Design and Development
The BT-9, designated NA-19 by the manufacturer, evolved from the North American NA-16, which first flew in April 1935. The BT-9 design first took to the skies in April 1936.
Fabric covered the movable surfaces on the tail and wings, as well as the sides of the fuselage from just behind the firewall to the tail. The remainder of the aircraft was metal-covered and featured fixed (non-retractable) landing gear. The Army Air Corps purchased a total of 199 BT-9s, BT-9As and BT-9Bs. Many foreign countries also used variants of this aircraft under North American's NA-16 designation.
The BT-14 (NA-58) and the similar NA-64 Yale represented a major aerodynamic improvement over the NA-16 series, with a longer all metal fuselage replacing the fabric covered fuselage of the earlier NA-16s. The BT-14 featured a Pratt & Whitney R-985 engine versus the Wright R-975 used on the BT-9 and NA-64. As well as metal skin replacing the fabric on the fuselage, the fin was changed from having a corrugated surface on the BT-9s to being a smooth stressed skin structure and was moved aft slightly, lengthening the rear fuselage while the engine was moved forward to maintain the CG. The rudder was also changed from the rounded shape used previously to one with a roughly triangle shape with the broadest part being at the bottom, and the canopy was redesigned. The new fuselage would provide the basis for the entire AT-6 family, when fitted with the larger Pratt & Whitney R-1340 engine, a new wing with retractable undercarriage and minor changes for a gunners position.
The BT-9 and NA-64 suffered from stall/spin problems and a variety of fixes were tried. The USAAC temporarily settled on using slats on the later versions of the BT-9 however these did not work well, and later developments would have the outer wing panels swept forward slightly so that they no longer had the straight trailing edges of the BT-9 and NA-64. The later swept forward wings were fitted to the BT-14.
Read more about this topic: North American BT-9
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