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XB-70 Valkyrie : The Granny of Supersonic

December 3, 2025 //

Prologue

In the Research & Development Gallery at the National Museum of the USAF near Dayton, Ohio stands an aircraft towering over all the other aircraft in the gallery, the centre piece, in the shadow of its wings stand several other aircraft appearing to shelter there.

This is the XB-70 Valkyrie, arguably one of the most influential aircraft of all time.

The XB-70 Valkyrie with the B-58 Hustler

The Nuclear Deterrence

Before we dive into the XB-70 Valkyrie, it’s important to understand the backdrop which led to her development.

The 1945 nuclear events of Hiroshima and Nagasaki clearly established the importance of nuclear deterrence. The cold war was ramping up and the modified B-29s used to deliver the nuclear ordnance were inadequate.

The Enola Gay & Bockscar the two B-29s that dropped the atom bombs on Hiroshima & Nagasaki.

By 1941 Britain was at the risk of falling to Nazi Germany. America was looking for a new bomber that had at least a 5,700 mile range ( Gander – Berlin roundtrip), the ability to deliver a 10,000 pound ordnence load and return. Furthermore the bomber had to have a service ceiling of 40,000 feet and a cruising speed of approx 275 mph. The events of Pearl Harbor ensured the B-36 would only enter service post WW2.

Convair won the contract and the aircraft was originally designated the B-35, later switched to B-36 to avoid overlap and confusion with the Northrup YB-35 flying wing. The aircraft first flown in 1948 was huge with a wingspan of 230 feet and a length of 162 feet, was propelled by six pusher props. Later models had four turbojets on the outboard wings making a total of ten power plants ‘ Six Turning & Four Burning’(the maximum on any production bomber aircraft ever), could carry over 80,000 pounds of ordnance.

The ten engined B-36 . Six turning & four burning

The B-36 was relegated to obsolescence with the advent of the MIG 15 over North Korea by 1950. It was too slow for the faster interceptors Russia was producing. America needed an all jet powered bomber that was quicker.

The B-47 Stratojet entered operational service in 1951. While the requirement goes back to 1943 for a jet powered reconnaissance bomber, the original model 424 was essentially a version of the B-29 . Following the 1945 inspections of captured top secret German documents on swept wings the jet powered game pivoted on its head. With a 35 degree sweep and a wingspan of 116 feet with wings mounted on the fuselage shoulder, the aircraft was powered by six turbojets. The nuclear capable bomber had a max payload of 25,000 pounds and a range of 2500 miles. With a cruise speed of approx 500 mph the B-47 was the backbone of the Strategic Air Command’s ( SAC) bomber fleet through the 1950s. (Note: The B-45 operated from 1947-59 however had many shortcomings that severely curtailed its usefulness)

The six turbojet engined B-47. The very first jet engined bomber ever.

While the B-47 operated in tandem with the B-36 there was a clear gap in the Range / Payload / Speed doctrine and most importantly reliability, enter the B-52.

The eight jet B-52 is a venerable veteran among bombers globally, first entering service in 1955 and still in active service to this day. With a wingspan of 185 feet and a length of 159 feet, the aircraft cruises at 525 mph, has a range of 8,800 miles and service ceiling of 50,000 feet. The aircraft can carry 70,000 pounds of ordnance and is nuclear capable.

The legendary B-52 with its eight engines. In service for 70 years and counting.

The B-52 prototype with a B-36 in the background.

Through the 1950s aircraft got faster and the push for air superiority quickly moved aircraft into the supersonic era. Starting with Gen. Chuck Yeager’s famous 1947 first in the Glamorous Glennis. Aircraft such as the F-86 Sabre and the F-100 Supersabre made sure that supersonic was here to stay. The Russians were making supersonic strides themselves with their MIG 19 ‘Farmer ‘ . Bombers needed to go supersonic.

The B-58 Hustler. The very first supersonic bomber.

The B-58 was designed with nuclear strike capability and was the very first operational Mach 2 bomber. While the B-58 was a clear statement of intent the aircraft had a limited range of 4,000 miles and payload capacity of approx 20,000 pounds. The delta wing (a recent innovation) made low speed handling very difficult and the aircraft had a high incident rate. SAC issued a fresh directive for new aircraft.

WS-110A

In 1955 the SAC issued ‘ General Operational Requirement No. 38 ‘ the foundation for an operational bomber that had the capabilities of both the B-52 and the B-58. The conventional fuel powered jet version of this requirement was called ‘ Weapons System 110A ‘ or WS-110A.

The specifications of the bomber was a cruising speed of Mach 0.9, 50,000 pound payload and a combat radius of 4,000 miles. Boeing & North American Aviation both were included in round one of the development along with other leading companies.

By the mid 1950s USSR in addition to its supersonic fighters such as the MIG-19 had SAMs (Surface to Air Missile). The missiles were a threat to a Mach 0.9 aircraft. The rules of engagement changed to a Mach 3 heavy strategic bomber and a cruising altitude of 70,000 feet.

The initial designs from both companies had take-off weights in excess of 750,000 pounds and both the proposals were dismissed ‘ being too large ‘. Gen Curtis LeMay, the commander in chief of the SAC is said to have commented on seeing one of the proposals “ this is not a bomber, it is a three ship formation!”

Both companies were told to refine designs.

The NAA & Boeing initial designs for the WS110A.

NACA Supersonic Studies

In 1951 Richard Whitcomb put forward the ‘ Area Rule’. His discovery stated that ‘ Total cross sectional area ‘ of the aircraft was responsible for drag in the transonic ( Mach 0.8 – 1.2) regime and not just the wing cross section. This finding resulted in the ‘coke bottle fuselage’ , a narrowing of the fuselage where the wing cross section came into play.

In 1956 A J Eggers & Clarence A Syverton published ‘ Aircraft configurations developing high lift-drag ratios at high supersonic speeds’. The principle investigated the design concepts of aircraft at high supersonic speeds. The long title would come to be known as compression lift or wave riding.

The 1951 ‘Area Rule’ was first tested on the redesigned F-102A Delta Dagger. The rule which required the original F-102 to be lengthened by 11 feet , with narrowed coke bottle design in the middle, a new canopy along with redesigned wings and a pushed back tail, resulted in a much faster , more stable aircraft that comfortably sustained Supersonic speeds.

A design schematic of the F-104A design changes over the F-104 implementing ‘Area Rule’

The 1956 internal memorandum was studied in detail by NAA and they figured compression lift had to be central to the WS-110A design philosophy along with area rule.

By early 1958 the WS-110A would be officially designated the XB-70. The Air Force had transitioned the project from a concept ( Weapons System or WS) to an experimental program (XB). The name Valkyrie was the winning name submitted by Sgt. Francis Seller in a naming contest held by the USAF. Valkyrie the Norse Goddess is the ‘chooser of the slain’, guides souls lost in battle to Valhalla(the hall of heroes). Valkyrie was chosen from over 20,000 suggestions.

The Canards & Forebody

The XB-70 experienced significant ‘ Mach Tuck’ at high Mach speeds. This was caused by the centre of pressure moving aft as the aircraft accelerated through the speed regime.

The automatic canards managed by the FACS (Flight Control Augmentation System) adjusted continuously to manage the tuck. With a span of 28 feet they were significant in trimming out pitch shifts and helped smooth shock transitions.

The canards & forebody of the XB-70. Sr-71 in the foreground.

The canards worked in conjunction with the elevons on the wing’s trailing edges.

The forebody of the XB-70 like most supersonic aircraft today was sharply tapered through to the canards. The underside as were the sides were not only flat and shallow, but also contoured to create the primary shockwave.

Behind the nose the contour widens and transitions towards the engine nascelles. It is here the coke bottle design is clearly visible.

Please be sure to read an about the evolution of the Flying & Blended Wings in the two part series here. http://theaviationevangelist.com/2025/09/13/the-evolution-of-the-flying-wing-part-one/

The windshield of the XB-70 in flight with the side windows. The second picture shows the XB-70 from 1964, the waisting clearly visible.

The XB-70 used a retractable windshield ( the first of its kind). The windshield serviced multiple purposes. The first was to create a clear aerodynamic line. The second was heat insulation for the cockpit at 600 degrees F (it did heavily restrict forward visibility). To augment visibility, the canopy had flat, heat shielded windows on the sides. Aircraft such as the Concorde and TU-144 followed a similar concept with their droop noses.

The Wings

The large & thin wing area with a high aspect ratio ( the wingspan divided by the mean distance between the leading & trailing edges of the wing a.k.a average chord) managed sub / transonic lift (aerodynamic lift).

The sculpted leading edges of the wing helped control vortices the delta wings generated. Vortex lift is important during high angles of attack (specific to delta wings) during take off and landing. Concorde is a famous example of using vortex lift.

A front view of the XB-70 clearly showing the sculpted leading edges and some elevations raised.

The flat undersurface of not just the wings but also engines ‘6 pack’ was critical to the XB-70’s most important design feature, ‘ Compression Lift’. The wings outer panels ( last twenty of the trailing edge on each side ) drooped by up to 65 degrees. The droop was important to trap the shock waves created off the sculpted engine intake splitter & the intakes themselves.

The sculpted inlet , the vertical splitter being the prominent feature. The second picture shows the six pack and the flat underbody of the aircraft enabling shockwaves to be trapped under the aircraft with the drooping wings.

While most of us think of shockwaves coming off a supersonic aircraft horizontally, the splitter was responsible for generating shockwaves vertically, these waves being trapped by the folded wingtips creating a wave cushion. The XB-70 generated up to 30% of required supersonic lift through compression lift. Shock waves would bounce into the engine inlets too. The folded wingtips improved yaw handling a great deal and the XB-70 needed much smaller vertical stabilizers as a result.

The XB-70 is the first aircraft to use three different kinds of lift across the speed regime. The swept back wings at 65 degrees reduced transonic drag and improved handling.

The wings flexed and bent considerably through the speed regime. To help keep the wing flexible the engineers at NAA intuitively integrated six elevons (combination flaps & airelons) on each wing and avoided binding the wing. Furthermore by doing so they managed extreme hinge and actuator loads inflight (hinge moments).

Detailed schematics of the XB-70 showing the crew capsule (top picture top left corner ) and elevons (both pictures).

The six elevon setup gave the FACS more flexibility as it managed pitch / trim (inboard elevons) and roll (outboard elevon). As the wingtips drooped (25 – 65 degrees) the two outboard elevons were faired to zero and became part of the folding wingtip. Lastly, having six elevons helped with redundancy. All hydraulics on the aircraft were at 4,000 psi.

AV1 had a flat wing with zero degree dihedral, while AV2 had a five degree dihedral as a design refinement. This gave AV2 better directional and roll stability over AV1 and also gave AV2 better compression lift efficiency.

AV2 was unfortunately lost on June 8 , 1966 during a formation photo flight. General Electric had a photo session using the XB-70, F-104 Starfighter, F-4 Phantom II & a T-38 Talon. All of them used GE engines.

Test pilot Joe Walker (the most experienced supersonic pilot then) in his F-104 was sucked into the starboard wingtip turbulence of the XB-70, flipped over the vertical stabilizers of the XB-70 and crashed in a fireball. The doomed XB-70 flew level for a few seconds before going into a steep spiral and crashing, taking with it co-pilot Carl Cross. Pilot Al white ejected using the crew escape capsule engineed for high altitude ejection or depressurisation while retaining control of the aircraft (in event of depressurization).

The Engine Nascelles & Intakes

The engine nacelles not only fed the engines with air but also were an integral part of the compression lift generated by the XB-70.

The entry was split by a vertical splitter fins. The engines were split three on each side. They also projected the airflow towards the drooped wing tips to trap shockwaves. The nacelles created oblique shock waves at the inlet lips as they began slowing air to about 400 mph from supersonic speeds as stable air was directed to the engines. This kept engine compressor pressure within a constant bandwidth. The trailing edges of the three moveable ramps behind the engine inlets hinged inwards or outwards (between 10 – 30 degrees or upto one foot) as per the Mach number and compressor requirement. The entire system including the ramp angles & bleed doors (for excess air) was continuously adjusted by the inlet control system.

The engine nacelle had a 2D rectangular configuration and had a maximum height of 4 feet. The length of the intake from the nacelle to the engines was approx 30 feet.

A front view showing the engine inlet and splitter in detail

Rare pictures of the inside of the intakes at different depths inside the 30 foot intake .

The underside of the entire intake ramp was flat as it aided in compression lift.

The Engines a.k.a ‘The Six Pack’

The XB-70 had six General Electric ( GE) YJ93-GE-3 turbojet engines.

Each axial flow engine generated 19,900 pounds of dry thrust and 28,800 pounds with afterburners. The engines had no thrust reversers and used drogue chutes as a stopping device. With eleven compressor stages and of which six were low pressure and five high pressure.

The engines were made of Nickel based alloys and stainless steel. Advanced blade cooling allowed the engine to survive high exhaust gas temperatures (EGT). The engines used high flash point JP-6 fuel.

The engine control system synchronised with automatic inlet control management to prevent compressor stalls and upstarts (happens when airflow to engines is unstable due rapid speed changes).

With so many different systems working in tandem on such a precision piece of engineering the YJ93 was a high maintenance product.

The Landing Gear

The XB-70 had the conventional hydraulic tricycle gear.

The rearward folding nosewheel had two wheels.

The main gear had two bogies with four wheels each. The main gear had a complex mechanism of folding the bogie in, then a twist and then folding into the wheel wells. The wells had a flap that closed and aerodynamically sealed the wheels inside.

The main gear each had one small wheel between the outer pair of wheels. This small wheel acted as a braking sensor was an early ABS mechanism. During rejected takeoffs the brakes could heat up to 1,000 degree F.

The landing gear struts were made of forged chromium-molybdenum steel for its exceptional strength and fatigue resistance. The struts were more than capable of handling the 500,000 pound gross weight during heavy landings at over 200 knots.

The tyres were made by Goodyear and had aluminium woven into them to withstand the high landing temperatures of over 300 degrees F. Each tire was Nitrogen inflated to over 250 psi.

The XB-70s brakes had a multiple disc setup. Each disc is made of forged steel. They were heat treated to resist warping and cracking under extreme thermal loads.

The Fuel System

The aircraft carried approximately 43 – 46,000 gallons of JP-6 Fuel. Everything about the system was about managing heat, aircraft stability & structural integrity in addition to feeding the engines optimally across the speed range.

Fuel was stored across eleven fuel tanks distributed across the fuselage and wings of the aircraft. The tanks themselves were constructed using the same honeycomb sandwich panels used for the fuselage skin. The honeycombing did throw up sealing issues which was resolved using advanced epoxy compounds. Although some tanks never properly sealed and hence were never used (ex: the tail tank).

Using the JP-6 fuel as a coolant was a first ! The fuel was circulated through ten heat exchangers throughout the aircraft to absorb and dissipate heat. The heat exchangers were part of the engines fuel pumping system enroute to ignition.

The tanks themselves had heatsinks within each of them to draw excess heat. Furthermore to prevent vapor ignition the tanks were inerted using 700 pounds of liquid nitrogen held in dedicated tanks. As fuel was consumed nitrogen filled the empty tanks to maintain pressure, displace oxygen and reduce fire risk at elevated temperatures.

The fuel management system was integral to the Centre of Gravity Management system. The system actively transferred fuel between tanks as Mach numbers increased. As speed increases the aerodynamic centre of the aircraft moves rearward. The centre of gravity needs to coincide with this to avoid a Mach Tuck. By drooping the outer wingtips in conjunction with its canards, the aircraft effectively moved the centre of pressure forward. The fuel management system worked in conjunction with the compression lift mechanism by moving fuel forward to balance the rearward move of the aerodynamic centre. The wing tanks were typically burned off first.

We observe here that all systems were dependent on each other to maintain stability.

Lastly the JP-6 fuel was specifically developed for the XB-70 program and its extreme speed regime. It addressed the issues of aerodynamic heating, high speed engine performance and safety & high altitude operation. The fuel performance exceeded all the XB-70 operating parameters and was developed as an alternative to zip fuels (high calorific value boron based fuels). Zip fuels had caustic byproducts that caused engine wear and posed toxicity risks.

Kerosene based JP-6 was the safe alternative that provided for all requirements without the byproducts.

The Materials of the XB-70

Over ninety percent of the external structure of the XB-70 ( fuselage skin, nacelles ) was made of type 321 stainless steel built as a honeycomb structure. The material and construction had high thermal resistance of up to 600 degrees F with minimal distortion at Mach 3. The structure itself was rigid, lightweight and thermally stable.

The hot areas such as engine bays & aft of bays & internal structure was made of a titanium alloy called Ti-6Al-4V also known as Grade 5 titanium. The alloy was 90% titanium, 6% aluminium, 4% vanadium and had excellent thermal resistance of over 1000 degrees F with an excellent strength to weight ratio.

High temperature adhesives used to bond the honeycomb structure were made of redux and epoxy adhesives. The honeycomb structure could not be riveted as it would weaken the structure.

Non heat zones such as avionics bays, hydraulic lines & non load bearing fuselage sections were made of aluminium alloys as they were light weight, easy to machine and cooler.

The engine and exhaust area materials were made of Inconel & Rene 41. These alloys can resist very high EGTs in the range of 1,800 degrees F.

All coatings and sealants had heat resistant coatings to prevent oxidation and surface degradation due high temperatures. The sealants protected the honeycomb edges from moisture intrusion & thermal cycling damage.

Strategic Bomber to Experimental Research Platform

By the late 1950s the US & Soviet SAMs were getting bigger, faster and more powerful. President Eisenhower was a proponent of the ICBMs (Inter Continental Ballistic Missiles). His take on the XB-70 program was that “ building the XB-70 was like fighting with bows & arrows in the era of gunpowder and guns” The XB-70 just could not cope with the banks of Soviet SAM systems coming online across the entire USSR.

Gary Powers was shot down in a U2 over the USSR at 70,400 feet and this would prove President Eisenhower’s prophecy.

The program was cancelled in 1959, however to salvage the considerable expenditure already incurred (over $300 mn) the Pentagon authorized the production of a single vehicle. AV1 was almost completely handbuilt.

Pic 1 shows the XB-70 with flaked off paint after a supersonic run. The second picture shows the XB-70 with the A-12 Oxcart.

The XB-70 program is a great example of how politics directs expenditure. As the political tug of war continued NAA was caught in the middle of a fierce battle. The Air Force continued to support the program and even attempted to reinstate it as a combat test vehicle.

The 1960 election of President Kennedy brought fresh impetus to a failing program, the President switched the program from a manned bomber to an experimental aircraft. A total of three were to be constructed, however only two were ever completed, the third was incomplete (the avionics and other systems were actually ready).

NAA should be commended for sticking through the program at each step. Finally there was consensus across all stakeholders including the Air Force, Politicians, NASA & of course NAA.

The XB-70 in the Air

Total flights – 129

AV1 – total flights83
Total flight time – 160hrs 16min

Mach 3 flights – 1.

AV1 had several design issues that restricted speed to Mach 2.5

AV2 – total flights 46

Total flight time – 92hrs 22min

Mach 3 flights – 9

On May 19, 1966 AV2 flew at Mach 3 for 32 consecutive minutes.

Combined, the XB-70 Valkyrie accumulated a total of 1hr 48min at Mach 3+.

Each flight of the XB-70 was an adventure and there were several incidents.

The Legacy of the XB-70 Valkyrie

The XB-70 was an aircraft of many firsts, later adopted for use by the Aviation / Aerospace Industry. Below are listed a few of them!

Variable geometry wings later adopted by aircraft such as the B-1A/B Lancer. Compression lift later used by the SR-71. The overall aerodynamic stability of XB-70 influenced several other projects.

Material and thermal management solutions advanced the development of heat-resistant structures and cooling systems, impacting aerospace exploration technologies.
Fuel and propulsion innovations directly contributed to the SR-71 and indirectly to modern jet engines and fuel systems, particularly for high-speed and high-altitude operations.
Avionics and automation laid groundwork for modern flight control and safety systems, enhancing reliability and reducing pilot workload in complex aircraft .
The XB-70’s strategic obsolescence redirected military aviation toward low-altitude and stealth technologies, while its test data shaped research and development for decades

Epilogue

Over 50 years after her last flight in 1969 the XB-70 at the National Museum of the United States Airforce, looks ready to take off and fly away to the clouds where she belongs. Makes you wonder what she would have been like in the air? A combination of size, speed, sound ,smoke & incredible power all coming together creating a show like none other.

Perhaps the Valkyrie’s greatest message to future generations is ‘ Always be innovating, it’s the only path forward’.

The XB-70 says good bye as she accelerates to Mach speed with her wingtips down to 65 degrees….

Credit for all pictures to the respective owners.

Please be sure to read about the Flying & Blended Wings, a two part series here. http://theaviationevangelist.com/2025/09/13/the-evolution-of-the-flying-wing-part-one/

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Tags B-29, B-36, B-47, B-58, Boeing, canards, cockpit, compression lift, construction, contouring, crash 1966, Design, engines, Experimental, fuel tanks, Mach3, moving windshield, NAA, Nuclear deterrence, paint, SAMs, six pack, SR-71, Strategicaircommand, Supersonic, Surface to air Missiles, USAF, USAF Museum, WS110A, XB-70Valkyrie, YB-35

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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.