Tuesday, 12 April 2016

Horten Ho229 - How to reach performance and radar question.Part 2 - compiled by Cesar Winklemann

Horten Ho-229 V3 captured by the United States Army, 1950
In his later life, Reimar Horten promoted the idea that the Horten Ho 229 V3 was intended to be built as a stealth aircraft, which would have placed this jet’s design several decades ahead of its time. Reimar Horten claimed that he wanted to add charcoal to the adhesive layers of the plywood skin of the production model to render it invisible to radar, because the charcoal “should diffuse radar beams, and make the aircraft invisible on radar” (Horten and Selinger 1983). This statement was published in his 1983 co-authored book Nurflügel (which translates as “only the wing”). While this statement refers to the never-made production model, it seems possible that the experimental charcoal addition could have been used on the Horten Ho 229 V3 prototype. The mere mention of early stealth technology sparked the imagination of aircraft enthusiasts across the world and spurred vibrant debate within the aviation community.

The British radar station
Horten brothers: Walter (left) and Reimar (right)
In 1983, Reimar Horten wrote in Nurflügel that he had planned to combine a mixture of sawdust, charcoal, and glue between the layers of wood that formed large areas of the exterior surface of the Ho 229 V3 production model. This was done to shield, he said, the "whole airplane" from radar as "the charcoal should absorb the electrical waves. Under this shield, then also the tubular steel [airframe] and the engines [would be] invisible [to radar]" (Horten and Selinger 1983, Russ Lee translation). While this statement refers to the never-made production model, it seems possible that the experimental charcoal addition could have been used on the Horten Ho 229 V3 prototype.
In 1943 the all-wing Horten 229 promised spectacular performance and the Luftwaffe (German Air Force) chief, Hermann Göring, allocated half-a-million Reich Marks to the brothers Reimar and Walter Horten to build and fly several prototypes. Numerous technical problems beset this unique design and the only powered example crashed after several test flights but the airplane remains one of the most unusual combat aircraft tested during World War II. Horten used Roman numerals to identify his designs and he followed the German aircraft industry practice of using 'Versuch,' literally test or experiment, numbers to describe pre-production prototypes built to test and develop a new design into a production airplane. The Horten IX design became the Horten Ho 229 aircraft program after Göring granted the project official status in 1943 and the technical office of the Reichsluftfahrtministerium assigned to it the design number 229. This is also the nomenclature used in official German documents.
The idea for the Horten IX grew first in the mind of Walter Horten when he was serving in the Luftwaffe as a fighter pilot engaged in combat in 1940 during the Battle of Britain. Horten was the technical officer for Jadgeschwader (fighter squadron) 26 stationed in France. The nature of the battle and the tactics employed by the Germans spotlighted the design deficiencies of the Messerschmitt Bf 109, Germany's most advanced fighter airplane at that time. The Luftwaffe pilots had to fly across the English Channel or the North Sea to fulfill their missions of escorting German bombers and attacking British fighters, and Horten watched his unit lose many men over hostile territory at the very limit of the airplane's combat radius. Often after just a few minutes flying in combat, the Germans frequently had to turn back to their bases or run out of fuel and this lack of endurance severely limited their effectiveness. The Messerschmitt was also vulnerable because it had just a single engine. One bullet could puncture almost any part of the cooling system and when this happened, the engine could continue to function for only a few minutes before it overheated and seized up.
Walter Horten came to believe that the Luftwaffe needed a new fighter designed with performance superior to the Supermarine Spitfire, Britain's most advanced fighter. The new airplane required sufficient range to fly to England, loiter for a useful length of time and engage in combat, and then return safely to occupied Europe. He understood that only a twin-engine aircraft could give pilots a reasonable chance of returning with substantial battle damage or even the loss of one engine.
Since 1933, and interrupted only by military service, Walter and Reimar had experimented with all-wing aircraft. With Walter's help, Reimar had used his skills as a mathematician and designer to overcome many of the limitations of this exotic configuration. Walter believed that Reimar could design an all-wing fighter with significantly better combat performance than the Spitfire. The new fighter needed a powerful, robust propulsion system to give the airplane great speed but also one that could absorb damage and continue to function.
The Nazis had begun developing rocket, pulse-jet, and jet turbine configurations by 1940 and Walter's role as squadron technical officer gave him access to information about these advanced programs. He soon concluded that if his brother could design a fighter propelled by two small and powerful engines and unencumbered by a fuselage or tail, very high performance was possible.

At the end of 1940, Walter shared his thoughts on the all-wing fighter with Reimar who fully agreed with his brother's assessment and immediately set to work on the new fighter. Fiercely independent and lacking the proper intellectual credentials, Reimar worked at some distance from the mainstream German aeronautical community. At the start of his career, he was denied access to wind tunnels due to the cost but also because of his young age and lack of education, so he tested his ideas using models and piloted aircraft. By the time the war began, Reimar actually preferred to develop his ideas by building and testing full-size aircraft. The brothers had already successfully flown more than 20 aircraft by 1941 but the new jet wing would be heavier and faster than any previous Horten design. To minimize the risk of experimenting with such an advanced aircraft, Reimar built and tested several interim designs, each one moderately faster, heavier, or more advanced in some significant way than the one before it.
The Junkers Jumo 004 jet engine
Reimar built the Horten VC and VC to evaluate the all-wing layout when powered by twin engines driving pusher propellers. He began in 1941 to consider fitting the Dietrich-Argus pulse jet motor to the Horten V but this engine had drawbacks and in the first month of 1942, Walter gave his brother dimension-ed drawings and graphs that charted the performance curves of the new Junkers 004 jet turbine engine [this engine was also fitted to these NASM aircraft: Messerschmitt Me 262, Arado Ar 234, and the Heinkel He 162]. Later that year, Reimar flew a new design called the Horten VII that was similar to the Horten V but larger and equipped with more powerful reciprocating engines. The Horten VI ultra-high performance sailplane also figured into the preliminary aerodynamic design of the jet flying wing after Reimar tested this aircraft with a special center section.

Jumo 004C engines mounted in the internal structure of |Horten Ho229
Walter used his personal connections with important officials to keep the idea of the jet wing alive in the early stages of its development. General Ernst Udet, Chief of Luftwaffe Procurement and Supply and head of the Technical Office was the man who protected this idea and followed this idea for the all-wing fighter for almost a year until Udet took his own life in November 1941. At the beginning of 1943, Walter heard Göring complain that Germany was fielding 17 different types of twin-engine military airplanes with similar, and rather mediocre, performance but parts were not interchangeable between any two designs. He decreed that henceforth he would not approve for production another new twin-engine airplane unless it could carry 1,000 kg (2,210 lb) of bombs to a 'penetration depth' of 1,000 km (620 miles, penetration depth defined as 1/3 the range) at a speed of 1,000 km/h (620 mph). Asked to comment, Reimar announced that only a warplane equipped with jet engines had a chance to meet those requirements.
In August Reimar submitted a short summary of an all-wing design that came close to achieving Göring's specifications. He issued the brothers a contract, and then demanded the new aircraft fly in 3 months. Reimar responded that the first Horten IX prototype could fly in six months and Göring accepted this schedule after revealing his desperation to get the new fighter in the air with all possible speed. Reimar believed that he had boosted the Reichsmarschall's confidence in his work after he told him that his all-wing jet bomber was based on data obtained from bone fife flight tests with piloted aircraft.
Official support had now been granted to the first all-wing Horten airplane designed specifically for military applications but the jet bomber that the Horten brothers began to design was much different from the all-wing pure fighter that Walter had envisioned nearly four years earlier as the answer to the Luftwaffe's needs for a long-range interceptor. Henceforth, the official designation for airplanes based on the Horten IX design changed to Horten Ho 229 suffixed with 'Versuch' numbers to designate the various prototypes.

All versions of the Ho 229 resembled each other in overall layout. Reimar swept each half of the
wing 32 degrees in an unbroken line from the nose to the start of each wingtip where he turned the leading edge to meet the wing trailing edge in a graceful and gradually tightening curve. There was no fuselage, no vertical or horizontal tail, and with landing gear stowed (the main landing gear was fixed but the nose wheel retracted on the first prototype Ho 229 V1), the upper and lower surface of the wing stretched smooth from wingtip to wingtip, unbroken by any control surface or other protuberance. Horten mounted elevens (control surfaces that combined the actions of elevators and ailerons ) to the trailing edge and spoilers at the wingtips for controlling pitch and roll, and he installed drag rudders next to the spoilers to help control the wing about the yaw axis. He also mounted flaps and a speed brake to help slow the wing and control its rate and angle of descent. When not in use, all control surfaces either lay concealed inside the wing or trailed from its aft edge. Parasite or form drag was virtually nonexistent. The only drag this aircraft produced was the inevitable by-product of the wing's lift.

Few aircraft before the Horten Ho229 or after it have matched the purity and simplicity of its aerodynamic form but whether this achievement would have led to a successful and practical combat aircraft remains an open question.

The structure of playwood used to build Horten Ho229
Building on knowledge gained by flying the Horten V and VII, Reimar designed and built a manned glider called the Horten 229 V1 which test pilot Heinz Schiedhauer first flew 28 February 1944. This aircraft suffered several minor accidents but a number of pilots flew the wing during the following months of testing at Oranienburg and most commented favorably on its performance and handling qualities. Reimar used the experience gained with this glider to design and build the jet-propelled Ho 229 V2.

Wood is an unorthodox material from which to construct a jet aircraft and the Horten brothers preferred aluminum but in addition to the lack of metalworking skills among their team of crafts persons, several factors worked against using the metal to build their first jet-propelled wing. Reimar's calculations showed that he would need to convert much of the wing's interior volume into space for fuel if he hoped to come close to meeting Göring's requirement for a penetration depth of 1,000 km. Reimar must have lacked either the expertise or the special sealants to manufacture such a 'wet' wing from metal. Whatever the reason, he believed that an aluminum wing was unsuitable for this task. Another factory in Reimar's choice of wood is rather startling: he believed that he needed to keep the wing's radar cross-section as low as possible. "We wished", he said many years later, "to have the [Ho 229] plane that would not reflect [radar signals]", and Horten believed he could meet this requirement more easily with wood than metal. Many questions about this aspect of the Ho 229 design remain unanswered and no test data is available to document Horten's work in this area. The fragmentary information that is currently
available comes entirely from anecdotal accounts that have surfaced well after World War II ended.
Range of British radar stations