Koolhoven FK-55


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20 November 2008
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Hello folks I'm looking for info of the prototype of the FK-55 fighter.
It was developed by the Dutch Koolhoven company at Waalhaven.
The engine of this fighter was added behind the cockpit,for better stability
it made one flight and flew never again.
I hope someone has drawings,pics or other info about this rare Dutch fighter.
best regards T-50
There is a Dutch book by Wesselink and Postma: 'Koolhoven, Nederlands Vliegtuigbouwer In De Schaduw van Fokker' (Haarlem, 1981) that has a chapter on this aircraft. If you cannot get hold of it, let me know and I can send you a scan of the relevant pages.

Part way down the page. Sort of a high winged P-39 with counter-rotating props
from "Kryl'ya Rodiny" #8-96


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Mockup and prototype


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there were also plans for a modified FK-55 with the wings mounted low on the fuselage
but this project stays on the drawing board
Wow :eek: Do you have an Ontwerp number for this low-winged FK-55 T-50?
hello Apophenia haven't a project number of this version of the FK-55 but a short description in a book that i have about Frits Koolhoven and its aircraft
the Dutch title is
Frits Kool hoven en zijn vliegtuig productie
the autor is Dik Top
on page 124 he describes the modified FK-55
it was mented that it shall have a stronger Lorraine Sterna engine, and a short legged under carriage.
Also was planned 4 machine guns with 500 rounds each the wing were placed lower on the fuselage
it was a very agile aircraft with a maximum speed of560 km a hour
cheers T-50
Thanks T-50, good stuff.

Flight (8 Dec 1938) says the Sterna intended for the original FK.55 would put out 900-1000 hp. Elsewhere, Flight says the Sterna would put out 900 hp at 5000 ft, 850 hp at 14750 ft. But they also say that this Sterna would be fuel injected (with a Planoil supercharger). I could be wrong, but the illustrated engine looks carburated to me.


Off topic, Flight also mentions a cancelled Lorraine project for a larger, 1300hp engine -- the Eider.


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hi apophenia interesting photos about the FK-55engine
thanks, it was better if the FK-55 has an German engine,becouse it was more advanced and powerful
but the idea was not bad
cheers T-50
Hi T-50,

It is hard to make a direct comparison between the planned FK.55 engines and German alternatives. Obviously, the Koolhoven's mid-engined arrangement is an issue. Comparable German engines were all inverted V-12s placing their crankshafts much higher in the airframe (and, remember, the pilot already has the full-rpm shaft running between his legs).

If the inverted-V layout was acceptable in a major redesign, the choice is between the much larger DB 600 (34L vs 30.5L for the Sterna) or smaller Jumo 210 (19.7L vs 28.7L for the Petrel).

There is a benefit to be gained in dry weight though. Like all French V-12s, the Lorraines were heavy. The Petrel weighed 1430 lbs. That was shaved to 1320 lbs for the Sterna. So, does Koolhoven go for the lower-powered Jumo 210 at 975 lbs or the higher-powered DB 600C at 1268 lbs?

Of course, the designer could simplify his life by shifting to other upright V-12 designs. We know that the Allison V-1710 and HS.12Y/Z worked in such mid-engined arrangements. No doubt the Merlin would have too.
To T-50 and other members

Thanks for bringing the FK-55 to my attention, as I have not heard of it before!
Some great drawings and pictures!!

I know that Koolhoven had more fighter projects such as the Koolhoven FK-58 special developed for France,a shame that the luchtvaart afdeling doesn't have any of these!
It was a figher of mixed construction,it was known under ontwerp=design 1166 in 1938
it has a enclosed cockpit and an retracktable under carriage.
It was first mend to equip this fighter with a advanced gullwing,but they build the prototype with a conventional low mounted wing made from wood.
The fuselage was made from metal and the aftsection was build with a metal skeleton covered with linnen like the Hurricane.
the first prototype had a star engine,the 14 cylinder Hispano Suisa 14 AA 1080 pk strong.
The later French versions were quipped with the Gnome Rhone engine of 1030pk
it was in my opinion a promising fighter
T-50: According to Flight, the LVA ordered F.K.58s powered by 900 hp Bristol Taurus radials (postwar sources usually say 1080 hp Taurus IIIs). Top speed was estimated at 320-345 mph at 16,400 feet.

Flight lists gross weight for the Taurus F.K.58 at 5533 lbs (disposable load 1543 lbs), ceiling 32,800 feet, wing loading 29.59, dimensions as per French-ordered F.K.58s.

Flight suggests numbers ordered as high as 40. Most sources say 36 (and there is a report that 40 Taurus were ordered from Bristol in September 1938 so that may be were the confusion lies).


BTW, the LVA was also considering Hurricanes (the ML KNIL seemed to favour US aircraft).
Flight published a powerplant drawing of the Koolhoven F.K.55 much the one reproduced by Justo. But the canopy is notable different in shape from the mockup or prototype. Does anyone know if this was an alternative concept or simply an inaccurate artist's impression?


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If you imagine the ends of the fuselage continuing with the curvature shown, it looks like it would be much shorter than the prototypes too. I imagine it is artistic license/error. Depending on how charitable you are feeling :)
Another photo, a plan and a cutaway of the F.K.55:


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From a Popular Mechanics drawing posted by hesham in the Fokker thread, there were
drawings showing what actually is an interpretation of the FK-55:


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Some more images:


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