Blohm & Voss Ha 137

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Ha 137
Type	Dive bomber
Manufacturer	Blohm & Voss
Maiden flight	April 1935
Produced	1935-1937
Number built	6

The Ha 137 was Blohm & Voss's entry into the contest to equip the re-forming Luftwaffe with their first purpose-built dive bomber. Although the contest would eventually be won by the infamous Junkers Ju 87, the Ha 137 demonstrated that B&V's Hamburger Flugzeugbau, not even two years old at this point, had a truly capable design team of their own.

Hamburger had already designed a biplane training aircraft of no particular distinction, the Ha 135 under their first designer, Reinhold Mewes. Mewes then left to join another small company, Fieseler. In his place Hamburger hired Richard Voit, who had been working for a decade with Kawasaki and was looking to return to Germany. Before leaving, Voit had been working on a new design for building wing spars, using a single chromaloy steel tube (often square or rectangular) that formed both the middle portion of the wing, and also served as a primary fuel tank.

When tenders were offered for the dive bomber program in 1934, Hamburger was not even invited to submit an entry. Nevertheless, Voit was convinced that his new construction method would deliver a plane of the required strength with better performance than traditional designs, so he started work on Projekt 6 and submitted it anyway. He also started work on a more conventional biplane design as Projekt 7.

Projekt 6 was essentially a scaled up version of his last design at Kawasaki, the Ki-5. Built entirely of metal and using a semi-monocoque fuselage, the design looked more like a fighter - specifically like the Heinkel He 112 - than a dive bomber. The wing used the tubular spar system, the inner portion of which was sealed as a fuel tank holding 270 litres. The design used fixed gear, so in order to reduce their length, and thus the drag, the wings featured a sharp reverse-gull bend at about 1/4 span. The wheels were mounted on two shock absorbers each, so the fairing around the gear was large enough to allow the mounting of a 7.92mm MG 17 machine gun for testing, and a 20mm MG FF if required. Two additional MG 17's were mounted in the fuselage decking above the engine.

Engines proved to be more of a problem. Voit originally submitted the design mounting the new BMW XV. The future of this engine was in doubt, however, and the RLM asked for the design to be re-submitted with the 650 hp Pratt & Whitney Hornet, then starting licensed production in Germany as the BMW 132. Voit's team then modified the design to use the Hornet as Projekt 6a, or alternately the Rolls-Royce Kestrel as Projekt 6b. The RLM found the resulting design interesting enough to fund construction of three prototypes.

The Hornet powered Ha 137 V1 first flew in April 1935, followed the next month by V2, and were shipped to Travemünde that summer. It quickly became apparent that the Hornet was so large that the visibility during diving was greatly affected, and the RLM then suggested that the third prototype be completed as a 6b with the Kestrel, delaying it slightly to change the engine mounts and add a somewhat odd looking radiator under the nose. By this point the definitive requirements for the dive bomber program had been drawn up, taken directly from Junkers' description of their own entry which had already been selected to win, calling for a two-seater arrangement. The Ha 137 was thus excluded, although realistically no other design had a chance to win anyway.

The RLM was nevertheless interested enough in the design to order another three prototypes with the new Junkers Jumo 210 engine. The radial powered versions retroactively became known as the Ha 137A, while the inline powered versions became the Ha 137B. Further testing continued during 1936, and they also took part in the "doomed" dive bomber contest in June 1936, but the design was still being considered for the close support role instead of dive bomber. However, when Ernst Udet took over the T-Amt later that year he considered the close support role unnecessary, and informed Hamburger that they should stop work on the design.

The three Jumo-powered prototypes were built anyway during 1936 and 1937, eventually being used as testbed aircraft at Blohm & Voss. V1 was destroyed in testing during 1935 when the ammunition for its guns exploded, and V6 crashed in 1937, but the remaining four were used for years until a lack of spare parts for their engines eventually grounded them.

Voit had also done some work on a navalized version of the design as Projekt 11, however the additional weight of the landing equipment, or floats as in the 11b, dramatically reduced range and made the design uninteresting.