And would it have been any good?

The writer Luke Turner shared a link with me via X leading to a sketched cutaway from 1945 of a formidably armed fighter-interceptor labelled as the ‘Westland Wildcat’. The twin-boom pusher design, reminiscent of the Saab 21, was plausible, but what was it?

The Westland aircraft company had created the revolutionary Whirlwind fighter design in 1938, was this sketch evidence of another innovative fighter that never made it into production? Sadly, this article is no exception to Betteridge’s law that the answer to a question in a headline is always ‘no’. The Wildcat, which is no relation to today’s helicopter of the same name, was an attempt at disinformation by an incarcerated Marine pilot.

According to the British National Archives “Guy ‘Griff’ Griffiths is a Royal Marine pilot and one of the first naval officers to be captured. He found distraction from captivity in dreaming up a scheme for feeding misleading intelligence to the German authorities whilst imprisoned in Dulag Luft – creating sketches of fake British aircraft which he leaves around the camp for guards to find.”

Guy Beresford Kerr “Griff” Griffiths (6 June 1915 – 12 July 1999) was a Skua pilot at the start of the war. Eleven days into the Second World War, three Skuas were dispatched from HMS Ark Royal to defend a merchant ship against a German submarine attack. This was the first British Naval bombing of the conflict, and one of the pilots was Griffith. Due to incorrect fuse arming, the Skuas bombs damaged the tails of two of the three aeroplanes, which crashed into the sea with the loss of both air observers. Griff, along with fellow pilot Thurstan, survived, becoming the first naval officers captured in the war. Griffiths’ spirit of defiance survived captivity and he used his artistic skills to create fake British warplane designs to confuse and waste the time of Nazi intelligence (he also forged documents to aid escape attempts). He was freed in 1945 and become the first Royal Marines officer to fly a helicopter. During the Korean War he visually identified the first downed MiG-15.

The Westland Wildcat

So would the Wildcat have worked in real-life? Jim Smith had significant technical roles in the development of the UK’s leading military aviation programmes from ASRAAM and Nimrod, to the JSF and Eurofighter Typhoon. He was also Britain’s technical liaison to the British Embassy in Washington, covering several projects including the Advanced Tactical Fighter contest. We asked him his opinion of this intriguing design:

“The Wildcat concept is a compact, single-engine, twin boom-design that shows its Westland
pedigree in the compact central fuselage nacelle, pusher engine and heavy armament, with nose-
mounted cannon supplemented by additional weapons in the front of the tail-booms. The nacelle-
form fuselage not only looks like the front end of the Whirlwind, it also resembles in layout the
fuselage of the 1931 Pterodactyl IV, and the projected Mk VI fighter. To me, it looks to be reflecting
an answer to the question “How can we deliver the punch of the Whirlwind in a design using a single
Merlin engine?”.

This would be desirable for two reasons. Firstly, the Peregrine engine powering the Whirlwind had
found itself at a development dead end, given the far greater urgency of continuously keeping the
Merlin competitive, because of its criticality to the performance of key fighter, strike, and bomber
aircraft. Secondly, moving to a single-engine design would have a greater chance of Air Ministry
approval than simply replacing the two Peregrines on the Whirlwind with two Merlin engines, given
the pressing demand for those engines.

Unsurprisingly, the proposed pod and boom layout offers both benefits and disadvantages – like any
other aircraft, there are trade-offs to be made between desirable characteristics like range,
manoeuvre performance, speed and handling. The most obvious attraction of the design is the
realisation of a small, yet heavily armed fighter, combining aerodynamic and structural efficiency
into what should be a fast, manoeuvrable and effective platform.

Where then, do the disadvantages or questionable aspect arise? Apart from the question mark
about whether Merlin engines would have been available, that is. Two aspects strike me as requiring
some thought – and possible revision.

The undercarriage arrangement is a particular problem as the ‘conventional’ tailwheel layout will
lead to low ground clearance for the propeller – or longer and heavier undercarriage legs will be
required to give adequate clearance. This problem is compounded by the positioning of the fin,
rudder and tailwheel in the centre of the tailplane. This results in fin, rudder and landing loads all
having to be sustained by the tailplane, inevitably adding weight to the structure.
A second related issue is the complexity of the control runs for the empennage. Positioning the fin
and rudder at mid-tailplane has the advantage of these surfaces being in the propeller slipstream,
but with the disadvantage of complexity of the control system, which has to pass from the cockpit
through the wing to the tail-booms, down the booms, and then across through the tailplane to the
fin.

I suggest that the use of a nosewheel undercarriage would result in better ground handling, and be a
lighter solution to providing adequate ground clearance. Similarly, moving to a fin and rudder carried
on each boom would not only simplify the control system, but also provide a more structurally
efficient transfer of fin loads into the structure. Of course, the resulting configuration would perhaps
then look like a piston-engined precursor to the Vampire, and, perhaps less like a quirky Westland
product.

Pilot escape might be another problem area, and the Wildcat might, perhaps, have been an early
ejection seat adopter should the design have gone into production. Another issue, often
encountered with pusher engine installations, is difficulty in cooling the engine. Arrangements for
engine cooling are not entirely clear, the drawing hinting at an under-fuselage radiator, rather like
that of the Hurricane. This is certainly an area which would need to be considered in development.

With a small design, as this appears to be, one does wonder what the combat range would be,
particularly as the available volume in the fuselage and wing appears limited. Timing is everything
here – as a shorter-range interceptor fighter the concept has some promise. But in the latter stages
of the war, larger, longer-range escort fighters were perhaps a greater priority.”

The National Archives bew exhibition, ‘Great Escapes: Remarkable Second World War Captives’ draws on The National Archives’ vast collections of wartime era documents and photographs – featuring not only the iconic stories that you may have heard of, but many stories of survival that have rarely been told.

Matthew Willis has written an excellent book on the most beautiful German aircraft ever flown, the Focke-Wulf Fw 200. We met him at an undisclosed location and plied him with Jägerbombs until he revealed the juiciest secrets of this classic design.

What was the Fw 200 and why was it significant to aviation and world history?

The Fw 200 was a late-1930s airliner that early in WW2 was hurriedly converted to a maritime patrol bomber. It was initially significant for being ridiculously modern at a time when aircraft like the Ju 52 were commonplace, and setting some much-publicised distance records. The war curtailed what would probably have been an illustrious civil career. Germany lacked a good long-range maritime patrol bomber at the outset of war and the Fw 200 was the only suitable type available. The straightforward military conversion was astonishingly successful in 1940-41 in attacks on Allied supply convoys, both in sinking ships in its own right and directing U-boats. For a single type with only a small number of airframes, it required an enormous effort by Allied forces to counter. It’s probably also worth mentioning that it was a popular VIP transport for high-ranking Nazis, and Hitler had his own personal version with a special escape system.

Why is it so much better looking than the Ju 52?

It’s tempting to say that this was purely a function of being a later generation of aeroplane, benefitting from better structural and aerodynamic knowledge, and better materials, not to mention wind-tunnel testing, which was still fairly rare at the time. But even for the late 1930s, it was among the best proportioned and most pleasingly lined airliners. Perhaps just luck, or perhaps the designers had an eye for art as well as function.

Not many were built were they?

Fewer than 300, in all variants.

What was its relationship with U-boats?

It was meant to be the U-boats’ ‘eye in the sky,’ reconnoitring for merchant ships and directing the submarines to their targets. And it did do this, and very effectively, but due to the power struggle between the Luftwaffe and Kriegsmarine, there was often too much focus on the Fw 200s attacking shipping directly, which was pleasing for Goering, but actually less effective overall than concentrating on reconnaissance.

Was it effective in World War 2?

Yes, in terms of sinking supply shipping, directing U-boats, and soaking up a huge expenditure of resources by the Allies to respond to it. It was also successful as a transport aircraft, making Rommel’s spring 1942 advance possible through petrol deliveries across the Mediterranean, and airlifting critical supplies to Axis troops in the Kuban pocket in 1944.

Was a long-range bomber version proposed?

It did actually act as a long range bomber early in the war, with some raids on a hydro-electric plant in Scotland, but it wasn’t particularly successful. The airframe wasn’t really suitable for development as a pure bomber as it had a very low strength factor. Focke-Wulf did start to develop a bigger aircraft with more powerful engines, the Fw 300, but even this was for essentially the same roles as the Fw 200.

Weirdest thing about the Fw 200 story?

British intelligence was contacted in 1941 by a man claiming to be the father in law of Hitler’s personal pilot, Hans Baur, who it was claimed wanted to use Hitler’s personal Fw 200 to kidnap the Fuehrer and deliver him to the Allies. The RAF made plans to receive the aircraft at Manston on the appointed date, but needless to say, Hitler never arrived. The alleged kidnapping was probably part of a convoluted plot to damage Baur’s influence with King Boris of Bulgaria.

Best and worst things about the 200?

Best – superb efficient design that gave it great range and made it the only available aircraft capable of carrying out a number of important roles.

Worst – its airframe was too lightly built for military use so it had to be manoeuvred with care.

Could it fly safely on two engines?

According to Focke-Wulf publicity when the aircraft was new, it could, though this capability was probably reduced somewhat by the significantly increased weight of military variants.

Wait – there was a BOAC 200?! What was the story?

One of the pre-war customers for the first airliner Fw 200s was the Danish airline DDL. One of its Fw 200s happened to be on a flight to the UK during the surprise invasion and occupation of Denmark in April 1940, and the aircraft was impounded and transferred to BOAC. It was stored for a while then refurbished by Cunliffe Owen from Spring 1941, but in July that year crashed on take-off and was written off. No Condor in ‘speedbird’ markings, sadly.

What was it comparable with?

As an airliner, it doesn’t seem to have an exact analogue – the French Bloch MB.160 is probably closest but I doubt many people even know what that is. I’d say it fits somewhere between the DH91 Albatross and the DC-4. As a maritime raider it isn’t like much else either – it had the same sort of role as the RAF’s Sunderland flying boats, and later on things like the Consolidated Liberator in Coastal Command service. All-in-all it was pretty unique.

What should I have asked you?

Where did the expression ‘Scourge of the Atlantic’ come from?

It is attributed to Churchill – a lot. The trouble is, despite a great deal of searching, I have not been able to find an actual source for it, just literally hundreds of ‘”the scourge of the Atlantic,” as Churchill called it’-type quotes. The closest I can get is this, from Churchill’s The Second World War: “To the U-boat scourge was added air attack far out on the ocean by long-range aircraft. Of these, the Focke-Wulf 200, known as the Condor, was most formidable though happily at the beginning there were few of them.” I suspect that the usual version is a misquote of the above, but it’s hard to confirm because the popular version is so very prevalent.

Where should people buy your book from?

It’s available from the publisher, Morton’s as well as Amazon. You might also find it at WHSmith, and it can be ordered from any regular bookshop.

“Willis’s Fw 200 book is meticulously researched, utterly readable and with extremely beautiful photography – an essential book for every aviation historian.” – Joe Coles, Hush-Kit Aviation Blog

If you like your aircraft small and characterful, then head to India. For your pleasure, we plucked the ten most beautiful, handsome or aesthetically arresting Indian flying machines and presented them below.

10. HAL HJT-16 Kiran

Not many military aircraft inspire protective instincts in the casual observer, but one cannot help it with the Kiran. Though totally bereft in badassery it wins points for cuteness, scraping it in at number 10.

9. HAL Ajeet

An Indian derivative or the British Gnat, the Ajeet was an appealing design but loses points for looking too much like the Gnat/Midge.

8. NAL Saras

Despite being the least attractive aircraft in the light transport pusher class (lacking the sleekness of the Avanti or Vector) the Saras is still a pretty machine. It loses points for an overly broad chord to the vertical stabiliser and too small wings protruding from a flabby underbody, but gains some for the t-tail, friendly windows and pusher PT6s.

7. HAL HF-73

An early 1970s Indian Aircraft requirement for a Deep Penetration Strike Aircraft (DPSA) required collaboration with the West German Messerschmitt-Bölkow-Blohm company. The resultant concept, with its wedge inlets and twin-tails looked like a tiny MiG-25 or perhaps a butch F-5E. Unsurprisingly considering MBB’s concurrent work on the MRCA, its forward fuselage and intakes were extremely Tornado-esque. Power would have come from the same engine type as the Torndao, though the smaller lighter Indo-German design would have enjoyed a far superior power-to-weight ratio. Studies featuring both single and double tails were produced but a lower risk option, the procurement of the Anglo-French Jaguar, replaced this extremely promising design. The HF-73, had it been actually built, may well have topped our list, but loses points for failing to happen. The extremely low canopy bow would have afforded an excellent downward view, a feature seen on the contemporary HAL HLFT-42 concept.

6. HAL Prachand

There is nothing wrong with a convex belly and thicker torso, with many people finding the ‘Dad bod‘ extremely attractive. Having said that, the Prachand looks best when concealing its stocky underside from the camera. Nose down and flying towards you, the Prachand (meaning ‘intense’ or ‘giant’ in Hindi) has a hungry predatorial look utterly appropriate for a light attack aircraft.

China’s Z-10 is sleeker and more futuristic, and the larger Apache and Ka-52 may cornered the market for the hideous-satanic-harvester-of-souls look, but the Prachand has a tall lumbering purposefulness all of its own as if British firm Avro still existed and decided to make a helo. The rather comical ‘grand piano’ style tail wheel adds a touch of humour so often sadly lacking in the world of military helicopters.

5. HAL HTT-40

Neither as freaky as an Orlik nor as exciting as a PC-21, the HTT40 is still undoubtably an attractive machine.

4. HAL HF-24 Marut

German Kurt Tank designed the exceptionally elegant Fw 200 airliner, the muscularly piscine Fw 190 fighter and the salaciously elongated Ta 152. Clearly Tank knew how to sculpt a beautiful aeroplane and his Marut was no exception. The first successful Asian jet aircraft did not, like the Spitfire for example, enjoy ‘all aspect’ beauty – and there are angles of looking at the design, where it seems incoherent or awkwardly proportioned. It would be rude to look at the Marut directly from above, where it becomes clear that the fuselage is far too thick and the wing too small. But the Marut, with its sleek sweeping fin and mass of exquisite natural metal design features is somewhat like a 1950s US Cadillac, it is kind of ridiculous – yet wonderful. These design features included a ravishingly space-age intake comprising a half-body and splitter plate, and the 1950s style split exhaust trough. Viewed directly from the front it looked uncannily like the later Mirage 2000 and 4000. Though not pretty from every angle, the Marut was the most charismatic of Indian aircraft.

Rival Willy Messerschmitt’s minute HA-300, a far more coherent-looking machine was more akin to a Fiat 850 Sport Spider.

3. HAL HT-2

Vishnu Madav Ghatage obtained his doctorate in Aeronautical Engineering at Gottingen in Germany under the famous Dr Ludwig Prandtl. A few years later he led the design of the the HT-2, a brilliantly pragmatic design with a rather lovely tailfin.

2. HAL HJT-36 Sitara

The curvaceously sexy canopy of the Sitara intermediate jet trainer is arguably the most appealing in production anywhere in the world, the simple tiny ‘ear’ intakes are rather cheeky and the petite dynamism of the Sitara is as refreshing an Italian ice cream next to Lake Como.

1. HAL Tejas ‘The Bangalore Matador’

“The moment had come,
I swallowed my gum,
We knew there’d be blood on the sand pretty soon.
The crowd held its breath,
Hoping that death
Would brighten an otherwise dull afternoon.”

– Tom Lehrer, In Old Mexico

Put aside the history, put aside a technical assessment and enjoy the Tejas from a purely aesthetic perspective. It’s not hard, as what could be more thrilling than a pocket-size Mirage 2000? Which is in many many ways what this tiny Indian fighter jet resembles. But as much as a Mirage 2000, the Tejas resembles the Spanish bullfighter, the matador. By ‘matador’ I do not mean the Spanish Harrier, but actual matadors, upholders of the Spanish tradition of the bullfight that rather too viscerally combines slaughter and spectacle. The epitome of the dainty deft killer, the Tejas is every inch the matador. Let’s start with the extremely pleasing taper of the fuselage from its widest point back to its neat little nozzle, reminiscent of the way the tight-fitting tights, or taleguilla, of a matador lead down to the zapatilla flat slippers. The extremely unusual LEVCONS are very much like the broad proud shoulders of the matador’s jacket (chaquetilla) and the stylish Viggen-like wing is like the matador’s cape, the capote de brega. Imagine an unfairly disadvantaged Su-30 (perhaps the pilot has been gored and denied missile usage) ‘fighting’ a Tejas in dissimilar air combat training gives you the closest aerial equivalent to the bullfight.

¡Ole! 

Return of the High-Level Bomber         

Stephen Liddle looks into the surprisingly long story that led to the first flight of the most advanced heavy bomber the world has ever seen.

The first flight of a completely new combat aircraft – especially the manned variety – doesn’t happen often these days. If we’re considering US heavy bombers, then the most recent prior to this month would have been the Northrop B-2A Spirit, in 1989. Before that, the B-1 in 1974 was preceded by the B-58 and B-52, both in the 1950s. It doesn’t happen often, which makes the stunningly lit baby steps of the new Northrop Grumman B-21A quite exciting.

The product of the Advanced Technology Bomber project and code-named Senior Ice, the B-2 was revealed not long after the F-117 and the whole concept of stealth was officially acknowledged. Attention was focussed inevitably on the seeming magic of the ‘invisibility’ to radar, but Low Observability is a suit of techniques and technologies that improve survivability. By limiting the opponent’s ability to detect and engage, the freedom to operate is vastly increased. In the 1980s, with the Cold War proper still in full swing, it had been two decades since the favoured method of penetrating the other side’s formidable air defence systems had become the Hi-Lo-Hi mission. The route into the target was only considered viable if low-level, terrain-following tactics were adopted. Aside from enemy action, these were dangerous for the crews of the existing heavy bombers adapted to the new the regime. As a USAF B-52G crew member once told me, “In the Gulf, we crossed the border at 250 feet. The minimum ejection height was 300 feet.” At least he had an ejector seat; as is well known, the three rear crew members on the RAF’s V-Bombers had to resort to unlikely manual escape.

A bomber that need not concern itself (overly) with what the enemy may try and do to it could avoid these pitfalls, but also take advantage of appropriate physics too. If it could fly at an optimum (high) altitude, then range would significantly benefit, supporting the idea of global reach and the USAF bomber’s raison d’etre. If outrunning defending fighters through speed too was deemed less relevant, then the aerodynamic efficiency of a subsonic jet pushed the range still further. However, whereas the B-52 and Vulcan conforming to this philosophy had been able to be aerodynamically dominated designs, the very technologies that might just allow high-flying subsonic bombers in the later Cold War exerted the strongest influence on external shape. Altitude – defeating visual detection from the ground as well as making both radar targeting and subsequent engagement more difficult – had to argue its priority in the LO toolkit.

Things should have moved on in the four decades since the B-2 was conceived, and indeed they have. The B-21 is familiar, but it clearly isn’t the same. As someone with a professional interest in aerodynamics, but from outside of the defence aerospace industry, I’ve been amusing myself by attempting to unpick some of what I’ve seen in the few available images. I claim no more than that.

The outstanding, fundamental difference in configuration between the new B-21 and the preceding B-2A Spirit, shown even in the early Northrop Grumman renders c.2015, is the simpler trailing edge shape. In fact, the ‘W’ of the new aircraft as opposed to the ‘Saw tooth’ of the old, would have been familiar to the designers in the early 1980s too. While the B-2 started life as a high-altitude bomber, the thinking behind ATB from both the USAF and ATB bounced between a pure optimisation in favour of 60,000ft penetration, and retention of the in-vogue low-level strike emphasis. Was the plan a stealthy FB-111, or B-52? The Request for Proposals (RFP) that stimulated the first design studies required quantification of the ‘fallout’ capability of the High-level design to perform a low-level mission if required, but without changes to the design to help this. That would change in April 1981 with a Modification Request to add significant low-altitude capability, as a ‘…prudent hedge against an ever-changing and maturing radar threat operational throughout the Soviet Union.’[i] In other words, with Stealth completely unproven operationally and even had it been, its longevity open to question, could the USAF afford put all of its chips on black?

Northrop conceptual design, c.1979 (Griffin et al)

The low-level penetration mission immediately added about 10,000lb to the expected structural weight of the ‘paper’ high-level design. It is worth noting in the light of what came after, that aircraft cost has been strongly correlated to weight.

While the airframe gained weight to meet new strength requirements, in terms of both fatigue and ride quality aeroelastic effects were more emphasised. Engineering work showed that much of the energy was absorbed with the first wing bending mode. On the baseline design, the node line ran over the planned outboard control surfaces. At the same time, ways were sought to improve the balance between carried at the front and rear of the wing carry-through box; 70% and 30% respectively. The final design introduced an IB control surface array, with the mode line running between them and the OB set. This meant the aeroelastic bending could be actively controlled by out-of-phase actuation of the two sets. The main gust alleviation work was in the hands of a powerful central control surface, which worked with sensor systems to pitch the aircraft into the local gust vector and minimise its effect, via a, “very aggressive flight control system which is designed to provide significant improvements in ride quality and load alleviation during low level contour flying.”[ii] The result was the familiar shape seen today, which exists only to adapt the high-level B-2 to the punishing low-level environment. On the other hand, it was noted during an investigation into the aircraft’s response to lateral gusts, that the flying wing shape, “…is sufficiently small in the vertical dimension that it can be considered planar, and the lack of vertical surfaces, nacelles, or external stores greatly reduces its sensitivity to lateral gusts.” So, it wasn’t all bad.

What does this tell us? It isn’t as simple as saying the B-21 is high level only. While the B-2 planform changes were driven by the low-level mission, there were other ways to skin the cat and in the intervening four decades, both structures and control have advanced. The B-2 was off the scale in terms of the proportion of composite structure it used for the time it was designed; it doesn’t necessarily follow that the structural modes of the B-21 follow the same pattern as its forebear.

B-2 Planform changes through both development and addition of low level requirements, c.1983 (Griffin et al)

The B-2 and B-21 obviously also share the overall stealth strategy of sharp parallel edges at oblique angles to the flight path, together with smoothly curved surfaces. From a cruise efficiency perspective, sharp leading edges were not a positive feature due to the loss in forward suction and hence a poor lift/drag ratio of the aerofoils. In the early 1980s, a new subsonic aircraft not considering LO would have inevitably used a supercritical aerofoil shape, with a relatively large radius leading edge, thick forward region and cusped trailing edge for aft loading. A compromise between the aerodynamicists and LO engineers was evolved, after the latter group were able to show that retaining sharp edges on the central region and tips should prove sufficient. This is a very obvious feature of the B-2, once one has noticed it anyway. Interestingly, it is much less clear from the images seen to date, that it has been incorporated on the B-21. It is certainly more subtle, but the need to align a sharper leading edge to the oncoming, upwashing flow at the nose has again resulted in the characteristic ‘beak’ shape. Potentially, an example of technology moving on and the aerodynamic restriction being lifted by advances in LO and the ability to model its effect.

A B-2A in flight. Note the open drag rudder surfaces, the outboard-most trailing edge controls. The leading edge appears to change sweep angle at the tip and near the centre; in fact, this is due to the change in leading-edge radius from an aerodynamically favoured relatively large section for the mid section, to the sharp LO-biased shape at root and tip.

Have a look at The Hush-Kit Book of Warplanes Vol 3 here

One thing that certainly isn’t evident on the B-2 is the level of leading-edge droop outboard on the B-21. Where have we seen that before though? Looking back to the 1950s and the similarly sized, high-altitude V-Bombers, it is well known that the Vulcan didn’t work aerodynamically in its initial incarnation. The natural tendency of the inboard, leading part of a swept wing to load the outboard part behind it, caused significant suction peaks to be developed there. Progressive LE droop was the answer, being made to work really well by an understanding of the ‘peaky’ mechanism and specific sectional shaping. The supersonic expansion followed by an initial compression using Mach wave reflections rather than a shock wave, resulted in a much more efficient flow at high subsonic speed. This is a fundamental characteristic of the supercritical aerofoils used on the B-2 as well. The rival Victor used a philosophy of ‘constant critical Mach’; the intention was the wing would aerodynamically adapt itself along the span. The OB was drooped, thinned and of reduced sweep. It may not be simple to build, but progressive full span LE droop can be a significant aid to transonic cruise efficiency. Are we seeing the ability to combine 202x design and simulation capability across the fields of LO and aerodynamics, vs. 198x era work on the B-2? This is further visible evidence of a quicker or better L/D bomber, less compromised by LO, as engineering has advanced.

We’ve discussed one reason for the outboard leading edge droop: transonic cruise efficiency. That’s definitely not the only reason one would see this though. For the vast majority of practical aircraft, a vertical fin and rudder provide the ability to account for the usual adverse yaw effect in a turn. In 1920, Prandtl and his Gottingen colleagues developed lifting line theory to analyse three dimensional wings. Constant downwash from an elliptical span load gave min induced drag. However, in 1933 this was extended to solve for given mass; a bell curve was superior. This 1933 shape implies a switch from downwash to upwash and hence thrust outboard. By deploying lateral control surfaces in this region, a local increase in load (down aileron) creates a yawing moment into the turn (Proverse Yaw), without requiring a rudder input. In 2016, Bowers and co-workers published a NASA report[iii] suggesting that this bell span load, proverse yaw inducing model applied to soaring bird flight. Birds don’t need a vertical tail for coordinated turns either. All of this leads us back to a place where the subsonic cruise efficiency (minimum induced drag) and lateral-directional control response in the absence of a fin (for stealth) are both served by a bell-shaped span load. Note that R T Jones and the Hortens all feature in this story, but are not necessary to the narrative. Please don’t write in and complain!

The B-2 users split drag rudders for directional control when not in stealth mode, while relying on differential thrust and the remaining control surfaces when LO is vital. The ability to mix these controls (together with a neutral to unstable configuration) was not available to Northrop in the 1940s, when such devices were used on the XB-35 flying wing bomber project. As well as increasing the aircraft’s general control authority, the strategy would specifically push the crosswind landing limits and hence allow missions to be launched in a wider range of conditions. The B-21 seems to have dispensed with the split surfaces entirely, which may be an indication of more confidence in the modelling of the aerodynamic derivatives and flight control system performance during the engineering phase. It might also hint at greater authority being available from the engines, with the combination now being able to meet all specified landing requirements. Of course, there are many more flight-validated data points available to the designers of a flying wing these days.

Another pillar of the LO shaping strategy for the B-2 was the shielding of engine inlets and exhausts from below, together with the highly reflective engine compressors themselves. The ideal of an S-shaped duct from the upper surface of the wing was far easier said than done. As Hans Grellmann, responsible for the aerodynamic design of the aircraft described the situation, “In essence, two supercritical airfoils had to be designed in series. The first being the wing surface where the flow expands to reach supersonic speed and then is recompressed to subsonic speed before it enters the inlet. The second “airfoil” is the nacelle between the inlet lip and the exhaust exit. In this region, flow accelerates over the inlet cowl to supersonic speed, while recompression becomes part of the compression region extending out to the outboard wing.”[iv] The impact of the upper surface nacelle configuration would make itself apparent towards the end of the flight test programme, as late as June 1994. Whilst expanding the Mach limit of the envelope at low altitude, a Residual Pitch Oscillation (RPO) was identified. The test involved checking the aircraft’s response to random small control surface inputs; the expectation was that any resulting oscillations would be damped and eliminated within a specified period. In this case, the oscillations continued at a low level, while the engineers were able to identify the trace of upper surface shock waves over the nacelles and inboard trailing edge notch, also oscillating with time. These had coupled with the structural modes and ultimately kept the vibration going. As the situation was outside of operational requirements, the solution was a Mach overspeed warning in order to give the pilots time to correct.[v]

Computational mesh from analysis work associated with correcting the RPO problem with the B-2. Note that the shock was located somewhere over the nacelle and affected the trailing edge notch region. Both of these features are avoided on the B-21. ( Jacobson et al)

The B-21’s nacelles are notably lower profile than the B-2 equivalent geometry, while the inlet plane is further forward. By reducing the pressure recovery demand over the top by these two geometry changes, the aircraft would likely be less susceptible to shock induced separation, in the manner that caused the B-2 RPO problem. The simpler trailing edge geometry has a strong part to play in this too, while the simulation challenges identified after the B-2’s problems came to light have been tacked with thirty years of transonic CFD tool development. The new bomber was starting from a much better place.

If the B-2 benefitted from moving the inlet rearwards and even then, required a boundary layer bleed duct underneath to remove the low total pressure flow, how has the B-21 team managed to move their inlet plane forwards? The answer, one suspects, is the work conducted in the intervening period on supersonic diverterless inlets, as featured on the F-35. These devices use careful shaping of the surface ahead of the inlet, usually via a bump, to control the local flow direction in the boundary layer itself by introducing a compression. While the B-21 itself is subsonic, the local flow on top of the wing leading edge will be marginally supersonic in the cruise. Images of this region are unclear at best, but the challenge of providing attached flow and maximum pressure recovery in the LO-compliant S-duct diffuser ahead of the engine is severe. It is inconceivable that attention has not been paid to adequate boundary layer control by some method such as this.

A few thoughts then on the B-21, as revealed so far.

• The fundamental planform and strategy are reminiscent of the initial B-2 proposals from Northrop, as accepted by the USAF for early development.
• Many of the planform differences between B-21 and B-2 can explained by the low altitude requirements introduced in the final Advanced Technology Bomber specification.
• Backing out of the Hi-Lo-Hi design certainly reduces weight (cost) and would have avoided a number of specific aerodynamic issues that were difficult to predict (time and cost).
• The B-21 geometry is consistent with advances in both predictive tools and confidence gained from related LO platforms. The elimination of the split rudders and potential exploitation of proverse yaw is an example.

Overall, there is a fascinating interplay between the conflicting aerodynamic and LO optimum solutions. As the ability of computational tools – particularly computational fluid dynamics (CFD) – to predict the physical phenomena associated with transonic flight regimes has advanced, then designers can have more confidence in pushing the shapes towards stealth-biased solutions. An example would the complex intake and exhaust geometries, which must retain healthy aerodynamic performance in terms of pressure recovery and minimal losses. Conversely, as analytical tools for the assessment of LO performance without the need for physical testing have matured, then the aerodynamic solution space widens. The clear variation in section shape and obvious rear loaded geometry of the B-21’s wing lower surfaces, looks to be a more geometrically refined and optimised transonic shape than the B-2, superficially at least. Things have indeed moved on, as much as the 1979 Northrop proposal may remind us of what has been recently revealed.

It would be remiss of me not to mention the great help in accessing background material given by David Lednicer – it was very much appreciated. All misinterpretations of the dataset are entirely my fault alone, however.

-Stephen Liddle is a must follow on X Twitter and is currently preparing a book on the aerodynamic development of the V-Bombers, that he hopes will be published Q2 2024.

You should reserve your copy of the fabulous Hush-Kit Book of Warplanes Vol 2 happen here

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[i] Griffin et al; B-2 Systems Engineering Case Study; Air Force Center for Systems Engineering; 2007

[ii] Crimaldi,  J.P., Britt R.T. and Rodden, W.P; Response of B-2 aircraft to nonuniform spanwise turbulence; AIAA Journal of Aircraft, Vol 30 No.5, Sept-Oct 1993

[iii] Bowers et al; On Wings of the Minimum Induced Drag: Spanload Implications for Aircraft and Birds; NASA-TP-2016-219072, 2016

[iv] Grellmann, H.W.; B-2 Aerodynamic Design; AIAA 90-1802, AIAA Aerospace Engineering Conference and Exhibit, Los Angeles, CA, February 13-15^th 1990.

[v] Jacobson, S.B., Britt, R.T., Dreim, D.R. and Kelly, P.D.; Residual pitch oscillation (RPO) flight test and analysis on the B-2 bomber; AIAA paper AIAA-98-1805, 1998.