A.Weyl Fokker: The Creative Years (Putnam)

Fokker’s next product, the M.20, was an attempt to better the Albatros types by using the same engine, the 160-h.p. Mercedes. Fokker had already tried to instal one such engine in the M.18, but the structural tests were so discouraging that the development was not pursued. Now it was essential to revive the idea, especially as the 160-h.p. Oberursel gave neither speed nor altitude performance and was not liked by pilots.
   The M.20 was little more than a rehashed Fok. D.I, modified to accommodate the larger and heavier engine and two machine-guns with their double supply of ammunition. To compensate for the increased side area forward of the centre of gravity a triangular fin with an area of 0-35 sq. m. was fitted. The prototype was provided with ailerons when it was presented for its type test early in October 1916. However, Fokker had not bothered to increase the structural strength of the airframe: wing spars, etc., were all identical with those of the Fok. D.I. This was asking for trouble.
   The new type was subjected to structural tests at Adlershof from October 2 to October 7, 1916. This was an uncommonly long time for any new type, particularly for an aircraft that was little more than a modification of three tested and approved designs.
   Fokker was again asked to attend the tests - he had complained about prejudice against his products - and he was there, morose and bitter. The tests did nothing to cheer him up: the wings collapsed in the Case A test (see Appendix III) when the sand loading reached 4-32 times the scheduled load. The acceptable minimum factor was 5. Failure occurred when a bolt that held the turnbuckle of a main bracing cable tore out. Closer examination revealed that both the cable and turnbuckle had been under strength. On top of this, Fokker was unable to deny that the turnbuckle did not even fit the cable; this, he explained, as if it were a valid excuse, was attributable to a “mistake” in the assembling of the aircraft.
   New components were obtained from Schwerin and the loading was repeated. This time another turnbuckle broke, in a cable attached to the fuselage.
   The engineers conducting the tests now urged that all major cables in the wing bracing be replaced by cables of 65 % higher strength. Even this could not produce a load factor better than 4-77, and further strengthening of the wing bracing was needed.
   Detail tests in the Adlershof laboratories proved beyond any doubt that the material of the bracing cables was so inferior that it should never have been used in any aircraft. Fokker had bought it, probably very cheaply, apparently from scrap.
   Until this shocking discovery was made, all cables, turnbuckles and bolts had been supplied by Schwerin. This was the accepted procedure during official strength tests. Now, however, the testing engineers replaced such Fokker-supplied components with material from government store. This was an unprecedented action. Braced with this good-quality cable and components, the M.20 wing cables could be loaded to a factor of 5-03 without undue deformation. The loading was then discontinued in order to preserve the aircraft for the other loading cases as required by the BLV. To achieve this minimum result, the Adlershof technicians had had to perform five major sand loadings and two subsidiary loadings, apart from many laboratory investigations.
   The report on this first stage of the M.20 testing contained a stern warning to Fokker that he ought to arrange for more efficient supervision of his production, and that the utmost care ought to be taken to ensure that no defective materials were embodied in any vital structure. Valuable lives might have been saved if Fokker had heeded this admonition. As it was, he departed fuming that those slide-rule wielders had again got the better of him.
   Structural tests for Cases B and D were resumed after the damaged wing had been replaced by a new one from Schwerin. This was actually a concession, because the minor damage to the wing ought to have been repaired and the subsequent tests conducted on the same set of wings.
   The required load factor of 3-5 in Case B was not attained: the rear spar buckled in one of the inboard bays of the new wing when the sand load corresponded to a load factor of 3-31. Again, Fokker was present.
   The IdFlieg now insisted on a stronger rear spar. When the wings were accordingly modified, a load of 3-58 was achieved in Case B and 3-00 in Case D without undue deformation or failure of any vital member.
   Structural tests of the tail unit revealed the need for further modifications. The elevator collapsed under only 72 % of the specified minimum loading, while the rudder stood up to 98% of its load. All the tests had to be repeated after the components had been strengthened.
   By this time a number of production aircraft had already been completed. Fokker had again hoped that they would be rushed to the front without waiting for structural testing. In all of these prematurely constructed aircraft, the centre-section bracing had to be reinforced by the addition of a diagonal strut on each side. The resultant N-struts became a characteristic of succeeding biplane types.
   Towards the end of 1916 the fuselage of the Fok. M.20 was tested, complete with a new fin. The structure collapsed at only 68% of the specified minimum tail load; what was worse, it collapsed through buckling of the lower longerons. This was not a local failure in an easily replaced member: it pointed to serious underdimensioning or shoddy material or negligent workmanship - not, as Fokker liked to explain apologetically, “merely a slip during assembly”. The IdFlieg demanded the use of stronger steel tubing in the fuselage structure and the immediate modification of all the aircraft of the type already built at Schwerin. A considerable part of Fokker’s big profits was going down the drain, but he had only himself to blame.
   The structural investigation established beyond doubt that the Fokker works relied on guesswork when designing new aeroplanes, that they did no experimental stressing, and that their products could not be trusted. All this was brought home to Fokker by the responsible officers of the IdFlieg who studied the reports.
   Fokker went back to Schwerin. He felt defeated, thanks to his weak spot - engineering. However, he had already entrusted Platz with the organization and running of a laboratory for the conduct of strength-testing materials, components and entire aircraft structures - at minimum cost, of course. Fortunately, Platz was good at this sort of thing: he was not only exceptionally skilled and interested, but could be relied upon to spend not a penny more than was absolutely necessary.
   What was equally essential was a new and better design. After Kreutzer’s death, Fokker tried two new designers, one after the other. Where these “aeronautical experts” had come from and where they went to, nobody knew. They were newcomers to aircraft design and lacked experience. Neither was able to satisfy Fokker; neither completed a single experimental aircraft before being sacked.
   Fokker was given an initial order for twenty Fok. D.IV aircraft; this was later increased to thirty. This was disappointing to him, of course, but he later admitted that the flying qualities of the Fok. D.IV had not been so good as those of the Fok. D.III. Nevertheless luck had not completely deserted him: the Albatros D.III fell into disrepute just when the first Fok. D.IV reached the Western Front. This allowed the D.IV to have some initial success in January and February 1917, notably with Jasta 2 (now named the Boelcke Squadron). But by April 1917 the Fok. D.IV had become useless for the Western Front. Very few went to the front, anyway, and of those only four were left by October 1917 to serve somewhere in Russia or Mesopotamia. Altogether thirty-three D.IVs were supplied, the additional three being special versions with wings of longer span and greater area.
   The D.IV was the last Fokker type to have side radiators. These were forbidden on new types after November 10, 1916. Wing-mounted radiators were to replace them.

J.Herris, J.Leckscheid Fokker Aircraft of WWI. Vol.3: Early Biplane Fighters (A Centennial Perspective on Great War Airplanes 53)

Fokker M.20

  The Fokker M.20 was probably an experimental monoplane in development at Fokker in April 1916. However, Fokker abandoned the project about three months later.
  In May, an all-wood wing was built at the Bruning company in Hanau to the design of the Swedish engineer Villehad Forssmann. It was to be ready towards the end of the month.


Fokker M.21 & D.IV
  
  The availability of the Mercedes D.III engine, rated at 160 hp, compared to the 120 hp of the Mercedes D.II which powered the M.18/Fokker D.I, prompted Fokker to quickly evolve a new airframe that was compatible with the more powerful engine.
  At least three experimental airframes powered by the more powerful engine, all under the company designation M.21, are documented. The first of these, carrying the Fokker Werknummer 782, made its initial appearance on the airfield at Schwerin-Gorries in August 1916, just as the first production Fokker D.I fighters were being shipped to equip the newly-formed Jagdstaffeln.
  And as if signaling the beginning of a race, at just the same time the first deliveries of the Albatros D.I, which was powered by the same engine, were also on their way to frontline units.
  The M.21 prototype carrying the Werknummer 782, was superficially similar to the Fokker D.I production aircraft. It featured an extended span and longer fuselage; besides these dimensional differences the most noteworthy design change was a new swept-back upper wing fitted with ailerons. The lower wing retained a straight leading edge, and the most important identifying features of this prototype were the interplane struts; the outer pair of struts was mounted straight between both wings, the inner pair was staggered forward.
  The swept upper wing design apparently required further development, since it was not retained on the other M.21 aircraft, which reverted back to the straight wing layout of the three previous D-types. But it would soon reappear in the Fokker D.V. The nose section of wn 782 was quite pointed, exposing almost the entire first cylinder of the engine to the propeller slipstream.
  Next in line came a M.21 prototype with a shorter span wing cellule that was not swept but still fitted with ailerons. This aircraft, carrying the Fokker Werknummer 927, is thought to have entered the evaluation phase in September 1916. It featured a shorter fuselage and a more bulbous nose when compared to wn 782 , and it can be seen as the true series-prototype. The production D.IV aircraft following it mainly differed in a different cabane strut layout and by having an additional drag wire added from the nose to the lower inner wing strut, just as it was mounted on the production D.I.
  The first production D.IV, carrying the military number 1640/16, was actually the third and final experimental aircraft in the M.21 family; compared to the previous prototype and the other production aircraft, it had a faired nose with large spinner. This one of a kind aircraft appeared in late September 1916 and was apparently strongly inspired by the aerodynamics of the Albatros D.I.
  In Mid-April 1916, the Fokker M.17 and two examples of the M.18 were demonstrated at Adlershof, as were new fighter designs of other manufacturers. Naturally, Anthony Fokker was also present at this important event, and when it was the turn of the Albatros company to demonstrate their new D.I prototype Fokker seized the opportunity to take photographs.
  The streamlined fuselage design of 1640/16 looked rather pleasing, and the aircraft underwent a long evaluation phase. Probably Fokker was hoping to bring its performance at least on a par with the fighters built by his competitors. Photographs indicate that different radiators may have been tried, but details are lacking. Eventually, this aircraft was accepted by the military authorities on 11 March 1917, and it was shipped from the factory 11 days later.
  The mandatory static load tests of the D.IV began on 2 October 1916, and Fokker personally attended these tests at Adlershof, an indication of their importance to him and a fact omitted from his autobiography. Right on the first day of testing, the D.IV wing cellule failed its initial load test (Case A - pulling of a dive); a load factor of 5.0 G was required and the D.IV failed at 4.32 G. The D.IV was strengthened and tested again,- on re-test it failed at 4.56 G. All turnbuckles were replaced by larger ones, and during the next test it failed again at 4.77 G. Again strengthened, the left drag cable failed at 4.17 G during its fourth test held on 6 October. Idflieg questioned why the structure failed at a lower load factor, and following an examination of various components, the cables were found to be made from sub-standard materials. After all cables were replaced by ones taken from Fliegertruppe supplies, the wing cellule passed on its fifth try, failing at 5.03 G on 7 October.
  However, load tests for Case B (gliding flight) and Case D (upward pressure, inverted flight) still had to be passed. On 13 October the rear upper wing spar cracked at a load factor of 3.31 G (3.5 G was required). On 26 October a strengthened wing cellule passed the Case B test at a load factor of 3.58 G, and the next day the wings passed the Case D load test.
  The engineer responsible for the wing tests warned on 8 November that "it was absolutely necessary in making the changes and during fabrication that the Fokker factory be monitored in the most precise manner."
  In parallel with the D.IV structural tests, on 5 October an E.I fuselage was load tested after a series of Eindecker crashes and found not strong enough to handle rudder and elevator loads. This caused Idflieg engineers to question the strength of Fokker's biplanes which shared a similar rear fuselage assembly. On 23 October a D.IV fuselage failed at 68% of the required stiffness. On 25 October the fuselage was re-tested after Fokker workers replaced the failed longeron with stronger tubing and passed. Fokker was notified that completed D.IV fuselages had to be modified with stronger tubing in the same way, and that the modifications had to be carried out at his expense. This clearly indicates that production of the D.IV was already underway at Schwerin. But even this did not end the story of structural failures.
  On 3 November the elevator and rudder failed their load tests. Now Idflieg required that the ribs in the tail surfaces use stronger tubing with wooden spacers at the weld points and the foot controls needed heavier sheet metal. Following the correction of these faults, the type finally passed the acceptance procedure. It had taken the D.IV about five weeks to pass the load tests, and it was exactly during this time that its main competitor, the Albatros D.I, achieved great success at the front with the pilots that were lucky to take them into combat. Even worse for Fokker was the prospect that, while his D.IV was trapped in time-consuming strengthening and re-testing circles, the Albatros company began to ship the first updated version of their fighter in the shape of the Albatros D.II, and by the end of October, the first 28 examples of this type were already serving at the front.
  In his autobiography Fokker claimed that he had a very difficult time getting the 160 hp Mercedes engine, but the truth was different. Even before the load test had begun, Idflieg issued an order for 40 production aircraft to Fokker (numbered 1640/16 to 1679/16) in early September. This was an obvious expression of the trust that the authorities still had in Fokker and his design team. The prolonged M.21 type testing compared to the immediate success of the Albatros D.I/II fighters was certainly the main factor why only this single, small batch of the type was ordered.
  What galled Fokker was by the time the D.IV contract was approved in October, about 700 160 hp Mercedes engines had already been allocated to Albatros for its fighters. But Idflieg had a very good reason to prefer the Albatros design, it was already proving its worth at the front while the Fokker D.IV failed the required load testing time and time again. The D.IV contract was Fokker's opportunity to build a fighter superior to its Albatros competitors; and he failed.
  The final curtain for the D.IV fell when two Fokker D.I fighters crashed at the front in August and December (as described in the M.18/D.I text). The subsequent load testing of Fokker fighters that had seen frontline service revealed considerable structural weakness in several areas. This was traced back to poor workmanship and a lack of quality control at the factory. As a consequence, on 6 December an order was issued that banned Fokker fighters from performing frontline duties. Those that were available and under construction were intended to perform "home duties" instead, such as serving with flying schools and home defense Kest units.
  Two D.IV fighters were listed in the frontline inventory on 31 August 1916, but this must have been a recording error. The two listed at the end of December were probably "leftovers" that had not yet been returned to an Armee Flug Park.
  Of the four first Fokker D-types, the D.IV enjoyed the shortest and most uneventful career. No records at all are available concerning actual use with the Jagdstaffeln, only a single photo showing a very early example in the markings of Ltn. Hans von Keudell is documented as the only D.IV having seen some kind of frontline use at all. Armament was two machine guns, in most cases LMG 08/15.
  Production was slow and ended in March 1917, but at the end of the year the type enjoyed a short and unspectacular revival. Four examples of the D.IV, said to have been build specifically for export, were sold to Sweden, intended for the defense of Stockholm, should this need ever arise. This quartet was powered by 150 hp Benz Bz.III engines, and the aircraft arrived in Sweden in March 1918. Of the four, only one example was actually armed and assembled while the remaining trio remained in storage in a dismantled stage, a seemingly fitting end for the type.


Fokker Early D-Type Specifications
Fokker D.I D.II D.III D.IV D.V
Fokker Type M.18 M.17 M.19 M.21 M.22
Engine 120 hp Mercedes D.II 100 hp Oberursel U.I 160 hp Oberursel U.III 160 hp Mercedes D.III 100 hp Oberursel U.I
Wing Span, m 9.05 8.75 9.05 9.7 8.75
Wing Chord, m 1.25 1.15 1.25 1.25 1.16
Wing Gap, m 1.34 - 1.35 1.32 1.20
Wing Area, m2 22 18 21.6 22.5 15.55
Length, m 5.7 6.4 6.3 6.3 6.05
Height, m 2.25 2.55 2.25 2.45 2.30
Empty Weight, kg 463 384 452 606 363
Loaded Weight, kg 671 576 710 841 566
Max Speed, km/h 150 150 160 160 160-170
Climb to: 1,000 m 4 4 3 3 3
   2,000 m 9 8 7 - 8
   3,000 m 15 15 12 12 16
   4,000 m 23 24 20 30 24
Roll Control Wing-warp Wing-warp Wing-warp, then ailerons Ailerons Ailerons
Armament 1xLMG 08/15 1xLMG 08/15 2xLMG 08/15 2xLMG 08/15 1xLMG 08/15
Note: Climb times in minutes.


Fokker Early D-Type Identification Attributes
Type Engine Notes Qty Built
D.I 120 hp Mercedes D.II Wing warping, 2-bay, 1-gun 90
D.II 100 hp Oberursel U.I (9 cyl) Wing warping, 2-bay, 1-gun 181
D.III 160 hp Oberursel U.III (14 cyl) Wing warping (ailerons on later), 2-bay, 1-2 guns 210
D.IV 160 hp Mercedes D.III Ailerons with horn balances, 2-bay, 2 guns 44
D.V 100 hp Oberursel U.I (9 cyl) Ailerons with horn balances, 1-bay, 1 gun 300


Fokker Early D-Type Production
Type Order Date Qty Serials
D.IV Oct. 1916 40 D.1640-1679/16
D.IV 1917 4 D.5850-5853 (for Sweden)

J.Herris Fokker Aircraft of WWI. Vol.6: Foreign Service (A Centennial Perspective on Great War Airplanes 56)

Fokker D.II (MAG) Series 04.5

  The aircraft ordered from MAG on 26 August 1916 included 34 Fokker D.II(MAG) fighters, increased to 42 aircraft on 19 April 1917. These fighters were numbered 04.51 to 04.92 and were powered by a 185 hp Daimler(MAG) engine. The MAG D.II was based on the Fokker M21 (the D.IV in Germany) but MAG made some small structural changes. Production started in January 1917 and 04.51, the first production machine, was inspected at Aspern on 24 March. Placement of the two Schwarzlose machine guns and radiator design delayed production. Deliveries began in June 1917 and ended in December 1917, four months later than planned. Prototype 04.51 was armed at Fischamend and returned to Matyasfold in July as an armament sample. Due to the gun shortage two aircraft were armed with only one gun and nine were delivered without guns. Flik 49 was assigned 04.52 as an "experimental fighter" in fall 1917 for testing on the Eastern Front. It was criticized for the weak undercarriage. A mid-1916 design, the D.II(MAG) was condemned as obsolete by five Fliks when it reached the Italian Front in January 1918. It was too slow and only useful as a fighter trainer. As a result it was used as a transition trainer on the Piave Front (Fliks 22/D, 37/D, 56/J, 57/Rb, 62/D, 65/D, and 66D), Albania (Flik 64/F), and the flying schools at Campoformido and Neumarkt.


Fokker D.II(MAG) Series 04.5 Specifications
Engine: 185 hp Daimler (MAG)
Wing: Span (Upper) 10.20 m
Span (Lower) 9.70 m
Chord (Upper/Lower) 1.23 m
Gap 1.30 m
Wing Area 21.4 m2
General: Length 6.70 m
Height 2.55 m
Track 1.70 m
Empty Weight 645 kg
Loaded Weight 865 kg
Maximum Speed: 175 km/h
Climb: 1000m 3.1 min
3000m 11.0 min
5000m 34.9 min



Swedish Fokker D.IV

  In January 1917 Sweden bought four Fokker D.IV fighters for the aerial defense of Stockholm. These were powered by the 150 hp Benz Bz.III engine instead of the standard 160 hp Mercedes D.III engine. The reason for this is unknown, but was likely due to German demand for the Mercedes for fighters. The German serial numbers were D.5850-5853/17 (work numbers 1662-1665). Prior to delivery, they were test flown at Schwerin by Swedish pilots.
  Arriving in Sweden on March 27, 1918, the aircraft retained their German camouflage and markings. Apparently none of these aircraft were flown in Sweden; they were held in reserve until struck off charge in April 1922.

J.Forsgren Swedish Military Aircraft 1911-1926 (A Centennial Perspective on Great War Airplanes 68)

Fokker D.IV

  The story of the four Fokker D.IVs is remarkable from several aspects; the aircraft were ordered for a specific purpose, to consistute the aerial defence of the Swedish capitol Stockholm, but, following arrival in late March 1918, all four were firmly locked away, with all requests for test flights, etc, being curtly denied! The bureaucratic notion behind this interesting (for lack of a better word) take on reality was that the fighters were to remain stored until required, ie in wartime. That pilots and mechanics out of necessity require training on the actual hardware was simply ignored by those in charge. Additionally, the fact that even if the aircraft remained dormant, they required regular maintenance. This appears to have been ignored as well.
  First flown in late 1916, 40 Fokker D.IVs (serial numbers D.1640-1679/16) were ordered by the Idflieg. Due to various reasons, Including doubts about the airframe s structural integrity, none of the D.IVs reached the front. Instead, they were utilized as advanced trainers. On January 24, 1917, four D.IVs were ordered through the Ministry of Fortifications, using funds collected by Foreningen for Stockholms fasta forsvar & Svenska Aeronautiska Sallskapet (The Society for the Fixed Defence of Stockholm & Swedish Aeronautical Society, SAS). The aircraft were intended for the aerial defence of Stockholm. The German serial numbers were D.5850-5853/17 (c/ns 1662-1665). The Fokker D.IVs were powered by 150 h.p. Benz Bz III engines, with the engine serials being 19636 (D.5850/17), 26546 (D.5851/17), 19672 (D.5852/17) and 19843 (D.5853/17). Quite why the Fokker D.IVs were fitted with Benz Bz III engines is unclear, instead of Mercedes D III engines, which was the usual powerplanet for the Fokker D.IV. Prior to delivery, the airplanes were test flown at Schwerin by Swedish pilots.
  All four Fokker D.IVs were shipped to Sweden, arriving at Trelleborg on March 27, 1918 for onward delivery to Barkarby north of Stockholm, where an airfield had recently been established. A hangar was also built. Amazingly, upon arrival at Barkarby, the D.IVs were put inside the hangar, with the hangar door being padlocked. No acceptance test flights were made following their arrival in Sweden, with the aircraft all retaining their German insignia and camouflage.
  In November 1918, the quartet of Fokker D.IVs were mobilized as part of the Stockholm Aerial Section. Several requests, all in vain, were made for the airplanes to be transferred to Malmen. Remarkably, permission to test fly the fighters did not arrive until February 1920. Following transfer by train to Malmen, the Fokker D.IVs remained firmly grounded due to obsolescence. When struck off charge in April 1922, none of the Fokker D.IVs had been flown in Sweden, or even received Swedish serial numbers or insignia. Presumably, the engines were subsequently reused (possibly in Friedrichshafen FF 33s used by the MFV).

Fokker D.IV Technical Data and Performance Characteristics
   Engine: 1 x 150 h.p. Benz Bz III
   Length: 6,3 m
   Wingspan: 9,7 m
   Height: 2,45 m
   Wing area: 21,00 m2
   Empty weight: 606 kg
   Maximum weight: 841 kg
   Maximum speed: 160 km/h

O.Thetford, P.Gray German Aircraft of the First World War (Putnam)

Fokker D I and IV

   With the eclipse of the E type monoplanes, the Fokker establishment brought forth its D I biplane in the summer of 1916. Evolved from a series of prototypes designed by Martin Kreutzer, it was an uninspired and singularly mediocre aeroplane and only passed into comparatively limited production, because little else was available to replace the outdated E III on the Western Front.
   Developed from the M 18 prototypes, the D I was the production version of the M 18z, the "z" indicating zweistielig (i.e. two-strutter, or two-bay). Being a two-bay biplane spanning some 30 ft., with only a 120 h.p. Mercedes D II engine, it was decidedly under-powered as a single-seat fighting machine. However, due largely to lack of any stiff opposition, it managed to give a reasonable account of itself, sufficient at least to impress the authorities (reinforced no doubt by Fokker's persuasive volubility) to place production orders for the type.
   On the Western Front the D I soon began to encounter doughtier opponents, and could not compete in manoeuvrability and climb with the lighter and more nimble rotary-engined Nieuports the Allies were bringing into use. Hence it soon fell into disrepute with the pilots who had to fly it, and it was relegated to the Eastern Front and to non-operational duties. Some machines were sold to the Austro-Hungarian authorities. In a final endeavour to extract the utmost use from the type, Fokker slightly modified and enlarged it and re-engined it with the 160 h.p. Mercedes D III, in which guise it was designated D IV (M 21). Performance increase, however, was only marginal, and the engines could be - and were - put to much better use by the Albatros firm in their D I type.
   There was nothing unconventional about the D I; it was an orthodox two-bay biplane. The fuselage followed the same style of welded steel-tube construction as in the E type monoplanes, braced in all planes with stranded cables to form a rigid-braced box-girder structure. The nose section had to be completely re-worked to accommodate the six-cylinder in-line motor, which was quite neatly enclosed within somewhat bulbous metal panels, with just the fore part of the cylinder block remaining exposed, which facilitated servicing. The cooling system consisted of two long, narrow, "honey-combed" radiator boxes on either side of the nose, adjacent to the leading edge of the wings. Aft of this point the fuselage was fabric covered and tapered to a horizontal knife-edge. Although slightly different in size and area, the complete empennage was identical to that of the E types in both profile and proportion: again there were no fixed fin surfaces.
   The wings were of straightforward parallel-chord layout, of equal span and with a slight angular rake to the tips. The upper wing was sited quite close to the fuselage on short steel-tube centre-section struts which were welded direct to the longerons. A peculiar feature on the centre-section was the raising of the line of the leading edge. A large angular cut-out in the trailing edge, together with the wing at approximately eye level, gave the pilot an excellent field of forward and upward vision. Interplane struts were of circular steel tube faired off with wooden fairings: the middle of the inboard rear struts was "notched out" to allow passage of the warp control wires, which passed over pulleys on the rear centre-section struts and so down to the control stick.
   The undercarriage was a conventional vee-type chassis of steel tube with wooden fairings, and sprung with elastic shock cord. The wooden tailskid was hinged to an inverted tubular pylon which also served as a pivot anchorage for the lower extremity of the rudder.
   With the installation of the 160 h.p. Mercedes D III power plant and a slight increase in overall dimensions, the D IV had an improved performance and the level speed increased slightly, but it still lacked the manoeuvrability expected of a fighter. As the Albatros D types became available in increasing numbers, the Fokker D Is and IVs lapsed into the obscurity of non-operational duties with the Fliegerschulen (Flying Schools). Total production of D I was twenty-live and of D IV thirty-three.
   With the formation of the Jagdstaffeln in the summer of 1916 the majority were inevitably equipped with a heterogeneous collection of single-seaters transferred from the escort duties with the various Fl. Abt. units. One such to have some Fokker biplanes on its establishment was Jasta 2 and Leut. Grafe from this unit was surprised in a D I by Capt. Albert Ball, V.C., on 21st September 1916, when he closed with it among clouds some 5,000 ft. over the Bapaume Cambrai road. He was able to approach, apparently completely unobserved, and put in an attack from below at exceedingly close range. He plainly saw his tracers entering the Fokker in the region underneath the engine and pilot's seat, and the machine disappeared into the clouds in a side-slipping dive. The subsequent crash was not observed, and Capt. Ball was not credited with this victory. However, it was eventually established that the Fokker did crash about a kilometre east of Bapaume and that the pilot was killed.

TECHNICAL DATA
   Description: Single-seat fighting scout.
   Manufacturer: Fokker Flugzeug-Werke G.m.b.H. (Fok.).
   Power Plant:
   One 120 h.p. Mercedes D II 6 cylinder in-line water-cooled engine (D I).
   One 160 h.p. Mercedes D III 6 cylinder in-line water-cooled engine (D IV).
   Dimensions:
   Span, 9.05 m. (29 ft. 8 3/8 in.). Length, 5.7 m. (18 ft. 8 1/2 in.). Height, 2.25 m. (7 ft. 4 5/8 in.). Area, 22 sq.m. (238 sq.ft.). ( D.I.)
   Span, 9.7 m. (31 ft. 10 in.). Length, 6.3 m. (20 ft. 8 in.). Height. 2.45 m. (8 ft. 0 1/2 in.). Area, 21 sq.m. (227 sq.ft.). (D IV.)
   Weights:
   Empty, 463 kg. (1,019 lb.). Loaded, 671 kg. (1,476 lb.). D I.
   Empty, 606 kg. (1,333 lb.). Loaded, 841 kg. (1,850 lb.), D IV.
   Performance: Maximum speed, 150 km.hr. (93-75 m.p.h.), D I; 160 km.hr. (100 m.p.h.), D IV. Initial climb, 1,000 m. (3,280 ft.) in 5 min., D I; 1,000 m. (3,280 ft.) in 3 min., D IV. Duration, 1 1/2 hr.
   Armament:
   D I one fixed Spandau machine-gun forward.
   D IV two fixed Spandau machine-guns forward. (Usually not more than one gun was fitted to obtain improved performance.)

W.Green, G.Swanborough The Complete Book of Fighters

FOKKER D IV (M 21) Germany

   A contemporary of the M 19 and the last of the Fokker fighters to be ascribed solely to Martin Kreutzer, the M 21 was, to all intents and purposes, the D I (M18) with twin-gun armament and the 160 hp Mercedes D III six-cylinder water-cooled engine. The M 21 was assigned the service designation D IV and two were at the Front on 31 August 1916, but saw no combat service. The performance of the D IV proved disappointing by comparison with contemporary types and the Idflieg considered that supplies of the 160 hp Mercedes should be assigned to other types. Production was, in consequence, restricted to 40 aircraft for the Fliegertruppen, one being experimentally fitted with a refined engine cowling and large propeller spinner. In addition, four D IVs were built for Sweden where they arrived in March 1918.

Max speed, 99 mph (160 km/h).
Time to 3,280 ft (1 000 m), 3.0 min.
Range, 137 mis (220 km).
Empty weight, 1,336 lb (606 kg).
Loaded weight, 1,852 lb (840 kg).
Span, 31 ft 9 1/10 in (9,70 m).
Length, 20 ft 7 8/10 in (6,30 m).
Height, 9 ft 0 1/4 in (2,75 m).
Wing area, 226.05 sqft (21,00 m2).

P.Grosz, G.Haddow, P.Shiemer Austro-Hungarian Army Aircraft of World War One (Flying Machines)

Fokker D.II(MAG) Series 04.5

  The remaining aircraft ordered on 26 August 1916 were 34 Fokker D.II(MAG) fighters, increased to 42 aircraft on 19 April 1917. These were numbered 04.51 to 04.92 and powered by a 185 bp Daimler(MAG) engine. The D.II(MAG) fighter was based on the Fokker D.IV (M 21) but MAG made some minor structural changes. Production began in January 1917 and the first machine, 04.51, was inspected at Aspern on 24 March. Acceptance was delayed pending choice of radiator and placement of the twin Schwarzlose machine guns. Acceptances began in June 1917 as planned and ended in December, four months later than scheduled. Test pilot Antal Feher's logbook shows that he performed 23 acceptance flights between October and December 1917.
  Aircraft 04.51, with guns installed by the armaments section at Fischamend, was returned to Matyasfold in July 1917 as a weapons sample. Owing to the gun shortage, two aircraft were armed with only one gun and nine delivered unarmed. In July 1917, aircraft 05.54 and 04.61 were assigned to Aspern for flight and performance test and to investigate the efficiency of various radiator installations.
  Flik 49 was assigned aircraft 04.52 as an "experimental fighter" in the fall of 1917 for testing on the Eastern Front. Structurally it was criticized, especially the weak undercarriage. When the Fokker D.II(MAG), a mid-1916 design, arrived on the Italian Front in January 1918, it was immediately condemned as obsolete by at least five Fliks. An army command report, dated 4 March 1918, regarded the type too slow and acceptable only as a fighter-trainer. Consequently, the D.II(MAG) served as a transition trainer at Fliks 22/D, 37/D, 56/J, 57/Rb, 62/D, 65/D, and 66/D on the Piave Front and Flik 64/F in Albania, as well as the field flying schools at Campoformido and Neumarkt.
  As of 20 August 1918, twenty D.II(MAG) fighters were carried in the frontline inventory. Fifteen fighters, two fitted with Mercedes engines, were offered to the Czechoslovakian government in 1920.
Fokker D.II(MAG) Series 04.5 Specifications
Engine: 185 hp Daimler (MAG)
Wing: Span Upper 10.20 m (33.46 ft)
Span Lower 9.70 m (31.82 ft)
Chord Upper 1.23 m (4.04 ft)
Chord Lower 1.23 m (4.04 ft)
Gap 1.30 m (4.27 ft)
Stagger 0.29 m (0.95 ft)
General: Length 6.70 m (21.98 ft)
Height 2.55 m (8.37 ft)
Track 1.70 m (5.58 ft)
Empty Weight 645 kg (1422 lb)
Loaded Weight 865 kg (1907 lb)
Maximum Speed: 175 km/hr (109 mph)
Climb: 1000m (3,281 ft) in 3 mm 5 sec
3000m (9,843 ft) in 11 min
5000m (16,405 ft) in 34 min 56 sec

E.Hauke, W.Schroeder, B.Totschinger Die Flugzeuge der k.u.k. Luftfahrtruppe und Seeflieger 1914-1918

01. - 010. Flugzeuge ausländischer Produktion (Самолеты иностранного производства)
04.51 - 04.92 Fokker D.II (Type M 20) Dm 185