Today we’re taking a look at the De Havilland DH.108 Swallow, a plane that was as innovative
as it was dangerous.
It was a supersonic jet that did much for the development of cold war aircraft in Great
Britain, in fact it was the first British jet to break the sound barrier, but its tragic
history has often overshadowed its accomplishments.
The development of the Swallow can be traced back to the second world war.
As early as 1942 steps were being taken to ensure that the British Aircraft industry,
which had grown considerably during the war, would still have a good supply of work – and
thus maintain a large number of jobs – once the war was over.
This resulted in the formation of the Brabazon Committee, the goal of which was to investigate
the needs of a post-war civil airline market in both Britain, its overseas territories,
and the commonwealth in general.
Eventually the first, and then second Brabazon committee would culminate in the drawing up
of specifications for the construction of seven new aircraft designs after the war.
The Brabazon committee is a complicated and deeply interesting topic – at least as far
as aviation is concerned – and it’s definitely something that will be covered in its own
video, but for now we will only focus on the bit that affects todays featured aircraft.
The type IV aircraft specification from the committee was the most advanced, and it called
for the design and construction of the world’s first jet airliner, which would eventually
evolve into the DH 106 Comet.
In the earliest stages of the design process, the 106 was originally intended to be a tailless
aircraft.
The concept of the original design was so advanced that De Havilland had the design
and development both the airframe and the engines, and it was the development of the
airframe that led to the DH 108.
Unsurprisingly, De Havilland was reluctant to stick 36 passengers in such a radically
new design without prior experimentation.
And so, in October 1945 a scale model of the design was ordered, one that would be used
for testing to find any potential problems with aerodynamics, hull stresses, and to learn
some of the techniques required for high-speed jet powered flight.
The result was the DH.108, and this mini comet was given the unofficial nickname of Swallow.
The development of the Swallow was covered under Air Ministry Specification E18/45, which
was drawn up in January of 1946.
This required the construction of two airframes for conducting full-scale experiments on high-speed
flights with swept-back wings, to evaluate aerodynamic and structural forces experienced
during high-speed flight, and to act as an approximately half-scale version of the proposed
multi-engine jet airliner that would become the comet.
De Havilland designer John Frost was put in charge of the project.
By this point he had gained valuable experience in the design and production of jet aircraft,
having worked on the team that developed Britain’s second jet-powered fighter: the DH.100 Vampire.
As this aircraft was already tried and tested, the Swallow was directly developed from it.
It made use of the fuselage and the powerplant of the Mk 1 version of the vampire, the powerplant
being a De Havilland Goblin 2, the tail booms were removed, and the wings were replaced
with ones that had a sweep back of 43 degrees.
The two Swallows ordered under the specification were built from two existing Vampire airframes,
these were being built on the production line operated by English Electric.
The first airframe was given the new registration of TG283 and the second was given TG306 – but
to save me repeating serials half a dozen times in a five-minute window I shall simply
refer to them as Swallow 1 and Swallow 2.
After being delivered to Hatfield in September of 1945, work on converting Swallow 1 from
a regular Vampire into its experimental layout began.
The original vampire fuselage pod was extended rearwards to make room for the vertical fin.
The pod had been built in wood, but the new additions to the airframe – the rear fuselage
and the fins – were all made from light alloys.
The swept wing used a main spar -plus auxiliary front and rear spars- and it also housed extra
fuel tanks to offset the change in dimensions affecting the fuel consumption.
Swallow 1 was designed to investigate low-speed handling.
For this it had wooden Handley Page slats that ran the full span of the wing leading
edge and over the air intakes, this restricted its maximum airspeed to approximately 280
mph – or 450 km/h- though some sources conflict and claim it could actually go as fast as
350 mph.
As it was designed to test stalling, the risk of uncontrolled spinning was duly appreciated.
Special anti-spin parachutes were installed to combat this, being fitted to the wingtips,
though as we will see later on their effectiveness was questionable.
The second swallow also had slats built into the wings, but these were retractable and
operated automatically as a safety feature as this aircraft was designed for high-speed
testing.
Converting the aircraft from Vampires to their experimental specifications was completed
in rapid time, and on the 15th of May 1946 Swallow 1 flew for the first time, taking
off from a secluded airfield at Woodbridge with Geoffrey de Havilland Jr at the controls.
After proving that the hastily converted Vampire could indeed fly, it was then taken to Hatfield
for manufacturer trials.
Not long after this, the second Swallow was ready for its first flights.
It first flew as a normal de Havilland Vampire in February 1946, but that was only to take
it to Hatfield for its conversion works.
After these were complete – again done in a record time that probably should have raised
several safety concerns – it took off in its experimental, swept-wing form in June.
This aircraft was used to assess the high-speed characteristics of the swept wings, and during
its initial trials progress was rapid, though not without some concern.
In many flights, the pilot began to encounter compressibility at airspeeds of 340mph at
35000 feet, which resulted in a phenomena known as ‘mach tuck’.
This was a tendency for the nose to pitch down as the plane approached its critical
mach number.
This was not unexpected, and with pitch correction the pilot found it relatively easy to maintain
control; however the aircraft was only put near its critical mach number at higher altitudes
as it was thought that the pitch oscillation caused by mach tuck would be increased at
lower altitudes where the air was denser.
As tests continued, the high speed performance of the second swallow began to exceed expectations,
so much so that it was decided to have a go at breaking the world air speed record, which
was currently held by a Gloster meteor that clocked in at 616mph or 991 km/h….and this
is where the Swallow’s story begins to turn sour.
On the evening of the 27th of September, during one of the trial flights made in preparation
for the record attempt, Geoffrey de Havilland Jr – who was flying the Swallow high over
the Thames estuary – took the plane into a shallow dive from 10,000 feet.
During this dive, the Swallow reached an estimated speed of mach 0.87 and it was here that it
suffered a violent pitch oscillation that caused the aircraft’s wing spars to crack
and fail at the roots, which of course led to the disintegration of the airframe.
I talked about critical mach in our recent video on the Bell X-1.
What had happened here was that the air flowing directly over the wing was already at supersonic
speeds – even though the plane was not-, the shockwave caused by this forced the airflow
away from the control surfaces, which at first reduced the effectiveness of the elevators,
then it went on to cause a reversal of pitch, a nose-down change in trim and a severe reduction
of longitudinal control.
All of this culminated the destruction of both the aircraft, and the death of de Havilland
junior, which was a tragic loss for the company.
The exact cause of the crash was not known until extensive wind-tunnel tests on several
models gave the full picture.
However, despite the loss, the value of the second swallow as a high speed aircraft had
been established and a third prototype was ordered and completed- being registered as
VW120.
This model featured several major changes over the first two swallows.
It had a more pointed nose, a stronger airframe, reinforced wing roots (for obvious reasons),
a more powerful Goblin 4 engine, powered control surfaces, and fully automatic leading edge
slats.
This sleeker, more aggressive looking Swallow flew for the first time on the 24th of July
1947.
It made its public debut at the SBAC air show at Radlett in September of the same year.
and Following this it was put into a series of high-speed trials, as well as entering
for several speed record attempts, of which on the 12t of April 1948 it set a new 100km
closed-circuit speed record, with an average speed of 605 mph.
However, there were still some problems.
flights at high speed and altitude revealed a loss of roll control at mach 0.99 and a
severe reduction of pitch control at mach 0.97.
Eager to avoid another public tragedy, de Havilland imposed mach limits on the aircraft
until it could engineer a solution.
these limits appeared to have been ignored, when on the 6th of September 1948 test pilot
John Derry put the aircraft in a dive from 45,000 feet.
During this dive the aforementioned control issues made their appearance and he lost control,
though thankfully he was able to regain control at around 23,000 feet.
During this chaotic descent a speed of between mach 1.02 and mach 1.04 was recorded, which
meant that the DH.108 Swallow had become the first British aircraft to break the sound
barrier – albeit while out of control.
Not long after this it then became the first British aircraft to go supersonic while under
actual pilot control, as well as being the first supersonic plane in the world that could
take off and land under its own power – the Bell X-1 may have broken the sound barrier
first, but it was air launched to save fuel.
On the 11th and 12th of December that year, the record-breaking plane was displayed to
the public at the Farnborough air show, and then the following year it went to Royal Aircraft
Establishment to join the first swallow – which had been quietly completing its slow-speed
handling tests.
Those who flew the swallows almost unanimously found them to be difficult to control, bordering
the line of being too dangerous in general to even consider flying.
One of the test pilots who flew the third swallow, Capt Eric Brown, was severely critical
of its handling.
It had a nasty tendency to stall suddenly, catching out even the most experience pilots,
and its tendency to oscillate violently at high speeds caused him to label the aircraft
as a ‘killer’.
His words would turn out to be prophetic.
On the 15th of February 1950, the third swallow crashed at Brickhill near Bletchley after
the test pilot, Sqdn Leader Stewart Muller-Rowland, lost control.
The aircraft had performed a high-speed descent from 27000 feet and it had broken up below
10,000 feet.
Witnesses recall it spinning violently out of control, one wing having snapped off, before
crashing into a wood.
Initially, it had been though that the loss of control was due to a fault in the oxygen
supply system that had incapacitated the pilot, but later on further investigations pointed
to a failure of the left-wing spar as the ultimate cause.
The loss of two out of their three test airframes convinced de Havilland to go for a less radical
design for their multi-engine jet airliner, though in a cruel sense of irony the de Havilland
comet would become as infamous for its tendency to disintegrate as its experimental forbear.
This left the original, and now singular, DH 108 swallow to be used as a dedicated low-speed
research aircraft.
This it did…for a about three months until it, too, was destroyed in an accident.
During a stalling trial at 15,000 feet, the aircraft entered an uncontrolled inverted
spin.
The anti-spin parachutes that were meant to save it failed to function properly, and the
test pilot Sqdr Leader George Genders prepared to bail out.
But at around 2000 feet he made a partial recovery, only to lose control again moments
later.
This time he did bail out, but his parachute failed to fully deploy due to the low altitude
and he was killed.
Despite their tragic safety record, something that was not at all uncommon in those early
years of high-speed jet aircraft, the de Havilland 108s achieved a number of firsts for British
aviation.
For Britain they would be the first swept-wing aircraft, the first tailless jet aircraft,
and the first supersonic aircraft.
The man behind the ill-fated Swallow, John Frost, moved to Canada not long after he completed
the initial design work in 1947, and here he would continue to work on some very interesting
designs, which included the UFO-like Avrocar.
But that’s a story for another day.
As always thank you all very much for watching, and a special thank you of course to the Patreons,
with a special shoutout to Kevin, Delliardo, Bane, FB, Christopher R, Tronathon, and Erik
Hindman, for their support as wing commander tier patrons.
Thank you all so much, and I’ll catch you all next time.