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The Theory of Stealth

October 22, 2025 //

Introduction

The F-117 & the B-2 have firmly ingrained the shape of stealth in our collective imagination. Stealth represents the pinnacle of technology, power & supremacy. To truly understand stealth we need to go back to the beginning.

Stealth is a form of camouflage. The earliest forms were natural cover for example in forests, armies marching by night or hunters disguising themselves to blend with their surroundings. In the early days of aviation stealth as a concept did not exist. Flying in WW1 was mainly at night or Zeppelins flew noiselessly at high altitudes or under cover of night.

The concept of Radar was first introduced in 1886 by Heinrich Hertz who observed that radio waves deflect off metal bodies. The development of Radar in the 1920s & 1930s by both Germany & Britain also began the idea of radar avoidance. There was little understanding of how Radars actually detected metal objects or the concept of RCS (Radar Cross Section).

The first experiments with stealth began in WW2 and the most famous example of stealth albeit accidental is the Ho-229 by the Horten Brothers. A flying wing that had a severely curtailed RCS due to its combination of design & materials. Another example of accidental stealth is the DeHavilland Mosquito, which had a low RCS because it was made of wood.

These aircraft are considered stealth generation zero some stealth and high maneuverability.

The Lockheed SR-71 and the Blackbird Family of aircraft are the first aircraft where stealth was conciously applied. The Blended Wing Fuselage (read : https://theaviationevangelist.com/2025/09/19/the-flying-wing-part-two-the-blended-wing-body/ ) and inward canted vertical stabilizers were intended to divert radio waves away from the emitter/receiver in addition to the fins being contructed from early composite materials.

*The most popular image of the Lockheed SR-71 in flight. One that I first saw in 1975. Pic Source : Wikipedia*

The paint on the aircraft was radar absorbing and consisted of iron particles to convert radar waves to heat, plasma stealth was experimented on the A-12 Oxcart where fuel contained a cesium based additive that created an ionized cloud of exhaust to absorb radar waves. The RCS of the SR-71 was bigger than a bird but smaller than a man. The SR-71’s biggest weapon was its speed at over Mach 3.0

The Blackbird Family of aircraft are the first generation of stealth combining speed and low maneuverability.

The Concept of Stealth was formalized by Russian electrical engineer & mathematical physicist Pyotr Ufemtsev when he published his epochal research now known as ‘Physical Theory of Diffraction’ (PTD) in 1962.

Pyotr Ufemtsev

While at Institute 108 (a key institute in the research of radio & radar technology) in 1954 Ufemtsev began his research into the reflection of electromagnetic waves and began developing a high frequency theory for predicting the scattering of electromagnetic waves from 2 & 3D objects.The shapes of the objects included discs, cylinders, cones, flat bases and wires in addition to several other geometrical shapes. These findings together came to be known as PTD and were published in his book ‘ Method of Edge Waves in the Physical Theory of Diffraction’ (PTD for short) in 1962.

The Russian think tank deemed the findings to have no significant military value and okayed his book to be published internationally. And the book languished for over a decade, before it was rediscovered by Denys Overholser of Lockheed. However this comes later.

Ufemtsev continued with his research in yet another gray forgotten facility in the former USSR for another twenty years. Gorbachev’s Perestroika gained traction in 1985 and the scientific funding that most of the institutes in the USSR received began to dry up. In the words of Ufemtsev, science took a back seat and political reform took center stage.

*Pyotr Ufemtsev the Father of Stealth. Pic Source : Wikipedia*

In 1989 Ufemtsev was in Stockholm to attend the Technology Symposium when in his own words he was surrounded by about ten Americans who said they were his students. You can imagine his shock as most of these ‘students’ looked middle aged and were professors themselves. One of them was from the University of California and offered him a position of visiting faculty. By this time USSR had almost fallen and Ufentsev was free to leave, which he did and Ufemtsev & his family found themselves in sunny California in University accommodation. It was here that could resume his research that was close to his heart. He was asked for a plan of reserach and funds were released direct from NASA to UCLA.

All this while Ufemtsev claims he was unaware stealth aircraft were being developed using his theory. His lecture at the 1992 National Convention of Science & Technology in Nice, France was one which was closely anticipated and watched by the Americans! Why? You might ask, the lecture was about how to defeat stealth.

Most radars of the time were monostatic radars. To put it simply, both the receiver and the transmitter of the radar are in one unit, so radio waves travel both and forth down the same corridor. At full absorption the radio energy is at coefficient zero or 50% absorption. He further explained that stealth aircraft were surrounded by a dispersed energy field, however there was no physical manner of decoding this.

The other manner of defeating stealth was to have the bistatic or multistatic radars, where the transmitters and receivers are separated by varying distances. This increases the complexity of the unit, however as stealth aircraft are primarily about deflecting the radio waves away from the transmitter, the chances of being caught in a multi static radar array are much higher. The technology has been around since the 1920s, however post Ufemtsev’s lecture and the continued rise of stealth, bi/multistatic radar has seen a resurgence since the 1990s.

Into his 90s now Ufemtsev is the father of stealth.

Project Harvey

The 1973 Yom Kippur war was a major wake up call for the US. The 19 day war was lethal for US made aircraft. Of the total 102 aircraft lost, 85 were American made, 32 F-4 Phantoms & 53 A-4 Skyhawks. The Soviet made S-75 Dvina missiles were part of an integrated air defense system supplied by the USSR to the Egyptian & Syrian forces were lethal against Israeli aircraft.

The war and result was a huge wake up call for DARPA (Defense Advanced Research Projects Agency) who initiated Project Harvey. Harvey the name from Harvey the Rabbit an invisible six foot tall white rabbit from the 1950 movie.

US strategic planners realized the vulnerability US aircraft & forces faced if they were ever to be in a similar high threat environment. In 1974 Chuck Myers, Director of Air Warfares Programs, espoused the ‘ Harvey ‘ concept to Robert Moore, Deputy Director of DARPA. They needed a new aircraft that would be invisible to enemy radar, infrared and other sensors. The idea almost immediately gained the support of the Pentagon and DARPA took the lead. This led to the highly classified ‘Have Blue’ program. DARPA invited McDonnell Douglas, Grumman, General Dynamics, Fairchild & Northrop. Lockheed was a surprise entry, they were initially not invited, however they leveraged their experience with the A-12 & SR-71 projects with DARPA and won an invite.

The final two projects accepted by DARPA were Northrop & Lockheed and after testing both model mockups (they were surprisingly similar) Lockheed won, and this would result in the legendary F-117 Nighthawk.

Have Blue & The F-117 Nighthawk

Ben Rich who succeeded Kelly Johnson as Director of Lockheed Skunk Works put together a team of talented individuals across several critical fields, they consisted of Bill Schroeder a veteran designer, Dick Sherrer preliminary designer, Denys Overholser among many famous names.

Denys Overholser recommended an aircraft with flat surfaces. In Overholser’s own words “Well, it’s simple, you just make it out of flat surfaces and tilt those surfaces over, sweeping the edges away from the radar view angle, and that way you basically cause the energy to reflect away from the radar”. Using Overholser’s recommendations, Dick Scherrer drew a preliminary aircraft with low RCS. The aircraft had faceted surfaces. The aircraft looked like anything but an aircraft and definitely did not look like it would fly.

*The original Hopeless Diamond. Pic Source : Wikipedia*

Over the next several weeks Overholser and Scroeder put together a computer team to create a prediction software called ‘Echo 1’. Ufemtsev’s calculations were incorporated into the software (Ufemtsev’s work had been translated by USAF Systems Command Foreign Technology Division which Overholser accidentally discovered). The final design would be a faceted delta wing that was stuck with the name ‘Hopeless Diamond’ a reference to the famous Hope Diamond at Smithsonian. The team took six months to convert Ufemtsev’s calculus to design. On March 7th 1974 at 02:47 a.m the code had its moment of epiphany, pyramids reflected no radar. These shapes were to be incorporated into Have Blue. Kelly Johnson who was sceptical of stealth said to Ben Rich “Our old D-21 drone (a mach 3.3 drone, Kelly Johnson loved speed) has a lower cross section than that goddam diamond”. The shape of Have Blue was limited by the computing power available at the time to create complex shapes, and hence the extremely faceted design.

In the summer of 1975 DARPA formally invited Northrop & Lockheed to develop the Experimental Survivable Testbest (XST). Both the shapes looked surprisingly similar. Northrop used a software called ‘GENSCAT’ similar to ‘Echo 1’. By Nov 1975 both Lockheed and Northrop were awarded $1.5 mn each to build a full scale mockup of their designs to be tested for their RCS signatures. It is important to remember here the designs were about stealth and not about aerodynamics. Lockheed won the round and were asked to build a flying test bed. Northrop were asked to continue with their own development, but were not part of Have Blue anymore.

*Northrop’s XST. Pic Source : Reddit User*

Lockheed were to build two demonstrators for testing and Ben Rich raised over $10 mn from Lockheed management for development. The demonstrators were ¼ th the size of the final F-117 Nighthawk. They had a wing sweep of 72.5° and had an inverse V tail. It had a wingspan of 22’6” and a length of 47’3” and a MTOW of 12,500 lbs. The small aircraft was powered by two GE J85 turbojets that developed 2950 pounds of thrust each giving it a max speed of 600 mph.

The shape of the aircraft meant it was extremely unstable and it had a quadruple redundancy fly by wire control system that gave the aircraft normal flying characteristics. However actual flights tended to be extremely wobbly, something the F-117 shared with Have Blue and came to be called Woblin Goblin. The two prototypes flew a total of 88 flights between them, before both of them crashed. HB1001 flew a total of 36 sorties and HB1002 flew a total of 52 sorties. Both the pilots were safe. The Have Blue shape was very similar to what Ufemtsev had envisaged.

*Overlap comparison of Have Blue & the F-117 Nighthawk. Pic Source: PICRYL*

The final shape of the F-117 Nighthawk had sides that were at least 30° off the vertical with multiple polygonal shapes. The aircraft now sported a V tail. The serrated edges that would be found on the F-117 Nighthawk were all about diverting radar waves away from the point of origin. The final radar cross section of the F-117 is about 0.001 m2 . The length was 65’11” and wingspan was 43’4”, the MTOW was 52,500 pounds with internal weapons bays.

The F-117 was powered by two modified GE F404 – F1D2 family of low bypass turbo fans. In the interest of stealth the engines had no afterburners. The engines were buried deep inside the aircraft to avoid radar waves hitting the fan blades as these give a very loud RCS return. Furthermore both the intakes were covered by a composite radar absorbing grill, so any radar waves that got in would not get out again. The exhaust was buried in a ‘platypus tail’ which was wide, rectangular and flat, furthermore exhaust heat was managed inside the long, buried exhaust duct lined with heat absorbing exhaust tiles. The bypass air was ducted over the tiles to cool them and further facilitate hot exhaust gas mixing with ambient cold air. Furthermore the hot exhaust air was directed at a very narrow angle just behind and above the aircraft.

*Rearview and top view of the F-117. Note the faceted shape and exhaust. Pic Source : Wiki Commons*

The F-117 had almost one ton of radar absorbent material (RAM) consisting of several composites. The RAM actually impacted load and trim. The first was iron ball paint which consisted of polymer based paint infused with ferrite. The second was several layers of carbon based layered composite materials. When radar waves hit the iron balls oscillate and convert the electromagnetic waves to heat which is dissipated by the aircraft body. The RAM was cut much like a linoleum sheet and glued to the aircraft skin. The gaps between the panels were filled with a putty called butter. This ensured a seamless face to radar with no gaps. The glass of the cockpit was coated with a special composite mixture to absorb radar waves as well. In fact at one point of time Skunk Works toyed with filling up the cockpit with Carbon Monoxide, which would mask the pilot, however this was firmly rebutted by the pilots themselves!

The F-117 had a hinged radio antenna that hinged back into the aircraft body. Once retracted it meant the aircraft was silent with no contact. The pitot tubes themselves were designed to divert radar away from the emitter. In fact they were so sharp that one could cut a finger on them. The main pitot tube speed incicator had one hole that flew directly into the path of oncoming air for airspeed. This was also right out front in clean air. One of the reasons the F-117 did not go supersonic was to avoid shockwaves coming off the tube.

While Have Blue first flew in 1977, it was a black project and was only publicly disclosed in Nov 1988 after the disclosure of the B-2.

The flyaway cost of each aircraft was $45 in 1981 dollars.

The F-117A is the second generation of stealth combining stealth with reasonable agility.

Tacit Blue & the B-2 Spirit

The Northrop Tacit Blue was a low observable stealth surveillance technology demonstrator aircraft. The aircraft could operate close or behind enemy lines with a high degree of survivability due its low probability of intercept radar (LIPR) and other sensors. The aircraft flew between 1982 – 85 but was publicly unveiled only in 1996.

The designation YF-117D represents an evolution on both Have Blue & F-117 programs. Tacit Blue was about demonstrating not only the next level of stealth design but other advances such as radar sensor technology was part of the ‘ Assault Breaker ‘ program that included technologies such as lasers, electro optical sensors, data processors etc that could break up ground launches working together in unison.

Tacit Blue, nicknamed ‘ the whale / alien school bus’ featured a straight tapered wing with a V tail.. The fuselage was curved in a manner to cut RCS, and ended in a rectangular edge all round that protruded. The engines had a single flush inlet on top of the fuselage that led to a S shaped curve as did the exhaust. The exhaust did not have the ceramic tiles the F-117 had but was instead lined with radar absorbent and heat resistant coatings. The exhaust nozzle itself was flat, wide and a curved arc that blended into the highly curved body. The nozzle was placed between the V tail. The setup ensured the engines would have no exhaust / intake signature on radar.

*Rearview of Tacit Blue. Pic Source: Wikipedia*

The aircraft featured concealed radar as mentioned earlier so it could be in touch with its ecosystem at all times. The radar did not interfere with its RCS signature. The shape of Tacit Blue was possible because of strides in computing power that in turn enabled more complex radar cheating shapes. Overall a look at Tacit Blue tells you of a shape is continuous with no breaks and very reminiscent of the B-2 Spirit.

The aircraft used two Garrett ATF-3-6 medium bypass turbofans that produced 5,440 pounds of thrust each. The engines propelled the whale at 290 mph, which is reasonable considering the aim of the aircraft was reconnaissance and not speed. The aircraft had a service ceiling of between 25-30,000 feet.

The materials used on the aircraft were aluminium for the fuselage and wings structure with Titanium in structural components. The empennage, wing & fuselage leading edges used proprietary Northrop radar absorbent composite materials. The RAM continued to use ferrite materials that were highly radar absorbent, however unlike the F-117 the material could be applied to the continuous curved surfaces and not have to use butter in between panels.

Tacit Blue flew a total 135 sorties and 250 hours over a three year period and gathered valuable data that was used in the B-2 Spirit.

The B-2 was the spiritual successor to Tacit Blue. It incorporated several technologies from both Tacit & Have Blue. The S shaped engine intakes and active exhaust gas management. The curved surfaces of Tacit Blue showed that improvements in computing power meant stealth aircraft shapes could be complex curves. The radar integration on Tacit Blue is on the B-2 as well.

The Advanced Technology Bomber program as the B-2 was known began in 1979. The black project codenamed Aurora narrowed on the Northrop/Boeing & Lockheed/Rockwell teams to begin preliminary work. Both the teams came up with Flying Wing designs with Northrop’s proposal codenamed ‘Senior Ice’ & Lockheed’s as ‘Senior Peg’. Northrop’s proposal won as they already had experience with flying wings from their YB-35/49 days , their aircraft was larger & was a pure flying wing. Lockheed’s proposal was more like its Have Blue program featured a faceted design and incorporated a small tail. Northrop was awarded the contract in late 1981.

*Senior Peg by Lockheed. Pic Source : TWZ*

For the Northrop the B-2 Spirit was the culmination of almost 40 years on flying wing aircraft ( read: https://theaviationevangelist.com/2025/09/13/the-evolution-of-the-flying-wing-part-one/ ). Flying wings are naturally stealthy along with the technologies validated by participating in the Have Blue and Tacit Blue programs.

In its final form the B-2 is a lambda wing and is made of mostly carbon graphite composite material, with buried engines that have S shaped ducts for intakes and active exhaust management. The RAM on the B-2 is even more advanced than on the F-117 & Tacit Blue. While the material continues to be classified we can speculate it uses alternate high frequency material to reduce maintenance post each sortie. The iron ball paint technology is probably used as well in addition to signature control materials such as sealants (butter?), and conductive tapes. All of this gives the B-2 a radar profile of 0.1 m2 .To protect the coatings the B-2 is stored in environmentally controlled hangers called B-2 shelter systems (B2SS).

*The B-2 Spirit. Pic Source : Wikipedia*

The length of the B-2 is 69 feet with a wingspan of 172 feet (same as the YB35/49) and a height of 17 feet. Its MTOW is 376,000 pounds. The engines on the B-2 are four GE F118 non afterburning low bypass turbofans that develop 17,300 pounds of thrust each. The cruise speed of the B-2 is 630 mph with a service ceiling of 50,000 feet and a range of 6,900 miles, all phenomenal statistics considering it is almost never seen!

By 2004 the total program had cost $44.75 bn. Calculating the cost over 21 aircraft produced the cost per aircraft comes to $2.1 bn!

The B-2 represents the third generation in the stealth technology evolution.

The YF-23 & the F-22 Raptor

By 1986 the Soviet Union had several emerging threats such as the Sukhoi Su-27, the Mikoyoyan MIG-29 fighters and the under development Beriev A-50 airborne warning & control system (AWACS) along with increasingly sophisticated SAMs. The US needed to replace its aging air superiority fighter the F-15 Eagle. The Advanced Tactical Fighter (ATF) would use the stealth technologies developed along with advanced avionics, fly by wire systems, and advanced propulsion systems. Lockheed and Northrop were invited by DARPA as lead contractors of their respective teams due to their previous experience with all of the above. Lockheed’s proposal was dubbed the YF-22 & Northrop’s YF-23. Both aircraft had very similar properties.

*The YF-23 with the B-2 coming in at Edwards AFB. Pic Source : Wikipedia*

The YF-23 nicknamed ‘ Black Widow II’ had two prototypes built. Northrop teamed up with McDonnell Douglas and the prototypes would run both Pratt & Whitney and General Electric engines. So while this was a competition between airframe manufacturers it was a competition between engine manufacturers as well.

Three design concepts were studied, the Agile Maneuverable Fighter (AMF) with two vertical tails, that had the best handling but the least stealth was the first. The second was the Ultra Stealth Fighter (USF) that had the best stealth characteristics called the Christmas Tree due to its design and the third was the High Stealth Fighter (HSF) which balanced stealth and maneuverability. It had diamond shaped wings and all moving V tail rudders. The third would eventually become the YF-23.Northrop received $691 mn as did Lockheed and given 50 months to create for demonstration & validation (Dem/Val) two prototypes each.

*The three designs Northrop assessed. Pic Source: Wikipedia*

The YF-23s faceted & blended fuselage with its diamond shaped wings had very good RCS return of 0.001m2 , the steeply canted V tail (at 50°) gave the aircraft the agility it needed while at the same time blocking out the engine exhausts. The tail itself had the span of a small aircraft. A top view of the aircraft shows an alignment of all the edges along a couple of axis, this is critical to control radar return in a very narrow corridor. The engine intakes (at the entrance had a porous section) again had S shaped ducts similar to the B-2 and the exhaust had active gas cooling with ceramic tiles similar to the F-117. All weapons and radars were carried internally. The RAM continued to use the ‘iron ball paint’. Critical parts such as the all moving tail were made of advanced composite materials with high radar absorption.

The YF-23 was tested with both the engine options. On the P&W YF119 option YF-23A number 1 flew a total of 34 flights for 44.3 hours. The maximum AoA at minimum speed was 25°, while the max speed was Mach 1.8 at maximum altitude of 50,000 feet..impressive numbers.

Prototype number 2 flew with the GE YF120 option, clocked 16 flights for a total of 21.6 flight hours. The max AoA at min speed was 20°, while max speed was Mach 1.72 at 50,000 feet max altitude.

Both sets of numbers are impressive, while both the engines had afterburners the YF-23 could cruise comfortably at Mach 1.4-1.6 without them. An F-16 needed to afterburn to keep up.

The final specs of the aircraft was a length of 67’5”, wingspan of 43’7” and height of 13’11”. A MTOW of 51,320 pounds. The engines tested were two P&W YF119 and two GE YF120 that generated 23,500 pounds of thrust each (YF120) dry or 35,000 pounds with afterburner. The prototypes had to be constructed within a 3 month month period as their 50 month deadline was running out.

Lockheed teamed up with Boeing and General Dynamics to create the YF-22 which would become the F-22 Raptor. As stealth took centre stage the design team at Lockheed Skunk Works led by Bart Osborne moved away from its SR-71 type designs and once again came up with a design very similar to the F-117s faceted shape as they used the same computer program Echo 1. This design gave very poor aerodynamic handling characteristics. They needed to get better. They needed a design more like Northrop’s curved blended aircraft.

The final design submitted by Lockheed and team was vastly improved on the faceted design, incorporating curved shapes and surfaces. While the computers were not able to handle the design, physical reliance on radar range testing improved RCS while the curved shapes helped aerodynamics. The final design submitted for Dem/Val designated 090P had an arrow head forward fuselage, trapezoidal wings that had a steep crank on the leading edge and four empennage tail surfaces. The engines used the now familiar S shaped inlet and the rectangular exhaust nozzles and this is where the magic happened.

*A screen grab showing the F-22 aligned edges. Pic Source: Western Museum of Flight*

The nozzles featured a 2D thrust vectoring system that pivoted the exhaust stream down by up to 20°. This in conjunction with the all moving horizontal tail planes in conjunction with the twin vertical stabilizers (canted at 28°) gave the aircraft incredible pitch and stability control. And this probably won the F-22 Raptor contract.

The aircraft was constructed of composites which have now become a trend in stealth aircraft in addition to leading / trailing edge alignment all round. The large fins continue to mask the engine exhaust. The RAM on the YF-22 is a proprietary formula developed by Lockheed and Boeing an advancement on the iron ball paint formula and is done in layers. All hatches and openings on the aircraft are serrated much like the YF-23 to deflect radar waves away. In addition the surface of the aircraft is multiple shades of gray, these are different materials bonded together to further deflect radar waves and reduce RCS.

The first prototype with GE 120 engine first flew in September 1990 while the second with the P&W engines first flew in Oct the same year. The prototype with the GE 120 engines did a total of 43 flights for a total of 52.8 hours, maximum AoA at min speed was an insane 60°, while maximum speed was over Mach 2 at max altitude of 50,000 feet. The prototype with P&W engines flew a total of 31 flights for 38.8 flight hours, max AoA at min speed was 20° while max speed was MAch 1.81 at 45,000 feet.

The YF-22 won, but it needs be said the YF-23 is a legendary aircraft in its own right. The F-22 Raptor final specs were a length of 62’1” and a wingspan of 44’6”. The MTOW is 83,500 pounds and is powered by P&W F119 engines that develop 26,000 pounds of thrust each dry and 35,000 pounds of thrust each with afterburner. The max speed is Mach 2.25 and range is 1,800 miles.

A total of 195 F-22 Raptors were constructed at a cost of $67.3 bn, at a unit program acquisition cost of over $350m each as the construction numbers were heavily curtailed from 750 as the costs proved to be prohibitively expensive and shifting priorities after the end of the Cold War.

The F-22 Raptor is the fourth generation of stealth incorporating speed and maneuverability.

The F-35 Lighting & The Democratization of Stealth

In 1993 DARPA launched the affordable Common Affordable Strike Lightweight Strikefighter (CALF) project to develop a stealth aircraft that would eventually repace the F-16, F-18 &Harrier across forces. The Joint Advanced Strike Technology project started in 1994. Congress order both of them to be merged and this became the Joint Strike Fighter (JSF). The fighter needed to a versitile fighter that was capable of Short Take Off & Vertical Landing (STOVL) and be capable of a supersonic dash while being stealthy. There were four submissions Mc Donnel Douglas, Northrop, Lockheed & Boeing.

By 1996 the two finalists were Lockheed and Boeing. Both were awarded $750 million each to build two concept demonstrators each. Boeing would develop the X-32 & Lockheed the X-35.

Boeing’s strategy was about keeping lifecycle costs down by minimizing variations across applications. The X-32 had a carbon fibre composite delta wing with a 55° sweep angle and a thick leading edge. This allowed the aircraft to have minimal transonic drag , aid lift at lower speeds and carry radio antennas internally.

The X-32 used a single P&W F119 powerplant that put out 28,000 pounds of thrust , dry and 43,000 pounds of thrust with afterburner. For STOVL the aircraft used thrust vectoring where the engine nozzle moved down upto 15°. The aircraft had a V tail canted to 58° each and an air intake that was directly beneath the cockpit reminicent of the F-16 and unfortunately could not achieve the kind of stealth required.

*The Boeing X-35 prototype. Pic Source: Wikipedia*

The final specs of the aircraft show a length of 45’0.1”, wingspan of 36’ & height of 17’3.8”. The MTOW of the aircraft was 38,000 pounds with all weapons to be carried internally. The max speed of the aircraft was 1,200 mph max range across profiles was 850 miles in the USAF profile.

The X-35 used many design elements from the F-22 Raptor (and does not need to repeated) and the VTOL exhaust duct from the Covair Model 200 from 1972. Furthermore Lockheed had purchased technical data from the cancelled Russian Yak-141 for examination of its swivel nozzle in 1991. The aircraft used helmet mounted display systems that had already been around to integrate into the hardware.

The stand out element of the X-35 was the shaft driven lift turbofan. The design was pateneted by Lockheed engineer Paul Bevilaqua and developed by Roll Royce. During normal flight the engine behaved as a normal medium bypass turbofan with afterburners. The turbofan also acted as a turboshaft engine where the engine produces shaft power instead of jet thrust a concept used in helicopters.

*The engine nozzle, shaft & hover fan assembly of the F-35. Pic Source: Wikipedia*

Where the X-35 differs is only a portion of the jet’s power is directed to the shaft, the rest of the power is still directed towards a thrust vectoring engine nozzle for hover mode. The nozzle can swvel to an astonishing 95° using the 3 Bearing Swivel Duct Nozzle (3BSD). The aircraft can transition from normal flight mode to hover inflight. The shaft in turn drove a two stage titanium lift fan that was 50” in diameter. The entire fan assembly and housing weiged in 1.2 tons, a deadweight during normal flight. In the interests of stealth the far was closed out from both top & bottom when not in use and was a dead weight during normal flight.

*The X-35 inflight. Pic Source: Wikipedia*

The final specs of the aircraft was a length of 50’5” , wingspan of 33’ & height of 13’3”. The MTOW was 50,000 pounds and powerplant was the P&W F119 turbofan that generated 25,000 pounds of thrust dry and 40,000 pounds with afterburner. The max speed was Mach 1.5+ and a max range of 1,400 miles with a service ceiling of 50,000 feet.

The X-35 won mainly on the basis of its extremely versitality and would become the F-35 Lightening. The final unit cost of an average of just over $100 mn a unit across variants represents a huge improvement in costs.

The F-35 represents the Fifth Generation of stealth aircraft.

Since the 2010s as stealth technology has been better undertood & costs bought down countries such as China & Russia we have seen a raft of stealth aircraft make their way into pubic awareness. Of these the J-50 & J-36 from China stand out as they incorporate All Moving Wingtips (AMT) along with tailess designs ( read here : https://theaviationevangelist.com/2025/10/02/lambda-wings-moving-wingtips-flying-wings-part-3/ ). The Chinese also have the J-20. Tailess aircraft being naturally stealthy. A late edit: A Chinese Team has recently unveiled an evolutionary new stealth material that has a durable and flexible coating that is extremely thin at just 0.1mm. Furthermore one of its properties is impedance tuning , adjusting dynamically to ambient surroundings including temperature. Such a property is critical to advanced stealth as it controls the material’s interaction with incoming electromagnetic waves from radars within range. This is a developing story and not many details are known yet.The Russian Su-57 is another example of stealth technology at work. The US has the B-21 Raider the the F-47 under development, both of them promise to be interesting developments in the evolution of stealth.

Stealth is the standard by which air superiority aircraft are measured.

The Future

The advent of drones like the loyal wingman, the Lockheed Vectiss is a stealth drone that flies ahead in high risk environments and ensure surviveability for airmen. This also represents the connected environment stealth enconpasses, it’s not just about stealth its about communicating effectively.

Stealth has spurred the evolution of radars. Bistatic & multistatic radars are getting more sophisticated (they are complex). RAM is optimized for high frequency but low frequency are getting better at stealth aircraft detection. Over the Horizon radars have the ability to monitor vast areas by bouncing their waves off the ionosphere. AI is playing a big hand in patching multiple data points to create a single picture this strenthens existing infrastructure.

Quantum radar is in its infancy. Quantum radar looks at subtle changes in photons through quantum entanglement , this will render current stealth technology ineffective. We are getting better at detecting Ufemtsev’s theoretical radiation bubble around stealth aircraft.

The next level of stealth is a cloaking device much like Star Trek!

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Tags 0.1mm, B-2 bomber, B-21 Raider, blackbird, Boeing, Chinese ultrathin material, composite, DARPA, F-117 Nighthawk, F-22 Raptor, F-35 Lightening, F-47, ferrite, Have Blue, Ho-229, Horten Brothers, impedance tuning, J-20, J-36, J-50, Lockheed, McDonnell Douglas, MIG-29, Northrop, nozzles, Project Harvey, prototype, RAM, shaft, SR-71, Star Trek, stealth, STOVL, Su-27, Su-57, Tacit Blue, thrust vectoring, X-32, X-35, XST, YF-22, YF-23

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Lambda Wings & Moving Wingtips – Flying Wings Part 3

October 2, 2025 //

The Shenyang J-50 / J-XDS

The leaked pictures of the Shenyang J-50 last week on its takeoff roll, with its sleek lines looking like something out of Star Trek raised quite a stir on Social Media.

While the J-50’s tailless design & lambda wings raised eyebrows worldwide, what really got the buzz were the all moving wingtips (AMT) .

Before we get into the evolution of Lambda Wings, let us first investigate the all moving wingtips.

*The Shenyang J-50. Note the wingtip angle. Source: X intelligence Pic*

The All Moving Wingtips (AMTs)

Tailless flying wings or low drag configurations have always had yaw and pitch control challenges typical only to them. The control challenges have two approaches in the flying wing ecosystem. The drag rudder (made famous by the B2- Spirit & the Ho-229) and the recently made famous all moving wingtip.

*The drag rudder schematic. Pic source: screenshot from slope dudes*

On the face of it both of them appear to do the same thing but in actuality they are different.

In AMTs the entire wingtip swivels or pivots, this gets them to act as mini wings, generating lift to create precise pitching/rolling movements with minimal drag.

Split rudders by opening to disrupt airflow create drag based yawing moments, they are effective for directional control in tailless aircraft but less agile for pitch control.

To sum up, on a fighter aircraft which has to be capable of high-alpha (high AoA) movements, AMTs are much more effective than drag rudders which are effective for stable level flight with gentler yaw moments much like a B2’s.

The evolution of AMTs

Right from J W Dunne’s early tailless flying wing designs in the early 1900s, they had high angle of attack (AoA > 15°) instability, a pitch up due to vortex formations and airflow flow separation.

Early stability was achieved through wing washout (refer part one of this series), some others such Jack Northrop’s N-1M from 1941 had manually adjustable wingtips that were adjusted preflight. These wingtips moved up and down on the vertical axis (altering the dihedral/anhedral angle relative to the main wing plane), however it was found that drooping wingtips did not contribute to lift and increased drag. Subsequently the wings were left flat inflight.

*The Northrop N-1M. Note the dropping wingtips set before flight. Pic Source: Northrop Grumman Corporation on FB*

The Ho-229 used elevons, a combination of elevators and ailerons. The elevons on the wingtips leveraged the wings outer sections for a greater moment arm (the perpendicular distance from the axis of rotation to the line of action of a force), improving control authority. The Ho-229’s elevons(which either deflected together or rolled differentially) on the wingtips can be considered an early precursor to AMTs.

The Short SB.4 Sherpa is considered a milestone in the evolution of AMTs. It had an aero-isoclinic swept wing (maintaining a constant angle of incidence despite wing flexing and air loads, preventing issues like torsional instability, aileron reversal, and tip stalling). It had a 42° leading edge sweep and was designed by G.T.R.Hill who had designed the Westland Pterodactyls & BWB(detailed in parts one & two). The SB.4 Sherpa was the very first aircraft with controllable AMTs and its all moving wingtips acted as elevons (only). The AMTs were 20% of the total wing area and were hinged at 30% chord (means that the pivot point for the elevon was not at the very front of the control surface but further back. This allows the elevon to act more like an all-moving stabiliser and less like a conventional aileron, increasing its control authority).

*The all moving wingtips set before on the SB.4 . Pic Source : Wikipedia*

The elevons had a symmetric rotation of ±15° and asymmetric rotation of ±10°. While the design was pathbreaking the electric actuators were slow and underpowered for larger aircraft, limiting scalability. The tests on the SB.4 that ran between 1953-64 confirmed 15% better control at high AoA vs flaps. The SB.4 Sherpa’s cancellation would only highlight the influence AMTs could have on future aircraft.

During the 1980s the concept of wingerons on RC gliders (C.R. Turbo Kit) came about. They adapted the AMT concept for lightweight, low speed gliders. It became popular in RC communities because it simplified construction and enhanced soaring efficiency. In these gliders either entire wings were pivoted or just wingtip sections around a central joiner (carbon rod). The AMTs improved glide ratios by approx 15% and the wingerons had great performance at high AoAs (greater than 20°). The wingerons were directly inspired by the Horten Brothers and the SB.4 Sherpa.

The 2023 paper ‘Numerical analysis of pitch-break and all moving wingtip aileron of lambda wing configuration’ (https://www.sciencedirect.com/science/article/abs/pii/S1270963823004054) reflects on decades of tailless aircraft research (ex: SACCON(Stability And Control CONfiguration), 1303 UAV(lambda wing unmanned combat aerial vehicle UCAV, configuration used for extensive aerodynamic and stability studies, notably its low-speed characteristics) and advances in computational fluid dynamics (CFD). It further proposes AMTs on aircraft such as the B-2 & X-47B. The reason for this is pitch break (cites the B-2 crash of 2008) over lambda winged aircraft. Pitch break on a lambda wing aircraft is a sudden and unstable pitch up motion that occurs at high AoA . This aerodynamic instability is caused by the complex flow patterns over the wing at high AoA, particularly flow separation over the outboard portion of the wing. The paper further suggested that Pitch-break on lambda wings is attributed to the combined effects of the leading edge vortex and the trailing edge pressure gradient at high angles of attack. A numerical analysis suggests that an all-moving wingtip (AMT) auxiliary aileron could provide more stable pitch control during the pitch-break zone compared to conventional ailerons, demonstrating engineering feasibility.

The paper’s AMTs or pivoting wingtip sections suggested them to be approx 10-15% of the wingspan, rotated symmetrically for a pitch of ± 10°. The differential rotation for roll of ±8°. To achieve precise control fly by wire is a must and the leveraged wingtips influence spanwise flow by adjusting lift and manipulating wingtip vortices, thereby enhancing aerodynamic stability. The paper references A. Schutte ( German Aerospace Centre (DLR)) & Sedat Yayla (Kocaeli University) among many others who researched aerodynamic challenges in tailless lambda wing configurations.

The Shenyang J-50 is a synthesis of all the knowledge gained over 70+ years and applied on a sixth generation fighter. The J-50 likely has its AMTs working in a similar manner as suggested above. The hydraulic actuators driven by fly by wire should have responses of <0.5s with composite hinges. The weight of each wingtip should be <100kg (speculation).

In summation the AMTs on the Shenyang J-50 overcame the challenges the tailless Horten Ho-229 & Sherpa SB.4 elevons faced. It takes the RC community idea of wingerons for low drag rotation and further synthesizes it with the 2000s research to deliver unmatched agility on a tailless lambda wing.

The Evolution of Lambda Wings

Without doubt the most famous lambda ( ƛ )wing is the legendary B-2 Spirit. Lambda wings get their name from the lambda shaped kink on their trailing edges. Almost 40 years after her first flight, she continues to inspire awe and respect. The B-2’s lambda wing design emerged from a combination of Jack Northrop’s (considered one of the fathers of the flying wing) flying wings from the 1940s, the need for stealth and desire for superior aerodynamics. To truly understand the lambda wing, let’s go back to the beginning.

*The amazing B-2 Spirit. Note the split rudder on the port (left) wing. The lambda shape clearly visible on the trailing edges. Pic Source : Wikipedia*

The basic supersonic shape of a wing is the Delta wing, it shaped like a ⍙ . Triangular in shape with a continuous wing sweep of between 35-60°. Double delta wings have kink on the leading edge a 50-60° sweep (vortex lift) and the outer wing (planar lift) has a sweep of approx 30-40°. The moderate kink blends delta and straight wing traits, improving low speed handling without really compromising high speed performance while maintaining aspect ratio.

The J-50 is a Cranked Lambda(Arrow) Wing which has a sharp kink on the leading edge. The inner wing has a sweep of between 60-70° and the outer wing has a sweep of between 30-40°. Furthermore the trailing edge has a kink (ƛ shaped) on it too and contributes to the J-50s stealth performance.

*The cranked lambda wings on the J-50. Note the kinks on the leading and trailing edges. The elevons & wingtips clearly visible. Pic source : China Weibo Image*

To fully understand Lambda Wings let’s go back to the beginning. Post WW2 early deltas such as Convair XF-92 (1948) and Dassault Mirage I (1955) had excellent high speed characteristics, but poor low speed handling. Engineers looked at hybrids to balance the two, the cold war dictated fighters capable of Mach 2 intercepts and be agile dog fighters. The answer was the double delta.

The Saab 35 Draken is considered a major milestone as the first operational double delta winged aircraft. It had an inner wing sweep of 80° and an outer wing sweep of 60°, this gave it excellent transonic performance and high maneuverability. The draken had only a vertical stabilizer. The kink improved lift to drag ratio by 10-15% at subsonic speeds compared to pure deltas. The Saab 37 Viggen had canards coupled close to the double delta wing, this boosted low speed agility even more and reduced the incidence of stall by generating additional vortices that interacted with the main wing flow.

*The Saab 35 Draken with the leading edge kink. Pic Source: Wikipedia*

Through the 1960s & 70s various aircraft experimented with variations of the double delta wings, and some of the notable examples were the XB-70 Valkyrie(1964, please read the detailed piece on it), the Tu-144(1969). These examples highlight design progression toward multi role versatility.

*The double delta wing on the TU-144. Pic Source : Aerospaceweb.org*

Other examples of evolution of the double delta (kink of leading edge) handling include the General Dynamics F-16XL (1982), developed under NASAs high speed research program. It modified a F-16s cropped delta into a cranked double delta with an inner sweep of 70° and an outer wing sweep of 50° and incorporated an S curve on the leading edge for a smooth flow transition. This design experiment improved fuel efficiency by 25% at subsonic speeds , increased range and had enhanced payload (weapons / fuel). The F-16XL had a vertical stabilizer, but proved double delta wing efficiencies. Through the 80s there were several experimental delta wings such as the Grumman X-29, a forward swept wing with close coupled canards that tested aerodynamic efficiencies.

*The F-16XL. Pic Source : Lockheed Martin*

The 1997 McDonnell Douglas/Boeing – NASA X-36 was a sub-scale tailless demonstrator with lambda wings and close coupled canards. The X-36 would directly influence UACV designs moving ahead.

In the early 2000s a generic lambda wing was used in transonic research to study complex vortical phenomena that occur at subsonic speeds (Mach 0.5-0.8) on lambda wings. The SACCON was used as a research test bed.

*The X-36 & X-45 along with other greats at the USAF Museum. Note the trailing edge kinks on both aircraft. Pic Source : USAF Museum*

The 2002 Boeing X-45 was a tailless UCAV demonstrator featuring a cranked lambda wing with a 65° inner wing sweep and a 30° outer wing sweep. The trailing edge kink was in the range of 30°.The wing’s leading edge had a sharp kink and swept back planform aligned edges aided RCS (Radar Cross Section) reduction. The design exhibited improved lift coefficients by 15% at transonic speeds, with vortex lift sustaining high-alpha (high AoA) maneuvers. The X- 45 proved autonomous technology along with lambda wing maneuverability.

The Northrop Grumman X-47B program that ran between 2011-2015 is another example of lambda wings. The X-47B was a lambda wing UAV to test carrier operations. The program was cancelled in 2015 as engineers struggled to balance stealth, aerodynamics & propulsion. The 2023 paper mentioned earlier recommended AMTs. The program validated the concept of unmanned carrier aviation.

Summation

The cranked lambda wings (a.k.a cranked arrow) represents a move forward and an evolution of the double delta wing design. The design features a more pronounced leading edge kink in the range of an inner sweep of between 60-70° and an outer wing sweep of 20-40° to optimize transonic efficiency and stealth. The crank enhances lift by between 5-10% in the transonic regime and improves vortex sustainability for better longitudinal stability. The lambda shaped kink (approximately 30°)on the trailing edge improves aerodynamic efficiency across the speed regime in addition to contributing to stealth properties.

The Shenyang J-50 a sixth generation Chinese fighter with it’s cranked lambda wing balances subsonic agility , transonic efficiency and supersonic dashes, with its wingtips enhancing high-alpha control by over 15% over traditional elevons. The wing further aid reduced RCS.

The fly by wire J-50 represents the next step in lambda wing evolution and it combines stealth, autonomy & adaptive surfaces and is definitely up there as far as sixth gen fighters go.

The J-50 prepares to take off. Note the wingtips. Pic Source: X

Please do read parts 1 & 2 of this series:

http://theaviationevangelist.com/2025/09/13/the-evolution-of-the-flying-wing-part-one/

http://theaviationevangelist.com/2025/09/19/the-flying-wing-part-two-the-blended-wing-body/ do keep scrolling down, and do share

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The Evolution of the Flying Wing – Part One

September 13, 2025 //

Prologue

On June 22nd 2025 seven B-2A bombers carried out strikes in Iran. The total sortie codenamed ‘Operation Midnight Hammer’ lasted 37 hours. They dropped a total of 14 GBU-57 bunker buster bombs on Iran’s Fordow Fuel Enrichment Plant & Natanaz Nuclear Facility .

The sortie only strengthened the B-2’s formidable reputation of striking distant targets unseen & unheard..

The B-2 Spirit is a major milestone in the ongoing evolution of the flying wing.

The Pioneers

Before we get to J W Dunne (considered the father of the flying wing) , we need to first acknowledge a couple of important milestones. The first is Sir George Cayley who in 1799 put forth the concept of a fixed wing ‘machine’, one that had separate systems for creating lift, propulsion & control surfaces. He is the first person to understand the forces that act on a flying machine, weight, lift, drag & thrust. Later in 1810 he worked out the importance of having a dihedral angle at wing roots. The upward angle of wings was “the chief basis of stability in aerial navigation”. It needs to be noted Sir George Cayley was more intent on sharing his knowledge rather than patenting them, and therein lies his greatness.

The second is Alphonse Penaud , best known for his 1871 ‘Planophore’ , a rubberband powered model plane. The design achieved stable flight for 40 seconds demonstrating the stability of fixed wing aircraft designs over any others (such as ornithopters). He teamed up with mechanic Paul Gauchot in 1873 and they patented the monoplane flying wing in 1876. The structure of the ‘flying wing’ had a slight dihedral angle which provided roll stability, they had unswept wingtips and a slight arch on the leading edge with elastic trailing edges, this gave the wing better flexibility when dealing with unstable airflow. The patent contained a detailed performance analysis and is an important milestone as it integrated fixed wings, propulsion & control surfaces into a flyable machine decades before powered flight became a reality.

Alphonse Penaud’s 1876 Flying Wing. Pic Source : Wikipedia

Between 1906 – 1913 J W Dunne ran his experiments on ‘inherent stability’. From a military background, Dunne collaborated with Col. J E Capper at the British Army’s Balloon Factory to test early designs of his tailless gliders with swept wings. In 1908 his D.1b glider exhibited stable glides after the earlier D.1 crashed unceremoniously. The D.1b ‘s ‘inherent stability’ was achieved through tailless wing design alone.

The D.5 & D.8 were powered tailless biplanes (flying wing configuration) with sweeps up to 30 degrees. The wings featured a washout (a twist reducing the angle of attack at the wingtips) for enhanced stability. The D.5 demonstrated a hands-off flight while Dunne was reading a newspaper). This was a first in the early days of powered flight. The D.10 saw further refinements with a more streamlined design. Dunne’s use of Elevons (combination elevators & ailerons) is considered a first. His designs definitely helped reduce pilot work load. His work would be foreshadowed for the next ten years.

In 1910 the same year Dunne filed his patents, Hugo Junkers too filed a patent for a ‘flying wing or Nurflugel (pure wing). It was for a hollow metal airliner where passengers would sit inside the wing structure (an early blended wing). The fuel and cargo would be in the wings too. Long-term he envisioned transatlantic flights in such airplanes. His designs had structural integrity due to all metal structures with cantilevered wings and small tails for stability. The G38 was an example of his ideology. He was thrown out of his own company in 1933 as he disagreed with Nazi ideologies.

The Junkers G-38 of 1929. Pic Sour : Wikipedia

Between the wars Germany was not allowed to build powered aircraft and this gave rise to a number of glider clubs . Alexander Lippisch worked at Junkers between 1925-27 and this inspired him to take up the ‘nurflugel’ torch. His Storch series of gliders were tailless with swept wings. His Delta series of gliders explored low aspect ratio wings for better roll control. His gliders featured wingtip rudders and elevons. By the early 1930s he had contracted with DFS (Deutsche Forschungsanstalt für Segelflug a.k.a German Institute of Glider Research, a Nazi front) for powered prototypes such as the 1931 DFS 40, a rocket powered tailless plane. Lippisch directly inspired two young brothers who would go on to create one of the most fabled aircraft of all time, enter Reimar & Walter Horten and the aircraft they would create the Ho-229.

The Lippisch, Storch & Delta gliders. Pic source Wikipedia

Mentions: The powered Cheyranovskii BICh-3 Tailless research aircraft of 1926 & G T R Hill’s Westland Pterodactyl series of tailless gliders between 1926-32. Hill would go on to design and construct the Westland Dreadnought, the very first purpose built Blended Wing Body.

The sequence of events mentioned above illustrates the evolution of wings over an almost 200 year period starting with Sir George Cayley. The unveiling of the B-2 Spirit in 1988 was 112 years after Alphonse Penaud’s patent of 1876. During the early years of aviation, patents filed in different countries could be viewed or accessed through scientific journals, world fairs & patent translating offices. Percolation of ideas was slow and the timespan mentioned above makes the point.

The Horten Brothers

Between the wars in Germany several ‘civil clubs’ sprung up where students trained on gliders under the supervision of WW1 veterans. By the mid-late 1920s, the young brothers, heavily influenced by Alexander Lippisch began experimenting with tailless gliders. Their recollections in later life mention they turned their bedroom, attic & basement at home into a workshop, cluttering their family home in Bonn with airplane models. The home based experimentation was important to their later productions. Their gliders were simple tailless constructions with a cocoon for a pilot integrated into the design. The models focussed on keeping parasitic drag (all objects experience drag through the air) down and had better performance than conventional designs.

By 1931 the brothers had moved their activities to Bonn-Hangelar Field, a gliding club where they had access to mentoring, tools & materials from more experienced aviators. Their first full scale glider was the Horten H-I from 1931 and had a 40 foot wingspan, it was constructed of wood and fabric. The design integrated swept wings & elevons.

By the mid 1930s (1933-1937) the Hortens were further refining their designs at Wassekrupp, Germany’s Mecca of gliding. They too had support from the DFS and constructed their subsequent designs (Horten H-II – H-IV).

Each model kept growing in size. The H-II had a 52 foot wingspan while the H-III & H-IV each had a 80 foot wingspan. The materials used got better with funding from DFS, for example they began using plywood i.s.o fabric. The internal structure moved from wood on the initial models to steel and aluminum in later ones.

H-III was a motorized version which had a 32hp VW engine driving a foldable propeller. Model H-IV was a high performance pure glider.

The models exhibited a bell shaped lift distribution curve across the wing. It is higher near the wing root and tapers off near the wingtips with a smooth non linear profile. This sort of lift balances efficiency & stability and is essential for gliders with no engine to compensate for inefficiencies. The non-linear curve is important as the bell has a flatter peak and falls off at the tips, meaning optimal lift is maintained for longer.

The gliders achieved this with swept wings of up to 30 degrees, the wingtips had washout built into them and had variable chord, meaning they tapered toward the wingtips from the wing roots. An example of this lift efficiency is when the H-III achieved flights as long as 300 km.

Reimar Horten’s focus on lift distribution gave their designs a glide ratio of 30:1 i.e they could glide thirty times their height in distance. The focus on lift distribution is also one of the possibilities of the Ho-229’s ‘ stealth properties’ which we speak of later. Ludwig Prandtl was the scientist credited with presenting the concept of spanwise lift distribution in 1919 , Reimar Horten adapted Prandtl’s insights fifteen years later.

The H-V (1937-1943) was an exception to the materials the Horten brothers used for their gliders. They used experimental plastics. The H-V is considered the very first composite materials aircraft, however the first prototype crashed on its very first flight and Hortens reverted to wood as their material of choice. The H-V had a 46 foot wingspan and was powered by two 79 hp Hirth HM 60 R engines from the mid 1920s, powering pusher propellers. Specs gleaned from wikipedia showed the H-Vb(the second of three built) had a cruise speed of 230 km/h and a landing speed of 70 km/h.

It is just about here we observe the iterative design approach the Hortens took. They alternated each glider model with a motorized version, the H-III had a motor as did the H-V, H-VII & H-IX.

The H-VI (1944) reverted back to pure glider form after the learnings from the H-V and had significant design improvements such as a very high aspect ratio of 32.4 and a 80 foot wingspan. The wings had a sweep back of 20 degrees. The refined control surfaces were drawn from H-V data. The model had extensive stall behavior examination using tuft tests done on it. A tuft test is where strings of yarn (tufts) are attached to the entire wing surface and the aircraft is tested either in a wind tunnel (or in the Hortens case in flight). Attached airflow shows the tufts align smoothly with the laminar airflow. Separated flow is when the tufts begin to flutter erratically. Using this test is important to identify stall onset, control effectiveness, drag data & tip stall mitigation.

The H-VII(1944) was once again the H-V under a new guise. It was powered by two 240 hp Argus AS 10C engines. The V8s powered propellers once again in pusher configuration. The pilot seating was side by side vs the H-Vs semi prone position. Key increments included better control surfaces (elevons, spoilers & drag rudders) and in general a more robust internal structure for longer operations.The H-VII had a cruise speed of 300 km/h and service ceiling of over 21,000 feet. (source wikipedia).

The H-VIII(1945) was an upscaled version of the H-VII. It was sold incomplete to the RLM (ReichsLuftahrtMinisterium a.k.a Ministry of Aviation). It had a 131 foot wingspan and was powered by six pusher propeller engines. Each Argus 10 engine developed 236 hp. It represented a clear step in the direction of military applications and was expected to have a 1000 km bombing radius. The incomplete aircraft was destroyed by the Allies.

The H-IX v3 or the Ho-229 was the aircraft that is responsible for the Horten legend. When the allies got to the Gotha factory they found an aircraft unlike any other they had seen. It had bat-like wings and jets for engines (largely unknown then). The H-IX was a direct evolution of the H-V & H-VII designs. It was powered by two Junkers Jumo 004 turbojet engines buried inside the wings. Each of the engines generated 1990 lbs of thrust. The H-IX had a 55 foot wing span and the wings had a 32 degree sweep. It could fly at 977 kmph and had a service ceiling of 49,000 feet. This aircraft was beyond anything the Allies had to offer in terms of speed and agility.

The tailless wing at such speeds did throw up control related challenges, and in the era before fly by wire computers the aircraft had as many as eight control surfaces for the pilot to manage. The aircraft had a total of four elevons (two per wing), two drag rudders (one per wing) to induce yaw, and two speed brakes to control dives (also known as dive rudders). The v3 was the third in the series after the v1 & v2 and is the only surviving example of the H-IX/Ho-229.

Jack Northrop

Northrop began his aviation journey as a young man in 1916 with the Loughead Aircraft Manufacturing Company. As a mechanical draftsman & engineer, during his first stint there (1916-17) he worked on multiple aspects of the F-1 flying boat. Importantly his work focussed on light weight and high strength structures which would further fuel his focus on efficiency.

By 1917 he was drafted into the US Army where he served as an infantryman, however he quickly transferred to the Signal Corps to analyse Curtiss flying boats. In 1918 Loughead secured his return from the army where he continued his aviation career co-designing the Loughead S-1 a small sports plane that used moulded plywood construction and was known for its drag reducing streamlined fuselage.

Between 1926-28 after stints with Douglas Aircraft, Jack Northrop rejoined Loughead Aircraft (soon to be Lockheed) as chief engineer and designed the Lockheed Vega made famous by Amelia Earhart and her 1932 Transatlantic solo flight. The Vega was known for its low drag coefficient of 0.02. His work on the Vega further refined his expertise and focus on lightweight aerodynamic airframes, contributing to his future work on flying wings.

In 1928 Jack Northrop founded the Avion Corporation focussed on developing all metal aircraft with tailless designs and by 1929 he built the Avion Experimental No 1 (Northrop Flying Wing a.k.a X-216H). While it was a flying wing, Northrop retained a twin tail boom, this was for added safety during testing (wings were still being understood). The wing was made of aluminium and was of stressed skin multi cellular construction. Such constructions distribute loads across the entire wing while reducing weight and maintaining structural integrity. The wing demonstrated low Cd of 0.015 but suffered from pitch and yaw instability. Aircraft company consolidation meant that Avion Corporation was acquired by William Boeing as part of UATC (United Aircraft Transport Corporation) and was renamed Northrop Aircraft. At the time Jack Northrop designed the Alpha, a conventional low wing monoplane mail carrier.

Around 1931 the depression played a major role with Jack Northrop and UATC merged Northrop Aircraft with Stearman in Wichita, Jack Northrop refused to relocate and quit. In 1932 with the backing of Donald Douglas, Jack Northrop founded the new Northrop Corporation as a Douglas subsidiary. He developed the Beta, a faster variant of the Alpha and Gamma between (1932-34). The Gamma was a 700 hp mail & research plane. The most famous was the ‘Polar Star” that was transported via ship to Antarctica. This was followed by the Delta which was intended for passengers, however regulations prohibiting single engined aircraft from carrying passengers at night or over rough terrain curtailed this aircraft. Technically it was a success.

Further to these aircraft Northrop’s multicellular wing design greatly influenced the legendary DC-3. By 1937 Douglas was acquired once again and Northrop who yearned freedom to chase his wing designs quit once again and founded the Northrop Aircraft Inc in Hawthorne California.

The N-1M(1940-41) made its first flight in 1940 as Northrop’s first flying wing. It had a 38 foot wingspan and two 65hp Lycoming O-145 pusher prop engines. The skin was laminated wood around a tubular steel frame. It had an adjustable wingtip with a 15 & 30 degree vertical sweep. Its glide ratio was 15:1 and it proved tailless flight stability.

In 1941 the USAF was looking for a new bomber and authorized Northrop to develop the YB-35 flying wing bomber. As a first step Northrop developed the N-9M(1942-45) a one third scale flying wing with a wingspan of 60 feet and two 400 hp O-540 engines. The aircraft had automatic trim, split flaps & drag rudders which were an improvement over the N-1Ms manual controls. The first airframe crashed in 1943 killing the pilot, the reason being pitch control failure, which prompted redundancies to be built into later aircraft. The aircraft had a cruising speed of 320 km/h and a service ceiling of over 21,000 feet with a glide ratio of 18:1. The numbers validated full scale construction of the XB/YB-35.

The YB-35 Dimensions. Pic source : Wikipedia.

With a wingspan of 172 feet and four contra rotating pusher props the X/YB-35 was a majestic sight. The aircraft used four Pratt & Whitney R-4360 radial engines. The contra rotating gear boxes caused excessive vibrations leading to mechanical failure and stress. The engines & propellers were owned by AAF ( United States Army Air Force) . None in the supply chain had checked the engines for compatibility with the Hamilton Standard propellers, furthermore nobody took responsibility for the shortcomings either. The XB-35 flew a total of 27 flights between the two aircraft and only one flight was deemed satisfactory. Of the 14 YB-35s built only one was completed and that flew a total of seven flights for a total of less than ten hours. The YB-35 continued to be plagued by the same engine problems that plagued the XB-35. Reverting the engines to single propellers resulted in the aircraft being underpowered resulting in low speed handling issues.

Jack Northrop grew frustrated with the engines and attempted corrections, however he had severe limitations as the engines and propellers were owned by the AAF. In the meantime the AAF had turned its attention to jets and ordered two of the YB-35s converted to the jet engined YB-49. By 1948 the troubled YB-35 was terminated, never reaching fruition for reasons beyond its control.

The YB-49 had eight Allison J-35-A-15 turbojet engines, each developing 4000 pounds of thrust.The aircraft immediately hit 40,000 feet and cruised at 587 km/h (wikipedia), however with eight engines instead of four the range effectively dropped to half the YB-35. While the specifications were the same, the YB-49 did have four small passive vertical fins on the wings to help with yaw control. The two wings completed approx 25 flights between them, however both crashed in 1948 & 1950 the first killing all its crew including Captain Glen Edwards after whom Edwards AFB is named.

One more YB-35 was converted to a YB-49A reconnaissance aircraft (with podded engines) however this was never completed either.

Jack Northrop’s dream project was abruptly cancelled in 1950. Northrop himself was deeply anguished to see his dream cancelled and retired in 1952. In 1979 Northrop mentioned the Flying Wing contracts were cancelled because he refused to merge with Convair. Hindsight shows the flying wing program was way behind execution deadlines and over budget, hindsight also shows there was always a place for the flying wing. Alas that was not to be and all the wings were scrapped and none exist today.

The Story of WW2 Stealth Myth vs Reality

The Indiana Jones style discovery of the Ho-229 v3 deep in the German countryside inside a dark deserted hangar created the myth. The fact it looked like no other plane before and was referred to as the batwing only added to the myth, the jet engines solidified it.

The aerodynamic properties of flying wings naturally make them stealthy. The glide ratios of all the powered wings (including the YB-35 & 49) had ratios in the range of 20-28:1 . This compares favorably with the B-2 which has a similar ratio. Physics dictates that all flying wings will look similar and flying wings through the decades attest to this.

The wings were built for speed, their aerodynamics being the enabler. This meant the speed of the Ho-229 was over 75% faster than convention fighters of the time.

The controversial 2009 Nat Geo documentary with Northrop Grumman where a representative replica was made and subjected to RCS tests, showed a 20% decrease in the RCS (Radar Cross Section) properties over conventional aircraft of the time. This combined with the speed of the Ho-229 / H-IX v3 is what would have made the aircraft difficult to counter. Point to note in the documentary was the Northrop Grumman team had difficulty replicating the complex aerodynamic surfaces of the original wing.

A step back from the Horten story tells you this was incomplete. The incomplete H-VIII which was delivered to the Ministry of Aviation highlights the state Germany was in and the increased pace of aircraft iterations (1943-45) along with the H-XVIII Amerika Bomber being just plans on paper point to the incomplete story (much like Northrop’s).

Reimar Horten’s 1983 claim in the book ‘ Nurflugel” about planning for the v3’s successors to be stealthy by mixing carbon in the binding elements & painting the aircraft with graphite sounds opportunistic in view that no hard evidence or documentation was ever found. The Ho-229 did not exhibit any carbon in its adhesives conclusively. The timing of the claim ties in well with the announcement of the B-2 Stealth bomber, and Reimar who for all his achievements was fading into insignificance perhaps wanted to make the best of the reflected glory and renewed interest in the Ho-229. This is the reality.

History finds stories like the Ho-229 irresistible, and there lies the fable.

The B-2 Spirit

By the 1970s military designers were chasing the concept of Stealth. Low RCS is achieved by a cross section of materials, aerodynamic design, electronics & of course masking engine thermal signatures & sound.

By 1979 Northrop’s Tacit Blue program had already proved that stealth was possible and the technology was incorporated in the B-2.

During the 1981 presidential race Ronald Reagan repeatedly dug into Jimmy Carter and his cancellation of the B-1A bomber. In response to this Carter on August 22nd 1980 disclosed the Department of Defence was working on the B-2.

While the development was a black program, the B-2 was less closely guarded than the Lockheed F-117 stealth fighter. The unveiling of the B-2 in 1988 was highly restricted. At least two Northrop employees went to prison for espionage during and after its development.

The B-2 dimensions. Pic source : Wikipedia.

That the wingspan of the B-2 is 172 feet, the same as the YB-35/49 is perhaps a happy co-incidence, however its capabilities are entirely intentional. Its cruise speed is 1010 km/h, range is 11,000 km, and service ceiling of 50,000 feet the numbers are very similar to the Ho-229/YB-49 (except range).

The control issues all the flying wings faced dissipated as computers took over the pilot’s work load and made continuous split second corrections for stable flight.

A very old Jack Northrop was shown a model of the B-2 a few months before his passing in 1981 and he poignantly commented “ I now know why God kept me alive for the last 25 years”.

The B-21 Raider had its first flight in Nov ’23. While it is smaller than the B-2 , it remains just as exciting.

Epilogue

In the centre of the Udvar- Hazy hall at Smithsonian sits the H-IX / Ho-229 v3. Everyday hundreds of spectators file past its still figure as if paying homage. The aircraft that launched a thousand dreams continues to do so.