M.Simons The World's Vintage Sailplanes 1908-45

THE YANKEE DOODLE AND LK 10 (TG-4)

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  The German successes with glider-borne attacks in the European war, led to a growing interest by the US Army in airborne troops. Jack Laister approached the authorities with a suggestion for a two-seat trainer based on his well-known Yankee Doodle and in the summer of 1941 received a contract to develop and produce three prototypes, one for structural testing only, the others for flight proving at the Army’s Wright Field. Still very youthful, Laister was put in touch with a St Louis businessman, John Kauffmann, who suggested that the gliders could be built in St Louis. The Laister-Kauffmann Aircraft Company was formed, at first with a bare dozen employees, and after considerable work the design for the two-seater was completed and presented to the Army. Construction of the prototypes began, with only 90 days allowed for delivery of the first to Wright Field. The midnight oil was burned a good deal toward the end of this period. The result was the Yankee Doodle 2, or Laister-Kauffman 10, which became, in the military parlance, the XTG-4.
  The most noticeable difference between the old Yankee Doodle and the XTG-4, apart from the two tandem seats, was the replacement of the ’gull’ wing by a straight wing of slightly greater span and area. The cockpit canopy was of a simpler, less streamlined form, and extended aft to the rear seat. A stronger undercarriage was fitted and the spoilers and wheel brake worked from the same lever in the cockpit. The tailskid was also strengthened. The whole structure was reinforced to carry the extra weight of the second pilot. A few changes were made after the structural tests, the main ones being to replace the stabiliser, originally designed to fold upward in two parts, by a one-piece tailplane and to lengthen the nose to improve cockpit space. This resulted in the LK 10A which immediately entered production as the TG-4A.
  156 TG-4As were built. In 1944 many of them were sold as war surplus, along with the Schweizer TG-2 and TG-3 and Pratt Read TG-32. Laister’s plans to bring out an improved Yankee Doodle 2 for civil use collapsed, as the market was then flooded with cheap sailplanes. Only three of the civilian LK 10B were built. The Company quickly went into liquidation and production of LK sailplanes ceased.
  The LK 10A, basically a military trainer, had only a modest soaring performance. The detail design was relatively crude and the best glide ratio was measured at 1 : 22, with a minimum rate of sink of 1.07 m/sec. Dene Miller, owner of a recently purchased war surplus LK 10A, decided to improve his sailplane by removing the rear cockpit and, with it, the whole of the upper part of the rear fuselage, which was not part of the primary structure. He replaced the front cockpit cover with a blown bubble canopy, reducing the fuselage cross-section and cutting skin drag. This aircraft was known as the Miller Flat-top. In 1948 August Raspet of Mississippi State University, took the LK 10 in hand, and developed a modification programme to improve both performance and handling. Part of the programme was the so-called ‘flat topping’. The various excrescences, aerodynamic horn balances on control surfaces etc were removed, all gaps were carefully sealed, the skid was reduced in size, and the wheel fairing improved. The wing profile was carefully filled, smoothed and polished. The LK 10 came out 50 kg lighter and flight tests now showed a best glide ratio of 25 : 1, the minimum sink having been reduced to 0.68 m/sec. Handling and stability were also greatly improved by attention to trim and control gearing. Many LK owners copied the Raspet modifications or incorporated some of them, though some chose to retain the second seat with a ‘bubble’ canopy for that too. Others added retractable wheels, lengthened the wings, rebuilt the tail unit and so on, until the final product became scarcely recognisable as an LK 10 at all.
  The Raspet study of this type revealed how much could be done, by attention to detail, to improve a sailplane, though it is true that the LK 10, as it came from the military stores, gave plenty of scope for improvement.
  By mid-1980 there were still said to be about 60 LK 10s, in various shapes, in use in the USA. In both single and two- seat categories they had broken many distance records, including the two-seat national goal-and-return record of 399 km held by Harold Hutchinson from 1956 until 1967.

  Technical data:
   TG-4A: Span, 15.24 m. Wing area. 15.438 sqm. Aspect ratio. 15.06. Empty weight, 232 kg. Flying weight. 414 kg. Wing loading. 26.8 kg/sq m. Aerofoils: root, NACA 4418. mid-span, NACA 4413 tips. NACA 4409 with 4 degrees washout. Best glide ratio, measured, 1:22 at 86 km/h, minimum sinking speed, 1.07 m/sec at 77 km/h.
   Flat-top LK 10A: As for TG-4 but empty weight, 195 kg. Flying weight, one pilot; 285 kg, two pilots; 390 kg. Wing loading, 18.5 kg/sq m. to 25.26 kg/sq m. Best glide ratio, one pilot 1:25.6 at 64 km/h. Minimum sink (one pilot) 0.68 m/sec, at 61 km/h.