The ultimate aerodynamic development of the Messerschmitt Me 262 – the HG III. It was to be powered by a pair of HeS 011 engines buried in its wing roots, its wings had a 45 degree sweep-back and its pilot sat beneath a low profile Rennkabine or ‘racing cabin’ canopy. Art by Chris Sandham-Bailey
Adolf Busemann had proposed swept wings as early as 1935. Messerschmitt researched the topic from 1940. In April 1941, Busemann proposed fitting a 35° swept wing (Pfeilflügel II, literally “arrow wing II”) to the Me 262, the same wing sweep angle later used on both the American F-86 Sabre and Soviet MiG-15 Fagot fighter jets. Though this was not implemented, he continued with the projected HG II and HG III (Hochgeschwindigkeit, “high-speed”) derivatives in 1944, which were designed with a 35° and 45° wing sweep, respectively.
Interest in high-speed flight, which led him to initiate work on swept wings starting in 1940, is evident from the advanced developments Messerschmitt had on his drawing board in 1944. While the Me 262 HG I actually flight tested in 1944 had only small changes compared to combat aircraft, most notably a low-profile canopy (tried as the Rennkabine (literally “racing cabin”) on the Me 262 V9 prototype for a short time) to reduce drag, the HG II and HG III designs were far more radical. The projected HG II combined the low-drag canopy with a 35° wing sweep and a butterfly tail. The HG III had a conventional tail, but a 45° wing sweep and turbines embedded in the wing roots.
Messerschmitt also conducted a series of flight tests with the series production Me 262. In dive tests, they determined that the Me 262 went out of control in a dive at Mach 0.86, and that higher Mach numbers would cause a nose-down trim that the pilot could not counter. The resulting steepening of the dive would lead to even higher speeds and the airframe would disintegrate from excessive negative g loads.
The HG series of Me 262 derivatives was believed capable of reaching transonic Mach numbers in level flight, with the top speed of the HG III being projected as Mach 0.96 at 6,000 m (20,000 ft) altitude. Despite the necessity to gain experience in high-speed flight for the HG II and III designs, Messerschmitt made no attempt to exceed the Mach 0.86 limit for the Me 262. After the war, the Royal Aircraft Establishment, at that time one of the leading institutions in high-speed research, re-tested the Me 262 to help with British attempts at exceeding Mach 1. The RAE achieved speeds of up to Mach 0.84 and confirmed the results from the Messerschmitt dive tests. The Soviets ran similar tests. No one tried to exceed the Mach limit established by Messerschmitt.
It was not only aircraft armament which was important for the further development of the Me 262. From February 16, 1944, three steps were taken to introduce a high-speed version of Germany’s standard jet fighter. The Me 262 high speed program encompassed three solutions including the Me 262 HG I, Me 262 HG II and Me HG III ( HG-Hochgeschwindigkeits-Jager (high-speed fighters),. of which the first flyable aircraft were under construction early in 1944. At the end of March 1944, the Projektburo at Oberammergau pushed ahead with plans to rapidly complete the Me 262 HG I, by using the Me 262 V9 (9th Me 262 prototype) for flight evaluation. It was modified with a low-drag cabin hood referred to as the Rennkabine (racing cabin) a new triangular addition to the inboard leading edge of the wing and a new horizontal tailplane with a 40-degree sweptback and a slight modification to the leading edge of the fin which slightly increased its area. By March 31, 1945, a total of 201 flights had been made by the Me 262 V9 some in conjunction with the HG I program. At this period, after sufficient information had been acquired with the Rennkabine, the aircraft once again reverted to a conventional tailplane.
The Me 262 HG II proposal was compiled between April and December 1944. In addition to new triangular fillets at the wing leading edges, a tailplane with a sweepback of 40-degrees and a modified streamlined canopy Rennkabine II were proposed. Additionally, tests with unmanned gliders were carried out to investigate and improve the design of the 35-degrees sweptback wing. Concurrently, a wind tunnel model and a full-scale mock-up were built. The first HG II (Werk-Nr. 111538) was still under construction at Lechfeld in April, 1945.
The HG III was the most radical design and differed from the HG II by its 45-degrees sweptback wings and two HeS 011As buried in the streamlined wing roots. The design underwent some modification as the program progressed, which resulted in a changed wing plan with larger and more direct engine intake cut-outs. Wind tunnel tests proved that drag was reduced, thanks to the clean aerodynamic layout. The Oberbayerische Forschungsanstalt stated that the performance of the Me 262 HG III fighter would match that of the single-engine Me P 1106 project.