Today
we’re taking a look at a pair of very unusual and interesting prototypes hailing from the United
Kingdom. Of a tandem wing configuration – they were certainly some of the more
unique-looking aircraft of the 1940s, though many still do not know about these strange
designs to this day! We’re talking about the Miles M.35 & M.39B “Libellula”.
Libellula, referring to a genus of dragonfly known for having two sets of similar span wings,
was a designation given to a series of Miles aircraft that experimented with the
same concept. From the get-go, this tandem wing layout was predicted to have a fair
amount of benefit over the conventional wing configuration - including a greater Centre of
Gravity range between the two mainplanes - which could help to reduce the required
fuselage length and wingspan of an aircraft. These characteristics, if properly implemented,
would be hugely beneficial for carrier-based aircraft, where every reduced metre of required
hangar space is considered very valuable. It could also negate the necessity of a
folding wing mechanism, which introduces more points of failure and weight, but is usually
necessary for most conventional carrier-based aircraft. The Libellula concept, therefore,
seemed to be a perfect candidate for carrier-born applications. Indeed, it was the problems being
experienced by the Royal Navy in 1941 -with their improvised Seafire and Sea Hurricane- that
inspired Miles towards this line of thinking. Miles Aircraft Ltd, an aircraft company originally
founded in 1928 under the name Phillips & Powis Aircraft, would become one of larger proponents
of the tandem-wing layout. This was despite their background in more conventional civil aviation,
where they had produced aircraft such as the Miles Hawk Major and the Falcon.
Under lead designer Frederick George Miles, development would begin on their first military,
tandem wing design in early 1942. In November 1941, George Miles had been inspired by the sight
of a heavily modified Westland Lysander. He, along with several other designs, had been looking for a
solution to the Navy’s carrier-fighter problem (the attrition rate from landing accidents was
becoming appalling), and he quickly realised that the tandem-wing design could be the answer.
In his mind, a design rapidly formed. The pilot could be located in the nose, which
would provide perfect visibility for takeoff and landing aboard a carrier. With vertical control
surfaces on the rear wings, the weight and drag of a tailplane could be removed to allow for the
installation of a rear-mounted engine. This not only further improved pilot visibility,
but also comfort, as the noise and vibration of the engine and propeller would be as far
away from the pilot as physically possible. The whole arrangement could be incredibly compact,
in terms of span, and the whole airframe would be easy to transport by land, and at sea it
would fit on the elevator of an Illustrious-Class carrier without the need for folding wings.
To avoid the bureaucratic complications that came with submitting an unorthodox design for
official consideration, and…because they weren’t technically allowed to do anything without the
Air Ministry’s say so anyway, the development of this new aircraft was done in total secrecy.
Miles opted to first build a small flying wooden mock-up. This aircraft - designated
the M.35 - featured two high-mounted front wings directly aft of the cockpit, and two larger,
swept back wings at the rear, to which the twin rudders were mounted. The elevator
would be in the front wings, and ailerons in the rear wings. A single de Havilland Gipsy
Major producing just 130 hp would drive the M.35 in a pushing configuration.
Owing to the secret nature of the project, Miles couldn’t exactly roll his new design into a wind
tunnel to assess its stability. Because of this, it was decided to build a scale mockup
of the scale mockup -built to ¼ scale- and tow it aloft for an experimental flight.
Said flight did not provide much in the way of encouragement. The model initially
climbed exceedingly well, so well in fact that it threatened to overtake the towing
aircraft – the prototype Miles M.28 – but then when the towline was released, the model
immediately stalled and plummeted vertically into the ground 150ft below with zero signs of
it trying to recover. Despite this preference for lithobreaking rather than gliding, Miles
was confident that the M.35 would behave better under its own power, and so work continued.
On the 1st May 1942 the M.35 would conduct its maiden flight. Early in the morning
it was towed through the streets of Reading towards the nearby airfield, Its short 20ft
wingspan allowing it to be transported fully assembled, but Upon its arrival, Mile’s chief
test pilot took one look at the prototype and said ‘Surely you don’t expect me to fly that?!’.
Not to be dismayed, George Miles himself would take over and conduct the first flight trials.
It was found that the aircraft was reluctant to take off, and to get it to leave the ground the
throttle must be sharply closed whilst at high speed, upon which the aircraft would
leap sharply into the air. After overcoming this quirky behaviour, and successfully taking off,
it was found that the aircraft was essentially uncontrollable, though George was fortunately
- and very skilfully - able to bring it in for a safe landing without incident. According to Miles
the aircraft was, in his words, catastrophically unstable and the brief flight had been a ceaseless
battle to prevent violent pitch oscillation Bizarrely, one official report states “The
aircraft flew successfully the first time; it took off, climbed to about 200ft, circled the aerodrome
and landed”. It then reached the somewhat contradictory conclusion that 'The aircraft
behaved quite well, with the exception that it was completely longitudinally unstable under all
conditions of flight’. It’s most likely that this second-hand report, , was not written by a witness
to the flight, and the author instead based their conclusions on nonchalant, and slightly biased,
comments made by the aircraft’s designer in a later meeting with Air Ministry officials.
A post-flight investigation found that the control issues had been a result of
a misaligned centre of gravity, which was promptly remedied with ballast for further flight testing.
Additionally, Miles reported that there had been issues with the engine overheating,
and to correct this an air scoop was added to the rear of the starboard fuselage.
In the following weeks the M.35 logged a number of additional flights, which were considerably more
successful, and the aircraft demonstrated promising performance and handling.
Buoyed by this, Miles then submitted a proposal for a naval fighter based on the M.35 to the
Admiralty and Ministry of Aircraft Production in April, who promptly chastised him for producing an
unauthorised mock-up of the aircraft, and outright rejected the proposal. Despite this setback,
George was not to be discouraged - as he considered the results of the M.35 to be
extremely promising. He immediately began work on an enlarged bomber design of the same principle
to meet specification B.11/41, which outlined a medium-altitude high speed bomber aircraft
that could carry at least 2,000 lbs of bombs. This would eventually result in the M.39B, which
would also be built in a small scale for initial flight tests and featured a different - though
just as unusual - design as the M.35. The single engine in the rear fuselage would
be replaced by twin de Havilland Gipsy engines in the rear wing, whilst a tall ventral vertical
stabiliser would be added to improve lateral stability. The general layout of the front wings,
cockpit, swept-back rear wings, and twin outboard vertical stabilisers would more or less remain the
same, albeit enlarged from the M.35. However, in order to prevent downwash and improve ground
clearance for the propellors, the previously high-mounted forward wings would be moved down,
and Inversely the previously low-mounted rear wings would now be mounted high.
The M.39B was completed in mid-1943, and immediately handed over to the Royal
Aircraft Establishment for formal flight testing - whereupon flight trials would
begin on July 22nd. These trials would be undertaken predominantly by George Miles and
Flight Lieutenant H. V. Kennedy, and despite some setbacks involving the landing gear collapsing,
overall handling characteristics were deemed to be fairly good. Some small alterations were needed to
improve stability during take-off and landing, mainly shifting the centre of gravity a little,
and some minor trim adjustments, but on the whole these changes were not major.
As a small scale mockup with placeholder engines, the aircraft was of course significantly slower
than its intended full-scale production version - reaching a maximum speed
of 164 mph. Its smaller size also brought with it another unexpected problem. It was reported
that due to the lightweight construction of the fuselage, the cockpit would experience alarming
lateral oscillations - the entire front of the fuselage would warp and bend from side
to side- which made for some hair-raising moments in its earlier flights, but this
was soon fixed by the addition of heavier gauge ply skin. The M.39B received plenty of praise,
with test pilot Don Brown reporting that the aircraft “showed no sign of longitudinal
instability or other undesirable characteristics”; on the contrary, it “handled perfectly normally,
like any other twin of its power, and with the additional advantage of no swing
on take-off and no wing-drop at the stall”. Though not officially tested with it, the aircraft
was planned to have a fairly impressive armament, capable of holding up to 6,000 lbs of ordnance
in the bomb bay, along with twin 20mm cannon in the canards. Carrying such a load was not to be
a problem, even during takeoff from a smaller airfield. A typically ingenious mechanism had
been devised by George, allowing the elevators to be operated normally over their full range on the
rear wing, but when the flap was deflected, the elevator mechanism was activated also, allowing
the flap and elevator to work in unison. This also ha the benefit of greatly reducing the stall
speed of the aircraft during take-off, which was often the most dangerous time for a bomber crew,
other than encounters with the enemy of course. Development of the M.39 would remain a private
venture until September 1943 (several months after its completion), when it piqued the interest of
the British Air Ministry, who offered a formal development contract. Perhaps optimistically,
attempts were also made to garner interest from the USAAF after it came to Miles’ attention
that they were working on the XP-55, which shared some design features with the M.39.
This would ultimately fall through however, as the XP-55s flight testing had left Wright
Field of the opinion that the design was not worth further development.
Despite the official backing from the Air Ministry, only a single prototype of the
M.39B would be produced. The specification to which it had been built was cancelled
before the aircraft was even complete, but instead of ordering it scrapped,
the M.39B was taken from Miles and put through an accelerated test program at RAE Farnborough.
This was done to explore the feasibility of the tandem-wing design in an envisioned high-speed
jet bomber instead – to which Miles may have still received a production contract. But At
the conclusion of said trials, the benefits of tandem-wing aircraft were now being nullified
by those of rear-facing jet engines, which allowed the same visibility and advantages
that the “Libellula” designs had boasted, and after a period of further testing back at Miles
the M.39 was eventually scrapped. In such a climate of rapid development,
the M.39B stood no real chance against its emerging jet-powered competitors.
The earlier M.35 might have had more luck, had the Air Ministry shown some initiative,
but its impossible to say how successful it would have really been. As such they remain an unusual,
and fairly unknown aircraft despite their striking aesthetics. A good concept that was simply
overshadowed by an emerging technology. Now if you want to find out more about these
fascinating designs, fear not. For I am working on one of my longer, deep-dive videos, where I
will cover the entire series of experimental Miles aircraft in detail, for there were quite
a few. But that is a topic for another day. As always, thank you all so much for watching,
and a big thank you of course to the patreons, who’s names you’ll see here. With Christmas just
around the corner I’ll be taking some time to myself, but there should probably be one more
video for December after this, so keep an eye out for that, and hopefully I’ll have some channel
news in the not too distant future. A big thank you as well to our wing commander tier patrons,
our top tier members. Once again patreon is being slow with updating my list,
so if I have missed anyone I apologise in advance, and I’m probably going to put up some polls this
week or next for some video ideas for next year, so stand by for further details. But, as always,
thank you all so much for your continued support, and I’ll catch you all next time. Goodbye,