Germany’s greatest fighter

What if the Spitfire – that most iconic British fighter – wasn’t? British, I mean. What if it’s sighed-over elliptical wing (which, as any attentive high school student could tell you, isn’t even an ellipse) owed as much to the Germans it fought against as the English it saved?

A masterpiece of aerodynamics

Okay, I admit it’s a deliberately hackle-raising question… After all, the Spitfire was conceived, designed, built and blooded in Britain. It is emphatically British.

But the fact remains that R.J. Mitchell’s glorious creation owes much of its aerodynamics, and that iconic wing in particular, to visionary Germans like Hugo Junkers, Alexander Lippisch and Ludwig Prandtl.

To further confuse things, the brilliant aerodynamicist who was their conduit into Supermarine wasn’t English either, but a largely forgotten Canadian called Beverley Shenstone.

He deserves to be as famous in England as Reginald Mitchell – and even more so in his native Canada.

Beverley Strahan Shenstone

Bev Shenstone was born into a middle-class Toronto family in June 1906. A love of small boats, and sailing in particular, grew into a fascination with aerodynamics. He graduated from the University of Toronto with a degree in Engineering in 1928, and was promptly invited to begin post-graduate studies into the stability of flying boats.

A natural gift for mathematics and an intuitive appreciation of fluid dynamics led him to achieve Canada’s first ever Master of Aeronautics from U of T. At the same time, he had joined the RCAF via the University and learned to fly.

However his academic prowess was attracting considerable attention and a network of contacts, beginning with his professor JH Parkin, led via the RCAF and the RAF to the British Foreign Office and, finally, the British Air Attaché in Berlin.

Already something of a specialist in flying boats, Shenstone hoped to secure a place in Claudius Dornier’s factory on Lake Constance. Instead, on November 11th, 1929, he found himself at the Junkers works in Dessau.

Expecting only to find the chunky, corrugated metal craft Junkers was known for, he quickly became enthralled by the experimental designs Junkers was developing privately, starting with the 107-foot span Ju.38 flying wing.

On the leading edge of aerodynamics

If it wasn’t clear any sooner, Shenstone now knew he had landed at the leading edge of aerodynamics. As the 1930s began, German aerodynamicists and aircraft builders were thinking far in advance of other nations – and even far in advance of their own customers, as the designs that ultimately saw combat a decade later would prove.

But in the meantime Shenstone had much to learn. In the summer of 1930, he travelled to the famous Wasserkuppe – the home of soaring flight. In the enthusiastic and inviting atmosphere that still distinguishes gliding communities everywhere, he learned to fly without a motor.

While there he was introduced to noted designer Alexander Lippisch, who was already building and flying tail-less gliders. The two aeronauts became friends, sharing ideas and knowledge into the night and putting them into practice during the day. By that winter, Shenstone had moved from Junkers to Lippisch’s drawing and design office on the Wasserkuppe.

Given the season, they would take breaks to go skiing on the mountain rather than gliding, but mostly Shenstone honed his knowledge of aerodynamics and the specialised calculus invented for wing analysis by yet another German, Ludwig Prandtl. It was a totally immersive environment where Shenstone and the team would investigate delta designs, as well as the difficult lift distribution, pitching moment, stalling behaviour and drag characteristics of various planforms.

Ultimately, Lippisch would overcome all the difficulties and perfect the delta wing. From the 1940’s DH.108 Swallow to the 1950’s F-102 Delta Dagger; 1960’s Concorde; and beyond, his discoveries would benefit aviation worldwide once Nazi Germany was defeated.

Back to Britain

As winter ended in early 1931, Shenstone travelled to Heidelberg and met with Ludwig Prandtl himself; the two men discussed boundary layer behaviours and the properties of different wing planforms for hours.

By the Spring of that year, a very much wiser Bev Shenstone returned to England and applied for work at Hawkers. There, the famously irascible Sydney Camm told him all fighters would be biplanes and walked out of the interview. So Shenstone approached the more forward-thinking Reginald Mitchell at Supermarine and was given a two-month trial.

After proving to Mitchell that he could save around half a ton’s weight from the wing of a new flying boat design (never built), he was given a full-time job. Being at Supermarine brought Bev Shenstone back to his earlier affection for flying boats – but the association would prove far more fruitful in other areas…

The first to be called ‘Spitfire’

From the drawing office, Shenstone rapidly rose to the position of Mitchell’s Chief Aerodynamicist. With his rare ability to read technical reports from Germany, as well as from Britain and the NACA in America, he was a valuable asset for Supermarine.

Shenstone had taken a junior role in the design of their heavy-set Type 224 fighter proposal of 1934 – the first to be called ‘Spitfire’. Unfortunately it didn’t fire at all, and ended its days being used as a gunnery target on Orford Ness.

But Mitchell and his team, which now starred Bev Shenstone, took the lessons from that exercise onto their next design – the Supermarine Type 300, which would inherit the 224’s name of ‘Spitfire’.

Using extended funding from the Type 224’s F7/30 contract, it was clear that Supermarine would have to get everything right on this next attempt. Mitchell wanted a leap, not a step. He specified a modern, monocoque fuselage design and a much thinner, more efficient wing.

Having just returned from a second visit to Germany and a tour of the United States, where he’d met with Theodor von Karman and studied the latest NACA 22-series thin wing airfoils, Bev Shenstone was full of ideas.

From Russia, with lift

Back in 1931 his German friends had exposed him to the work of Russian pioneer Nikolai Zhukovskii (Joukowski) and his research into elliptical wings from 1902 to 1920. At the same time, Alexander Lippisch had given him the mathematical tools to plot and perfect the troublesome ellipse. Meanwhile, the rapid perfection of cantilever monoplanes was finally making it possible to build such a highly efficient design.

Zhukovskii had found that an ellipse would provide considerable efficiency improvements because the reducing chord concentrated lift inboard, and distributed overall lift far better than a straight taper. Reducing lift at the wingtips helped reduce lift induced drag at the extremities and the production of draggy tip vortices as a result.

With its curved outlines, an ellipse was also better at distributing the stresses of form drag (basically wind resistance).

Taming the vicious tip stall

The down-side was that the elliptical wing had a vicious tip-stall built into it.

The huge increase in drag that comes with a stall is  proportionally larger at the normally low-drag tip of an elliptical wing. With a large loss of lift inboard and a particularly marked rise in drag outboard, the wing drops down and back – quickly pulling the aircraft into a spin.

While it would have been relatively simple to control the ellipse’s worst vices with an aerodynamic fence and stall strip, Shenstone used his mathematical skills to plot and perfect a more efficient washout – twisting the wing along its length to lower the tip angle of incidence, thus forcing the inboard section to stall first while the tip stayed ‘flying’.

At the same time he modified the pure mathematical ellipse to optimise mid-span lift across a broad speed range – from 50mph to 500mph, no less – giving the Spitfire notably better low-speed handling than the Bf.109, which needed leading-edge slats to even come close.

Covering the guns?

Of course, the elliptical wing wasn’t Shenstone’s unique domain. There’s plenty of evidence that the planform was on the table from the earliest stages of the Type 300 design. And other aircraft had used, or would use, an elliptical wing – including Heinkel’s He.70 to He.111 family and, later, the Republic P-47.

Against this is Shenstone’s account of Mitchell jokingly telling him ‘I don’t give a bugger whether it’s elliptical or not, as long as it covers the guns’. It’s a revealing story.

Shenstone was always modest about his contribution to the Spitfire and, like the rest of Supermarine, talked in terms of ‘we’ and ‘the team’. This anecdote paints him as the ellipse’s leading advocate – and perhaps a slightly over-enthusiastic one on that occasion.

What’s more, the Air Ministry increased the number of guns from four to eight in April 1935, four months after they’d written Specification F10/35 around the Spitfire design and six months after the start of detailed drawing work.

Clearly the curved trailing edge was chosen for more than its ability to cover four, and then eight, guns.

Ellipsoid, not elliptical

In the end though, the solution wasn’t a true ellipse at all.

More accurately, it was a double-ellipse – the two component ellipses, with their vastly different short axes, aligned span-wise between the wingtips and divided chord-wise by the fuselage.

Even then, those four quarter-ellipses aren’t geometrically perfect. The long thin ellipse of each leading edge is flattened for a good deal of its length; while the much wider trailing quarter-ellipses form highly customised ellipse-like curves that then reflex back into the wing-fuselage fillets.

Those wide, sweeping fillets  were a further touch of brilliance, saving huge amounts of interference drag at those critical (and problematic) intersections.

Using Prandtl’s calculus to plot its performance, Shenstone shaped the entire wing to get exactly the lift/drag and stall characteristics he needed, even compromising a little high speed performance to optimise the Spitfire’s turn rate and low speed handling.

A more advanced spar

Another, hidden secret of the design is the main spar position. Joe Smith’s thin, strong spar design, built up from telescoped square sections that reduce in cross-section, weight and load-carrying along their length, are a work of engineering genius in their own right. But they’re also placed  forward of the wingtips, so they’re not actually at the widest part of the span as you might expect.

This more advanced position helps the spars resist the wide aerofoil’s natural pitching moment and, critically, creates adequate space for the wheel wells through the thickest part of the wing.

Shenstone worked closely with Mitchell on the Spitfire’s empennage too, using similar ellipsoid shapes and showing the great man how having better flow control off the main planes meant the tail components could be substantially smaller.

In 1935, Air Ministry observers would try to force a much larger biplane style tail onto the Spitfire prototype, more like the Hurricane’s and fully 40% larger than the original design. Mitchell, no doubt better informed by his Chief Aerodynamicist’s mathematical proof, held firm.

The world’s aeroplane

And so the prototype Supermarine Type 300 to Air Ministry specification F10/35, the Spitfire, was born.

The final result was incredibly efficient, incredibly responsive, and evocatively beautiful. But it was also a surprising combination of German, British and Canadian talent, with some Russian and American research for good measure.

As much as the Spitfire would help deliver the world from tyranny, it was the world’s aeroplane.

North American matters

With the Spitfire in production and Mitchell retired with terminal cancer, Shenstone made the difficult decision to leave Supermarine in 1938.

He managed another trip to Germany later in the year, where he met with Willi Messerschmitt and was shown the still-secret Bf.110. From this glimpse he was able to give a disbelieving British Air Ministry surprisingly accurate predictions about the Zerstörer’s performance.

He was then seconded to the Bristol company by the all-powerful Ministry of Aircraft Production, where he worked on the Bristol Beaufort and Bristol’s radial engines.

In 1940 he was appointed to the British Air Commission in the US, overseeing the improvement of American aircraft for Britain, based on hard lessons learned by the RAF in conflict.

In this role he helped North American Aviation resolve a troubling issue with their new ‘Mustang’ fighter. While the design made brilliant use of Meredith Effect thrust from its cooling ducts, tests were revealing loud (and very disconcerting!) thumps coming from below the plane at certain speeds.

Thanks to his conversations with Prandtl about boundary layer behaviour, Shenstone was able to deduce that air was building up in front of the cooling inlet duct then flooding into it with sudden force.

He recommended stepping the inlet away from the lower fuselage skin to leave room for the boundary layer to flow smoothly past. It worked.

In solving the Mustang’s problem, Shenstone added another family connection between the war’s two great Merlin-engined fighters – and a little extra German DNA to the P-51’s pedigree.

The future of air travel

As the war progressed, Bev stayed with the BAC as its attentions moved from aircraft production to the future of commercial air travel. Despite his earlier passion for flying boats, repeated trips across the Atlantic in both land planes and Boeing Clippers soon convinced Shenstone that flying boats were doomed.

Returning briefly to Canada, he played a significant role in the development of the Canadair North Star – an extraordinary 1946 airliner that fused a Douglas C-54 with the DC-6 nose and four Merlin 622 engines, to achieve a cruising speed 100 mph faster than that of stock C-54s.

From 1964 to 1966 he worked as Chief Technical Director of BOAC, which included planning for the instigation of Concorde SST services – despite his deep (and sometimes very public) scepticism about them ever being profitable…

At long last, Bev Shenstone retired in 1966 and built himself a boat, which he used to cruise the waterways of England and the continent. He would summer on Cyprus, where he provided part-time technical advice to Cyprus Airways until he passed away on in November 1979.

Shenstone’s legacy

Beverley Shenstone’s lifetime contribution to British aviation is beyond measure. The aerodynamics he learned from Germany’s great pioneers would prove vital in the preparations for war, as would his fluency in German technical writing. He also helped shape several game-changing airliners, including some of Britain’s most successful and influential post-war designs.

But still, his contribution to the way the Spitfire looked and flew and fought would help to write history. And the reason the airplane is such a recognised icon to this day is largely thanks to Bev Shenstone and that ellipsoid wing.

Given his talent and experience, Reginald Mitchell would always have designed a superb fighter – but along with his brilliant colleague from Canada, he led a team that created something immortal.

It’s as much Beverley Strahan Shenstone’s legacy as anyone’s.

Sources:

Secrets of the Spitfire by Lance Cole ISBN 978 1 84884 896 2

Beverley Shenstone on wikipedia

Title quote from Blackadder Goes Forth, BBC