Koolhoven FK23

Koolhoven FK23During the First World War Frits Koolhoven, dutchman by birth, worked as a designer for several aircraft factories, ending up with BAT in London in 1918. After a number of preliminary designs he produced the BAT FK23 Bantam with the all new 170 hp. ABC Wasp engine. This was a tiny fighter for which expectations were very high. Compared with this plane and its engine the opponents aircraft were at least 50 percent heavier.
A prototype-series of 12 aircraft were ordered by the Royal Air Force. The first three were delivered in the summer of 1918. Throught its testflights the aircraft to performed very satisfatory but the engine proved to be its weak spot and had to be replaced regularly. ABC had noticed this and already worked on an improved version.
Koolhoven had nearly convinced the allied forces to decide for the FK23 as their standard fighter but the end of hostilities led to the dissolution of all contracts. The RAF completed construction of two but the factory was left with the remaining aircraft for which it tried to find a market by participating in displays and airraces.
As Koolhoven had several contacts in the Netherlands, he was able to show his aircraft at the 1919 Amsterdam ELTA (abbreviation for First Aviation Display Amsterdam). One aircraft was on static display, another gave several spectacular demonstrations daily. A third was added later which incidentally was kept as private property by Koolhoven after the ELTA closed.
In 1922 Koolhoven became chief constructor for the newly founded Dutch Aircraft Industy (NVI). On Rotterdam's Waalhaven airport testflights with the improved Wasp engine were made to obtain a certificate of airworthiness. The aircraft was offered to the Dutch airforce but this did not result in any orders. By that time ABC decided not to continue development of the Wasp engine.
Following the demise of the NVI in 1926 Koolhoven started an aircraft factory under his own name, which was to become the second largest in the Netherlands, Fokker leading the ranks. Customers ar the Dutch Airforce, KLM and several foreign companies. Just before the 2nd World War brok out, Koolhoven signed a huge contract with the French Airforce. The aircraft were partly completed but sadly never delivered. The Rotterdam Waalhaven factory was completely bombed during the May invasion of German troups and never rebuilt. Some Koolhoven aircraft have seen service during and after the war but none have survived.

Production: 12
Engine: ABC Wasp 170 hp at 1750 rpm
Speed: 223 km/hr, range 724 km (3,4 hr)
Weight: empty 378 kg, max. 599 kg
Dimensions: wingspan 7.62 m, length 5.58 m, height 2,03 m

The 'Early Birds' Koolhoven FK23
At the end of its existence in 1920, the BAT factory's then existing aircraft were sold to a merchant who was left with them because no buyers could be found. At his death in 1953 the Shuttleworth Collection of Old Warden aquired the remains, storing them in a barn. In 1986 the Koolhoven Foundation was founded and with assistance of Early Birds, succeeded in buying the remains and transporting them to the Netherlands. The collected bits and pieces add up to about two aircraft, one being fairly complete, and a number of parts. The complete aircraft has been totally restored by the Koolhoven Foundation and is now on static display in the Aviodrome in Lelystad (see picture).

Since none of the original drawings and specifications survived, every single part was measured and documented during the restoration. The resulting drawings are now basis for the construction of a flying Koolhoven by Early Birds, using as many original parts as practical. This aircraft will bear the registration of the second remaining Koolhoven from the Shuttleworth Collection: F1657.

Mid-2005 a team of two (now 3) Early Birds staff started construction of the Koolhoven. It was soon decided that an original Wasp engine would be less than reliable for flying operation. Studying a number of eligible engines it was found that the 145 hp Warner radial would be a suitable replacement, leaving the original design and 'look' of the plane intact to the centimeter. The dimensions of this engine also had the advantage that the cockpit will be much larger, leaving space for carrying out maintenance, a safety-related item which the original design all but lacked. The Koolhoven Foundation will calculate and design the necessary new engine mount.
To put this concept to the test a mock-up was built and tested with positive results.

In early 2007 building of the wings started. One of the Early Birds employees digitized the drawings of the wings so that deviations from the -at that time- more than 80 year old remains have been eliminated. This made it possible to cut out the metal brackets with modern machines by a specialized company and to bend them at Fokker in Hoogeveen. It is hard to imagine that this used to be done by hand, because every bracket is a piece of art by itself. The same applies to the laminated spars, which are glued by the team and then finished by a specialized company with the right chamfers, roundings and recesses. All the ribs are cut out by the team itself and sanded to size. Also, all cap-strips and wingtips are steamed and bent.

During construction it became clear how labour-intensive the construction of the wings is. Each rib gets extra adhesive strips and reinforcing strips glued on it. At places where there is a bolt of the brackets, recesses must be made in the ribs. After mounting each rib gets a capstrip glued and nailed on it. On the leading edge of the wings also to be mounted a huge number of form slats. It is clear that - at that time - labour was not an issue and Koolhoven has apparently not paid any attention to the amount of work, but only made beautiful constructions. (See the pictures).

However, the wings, the ailerons and the stabilizer are now (early 2016) almost ready to be covered. The team will now focus on the construction of the hull.

This project was acknowledged as culturally important and is financially supported by the Prins Bernhard Cultuurfonds.

More pictures:

Koolhoven FK23Koolhoven FK23Koolhoven FK23foto2 dsc04744 dsc04760 koolhoven 024

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