Koolhoven F.K. 55, the "Pregnant Duck"

Would you like to see the Koolhoven F.K. 55 ingame?
  • Yes.
  • No.
0 voters
In what techtree would you like to see this aircraft be added in?
  • In a future Dutch/BeNeLux techtree.
  • In the French BeNeLux sub-tree
  • Other (Please explain in the comments).
  • I said “No” in the first question.
0 voters

Hello everyone! Today I’m going to suggest a rather weird plane from the Netherlands, the Koolhoven F.K. 55!

image
The F.K. 55 Prototype
Source: nhungdoicanh: Koolhoven FK-55


Quick note:
This aircraft went through two main stages. There was the mockup stage, and there was the prototype stage. Now the mockup and prototype look nothing alike, the mockup design was way to advanced so the prototype had to cut back on a lot of features.
As far as War Thunder is concerned, this suggestion is for the Prototype model! But I will also talk about the Mockup stage since it is an interesting side of this aircraft project.


History of the Mockup

Spoiler


A blueprint of the F.K. 55 mockup, note the triangle shapes fuselage and high mounted wings
Source: Modelbrouwers.nl modelbouw • Toon onderwerp - Koolhoven F.K.55 proto, een ongewoon vliegtuig

Around 1936 the F.K. 55 project was started. And this new plane was planned to be one of the most advanced aircraft of the time, featuring many things that were very advanced for the time. The aircraft was to be powered by a 860 hp Lorraine Pétrel 12Hars engine, that was mounted in the center of the aircraft (behind the pilot).
The engine shaft would go under the pilot and come up into a into a special gearbox that would transfer the power to two propellors that would counter rotate from eachother. Futhermore the airframe itself would have a triangle shape with high mounted wings.

The high mounted wings did pose a huge problem. Because the engine was located in the middle of the fuselage, there was no room to mount any kind of retractable gear in the lower fuselage. And so a huge almost 2 meter tall landing gear was planned that would retract into the wings.


The F.K. 55 Mockup in a factory, note the super long landing gear that was made to retract into the high mounted wings
Source: Koolhoven FK.55 Fighter | Old Machine Press

Planned weaponry was a 20 - 37 mm cannon firing through the propellor hub, and machine guns in the wings. On the blueprint we see four machine guns in total, two per wing. But on the Mockup as it was displayed at the Paris Airshow only shows two machine guns, one per wing.


The very realistic mockup under construction
Source: Modelbrouwers.nl modelbouw • Toon onderwerp - Koolhoven F.K.55 proto, een ongewoon vliegtuig

Another advanced idea Koolhoven came up with was to not use ailerons in the wings. As can be seen on the blueprint above the wings are completely smooth and lack any kind of ailerons or flaps. Instead each wing had a thin slot that went from the tip to the middle of the wing. Instead Koolhoven had planned an all new kind of system where small slots would come up out of the wing and restrict airfrow, which would then result in the aircraft starting to roll in that direction.

If you want to see an ingame example of this, then have a loot at the French premium SNCASO SO.8000 Narval. This plane uses a similar system.

The goal of the Mockup was to atract a lot of attention, and it got that during the 15th Paris Airshow. For that reason the Mockup was made to look just as real as a real aircraft. And for the normal visitors of the Air Show it was impossible to tell it apart from a real aircraft.
Another goal of the mockup was to sort of trial out the various advanced ideas on a wooden model to see what could realistically be done and what couldn’t.

The Mockup returned to the Netherlands after the air show and would be displayed in another show in the Netherlands. After that it returned to the Koolhoven factory.

Sadly the mockup was destroyed during the German bombing of the Koolhoven factory on the 10th of May 1940.


The F.K. 55 Mockup as it was displayed at the 15th Paris Airshow
Source: https://www.reddit.com/r/WeirdWings/comments/18d5zr0/koolhoven_fk55_single_seat_fighter_prototype_with/


History of the Prototype

Spoiler


The F.K. 55 Prototype with the engine cover removed
Source: Koolhoven FK.55 Fighter | Old Machine Press

Following the construction of the Mockup model, work on an actual Prototype properly started in 1937. As was expected, the various advanced ideas the Mockup model tried to show of could in reality not be made.
The first thing to go was the complex triangle shaped fuselage. It turned out that this shape was actually a little bit to small to fit the engine and landing gear arms. Instead a more round fuselage was made that gave the aircraft much more of a stubby fat look. One commentor even said it looked like a Pregnant Duck!

The really high mounted wings were kept mainly because the founder of Koolhoven hated low mounted wings on aircraft, saying that “low mounted winged aircraft fall over”. He insisted that the high wings were kept. This also meant the super tall landing gear had to be kept. This too was just to much work to actually make, so it was decided the Prototype would be given a fixed landing gear for now.

And at last the weird slot system to replace the ailerons was also not pursued since the prototype was given just normal ailerons.

More problems arose with the Lorraine Pétrel engine. The Pétrel had it’s first run in 1932, and now it was 1937 already and so the engine was already considered a bit dated. Lorraine was busy with various other orders so the Pétrel for the F.K. 55 was not on a very high priority.
Even worse, when the engine was finally delivered, it ofcourse had the complex drive shaft and gearbox that would allow the engine to be mounted in the middle of the fuselage. But this one of a kind system had only been test run for 3,5 hours by Lorraine before they send it over to the Netherlands. Again showing a lack of interest by Lorraine to spend time on this engine.

image
The F.K. 55 Prototype
Source: 1938 SN de Construction de Moteurs Brochure (EN)

On the 30th of June 1938 it was finally time to fly the prototype. The aircraft was flown by Thomas Copper, a test pilot that had already flown other Prototype aircraft. After going down the runway and doing small hops with the aircraft to get an idea of how it felt, it was time to do a proper flight.

Coppers hit the throttle on the aircraft and took of. He flew it up to an altitude of 50 meters and started to make a turn. But here something went wrong, and Coppers landed the aircraft right away. After getting out of the aircraft Frits Koolhoven ran up to Coppers to ask what happend. What exactly was said between the two men is not known, but something that is known is that the aircraft never flew again after that point.

There are sources saying the aircraft had a fuel injection breakdown, other sources say the cockpit became super hot due to the pilot sitting just above the engine, any of them can be true.

The aircraft was put somewhere in a corner of the Koolhoven factory, and simply left there. Many people blamed Frits Koolhoven and his insistence on a high mounted wing that doomed the project. And so sometime after the project had been stopped, a few staff members decided to, behind Frits his back, make a redesign of the aircraft that featured a low mounted wing, and a 1,200 hp Lorraine Sterna engine. The fuselage was to be made with help from Lorraine themselves and would be wide enough to properly fit the experimental engine.

When this proper redesign was shown to Frits Koolhoven, he cursed and wiped the idea aside. Which is a real shame because the aircraft had the potential to be one of the most advanced, fasted and powerfull aircraft of the time. The minister of defence at the time noted how the Air Force lacked any proper fast fighters, and made a reference to the Koolhoven project saying that aircraft of that performance should be in service with the Air Force by now.

image
The F.K. 55 Prototype before going for it’s first and final flight
Source: nhungdoicanh: Koolhoven FK-55


Design (Of the prototype)

Spoiler


The F.K. 55 Prototype and a photo of the engine and shaft gearbox
Source: Koolhoven FK-55 | Secret Projects Forum

The F.K. 55 Prototype was a high wing, single engine, contra-potating propellor driven aircraft. The engine was located in the middle of the fuselage, just behind the pilot. Due to the very high wing, a very long landing gear was planned that would retract into the bottom of the wings. But due to the long development time of such an landing gear it was decided to give the F.K. 55 Prototype a simple fixed landing gear for the time being.

It was powered by a 860 hp Lorraine Pétrel 12Hars engine, giving the aircraft a top speed of 520 km/h at 4,400 meters. And due to the engine being in the middle of the aircraft, this benefitted the center of gravity of the aircraft. Visibilily for the pilot was also improved due to there not being an engine in the line of sight of the pilot. The cockpit of the F.K. 55 was fitted forward of the wings, and the nose of the aircraft was quite short giving the pilot an amazing field of view.

Weaponry of the aircraft was planned in the form of an 20 - 37 mm cannon in the nose of the aircraft, firing through the propellor hub. Four extra machine guns would have been put in the wings, with two guns per wing. The exact weapon types are not clearly specified since the aircraft didn;t get to the weaponry trial stage.

But by looking at cutaway blueprints of the nose section of the Mockup and Prototype models we do see an autocannon depicted in the nose. And it’s design matches that of the Danish 23mm Madsen autocannon. At the same time this F.K. 55 Mockup was made, the Dutch Fokker G.1 Prototype was also flown, and it was armed with two 23mm Madsen autocannons.
So the F.K. 55 getting armed with a 23mm Madsen autocannon is very possible, and was a weapon type tested by the Dutch Air Force.

As for the exact machine guns this is something that is not detailed in blueprints. If the F.K. 55 were to be armed with the Dutch Air Force in mind then the guns chosen would almost certainly have been the 7.92 mm FN-Browning machine guns.


A simple blueprint of the F.K. 55 Prototype
Source: nhungdoicanh: Koolhoven FK-55


Specifications

Spoiler


A blueprint of the F.K. 55 Prototype
Source: Modelbrouwers.nl modelbouw • Toon onderwerp - Koolhoven F.K.55 proto, een ongewoon vliegtuig

Crew: 1 Pilot
Length: 9.25 m
Wingspan: 9.60 m
Wing area: 16.00 m2
Height: 2.60 m
Empty weight: 1,600 kg
Full weight: 2,200 kg
Powerplant: Lorraine Pétrel 12Hars (860 hp at 4,000 meters)


Maximum straight line speed: 520 km/h at 4,400 meters


Armament
Guns:
1 × 23 mm Madsen autocannon
4 × 7.92 mm FN-Browning machine guns

image
Another blueprint of the F.K. 55 Prototype, this one with a cutaway of the fuselage
Source: Koolhoven FK-55 | Secret Projects Forum


Ingame

Spoiler

As usualy with my Dutch vehicle suggestions, I ofcourse want to see this ingame in a Dutch or BeNeLux techtree. The F.K. 55 is such an weird plane it would in my opinion be a very nice Premium or Event aircraft

The F.K. 55 would be a very nice addition of a Dutch/BeNeLux techtree. You see, not many indigenous aircraft of the Dutch Air Force were actually armed with autocannons of the size of 20mm or more. So the F.K. 55 having a 23mm autocannon is quite special.

Ofcourse the BeNeLux has been added to France, so this is now the only option where Dutch Aircraft will go to. But it must be said that the Netherlands and France have nothing in common, and do not make sense to combine together ingame.


A front view of the F.K. 55 Prototype
Source: Koolhoven FK.55 Fighter | Old Machine Press

So there you have it! Please make sure to vote in the poll above, and let me know what you think of this very odd aircraft!


More photos (Of the mockup)

Spoiler

image
Source: nhungdoicanh: Koolhoven FK-55


Source: Koolhoven FK.55 Fighter | Old Machine Press


Source: Modelbrouwers.nl modelbouw • Toon onderwerp - Koolhoven F.K.55 proto, een ongewoon vliegtuig


Source: Koolhoven FK.55 Fighter | Old Machine Press


Source: Koolhoven F.K.55 scheme | kitchener.lord | Flickr

image
Source: Koolhoven FK-55 | Secret Projects Forum


Source: Modelbrouwers.nl modelbouw • Toon onderwerp - Koolhoven F.K.55 proto, een ongewoon vliegtuig


Source: Modelbrouwers.nl modelbouw • Toon onderwerp - Koolhoven F.K.55 proto, een ongewoon vliegtuig

image
Source: Koolhoven FK-55 | Secret Projects Forum

image
Source: nhungdoicanh: Koolhoven FK-55


Sources

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Main sources:

Secondairy sources:

Extra sources:

Photos:

1 Like

The original mockup is actually quite beautiful. The prototype… not so much. :D
+1 for either, though if it could be closer to the original design, I’d much prefer it!

I think the best and most accurate option for an ingame model would be the Prototype model with a couple of Production version features.

The main things would be the retractable landing gear, and ofcourse the actual weaponry.

1 Like