The Stearman Model 70

Stearman Prototype
Stearman Model 70 - Prototype of the "Kaydet"

Above, the 1933 Model 70, with small rudder and unfaired landing gear. The single-leg landing gear did much to distinguish the Model 70 from other open-cockpit biplanes of the era.

Kaydet Star 

A question arises as to whether or not Lloyd Stearman had anything to do with the design of the "Kaydet." Indirectly, he did, even though he had left the company more than two years before the Model 70 was designed.

Harold Zipp
Harold Zipp, co-designer of the Stearman Model 70.

Model 70, c/n 70001, registration X571Y was not a new design that started with a clean sheet of paper. Rather it began as doodles by engineers Harold Zipp and Jack Clark on a three-view drawing of a cleaned-up Cloudboy (Model 6) that had been designed by Lloyd Stearman.

Cloudboy had demonstrated all the details of size, proportion, aerodynamics and equipment that met the requirements of Model 70, which was intended to be a military primary trainer. It was designed and built in only 60 days at a time when the two-seat open cockpit biplane was phasing out of U.S. aviation. The military procurement by both services was known to be impending and Stearman wanted a part of that market.

The Kaydet, at the time of its introduction in December, 1933 would be built in greater numbers than any American biplane, and would only be exceeded in world production by two Russian designs, the Polikarpov PO2, (1930-1941) and the Antonov AN-2 of 1947 which is still in production and approaching 20,000 examples.

Cloudboy 6L
Cloudboy, Model 6L

Although retaining the basic structure and proportions of Cloudboy, Model 70 featured a great refinement of line. Wingtips and tail were more rounded, and the fuselage was rounded out by aluminium-frame stringers. Significant new features were the single-strut landing gear and the elimination of the moveable stablizer for pitch trim. This was replaced by a pilot-controlled trim tab on the elevators. As on the Cloudboy, the structure was designed to higher load factors than would be encountered with the 200-225 hp engines that the Army and Navy were interested in at the time.

When the Army tested Model 70, it was displeased with the stall characteristics which it considered too gentle for a military trainer. To make the stall more abrupt, Stearman added small stall strips, or spoilers to the outer leading edges of the upper wing. The effect pleased the Army so well that full span strips were incorporated on production Model 75's.

The end of World War II virtually ended the US military career of the Kaydet, although it soldiered on for many years in small nation air forces. The US Army switched to more advanced designs for primary training, but the Navy retained some N2S-5's as late as 1948.

Model 70 drawings by Lloyd Stearman
Lloyd Stearman's marked-up three-view drawing of the Model 6L-1 which was a refined Model 6 Cloudboy

Text and images from: Wings of Stearman by Peter M. Bowers. Available at [] Published by Flying Books International, 1401 Kingswood Road, Eagan, Mn 55122