Throughout the long history of aviation, many companies (or individuals), have proposed new,
radical, and innovative designs only to find that their ambitions on paper rather outstripped their
capacity to make things work in practice. The Koolhoven FK.55 is one such example.
The mid-1930s was a period of rapid development in the field of aviation technology;
not only was the expanding and ever competitive commercial industry stimulating development in
the form of more efficient, safer, and sturdier aircraft, but various conflicts around the world
(particularly the Sino-Japanese war and the Spanish Civil war) were accelerating
the development of military aircraft. In a period of less than ten years,
things had gone from fabric-covered biplanes to all-metal monoplane designs, complete with
features such as retractable landing gears, fully enclosed cabins, and various flight control aids:
things that we take for granted nowadays, but were considered cutting-edge back then. During
this period, many manufacturers attempted to re-write the book on how military aircraft
should be designed, and one of the best ways to showcase their so-called innovations was to
display them at the various airshows that formed the beating heart of the aviation community.
The Paris Air show was one such event, and it was during the 1936 show that a Dutch Aircraft
manufacturer called Koolhoven surprised many by showing off a sleek, new fighter.
It was not, in fact, a working fighter aircraft, but rather a mockup of what the
company intended to build: the Koolhoven FK.55. Designed by Frederick Koolhoven,
the company’s founder, it was a stark departure from his previous, more conservative designs,
and it was a strong indicator of the growing need for new fighter aircraft,
or more specifically, the growing desperation of various European countries trying to hastily
re-arm in the latter half of the 1930s. The fk.55 mockup showed-off several innovative
features. The engine, which was a liquid-cooled V-12, was installed behind the cockpit near the
aircraft’s centre of gravity. This allowed the pilot to be seated further forward, and
combined with a clean canopy design it provided excellent visibility. At the nose, and housed in a
streamlined spinner, was a pair of contra-rotating propellers; these were driven by a shaft that ran
from the engine, under the pilot’s seat, and through to a reduction gear unit. Also mounted
in the nose was a 20mm cannon that fired through the propeller hub, and this was to be complimented
by four wing-mounted 7.7mm machine guns. Other innovative features of the FK.55 was
its streamlined design and unorthodox control surfaces, though the latter didn’t remain in this
form for very long. When viewed from the front, the fuselage seemed to almost have a triangular
cross-section, and the shoulder-mounted wings blended smoothly into it. This streamlined design,
combined with the benefit of contra-rotating propellers doing away with the danger of p-factor,
was hoped to make the FK.55 an easy aircraft to learn for rookie pilots. And To improve strength,
particularly in high-speed manoeuvres, the centre section of the wing was built
integral with the fuselage, with the remaining section connected by spars.
Unfortunately, not much information can be found on the exact design of the wings,
so I can’t go into detail. But I do know that initially it was proposed to do away with ailerons
in favour of so-called “slot-spoilers”. These appeared to have been envisioned as rotating,
slotted cylinders that would extend into the slipstream to control the aircraft’s roll, but
the idea was dropped by the time of the Paris Air show as they were covered over in the mockup, and
they never featured on the prototype itself. Now, as for the actual prototype. Oh dear,
this is where things begin to go completely sideways for this poor little thing. It was
one thing to display a new, innovative design as a static concept piece, but to put it into
practice was another matter entirely. At the end of 1936, or at least during the
spring of 1937, work had begun on building a fully functional prototype of the FK.55,
and almost immediately the design team ran into a proverbial minefield of problems.
During the early phases of construction, it must have been apparent that the cooling
system for the aircraft was inadequate, to put it nicely, as there were multiple changes to
both the design of the air intakes, and the physical dimensions of the engines radiators.
The resizing of the radiators was particularly detrimental as it required re-designing the
shape of the fuselage, replacing its sleek lines with something far more cumbersome.
The added weight from this change also meant that the landing gear decided to break. Its
effectiveness was already under question before the changes, as landing gears on high-mounted
wings often presented difficulties during this time, and the design team was forced to install a
fixed landing gear as a temporary measure until the original could be strengthened….if
it could be strengthened at all. Owing to the re-design of the fuselage,
and the deletion of the retractable gear, the wing also had to be adjusted. As they it longer
benefited from the support of the landing gear struts on the ground, the wing had to
be reinforced, which made it slightly thicker, and it no longer seamlessly blended into the
fuselage like it had on the concept model. All of this, of course, would increase drag.
All of this meant that, Even though the prototype was completed without armament, it was still
considerably heavier than originally planned. The mockup had had a projected loaded weight of 1,650
kg, but the prototype weight 2,280 kg. From what I can find in the limited information available,
the engine was unchanged: a 680 or 700 hp Lorraine Pétrel water-cooled V-12. This is somewhat
supported by the gearing of the propellers, as the alternative engine, a 1,000hp Sterna,
spun its propellers in the opposite direction. But again, sources on this are hazy at best.
But, what we do know is the FK.55, in its new ungainly form, did not impress.
On the 30th of June 1938, it was taken to Welschap Airfield for its first flight.
Koolhoven test pilot Thomas Coppers first put it through a series of ground tests, taxiing
about at ever increasing speeds, and eventually completing several test-hops at take-off speed
before attempting a full flight. He did not have high opinions of the aircraft, and had already
complained about engine heating before prepping for take-off, but he still took the FK.55 for its
first, and as it turned out, final flight. He was in the air for a grand total of two
minutes. Not long after taking off, and flying at a relatively low altitude,
Coppers immediately turned around and landed. What followed was an apparently animated and
heated argument between him and Frits Koolhoven; the latter wanting to know why he had abandoned
the test flight, and the former explaining that the engine was heating up the cabin at such a
rapid rate that he would have been broiled alive, and that the plane handled poorly.
Which was a nicer way of saying that it was so fat, sluggish, and unstable that it was
borderline unflyable. All of the design changes, combined with the centrally mounted engine,
had produced an airframe that needed exceptionally skilled hands to manage.
Following this, the team at Koolhoven made attempts to improve the design, though,
unfortunately, nothing came of it. Frits Koolhoven himself had lost interest in the
project, believing a complete re-design to be a waste of money and resources,
and the onset of the war in Europe curtailed all further development.
As if to rub salt in the wound, the prototype, as well as the entire Koolhoven factory was
then destroyed by a German bombing raid on the 10th of May 1940. And so,
the FK.55 had gone from a promising concept that looked not unlike a Dutch equivalent of the P-39,
to an overweight, underpowered, and dangerously unstable piece of burning wreckage.
As always thank you all so much for watching, and a big thank you of course to the Patrons.
Now I’m going to be away for the latter half of November, so at some point after
the 17th I wont be updating the end page of the video again until the end of the month,
so just a heads up for that. A special shoutout as always to the wing commander tier patreons,
the highest tier members of this special group. Now I’ll be putting up this months Q&A thread
on patreon soon so keep an eye out for that. And as always I’ll catch you all next time. Goodby.