The Heinkel 177 Greif (Griffin) was Germany’s only heavy bomber. Designed in 1936, it was in point of fact, a four engine bomber; an unrealistic demand by the Luftwaffe High Command was placed upon its design. The result was that instead of four engine nacelles, it had two engine nacelles using coupled engines.
These were inline engines but with front facing radiators, giving it a radial engine appearance. It proved to be a major problem; repeated overheating and engine fires plagued the design, and was never fully overcome.
The Bomber A aircraft specification required the plane to carry a bomb-load of at least 1,000 kg (2,200 lb) over a range of 5,000 km (3,100 mi), with a maximum speed of not less than 500 km/h (311 mph) at altitude.
This was a formidable specification, calling as it did for an aircraft able to outrun any modern fighter – as was expected with the top speeds of the main force Schnellbomber concept – and outperform, by a considerable margin, any bomber then in service. On 2 June 1937, Heinkel Flugzeugwerke received instructions to proceed with construction of a full-scale mock-up of its Projekt 1041 Bomber A. That was completed in November 1937, and on 5 November 1937 it was allocated the official RLM airframe type number “8-177”, the same day that the Luftwaffe High Command (OKL) stipulated that the new design should possess sufficient structural strength to enable it to undertake medium-degree diving attacks. Heinkel Flugzeugwerke’s estimated performance figures for Projekt 1041 included a top speed of 550 km/h (342 mph) at 5,500 m (18,050 ft) and a loaded weight of 27,000 kg (59,500 lb). In order to achieve these estimates, Heinkel’s chief designer, Siegfried Günter, employed several revolutionary features.
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I prefer this video’s still shots; it shows more than one variant of this plane, such as one with remotely operated dorsal gun turrets, another with two types of unidentified guided rockets (another subject entirely) and maintenance of its coupled engines. You might want to mute the soundtrack… (snark)
This one describes the coupled engine problems, as well as a 277 (with four engine nacelles) that ought to have been the original design, without the previously mentioned demands imposed: