Barely had the Wright brothers proven that flight for heavier-than-air vehicles was possible, than designers began to think on how to improve aircraft performance. And the most logical way to accomplish that was more power, either with engines offering greater horsepower or just fitting more engines.
Of course, more engines also meant more drag, as multi engine layouts increased an aircraft’s wingspan to accommodate them. Which led to another idea – the “puller-pusher” layout.
With this, an aircraft minimised the aerodynamic drag of utilizing multiple engines by fitting them inline with each other, one acting as a “tractor” motor, pulling the aircraft, the other pushing it.
Because of the theoretical advantages of this layout, most designers and aero companies did at least experiment with the concept. One of these was Antony Fokker, who in 1915 built the K.1, a three-seat aircraft that seated the pilot in a nacelle in the centre of the aircraft with an engine both in front and behind him.
Like most others, Fokker found that the puller-pusher configuration required a lot more development to get correct and he abandoned the idea for more conventional designs.
Though several aircraft of this configuration did ultimately get built and see both military and civilian service, the complications in getting the aerodynamics correct meant the concept was largely abandoned by most, and puller-pusher aircraft primarily became the trademark of the Dornier company, who built several models of flying boats using the principle.
But evidently Antony Fokker also never completely wrote off the idea of building a puller-pusher -and not just a stately flying boat, but as a high-performance fighter.
In mid-1930’s the Dutch authorities realized that the security situation in Europe was looking rather worrisome and that they should probably take steps to shore up the defence of the Netherlands. One of the critical weak spots was the Dutch Air Force which was composed of just a single fighter squadron of Fokker D.XVII biplanes.
In 1937 a four-year plan was announced that would see the establishment of eight fighter squadrons, two of which should be high performance interceptors.
The problem was, the Dutch aero industry didn’t produce such an aircraft. The best indigenous fighter available was the Fokker D.XXI, which had been designed as a rugged colonial fighter.
Though a solid aircraft, and one which would do surprisingly well in the coming war, it was inferior to modern aircraft then taking to the skies. As a result, the Dutch would give serious consideration to buying Hawker Hurricanes, Supermarine Spitfires and the Heinkel He 112 to meet this requirement.
Naturally, the Dutch-based Fokker company were not too thrilled on this plan. The Netherlands had traditionally followed a policy of buying home-grown aircraft up until that point, which had suited Fokker perfectly.
But with the threat of losing that monopoly, Fokker’s needed to up their game. So, they set about designing a new aircraft that would not only match their foreign rivals but would re-establish Fokker as one of the premier fighter aircraft companies in the world. To achieve that, they needed an aircraft that was technically well in advance of the competition.
And so was created the Fokker D.XXIII.
This utilized the pusher-puller layout by having two Walter-Sagitta I-SR air-cooled inline engines, one located in front of the single-seat cockpit, one behind, which each produced 535 hp.
The D.XXIII had a twin-boom configuration, was of all-metal construction and also had a tricycle landing gear.
There are some discrepancies on the proposed armament and as it was never ultimately fitted it is likely that the question was never resolved, but it seems that it was either to be six 7.92mm FN Browning machine guns or else two 7.92’s and two 13.2mm FN Browning heavy machine guns, which would have been fitted in the root of each boom.
The prototype was publicly displayed at the Paris air salon in 1938 before it had even flown, where it was subject to much interest.
On May 30th, 1939, the aircraft flew for the first time.
Obviously, for an aircraft that incorporated so many new ideas, there were problems. The rear engine was found to overheat, largely because of air flow problems as the turbulence created by the front propeller disrupted a cooling flow over the rear one.
There were also concerns about how the pilot was expected to bail out in the event of a problem, as he would most likely be hit by the rear propeller if he did so. This led to proposals for the fitting of an ejection seat, another advanced concept that was in preliminary exploration at the time.
Naturally, all of this meant that development was a gradual process and the D.XXIII was to be subject to several alterations, principally to the rear engine panelling and cowling to try to resolve the heating issues.
In total the prototype made eleven flights, and though it seems to be unconfirmed, it is reported that top speed was 326 mph (525 km/h). This wasn’t bad, though any service aircraft would have been much slower once fitted with armament and military equipment.
However, the prototype was intended to be more of a proof of concept and any production aircraft was expected have substantially more powerful engines. Fokker proposed that production aircraft could have either the Junkers Jumo 210 or Rolls Royce Kestrel engines, both of which produced around 750hp.
These engines were of the previous generation to the Merlin and DB 600-series engines that both the German’s and British were equipping their then-current fighters with. As a result, Fokker didn’t anticipate issues acquiring either of these engines from their producers as they weren’t critical for their respective countries own defence plans. Additionally, these engines were considerably cheaper than the later models and costs were a major issue for the Dutch defence ministry.
Despite these lower-powered engines, the D.XXIII’s configuration meant that top speeds for production aircraft equipped with them was anticipated to be around 350mph (563 kp/h). These speeds, if accurate, would have put the aircraft in the same league as the Spitfire and the Messerschmitt Bf 109 models then flying.
Additionally, Fokker was thinking actively about the possibility of using even more powerful engines. Should Roils Royce Merlin’s or DB 601’s be available for purchase or licence production and be fitted to the D.XXIII, then top speeds were projected to be over 380 mph (612 kp/h).
In fact, that almost certainly would never have occurred. The outbreak of the Second World War in September 1939 meant that no European nation was willing to supply engines as they were all desperately trying to build as many aircraft for their own needs as they could.
Despite this, the Dutch still needed their interceptor, and having left it too late to acquire a foreign aircraft they continued to run tests with the D.XXIII until April 1940, when the undercarriage suffered damage. This meant that when the German invasion of the Low Countries occurred a few weeks later, the Fokker D.XXIII was sitting in a hanger.
It was reportedly badly damaged in the fighting and subsequently vanished into history, an interesting aviation “what-if”.
Related:
https://www.fokker-history.com/en-gb/d-xxiii
https://en.wikipedia.org/wiki/Fokker_D.XXIII
https://ospreypublishing.com/blog/holland_1940/
The Fokker D.XXI; Dutch Defender that Served a Surprisingly Long Time
The Fokker G.I Heavy Fighter; Dutch Grim Reaper
The Koolhoven F.K.58; A Dutch Fighter Used by Poles Fighting for the French
