Boeing Model 21 / NB
|PRIMARY TRAINER||Virtual Aircraft Museum / USA / Boeing|
Having become recognised as a designer of military aircraft, following the supply of Model 15 to both the US Army (PW-9) and US Navy (FB-1), the company designed the Boeing Model 21 to meet a US Navy requirement for a primary trainer. This was an equal-span biplane, incorporating an unusually wide wing centre-section, and introduced N-type interplane struts to eliminate wing incidence-bracing wires. The divided-axle main units of the tail-skid landing gear had rubber bungee shock absorbers, plus provision for easy conversion to operation with floats. The pilot and pupil were accommodated in tandem open cockpits. Power was provided by a Lawrance J-1 radial engine.
US Navy testing of the Model 21 was carried out with the prototype aircraft, under the designation VNB-1. From the potential operator's point of view, however, the type was unsuitable for the required role, being unspinnable and too easy to fly. On the understanding that modifications would be introduced to make the aircraft a little more demanding, and spinnable, 41 production aircraft were ordered under the designation NB-1. The first of these was delivered on 5 December 1924, and it was soon discovered that Boeing's spin modifications were too effective, since it was possible to get into a flat spin from which recovery was virtually impossible.
Further modifications produced an acceptable compromise. Some NB-1s had Lawrance J-2 or J-4 engines, and several aircraft had 164kW Wright J-5 engines installed at a later date. Following delivery of the NB-1s an additional 30 were ordered of the NB-2 variant, this differing only by having war-surplus 134kW Wright-Hispano E-4 engines, installed at the US Navy's request to utilise some of the very large number of these licence-built engines held in naval stores.