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Join Date: Jun 2007
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Country:  | During 1936 the Luftwaffe issued details of its requirements for the new "Bomber A". From the start it was a demanding specification, and called for an aircraft with a maximum speed of 335 m.p.h., able to carry 4,400lb of bombs to a target 1,000 miles away or 2,200lb to a target 1,800 miles away. Moreover, it had to be strong enough to make medium-angle dive attacks, descending at 40-50°.
This formidable specification demanded an aircraft able to out perform, by a considerable margin, any bomber and outrun any fighter then in service. To meet the challenge the Heinkel design team displayed great ingenuity in their P.1041 design study for a well-proportioned aircraft of all metal construction, with an exceptionally clean airframe. Early on, however, its advanced features presented serious problems.
Power for the new bomber would be from a pair of Daimler Benz DB606 coupled engines, Each DB606 comprised two DB601 liquid-cooled 12-cylinder inverted-vee inline motors, mounted side-by-side and driving a single four-bladed propeller through a connecting gear train. Each combined engine gave an output of 2,600 h,p, A clutch arrangement allowed either engine in a pair to be shut down in flight, so the aircraft could cruise on two engines to extend its endurance. Installing the engines in two nacelles produced an aerodynamically cleaner arrangement than placing the four engines in individual nacelles. Moreover, concentrating the weight of the propulsion system well inboard gave the bomber increased manoeuvrability for the medium-angle dive-attack role.
In a further move to reduce the drag of the engine installation, the designers sought to dispense with the drag-producing radiators usually employed with liquid-cooled engines. For an attempt on the world air speed record title Heinkel company had pioneered an evaporative cooling system.The water coolant was pressurised, allowing the temperature to rise to 1,100C before steam formed. The coolant was then ducted away and depressurised. at which point steam formed. The water was separated off and returned to the engine, while the steam was piped to condensers in the wing leading edges cooled by the airflow, There the steam condensed and the coolant was pumped back into the engines.
The system worked well enough during a record-setting flight lasting about 15min, and in March 1939 a much-modified Heinkel He 100 fighter raised the world air speed record to 463 m.p.h.
The cooling of a D6606 running for several hours on end, was a different matter It quickly became clear that dissipation of the heat generated by the D8606s was far beyond the ability of the evaporative cooling system. The designers reverted to conventional radiators, increasing drag.
In November 1938 tile Helnkel company received an order for six prototypes. At the same time the type received its official designation - Heinkel He 177. On November 19, 1939, the He 177V1 prototype made its maiden flight. The chief of the Rechlin flight test centre, Dipl-Ing Franke, took the bomber into the air but almost immediately the engines started to overheat and he returned to the airfield after just 12min airborne. Franke reported favourably on the bomber's handling, apart from the engine overheating, although he also complained of the inadequate effect of the tail surfaces, undue
vibration from the propeller shafts and a degree of aileron flutter. To cure the flutter the prototype was fitted with a new fin and tailplane of 20 per cent greater area.
As pilots explored the bomber's performance it became clear that its maximum speed was disappointingly low; a mere 286 m.p,h. Moreover, they found that the He 177’s range was little more than 3,100 miles, a quarter less than specified.
A few weeks later He 177V2, the second prototype, joined the test programme. This aircraft was generally similar to the V1 but retained the original, smaller tail surfaces. During an early test dive the V2 developed severe control flutter and broke up in mid-air, killing the pilot Consequently the \/3, V4 and V5 prototypes, then nearing completion, were all given the enlarged tail surfaces. Each measure taken to improve handling added more weight. and each successive prototype was somewhat heavier than its predecessor.
Diving trials resumed using tile V4 prototype, but during one of these the aircraft suffered a malfunction of the propeller-pitch mechanism, failed to recover from its dive and crashed into the Baltic.
The last aircraft of the prototype batch, the V5, was used to test the bomber's armament. To save weight this aircraft had its 13mm machineguns replaced by 7-9mm weapons. Early in 1941, during a simulated low-level attack, the V5 suffered a double engine fire, flew into the ground and exploded.
Thus, during the early test programme three of the first five He 177 prototypes crashed, taking their crews to their deaths.
As the test programme continued there remained the difficulty of meeting the requirement for the bomber to make precision attacks in dives of up to 60°. Pilots found that to get the He 177 into a diving attitude they had first to reduce speed, then very slowly ease forward on the control column to get the bomber's nose to drop, Almost as soon as the bomber was established in its dive it was time to initiate a relatively gentle pull-out Even with this gentle handling the He 177 was easily overstressed and would pop rivets, It was far too large for steep dive attacks,
However, the new Lotfe tachometric bombsight, working on a similar principal to the American Norden bombsight entered Luftwaffe service, It produced accuracies in horizontal bombing attacks similar to those obtained in diving attacks, so the He 177's dive-bombing role was quietly dropped, and after the initial pre-production batch the dive brakes were omitted.
In an effort to get the new bomber into service the sixth prototype was sent to Bordeaux-Merignac in France for initial operational trials during the summer 1941. But these revealed several shortcomings and the aircraft was returned to Germany.
The Luftwaffe began receiving the pre-production A-O series, totalling 35 aircraft in November 1941. These aircraft explored the performance envelope and technical features,
It became clear that the problem of engines overheating and sometimes catching fire had more than one cause. Some cases were blamed on the common exhaust manifold which carried the exhaust gases away from the two inner cylinder blocks, and became excessively hot in flight. Moreover, if spilt oil collected in the bottom of the cowling it could ignite. Sometimes the fuel injectors delivered too much fuel if the pilot throttled back too quickly. If the fuel connectors were insufficiently tight, or were defective, excess fuel might spill on to a hot part of the engine.
Owing to the bid to save weight the He 177 lacked firewalls at the rears of the nacelles. Thus even a small fire could quickly run out of control and reach a wing fuel tank.
It was found that some highly stressed engine parts had poor lubrication. In extreme cases this led to connecting-rod bearings seizing-up and smashing through the crankcase. Lubricating oil spilling on to a hot exhaust pipe could also cause a fire. Daimler-Benz launched a major effort to diagnose and cure the DB606’s failings, but the problem of engines overheating and catching fire would run for a lot longer. |
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