Horten Ho 229 Flying Wing

In the last months of World War Two, Nazi Germany tested an experimental fighter more spaceship than aircraft. Only now are we realizing how inspired it was, The future looks of the Horten Ho 229, one of aviation’s most futuristic designs.

In December, US aircraft maker Northrop Grumman unveiled a revolutionary design for a future fighter aircraft that could, theoretically, fly over the war zones of the coming century.

Their concept looks more like a flying saucer than a fighter plane – it is what aviation experts call a ‘flying wing’, a design which ditches the traditional tail fin at the back. This design helps reduce the aircraft’s size, and creates a smoother shape – one less likely to bounce back radar signals being sent out to detect it.

Northrop Grumman's concept for a flying wing fighter has similarities to the Hortens' innovative design (Credit: Northrop Grumman)

It looks about as futuristic as fighter aircraft can get, but its genesis goes far further back than you think – to a truly groundbreaking jet fighter design built and flown in Nazi Germany in the dying days of World War Two.

That aircraft – the Horten Ho 229 – might be a footnote in aviation history, but it was so far ahead of its time that its aerodynamic secrets are still not completely understood. In fact, there’s a chief scientist at Nasa still working to discover just how its creators managed to overcome the considerable aerodynamic challenges that should have made it unflyable.

The Ho 229's design was incredibly advanced for its time (Credit: Malyszkz/Wikipedia/)

The ‘flying wing’ design isn’t an everyday sight in our skies because it’s incredibly hard to make work. By getting rid of the tail – which helps keep the aircraft stable and stops it ‘yawing’ from side to side – the aircraft is a lot harder to control. So why would you try to build something that was inherently difficult to fly?

If you can make a flying wing work, it has several benefits. The resulting plane becomes difficult to spot on radar, partly because it has no tail fins that will bounce back radar waves. The smooth shape also means the aircraft has as little drag as possible, which means it can be lighter and more fuel-efficient, and possibly fly faster than a more conventionally shaped aircraft using the same engine.

The Hortens developed their flying wing approach with increasingly effective results

All of that looks good on paper – but getting it to work in the real world is a lot more difficult. Flying wings have proved to be a headache for aircraft designers stretching back almost to the time of the Wright Brothers. All of which makes the achievements of the German Horten brothers so impressive.

The Hortens – Walter and Reimar – began designing aircraft in the early 1930s, while Germany was officially banned from having an air force under the constraints of the Treaty of Versailles following World War One. The brothers had joined sporting air clubs, set up as a way to get around such restrictions, and which were a foundation for what could become Nazi Germany’s air force, the Luftwaffe.

Many of the amateur aviators who would later become Luftwaffe pilots cut their teeth flying various gliders and ‘sailplanes’, unpowered aircraft which taught them the rudiments of flying. The Horten brothers combined flying with designing aircraft as well – turning the family’s lounge-room into a workshop to work on new designs, according to the aviation website Aerostories.

New fighter

The pair followed some of the esoteric ideas of unconventional aircraft designer Alexander Lippisch, who was a pioneer of delta-wing aircraft designs; another radical form that came into its own once jet engines had been developed. The Hortens developed their flying wing approach with increasingly effective results, ending in their Horten Ho IV glider, in which the pilot lay prone in the aircraft, which meant the cockpit canopy didn’t jut so far out from the fuselage and create aerodynamic drag.

By the time the Ho IV glider was being tested, Walter Horten had already served as a Luftwaffe fighter pilot during the Battle of Britain. Russ Lee, a curator at the Smithsonian Air and Space Museum in Washington DC, says this was a turning point. “The Germans, of course, lost the Battle of Britain, and Walter realised that Germany needed a new kind of fighter aircraft. And an all-wing aircraft might make that good new fighter.”

At the same time, the head of the Luftwaffe, Hermann Goring, had requested designs in a project called ‘3x1000’ – aircraft that would be able to carry a 1,000kg (2,200lb) bombload 1,000 miles (1,600 kilometres) at 1,000km/h (625mph). That led the Hortens to develop what would eventually become the Ho 229 prototypes. The first of the three prototypes was an unpowered glider, built to test the aerodynamic design. The second added jet engines, and flew successfully on 2 February 1945, though it crashed after engine failure on another test flight a few weeks later, killing its test pilot. But the tests proved, says Lee, that the aircraft could take off, cruise and land, and the aircraft’s basic design was sound.

The prototype Ho 229 is currently undergoing restoration (Credit: BrettC23/Wikipedia/CC BY-SA 4.0)

Lee has a good reason to know the Ho 229 backstory so well; he’s responsible for preserving and restoring the only other Ho 229 to have been built, the third, partially completed prototype, known as the Ho 229 V3. It was taken – like many other examples of cutting-edge German aircraft design – to the US after World War Two. Along the way, it spent a brief time at the British testing facility at Farnborough, near London.

“The word revolutionary is not inappropriate when you’re talking about the Ho 229,” says Lee. “The Hortens were more advanced in this area than anyone else in the world.”

The Northrop B-2, the aircraft that is at the forefront of the US nuclear deterrent, looks at first glance like an obvious descendent of the Hortens’ design genius. So much so, that some commentators described the Ho 229 as the “world’s first stealth bomber” – though its role would have been to shoot down the fleets of Allied bombers that were attacking German industrial targets and cities.

One of the hardest things is getting an aircraft without a tail to be flyable during a stall – Russ Lee, Smithsonian Air and Space

“Just getting one of these things to fly, well you had to make the wing do all the work, and end up with a plane that behaved as well as a conventional plane with a tail.”

Besides the tendency to “yaw” side to side at the best of times, a tailless plane can become virtually uncontrollable when the engine cuts out. “One of the big things with this aircraft was its stability in flight. One of the hardest things is getting an aircraft without a tail to be able to be flyable during a stall, and that’s something every aircraft has to be able to complete,” says Lee.

The Hortens were able to keep their aircraft stable by making the wing long and thin (known as a high aspect ratio wing). This spread the weight of the aircraft over a greater surface area, and also decreases the proportion of air that creates a vortex around the wing – a mini whirlwind that creates drag – slowing the aircraft down.

Radical shape

Reimar Horten may not have been fully aware that he was solving these two crucial aerodynamic problems in one fell swoop. That’s what Al Bowers, a Nasa chief scientist at the Neil A Armstrong Flight Research Center in California believes. Bowers has been testing the Hortens’ design principles for many years. Bowers says Reimar Horten’s genius was in using a ‘bell-shaped’ wing to cancel out the yawing issues an aircraft without a tail usually suffers, but which also reduced drag.

The Ho 229’s wings are radically different to the kind of elliptical shape which was thought to generate the most lift and reduce drag (look at the wing of a World War Two Spitfire fighter below, an example of a conventional elliptical wing).

In the 1940s, most aircraft had elliptical wings, like the Spitfire seen here (Credit: iStock)

Horten’s wing design echoed the principles of another pioneering German designer, Ludwig Prandtl, who was the first aerodynamic scientist to stress that the shape of a wingtip could massively affect an aircraft’s flying ability. He’d also come up with the bell-shaped wing in the early 1930s but had done so to reduce drag, not realising that it would also solve the yawing problems in a tailless aircraft. The wing, in many ways, is doing what a bird’s wing does in flight; evolution hasn’t yet felt the need to put an upright tail on a bird, after all.

“The Ho 229 was decades ahead of its time,” says Bowers. “I believe it will be shown as the progenitor of the future of aviation.”

Flying wing designs gained some credence in the 1950s, mostly due to the efforts of Jack Northrop, who had been inspired by seeing some of the Horten’s sports gliders in the 1930s. The captured Ho 229 may also have encouraged him. Northrop’s unsuccessful YB-35 flying wing bomber design of the late 1940s, was hamstrung by massive vibration problems caused by the propeller-driven engines, showing that the Hortens were right to have used jets in the Ho 229. Northrop’s later jet-propelled YB-49 design used jet engines, and while it never went into service, it paved the way for the company’s B-2 Spirit stealth bomber decades later, a design which certainly shares some physical similarities with the Ho 229.

Reimar Horten was on the right track. He never saw the full potential of his ideas – Al Bowers, Nasa

Bowers has been using the principles in the Ho 229 and from Prandtl’s earlier experiments into a Nasa design, the Prandtl-D flying wing concept, an unmanned flying wing design that could one day be used to explore Mars.

The Prandtl-D would be used on Martian research missions, possibly launched from a high-altitude glider, flying under its own power for some 10 minutes before gliding down to land on the planet’s surface. The Prandtl-D won’t be anywhere as big as the Ho 229 however – it’s expected to have a wingspan of only 2ft and weigh little more than 1.3kg (3lb).

The Ho 229's design has influenced a Nasa project for a small flying wing which could explore Mars (Credit: Tom Tschida/Nasa)

“We believe that Prandtl’s solution (and Horten’s) is the answer we’ve been looking for all along,” says Bowers. “It explains so many things about the flight of birds, and minimising drag, and superior efficiency possible in future aircraft. It is my belief that we can improve aircraft efficiency by at least 70%. And my own work is just a scratch of the surface. Reimar Horten was on the right track. He never saw the full potential of his ideas. Yet I suspect if he could see where we are today, he would be pleased. Perhaps not so pleased by the pace of our progress, but that we are finally listening.”

As for the Smithsonian’s example of this inspired design? Lee says the work to preserve this pioneering design is gradual and painstaking, and unlikely to be finished until the early 2020s. Then, this inspiring, overlooked design will be on public display – and the Hortens’ aerodynamic genius can be appreciated by a wider audience.

The Horten H.IX, RLM designation Ho 229 (or Gotha Go 229 for extensive re-design work done by Gotha to prepare the aircraft for mass production) was a German prototype fighter/bomber initially designed by Reimar and Walter Horten to be built by Gothaer Waggonfabrik late in World War II. It was the first flying wing to be powered by jet engines.^[1]

The design was a response to Hermann Göring's call for light bomber designs capable of meeting the "3×1000" requirement; namely to carry 1,000 kilograms (2,200 lb) of bombs a distance of 1,000 kilometres (620 mi) with a speed of 1,000 kilometres per hour (620 mph). Only jets could provide the speed, but these were extremely fuel-hungry, so considerable effort had to be made to meet the range requirement. Based on a flying wing, the Ho 229 lacked all extraneous control surfaces to lower drag. It was the only design to come even close to the 3×1000 requirements and received Göring's approval. Its ceiling was 15,000 metres (49,000 ft).^[2]

Design and development

In the early 1930s, the Horten brothers had become interested in the flying wing design as a method of improving the performance of gliders. The German government was funding glider clubs at the time because production of military and even motorized aircraft was forbidden by the Treaty of Versailles after World War I. The flying wing layout removed the need for a tail and associated control surfaces and theoretically offered the lowest possible weight, using wings that were relatively short and sturdy, and without the added drag of the fuselage. The result was the Horten H.IV.^[3]

In 1943, Reichsmarschall Göring issued a request for design proposals to produce a bomber that was capable of carrying a 1,000 kilograms (2,200 lb) load over 1,000 kilometres (620 mi) at 1,000 kilometres per hour (620 mph); the so-called "3×1000 project". Conventional German bombers could reach Allied command centers in Great Britain, but were suffering devastating losses from Allied fighters.^[3] At the time, there was no way to meet these goals—the new Junkers Jumo 004B turbojets could provide the required speed, but had excessive fuel consumption.

The Hortens concluded that the low-drag flying wing design could meet all of the goals: by reducing the drag, cruise power could be lowered to the point where the range requirement could be met. They put forward their private project, the H.IX, as the basis for the bomber. The Government Air Ministry (Reichsluftfahrtministerium) approved the Horten proposal, but ordered the addition of two 30 mm cannons, as they felt the aircraft would also be useful as a fighter due to its estimated top speed being significantly higher than that of any Allied aircraft.

The H.IX was of mixed construction, with the center pod made from welded steel tubing and wing spars built from wood. The wings were made from two thin, carbon-impregnated plywood panels glued together with a charcoal and sawdust mixture. The wing had a single main spar, penetrated by the jet engine inlets, and a secondary spar used for attaching the elevons. It was designed with a 7g load factor and a 1.8× safety rating; therefore, the aircraft had a 12.6g ultimate load rating. The wing's chord/thickness ratio ranged from 15% at the root to 8% at the wingtips.^[1] The aircraft utilized retractable tricycle landing gear, with the nosegear on the first two prototypes sourced from a He 177's tailwheel system, with the third prototype using an He 177A main gear wheelrim and tire on its custom-designed nosegear strutwork and wheel fork. A drogue parachute slowed the aircraft upon landing. The pilot sat on a primitive ejection seat. A special pressure suit was developed by Dräger. The aircraft was originally designed for the BMW 003 jet engine, but that engine was not quite ready, and the Junkers Jumo 004 engine was substituted.^[1]

Control was achieved with elevons and spoilers. The control system included both long-span (inboard) and short-span (outboard) spoilers, with the smaller outboard spoilers activated first. This system gave a smoother and more graceful control of yaw than would a single-spoiler system.^[1]

Given the difficulties in design and development, Russell Lee, the chair of the Aeronautics Department at the National Air and Space Museum, suggests an important purpose of the project for the Horton Brothers was to prevent them and their workers from being assigned to more dangerous roles by the German military.^[4]

Operational history

Testing and evaluation

The first prototype H.IX V1, an unpowered glider with fixed tricycle landing gear, flew on 1 March 1944. Flight results were very favorable, but there was an accident when the pilot attempted to land without first retracting an instrument-carrying pole extending from the aircraft. The design was taken from the Horten brothers and given to Gothaer Waggonfabrik. The Gotha team made some changes: they added a simple ejection seat, dramatically changed the undercarriage to enable a higher gross weight, changed the jet engine inlets, and added ducting to air-cool the jet engine's outer casing to prevent damage to the wooden wing.^[1]

The H.IX V1 was followed in December 1944 by the Junkers Jumo 004-powered second prototype H.IX V2; the BMW 003 engine was preferred, but unavailable. Göring believed in the design and ordered a production series of 40 aircraft from Gothaer Waggonfabrik with the RLM designation Ho 229, even though it had not yet taken to the air under jet power. The first flight of the H.IX V2 was made in Oranienburg on 2 February 1945.^[3] All subsequent test flights and development were done by Gothaer Waggonfabrik. By this time, the Horten brothers were working on a turbojet-powered design for the Amerika Bomber contract competition and did not attend the first test flight. The test pilot was Leutnant Erwin Ziller. Two further test flights were made between 2 and 18 February 1945. Another test pilot used in the evaluation was Heinz Scheidhauer (de).

The H.IX V2 reportedly displayed very good handling qualities, with only moderate lateral instability (a typical deficiency of tailless aircraft). While the second flight was equally successful, the undercarriage was damaged by a heavy landing caused by Ziller deploying the brake parachute too early during his landing approach. There are reports that during one of these test flights, the H.IX V2 undertook a simulated "dog-fight" with a Messerschmitt Me 262, the first operational jet fighter, and that the H.IX V2 outperformed the Me 262.

Two weeks later, on 18 February 1945, disaster struck during the third test flight. Ziller took off without any problems to perform a series of flight tests. After about 45 minutes, at an altitude of around 800 m, one of the Jumo 004 turbojet engines developed a problem, caught fire and stopped. Ziller was seen to put the aircraft into a dive and pull up several times in an attempt to restart the engine and save the precious prototype.^[5] Ziller undertook a series of four complete turns at 20° angle of bank. Ziller did not use his radio or eject from the aircraft. He may already have been unconscious as a result of the fumes from the burning engine. The aircraft crashed just outside the boundary of the airfield. Ziller was thrown from the aircraft on impact and died from his injuries two weeks later. The prototype aircraft was completely destroyed.^[5][6]

Despite this setback, the project continued with sustained energy. On 12 March 1945, nearly a week after the U.S. Army had launched Operation Lumberjack to cross the Rhine River, the Ho 229 was included in the Jäger-Notprogramm (Emergency Fighter Program) for accelerated production of inexpensive "wonder weapons". The prototype workshop was moved to the Gothaer Waggonfabrik (Gotha) in Friedrichroda, western Thuringia. In the same month, work commenced on the third prototype, the Ho 229 V3.

The V3 was larger than previous prototypes, the shape being modified in various areas, and it was meant to be a template for the pre-production series Ho 229 A-0 day fighters, of which 20 machines had been ordered. The V3 was meant to be powered by two Jumo 004C engines, with 10% greater thrust each than the earlier Jumo 004B production engine used for the Me 262A and Ar 234B, and could carry two MK 108 30 mm cannons in the wing roots. Work had also started on the two-seat Ho 229 V4 and Ho 229 V5 night-fighter prototypes, the Ho 229 V6 armament test prototype, and the Ho 229 V7 two-seat trainer.

During the final stages of the war, the U.S. military initiated Operation Paperclip, an effort to capture advanced German weapons research, and keep it out of the hands of advancing Soviet troops. A Horten glider and the Ho 229 V3, which was undergoing final assembly, were transported by sea to the United States as part of Operation Seahorse for evaluation. On the way, the Ho 229 spent a brief time at RAE Farnborough in the UK,^[3] during which it was considered whether British jet engines could be fitted, but the mountings were found to be incompatible^[7] with the early British turbojets, which used larger-diameter centrifugal compressors as opposed to the slimmer axial-flow turbojets the Germans had developed. The Americans were just starting to create their own axial-compressor turbojets before the war's end, such as the Westinghouse J30, with a thrust level only approaching the BMW 003A's full output.

Surviving aircraft

The only surviving Ho 229 airframe, the V3—and the only surviving World War II-era German jet prototype still in existence—has been at the SmithsonianNational Air and Space Museum's Paul E. Garber Restoration Facility in Suitland, Maryland, U.S. In December 2011, the National Air and Space Museum moved the Ho 229 into the active restoration area of the Garber Restoration Facility, and it is being reviewed for full restoration and display.^[8] The central section of the V3 prototype was meant to be moved to the Smithsonian NASM's Steven F. Udvar-Hazy Center in late 2012 to commence a detailed examination of it before starting any serious conservation/restoration efforts^[9] and has been cleared for the move to the Udvar-Hazy facility's restoration shops as of summer 2014, with only the NASM's B-26B Marauder Flak Bait medium bomber ahead of it for restoration,^[10] within the Udvar-Hazy facility's Mary Baker Engen Restoration Hangar. As of early 2018, the surviving Horten Ho 229 is currently on display at the Udvar-Hazy Center.

Claimed stealth technology

After the war, Reimar Horten said he mixed charcoal dust in with the wood glue to absorb electromagnetic waves (radar), which he believed could shield the aircraft from detection by British early-warning ground-based radar that operated at 20 to 30 MHz (top end of the HF band), known as Chain Home.^[11] A jet-powered flying wing design such as the Horten Ho 229 has a smaller radar cross-section than conventional contemporary twin-engine aircraft because the wings blended into the fuselage and there are no large propeller disks or vertical and horizontal tail surfaces to provide a typical identifiable radar signature.^[12][5]

Engineers of the Northrop-Grumman Corporation had long been interested in the Ho 229, and several of them visited the Smithsonian Museum's facility in Silver Hill, Maryland in the early 1980s to study the V3 airframe, in the context of developing the Northrop Grumman B-2 Spirit. A team of engineers from Northrop-Grumman ran electromagnetic tests on the V3's multilayer wooden center-section nose cones. The cones are 19 mm (0.75 in) thick and made from thin sheets of veneer. The team concluded that there was some form of conducting element in the glue, as the radar signal attenuated considerably as it passed through the cone.^[12] However, a later inspection by the museum found no trace of such material.^{[citation needed]}

Northrop-built reproduction

In early 2008, Northrop Grumman paired up television documentary producer Michael Jorgensen and the National Geographic Channel to produce a documentary to determine whether the Ho 229 was, in fact, the world's first true "stealth" fighter-bomber.^[12] Northrop Grumman built a full-size non-flying reproduction of the V3, constructed to match the aircraft's radar properties. After an expenditure of about US$250,000 and 2,500 man-hours, Northrop's Ho 229 reproduction was tested at the company's (RCS) test range at Tejon, California, where it was placed on a 15-meter (50 ft) articulating pole and exposed to electromagnetic energy sources from various angles, using the same three HF/VHF-boundary area frequencies in the 20–50 MHz range used by the Chain Home.^[12]

RCS testing showed that a hypothetical Ho 229 approaching the English coast from France flying at 885 kilometres per hour (550 mph) at 15–30 metres (49–98 ft) above the water would have been visible to CH radar at a distance of 80% that of a Bf 109. This implies a frontal RCS of only 40% that of a Bf 109 at the Chain Home frequencies. The most visible parts of the aircraft were the jet inlets and the cockpit, but caused no return through smaller dimensions than the roughly ten-meter CH wavelength.^{[clarification needed]}

With testing complete, the reproduction was donated by Northrop Grumman to the San Diego Air and Space Museum.^[12][13] The television documentary, Hitler's Stealth Fighter (2009), produced by Myth Merchant Films, featured the Northrop Grumman full-scale Ho 229 model as well as CGI reconstructions depicting a fictional wartime scenario, where Ho 229s were operational in both offensive and defensive roles, armed with "protruding" cannon barrels, an allusion to the proposed fitting of a pair of the existing, long-barreled (1.34 meters, 52-3/4 inch) MK 103 cannon proposed for the Ho 229.^[14]

Expert Debunking of Stealth Claims

Due to the popularity of this documentary project, the Smithsonian has since posted an extensive debunking of these claims citing their own research and the paper published and presented at the 10th American Institute of Aeronautics and Astronautics Aviation Technology, Integration, and Operations (ATIO) Conference held September 13 through 15 in 2010, in Fort Worth, Texas by the Northrop Grumman team followed in the Myth Merchant documentary.^[15]

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 “Flying 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 stealth myth has been growing since the 1980s and was invigorated when the National Geographic Channel, in collaboration with Northrup Grumman, produced a documentary called "Hitler's Stealth Fighter" in 2009. The program featured the Horten Ho 229 V3 as a potential "Wonder Weapon" that arrived too late in the war to be used (Myth Merchant Films, 2009). The documentary also referred to the jet's storage location as "a secret government warehouse," which added to the mystique of this artifact. Since the airing of the documentary, public pressure has increased to remove the jet from its so-called secret government warehouse and put it on display. In fact, this secret warehouse is the Museum's Paul E. Garber Facility in Suitland, Maryland where a team of conservators, material scientists, a curator, and aircraft mechanic has been evaluating the aircraft.

The Smithsonian has performed a technical study of the materials used and determined that there is "no evidence of carbon black or charcoal in the Horten jet" thus invalidating the proposed mechanism for an essentially non-existent radar absorbent property as compared to the control sample of plywood used in the original testing.

"The Ho 229 leading edge has the same characteristics as the plywood [control sample] except that the frequency [do not exactly match] and have a shorter bandwidth. This indicates that the dielectric constant of the Ho 229 leading edge is higher than the plywood test sample. The similarity of the two tests indicates that the design using the carbon black type material produced a poor absorber."

Dobrenz and Spadoni use the term 'absorber' to refer to the ability of the Ho 229 leading edge to absorb the radar signal rather than reflecting it back to the antenna receiver. More absorption means less reflected signal and greater stealth. The authors assumed in their paper that crafts persons used the "carbon black material" to lower the RCS, however, our technical study findings described above found no evidence of carbon black or charcoal in the Horten jet.

Variants