Country of origin:

United States of America

Description:

Single-seat agricultural biplane

Power Plant:

One 336 kw (450 hp) Pratt & Whitney R-985 Wasp Junior nine-cylinder air-cooled radial engine

Specifications:

History:

The National NA-75 was a conversion of the Boeing PT-17 Stearman two-seat training biplane, of which many became available after World War II from the US services, and a number of schemes were put in place to convert them for agricultural use. In its original form the Stearman was powered by a 164 kw (220 hp) Continental radial engine but many of those converted were fitted with larger engines, which were also readily available, including the 168 kw (225 hp) Lycoming R-680-B4, the 224 kw (300 hp) Lycoming R-680-E3, and the 317 kw (425 hp) Wright R-975.

The NA-75 was produced by the National Aircraft Corporation of Van Nuys, California, the design being radically modified from the PT-17, the main change being the installation of a 336 kw (450 hp) Pratt & Whitney Wasp Junior engine driving a Hamilton Standard constant speed propeller. New, all-metal structure, wings which were fabric covered were fitted, these being designed around a Gottingen 398 series aerofoil section, as were end-plates (known as wing splates by the manufacturer), at the wing tips. Flaps were not installed. New high-lift wings were designed and built for the aircraft, ailerons being installed on both upper and lower mainplanes. All wings were of constant section from root to tip and all ribs were completely interchangeable.

A hopper/spray tank 1.16 m³ (41 cub ft) in capacity was installed where the old front cockpit had been, and the fabric covered part of the fuselage was replaced by removable metal panels. On VH-FBA, which was one of two imported to Australia, no engine starter was fitted and the propeller had to be hand swung. The wheels and brakes of a Vultee BT-13 were also fitted. Equipment included a 0.897 m² (31.7 cub ft) reinforced fibreglass dustspray hopper, a spray pump and a dust agitator. The aircraft had a tailwheel undercarriage and the tailwheel was steerable via the rudder pedals.

In 1957 Crop Care Pty Ltd, an Australian offshoot of Crop Care Inc of California, USA imported two examples of that Company’s conversion, these being combination duster/sprayers and were known as the National NA-75. VH-FBA (c/n 75-4319 – ex N63267, NR63267, 42-16156) was a single seater, and VH-CCI (c/n not known – ex N56900) was a two seater and was to be used for training agricultural pilots, not being fitted with agricultural equipment but described as having the same flying characteristics. The importers expected that operating under Australian conditions an all-up weight of 2,087 kg (4,600 lb) would be permitted, thus allowing an agricultural load of 680 kg (1,500 lb). During tests a rate of climb of 190 m/min (625 ft/min) was achieved whilst operating at 2,087 kg (4,600 lb) and on take-off over a 15 m (50 ft) obstacle the take-off run was 396 m (1,300 ft). The type, when operated in the United States, operated with weights up to 2,000 kg (4,409 lb).

As noted, the two-seat variant VH-CCI was aimed at training agricultural pilots and, apart from having the new high-lift wings, new metal centre section and instruments installed in the wing trailing edge, it had the 336 kw (450 hp) engine, a constant speed propeller, and retained most of the features fitted to the original PT-17 aircraft. The aircraft was mainly fabric covered, had standard PT-17 windscreens and was fitted with an electric starter. It was also fitted with the original braking system which was toe-operated and hydraulically operated.

After assembly at Bankstown, NSW the two aircraft were flown by Department of Civil Aviation pilots with a view to obtaining an Australian Certificate of Airworthiness to work in the agricultural field but in the event testing was not completed and the registrations were not taken up, although they were painted on the aircraft. It is known testing was carried out at Armidale and Hoxton Park, NSW and some interest was shown in the aircraft by Australian operators. It is said the Department of Civil Aviation would not approve the agricultural payloads that were needed to be carried to make the aircraft economical to operate. The two machines were repacked and shipped back to the United States, VH-FBA later becoming N63496 and VH-CCI later becoming N56900.