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The Amazing Blackbirds

November 21, 2025 //

SAMs , Satellites & Unseen Speed

The very first SAMs were the German V2’s from WW2. While their value was being understood they still had a long way to go as on the range & accuracy parameters.

The US began developing its missiles from the late 40s onwards and by the mid 50s had batteries of Nike Ajax missiles to guard against Soviet bomber attacks. By 1955 the Soviets themselves had the S-25 Berkut system and the famous S-75 Dvina came into being by 1957 having range, speed & accuracy.

The Soviet Sputnik launch of 1957 started off a whole new Cold War race and it was dominance from space. However satellites were still in their infancy and the CORONA & GAMBIT missions were still between 5 and 10 years away. Missile technology had a head start over satellite tech.

By 1956 the recently released U-2, Dragon Lady was already being painted regularly by Soviet defense systems, however the U2’s cruising altitude of 70,000 feet was still thought to be out of range of Soviet missile systems, even at its subsonic speed. Gary Power’s being shot down in 1960 only reinforced the need for speed, altitude and agility, the need for a Blackbird (which was already in development).

However even before the 1960 incident a fresh thought went through the US Armed services and it was speed. The recently launched B-58 Hustler had shown that Mach 2 was possible (if a little dangerous) and the various wings of the armed forces and CIA began to look at Mach 3 as the speed benchmark, stealth was not yet in the picture.

The WS-110A or what would become the XB-70 Bomber already underway was in trouble even before it got off the ground as it was believed Soviet SAMs could take down a large bomber with no stealth capabilities, the XB-70 would become an experimental aircraft as an attempt to just their cost of over $1.5 Bn for two vehicles or $750 Mn a pop! (Read here about the XB-70)

(https://theaviationevangelist.com/2025/10/10/xb-70-valkyrie-the-grand-daddy-of-supersonic/ ).

At around the same time and in parallel to the XB-70 program the CIA went to Lockheed to develop a Mach 3 capable reconnaissance aircraft that flew at over 90,000 feet (considered untouchable by SAMs) and would be difficult to detect by Radar. Lockheed with previous experience developing the U2 Dragon lady which had a service ceiling of 70,000 feet albeit at subsonic speed looked the right outfit to build such a plane.

This is the story of the Blackbird Family of aircraft and it all started with the A-12 Oxcart, an ironic name considering the A-12 was the exact opposite of an Oxcart.

*A pencil sketch of the Blackbird by my Daughter from a few years ago. Hung in my office.*

Project Archangel

In Apr ’58 Kelly Johnson the legendary head of Skunk Works said ‘ I recall having long discussions with (CIA Deputy Director for Plans) Richard M. Bissell Jr. over the subject of whether there should be a follow-on to the U-2 aircraft. We agreed … that there should be one more round before satellites would make aircraft reconnaissance obsolete for covert reconnaissance ‘.

The CIA’s hunt for a U-2 successor was called Project Gusto and by 1958 the two finalists were Convair with their Kingfish and Lockheed with their Archangel. Convair’s Kingfish had a lower cross section than Lockheed’s A-3 concept. Both companies were asked to refine their designs and here is where Lockheed pulled into the lead.

*The A-11 that would be modified to become the A-12. Pic Source : Wikipedia*

Following the A-3, (the A stands for Archangel) Lockheed’s iterations A-4 to A-6 used Blended Body Fuselage (BSF) designs along with turboramjet (more on this later) & rocket propellant, but they fell well short of the range requirements. Iterations A-7 to A-9 used a single J58 engine (just the turbojet) with two Marquat XPJ-59 Ramjets that used J-150 fuel, a highly classified type of fuel the JP stands for Jet Propellent and was expected to improve range, however still well short. The A-10 used two GE J93 turbojets (same as the XB-70 Valkyrie) with underwing inlets for better range, however the iterations continued to fly short of the required parameters. Iterations on the A-11 Lockheed added twin inward canted fins that were angled inwards at 15° made of composite materials, other leading edge surfaces featured composites as well, together the improvements went a long way towards improving RCS ( Radar Cross Section) of the aircraft. To add to these design improvements the wings were extended through chines that went right upto the cockpit and the bottom of the aircraft flattened with the wings blended into the fuselage, the improvements won Lockheed a $96.6 Mn contract to construct a dozen A-12s. The dozen airframes would extend to 18 if you include the three airframes used for the YF-12 , one trainer and two M-21 aircraft. Project Archangel / A-12 was underway.

*The A-12 design: Pic Source : Wikipedia*

The J-58 Engine

While the A-12 was an amazing aircraft design that is yet to be replicated almost 70 years on, it is the insane engineering that went into the engines of the aircraft that needs to be spoken of first.

The J-58 Turboramjet!

The external dimensions of the engine were a length of 17.10’ a diameter of 4.9’ and weight of 6,000 pounds might feel puny by today’s standards, the engineering that went into them is unique.Pratt & Whitney JT-11 Mach 3+ jet engine (J58) . (strongly recommend a watch). The engine generated 30,000 pounds of thrust with afterburners and had 8 compressor stages.Pratt & Whitney J58 (JT11D-20) Turbojet Engine | National Air and Space Museum

Sometime between 1956-58 the US Navy approached P&W to develop a Mach 3 capable engine for their planned Martin P6M Jet Seaplane. P&W had begun testing their prototype when the Navy realized the costs involved did not justify an aircraft when their main weapons were ships, submarines & missiles. The Navy pulled out by 1958ish. The CIA, which already had Lockheed in advanced design stages of their project Archangel/A-12 had obviously heard of this engineering marvel and approached P&W to continue development on the J-58…and the rest is history.

*The J-58 engine. Note the three pipes heading towards the afterburner. The plate right upfront on the side is the hydraulic computer. Pic Source : Air & Space*

Coming back to the turboramjet, a couple of definitions.

Turbojet definition : In a turbojet all the air that goes in the front is sent through the compression stages, fired up in the combustion chamber and the resulting exhaust gases generate thrust.

Ramjet definition : A ramjet is a type of engine that uses the forward motion of the aircraft to compress air and fire it up. Such an engine has no moving parts and aircraft using such engines need to be launched off another transport aircraft generally.

So why did the J-58 need both?

The J-58 was optimized for Mach 3.2 cruise and such high speed generates heat in the excess of 750°F which would melt the internals of the J-58 turbojet. A solution was required and here lies the engine’s unique feature, the frontal spike and six tubes running (three on each side) from the stage four compressor straight back to the afterburner section (a type of bypass).

The J-58’s variable geometry spike is where over 50% of the engine’s thrust is generated, but first another bit of information. At Mach 3.2 the compression at the engine’s inlet was almost as high as the thrust generated out the back, the engine would be in a neutral state of thrust, and in some cases negative (this is where the inlet management is critical). The pressure recovery on the J-58 is at 88% showing it is highly efficient at Mach 3.2.

The spike moves front and back by 26”. Right up to Mach 1.6 the spike stays in the full front position and the engine operates as a normal turbojet. At Mach 1.6 the engine begins moving back 1.6” for every increase in speed by Mach 0.1. The spike itself moves backwards into a conical receptacle and the backward movement of 1.6” for Mach 0.1 increase in speed maintains the ‘normal’ just behind the throat of the spike receptacle. The normal is the point where dynamic pressure switches to static pressure, and the movement of the normal is carefully calibrated by the spike to maintain optimal thrust across the speed Mach 1.6 – Mach 3.2 range.

At approx Mach 2.2 sensors detect that airflow and temperature are right to begin turboramjet operation by opening up the compressor bleed bypass valves, these valves are placed at the fourth compressor stage, and direct ram air through the tubes direct to the afterburners. The air is approx 400°F and helps keep the combustion chamber and turbines relatively cooler and within thermal limits. The afterburner fires more efficiently as a result of the cooler air.

*A schematic showing the various engine regimes. Pic Source: Wikipedia*

At Mach 3.2 the engine’s spike aligns the shockwave with the engine’s nacelle perfectly. The engine has a series of doors that maintain optimal pressure through the entire length. The cowl bleed doors is a porous strip on the inlet’s inner surface, the purpose is to bleed off excess boundary layer air and prevent an unstart at high speed. Further back the engine has suck in doors, these doors open up at low speeds (below Mach 0.5) such as the beginning of a take off roll to feed the engine with more air and aid low speed thrust generation. Furthermore at low speeds right at the afterburners are tertiary doors that automatically open and close as per ambient pressure relative to the exhaust gases, these doors let in additional air as required. The spike itself has a porous strip that manages slow moving boundary layer air. At low speeds the engines are extremely air hungry and this creates a low pressure area at the engine nacelle, the strip pulls in the air into the centre body and vents it out through centre bleed louvres. The air reverses direction at approx Mach 1.5 the air inside the spike centrebody duct reverses.

There is a story of a SR-71 pilot who decided to speed check his bird and got up to Mach 3.4 before he swallowed his own shockwave, flaming out both engines at 80,000 feet! He recovered one at 65,000 feet and the other at 25,000 feet. There was of course a discreet rap on the knuckles!! This story does highlight the fine balance within the engine and how it was optimized for Mach 3.2.

A look at the engine shows a tremendous amount of plumbing, not all of it is air, oil or fuel!. On each side of the engine nacelle is a hydraulic computer, yes hydraulic! The plumbing you observe is the computers optimizing engine operation. One of the computers is to manage the afterburner and the other is for the engine. The J-58s were created when computers were in their infancy and a solid state system was required that could withstand high temperatures and work optimally, hydraulic computers were the option.

The operating temperatures expand the engine by 6” in length and 2.5” in diameter and this sort of expansion and contraction needs exotic metals. The very front of the engine at the nacelle is titanium, the rest of the engine is made of iron nickel alloys such as Waspalloy, Inconel & Astrology. All the metal in the engine is directionally solidified so the metal expansion is directional and can be managed. The plumbing on the engine is made of steel 321 and 347 and there are over 600 pieces of plumbing on the engine.

The oil used in the engine is synthetic, made of polyphenyl ether and is stable at 650°F. The oil is maintained at 400°F by routing through a fuel oil cooler, a heat exchanger where the oil contacts with the cooler fuel heating it up and cooling itself before the fuel is routed into the engine.

The complex system was started by two V8 Buick Hellcat motors which were a petrolhead’s delight, apparently the crew blew through most of the Buick motors that salvage yards across the United States had with them. The two motors would spool up to 6000 rpm and the crank interfaced with a gearbox at the bottom of the engine and needed to retract as the aircraft engines got to 3000 rpm, the J58s fired up at 4000. The crew got so carried away with revving the Hellcats that they delayed retraction blowing their engines up! Once the Hellcat stock was run through the crew moved to Chevy 454 cu.in engines, but they were not the same.

At Mach 3.2 over 50% of the engine’s thrust was created at the inlet and an additional 28% at the afterburner. This left just about 20% of thrust needed from the turbojet! While the first A-12s flew with less effective J-75 engines, once they cutover by 1963-64 to the J-58, the blackbirds never went back to anything else.

The Design

The external dimensions of the A-12 Oxcart (the foundational Blackbird) was a length of 101.7’, wingspan of 55.7’ and a height of 18.6’. The MTOW of the aircraft was 124,600 pounds.

A front view of the aircraft showed off a flattish underbelly with blended in wings at the fuselage. A sharp angular cockpit at the very front and twin tail canted in at 15° each. The flow of the wing’s leading edge was interrupted by heavily integrated engines on each wing right in the centre.

A *front view of the SR-71 note the canted fins, the flattish underbelly and the blended wing fuselage. Pic Source: Reddit User*

The nose of the A-12 looks more conventional than the Blackbirds that followed. While it slopes up towards the angular cockpit windows in a more or less conical manner, the bottom is more flattened to merge with the rest of the flattish underbelly. This sort of contouring is necessary to manage shockwaves and keep the aircraft aerodynamically optimized.

While Blended Wing Bodies have existed since the early days of flight, they had never been used practically. The blackbirds are not traditional BWBs (as we know them since the 1990s) in the new sense they are what is called a Blended Wing Fuselage. Read here (https://theaviationevangelist.com/2025/09/19/the-flying-wing-part-two-the-blended-wing-body/ ).

The chines that begin on each side of the cockpit at a sharp angle of approx 70-80° and swept back towards the delta wing were an integral part of the BWF serving multiple functions. The first was stealth (yes the A-12 is the very first purpose built stealth aircraft https://theaviationevangelist.com/2025/10/22/the-theory-of-stealth/ ). The specially designed edges with their composite materials reflected radar waves away from the source and reduced the aircraft’s RCS to about 10m2 or a largish bird, a big improvement of over 90% over the RCS signatures of preceding aircraft Reducing the A-12 Blackbird’s Cross Section. The second purpose was the chines served had a critical to the aerodynamics of the aircraft and that was to generate lift. They worked to generate approx 17-20% of total aircraft lift in two ways. The first was the creation of vortices over the chines, inner wing and fuselage, delta wings with a sharp leading edge sweep, at high Angles of Attack (AoA) rely on vortex lift . The second is the blended and flattish underbelly works as a lifting body and contributes towards the 17-20% lift. This means the load is off the wings and more evenly distributed which is critical at high Mach numbers. The reason the chines were terminated at the cockpit i.s.o going right to the nose like the SR-71 was the A-12 was a single pilot aircraft and the chines terminating at the cockpit saved weight and were optimized for higher speeds at altitudes of up to 95,000 feet.

The chines blended into a delta wing with a leading edge sweep angle of 60°. The edge of the wing was interrupted in the middle by the engine nacelle.Close observation of the leading edge and the engine shows up a gap on both sides of the engine, this was to accommodate the 2.5” expansion of the diameter of the engine and boundary layer control. On the trailing edge the gap is more pronounced as this was the business end of the engine with the hot exhaust gases. Other than this the wing was fairly standard in the front view profile! A top view of the wing shows a second chime that comes off the outboard engine cowling on both wings blending back into the leading edge, these chines increase the aspect ratio of the swept back delta improving lift.

The trailing edges of each wing had a pair of elevons, one inboard and one outboard of the engine. In tailless delta wings the elevons serve the purpose of the elevators and ailerons. When they move together they control pitch and when they work opposite to each other, they control roll on the aircraft.

Further back is a pair of twin fins each canted in 15° as mentioned earlier, the canting is part of the aircraft’s stealth and the original fins were made of composite (because of they non reflective properties), however most of the aircraft in the entire Blackbird fleet used titanium fins with composite accents.

The entire Blackbird was a flying fuel tank. Fuel was stored in six tanks throughout the body and wings including the chines. The fuel was burned in a specific sequence as the center gravity moved significantly rearwards at higher speed numbers. The Blackbirds famously had wet wings. That is the skin of the wings and body of the aircraft was the fuel tank itself. In the interests of saving weight and the fact the titanium skin of the aircraft was heat resistant, the fuel was stored directly. The thermal expansion in flight meant the panels had gaps on ground and there were thresholds by area of the aircraft as to the number of fuel drops falling per minute that was acceptable. The same gaps sealed in the air as the metal expanded.

*Acceptable fuel leak range by zone of the aircraft. Pic Source : Reddit User*

The aircraft had a tricycle landing with the main gear having three wheels in parallel. The main nose gear had a single two wheel bogie. The Goodrich tires were infused with aluminium for thermal resistance and were inflated with nitrogen, a non combustible inert gas for safety.

Most of the aircraft was constructed of titanium because of its thermal resistance, however titanium is extremely hard to work with and a specialized set of rigs and tools had to be created to work with the metal. At the time the Blackbirds were being constructed the Soviet Union was the largest exporter of titanium and the CIA procured the required titanium through a series of shell companies making the final buyers (the CIA) untraceable.

The wings of the aircraft had corrugation on the top and bottom prompting jokes that Kelly Johnson was building a Mach 3 Ford Trimotor (an early airliner). The corrugation was to aid thermal dissipation and while there was a drag penalty at lower speeds which was powered through, at Mach 3 and over 80,000 feet the drag was minimal.

The aircraft was painted black with iron ball paint. The paint helps with stealth by converting radar waves to heat and dissipating it. Furthermore according to Kirchoof’s Law of Thermal radiation a good absorber of thermal radiation is also a good emitter, means that the black iron ball paint is the right color to repel heat by emitting it!

With a first flight in April 1962 the A-12 quickly demonstrated its capabilities even with the less capable J-75 engines. The USAF which was initially part of project GUSTO quickly realized this was an aircraft that was the answer to its need for a high speed aircraft. They put out the requirements for RS-71 (Reconnaissance Strike) by approx 1963, it was President Johnson who called the aircraft SR-71 erroneously and the name stuck. Furthermore the A-12 needed to be kept classified (which it was until 1990) and the USAF’s requirements for a high speed aircraft made a great cover story in 1964 when the SR-71 and YF-12 projects were announced. The M-21 Tagboard was never officially announced during its active life. TheYF-12 and the M-21 aircraft had approx same dimensions as the A-12 Oxcart while the SR-71 was longer and bigger. The M-21 aircraft had a pylon on top between the two fins to fit a D-21 drone on it. Of the two prototypes built, one crashed in 1966 when the the D-21 drone collided with the fins after separating, the plane crashed while the pilot survived, the M-21 was cancelled immediately after this and the surviving prototype is at the Museum of Flight in Seattle. Lockheed M-21 (Blackbird) | The Museum of Flight .

*The M-21 with the D-21 drone. Pic Source : Wikipedia*

The YF-12 took spots 7-9 on the A-12 Oxcart assembly line and was a Mach 3 interceptor prototype. It was to be a replacement to the F-106 Delta Dart, however severe cost cuts in view of the Vietnam War resulted in the program being scrapped. The main modifications was cutting the A-12’s nose chines to accommodate radar and infrared tracking equipment. The chines of the YF-12 show a clear indentation. Today of the three aircraft constructed only one survives at the USAF Museum in Dayton Ohio, it flew with NASA until 1979 after the YF-12 program was cancelled in 1967.

*The YF-12 Interceptor. Note the truncated chines. Pic Source: Wikipedia*

The YF-12 with modified chines to accommodate the radar equipment. Pic Source : Wikipedia

The Lockheed SR-71 Blackbird is a fairy tale of an aircraft, it has been immortalized in movies, books, articles like this and forum across social media with a huge fan following even 60 years after its first flight. Where the A-12 was heavily classified decades after its operation, the SR-71 was heavily publicized (to cover the A-12) and this is why the SR-71 is considered the most famous of the Blackbirds. Lockheed SR-71 Blackbird | Military Wiki

The SR-71 was to have a two man crew as against the A-12 single pilot. And where the A-12 carried a high resolution camera system the SR-71 carried a sensor array that included Side Looking Radar (SLR) and Electronic Intelligence Systems (ELINT). Where the A-12 was about covert photography for the CIA (the aircraft was disguised in USAF markings) the SR-71 was more about strategic reconnaissance (SR) for the USAF. To accommodate the radar installations, the chines were extended to the nose in the manner we know so well. The chine extensions on the SR-71 had the same lifting and stealth properties of the A-12, where lifting contribution remained at the same 17-20% as the A-12, the RCS was slightly higher than the A-12 but not by much (it was the larger bulk).

In case you are wondering why the A-12 on the USS Intrepid has the chines right to the nose tip, it’s because it was used as a radar object when understanding the stealth characteristics of the SR-71.

*A front view of the A-12 at the USS Intrepid. Pic Source: Wikipedia*

The SR-71 was longer than the A-12 by six feet to accommodate the second crew member and had a length of 107.5’. The wingspan and height of the aircraft was identical to the A-12.The dry weight of the aircraft was 6 tons heavier than the A-12 and MTOW was 22 tons heavier than the A-12. The additional bulk and mass made the SR-71 slower than the A-12 whose max speed was Mach 3.35 vs the SR-71s Mach 3.2. The SR-71s service ceiling was 85,000 feet vs the A-12s 95,000 feet. The range of SR-71 was 3250 miles vs the A-12s range at 2500 miles.When we see a comparison of the numbers we realize the A-12 Oxcart is just not celebrated enough.

Project Nice Girl

Project Nice Girl was the face off between the A-12 & the SR-71. The costs of running multiple high cost projects for the various services was getting out of control and in the autumn of 1967 the A-12 & the SR-71 had a play off. While the A-12 had superior speed and altitude , it was hampered by cloud cover during the fly off and the high resolution panoramic cameras on the A-12 were beaten by the SR-71s sensors that could peer past the clouds and collect valuable accurate data. The dividing factor was beating the weather and the A-12 was retired in 1968, the project was only declassified in 1990 and the aircraft handed over to museums across the United States.

Summation

As satellites got better and were in a position to take over from the considerable duties the SR-71, the amazing bird saw its days numbered. Additionally astronomical sosts of keeping the birds in the air just did not make sense to keep them flying and the decision was taken to retire the program.

Over sixty years after it first flew the SR-71 and the Blackbird Family of Aircraft continue to inspire awe, several of the projects they were involved in continue to be classified and this is what contributes to their enduring legacy. Their speed and altitude records intact over 35 years after the last flight of a Blackbird.

The peak of innovation…

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Tags A-12, aerodynamics, aluminum dust, blackbird, Blended Wing Body, blended wing fuselage, boundary layer, Buick, BWB, bypass, chine, CIA, D-21, delta wing, Drone, engines, expansion, J-58, landing gear, Lockheed, M-21, NASA, nascelle, Navy, nitrogen, Radar Cross Section, ramjet, RCS, resistance, SR-71, stealth, thermal, thrust, tires, turbojet, turboramjet, USAF, V8, YF-12

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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.