A.Weyl Fokker: The Creative Years (Putnam)

The Fokker V.36

   Fokker’s own development of the basic D.VII produced new types in the V series. Of these, the V.22 and V.24 have already been mentioned.
   In the summer of 1918, the Fokker V.34 and V.36 (Factory No. 2656) appeared. It seems probable that their development proceeded more or less simultaneously. It is believed that the V.36 at least was originally powered by the high-compression 220/240-h.p. Benz Bz.IVu engine, but was soon converted to have the 185-h.p. B.M.W.IIIa. In this latter form of the V.36, its fuselage was apparently identical with that of the V.34, which also had the B.M.W.IIIa.
   In these two aircraft, the B.M.W. installation had an oval radiator of a new type, and the engine was more cleanly cowled. The wings had the same span as those of the Fok. D.VII, but were of smaller area; the new aircraft were also shorter, the V.36 being roughly the same length as the D.VI.
   The V.34 had a rudder of the same shape as that of the V.33, whereas the vertical surfaces of the V.36 resembled those of the Fok. D.VII. The undercarriage of the V.34 appeared to be virtually identical with that of the D.VII.
   The V.36 aircraft had a better climbing performance than the D.VII, and was Platz’s final development of the basic V.11 before the end of the war.
   One major innovation distinguished the V.36 from its predecessors. In this aircraft, the radical step of transferring the main fuel tank from the fuselage to the undercarriage axle fairing had been taken. This would have been warmly welcomed by every pilot. By 1918, owing to the widespread use of tracer and incendiary ammunition, the number of aircraft shot down in flames had increased alarmingly.
   In face of this obvious threat to morale, the IdFlieg was pressed to do something to reduce the fire risk. Inventors were encouraged to produce ideas, and the IdFlieg stipulated that all future fighter aircraft should embody adequate safeguards against fire, both from enemy action and from crash damage.
   Platz suggested the transformation of the undercarriage wing into the fuel tank. Fokker thought it a good idea. If such a tank were struck by incendiary bullets, no burning petrol would reach the pilot or damage vital parts of the aircraft, such as the tail unit. This offered a good chance of escape from such a situation; and in the event of a crash landing the tank was too far from the engine for there to be a risk of fire. It seemed an excellent solution.
   Fokker conducted ground tests at Schwerin, using coloured water and fuel for the purpose. These showed that no escaping petrol was likely to reach the cockpit or tail surfaces. The V. 1 fuselage was resurrected for the tests. Fokker also seems to have made some demonstrations in flight, using coloured water.
   He took out several patents on undercarriage tanks. His German D.R.P. 332,048 of April 6, 1918, related to the location of fuel tanks within the axle fairing; there were two tanks, one in front of and one behind the axle. D.R.P. 356,749 of December 24, 1918, covered the final version in which the tank was a one-piece component of streamline cross-section, and the axle passed through it in a box-type housing. All undercarriage tanks had a streamline section; the lifting-aerofoil section of the early axle fairing was abandoned.
   There was, of course, one obvious disadvantage in this arrangement of the petrol tank: considerable pressure was required to force fuel up into the small gravity tank. Submerged fuel pumps were unknown at that time; had they been available, they would have provided an ideal solution. Another fault that emerged after service experience had been gained by the Dutch Army was a proneness to leak after rough landings.
   However, when Fokker demonstrated the V.36, its undercarriage tank was welcomed as the solution to a difficult problem. It was sufficient to make the aircraft and subsequent types popular. Influential pilots made great efforts to secure undercarriage tanks for their own aircraft, but as far as is known, no Fokker aircraft was ever flown operationally with such a tank.
   The V.36 participated in the third fighter trials held at Adlershof during October 1918, and proved to be distinctly better than the Fok. D.VIIF. It was faster at altitude, and its flying qualities were better, although the aileron control was still considered heavy. In climb it was inferior only to the Rumpler Ru.D.I. This was a biplane of exceptionally low weight and elaborate structural features; it had been designed by J. Spiegel in collaboration with F. Budig and E. Rumpler (but was claimed to be Rumpler’s personal brainchild). The final version of the Ru. D.I was scheduled for quantity production at the time of the Armistice.
   Good though the V.36 was, it was not ordered in quantity before the war ended. In any case, its production was unlikely, because much faster fighters would have been needed in 1919.

After the war

   For all the victors’ insistence on the handing over of all Fok. D.VIIs, they made little enough use of the examples they acquired. Britain apparently took no notice of this product of a defeated enemy, but stuck to wire-braced wing structures with thin aerofoils for nearly twenty years. Many British aerodynamicists regarded thick aerofoils as bad; structural experts regarded cantilever wings as dangerous; and the A.I.D. cordially disliked welding, for had not a welded rudder failed on a B.E.2 in 1914?
   In France, welding was accepted, but the advantages of the welded steel-tube fuselage were not appreciated. French designers in the post-war years were more interested in developing metal structures for wings, consequently they took no real interest in the Fok. D.VII.
   The D.VII’s influence on American design thinking was considerable, however. In 1918-19, a total of 142 Fok. D.VIIs were shipped to the U.S.A., where they were thoroughly tested in a variety of forms and with a variety of engines. Some had the 230-h.p. Liberty 6, one the Packard 1A-1116. A report on the Liberty-powered D.VII dated February 13, 1920, by the Service test pilot L. P. Moriarty, stated that the flying qualities of the aircraft were exceptionally good. Manoeuvrability and control response were very good; the high stalling angle permitted near-vertical firing; and the field of view was better than that provided by most biplanes. The performance at altitude was somewhat inferior to that of the original Mercedes-powered Fok. D.VII.
   The German radiator was not big enough for the Liberty engine, however, and it lacked shutters. Lubrication suffered from surging because the oil pump was at a higher level than the tank.
   Two Mercedes-powered D.VIIs took part in the U.S. Army Air Service’s First Transcontinental Reliability and Endurance Test, held between October 8 and 31, 1919. The final report recommended aircraft designers to study the features of these aircraft; and the Mercedes engines were reported to have given “a wonderful performance”.
   In 1922 the Dutch Fokker works were asked to build for the U.S.A, a Fok. D.VII development with the 300-h.p. Hispano-Suiza engine, which was then being manufactured in the U.S.A, under licence by the Wright-Martin Co. The resulting aircraft was designated Fokker D.IX by its makers. It was slightly larger than the D.VII, had a flat frontal radiator and the undercarriage petrol tank, and its angular rudder was somewhat like that of the V.33 and V.34; it had no fin. To the U.S. Army Air Service it was known as the PW-6.
   The influence of the Fok. D.VII structure on American design thinking was far-reaching. Many of the biplanes built in the U.S.A, until the mid-1930s followed the box-spar-and-N-strut formula of the Fokker.
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