Development of the He 70 commenced in the early 1930s in response to a request from Deutsche Lufthansa for a fast mail plane. Heinkel designed a low-wing cantilever monoplane that incorporated various measures to minimise drag, including its use of an aerodynamically efficient elliptical wing, a smooth external finish, and a retractable undercarriage. The He 70 was powered by a single nose-mounted BMW VI 7.3 Z engine and cooled via a relatively compact retractable radiator as a further drag-reducing measure. The first prototype performed its maiden flight on 1 December 1932 and set eight separate world speed records over the following months.
The He 70 was adopted by Deutsche Lufthansa in 1934, although its commercial career was relatively brief before the type was eclipsed by larger and more capable aircraft. In addition to its civilian use, the He 70 was also adapted for military purposes, in which capacity it was flown in both the bomber and aerial reconnaissance capacities. In addition to the Luftwaffe, militarised models were operated by both the Royal Hungarian Air Force and the Ejército del Aire. The type was already relegated to secondary roles, such as training, by the outbreak of the Second World War. The He 70 is perhaps best known as being an ancestor to the Heinkel He 111, a larger bomber that featured similar elliptical wings and a streamlinedfuselage.
Design and development
The Heinkel He 70 Blitz (Lightning) was designed in the early 1930s as a mailplane for Deutsche Lufthansa in response to a request for an aircraft faster than the Lockheed Model 9 Orion (used by Swissair) to service short routes. While German officials initially specified a desired top speed of 250 kmph in 1931, this was subsequently superseded by an increased top speed of 320 kmph.[2] Around this time, Germany was already experimenting with producing high-speed mail planes that could compete with American offerings; furthermore, it was generally recognised that the speed of an aircraft was a major competitive advantage over other modes of transportation.[3] It was also recognised that to achieve such performance, engine power alone would not suffice, the airframe would have to possess favourable aerodynamic performance as well.[4]
On 12 February 1932, a formal order was placed for the design and construction of an aircraft designated He 65, which had a guaranteed top speed of 285 kmph.[2] However, in July of that year, it was decided to modify the design so that it could achieve a greater maximum speed; this redesign received the He 70 designation. The redesigned aircraft had a guaranteed top speed of 312 kmph while maintaining identical cabin dimensions, wing loading, and landing speeds to its American-built competitors.[2] The aircraft's speed was prioritised over all other potential improvements.[2]
A low-wing cantilever monoplane configuration was selected in order to achieve low drag along with a permissible fuselage cross-section for the desired cabin space, which needed to accommodate two pilots, five passengers, and their baggage.[2] The arrangement was distinct from that of those of the rival aircraft manufacturer Junkers, possessing greater aerodynamic performance for the fuselage section in proportion to its usable cabin space. The mounting of the wing in relation to the fuselage was such that the wing's upper surface and the side of the fuselage formed a relatively obtuse angle that had a pronounced anhedral that progressed into a dihedral in a manner that ensured sufficient lateral stability.[5] Wing flaps were initially not used in order to save weight, increase simplicity, and pursue greater top speeds; while the aircraft maintained satisfactory landing characteristics during testing, it was decided to add compact flaps so that shorter landing strips could be used by the He 70. Various configurations were trialled.[6]
Various wing configurations were considered, the design team having to balance pure aerodynamic performance against various other factors and characteristics, such as roll damping, weight, and the need to accommodate the retractable undercarriage.[7] Particular care was paid to obtain a high level of torsional stiffness and a sufficient margin against any oscillations, which was a particular problem for cantilever wings. The selected wing's profile thickness tapered considerably towards the wing tips while the camber corresponded directly with wing thickness and determined mathematically.[8] The wing was composed of wood and had a twin-spar structure that extended into two box-shaped recesses in the fuselage where they were secured to the main frames using bolts. The ribs were made of spruce and the webs were of laminated birch while the exterior of the wing was covered in plywood.[9] The torsional stiffness was relatively high on account of the continuous wing covering.[10]
A spindle-shaped monocoque fuselage composed of duralumin was adopted.[11] The longerons, bulkheads and stiffeners were all open channel sections; the combination of frame bulkheads and longitudinal channel sections permitted the presence of unobstructed compartments. The cabin extended across four primary bulkheads; all channels in close proximity to the cabin were riveted to the skin.[10] The bulkheads themselves were not directly connected to the skin. The fuselage terminated in a series of longitudinal channels that rested on circular bulkheads and was riveted to the skin.[10] While not particularly resistant to buckling, the amount that did occur even under high stresses was considered to be reasonably minor. Thick shell plates for conveying local stresses were only present in a few key locations, such as near to the main wing fittings.[12]
The fuselage, wings, and control surfaces were all shell-plated and flush riveted to create a relatively smooth surface to minimize drag and help fulfil the aircraft's demanding speed requirements. For similar reasons, all fittings, including the foot steps and door knobs, were inset and flush windows fitted.[9] Another drag-reduction measure was the fully retractable main undercarriage that made use of the space in between the wing spars. The ailerons were precisely balanced to prevent flutter.[13] The cantilever flight control surfaces were elliptical in planform. No wind tunnel testing was conducted prior to the completion of the first He 70 due to alleged calculation complications between models and real aircraft; several wind tunnel tests were performed after the aircraft's completion however.[14]
It was typically flown by a crew of two, a pilot and radio operator, who were seated in a tandem configuration in a fairly central position within the fuselage that was slightly elevated for better visibility.[15][16] Visibility was further bolstered by the presence of a transparent movable cabin roof and an adjustable seat. The elevators and ailerons were actuated using wheels while the rudder was controlled using a pedal.[16] A third seat, intended for use by a mechanic or a single passenger, was also present in the cockpit. Aft of the flight crew's position was a compact cabin that seated up to four passengers in pairs facing each other.[15][16] The passenger cabin was provisioned with air-based heating and ventilation and large windows that could be used as emergency exits. Aft of this cabin is a baggage hold.[16]
It was powered by a single BMW VIV-12 engine, capable of producing up to 470kW (630hp) at 1,600 rpm.[17] Unusually, this engine was cooled using ethylene glycol rather than water, which enabled the use of a more compact radiator. This radiator, which was located beneath the fuselage, had one-third of the frontal surface area of a conventional counterpart and could be retracted when the aircraft was flying at high speeds to further reduce drag.[9] The bottom of the oil tank, which was partitioned, was used for cooling; a wing pump was used for oil circulation. Both the engines and their cowlings were streamlined wherever practical to do so.[18] the aircraft was considered to be capable of achieving even higher speeds if it were to be outfitted with more powerful engines.[19]
On 1 December 1932, the first prototype performed its maiden flight.[20] Flight testing proved it to possess excellent performance, setting eight world records for speed over distance, and reaching a maximum speed of 377km/h (234mph).[21]
During the late 1930s, 28 aircraft were sent to Spain as part of the German-manned Legion Condor. These aircraft saw action during the Spanish Civil War, typically as fast reconnaissance aircraft. There they were known as the Rayo, Spanish for "lightning".[citation needed]
A single example was bought by Rolls-Royce for use as an engine testbed: it continued in use until 1944.[citation needed]
He 170
The He 70K (later given the RLM number: He 170) was a fast reconnaissance airplane export variant used by the Hungarian Air Force. Powered by a Gnome-Rhône Mistral Majorradial engine, the engines were built under license in Hungary as the WM-K-14, but the airframe manufacture and final assembly took place in Germany.[23] The new engines raised the top speed of the aircraft from 360 to 435km/h (224 to 270mph).[24] 18 were used by the Royal Hungarian Air Force from 1937 to 1942.
Weaknesses
A major weakness of the He 70 in military use was the fire risk. Parts of the airframe were made out of an extremely flammable magnesium alloy called "elektron", though the majority of the monocoque fuselage was duralumin. Elektron is very light yet strong, but burns readily when ignited and is difficult to extinguish. Moreover, each wing contained a non-self-sealing 210-litre (47-imperial-gallon) fuel tank, which may have further added to the aircraft's reputation for catching fire.[citation needed]
Other problems included poor defensive armament, short range and poor view from the cabin, all of which led to the Hungarian He 170A fleet being prematurely retired and replaced with obsolescent Heinkel He 46 parasol-wing monoplanes, until Focke-Wulf Fw 189 "Uhu" medium altitude observation aircraft could be introduced.[citation needed]
Influence
While the He 70 saw only limited service in training capacities during the Second World War, it was the Luftwaffe's first Schnellbomber and served as the antecedent for some of the bombers involved in the Battle of Britain.
German designs
The He 70 is known mainly as the ancestor to the Heinkel He 111, which had similar elliptical wings and streamlined fuselage in a twin-engined configuration. The He 111, which began service with the Luftwaffe in 1936, went on to become the most numerous bomber type of the Luftwaffe – with just over 5,600 examples produced during the war in total[25] – in the early years of World War II.
The He 70 was essentially scaled down to produce the He 112 fighter which lost out on competition against the Messerschmitt Bf 109, but was nonetheless built in small numbers.
Japanese designs
An He 70 was exported to Japan for study and inspired the Aichi D3A ("Val") carrier-launched light bomber.[26] This aircraft shared the He 70's distinctive, low-mounted elliptical wing.
British designs
Beverley Shenstone, R.J. Mitchell's aerodynamic advisor denied that the Spitfire wing was copied from the He 70. Shenstone said:
It has been suggested that we at Supermarine had cribbed the wing shape from that of the He 70 transport. This was not so. The elliptical wing had been used on other aircraft and its advantages were well known. Our wing was much thinner than that of the Heinkel and had a quite different section. In any case, it would have been simply asking for trouble to have copied a wing shape from an aircraft designed for an entirely different purpose.[27]
The Focke-Wulf Fw 200 Condor, also known as Kurier to the Allies, is an all-metal four-engined monoplane designed and produced by the German aircraft manufacturer Focke-Wulf. It was the first heavier-than-air craft to fly nonstop between Berlin and New York City, about 4,000 miles (6,400 km), making the flight from Berlin-Staaken to Floyd Bennett Field on 10/11 August 1938 in 24 hours and 56 minutes.
The Heinkel He 111 is a German airliner and bomber designed by Siegfried and Walter Günter at Heinkel Flugzeugwerke in 1934. Through development, it was described as a "wolf in sheep's clothing". Due to restrictions placed on Germany after the First World War prohibiting bombers, it was presented solely as a civil airliner, although from conception the design was intended to provide the nascent Luftwaffe with a heavy bomber.
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The Heinkel He 112 is a German fighter aircraft designed by Walter and Siegfried Günter. It was one of four aircraft designed to compete for the 1933 fighter contract of the Luftwaffe, in which it came second behind the Messerschmitt Bf 109. Small numbers were used for a short time by the Luftwaffe and some were built for other countries, with around 100 being completed.
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1 X as second letter is for experimental aircraft or imported technology demonstrators not intended for service, 2 Hyphenated trailing letter (-J, -K, -L, -N or -S) denotes design modified for secondary role, 3 Possibly incorrect designation, but used in many sources
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