Heinkel He 177

The Heinkel He 177 Greif (Griffin) was the only operational long-range bomber to be flown in combat by the Luftwaffe. Starting its existence as Germany's first purpose-built heavy bomber just before the war, and built in large numbers during World War II, it was also mistakenly tasked, right from its beginnings, to perform a milder version of the precision dive bombing the Junkers Ju 87 had pioneered during the Spanish Civil War. This requirement for a dive-bombing capability in such a large aircraft resulted in a design possessing considerably lower drag than any other "four-engined" heavy bomber of its time, in order to be able to perform the task in any measure, resulting in many major deficiencies being exposed in its general design, and hindering its widespread adoption for strategic bombing. Luftwaffe aircrew nicknamed it the Luftwaffenfeuerzeug (Luftwaffe's lighter) or the "Flaming Coffin" due to the serious engine problems on initial versions of the aircraft, many of these from the powerplants' installation in their wing nacelles. When these problems were later rectified, the type was successful, but it could not be deployed in large numbers due to Germany's deteriorating situation in the war.

Design and development
In 1936 the company of Heinkel Flugzeugwerke received details of the new Bomber A specification from the Reichsluftfahrtministerium (RLM). This specification, first proposed by the RLM on 3 June 1936 - the same day that, ironically, General Walther Wever, the main proponent for the Luftwaffe to have a strategic bomber force in the first place, lost his life - called for an aircraft more advanced than the Dornier Do 19 or Junkers Ju 89 "Ural bomber" prototypes that General Wever had championed. The Bomber A aircraft specification required the capability of carrying a bombload of at least 1,000 kg (2,200 lb) over a range of 5,000 km (3,100 mi), and possess 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. The mock-up was completed in November 1937, and on 5 November 1937, allocated the official RLM type number "8-177", the same day that the Luftwaffe High Command (OKL) first stipulated the requirement for the new design to 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.

Engines


The He 177 required at least a pair of 2,000 PS (1,973 hp, 1,471 kW) engines to meet performance requirements. However, no engine at the time developed such power. A four-engine version would have been possible with existing engines like the Daimler-Benz DB 601, but the four-engine layout would imply higher propeller drag for dive bombing. The use of only two propellers on a heavy bomber offered many advantages such as a substantial reduction in drag, reduction of dive instability, and a marked improvement in maneuverability. Indeed, the initial prototypes and pre-production models of the He 177 displayed an airspeed and maneuverability comparable to many heavy fighters of the time.

For the He 177, Günter decided to employ two of the complex Daimler-Benz DB 606 "power system" setups for propulsion. He had already employed these engines on the record breaking Heinkel He 119 reconnaissance aircraft prototypes. They consisted of a pair of DB 601 liquid-cooled 12-cylinder inverted-vee inline engines mounted side by side in a single nacelle - for the He 119, centrally within the fuselage, just behind its heavily glazed cockpit enclosure - driving a single propeller. The two component engines were inclined inwards by 30° so that the inner cylinder banks were disposed almost vertically. A single gear housing connected the front ends of the two crankcases, with the two crankshaft pinions driving a single airscrew shaft gear. The starboard DB 601 had to be fitted with a mirror-image version of its mechanically driven centrifugal supercharger, drawing air from the starboard side of the engine. Two of the DB 606s, each of which initially developed 2,600 PS (2,564 hp, 1,912 kW) for take-off and weighing some 1,515 kg (3,340 lb) apiece, were to power the He 177.

Surface evaporation cooling
For aerodynamic cleanliness, Günter decided to dispense with the usual system of drag-producing engine radiators and to employ a surface evaporative cooling system. Such surface cooling - in the form of simpler surface radiators instead - had been used on British high speed racing seaplanes as early as 1929. This sort of system was also pioneered on the He 119, and was also intended for use on the He 100 high-speed fighter project. In this system the coolant water is pressurized, raising the coolant's boiling point, in this case about 110°C (230°F). As the superheated water leaves the engine it enters an expansion area where the pressure drops and the water flashes to steam. The steam is then cooled by running in pipes along the outer skin of the fuselage and wings.

Before the design of the He 177 was finalized, it was clear that in practice such a system would be incapable of dealing with the vast amount of heat generated by each of the twinned pairs of DB 601 powerplants. As a result, the evaporative cooling system had to be abandoned in favor of conventional annular radiators fitted directly behind each propeller. These resembled, but were larger than, those fitted to the Junkers Ju 88 A bomber. These radiators added significantly to the He 177's weight and drag.

Defensive armament
Günter's original intention had been to equip the He 177 with three cockpit-controlled remote gun turrets, with two of them intended to come from the Junkers Ju 288 program, and only a single manned position in the tail. Compared with the manned turret, the idea of remotely controlled, turreted defensive armament traded technical complexity for reduction of size, weight, and drag. Furthermore, it held the advantage that the gunner could be installed in a protected position where he would have the best possible view, and where he would be less likely to be blinded by the flash from his own guns. Although work on remotely controlled aircraft defensive systems had reached a relatively advanced stage in Germany in the late 1930s, progress in this field was to prove insufficient to keep pace with the He 177. As a result the He 177 had to be modified to accommodate larger and heavier manned positions, such as the rear dorsal turret usually fitted to almost all examples of the Greif, armed with a single 13 mm MG 131 machine gun, this installation requiring that the fuselage receive structural strengthing in several locations. Eventually, most of the later production aircraft did receive a single remote forward dorsal turret, the Fernbedienbare Drehlafette (translating as "remotely operated rotating gun-mount", and abbreviated "FDL") 131Z, armed with two MG 131 machine guns, located at a point on the fuselage directly above the wing root's leading edge, with its rotating hemispherical sighting station's dome located a short distance forward of the turret itself and slightly offset to starboard, just behind the forward cabin area.

A compact tail gun position was fitted from the beginning for rearward defense, and armed with a single MG 131 machine gun, but its streamlined glazing demanded a prone gunner accommodation, which severely restricted the gunner's comfort on long duration missions. To improve the tail gunner's effectiveness, a revised tail gun position with a bulged upper glazing design was fitted to the He 177 A-3 and later models, which permitted the gunner to sit with his upper body upright. This revised emplacement design required a reduction of the lower end of the rudder surface for clearance. The single MG 131 gun would often be replaced with a 20 mm MG 151 cannon for a heavier defensive capability, or in a very few instances a semi-experimental twin MG 131Z mount, with the twinned 13 mm calibre guns mounted one above the other at the rear of the standard bulged upper glazing emplacement if the single 20mm autocannon was not used.

Usually, a single 7.92 mm MG 81 machine gun in a flexible mount was mounted in the upper starboard side of the cockpit nose glazing, for defense from a direct frontal fighter attack.

The undernose, inverted-casemate Bola gondola (a common ventral armament fitment concept for many German bombers), which was the full width of the fuselage where it emerged from under the nose, and centered under the forward cabin, usually had a flexibly mounted, drum-fed 20 mm MG FF cannon at the front end as added forward defense along with the aforementioned MG 81 in the nose glazing, and a flexibly mounted MG 81 machine gun in the rear for the initial A-1 version. An MG 151 cannon would replace the forward MG FF cannon in later production models, with a single MG 131 replacing the MG 81 for rearwards ventral defense.

Wing
Among the He 177's noteworthy features were its Fowler-type extensible trailing edge flaps which occupied the entire wing trailing edges, including those portions covered by the ailerons. Each aileron comprised upper and lower portions, the latter arranged to slide rearwards with flap extension while the upper part retained its function of providing lateral control for takeoff and landing. The original wing design did not take into full account the stresses resulting from the operation of the Fowler flaps. A Rechlin report dated 9 October 1942 stated: "The examination has shown that the strength of the He 177's wings is one-third below that estimated by Heinkel. The reason for this is the uneven rigidity of the individual members with consequent deformation under load. This condition was not recognized by Heinkel at the proper time, the failure tests having been undertaken too late in view of the size of the structure." Tests on the 40th production A-1 aircraft in September 1942 revealed serious outer wing panel component damage after only some 20 flights due to the aerodynamic stress from diving attack exercises, and very costly and extensive strengthening was needed to solve the problem. This significantly increased the aircraft's weight. Starting with the later versions of the He 177 A-3, the Fowler flaps along the outboard wing sections were deactivated and removed and an additionally strengthened wing design was introduced on the He 177 A-5.

Dive bombing
Accuracy with horizontal bombing during the years of the Ural bomber program demonstrated the weaknesses in existing German bombsights. This in itself called into question the effectiveness of horizontal bombing, whether by a medium or heavy bomber force. Some in the Luftwaffe believed that dive bombing was a more effective way to destroy targets. Technical data supported the accuracy of Stuka strikes achieving greater target destruction over Dornier Do 17s or Heinkel He 111s. The experience of the Condor Legion in Spain supported the theories of dive bombing and led some to believe that pinpoint accuracy was possible, consequently diverting them away from the idea of horizontal bombing in favor of dive bombing.

During the final inspection of the Projekt 1041 mock-up on 5 November 1937, Ernst Udet stated the OKL's new divebombing requirement to Ernst Heinkel, who replied that the aircraft would never be capable of it. The He 177 had to be strengthened to support the stresses imposed by the pull-out from a dive. Unfortunately, the dive bombing requirement was later increased to 60° attacks rather than the medium angle dives originally called for, resulting in further structural strengthening and an alarming increase in weight. Nevertheless, the requirement to dive bomb up to 60° was never satisfactorily solved, due to the constant increases in loaded weight. Despite the specially strengthened airframe, it was still possible to overstress the airframe during a dive attack maneuver. While the German bombsights of the 1930s were quite lacking, the follow-on versions of the Lotfernrohr 7 proved to be arguably as accurate as the American Norden bombsight. With the introduction of the Lotfe 7, which offered an average error of 20 m to 30 m (65 ft to 98 ft) from a release altitude of 3,000 m to 4,000 m (9,842 ft to 13,123 ft), and Hermann Göring's rescindment of the dive attack requirement in September 1942, the barred-gate type dive brakes, on the wing's lower surfaces and placed just forward of each of the outboard ends of the Fowler flap panels, were omitted from all He 177As built after the initial A-0 pre-production batch. One photo of one of the 12 "destroyer" He 177A-1/U2 heavy-cannon-armed test airframes, however still showed the retracted dive brake panel fitted on the undersurface of the outer starboard wing.

Undercarriage
During its initial development, the anticipated weight of the He 177 had increased so alarmingly that the provision of a main undercarriage design of sufficient strength to handle a 32-tonne (70,550 lb) loaded weight airframe began to pose a major problem. Neither the engine nacelles nor the wings provided much stowage space for the main undercarriage members, which needed to be of a longer-than-usual design to allow ground clearance for the large diameter four blade propellers of the aircraft's powerplants, and after several extremely complex arrangements had been considered during the aircraft's initial design stages, a rather novel, but still quite complex, system was adopted. Instead of the originally projected single wheel leg under each engine nacelle, two massive single wheel legs were attached to the main spar at each engine nacelle, the outboard legs retracting upward and outward into shallow wing wells, the inboard legs swinging upward and inward into similar wells in the wing roots, all units being completely enclosed by flush fitting wheel and strut doors, and almost "meeting" under each engine nacelle when fully extended. The lever-action lower gear strut sections, on which the wheels were mounted onto their axles, also had to pivot to a 90° angle (from their 120° angle while fully extended) to the main gear leg during the retraction cycle, just to fit into their wheel wells. A more conventional single-leg twin wheel arrangement for each main gear was actually used on the two prototype examples built (one during the war, one post-war) of the He 274 in France, and a few developments that only existed as drawings actually had tricycle gear setups being fitted to the Amerika Bomber entry version of the paper-only He 277, which were also depicted as using single main gear struts with twin wheels. The two hour time that it could take to change just one damaged main gear tire, using special Heinkel-provided 12-tonne capacity main gear jackstand blocks, which were in short supply, was just one of the myriad of problems that the He 177 A's complex main gear format brought about.

Prototypes
On 9 November 1939, the first prototype, the He 177 V1 was flown for the first time with Dipl. Ing. Leutnant Carl Francke, then the chief of the Erprobungsstelle Rechlin central flight test center, at the controls. The initial flight terminated abruptly after only 12 minutes as a result of overheating engines. Francke referred favorably to the general handling and landing characteristics of the prototype but complained of some vibration in the airscrew shafts, the inadequacy of the tail surfaces under certain conditions, and some flutter which accompanied any vigorous movement of the elevators. The He 177 V2 made its first flight soon afterwards.

Following Francke's initial flight, the He 177 V1 received several modifications suggested by the initial trials, including a 20% increase in the tail surface area. These modifications were not applied to the He 177 V2 when another test pilot undertook the first diving trials. During the diving trials, the V2 developed severe control flutter and broke-up in the air. Following this incident, the tail surfaces of the V3, V4, and V5 prototypes were modified in a similar fashion to those of the He 177 V1. The He 177 V3 was allocated the task of power plant development. The V1 through V3 prototype airframes were all equipped with two counterclockwise rotating DB 606 A powerplants, while the V4 prototype and all later aircraft, throughout the production run of the A-series, used a DB 606 A or DB 610 A engine on the starboard wing and one clockwise rotating B-version of the same powerplant on the port wing, so that the propellers rotated "away" from each other at the tops of the propeller arcs. The He 177 V4 was retained at Heinkel's test field where it undertook diving trials. While flying over the Baltic, the He 177 V4 failed to recover from a moderate dive, crashing into the sea near Ribnitz. It was later discovered that the accident had resulted from the malfunctioning of an airscrew pitch control mechanism.

Before the He 177 V3 and V4 prototype airframes had even been started, on 17 November 1938, Ernst Heinkel had personally requested permission from the RLM for the V3 and V4 airframes to be set aside for a trial installation of four separate Junkers Jumo 211 powerplants to address the concerns that the RLM Technischen-Amt technical department's director Ernst Udet and Heinkel had expressed over the RLM's dive-bombing priority for the He 177A, but was turned down for the trial fitment.

The He 177 V5 incorporated a number of changes which were principally concerned with defensive armament installations. Early in 1941, during a simulated low-level attack, both DB 606 engines burst into flames, the V5 hitting the ground and exploding. The He 177 V6 was the first aircraft equipped with main production type DB 606 A/B engines instead of the pre-production units which offered a slight increase in takeoff power by 100 PS to 2700 PS (2,663 hp, 1,986 kW). The He 177 V7 featured a revised nose section which, while generally following the contours of the nose sections employed by the previous prototypes, was considerably reinforced and embodied fewer glazed panels. In September 1941, the He 177 V8, the last of the aircraft to be built as prototypes from the outset with a different, almost "bulletlike" cockpit shaping and construction from the production He 177A series aircraft, was made available for engine tests, but owing to the urgency of other development work it was returned to Heinkel after only 40 days, and it was not possible to resume engine tests in the air until February 1942. The He 177 V1 to V8 and the A-0 production prototypes are notable for having a broad-bladed set of four-bladed propellers, with blade shapes and profiles similar to those used on the Junkers Ju 88 medium bomber, which were not used on the production He 177A series aircraft.



Photographs of the first eight prototypes show a largely circular fuselage cross-section, especially forward of the wing root, with the A-0 series possessing flatter sides, dorsal and ventral surfaces of the main A-series production aircraft. The choice of what was called the "Cabin 3" cockpit design on 20 September 1939 for the production A-series run, placed a well-framed hemispherical "fishbowl" nose onto the He 177 A-0, giving it the generic "stepless cockpit", without a separate windshield for the pilot and co-pilot, that almost all German bomber aircraft had in World War II. The He 177A's "stepless" cockpit's forward glazing had each set of its characteristic framing members, of four supporting frame members per set running in each orthogonal direction, running as the parallels and meridians on a globe would. Two sets of four roughly square windows, themselves arranged in a square of four windows each on each side of the upper cockpit, just behind the "fishbowl's" rear edge, provided sideways vision from the cockpit for the pilot and crew. These side windows are known from photographic evidence, to have been produced with slight differences in external appearance between those built by Heinkel (with its primary headquarters, Heinkel-Nord plant near Rostock, and the satellite Heinkel-Süd plants around Vienna) and those built by Arado Flugzeugwerke, the only major subcontractor for the He 177A's airframes. Often, the two lower rows of the "fishbowl's" windows in the lower nose were made opaque, with the exception of the bombardier's protruding bombsight window offset to starboard in the lower nose glazing, either by painting them over or replacement with metal panels that performed the same function.

Production
Eight prototypes were completed, followed by 35 pre-production He 177 A-0s (built by Arado and Heinkel) and 130 Arado-built He 177 A-1s. The early aircraft in this batch were used for further trials, and after a brief and unhappy operational debut the remainder were also withdrawn from service. From late 1942 they were replaced by 170 He 177 A-3s and 826 A-5s, both later models having 1.60 meter longer rear fuselages and slightly lengthened engine nacelles for use with the newer DB 610 "power systems".

Note - One A-0, one A-3, and two A-5 rebuilt as He 177B prototypes from before December 1943 to July 1944.

Engine difficulties
The tendency of the 1.5 tonnes-apiece DB 606 "power system" engines to ignite became increasingly serious as the test programme progressed and many of the He 177A-0 series of pre-production prototypes were destroyed in accidents or engine related causes. The DB 606 engine had first been introduced on the Heinkel He 119 and later used on other aircraft such as the Messerschmitt Me 261 where they functioned as intended, but the extremely tight cowlings on the He 177A led to considerable problems, the most common being in-flight engine fires and engine overheating. There were several reasons for the flammability of the DB 606 engine as installed in the Greif's engine nacelle accommodations, one of which was the common "central" exhaust manifold, serving a total of 12 cylinders, on the two inner cylinder banks of the twinned DB 601 component engines making up a DB 606. This central exhaust system would routinely become excessively hot and often caused the usual accumulation of oil and grease in the bottom of the engine cowling to catch fire. When the pilot throttled back there was a tendency for the injection pump to deliver more fuel than was required by the engine, in addition to which the injection pump connections leaked. To reduce the aircraft's weight no firewall had been provided, and the aft end of each DB 606 was fitted so close to the main spar — to the point that the rear two-thirds of the component powerplants' engine blocks were located behind the wing's leading edge — that there was insufficient space for the fuel/oil pipelines and electrical leads. The engine was frequently saturated by fuel and oil from leaking connections. At high altitude the oil tended to foam due to a badly designed oil pump, reducing its lubricating qualities as the oil circulated in the engines. Insufficient lubrication resulted in the disintegration of the connecting rod bearings which could burst through either one of the component engine crankcases, puncturing the oil tanks which poured their contents on to the hot central exhaust pipe collector. The tightly packed nature of the "power system" engine installations on the He 177A, with the extreme rearwards location of the component engines in their nacelles, also led to poor maintenance access to the engines as well as very poor ventilation. As a result of these factors, as well as a lack of routine maintenance in the field, the DB 606 powerplants frequently caught fire in flight. The effort to create an adequately powerful engine for the He 177 by mechanically coupling two pairs of lower-power engines into two heavy "power system" powerplant setups, while theoretically sound, proved to be difficult and time consuming to perfect, leading to numerous engine complications especially on the initial production models.

Reichsmarschall Hermann Göring, angered at the apparent slowness with which the He 177A was having its powerplant problems researched and solved late in August 1942, responded thus on August 13 to one Oberst Edgar Petersen's report (the Kommandeur der Erprobungstellen) on the He 177A's powerplant troubles, centred on remarks about the issues that the compromised installation, and the resultant poor maintenance access, that the He 177A's original DB 606 powerplant accommodation design produced:

Starting with the He 177 A-3/R2, a modified engine nacelle with a new "power system", the Daimler-Benz DB 610, each of which consisted of a pair of Daimler-Benz DB 605s set up to work as one as the DB 606 had been, was used to eliminate the tendency for engine fires. With the introduction of the DB 610 came several improvements including the relocation of the engine oil tank, the lengthening of the engine mountings by 20 cm (8 in), the complete redesign of the exhaust system which also facilitated the installation of exhaust dampers for night missions, and the setting of a power limitation on the engines which resulted in greater reliability. These modifications, supposedly numbering 56 of both major and minor varieties, were successful as far as eliminating engine fires were concerned, but other minor problems with the transfer gearbox between the two component engines of each "power system" and their shared propeller remained.

Oberst Petersen, as well as one Major Mons, through the Erprobungsstellen personnel and establishments were responsible for backing the substantial numbers of upgrades to the He 177A from the time of the rescindment of its dive-bombing requirement onwards in September 1942.

Experimental weapon loads
In addition to carrying a variety of bombs, torpedoes, and guided weapons the He 177 was tested with a number of unorthodox offensive armaments. The first of these experimental weapon schemes known to have been tested were the 12 examples of the He 177 A-1/U2 Zerstörer variant, which was armed with a pair of limited-traverse 30 mm MK 101 cannons in the extreme front of a dramatically enlarged Bola ventral gondola, and intended for ground attack, train busting, and possibly long-range anti-ship raids. Later, when assigned to flak-suppression sorties in the area of Stalingrad during the winter of 1942, Luftwaffe forward maintenance units modified a small number of He 177A-3s, fitting a 50 mm Bordkanone BK 5 cannon within the aircraft's undernose Bola gondola, with the long barrel protruding well forward, beyond the glazed "fishbowl" nose. This variant was unofficially dubbed the Stalingradtyp. Although a small number of He 177 A-3/R5 models were to be built from scratch, with the larger PaK-40-based, autoloading 75 mm Bordkanone BK 7,5 ventral cannon, structural problems caused by the weapon's recoil meant that the Stalingradtyp did not see combat use outside of the original, BK 5-armed improvised handful.

Five He 177 A-5s were experimentally equipped in January 1944 with batteries of 33 obliquely mounted 21 cm (8-1/4 in) calibre rocket mortar tubes, similar in their physical nature to the Werfer-Granate 21 single units already in use with single and twin-engined Luftwaffe fighters for bomber destroyer missions, and also likely to have been similarly derived from components of the Nebelwerfer infantry barrage rocket system. The nearly three dozen launch tubes placed in a Greif's fuselage in such a manner was meant to create the Grosszerstörer ("Big Destroyer") flying battleship, meant to break up and destroy the tight combat box defensive formations used by USAAF daylight bombers over Germany. The bomb bays and fuselage-housed auxiliary fuel tanks were removed on these aircraft in order to provide space for the spin-stabilized 21 cm (8 in) rockets and their launch tubes. The tubes were inclined to fire upward at an angle of 60° to the horizontal axis of the aircraft and slightly to starboard. The tubes could be fired individually, simultaneously, or in two salvoes of 15 and 18. Tests with fixed balloon targets showed the potential of this system, and limited operational trials against US Eighth Air Force bomber streams were authorized. The aircraft were operated by Erprobungskommando 25, flying out of the Baltic coastal Erprobungstelle facility at Tarnewitz. The intended mode of operation called for the Grosszerstörer He 177s to follow the enemy bomber formations, passing below (as with a Schräge Musik cannon fitment) and to port of the target, maintaining a difference of altitude of 2,000 m (6,560 ft) beneath the targets at the time of the attack from below. A few trial daylight operations were flown but no contact was made with Allied bomber formations, and as the escort fighters were becoming ever more numerous - in the manner of air superiority-purpose "fighter sweeps" well ahead of the massed USAAF bomber formations, starting in early 1944 - the entire scheme was abandoned.

Experimental defensive weapons fitments were also tried on small numbers of 177s set aside for such trials, mostly at the Erprobungstellen test detachment fields. One such fitment was to an He 177A-1, s/n 15155 with Stammkennzeichen GI+BP, which was the first-ever example of an He 177 to be fitted with an experimental, remote control twin-gun "chin turret" at the front of its Bola undernose gondola. The type of guns to be fitted was not recorded, but the date of which GI+BP was written off from a mishap in May 1943 would place the fitment of its experimental "chin turret" simultaneously with the lead-up to the May 1943 service introduction of the "gunship" USAAF Flying Fortress, the YB-40, which pioneered the same type of forward defensive armament on the best known American heavy bomber to attack Nazi Germany. Similarly, the much-anticipated Hecklafette HL 131V "quadmount" manned tail turret, armed with a quartet of MG 131 machine guns, was first tried in the late spring of 1943 through the summer of that year on a trio of A-3 examples set aside as the V32 through V34 prototypes, but never made it to production status.

Airworthiness and handling
The initial production version of the Greif, the He 177 A-1, demonstrated a tendency for instability in the yaw and pitch axes during August 1942, during flight tests, that would have led to poor bombing accuracy in action. Shortly after these tests, the third production A-1 example (factory serial number 15153, with Stammkennzeichen of GI + BN) had its fuselage lengthened by 160 cm (63 in) just aft of the trailing edge of the wing, and tests of the modified aircraft, from the longer distance of the "tail moment" that resulted, gave a marked degree of improvement in the yaw and pitch axis stability, enough to mandate the construction of the He 177 A-3 and all later models of the He 177 with the lengthened fuselage.

In early September 1944, the Royal Aircraft Establishment was ordered to supply an aircrew for a He 177 that the French Maquis and Allied units in Vichy France would take control of at the airfield at Blagnac near Toulouse, where elements of both the He 177A-equipped KG 4 and KG 100 Luftwaffe bomber wings were based. A transport and two escort fighters from the RAE flew to the area to leave the Chief Test Pilot and a flight engineer with the commando group. On 10 September, as Operation Dragoon was wrapping up in the southeast of France, the aircraft was seized and flown back to the UK. Soon afterwards, Eric Brown, a RN pilot then posted to the RAE as a test pilot, flew the He 177. He wrote about the in-flight handling characteristics of the He 177 A-5:. "... positive about all axes, but the controls were all remarkably light for such a large aircraft. Indeed I had the feeling that the elevator was dangerously light and I was all too aware of the intelligence reports of He 177s breaking up in the air so I decided to treat this control very gently... The aircraft had an automatic pull-out device and an acceleration warning apparatus fitted, but it really was nailbiting to have to treat a giant like this immense Heinkel bomber as if it was made of glass. The stalling characteristics with flaps and undercarriage lowered, the aircraft buffeted violently at 140 km/h (87 mph) before the nose dropped at 135 km/h (84 mph). The buffet experienced was so violent that I had some concerns over structural damage. Somehow the He 177 always conveyed an impression of fragility despite its size."

He said it was "one of the very few German aircraft of the period that I tested that I did not enjoy flying".

Further development-the Heinkel He 177B
Due to continuing problems with the DB 606's configuration, much development work was being done in order to rectify engine complications, including a complete redesign of the original He 177, primarily through newer wing designs and layouts to accommodate them, intended towards the creation of a four-engined version of the Greif's airframe. The first occurrence of such concerns over the coupled-engine vs. four separate engine issue for the He 177 emerged in mid-November 1938, as Ernst Heinkel had requested that he wished for two of the requested eight He 177 prototypes to be fitted out with four individual engines in place of the coupled-engine arrangements, eventually specifying that the V3 and V4 airframes get four individual Junkers Jumo 211 engines each in a 17 November in-plant corporate meeting - the same exact type and number of engines actually used for the first flights of the Me 264 V1 Amerika Bomber contract contender prototype's first flights in late December 1942. Ernst Udet was also critical of the coupled DB 606 powerplant choice for the He 177 from before the war's start, with Göring adding his input from his own frustrations with the seemingly interminable engine problems delaying the introduction of the He 177A into service. Göring was reported as stating in late August 1942, following his earlier complaints to Oberst Petersen on the 13th of the month: "I had told Udet from the start that I wanted this beast with four engines. This crate must have had four engines at some time! Nobody had told me anything about this hocus-pocus with welded-together engines."

Nearly four years after Herr Heinkel had unsuccessfully requested two of the prototype He 177 V-series airframes to be built with four individual powerplants, the RLM's requirement for the He 177 to perform diving attacks was finally rescinded in September 1942 by Goering himself, and with that decision finally rendered, Heinkel's design work on the pair of "separately" four-engined versions of the He 177A, the A-8 and A-10, collectively renamed the He 177B in August 1943 were then able to progress, meant to be powered with four individual Daimler-Benz DB 603 engines on new longer-span wings, with each liquid-cooled DB 603 fitted with a Heinkel He 219-style annular radiator right behind the propeller for engine cooling. This task was accomplished in a considerably later timeframe than British aircraft designer Roy Chadwick had done in similarly converting the Avro Manchester. The Manchester, like the A-series Greif (with its coupled DB 606s and 610s) had depended on two very powerful but in practice troublesome 24-cylinder powerplants, the British Rolls-Royce Vulture, but by 1941 had been redesigned with four Rolls-Royce Merlins, as the Avro Lancaster.

By August 1943 much of the detail work for the He 177B series aircraft was well on its way to completion, and Erhard Milch eagerly approved the creation of three He 177B prototypes, designated He 177 V101 to V103, stating on 10 August: "The He 177A-4 and A-5 will be produced as before. The He 177B-5 will be tackled with vigor. It will be built in series as soon as possible."

The B-5's first-built prototype, the He 177 V101, was converted from a mid-production He 177 A-3 airframe (number 535550, with Stammkennzeichen of NN + QQ), the V102 being converted from the eighth He 177 A-0 production prototype aircraft (which required lengthening to the He 177A-3 and later fuselage specification), and the V103 being converted from an existing, early production He 177 A-5 airframe, with all three airframes initially retaining the production 177A-style single vertical tail surfaces. Even with no photographs known to exist verifying their fitment, the general arrangement drawing for the V101 airframe showed that it was intended to be uniquely fitted with a small-area matching vertical pair, one per side, of so-called pivoting "drag rudders" mounted a short distance in from the horizontal stabilizers' tips, directly inline with the inner engine nacelles, to simulate "engine-out" conditions. Each of the pivoted "drag rudders" were to have their area divided equally above and below the plane of the stabilizer. Because of the 177A-style single-tailed V101 prototype having increasingly serious stability problems with higher airspeeds in its flight testing, the second prototype, the V102, was both the first He 177B example to fly with the quartet of DB 603 engines on 20 December 1943, in combination with a brand-new empennage of twin tail configuration, fitted to it during the early autumn of 1943. When the V102 was tested later that autumn while still flying with its A-series wing and powerplants before its own pair of B-series "four engine" wing units were ready, the new twin vertical tails gave the V102 significantly better in-flight handling when compared to the original He 177A's single tail design, except during the landing approach when the Fowler flaps were extended during its own initial flights with the twin tails in November 1943. By late February 1944, as the USAAF's Big Week strategic bombing campaign against Nazi Germany was initiated, at a meeting held at the Wiener Neustadt military airfield, visited by Erhard Milch, fellow guests Oberst Edgar Petersen and Oberstleutnant Siegfried Knemeyer each had a chance to fly the now four-engined V102 prototype after the B-series set of wings had been fitted, with Knemeyer stating that he could not believe a four-engined heavy bomber could possess the "excellent handling qualities" the V102 machine displayed. The only verifiable wartime photograph that exists today of any of these He 177B prototypes in an intact condition is one of the V101, parked outdoors on a foggy German airfield, most likely the Heinkel-Sud factory airfield at Schwechat. One additional surviving photo that is known to exist, showing what looks like an He 177B-series prototype from the right side with a production-style A-series single vertical tail surface set, and bearing the Stammkennzeichen code of NE+OD, does not match any item of the surviving documentation for the four known 177B-series prototypes ordered, built or flown before the end of the war.



The He 177B was also intended to introduce a slightly enlarged, somewhat more aerodynamic fully glazed nose that somewhat echoed the lines of the nose glazing from the Airspeed Horsa British troop glider in a sideview comparison, first meant for use on the production A-7 version, that could incorporate a remotely controlled power chin turret at the front of its Bola for forward ventral defense, mounting either a pair of MG 131 machine guns or MG 151 cannon, but the new nose design was only tested on the He 177 V15 production prototype (converted from an A-3, factory serial 355 001), without the chin turret, and was never fitted on any of the He 177B prototypes, which used the standard "Cabin 3" He 177A's well-framed nose. No photographs of this new nose design are known to have survived the war and only drawings exist of it in modern archives, with the V15 airframe itself wrecked in a crash on 24 June 1944. The remaining defensive armament for the B-series design generally remained similar to the He 177A, particularly the twin dorsal gun turrets for the He 177 B-5, with the aft manned dorsal turret being deleted on the planned He 177 B-7 (as on the He 177 A-7) to reduce weight, and a fully powered manned Hecklafette HL 131V tail turret, carrying a quartet of MG 131 machine guns, was intended for installation on the prototypes. The Hecklafette HL 131V four-gun manned tail turret system would have been standardized on the production B-series aircraft, but never went beyond the mockup and working prototype stage, with a trio of the prototype tail turret units documented as being fitted to the He 177 V32 through V34 A-series DB 610-powered prototype airframes for trials. The cumbersome four-strut main landing gear of the A-series was retained intact for the B-series prototypes, even though their height, meant to allow clearance for the A-series' pair of large four-blade propellers, was not changed - the outer edge of the DB 603's inner engine nacelle/wing surface juncture was located right at the "centreline" of each of the twin pairs of A-series main gear strut locations, on all four of the B-series prototypes.

The first flights of the He 177B prototypes, starting with the He 177 V102 on 20 December 1943, occurred between late December 1943 and early January 1944 in the vicinity of the Vienna-Schwechat airfield, at the firm's Heinkel-Süd southern production facility, where an additional prototype, the V104, whose purpose was to be the "finalized" production prototype for the He 177B-5, and also meant to be a twin tailed prototype like the earlier V102, was being completed by order from the RLM, converted from an early production He 177 A-5.

However, from 23 April, through July 1944, repeated Fifteenth Air Force bombing raids on German aircraft production facilities in Vienna destroyed the airworthy V103 and the incomplete V104 at the Floridsdorf and Zwölfaxing satellite plants of the Heinkel-Süd complex during a 8 July raid, setting back plans of producing any series examples of the B-5 version. Arado Flugzeugwerke, which had been the major subcontractor for the A-series Greif airframes, at that time was itself fully involved with the production of its own, much more advanced Arado Ar 234B turbojet-powered reconnaissance-bomber, and was not able to handle the anticipated demand from Heinkel to produce the B-5 by October 1944. The Arado firm would not have been able to have started the He 177B-5's production for another month (November 1944) from its own focus on the Ar 234B. The last known official accounts of the whereabouts of the two He 177B prototypes that escaped heavy bombing had the V101 still at the Heinkel-Süd plant's airfield at Schwechat near Vienna, and the V102 also there at Schwechat as late as February 1945, after damage sustained from a bad landing in April 1944 while evading one of the initial USAAF 15th Air Force raids on the area, which had kept it from being flown north to Luftwaffe's well-known central Erprobungstelle test facility at Rechlin for safety. It is possible, from some accounts, that the V101 prototype might have survived until at least February 1945 (as the V102 had) before it was scrapped, as at least a pair of photos of what is stated as the wrecked V101 place it at Cheb in May 1945, and allegedly showing that the V101 had even been test-fitted with a quartet of Junkers Jumo 222 engines, if verified, would conflict with Heinkel records as to the V101 having been scrapped. One of the Czech photographs, does show what is thought to be the V101 with four bladed propellers instead of its earlier three-blade units used with its four DB 603 engines, a combination of prop type and powerplant also used for the Fw 190C fighter prototype, and on closer examination the forward areas of the V101 engine nacelles' hinged upper cowling access panels themselves just behind the annular radiators as revealed in the Czech photos appear to be very close in appearance, and especially from their outlines — as possible Kraftei unitized engine installations — to those used on the He 219 night fighter., whose own earlier prototypes also used four-blade propellers on their DB 603 powerplants.

The adoption of the Emergency Fighter Program in early July 1944 dealt the final blow to the entire He 177B development program, with the Heinkel He 162 jet fighter being the only new Heinkel aircraft design that would be allowed into production.

Operational history
Beset by technical difficulties in development, the He 177 had a troubled service. Overly optimistic design requirements of long range, high speed, heavy bomb load, and dive bombing didn't help. Although the He 177 entered service in 1942 it was far from operational. In an assessment of the aircraft on 9 April 1942, the newly activated Erprobungsstaffel 177 reported that the Greif had good flying characteristics, but had unacceptable engine troubles and deficits with the airframe strength. As an emergency measure it was used to supply the encircled 6th Armee at Stalingrad where it was found to be unsuited for the transport role, carrying a little more cargo than the smaller, more reliable Heinkel He 111, and was useless for the evacuation of wounded. As a result the He 177s reverted to bombing and flak-suppression missions near Stalingrad. Only 13 missions were flown and seven He 177s were lost to fire without any action attributable to the enemy.

As the war progressed, He 177 operations became increasingly desultory. Fuel and personnel shortages presented difficulties, and He 177s were sitting on airfields all over Europe awaiting new engines or engine related modifications. During Operation Steinbock, of the 14 He 177 sent out, one suffered a burst tire, eight returned with overheating or burning engines and of the four that reached London one was lost to night fighters. These aircraft were brand new, delivered a week before the operation and not fully flown in, as the air unit had moved to a new airfield the day before and lacked sufficient maintenance personnel and material. Constant attacks against Luftwaffe long-range combat units in France made continuous operations difficult. While Steinbock was unsuccessful, the He 177 did achieve some successes. They typically carried two 1,800 kg (3,970 lb) and two 1,000 kg (2,200 lb) bombs. Climbing to 7,000 m (22,965 ft) while still over German territory, the He 177s approached the target in a shallow dive, each aircraft throttled back, the pilot putting his aircraft into a gliding descent to take it across the bomb release-point at about 4,500 m (14,760 ft). After releasing the bombs the pilot re-opened the throttles, but continued the descent at approximately 200 m (656 ft) per minute. The bombers typically re-entered German airspace at an altitude of 750 m (2,460 ft), and headed back to base. By such means, the He 177s were able to keep up speeds of about 600 to 700 km/h during their withdrawal phase. The higher speed and constant change of altitude made interceptions difficult, increasing the survivability of the aircraft, but decreased accuracy. With an average loss rate of 60% for all types of bomber used in Operation Steinbock, the He 177's loss rate below 10% made them the most survivable bomber in the campaign.

During operations on the Eastern Front in early 1944, often carried out in daylight at about 6,000 m (19,690 ft) or higher, losses were relatively light. The Soviet Air Force, equipped mainly for low-level interception and ground-attack roles, was able to do little to hinder the high-flying bombers.

In common with most German bombers, the He 177 was grounded from the summer of 1944 on as Allied bombing crippled German fuel production. The He 177 can be compared with the Boeing B-29 Superfortress which also took about two years to have its problems ironed out, after which it found success. However the He 177 was never to achieve its full potential.

Variants

 * He 177 V1 to V8
 * 8 prototypes built in total. He 177 V4 and subsequent aircraft powered by DB 606 A/B engines.


 * He 177 A-0
 * Pre-production series, 35 built. First to use the "Cabin 3" cockpit with "fishbowl" framed glazed nose, as with production A-series.


 * He 177 A-1
 * First production series, 130 built. Armed with a single MG 81 in the nose, a single MG FF/M cannon in the forward end of the Bola ventral gondola, a remote controlled dorsal turret with a single (later twinned) MG 131, and a single tail mounted MG 131.

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 * He 177 A-1/Rüstsatz 1 (/R1)
 * Equipped with a "twinned-up" pair of aft firing MG 81Z machine guns in the rear of the Bola ventral gondola.


 * He 177 A-1/R2
 * Experimental version only, equipped with a sighting station in the rear of the Bola ventral gondola for a remotely controlled ventral turret housing a single MG 131.


 * He 177 A-1/R4
 * Equipped with a supplementary aft firing MG 131 in the rear of the Bola ventral gondola and a manned aft dorsal turret containing an MG 131.


 * He 177 A-1/Umrüst-Bausatz 2 (/U2)
 * Zerstörer heavy fighter with a pair of limited-traverse 30 mm MK 101 cannon in enlarged Bola lower nose mount, 12 conversions.


 * He 177 A-2
 * Proposed four-man pressurized variant with reduced defensive armament of six MG 81 and a single MG 131, never built.


 * He 177 A-3
 * Second production series, 170 built, with 1.6 meter-longer lengthened rear fuselage. Sixteenth and subsequent aircraft powered by DB 610 A/B engines.


 * He 177 A-3/R1
 * Powered by two Daimler-Benz DB 606 A/B engines, 15 built.


 * He 177 A-3/R2
 * Improved electrical system. MG FF cannon replaced by an MG 151 cannon in the Bola ventral gondola. Larger redesigned tail position, MG 131 replaced by MG 151 cannon in the tail position.


 * He 177 A-3/R3
 * Anti-shipping version capable of using the Henschel Hs 293, equipped with Kehl control gear.


 * He 177 A-3/R4
 * Bola Ventral gondola lengthened by 1.2 m (3 ft 11 in) to provide room for the FuG 203b Kehl III missile-control equipment.


 * He 177 A-3/R5
 * Planned, never-built Stalingradtyp version armed with a 75 mm Bordkanone BK 7,5 cannon based on the 7.5 cm PaK 40 installed in the ventral Bola gondola, also used on the Junkers Ju 88 P-1, based on a small number of 177As actually field-equipped as A-3/Rüstsatz 5 machines, with the KwK 39-based Bordkanone BK 5 cannon.


 * He 177 A-3/R7
 * Torpedo bomber version abandoned in favor of the He 177 A-5, only three built.


 * He 177 A-4
 * Proposed high altitude pressurised version, never built under the designation, and later developed into the Heinkel He 274.


 * He 177 A-5
 * Main production series, 826 built. Standardized the A-3's longer rear fuselage, strengthened wing, shortened undercarriage oleo legs, increase in maximum external bombload.


 * He 177 A-5/R1
 * Version optimized for Fritz X and Hs 293 guided bombs, equipped with Kehl control gear.


 * He 177 A-5/R2
 * Armed with a single MG 81 in the nose, a single MG 151 cannon in the forward end of the Bola ventral gondola, a pair of MG 81 in the rear end of the ventral gondola, a pair of MG 131 in an FDL 131Z remotely controlled forward dorsal turret, a single MG 131 in a manned aft dorsal turret, and a single tail mounted MG 151 cannon.


 * He 177 A-5/R4
 * Simplified bomb rack installation, equipped with Kehl control gear.


 * He 177 A-5/R5
 * Tested with a supplementary pair of MG 131 in an FDL 131Z aft ventral remote turret aft of the rear bomb-bay, only one built.


 * He 177 A-5/R6
 * Replacement of the forward and central bomb-bays with enlarged, full-fuselage-depth fuel tanks.


 * He 177 A-5/R7
 * Pressurised cockpit study with a projected ceiling of 15,200 m (49,869 ft) and similar reduced armament to the He 177 A-2.


 * He 177 A-5/R8
 * Armed with FDL-series remote gun turrets. Abandoned as a result of difficulties with the turrets, only one built.


 * He 177 A-5 Grosszerstörer
 * Anti-bomber variant based on the He 177 A-5, armed with up to 33 spin-stabilised 21 cm calibre rockets obliquely mounted in fuselage, replacing bomb bays and auxiliary fuel tanks, and most likely based on components of the 21 cm Nebelwerfer 42 infantry barrage rocket system. Five examples delivered in January 1944 for operational trials. Abandoned due to increasing numbers of Allied escort fighters.


 * He 177 A-6
 * Meant to be a "32 metric-ton" loaded-weight long-range bomber, as a planned improvement over the A-5 version, the A-6 dispensed with the rear manned dorsal turret, and retained the A-5/R2's single MG 151 flexible cannon at the front of the Bola, the flexible ball-mount MG 81 in the "fishbowl" nose glazing, along with the regular A-series FDL 131Z remote forward dorsal turret, and standardized the rear armament with the planned Hecklafette manned HL 131V quadmount MG 131 machine gun turret for the first time. Not produced, due to building volume of design work on the He 177B-series four-engined aircraft.


 * He 177 A-6/R1
 * Replacement of the forward and central bomb bays with full-fuselage-depth fuel tanks (as on the A-5/Rüstsätz 6 modification) and the addition of external bomb rack under the new fuel tank bays, capable of carrying a single 2,500 kg (5,511 lb) bomb or Fritz X/Hs 293 in addition to the rear bomb-bay loadout, if equipped with Kehl control gear. Range of 5,800 km (3,604 mi), only six test conversions built, from A-5 versions.


 * He 177 A-6/R2
 * Equipped with a redesigned fuselage nose of improved aerodynamic form, abandoning the earlier "Cabin 3" A-series cockpit, with the new nose being generally the same as intended for He 177A-7 and all He 177B development versions. Retained the FDL 131 remotely controlled forward dorsal turret, a single flexible-mount MG 131 in the rear of the Bola, a pair of MG 151 cannon in a remotely controlled FDL 151Z "chin" turret (to be standardized on the B-version) at the front of the Bola, and a manned HL 131V MG 131 "quadmount" tail turret. Similar bombload and range to He 177 A-6/R1. Only one test airframe converted from an He 177A-3 to test the new cockpit/nose, as the He 177 V15, of which no photos are known to survive, and which itself was wrecked in a mishap in late July 1944.


 * He 177 A-7
 * High-altitude bomber with an extended wing spanning 36 m (118 ft 1⅓ in) and with DB 610 A/B engines instead of the intended 3,800 PS (3,748 hp, 2,795 kW) DB 613 "power systems", which never emerged from testing and used pairs of twinned DB 603 engines for each "power system". Six examples, for wing tests, converted from He 177 A-5 airframes, but never fitted with the intended He 177 B-series advanced cockpit. One converted He 177A-5 example, Wk. Nr. 550 256 captured by American forces, scrapped postwar and believed buried under the grounds of Chicago's O'Hare International Airport.


 * He 177 A-8
 * First proposed He 177 design to feature four individual engines, using the A-3 or A-5 fuselage with a new wing design, and either Daimler-Benz DB 603 or Junkers Jumo 213 engines with Heinkel He 219 style annular radiators. Remained a paper project only, before re-designation as the "He 177B-5" by August 1943.


 * He 177 A-10
 * Proposed four-engined He 177 design, similar to the He 177 A-8, but based instead on the He 177 A-7 definitive production fuselage, with manned rear dorsal gun turret omitted, and re-designated as the "He 177 B-7" in August 1943.


 * He 177 B
 * Developed as the direct, "separate four-engined" development of the "coupled engine" powered He 177A-series, four prototypes ordered (He 177 V101 to V104) with three built and flown under DB 603 power. Originally postulated in postwar aviation books to have been a "cover designation" for the never-produced, paper-only He 277 Amerika Bomber design competitor.


 * He 177 H
 * Initial project designation for the Heinkel He 274.


 * Special variants
 * He 177 V38
 * An A-5 (Werknummer 550 002, bearing Stammkennzeichen of KM+TB) – documented use was as testbed for FuG 200 Hohentwiel ASV maritime patrol radar with flexible MG 131Z nose gun installation, speculated to have been intended for the installation of an enlarged bombbay to be used in the Junkers Ju 287. A common myth claims V38 was the prototype for a German "atomic bomber" (purportedly capable of carrying a fission device as a droppable weapon). Remains found at Prague's Rusiye field on V-E Day.

Operators

 * 🇩🇪 Germany:
 * Luftwaffe
 * Fernkampfgeschwader 50
 * Kampfgeschwader 1
 * Kampfgeschwader 4
 * Kampfgeschwader 10
 * Kampfgeschwader 40
 * Kampfgeschwader 100
 * Kampfgeschwader 200
 * Flugzeugführerschule (B) 15
 * Flugzeugführerschule (B) 16
 * Flugzeugführerschule (B) 31
 * Wekusta/OBdL


 * French Air Force operated at least two He 177 A-3s left behind by the Germans and rebuilt by SNCASE at Blagnac.
 * French Air Force operated at least two He 177 A-3s left behind by the Germans and rebuilt by SNCASE at Blagnac.


 * Royal Aircraft Establishment
 * the He 177 A-5 (Geschwaderkennung code of F8 + AP from 6./KG 40) that had been taken from Toulouse-Blagnac airfield in September 1944. Repainted with British markings and given the serial TS439.

Potential for surviving aircraft
As of the early 21st century, no examples of the He 177 in any version exist in aviation museums, as all surviving He 177A aircraft post-war, including the photographed He 177B wreck at Cheb and both He 274 airframes completed in France post-war are known to have been reduced to scrap by the end of the 1950s. The potential likelihood of relatively intact surviving He 177A airframes, which could exist anywhere a problem-stricken Greif is likely to have been successfully force-landed during wartime on a fresh-water lake in wintertime on the lake ice, as documented occurrences in surviving Luftwaffe records - and then with the aircraft sinking to the lake bottom during a spring thaw - does exist, but none have been searched for or discovered as yet.