| He 70 | |
|---|---|
 | |
| General information | |
| Type | Mail plane,Passenger |
| National origin | Germany |
| Manufacturer | Heinkel Flugzeugwerke |
| Primary users | Deutsche Luft Hansa |
| Number built | 324 |
| History | |
| Introduction date | 1933 |
| First flight | 1 December1932 |
| Retired | 1954 (Spanish Air Force[1]) |
TheHeinkel He 70Blitz ("lightning") was a fastmonoplane aircraft designed and produced by the German aircraft manufacturerHeinkel Flugzeugwerke. It was the firstSchnellbomber operated by theLuftwaffe.
Development of the He 70 began in the early 1930s in response to a request fromDeutsche Lufthansa for a fastmail plane. Heinkel designed a low-wingcantilever monoplane with various measures to minimisedrag, including an aerodynamically efficientelliptical wing, a smooth external finish, and aretractable undercarriage. The He 70 was powered by a singleBMW VI 7.3 Z engine and cooled by a compact retractableradiator as a further drag-reducing measure. The first prototype made itsmaiden flight on 1 December 1932 and set eight separateworld speed records over the following months.
The He 70 was adopted by Deutsche Lufthansa in 1934, although its commercial career was relatively brief before it was replaced by larger and more capable aircraft. In addition to its civilian use, the He 70 was adapted for military purposes, as a lightbomber andreconnaissance aircraft. Militarised versions were operated by theRoyal Hungarian Air Force and theEjército del Aire. The type was already relegated to secondary roles, such as training, by the outbreak of theSecond World War. The He 70 is perhaps best known as being an ancestor to theHeinkel He 111, which had a similar elliptical wing.
The Heinkel He 70Blitz (Lightning) was designed in the early 1930s as a mailplane forDeutsche Lufthansa in response to a request for an aircraft faster than theLockheed Model 9 Orion (used bySwissair) to service short routes. While German officials initially specified a desired top speed of 250 kilometres per hour (160 mph) in 1931, this was subsequently superseded by an increased top speed of 320 km/h (200 mph).[2] Around this time, Germany was already experimenting with producing high-speed mail planes which could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 needed to be aerodynamically refined.[4]
On 12 February 1932 a formal order was placed for the design and construction of an aircraft designatedHe 65, which had a guaranteed top speed of 285 kilometres per hour (177 mph).[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 was designated theHe 70. The redesigned aircraft had a guaranteed top speed of 312 km/h (194 mph) while maintaining identical cabin dimensions, wing loading, and landing speeds to its American-built competitors.[2] The aircraft's speed was prioritised .[2]
A low-wingcantilever monoplane configuration was selected in order to achieve lowdrag along with adequate fuselage cross-section for the desired cabin space, which had to accommodate two crew plus five passengers and their baggage.[2] The arrangement was different from that of those of the rival aircraft manufacturerJunkers, possessing greater aerodynamic performance for the fuselage section in proportion to its usable cabin space. Wingflaps were initially not used in order to save weight, increase simplicity, and achieve a greater top speed; although the aircraft had satisfactory landing characteristics, it was decided to add compact flaps so that shorter landing strips could be used. Various configurations were tested.[5]
Various wing configurations were considered, the design team having to balance pure aerodynamic performance against various other factors and characteristics, such asroll damping, weight, and the need to accommodate the retractable undercarriage.[6] 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 thecamber corresponded directly with wing thickness and determined mathematically.[7] The wing was made 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. Theribs were made ofspruce and the webs were oflaminated birch; the exterior was covered inplywood,[8] adding to the wing's torsional stiffness.[9]
Aspindle-shapedmonocoque fuselage composed ofduralumin was adopted.[10] Thelongerons,bulkheads and stiffeners were all open channel sections; the combination of frame bulkheads and longitudinal channel sections permitted unobstructed compartments. The cabin extended across four primary bulkheads; all channels near the cabin wereriveted to the skin.[9] 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.[9] While not particularly resistant tobuckling, 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.[11]
The fuselage was flush riveted to minimize drag. For similar reasons, all fittings, including the foot steps and door knobs, were inset and flush windows were fitted.[8] Another drag-reduction measure was the fully retractable mainundercarriage which made use of the space in between the wing spars. Theailerons were precisely balanced to preventflutter.[12] The cantilever flight control surfaces were elliptical in planform. Nowind tunnel testing was conducted prior to the completion of the first He 70 due to alleged calculation complications between models and real aircraft, although several wind tunnel tests were performed after the aircraft's completion.[13]
It was typically flown by a crew of two, a pilot and radio operator, who were seated in atandem configuration in a fairly central position that was slightly elevated for a better view.[14][15] The view was further improved by the presence of a transparent movable cabin roof and an adjustable seat. 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.[14][15] The passenger cabin had air-basedheating and ventilation and large windows which could be used as emergency exits. Aft of the cabin was a baggage hold.[15]
It was powered by a singleBMW VIV-12 engine, capable of producing up to 470 kW (630 hp) at 1,600 rpm.[16] Unusually, this engine was cooled usingethylene glycol rather than water, which enabled the use of a more compactradiator. This was located beneath the fuselage and had one-third of the frontal area of a conventional counterpart, and could be retracted when the aircraft was flying at high speeds to further reduce drag.[8] The bottom of the oil tank, which was partitioned, was used for cooling; a wing pump was used for oil circulation. Both the engine and itscowling werestreamlined wherever practical to do so.[17] The aircraft was considered to be capable of achieving even higher speeds if it were to be fitted with a more powerful engine.[18]
The first prototype made itsmaiden flight on 1 December 1932.[19] Flight testing proved it to possess excellent performance, setting eight world records for speed over distance, and reaching a maximum speed of 377 km/h (234 mph).[20]
Luft Hansa operated He 70s between 1934 and 1937 for a fast service which connectedBerlin withFrankfurt,Hamburg andCologne, as well as between Cologne and Hamburg. Between 1934 and 1936, the type was also flown internationally fromStuttgart toSeville; this route was part of theSouth American mail service provided by Luft Hansa which continued viaBathurst,The Gambia toNatal,Brazil, usingJunkers Ju 52/3m andDornier Wal flying boats.[21]
All remaining aircraft were transferred to theLuftwaffe during 1937.
Starting in 1935, theLuftwaffe operated the He 70, initially as alight bomber andaerial reconnaissance aircraft. As soon as more capable purpose-built designs became available, it was relegated to use as aliaison andcourier aircraft.[citation needed]
During the late 1930s, 28 aircraft were sent to Spain as part of the German-mannedCondor Legion. These aircraft saw action during theSpanish Civil War, typically as fastreconnaissance aircraft. There they were known as theRayo, Spanish for "lightning".[citation needed]
A single example was bought byRolls-Royce for use as an engine testbed: it continued in use until 1944.[22]
The He 70K (later given theRLM number: He 170) was a fast reconnaissance export variant used by the Hungarian Air Force. Powered by aGnome-Rhône Mistral Majorradial engine, the engines werebuilt under license in Hungary as theWM-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 435 km/h (224 to 270 mph).[24] 18 were used by theRoyal Hungarian Air Force from 1937 to 1942.
A major weakness of the He 70 in military use was the fire risk. Parts of the airframe were made out of an extremely flammablemagnesium alloy called "elektron", though the majority of themonocoque fuselage wasduralumin. 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 obsolescentHeinkel He 46 parasol-wing monoplanes, untilFocke-Wulf Fw 189 "Uhu" medium altitude observation aircraft could be introduced.[citation needed]
While the He 70 saw only limited service in training capacities during the Second World War, it was the Luftwaffe's firstSchnellbomber and served as the antecedent for some of the bombers involved in theBattle of Britain.
The He 70 is known mainly as the ancestor to theHeinkel He 111, which had similar elliptical wings and streamlinedfuselage 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 ofWorld War II.[citation needed]
The He 70 was essentially scaled down to produce theHe 112 fighter which lost out on competition against theMesserschmitt Bf 109, but was nonetheless built in small numbers.
An He 70 was exported to Japan for study and inspired theAichi D3A ("Val") carrier-launched light bomber.[26] This aircraft shared the He 70's distinctive, low-mounted elliptical wing.
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]
TheGünther brothers had already used an elliptical wing for theBäumer Sausewind sports aircraft[citation needed] before they joined Heinkel.
Shenstone said that the He 70's influence on the Spitfire design was limited to use as a benchmark for aerodynamic smoothness.
Data fromThe Beautiful Blitz[29]
General characteristics
Performance
Armament
Related development
Aircraft of comparable role, configuration, and era
Related lists
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