|The Bell L-39 making a simulated carrier approach.
Following the end of the Second World War, captured German aerodynamic research had indicated the high-speed benefits of swept wings and many designs under development in the mid to late 1940s were revised to incorporate swept wings- two such examples being the North American F-86 Sabre and the Boeing B-47 Stratojet. While the USAF might have been enthusiastic about the benefits of swept wings, the US Navy still had its reservations- the Bureau of Aeronautics (BuAer), the Navy organization tasked with the development and support of naval aircraft, had concerns about the low-speed handling characteristics of swept wings as it was this particular flight regime that was critical in flight operations aboard aircraft carriers. While there was no questioning the high-speed benefits, the Navy didn't have the luxury of long runways to land at high speeds. To determine the scope of the problem, BuAer solicited bids from industry for a swept-wing flying demonstrator to explore the low-speed handling qualities of swept wings. Grumman tendered two proposals, one based on a modified F4F Wildcat as well as an all new aircraft that would have allowed wings of differing sweep to attached to the fuselage. Bell submitted a modification of its P-63 Kingcobra which won the contract as it offered lower development costs using two P-63 aircraft. The aircraft would be designated L-39. In those days, the Navy designated its research aircraft with a letter denoting the manufacturer followed by the manufacturer's model number- as exemplified by the more well-known D-558 Skystreak and later Skyrocket- "D" for Douglas, Model 558. In the case of the L-39, "L" was Bell Aircraft's letter designation and the swept wing demonstrator had the company designation Model 39. The design had really only a tangential relationship to the P-39 Airacobra (more on this in a bit).
The wings were basically P-63 Kingcobra outer wing panels that were modified to be swept to 35 degrees and attached to an unswept center stub section. This was done for aerodynamic balance purposes. The wings were further modified with slats which could be positioned before flight either open or closed. Because of the wing modifications, the landing gear was non-retractable, but since BuAer was more interested in low speed landing, this was of no consequence. Two L-39s were built, differing only in the size of the slats.
|The swept outer panels attached to an unswept center section.
It was quickly determined that the swept wing with the slats closed possessed entirely unacceptable stall characteristics- namely it was abrupt and caused the aircraft to roll to one side. However, if the wing were slatted, then the stall characteristics become acceptable. Simulated carrier approaches and landings were made by both BuAer test pilots and even Corky Meyer, Grumman's chief test pilot (as Grumman was in the process of designing swept wing aircraft for the Navy). Handling and stall characteristics in the low speed regime around the carrier were quickly determined to not be an issue as long as the swept wing were slatted and the L-39 flight test program concluded in August 1946.
|Close up of the L-39's wing slats.
Source: U.S. Naval Air Superiority- Development of Shipborne Jet Fighters 1943-1962 by Tommy H. Thompson. Specialty Press, 2008, p69-73.