attract the interest of the Army in a proposed light scout and observation vehicle using a ramjet-driven rotor, they turned their attention to the agricultural market in the belief that an easily maintained, low cost helicopter would find a ready market.
As described in a specification report dated 26 September, 1950, the Model 79 was essentially similar in concept and construction to the Model 38 (XH-20) but was somewhat larger with the diameter of the two-bladed rotor being increased from 5.49m to 8.23m. It differed further from the Model 38 in having tail surfaces consisting of a fixed ground adjustable tailplane and a universally mounted movable rudder. Open accommodation was provided for a pilot, and alternate alighting gears, consisting of either a tricycle gear or twin skids, were offered. Power was provided by a pair of McDonnell 8RJ4 ramjets, one at the tip of each rotor blade, and 379 litres of fuel was carried in two tanks. For ferry purposes auxiliary tanks could be fitted to increase fuel capacity to 681 litres.
Designed as a utility helicopter, the Model 79 was intended to be used for pest or weed control, defoliation, fertilizing, seeding, or many other varied uses such as carrying mail or cargo. To that end, it could be fitted with either (1) dual liquid chemical tanks with a total capacity of 439 litres and dual spray booms; (2) dual dust or seed bins with a total capacity of 0.44m3 and a dust outlet pointing downward and aft of the rotor disc; or (3) dual mail or light cargo bins with a total volume of 1.13m3. As a further alternative, the design provided for the installation, either during construction or as rapid modification kits, of cockpit floor extensions and removable plastic panels to convert the single-seat Model 79 into the two-seat Model 79A with dual controls.
Charles R. Wood Jr. first flew the single seat Model 79 demonstrator, which was registered N12M, on 26 March, 1952. Although trials proved relatively uneventful and confirmed that the basic design was sound and easy to fly and maintain, development of the Model 79 had to be abandoned in mid-1953 as the type had failed to attract commercial interest due to high fuel consumption rate and noise level.
Rene J. Francillon "McDonnell Douglas Aircraft since 1920: Volume II", 1997
