Classic German Fighters – The Fokker Dr I

The Fokker Dr I  in flightWhen the Sopwith Triplane entered British service over the Western Front in the spring of 1917, in the hands of squadrons of the Royal Naval Air Service, German pilots who encountered the type were amazed by the phenomenal combination of climb rate and agility displayed by the ‘Tripehound’. This resulted from the combination of a large wing area with compact overall dimensions as the result of the use of a triplane rather than biplane wing cellule. Reports were soon submitted to the Id.Flieg. (Inspektion der Fliegertruppen, or inspectorate of flying troops), and this body seems to have concluded that there must be some inherent advantage in the triplane layout for in July 1917, just as the Triplane was being replaced in British service by the Sopwith Camel biplane, the Flugzeugmeisterei responsible for the design and procurement of aircraft issued a circular letter to German aircraft manufacturers calling for a triplane fighter and inviting all interested parties to examine a captured example of the Triplane.

Genesis
The enthusiasm for this project was such that most German aircraft manufacturing companies seem to have prepared triplane fighter designs, and prototypes were ordered from AEG, DFW, Euler, Hansa-Brandenburg, LFG, Pfalz, Schütte-Lanz and Siemens-Schuckert. The triplane fad also spread to Austria-Hungary, where fighters of this configuration were designed by Lohner, Oeffag, Oesterreichische-Aviatik (Berg) and Wiener Karosserie und Flugzeugfabrik. A march had been stolen over all these companies by the Fokker company, however, for in April 1917 Anthony Fokker had visited Rittmeister Manfred Freiherr von Richthofen’s Jagdstaffel 11 (fighter squadron 11) on the Western Front and from a front-line observation post had seen Triplane fighters in action. Fokker was also afforded the opportunity to investigate a Triplane brought down on the German side of the front line, and to speak to Jasta 11 pilots with striking evidence of the Triplane’s capabilities, especially in a combat of early April when a single Triplane flown by Lieutenant R. A. Little of No. 8 Squadron of the RNAS had completely outmanoeuvred 11 well-handled examples of the Albatros D III biplane that was currently the most advanced fighter in German service.

Development
Fokker returned to his factory at Schwerin outside Berlin and instructed Reinhold Platz, his chief designer, to produce a triplane fighter with a rotary-engined powerplant. This gave great design discretion to Platz, who did not wholly approve of the triplane concept as it offended his concepts of aerodynamic cleanliness and simplicity. Platz nonetheless followed the instructions of his employer, whose decision to use a rotary engine was conditioned by the fact that he knew that the German authorities had in stock a considerable quantity of a French engine type, the Le Rhône 9 air-cooled nine-cylinder rotary engine, built in Sweden to a high standard but delivered after the utility of such engines in the 110-hp (82-kW) power rating had been called into question. Fokker had also acquired a major shareholding in the Oberursel company that was working on a copy of the Le Rhône 9 engine, and felt that the use of such an engine (initially licence-built Swedish and then his own German-produced unit) would provide his design with an advantage in that it would place no further demand on Germany’s already straitened production of water-cooled engines.

Platz was also given the opportunity to examine the Triplane, so his design was wholly original except in its basic configuration, and even this was modified to accord with Platz’s concepts of design and structure. The aeroplane was thus of mixed construction covered mainly with fabric, and was based on a fuselage of welded steel tube construction. This fuselage was of rectangular section with a rounded upper decking and carried, from front to rear, the powerplant, fuel and oil tanks, open cockpit, and tail unit. This last was of fabric-covered welded steel tube construction and comprised single horizontal and vertical surfaces: the former included a large tailplane with acutely swept leading edges and a single strut on each side bracing it to the relevant lower longeron, and this carried the plain elevators; and the latter included only a horn-balanced rudder of the ‘comma’ type that was hinged at its lower end to the vertical knife-edge in which the fuselage terminated.

The wing cellule was based on fabric-covered surfaces of plywood and wood construction, and these surfaces were notable for the fact that they were of the cantilever type already validated in the V.1 and V.2 biplane prototypes, in which the V stood for Verspannungslos (without bracing, or cantilever). The triplane prototype was the V.3, and in its original form as first flown by Fokker himself in June 1917 had three cantilever wings in the form of equal-span bottom and middle wings, which were attached directly to the lower and upper surfaces of the lower and upper longerons respectively, and a longer-span top wing that was carried above the fuselage by two outward-canted cabane struts of the inverted-V type. The bottom and middle wings were of constant thickness and chord to their basically square-cut tips that were rounded slightly, the middle wing had a cut-out in its trailing edge adjacent to the fuselage, and the top wing was of the same basic type but with a cut-out of comparatively wide span in its centre section as well as plain ailerons on the outboard ends of its trailing edge.

The airframe was completed by the landing gear, which was of the fixed tailskid type with a main unit of the through-axle type in which the two-wheel axle was bungee-bound into a spreader bar arrangement faired to an aerofoil section with plywood and carried at the closed ends of two wire-braced V-type struts extending downward and outward from the lower longerons. The powerplant was based on one Thulin (Le Rhône 9) engine rated at 110 hp (82 kW) and driving a two-blade wooden propeller of the tractor type: this engine was installed at the front of the fuselage inside a light alloy cowling that left the bottom of the engine exposed.

Fokker found the V.3 to be exceptionally manoeuvrable and to possess only adequate performance, although this latter was deemed acceptable as the new fighter was planned for defensive air interception purposes over German-held territory rather than offensive operations over Allied-held territory. Initial tests also revealed, however, that the V.3 had unacceptably high control forces as a result of its unbalanced ailerons and elevators. The two major changes therefore demanded by Fokker were the introduction of horn-balanced elevators and ailerons, and the addition of interplane struts to prevent the wing cellule’s tendency to vibrate in flight. Instead of submitting the V.3 for a official type test, Fokker ordered the construction of a revised V.4 prototype with longer-span wings and horn-balanced ailerons and elevators.

Further development
Fokker had meanwhile received a contract for two V.5 prototypes with a powerplant based on the Le Rhône 9 engine that was entering production in Germany as the Oberursel Ur.II. Platz went further than Fokker’s demand, however, in the V.4 and V.5 prototypes in which the V came to stand for Versuchsflugzeug (experimental aeroplane) or Versuchstyp (experimental type): in the V.5 aircraft each wing was increased in span, with the middle wing intermediate in length between the bottom and top surfaces; horn-balanced elevators and ailerons were fitted, two-piece single struts were added to brace the outer wing panels, and twin 0.312 in (7.92 mm) LMG 08/15 synchronised machine guns were fitted on the upper part of the fuselage ahead of the cockpit.

On 14 July 1917, the Id.Flieg. issued an order for 20 pre-production aircraft. The V.5 prototype serial 101/17 was tested to destruction at Adlershof on 11 August 1917. The first two pre-production aircraft received the designation F I, and could be distinguished from production Dr I aircraft by a slight curve on the tailplane’s leading edge. These aircraft, serials 102/17 and 103/17, were the only machines to receive the F I designation, and were sent to Jastas 10 and 11 for combat evaluation, arriving at Markebeeke, Belgium on 28 August 1917: these were elements of von Richthofen’s Jagdgeschwader Nr I (1st Fighter Wing), whose Jagdstaffeln 4, 6, 10 and 11 were disposed around Courtrai, for operational evaluation. The full production form, the V.5, was designated as the Dr I in the new Dreidecker (triplane) category, the contract of July 1917 comprising 320 aircraft including three prototypes, of which the V.4 was regarded as the first and the other two differing from this in the design of their horizontal tail surface with the elevator balance area included in the basic triangular planform of the whole surface rather than outside it as in the V.4. The only change demanded as a result of prototype testing was the use of slightly larger-diameter steel tube in the rear fuselage.

The evaluation of the F I was short-lived but highly successful, and production aircraft were delivered from October 1917. Pilot enthusiasm for the new type soon waned, however, when two aircraft broke up in the air, resulting in the deaths of both pilots. An official investigation was ordered, but von Richthofen jumped the gun and instructed his ground staff to began an immediate inspection of the surviving aircraft.

Shoddy workmanship
This revealed unmistakable evidence of shoddy workmanship, in the form of faults in the assembly of the wings, but also clear proof of damp. As a result, the deeply unhappy Fokker was ordered to complete new and stronger wings at his own expense and to undertake a full examination and repair of wing sets that had already been completed but not used.

Once this wing problem had been addressed and solved, the Dr I reacquired its popularity even though it lacked the speed to chase and catch the latest Allied warplanes. Most of the German fighter arm was now employed on defensive air operations, however, and in such fighting the Dr I’s lack of speed was more than balanced by its superb agility and phenomenal climb rate. The Dr I finally entered full-scale service in November 1917, initially with the Thulin-built Le Rhône engine but from a time later in 1917 with the Oberursel Ur.II that was basically the same engine made in Germany rather than Sweden. The German-made engine was inferior to the Swedish unit in materials and workmanship, however, and the better German fighter pilots tried whenever possible to secure a Swedish engine for their aircraft.

The Dr I became renowned as the aircraft in which von Richthofen gained his last 19 victories, and in which he was killed on 21 April 1918.

The search for improved performance
It was clear from an early date that additional performance would be useful, and that such performance could only be provided by the introduction of a more powerful engine. A number of high-rated rotary engines were evaluated, resulting in the V.6 enlarged prototype with the Mercedes D.II water-cooled six-cylinder inline engine, the V.7 prototype with the Siemens-Halske Sh.III engine and the V.10 prototype with the Oberursel Ur.III air-cooled 11-cylinder rotary engine. The best result were achieved with the remarkable Sh.III engine rated at 160 hp (119 kW): this was an 11-cylinder unit with a gearing system so that the cylinders and propeller turned in opposite directions in an effort to mitigate torque-reaction problems, and in the Dr I produced an even better climb rate and a truly remarkable ceiling. The Sh.III had many development problems, however, and neither this nor any other uprated rotary engine was used in the Dr I for operational purposes.

By a time early in 1918 it had been appreciated that the Dr I, despite its successes in the hands of the most skilled fighter aces, was obsolete, and production ended in May 1918 after the delivery of the 320th aeroplane: in the first day of that month there were 171 Dr I fighters in service, the largest number at any one time. From this time onward the importance of the Dr I as a first-line warplane declined steadily, especially after the superb Fokker D VII started to enter service in June 1918, but several of the type were still operational at the time of the Armistice that ended World War I in November 1918, although most of the survivors were by that time in use as trainers, many of them in unarmed form with the revised powerplant of one Goebel Goe.II rotary engine rated at 110 hp (82 kW).

Specification

Fokker Dr I

Type: fighter

Accommodation: pilot in the open cockpit

Fixed armament: two 0.312 in (7.92 mm) LMG 08/15 fixed forward-firing machine guns with 500 rounds per gun on the upper part of the forward fuselage with synchronisation equipment to fire through the propeller disc

Disposable armament: none

Equipment: standard navigation equipment, plus an optical gun sight

Powerplant: one Thulin (Le Rhône) 9 or Oberursel Ur.II air-cooled 9-cylinder rotary piston engine rated at 110 hp (82 kW) for take-off

Fuel: 15.8 Imp gal (19 US gal; 72 litres)

Dimensions: span 23 ft 7.125 in (7.19 m); area 200.86 sq ft (18.66 m²) including landing gear aerofoil surface; length 18 ft 11.125 in (5.77 m); height 9 ft 8.125 in (2.95 m); tailplane span 8 ft 7.125 in (2.62 m); wheel track 5 ft 5.75 in (1.67 m)

Weights: empty 895 lb (406 kg); normal take-off 1,292 lb (586 kg)

Performance: maximum level speed 100 kt (115 mph; 185 km/h) at sea level declining to 89 kt (102.5 mph; 165 km/h) at 13,125 ft (4000 m); climb to 9,845 ft (3000 m) in 10 minutes 5 seconds; service ceiling 20,015 ft (6100 m); range about 162 nm (186.5 miles; 300 km); endurance 1 hour 30 minutes

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