Design and Development
By the late 1930s, Beechcraft management speculated that a demand would exist for a new design dubbed the Model 18 which would have a military application, and increased the main production facilities. The design was mainly conventional for the time, including twin radial engines, all-metal semi-monocoque construction with fabric covered control surfaces and "taildragger" undercarriage, while less common were the twin tail fins. Upon an immediate glance they can be mistaken for the larger Lockheed Electra series of airliners which closely resemble the Model 18. Early production aircraft were either powered by two 330 hp (250 kW) Jacobs L-6s or 350 hp (260 kW) Wright R-760Es. The 450 hp (336 kW) Pratt & Whitney R-985 became the definitive engine from the prewar C18S onwards. The Beech 18 prototype first flew on 15 January 1937.
Prior to the Pearl Harbor attack, the Beech 18 was outsold by the Lockheed 12 by 2-to-1. However, war priorities forced Lockheed to concentrate on its heavier aircraft, and Beechcraft received a major boost through wartime contracts.
The aircraft has used a variety of engines and has had a number of airframe modifications to increase gross weight and speed. At least one aircraft was modified to a 600 hp (447 kW) Pratt & Whitney R-1340 powerplant configuration. With the added weight of approximately 200 lb (91 kg) per engine, the concept of a Model 18 fitted with R-1340 engines was deemed unsatisfactory due to the weakest structural area of the aircraft being the engine mounts. Nearly every airframe component has been modified.
In 1955 deliveries of the Model E18S commenced; the E18S featured a fuselage that was extended 6 inches (150 mm) higher for more headroom in the passenger cabin. All later Beech 18s (sometimes called Super 18s) featured this taller fuselage and some earlier models (including one AT-11) have been modified to this larger fuselage. The Model H18, introduced in 1963, featured optional tricycle undercarriage. Unusually, the undercarriage was developed for earlier-model aircraft under an STC by Volpar, and installed in H18s at the factory during manufacture. A total of 109 H18s were built with tricycle undercarriage, and another 240 earlier-model aircraft were modified with the undercarriage.
Construction of the Beechcraft Model 18 ended in 1970 with a final Model H18 going to Japan Airlines. Beechcraft set a record that still stands today for longest continuous production of a piston engine aircraft. Through the years, 32 variations of the basic design had flown, over 200 improvement modification kits were developed, and almost 8,000 aircraft had been built. In one case the aircraft was modified to a triple tail, tri-gear, hump backed configuration and appeared similar to a miniature Lockheed Constellation. Another distinctive conversion was carried out by PacAero as the Tradewind. This featured a lengthened nose to accommodate tricycle undercarriage, and the Model 18's twin tails replaced with a single fin.
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