Aircraft Production in Thailand
The distinction of being the first aircraft of Siamese design to be built and flown in Siam goes to the Boripatra. The development of the Boripatra started after the price of the Renault engines for the Air Service's Breguet 14s soared. During the late 'twenties, two types of aircraft were designed and built by the RAS, the two-seat Boripatra reconnaissance/light bomber and the Prajadhipok fighter. The Boripatra first flew on June 23, 1927, the construction having begun on April 5. It was a, for its time, conventional looking biplane, its layout being similar to the Breguet 19. The designer was Lt Col Luang Vejayanrangsit. The Boripatra was a biplane of mixed wooden and metal construction with unequal span wings. It was powered by a BMW VI liquid-cooled in-line engine. The initial tests were not deemed as successful. A Curtiss D-12 engine was installed in a second Boripatra. Some modifications were made to the radiator assembly, but otherwise the two Boripatras were similar in appearance. On January 31, 1928, King Rama VII, accompanied by the Queen, visited the Aeronautical Workshps at Don Muang. The Boripatra was then declared as being in operational service with the RAS. A third Boripatra, powered by a Bristol Jupiter VI radial engine was built in 1929. This Boripatra featured a redesigned front fuselage, and slightly different tail and rudder assemblies. Extensive comparative tests were performed on each of these three aircraft. Also in 1929, a three Boripatras made an eventful flight to India. Later, a belated reciprocal flight to Hanoi was made. Despite its superior performance to the Breguet 14, the Boripatra was not produced in quantity. When the development of the Boripatra became known by the French supplier of the Renault engines, they lowered the price of the engines. At least two Boripatras, possibly more, were built during 1928-1929. The reasons for not building more Boripatras were largely financial. Apparently, there were plans to build enough Boripatras to equip one squadron, but these were never realized.
The Boripatras were used as engine testbeds. At least one, although not airworthy, was still at Don Muang in 1938. It was described by a visiting RAF officer as being a "twin seat Bristol Bulldog, complete with a Bristol Jupiter engine." This was possibly the same Poripatra that survived World War II, albeit in derelict condition. During the Bangkok bicentennial celebrations in 1982, a ¾ scale replica was constructed. Two more replicas were built during the late 'eighties. They are currently on display at the RTAF Museum at Don Muang.
The Prajadhipok fighter first flew in 1928, and was used in comparative tests against the Nieuport Delage NiD 29C.1. It was named in honour of the Siamese King, Rama VII, and was designed by Lt Col Luang Naramit Baijayonta. The tests showed no significant improvement over the NiD 29C.1, and only one was built. The Prajadhipok was powered by a 600 hp BMW liquid-cooled inline engine. Both of these designs, the Boripatra and the Prajadhipok, can be considered to have been experimental designs. They were designed for test purposes, and were used extensively in that function.
The workshops at Don Muang were both cramped and inadequate for the needs of the Royal Aeronautical Service. At the end of 1929, a British company was contracted to build six steel-framed workshops, including a forge & foundry shop, a plywood shop and a carpentry shop. Each of these were 41,5 metres (136 feet) long and 16,8 metres (55 feet) wide. The six shops also included an even larger machine shop and two general workshops. The organization was simply by trade. The site chosen was Bang Sue, a couple of miles south of Don Muang. The cost of these shops was USD 173,000. By 1930, the Bang Sue aircraft construction factory was operational. Engine testing and repair shops remained at Don Muang. Final assembly also took place at Don Muang. A heavy work load was placed on the factory. Not only was the factory to produce new aircraft, but also conduct repairs and overhaul damaged aircraft. The attrition suffered by the RAS was at times severe. For example, between August 1930 and May 1931, the Air Service lost nine pilots killed and 11 aircraft destroyed.
The need to replace the old French training aircraft became apparent during the late 1920s. Two different types were ordered for evaluation purposes, the US Consolidated PT-1 (of which four were ordered) and the British Avro 504N. After evaluation, the Avro 504N was chosen to become the Air Service's new primary trainer, primarily on account of its wooden construction, which was better suited to license production in Siam. Twenty aircraft were ordered from Avro, their cost being, including various equipment 511.960, 75 Baht. Additionally, the cost for the licence was 61.052,44 Baht. In total, about 50 Avro 504N primary trainers were produced in Siam, the first being delivered in 1930. Amazingly, 30 (seven operational) were still in service in April 1945.
The standard reconnaissance and light bomber in RAS service during the mid- and late 'thirties was the Vought V.93S and V.93SA Corsair. Nine V.93S and three V.93SA multi-purpose aircraft were delivered in 1934. The V.93 Corsair was a two-seat biplane of mixed wood and metal construction, being typical in design and performance of the era. Its maximum speed was 191 mph over a range of 736 miles, the service ceiling being 23,700 feet. The Corsairs arnament consisted of five 8 mm Vickers machine guns, as well as the ability to carry a small bomb load.
The exact number of Corsairs built in Thailand is not known, but it was probably between 72 and possibly as many as 100. The first batch of 25 Bang Sue built Corsairs were delivered in 1936, with a second batch of 25 being completed in 1937. Some Corsairs from the first batch had a Townend ring cowling instead of the standard NACA cowling. The Siamese built Corsairs conformed to the V-93S "Super" series, being powered by a 675 hp Pratt & Whitney Hornet S5E radial engine. Fifty more Corsairs were ordered in 1939. Stockpiled Pratt & Whitney engines, of which 24 had been received between June 1938 and December 1939, were to be used. The Corsair was the most numerous type of aircraft in RTAF service during WW II. A total of 53 (28 serviceable) were listed as being in service in April 1945. The last RTAF Corsairs were not withdrawn from use until 1950.
After protracted testing of several types of fighter aircraft, the Curtiss Hawk II (the export variant of the US Navy's BFC-2 Goshawk) was chosen as the replacement of the NiD 29s. Among the aircraft tested were two Boeing 100 (the export variant of the Boeing P-12E), two Bristol Bulldog II and one Heinkel He 43D. Twelve Curtiss Hawk IIs were ordered, and delivered in 1934. A refined model, the Hawk III, an export variant of the Curtiss BF2C-1, which featured a retractable landing gear, was some time later selected to become the standard RAS fighter aircraft. For its time, the Hawk III had adequate performance for a fighter, its maximum speed being 248 mph over a range of 577 miles. The service ceiling was 29,700 feet. The Hawk III had two 8 mm machine guns, and was able to carry one 474 lb bomb.
The first 24 were built by Curtiss, at a cost of 63,900 Baht each, with the first batch of 25 Bang Sue built Hawk IIIs being ordered in 1937. As the Hawk III was of more complex construction than previous aircraft, including the Corsair, numerous problems were encountered. Wright Cyclone R-1820-F-53 engines and raw materials had been ordered in 1937, but a lack of proper jigs and tooling created severe problems and production did not start until the autumn of 1938. The technical manuals and drawings supplied by Curtiss were not always clear, which meant that some items had to be hand-built, adding even further to the delays. By mid-1939, the Bang Sue facility had yet to complete a flyable Hawk III. However, the first deliveries to an operational squadron took place during late 1939. At the end of 1939, a second batch of 25 Hawk IIIs was ordered. The Hawk III was the most numerous fighter of the RTAF during WW II. The Hawks saw active service during the Franco-Thai air war in 1940-1941 as well as during the Japanese invasion during December 1941. Forty-two (of which 26 were serviceable) were still in service in April 1945. Some Hawk IIIs were rebuilt, and soldiered on until the late 'forties.
In August 1937, the British Naval Attache visited the Bang Sue facility. He noted that the factory was well equipped for its purposes, and probably the best in Asia, with the notable exception of Japan. The Corsairs that were being produced were indistinguishable from American-built Corsairs. The factory was not yet able to produce all-metal stressed skin aircraft, but would shortly be able to build such aircraft. The problems experienced during the production process of the Hawk III showed a lack of trained technical staff. As Thailand only had a limited industrial base and infrastructure, a four year aircraft mechanics course was initiated by the RAS. Twenty-five students were accepted annually. The factory itself was reorganized into four sections, with No 1, at Bang Sue, being a technical department responsible for all design and experimental work, flight testing, writing technical manuals and text books as well as conducting training courses. The No 2 section was the factory itself, constructing airframes and doing final assembly. No 3, at Don Muang, was responsible for all major engine and airframe repair and overhaul. No 4 was a central depot, providing spare parts to both the factory and the operational squadrons. In 1939, some 700 workers were employed at Bang Sue. Under normal circumstances, 25 aircraft could be produced annually. The factory could also, given enough funds and skilled workers, expand even further.
After the Curtiss Hawks and Corsairs were introduced into service, it became obvious that the step between the Avro 504N primary trainer and the operational aircraft was too great. Therefore, a dual-control basic training version of the Corsair was ordered. The changes to the basic airframe included a new engine, a 235 hp Wright R-760 Whirlwind, and a new propeller. To maintain the centre of gravity, a new fuselage section was inserted in front of the cockpit. Ten were built during 1939, and delivered to the flying school at Don Muang. It is possible that more were built, or converted from other Corsairs, as 12 (of which five were operational) are recorded as being in service in April 1945.