Heinkel He 100

Design & Development

In order to get the promised performance out of the aircraft, the design included a number of drag reducing features. On the simple end was a well faired cockpit, the absence of struts and other drag inducing supports on the tail. The landing gear (including the tailwheel) was retractable and completely enclosed in flight.

There was also a serious shortage of advanced aero engines in Germany during the late 1930s. The He 100 used the same Daimler-Benz DB 601 engine as the Messerschmitt Bf 109 and Bf 110, and there was insufficient capacity to support another aircraft using the same engine. The only available alternate engine was the Jumo 211, and Heinkel was encouraged to consider its use in the He 100. However, the early Jumo 211 then available did not use a presurized cooling system, and it was therefore not suitable for the He 100's evaporative cooling system. Furthermore, a Jumo 211-powered He 100 would not have been able to outperform the contemporary DB 601-powered Bf 109 because the supercharger on the early Jumo 211 was not fully shrouded. From a practical matter, the He 100 used a novel cooling system that was complex, dependent upon many small pumps and difficult to maintain under field conditions. In order to reduce weight and frontal area, the engine was mounted directly to the forward fuselage, which was strengthened and literally tailored to the DB 601, as opposed to conventional mounting on engine bearers. The cowling was very tight fitting and as a result the aircraft has something of a slab sided appearance.

In order to provide as much power as possible from the DB 601 engine, the 100 used exhaust ejectors for a small amount of additional thrust. The supercharger inlet was moved from the normal position on the side of the cowling to a location in the leading edge of the left wing, which was also a feature of the earlier He-119. Although cleaner looking, the long, curving induction pipe most likely negated any benefit.

For the rest of the designed performance increase, Walter turned to the somewhat risky and still experimental method of cooling the engine via evaporative cooling. Such systems had been in vogue in several countries at the time. Heinkel and the Gunter brothers were avid proponents of the technology, and had previously used it on the He-119 with promising results. Evaporative or "steam" cooling promised a completely drag free cooling system. Unfortunately, the systems also proved complex and terribly unreliable in practice. Huge expanses of the airframe's outer skin had to be devoted to cooling, which made such systems succeptible to combat damage. The DB 601 was a pressure cooled engine in that the water/glycol coolant was kept in liquid form by pressure even though its temperature was allowed to exceed the normal boiling point. Heinkel's system took advantage of that fact and the cooling energy loss associated with the phase change of the coolant as it boils. Following is a description of what is known about the cooling system used in the final version of Heinkel's system. It is based entirely on careful study of surviving photographs of the He 100 since no detail plans survive. The earlier prototypes varied, but they were all eventually modified to something close to the final standard before they were exported to the Soviet Union.

Coolant exits the DB 601 at two points located at the front of the engine and at the base of each cylinder block casting immediately adjacent to the crank case. In the Heinkel system, an "S" shaped steel pipe took the coolant from each side of the engine to one of two steam separators mounted alongside the engine's reduction gear and immediately behind the propeller spinner. The separators, designed by engineers Jahn and Jahnke, accepted the water at about 110 degrees Celsius and 1.4 bar of pressure. The vertically mounted, tube shaped separators contained a centrifugal impeller at the top connected to an impeller-type scavenge pump at the bottom. The coolant was expanded through the upper impeller where it lost pressure, boiled and cooled. The by product was mostly very hot coolant and some steam. The liquid coolant was slung by the centrifugal impeller to the sides of the separator where it fell by gravity to the bottom of the unit. There, it was pumped to header tanks located in the leading edges of both wings by the scavenge pump. The presence of the scavenge pump was necessary to ensure the entire separator did not simply fill up with high pressure coolant coming from the engine.

Existing photographs of the engine bay of the final pre-production version of this system clearly show the liquid coolant from both separators was piped along the bottom left side of the engine compartment and into the right wing. The header tanks were located in the outer wing panels ahead of the main spar and immediately outboard of the main landing gear bays. The tanks extended over the same portion of the outer panel's span as the outer flaps. Coolant from the right wing header tank was pumped by a separate, electrical pump to the left wing header tank. Along the way from the right to left wing, the coolant passed through a conventional radiator mounted on the bottom of the fuselage. That radiator was retractable and intended for use only during ground running or slow speed flight. Nevertheless, coolant passed through it whenever the engine was running and regardless of whether it was extended or retracted. In the retracted position, the radiator offered little cooling, but some heat was exchanged into the aft fuselage. Finally, a return tube connected the left wing's header tank to that on the right. This allowed the coolant to equalize between the two header tanks and circulate through the retractable radiator. The engine drew coolant directly from both header tanks through two separate pipes that ran through the main landing gear bays, up the firewall at the back of the engine compartment and into the usual coolant intakes located at the top rear of the engine.

The steam collected in the separators was vented separately from the liquid coolant. The steam did not required mechanical pumping to do this, and the build up of pressure inside the separator was sufficient. The steam was piped down the lower right side of the engine bay and led into the open spaces between the upper and lower wing skins of the outer wing panels. There, it further expanded and condensed by cooling through the skins. The entire outer wing, both ahead of and behind the main spar, was used for this purpose covering that portion of the span containing the ailerons (the fuel was also carried entirely in the wings and occupied the areas behind the main spar in the center section and immediately ahead of the outboard flaps). The condensate was scavenged by electrically driven centrifugal pumps and fed to the header tanks. Sources indicate as many as 22 separate pumps were used for this, but it is not clear whether that number includes all of the pumps in the entire water and oil cooling systems or merely the number of pumps in the outer wing panels. The former is generally accepted.

Some sources state the outer wing panels used double skins top and bottom with the steam being ducted into a thin space between the outer and inner skins for cooling. A double skinned panel was used in the oil cooling system, but surviving photographs of the wings demonstrate they were conventionally single skinned, and the coolant was simply piped into the open spaces of the structure. Double skinning over such an extensive area would have made the aircraft unacceptably heavy. Furthermore, there was no access to the inner structure to repair damage, such as a bullet hole, from the inside as would be needed if the system used a double skin. A similar system was used by the earlier Supermarine Type 224. Contrary to assertions in some references, all of the He 100s that were built used the evaporative cooling system described above. A derivative of this system was also intended for a late war project based on the He 100, designated P.1076.

Unlike the cooling fluid, oil cannot be allowed to boil. This presented a particular problem with the Daimler-Benz DB 601 series of engines, because oil is sprayed against the bottom of the pistons resulting in a considerable amount of heat being transferred to the oil as opposed to the coolant. The He 100's oil cooling system was conceptually similar to the water cooling system in that vapor was generated using the heat of the oil and condensed back to liquid by surface cooling through the skins of the airframe. A heat exchanger was used to cool the oil by boiling ethyl alcohol. The oil itself was simply piped to and from this exchanger, which was apparently located in the aft fuselage. The alcohol vapor was piped into the fixed portions of the horizontal and vertical stabilizers and into a double skinned portion of the upper, aft fuselage behind the cockpit. This fuselage "turtle deck" panel was the only double skinned portion of the aircraft's cooling system. The use of a double skinned panel was possible here because the inside of panel was accessible in the event of repair. The retractable radiator below the fuselage was not used for the oil cooling system. Condensed alcohol was collected by a series of bellows pumps and returned to a single header tank that fed the heat exchanger. Some sources speculate that a small air intake located at the bottom front of the engine cowl was used for an auxiliary oil cooler. No such cooler was fitted, nor was there room for one at that point. This small inlet served simply to admit cool air into what was a very hot portion of the engine bay. Immediately above this vent were the two steam separators and immediately behind it were the hot coolant pipes coming from the separators.