Bell L-39 Sweep Cobra
After the capitulation of Germany in 1945, US forces captured large amounts of documents on all types of new aeronautical developments. One of these which were tested extensively by German scientists was the swept wing configuration. To test such a configuration at low speeds, Bell received from the U.S. naval air service a contract for the development and construction of two flying test models based on the existing Bell P-63 Kingcobra fighter.
Because of centre of gravity problems with the swept wing configuration, it was decided to replace the original P-63 propeller by the lighter and smaller propeller of the P-39 Airacobra fighter. The conversion of the P-63 was designated by Bell as the L-39 with the appropriate name Sweep Cobra. Both Sweep Cobras were fitted with completely new designed wings with 35° sweep. As such, they were in fact the first US planes with swept wings. The Sweep Cobra was flown in by test pilot Tex Johnson. Later, also Bell test pilots Slick Goodman and U.S. Navy test pilot Robert Champine flew the Sweep Cobra at NACA’s Langley airfield. The Sweep Cobra was regularly flown fitted with the well-known wool tufts for turbulence research.
According to existing pictures, the Sweep Cobra no. 1 flew initially with a 3-bladed propeller. Later, it was replaced by a 4-bladed prop. The Sweep Cobra was also fitted with an additional small vertical stabilizer under the rear fuselage. Flight trials revealed that the flight efficiency decreased quickly at lower speeds. This was later improved by fitting leading edge slats and a larger wing area.
Photos- Nico Braas, NASA Langley Color profile- Srecko Bradic