Heinkel He 116

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Heinkel's He 116 was an extremely long-range mailplane designed to deliver airmail between Germany and Japan. Several examples were built for this role, as well as a small batch to be used in the long-range reconnaissance role as well.

The design started in response to a 1936 request by Lufthansa, who planned a route over the Pamir Mountains in Afghanistan. This was the primary difficulty in producing an aircraft able to meet the range requirements, because the aircraft would have to lift its large fuel load to 7,600 m (25,000 ft) to clear the mountains. At the time there were simply no engines available with that sort of altitude performance, although Hirth was working on one in the 500 hp (370 kW) class. The Günter brothers proposed to adapt their basic He 70 Blitz airframe to carry four of these engines to provide enough power for the massive fuel load.

The Heinkel He 116 in flight.

The He 116 would use a modified version of the He 70's two-spar elliptical planform, skinned (like the He 70) with plywood. The fuselage was all-new, consisting of a semi-monocoque duralumin body with watertight bulkheads in case the aircraft were forced down over water. The design shared enough construction with the He 70 that the first prototype He 116 V1 was completed in early 1937. The new engines were not ready at this point, so the prototype was instead fitted with the much smaller HM 508C of 270 hp (200 kW). Even in this much less powerful form the design had some promise. V2 and V4 were delivered to Lufthansa in 1938, where they were known as the Schlesien and Hamburg. V5 and V6 were delivered to Japan in April 1938 in a six-day flight covering 15,251 km (9,477 miles) in 54 hours 17 minutes of flight time.

V3 was removed from the line to be converted into a record-breaking prototype. The modifications included a larger 75.6 metre² (814 ft²) wing with a 25 metre (82 ft) span, and increased fuel tankage in the fuselage. The design did not need the same sort of altitude performance, so the HM 508H was installed in place of the C, with a slightly lower RPM and somewhat improved fuel economy, but only 240 hp (179 kW). The lower power and increased fuel made takeoffs difficult, so the plane, renamed the He 116R Rostock, was fitted with four RATO units. On its first test one of the rockets tore loose and hit the wing, requiring extensive repairs. After these were completed a second attempt was made on June 30, 1938, covering 9,942 km (6,178 mi) unrefueled, at an average speed of 214 km/h (133 mph).

By this point the engine project had been cancelled outright, but additional orders continued with the 508s. V7 and V8 were modified for the long-range reconnaissance role with a "glass nose" similar to what would become common on most of the Luftwaffe's bomber designs. These were tested during 1938 with generally favourable results. Six additional aircraft based on the V7 pattern were then ordered, known as the He 116B-0, although they also received prototype numbers V9 through V14.

The He 116Bs were intended to operate at extremely long range, outside the range of enemy fighters, and therefore fit no defensive armament. All eight (V7 through V14) were issued to recon units prior to the war, but by that time the idea of a slow-moving unarmed plane providing any useful information seemed unlikely. Instead they were used over German territory providing mapping services.