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THK-13: Challenge Faced by the Turkish Aeronautical Association, and the Reality

Issue 10 - 2022
THK-13: Challenge Faced by the Turkish Aeronautical Association, and the Reality

Known as “stealth” in engineering terminology, this aircraft feature is an essential part of the fifth-generation fighter class in the 21st century. The first models that come to mind when talking about “stealth aircraft” are undoubtedly the F-117 of the U.S. Lockheed Martin, the maiden flight of which took place in 1981, and the B-2 aircraft of the U.S. Northrop Grumman that first flew in 1989. The partial similarity of THK-13, developed by the Turkish Aeronautical Association (THK) with the B-2 caused serious information pollution in the Turkish media and led to some myths that are irrelevant to reality. In order to eliminate such conceptual confusion, it is necessary to shed light on the technical details.

As per the official studies made, no object can be made entirely invisible. The radar cross-sectional area of every conventional aircraft appears on the screen when the radar wave is sent to the aircraft and then captured by the radar after hitting the aircraft surface and returning to radar. With the “stealth” method, electromagnetic waves hitting the aircraft surface are tried to be prevented from being manipulated and transmitted to the radar. In addition, activities are carried out in other areas such as the aircraft geometry enabling wave dispersion, the integration of the wave absorbing or dispersing materials into the body, the minimization of the acoustic projection, the reduction of the infrared signatures from the hot engine parts. To summarize, the “ghost plane” term used in the popular media is technically based on the "low visibility" concept.

Due to the low radar cross section, the concept’s (defined as flying wing in the past) fit for "stealth" design was also elaborated. Accordingly, the history of flying wing design should also be examined in terms of low visibility studies in aviation. The first known idea of flying wings dates back to the 19th century. The German zoologist and physicist Friedrich Ahlborn discovered the aerodynamic properties of the plant Alsomitra macrocarpa, known as the gliding seed, and dealt with the subject in detail in his book "Über die Stabilität der Flugapparate" published in 1897. Austrian pilot-engineer Ignaz Etrich also put this idea into practice. Etrich, with his engineer partner Franz Wels, succeeded to fly the first model of the patent (no: 23465) named "Flugmaschine" on November 29, 1909, which he obtained from the Austrian Patent Office on March 3, 1905. Interestingly, the first aircraft used in the history of world warfare was this flying wing named "Etrich-Rumpler-Taube", which was used by Italian pilot Giulio Gavotti during the Italian invasion of Libya in 1911 and dropped bombs on Turkish troops.

The related feature of Alsomitra macrocarpa attracted the attention of German engineer and aircraft designer Hugo Junkers, and he applied to the German Imperial Patent Office on February 1, 1910 with his own design. Junkers patent 253788 was approved on November 14, 1912. From the 1920s, tailless designs started to become popular. At the Soviet side, engineers such as Boris Cheranovski and Vladimir Chizhevski carried out activities on motor gliders. The BICh-26 model, designed by Cheranovski in 1948, was claimed to be the world's first supersonic flying wing aircraft. One of the most important projects of the world-famous Ukrainian aircraft designer Oleg Antonov, when he was the chief designer of the Moscow Glider Factory, was the tail-supported flying wing, named LEM-2, designed in 1937.

During World War II, three important names came to the fore in Germany working on this subject: Alexander Lippisch, the pioneer of the delta wing, which was also used in the Concorde model, and Reimar and Walter Horten, aka Horten brothers, who were considered to have laid the foundations of modern low-visibility aircraft. Lippisch's idea of a triangular delta wing dated back to 16th-17th century. At that time, it was determined that German engineer Conrad Haas and Polish general Kazimierz Siemienowicz used rockets with delta wing stabilizer elements. The world's first jet-propelled flying wing Ho IX / Ho 229, designed as a fighter and bomber, was the improved version of the Horten IV glider model previously designed by the Horten brothers. Before Ho 229 was mass-produced, the German government surrendered, and the war ended.

At the American side, Jack Northrop applied for a patent for the N-1M flying wing with the name "Design for an Airplane" on November 25, 1941, and this design was patented with the no D-143.853 on February 12, 1946, almost five years later. The Americans, not showing interest in the flying wing concept during the war, changed their minds after 1945. Theodore von Kármán, Chairman of the Scientific Advisory Board of the American Air Force, and his team, who were appointed to examine the strategic industrial facilities in Germany, after its defeat in the war, had a critical meeting with Alexander Lippisch, a German engineer who surrendered to the Allied forces. Simultaneously, the Horten brothers' workshops were also seized by the American soldiers. In the aftermath, interest in flying wing design exploded. The development of the YB-35 (1946) and YB-49 (1947) models, considered to be the ancestors of the Northrop B-2 bomber, coincided with this period. Lippisch was one of the German scientists abducted by the Allied forces at the Operation Overcast but later renamed Operation Paperclip, and in the following years he took charge at Convair and Douglas companies and lectured at the University of Iowa. Many German engineers who were brought to the USA like him, strengthened the Americans' hand against the Soviet Union in the technical field in the post-war period.

During Cold War, something unusual happened. Soviet physicist Pyotr Ufimtsev demonstrated that the radar projection was related to the surface and edges of the planes in his physical theory of diffraction published in 1962, using the edge wave method. Nonetheless this idea did not attract attention at the Soviet military side. When the Americans, following the scientific studies of their rivals, translated this work from Russian into English with the title of "Method of Edge Waves in the Physical Theory of Diffraction" in 1971 and analyzed it, they realized that the proposed solution they were looking for was put forward by Ufimtsev years ago. Ufimtsev developed a mathematical model that the degree of reflection to the radar would decrease if electromagnetic waves were scattered in other directions. Based on this idea, the Americans focused on sharp-edged designs and developed the F-117 Nighthawk, starting with the Have Blue project. The B-2 bomber model, introduced after the F-117, was produced using similar methods. Ufimtsev, on the other hand, moved to the USA with the invitation from the California Institute of Technology after the collapse of the Soviet Union and continued his studies there.

As it can be seen, a plant species that attracted the attention of a German zoologist at the end of the 19th century inspired the world of technology under the name of flying wing and became a part of the aviation industry especially between 1910-1945. The low visibility in the fifth-generation combat aircraft developed in the 21st century, including Turkey, is tried to be achieved not with the flying wing, but with the methods such as the sharp-edged geometry explained above. In other words, the equivalent of today's stealth technology is not just a flying wing.

When looking at the Turkish side, studies on the flying wing were initiated within the Turkish Aeronautical Association (THK) started in 1947, after World War II. The prototype of THK-13 first appeared on the local media on Saturday, April 10, 1948. Ankara-based Ulus newspaper shared the first image of this design with the headline "Tests of the flying wing model conducted yesterday" on the first page. In the text of the news, it was also stated that the American company Northrop produced a 9-crew flying wing model that could reach a speed of 400 kilometers per hour. This information was obtained from THK engineers during the interview. This naturally shows that Turkish engineers are aware of the equivalent designs developed previously. In short, when we look at the first newspaper article about THK-13, it is immediately understood that the "The Americans built the million-dollar Northrop B-2 stealth bomber after inspired by the Turks" and similar headlines are incorrect. We also need to state that the THK-13 was not a stealth aircraft and not developed for this purpose. The THK-13 can be defined as an experimental glider with a flying wing design.

The first flight test of the Turkish type flying wing THK-13 was made on August 20, 1948.1  THK-13, attached to the back of an off-road vehicle, proved that it could take off by making short 10-meter bounces. However, according to the statement of pilot Kadri Kavukçu, the vertical rudder control and aileron failed to show the expected performance. The short bounce trials continued in the following days, and the glider was able to reach an altitude of 300 meters in the last attempt on August 25, 1948, and after a short glide, it landed again with an S movement. In the report of Pilot Kavukçu dated August 25th, it was repeatedly stated that there was a problem with the controllers.

A long flight over Ankara was planned for the following day. The glider took off with a tow plane used by pilot Bahaeddin İdemen and while it was hovering over Çankaya skies, an unexpected event occurred near the Presidential Palace. The hook connecting the glider to the towing plane broke for an unknown reason and the pilot Kavukçu wanted to land the THK-13 at a suitable spot. While descending around Seyranbağları, the nose of the glider was crushed and one of its wheels was broken. After the THK engineers arrived at the scene and made the necessary repair, it took off again. After the second attempt on the same day, THK-13 made a forced landing because of an off-road vehicle that suddenly came across while it was gliding at low altitude around the Armored Corps School. When the engineers went to inspect for the second time, they witnessed that both of their vertical rudders were broken, and the wings and wheels were seriously damaged. Pilot Kadri Kavukçu was injured in his head and ankles, but in stable condition, and he recovered in a short time.

Such accidents were reported impartially in most of the local newspapers in the following days, but the news in Yeni Sabah newspaper dated September 5, 1948, intended to spread the message that Turkish Aeronautical Association (THK) wasted money on unnecessary projects. It was stated that while the country needed military and passenger planes, dealing with flying wing was an extravagant expense, there were no necessary infrastructure facilities for tests, there were technical errors in the design and such errors were not fixed despite the warnings of the pilots. Thereupon, on September 16, 1948, a disclaimer was published in the newspaper Yeni Sabah with the signature of the THK President, General Seyfi Düzgören. Düzgören firmly stated that he trusted his engineers and pilots, that false news should be disregarded, and that THK would continue its activities. An interview of an anonymous THK employee was published in the Kudret newspaper dated September 9, 1948. According to the statements of chief engineer Yavuz Kansu, some information was leaked to the press by Bahaeddin İdemen, one of the THK pilots, or someone else from the association.

The damaged THK-13 was brought to the factory and overhauled for weeks, and on September 29, 1948, bounce tests were restarted. The pilot of THK-13 this time was Cemal Uygun. After the successful tests, it was decided to make another flight attempt in the evening of the same day with a tow plane used by the pilot Cemalettin Aytaş. When they took off one after the other, the glider started to pull to the right and suddenly hit the runway on the nose. Pilot Uygun suffered a head injury and fortunately survived. THK-13, on the other hand, was shattered into pieces. Following the examinations, the pilot Cemal Uygun explained the problem as follows: A clamp was forgotten in the right wing, this piece was displaced during the take-off and the glider tilted to the right due to a sudden weight shift. In short, the THK-13 crashed because of foreign object damage.

After the accident, THK Aircraft Factory Manager Selahattin Beler wrote a letter to the American Northrop Aircraft Inc. to take counsel and ask for their examination. The reply received on December 15, 1948, was quite plain and diplomatic. Writing the letter on behalf of Jack Northrop, Engineering Director Walter J. Cerny stated that they admired the design of the THK-13, liked the mechanism in the landing gear and appreciated THK’s forward-thinking ideas. As it is understood from the letter, the main questions of THK remained unanswered and they only sent a brochure. It was also added that the technical drawings and reports sent by THK were put in Northrop's institutional archive.

Although the construction of a new THK-13 started after the second accident, no official approval was received from THK Headquarters. The production of the second glider was concluded in August 1949 and this time the errors detected in the first flights were corrected. THK-13's chief engineer, Yavuz Kansu, noted that the second glider could not fly due to the unavailability of a tow plane. There is no other information about the project in official written sources.

THK-13 is an innovative project that was developed by the THK team after examining German and American flying wings in the years of economic difficulties in the post-war period. Since the technical training and R&D understanding had not yet been achieved in Turkey at that time, the problems faced in the testing process were reported incorrectly in the press and described as a "waste of money, a luxury project" in a newspaper. After the death of THK President General Seyfi Düzgören on 28 December 1948, who defended the engineers and pilots against the press, no project defender remained at the senior management level.

The story of THK-13 reveals how serious the testing stages in engineering are and that such processes must be accomplished with determination in order to achieve progress. Technical negligence before the second test flight caused foreign object damage and a serious accident. The reports prepared by the pilots after the tests clearly indicate that the technical problems in the control and rudder mechanism of the glider could not be resolved2  and such problems were under the responsibility of the engineering team. This process that could be accomplished after repeated tests with the healthy communication of the pilots and the technical team could not be sustained due to the inability to conduct new tests.

The management mistake of THK when establishing the factories and the suspension of aircraft order from THK in line with the decisions taken by the government adversely affected the institution, and the factories were transferred to the state at THK's own request. Consequently, the first Turkish type flying wing THK-13 project, unfortunately, could not persist due to the turbulent period and remained an idle venture 

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