Argentine Aircraft: Horten's Delta Wings

Museo Nacional de Aeronáutica
Aeronautics National Museum

Inauguration of the Reimar Horten Room

30 years after the physical disappearance of Doctor Ing Aer. Reimar Horten, the National Museum of Aeronautics pays tribute to this world-class genius, through the inauguration of a thematic Room in his honor. Dr Reimar Horten was part of the Military Aircraft Factory carrying out several innovative projects specializing in delta wings. In addition, he was a professor at the Aeronautical University Institute of Córdoba, a province in which he lived until his death.

REIMAR HORTEN'S FMA IA-37 AND HIS QUEST TO CROSS THE SOUND BARRIER

By Hernan Longoni



Reaching that distant date of October 1, 1954, which marked the historic first flight of the IA-37P full-scale wooden glider, towed into the skies, was the result of many decades of research and development. It's important to note that not all of this research took place within the confines of the IAME.

In fact, Dr. Reimar Horten, the mastermind behind this prototype, firmly believed that the state or a substantial investor should financially support the development of prototypes, owing to their high cost. Nonetheless, Dr. Horten had been privately conducting experiments, from a young age, on devices capable of achieving transonic and supersonic speeds.

Together with his brother Walter, they explored a wide range of flying wings and tailless aircraft, primarily due to the advantage of their low drag coefficient, which allowed for more efficient utilization of the engines available during that era.

This ingenious designer held a theory that aircraft evolve in tandem with the progress of technology and their historical context. Building upon their experiences with the HO-II, they concluded that to break the sound barrier (a considerable mystery at the time), the aircraft needed to possess a high aspect ratio and lack a tail, or more specifically, a horizontal empennage.

It wasn't long before they determined that the delta wing configuration was the most effective for achieving high speeds. This design reduced the aerodynamic drag's impact on flight performance, particularly in fast flight, where the induced component of aerodynamic drag was minimal.

These discoveries were put to the test with the HO-V and VII, eventually leading to the presentation of the HO-IX to German authorities. This aircraft boasted an arrow-shaped design, moderate elongation, and taper, and was equipped with a small turbine, marking a pivotal moment in aviation history.



Reaching the significant date of October 1, 1954, when the IA-37P full-scale wooden glider was towed into the skies for the very first time, was the result of decades of research and development. It's important to note that not all of this research was conducted under the banner of the IAME.

In fact, Dr. Reimar Horten, the brilliant mind behind the prototype, firmly believed that the state or a well-endowed investor should subsidize the development of prototypes due to their substantial costs. However, from a young age, Dr. Horten had been privately experimenting with devices capable of achieving transonic and supersonic speeds.

Together with his brother Walter, they delved into the realm of tailless aircraft, specifically flying wings, as these designs offered a lower drag coefficient, enabling more efficient utilization of the engines available at the time.

This visionary designer held the theory that aircraft are products of their time and thus their development is intimately tied to the era and the level of technology available. Building on their experiences with the HO-II, they concluded that to break the sound barrier (a formidable challenge in those days), an aircraft needed a high aspect ratio and a tailless configuration, or even better, a lack of a horizontal empennage.

It didn't take long for them to determine that the delta wing variant was the most effective for achieving high speeds. This design reduced the aerodynamic drag's impact on flight performance during high-speed flight since the induced component of aerodynamic drag was minimal.

Their findings culminated in the presentation of the HO-IX to German authorities. This aircraft featured a delta wing with moderate elongation and taper, equipped with a small turbine, marking a pivotal moment in the history of aviation.



Thus, in 1953, a series of tests were conducted using scale models for free flight. These models were launched over Lake San Roque from a platform employing elastic ropes, a design that I had a hand in developing. Launch speeds reached up to 200 km/h, and a significant number of tests were performed. These tests helped establish conditions similar to those found in wind tunnel experiments.

Stability assessments were carried out in the IAME pilot wind tunnel under the guidance of Engineer Jose Krasinski. He determined that "...unlike a straight wing, a swept wing's taper can, in certain cases, enhance stability during slow flight," as documented in IAME publications and reports.

Furthermore, assessments of the IA 37 were conducted in the pilot wind tunnel. In April and July 1954, Engineer J. Maggipinto conducted these tests using two different wing configurations. In September and November 1959, Engineer D. Villareal investigated the impacts on the three longitudinal components resulting from modifications to the nose and toe.



With the foundational work complete, which resulted in a 60-degree arrow, a symmetrical profile, and significant taper, we advanced to the next phase of the project in 1954. This phase involved the development of a scaled model of the IA-37, which was both unmanned and made of wood. This model was known as the IA-37P. In its initial configuration, it featured a cockpit where the pilot assumed a prone position. This unconventional arrangement was necessitated by the strong wing symmetry and the initial absence of a more conventional fuselage, unlike the HO-III, which had one.

The reason behind this design choice was the shock wave that occurred at transonic speeds. This shock wave impacted the turbine's air intake and had the potential to shut down the engine due to supersonic speeds affecting the ignition. To counter this, a conventional cabin was later installed. This cabin was positioned to divide the air intake, which was set at an elevated angle in relation to the direction of travel, and this served to decelerate the air entering the turbine.

In practical terms, for operational efficiency and combat purposes, the final prototype featured the latter cockpit configuration, with the original one being retained for testing.

In this manner, towed by a Junkers Ju.52 aircraft, the IA-37P took to the skies for the first time on October 1, 1954.



In 1955, the IAME began preparing a metal prototype, intended to be equipped with a Rolls Royce Derwent turbine. This prototype was envisioned for conducting autonomous flight tests (no longer requiring towing) and for undergoing static testing. The designer of the IA-37 saw it as a preliminary step, relegating it to a role as a prototype, with the definitive and supersonic model planned to be powered by two Rolls Royce Avon turbines.

The events of September 1955 represented a setback in the technological journey of the IAME. Despite these challenges, the DELTA WING program continued its forward progress.

This brings us to August 6, 1957, a significant date when Engineer Jose Elaskar presented a note to the Director of the Aerotechnical Institute. This note contained a report prepared by Dr. Horten himself, outlining the timelines, models, characteristics, and construction costs of the aircraft.



Two fundamental versions of the aircraft were in the planning stage. The first version was designed for studying and testing subsonic speeds, with the aim of laying the groundwork for a future pure supersonic delta aircraft.

The second version served a dual purpose: as a training aircraft for pilots transitioning to the new pure delta configuration and as a two-seat variant (in this case, without ejection seats) for training pilots in jet aircraft operations, as well as for conducting hunting and attack maneuvers at maximum diving speeds of 1100 km/h.

This aircraft featured a delta wing design with a 63.5-degree arrow, no dihedral or warping, a wing surface area of 48 square meters, and a thickness of 10% at 40% of the chord. It also included a central drift and a hydraulically retractable landing gear.

The plan involved equipping it with a Martin Baker D/Sk/2511 ejection seat and installing two 20 mm Hispano Switzerland 804 cannons on the cockpit floor. Furthermore, the generous width of the transparent surfaces allowed for the installation of reconnaissance cameras.



The aircraft had a wingspan of 10 meters, a length of 11 meters, a height of 2.45 meters, and a weight of 3000 kilograms, with a maximum takeoff weight of 4500 kilograms.

As previously mentioned, the initial design underwent a transformation, gradually evolving into its definitive form, which incorporated a conventional cabin. This aircraft was ultimately designated as the IA-37 "Delta Wing," following the nomenclature commonly used for aircraft designed by Engineer Horten.

Drawing upon the insights gained from flight tests of free flight and glider models, particularly the findings of Engineers Maccipinto and Krasinski in the wind tunnel, documented in Technical Note Number 26 from July 1954, the theoretical performance of the aircraft was determined. This performance assessment included a maximum speed of 800 km/h at an altitude of 8000 meters, a climb rate of 17 meters per second, a landing speed of 110 km/h, depending on the landing gear configuration (which will be discussed later), and a practical service ceiling of 11,600 meters. The latter figure was established because, at that altitude, the aircraft could still ascend at a rate of 2 meters per second, considered adequate for combat maneuvers.



As previously mentioned, the events of 1955 brought the project to a halt. However, by April 1956, a team of three engineers, under Horten's leadership, resumed work. In February 1957, they were joined by ten draftsmen, forming the project's drafting office.

With this expanded workforce, Horten submitted a request to the Director, seeking to add 150 workers. This workforce would be essential for producing both a finalized and motorized prototype and a cantilever static testing facility. He estimated that with this level of manpower, the entire project stage could be completed between January 1 and December 1, 1958.

It's important to note that this project, like many undertaken locally in general, required a substantial proportion of imported materials. These materials included the engine, gearbox, ejection seat, oxygen regulator, and various minor systems. Procuring these components was challenging given the global political context at the time.



In a report submitted by Dr. Horten to the DINFIA Directorate, he concludes that "if the 150 workers mentioned earlier, along with the engineers, can work diligently without interruptions throughout 1958, it would be feasible to conduct both static tests and the first flight within the same year."

Dr. Horten's theories regarding the landing gear and how an aircraft should profile during takeoffs and landings were integral to the model. This aspect of the project is underscored in a study dated March 11, 1960, authored by the Designer, which delves into the specifics of the landing gear.

It's worth noting that the original plan had slated the glider's maiden flights for 1958. However, due to development delays, this timeline was not met as originally intended.

The initial scheme of the first IA-37P prototype, the wooden glider, featured a cockpit configuration with the pilot in a horizontal position, with the chin resting on a fork. In this setup, the landing gear was fixed and configured as a tricycle, as evident in the diagrams and photographs from that period.

However, the construction of the second prototype, which was intended to be motorized (as mentioned earlier), saw modifications to the cockpit, shifting it to a more conventional position. This change was not an evolutionary development but was made with this configuration in mind from the outset.



This aircraft must have already had the final morphology, despite which the towed prototype maintained the fixed gear of its predecessor.
The landing gear calculations were carried out by Eng. Della Vedova, and the project by Eng. Heine.
In his analysis, Dr. Horten explains that fast airplanes with reduced aspect ratio, especially deltas, to obtain maximum lift must obtain an angle of attack between 35 to 38 degrees. Once this physical condition is achieved in takeoffs or landings, it results in a component of the turbine jet that counteracts part of the weight of the airplane, considerably reducing the wing loading.



Therefore, it is advisable to obtain a landing gear that tends to place the device on the ground at these angles, in order to reduce the takeoff and landing run. As an example, we mention that the report states that the takeoff run of the I.Ae-33 Pulqui II was 600 meters, while that projected for the IA-37 was 307 meters.
Despite what was said, Horten was aware that most delta configuration aircraft of the time did not respect this principle. In his comparative analysis, he uses aircraft such as the AVRO Arrow CF-105, the AVRO Vulcan – an aircraft in which, while imprisoned in England after the Second War, he had the opportunity to participate in its design – and the Convair B. -58 Hustler.



It was determined that to maximize the lift for the IA-37, the landing gear needed to create an angle of attack of 23 degrees on the ground. Achieving this required positioning the main landing gear away from the aircraft's center of gravity, leaning toward the tail.

This configuration, with a high track and a low center of gravity (1.5 meters from the ground), endowed the aircraft with optimal theoretical ground handling characteristics, even surpassing those of the I.Ae-33. In the report, a comparison was made with the Hustler in this regard. The entire theory was put to the test with the glider, and the results matched the expectations. The landing gear behaved as planned, and the aircraft displayed no inclination for corkscrewing, making approaches safer for the pilot.

The end

In 1956, budget constraints once again became a significant issue. Notably, in 1951, due to the inability to pursue both projects simultaneously and given its contemporaneous development with the I.Ae-33 Pulqui II, the IA-37 was overshadowed by the IA-33.

Furthermore, the notion that the IA-37 served as a preliminary step for the real personal interceptor (the twin-engine IA-48) diminished its prospects, as it lacked an engine capable of delivering the necessary thrust. In the case of the IA-37, initially designed for the Rolls Royce Derwent, the aircraft's purpose was primarily to serve as an operational transition and to gain valuable theoretical experience.

By 1960, the feasibility of the project was dwindling, as the extended years of development rendered it operationally unnecessary, especially given the obsolescence of the IA-48, which never came to fruition.



The shift towards economic self-sufficiency and the decline of the Peronist "third position" ideology led to the perception that investing in the design, production, and operation of aircraft using domestic technology was no longer necessary.

It's worth noting that by the time delta-winged aircraft like the IA-37 were being considered, the prototype of the famous Mirage III had already completed its first flight in 1956, and similar aircraft were being produced and flown in the United States. This trend was mirrored behind the so-called "iron curtain."

For a combination of technical, industrial, economic, and political reasons, the development of the IA-37 aircraft was abandoned. The IA-37 prototype was stored alongside other iconic examples of post-war local aeronautical development, such as the IA-38 Naranjero, I.Ae-27 Pulqui, IA-45 Querandi, and others. Ultimately, these aircraft fell into obscurity and were eventually scrapped.

The author would like to express gratitude to Aeronautical Engineers Carlos Paoletti and Gonzalo Rengel, as well as Mr. Diego Horten, whose contributions were essential for the completion of this work. The digital illustrations accompanying this document were created by Alejandro Klichowski.


* This note, in an edited version, was part of issue No. 615 of AEROESPACIO Magazine. We greatly appreciate your publication in that prestigious medium.
** This note was originally published on the Historia de Aviones blog under the title “IA-37, on the way to breaking the sound barrier”, in which you can find other very interesting articles by Dr. Hernan Longoni on aeronautical developments of the FMA and historical facts of the Argentine Air Force. From this blog we recommend the author's latest book, in PDF format and free to download: DINFIA IA-50 GUARANI: THE INDUSTRY THAT WAS
Colored Digital Chief Petty Officer GNA Gerardo Miguel Gimenez, Prosecretary corresponding member.