Ryan ST The Ryan STs were a series of two seat , low - wing monoplane aircraft built by the Ryan Aeronautical Company . They were used as sport aircraft , as well as trainers by flying schools and the military of several countries . Design and development . T . Claude Ryan was the founder of the Ryan Aeronautical Company , the second incarnation of a company with this name , and the fourth company with which he had been involved to bear his name ( the first , Ryan Airlines , was the manufacturer of the Ryan NYP , more famously known as the " Spirit of St . Louis " ) . He began the development of the ST ( for " Sport Trainer " , and also known as S - T ) , the first design of the company , in 1933 . The ST featured two open cockpits in tandem in a metal semi - monocoque fuselage of two main frames - one steel , the other half of steel and half of aluminium alloy ( alclad ) - to take the loads from the wing spars and six more alclad frames ; and alclad skin . It had wings in three sections of hybrid construction ; the centre section integral with the fuselage had tubular steel spars , the front spar a simple tube with an external brace to the upper fuselage , and the rear spar in the form of a parallel chord truss . The two outer wing panels had wooden spars and alclad ribs , with diagonal rods bracing the wings internally . Alclad sheet was used to form the leading edges , and fabric covered the whole structure . When attached the outer wings were braced with flying wires to the fixed conventional landing gear and landing wires to the upper fuselage . Five STs were built before the follow - on ST - A ( also S - T - A ) ( A for Aerobatic ) was developed with a more powerful engine . A single ST - B was produced , this being an ST - A with only one seat and an extra fuel tank where the front cockpit normally was ; this aircraft was subsequently converted back to ST - A standard . The ST - A was further developed as the ST - A Special , with an engine of increased power . In 1937 the ST - A Special was developed into a military version , the STM ( also ST - M ) series . Changes included wider cockpits to enable military pilots to enter and exit while wearing parachute s , and provision for a machine gun on some examples . Variants in the series included the STM - 2P single - seat version armed with a machine gun delivered to Nationalist China ; and the STM - S2 , which could be fitted with landing gear or with EDO Model 1965 floats . After the ST - M came the ST - 3 , a substantial redesign in 1941 partly brought about by the unreliability of the Menasco engines fitted to STs to that point . The United States Army Air Corps ( USAAC ) had purchased several dozen ST - M variants under various designations and had Ryan Aeronautical re - engine most with Kinner R - 440 radial engines . The USAAC found the modification to be beneficial and asked Ryan Aeronautical to design a variant with this engine as standard , and with airframe modifications considered desirable from in - service experience . The ST - 3 that resulted featured a longer and more circular wider fuselage , this being suggested by the circular radial engine . Other changes included a revised rudder , balanced ailerons and elevators , and strengthened main landing gear with the legs spaced further apart . The streamlining spats covering the mainwheels , found on ST series aircraft to that point , were deleted as well . The ST - 3 served as the basis for military versions ordered by the USAAC and the United States Navy ( USN ) . The ST - 3 gave rise to another model developed in 1941 and early 1942 , this was the ST - 3KR ( for Kinner Radial ) . The ST - 3KR had a more powerful Kinner R - 5 engine fitted and became the definitive model ; more than 1 , 000 military versions were built during World War II as PT - 22 Recruits . The final variant was the ST - 4 , which was a version of the ST - 3 with a wooden fuselage , developed in case a shortage of " strategic materials " ( i . e . of metal ) developed . Such a shortage did not eventuate and the ST - 4 was not put into mass - production . Some U . S . Navy versions of the ST - 3 , the NR - 1 , were converted to specialized ground trainers to teach cadets how to taxi aircraft when on the ground or after landing , and especially in crosswinds . The main wing was clipped back to the landing gear ; a small nose wheel added to prevent ground loops ; a roll cage between cockpits to protect the pilot and cadet ; and the throttle modified so the engine could not go over a certain RPM . The first Ryan ST flew for the first time on 8 June 1934 and production began the following year , when nine aircraft were delivered . Except for 1937 ( when 46 aircraft were built ) , production rates remained low for several years , at about one aircraft every two weeks . This changed in 1940 when deliveries to military forces began in earnest ; production that year was just under three aircraft per week . Total production of civil and military aircraft prior to the entry of the United States into World War II amounted to 315 . Another 1 , 253 military versions were produced in 1942 and 1943 , for a total of 1 , 568 aircraft of all models . Operational history . Most civil aircraft in the ST series were delivered in the United States , although a few were exported to South Africa , Australia and various countries in Latin America . An example of the ST - A was procured by the USAAC in 1939 for evaluation as the XPT - 16 . This was followed by 15 YPT - 16s , the first time the USAAC had ordered a monoplane trainer . These were the first of more than 1 , 000 Ryan STs to serve the USAAC , its successor , the United States Army Air Forces ( USAAF ) and the USN . A large number of STMs were exported in the 1930s and early 1940s ( prior to the entry of the United States into World War II ) to various Air force s , with the biggest customer being the military of the Netherlands East Indies ( NEI , now Indonesia ) . The NEI Army and Navy took delivery of 84 STM - 2s and 24 STM - S2s in 1940 and early 1941 . Another 50 STM - 2Es and STM - 2Ps were exported to Nationalist China , while others were exported to Bolivia , Ecuador , Guatemala , Honduras , Mexico and Nicaragua . After the Japanese invasion of the NEI many Ryans in that country were pressed into combat , especially in reconnaissance roles , and large numbers were shot down or destroyed on the ground . Surviving STM - 2s and STM - S2s that were not captured by the Japanese were shipped to Australia , where 34 entered service in the Royal Australian Air Force as trainers . Many of those that survived until the end of World War II were then placed on the civil register in Australia and elsewhere , and some are still flying almost 70 years after they were built . Variants . US military designations . USAAC / USAAF . PT - 16 Operators . Numbers used from " World Air Forces "