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1 DESIGNING FOKKER AEROPLANES Part One - The Early Aeroplanes By Gary Sunderland INTRODUCTION Many, probably too many, books and articles have been written about Tony Fokker and the aeroplanes which have born his name. Some of these are fanciful accounts from people who obviously have little knowledge of an aircraft design organisation and how it works, or the enormous amount of effort which is required to produce just one prototype, let alone a whole series of successful aeroplanes. This account is not based on any new or original research and credit is due to the late Alex Imrie and Peter Grosz for identifying many of the individuals involved in the Fokker organisation. The author s sole contribution has been to work out who did what, when and why, and is based on fifty years experience in the aircraft industry. 1 Any errors are mine alone. However, I would like to dedicate this article to Peter Grosz, who provided the inspiration for me, and hopefully others, to follow. Credit is also due to Geoff Richardson and Colin Owers for supplying some ancient technical references, which proved useful in establishing the level of ignorance which existed about things aeronautical in World War One. Back in 1914, there were virtually no design standards, there were no specifications for materials to be used in aircraft, and few systems for testing and acceptance. Anything which could fly was bought by the military and the service pilots had to adapt these flying machines to warfare by trial and error. These pilots continue to be the major focus of aero-historians to this day, but the story of the development of the aircraft industry, also by much trial and error, is equally important but usually neglected. In the early wartime years the government agencies and the aircraft manufacturing industry developed standards for design, materials and testing, usually with scientists and suppliers working directly with aircraft design engineers. These industry standards were later taken up by government supply organisations, particularly in Great Britain, where the expansion of aircraft manufacture by a multitude of contractors created the need for an Air Board, subsequently the Air Ministry, to among other things institute government standards and the control of aircraft manufacture. In contrast the Central Powers, mainly Germany and the Austro-Hungarian empire, were outnumbered in the air by the combined French, British, Russian and Italian air forces throughout the war. The Germans sought to make up for this numerical disadvantage by technical superiority. The small size of their industry encouraged this approach as scientific and technical improvements in aircraft were swiftly put into production and into combat. The Fokker works was particularly successful in their ability to make small but significant improvements to their aeroplanes throughout the war, as the theory and practice of air combat

2 evolved. The first part of this study will look at the early years, when Fokker aeroplanes varied little from others of the time but combined features which provided a decisive edge in combat. The second part will examine the development of the cantilever wing V-series aeroplanes and the many technical problems which had to be overcome in their design and construction. THE SPIDERS The first Fokker aeroplane was based on model aeroplanes which Tony Fokker developed as a boy. With ten degrees of dihedral and ten degrees of wing sweep this monoplane was unlike any other aeroplane at that time. 2 Assembled at the Automobil-Fachschule, Mainz, many others contributed to the design and construction. Von Daum provided the finance, and Goedecker directly or indirectly designed the fuselage, tailplane and other major structural parts. Tony Fokker did all of the testing, or attempts at flight, learning to fly in the process. In December 1910 he flew 100 metres, before von Daum later crashed it into a tree. A second Spider was built by Goedecker with the same engine, a lighter structure and with ailerons replacing wing warping. Tony Fokker learned to fly on this machine and passed his flying test on 16 May 1911, being issued Pilots Certificate No.88 by the German aero-club. 3 Von Daum crashed this second Spider shortly after, and gave up flying. A third Spider was built by Goedecker, smaller and lighter again, but this time Fokker had the ailerons deleted altogether, depending on the secondary effect of rudder for control in roll. 4 Tony Fokker then became a display pilot and a flying instructor, on Goedecker Taubes as well as his own Spiders. These were known as Spin in German, or Spinne in Dutch, possibly names derived from the web of the spider, given the number of bracing wires used in the construction of the early Fokker monoplanes. Left: A 100hp Spider in a steep bank over the Rumpler sheds at Johannisthal aerodrome. This model was to become the M.1, the first being delivered in The late Alex Imrie Anthony Fokker in his third Spider. Note the exposed cockpit and surfeit of wires. P.M. Bowers

3 Major components of the early Spiders were made at the Goedecker factory and such drawings as were required were likely made by them. The parts were assembled in Fokker s rented shed at Johannisthal airfield near Berlin. The Spider was demonstrated in Holland and Russia by Fokker but an accident in Germany, when a wing bracing-wire broke, resulted in the death of a passenger, Ltn von Schlichting. Fokker was not injured. As a result of this accident the hard wire bracing of the Spider was changed to stranded cable. This was early in 1912 and by late 1912 the Fokker firm had been registered, a draughtsman named Palm was engaged, and also a welding specialist, Reinhold Platz, was employed. Fokker clearly had plans to manufacture and sell aeroplanes to the German military and to do so the crude design and construction of his aeroplanes would require many improvements. The Spiders featured steel tube wing spars and Fokker was obviously thinking ahead in favouring steel construction methods, including welding, which was a novelty at that time. THE WATER-PLANE The Fokker Spin 3 Joe Sewell In 1912 the German Navy requested that aeroplane builders supply machines to operate from water. Fokker s response was a radical flying boat, based on a wooden speedboat hull, below a large swept and dihedralled wing, once again without ailerons. To brace this large wing a smaller lower wing was mounted on top of the hull with a biplane truss structure between. The upper wing overhangs were cabane braced in a similar manner to some contemporary Farman aeroplanes. A Renault 70hp pusher engine was installed behind the top wing and there were two seats forward in the hull. The tail surfaces were small and later proved to be inadequate. Fokker sub-contracted a local boat maker to design the hull and also manufacture the rest of the wooden airframe and fittings. The design of the latter would be based on Fokker s earlier practical experience working at the Goedecker factory. As was usual at the time few structural design calculations were made. Fokker would have instructed the unknown firm s draughtsman as to the typical dimensions for spars struts and other wing fittings. The design was named the Fokker W.1 and Tony Fokker held patents for the Spider wing form and also details of the hull design. None of the workers at Johannisthal were aware of the existence of the W.1 until it turned up and was erected there. 5 The water plane was completed by February 1913 and tested shortly afterwards on a river nearby. Fokker managed to get it into the air but crashed back into the water. The W.1 was then abandoned. This history tells us a lot about Tony Fokker. His projects were original and pursued at maximum speed. Like all designers he tried to adapt other peoples ideas and experience into his own projects. A more cautious approach to testing may have led to a re-design, with much enlarged tail surfaces, before attempting to fly. Even after the crash the project may have been worth salvaging and re-designed to provide a flyable aeroplane. It was typical of Fokker that each new project was pursued with much enthusiasm and then abandoned abruptly when the flight characteristics did not come up to expectations. THE MILITARY AEROPLANES Fokker turned his attention to cleaning up the Spider to meet German army requirements. The M.1 was simply a Spider with two seats in a faired fuselage

4 For the M.2 Fokker had Platz weld up a steel tube fuselage, based on the Jeannin, and this was carefully streamlined with stringers and fabric to reduce drag. 6 The wings were also improved. The M.1 had the three-bay wing bracing of a typical Spider, meaning three sets of wires each side of the centre line, with steel tube spars. The M.2 had two-bay wing panels, with wooden spars, based on the Goedecker or W.1 wings. However the wings were still low set, with sweep and dihedral, without ailerons. The M.3 was similar, but with a slab-sided fuselage. Much of the design detail was worked out by Platz and Palm, based on Fokker s instructions. The M.3 first flew on 26 September 1913, the last of the Spider derived monoplanes. The M.4 was an orthodox cabane-braced monoplane with a straight wing and ailerons. This was also unsuccessful. THE MONOPLANE PROBLEM There were many flying accidents worldwide in , many involving structural failures and a large proportion involving monoplanes. Ref.G This led Blériot to strengthen his wing bracing wires against downloads and his test pilots engaged in displays of inverted flying, loops and other aerobatic flying to demonstrate the safety of Blériot aeroplanes. At the time there was little understanding of the wing torsional loads in flight, particularly those resulting from the highly cambered aerofoils which were then in use. There was no monoplane ban as such and Blériot and Morane monoplanes continued to fly in France and Britain until However for all aeroplanes the national governments began to require that manufacturers supply engineering analysis and test data for each type, or carry out their own check to establish that each design was able to carry flight loads. When the Wright brothers designed the first aeroplanes they assumed a load factor of three. That is, the airframe could withstand three times the load it would experience in level flight. This factor seems to have been adopted thereafter. Scientific investigations conducted in about 1913 showed that an aeroplane in accelerated flight at high angles of attack, centre of air pressure forward, would be subjected to higher loads and a factor of four or more was specified for this case. In Germany the army were experiencing similar problems, partly the result of their requirement that aeroplanes should be readily demountable (de-rigged) to enable them to be transported by road or rail. These A-category aeroplanes were mainly of the Taube, or dove, planform wing configuration. The original Etrich Taube structure featured an external bridge-type truss under the wings which did not lend itself to disassembly. As a consequence the later German monoplanes were cabanebraced and some of these failed in flight. The French monoplanes had relatively short wings which provided a steep angle to the bracing wires. In contrast the Taube s long wing resulted in a shallow angle to the outer bracing wires and high compression loads were generated in the spars. Consequently the wooden wing spars were subjected to simultaneous high bending and compression loads, whose interaction could not be readily calculated at the time. The German Army ordered structural tests on examples of A-type aeroplanes in service during Ref.H Several failed the load test, including the latest model Rumpler. Fokker would have been pleased that the Fokker M achieved a factor of 4.54 at failure, better than any of the Taubes. From these results the Germans decided that formal structural and flight tests were to be required as proof of airworthiness for all new types of aircraft. The required tests were to be limit loads only for each case, that is, the test airframe was to be mounted inverted at the appropriate incidence, the factored weight applied by sandbags or similar and, when the load was removed, the airframe should not be damaged. A failure of any part meant that the aircraft failed. In contrast to the British, the Germans required also that each new manufacturer of a type should submit an example for type testing. For example, a full test was required for the first Albatros C.I built under licence by Bay Flugzeug Werke. In the absence of government specifications and process controls this proof test gave some measure of quality control, demonstrating that the new manufacturer was able to obtain the correct materials and follow the processes specified by the original manufacturer

5 MARTIN KREUTZER By late 1913 the Fokker works had moved to Schwerin and expanded from 25 to 35 employees, with three in the drawing office. They were now joined by a young mechanical engineer, Martin Kreutzer, who initially worked as assistant to Palm. Kreutzer was a product of the excellent technical high school system and qualified to determine loads in structures and conduct stress analysis of beams, columns and other elements. Their first project after his arrival was another water plane, the W.2. Clearly Fokker recognised the need for engineering input to the design process, but he remained in control of the project overall. The W.2 had the sesquiplane wing and bracing reminiscent of the W.1, but otherwise was completely different, being mounted on twin floats, with a Jeannin-Platz steel tube fuselage. The long upper wings were straight and fitted with ailerons. After a couple of brief flights Fokker abandoned the project. Kreutzer learned to fly the Fokker Spin, Palm left or was sacked, and Kreutzer was put in charge of the design and drawing office. THE M.5 MONOPLANE Early in 1914 Tony Fokker purchased a Morane-Saulnier Type H with a 50hp Gnôme rotary engine. This was erected at Schwerin and flown by Fokker and some close friends. Fokker was impressed and determined to build an improved version, for flying displays and for private sport flying. While the German military were mainly interested in large two-seat aeroplanes, there was some interest in fast scouts, mainly because the French saw the need for such aircraft. The decision to go ahead was influenced by the news that the Gnôme rotary engine was to be manufactured in Germany under licence by the Oberursel company. At least one other firm had built a copy of the Morane in Germany and the Pfalz works in Bavaria also obtained a licence from Morane-Saulnier to build both the monoplane and the parasol. 7 The small team involved in the design of the M.5 included Tony Fokker, in direct and overall control, Reinhold Platz, who had the task of translating the wooden structure of the Morane fuselage, tail and other parts into steel tubing, and Martin Kreutzer, who was responsible for calculating the loads in components and ensuring they were strong enough, but no heavier than necessary. In this project they were assisted by at least three draughtsmen who documented each part, and any changes, as the design and construction progressed. At an early stage it was decided to build two versions, the M.5K with a span of 8.95 metres and the long-span M.5L at 10.9 metres. It is worth recording that the same fuselage was used for both versions, with the provision for a second occupant, seated close behind the pilot on a plank, with a leg either side of the pilot s bucket seat. 8 Alternatively a rear fuel tank could be located in this area. Either way an additional load of approximately 70 Kg would be included in the design. The load factor used for design was claimed to be more than the type M, which indicates a factor of at least five, once again about standard at the time for aerobatic aircraft. Platz and Kreutzer did an excellent job in designing the metal fuselage, which is reputed to have been lighter than that of the wooden Morane-Saulnier Type H. Platz is said to have originated the steel tube corner loops for the wire bracing, but these were likely developed from the Blériot- Saulnier U-bolts, used for bracing the original wooden monoplanes. Even so, Platz deserves credit for a simple solution, where other designers resorted to complicated brackets and fittings. The wing design was also a marked improvement. The Morane-Saulnier, like most other aeroplanes at that time, had substantial wooden compression ribs to resist the internal bracing loads. In a warping wing these heavy ribs had to bend and twist, with consequently high operating loads at the pilot s control column. The M.5 wings featured steel tube compression members joining the spars with pivoting end attachments to greatly reduce the stick loads in roll. Even so, these loads were still much greater than the very light elevator loads from the all moving tail. The other hardware was also very neat, with slotted pressed steel dome fittings to take the turnbuckle ends, so bolts, nuts and split pins were not required to attach the landing and flying wires. Fokker may have adapted these from similar fittings on the Albatros biplanes of the time

6 Figure 1: Typical Fokker fuselage structure showing a segment of steel tubing welded into the corner crosspieces. All of the bracing wires are lopped around this one corner fitting. A lifting handle is also shown clamped to the lower steel tube longeron. Figure 2: The Fokker A and E series spar fitting is shown at left. The diagram at right (not to scale) illustrates how the fitting barely protrudes above the wing surface. The Fokker M.5, from which the M.5K and then M.5L were developed. The late Alex Imrie Grosz has evidence Ref.5 that some simple tests were carried out at the factory, such as loading on a completed rib. This is quicker and easier to perform than any structural analysis, and undercarriage tests are in the same category. It is notable that Tony Fokker signed the photo of the rib test, indicating he was in charge of the design process, not Kreutzer and certainly not Platz! The M.5K first flew in April 1914 with the 50hp Gnôme, the M.5L with the 70hp Gnôme shortly afterwards. By May the new 80hp Oberursels were installed and Fokker was delighted with the performance of both prototypes. He performed his first loop in the M.5L and was soon engaged in many flying displays for cash, but also to advertise his new aeroplanes. Officers from Idflieg, the German Army technical department, visited the factory and witnessed Fokker s aerobatic displays. This was an opportune time as the Army and Navy were phasing out the old Taubes and looking to replace them with new fast monoplanes

7 It has been claimed that the M.5 was never tested by Idflieg, and it was accepted on the basis of Fokker s spectacular flying displays. This seems unlikely for a number of reasons, not least because that body of soldiers and professional engineers was set up to conduct such tests of new aeroplanes and the safety of service pilots depended upon them doing just that. It is known that Fokker had a batch of five M.5 airframes under construction, and these may have been completed, awaiting engines. It would have been a simple matter for Tony Fokker to loan one of these M.5 airframes to Idflieg for testing. After all, he was the boss and had no need to tell anyone else what was going on. The test was probably conducted on an M.5L at maximum weight, two occupants plus fuel, factor three in level flight and factor five at the high angle of attack. Remember, these were limit tests to prove the structure. 9 The M.5L would have survived the test and returned to the factory without the employees being any the wiser. The result was a conversion course for three army officers on the M.5L, during which Fokker taught all three how to perform a loop! The M.5L was recommended for service and an order for ten was placed. According to Grosz, Fokker delivered fourteen M.5L aircraft to the German Army and one to the Austro-Hungarian K.u.K.. THE TWO-SEATERS The German Army and Navy still had an urgent need for genuine two-seat aeroplanes and Fokker responded with the M.6 and later the M.8. In this period the war had begun and the army ordered forty M.8 aircraft off the drawing board. Type acceptance was based on that of the M.5L, as the M.8 fuselage was just widened, to provide room in a normal rear seat for an observer. The wing chord was increased to keep the wing loading down to that of the M.5L flown solo. The production rate was slow and the Halberstadter works were licensed to build the M.8. This allowed Fokker to concentrate on the new M.5K/MG development. MACHINE GUNS In 1911 Franz Schneider, the Swiss chief engineer at L.V.G., patented a fixed machine gun synchronised to fire through the propeller disc of a tractor aeroplane. No progress was made of a practical nature because the army would not make a machine gun available. German aeroplanes were being shot down by machine gun equipped Farman and Voisin pushers early in the war and in 1915 the German Army issued a specification for C-category aeroplanes with two seats, 150hp engines and armed with at least one machine gun. As part of this exercise they would have made machine guns available to aircraft firms, as required to design their mountings. Apart from a few pusher types and the Aviatik, the C-types were generally tractors with the observer moved to the rear seat and equipped with some form of rotating mount and means for elevating the gun. At this time Schneider would have been fully occupied designing the new C- class aeroplanes, with little time for synchronising systems. 10 Tony Fokker was in a completely different situation. His small, lightweight A-category machines 11 were soon to be obsolete, with no more orders in prospect, and a 150hp rotary engine was at least a year away. While he now could obtain a machine gun the only logical way to install it would be as suggested in the Schneider patent. With six M.5K airframes completed, possibly awaiting engines, the logical move would be to develop some sort of tractor installation for a machine gun, and this would involve expertise in guns and precision engineering not then available within the Fokker factory. It is therefore logical that Tony Fokker obtained expert help from outside his organisation, possibly through contacts among his many friends in the army, and the whole deal would have been kept confidential for obvious reasons. Some commentators have published criticisms of Fokker for stealing other peoples ideas. Every aircraft designer will take advantage of all relevant information and incorporate as much proven technology as is known in his next design. That is the nature of the business, but every successful designer must do more than just copy, the art is to build something which is better, as Fokker demonstrated with the M.5. In the case of the machine gun installation, this was completed on the M.5K/MG by April 1915 and Fokker invented a story of development in a few hours, to give the impression that it originated from the Garros installation and not the Schneider patent. This was not just to avoid the patent but also to cover the activities of those contacts in the army and the armaments industry who designed and built the hardware. This sequence of events would explain why the first few M.5K had the Parabellum LMG14 observer s gun installed, rather than the Spandau MG

8 What is undisputed is that Fokker took the M.5K/MG demonstrator to Döberitz and on Wednesday 19 May 1915 began firing trials, witnessed by Idflieg representatives. Subsequently the remaining five M.5K/MG aircraft were delivered to the army, where they were given a new E-type category identification, indicating an armed eindecker, or monoplane. These first E-types had the fittings for a plank seat behind the pilot, which would have been used for familiarisation flights and pilot conversions at the front, and also for joy rides for nurses, which might explain how Oswald Boelke gave nurse Blanka a flip. THE EINDECKERS The first production eindeckers varied little from the M.5K and retained the Oberursel seven cylinder 80hp engine, with a modest top speed of 80 mph. They can usually be identified by the location of the feed belt, outside the starboard longeron and covered by an aluminium fairing. The starboard wing root also had a rectangular cut-out, visible when the wing was removed, to clear the feed. When the nine-cylinder 100hp engine became available the fuselage and cowlings were redesigned to accommodate the bigger engine. With a slightly wider fuselage the span was increased slightly with the same, eleven rib wing panels. The wings were also lowered slightly to improve the forward view. The ammunition feed was now inside the fuselage. Given the factory designation M.14, the new type became the E.II in service. Tony Fokker took the prototype M.14 on a demonstration tour at the front. Painted white, with Fokker painted on the fuselage, Fokker gave the first demonstration at Stenay before the Crown Prince on 13 June 1915, followed by a visit to Douai on June. Himself only 26, Tony Fokker impressed the young potential fighter pilots with his spectacular low level aerobatic climbs, dives and rapid reversals. This demonstrated the way to bring the gun to bear down on the target and avoid any return fire. The Fliegertruppe knew how to fly but Fokker taught them how to fight. 12 The next variant was the type M.15, later designated as the E.IV. This had a 160hp two-row engine of virtually two 80hp seven-cylinder engines joined together. The first multi-gun scout on the German side, this had the three bay wings of the M.5L or possible the M.8. The prototype M.15, werke nr.298, originally had three MG08 machine guns and was accepted on 19 September Tony Fokker demonstrated this at Essen later that month. When firing the guns in flight, he shot away the propeller and made a hasty forced landing, but without injury to himself. Once again his luck held. The damaged prototype E.122/ 15 was returned to the factory and rebuilt, but with only two machine guns. This remained the standard for the few remaining M.15 aircraft constructed, limited in number by the few 160hp engines being available. THE E.III The final version was produced by extending the wing span of the M.14 with new thirteen-rib, two bay wings. This greatly improved the climb rate and ceiling of the M.14 and the army designated this variant as the E.III. This version was produced in greater numbers than the others and it is likely that damaged E.II machines were rebuilt at the factory with E.III wings. Up to this time all of the eindeckers had been accepted on the basis of the M.5L. However, the E.III wings were structurally different from the previous designs and Idflieg required an E.III airframe for tests, which proved satisfactory. Following some crashes at the front a fuselage was also tested to destruction to verify the welding design and construction, which was also determined to be very satisfactory. In summary, the eindeckers were relatively slow and had a low ceiling because of their obsolescent engines. However, they proved very effective against the slow two-seaters of the time, when used to intercept such intruders over the German side of the lines. If attacked by an Allied scout they could usually escape with a steep dive at high speed, quite safe for the Fokker but which an opposing Nieuport 11 or DH2 could not dare to follow. No ailerons meant no aileron flutter!

9 Fokker E.I Fokker E.II Fokker E.III Fokker E.IV Joe Sewell

10 Fokker E.I 5/ 15 prepared for a flight. The pilot in the cockpit is Ltn Kurt Wintgens of FFA 6b. Note that the aircraft is armed with a Parabellum MG rather than the Spandau. P.M. Grosz Ltn Bruno Lörzer of FFA 25 taking off in Fokker E.II 20/ 15. The late Alex Imrie Uffz Eduard Böhme of FFA 9b with his Fokker E.III 400/ 15. Böhme was one of the early successful pilots when flying the Fokker monoplanes. Colin Owers Ltn Wentzel (in flying clothing) with his Fokker E.IV. Note the extra cooling holes added to the front of the cowling, as well as the hump covering the twin Spandau machine guns. Colin Owers

11 In service the Fokker E gained a reputation for crashing from less than perfect landings. The problem was not in the undercarriage, which met all the requirements for strength, but in a heavy landing, with the undercarriage fully deflected up, the lower forward lift wires are dangerously close to the wheels. If a slack cable touched to top of a tyre it would quickly wrap around the wheel and demolish the wing and undercarriage. VARIATIONS ON A THEME While the production side of the Fokker works were rapidly expanding, to cope with eindecker construction, Fokker also had the design office hard at work, responding to his creative enthusiasm for new aeroplane projects. The delayed M.7 naval two-seater saw Fokker return to the sequiplane layout of his W.1 and W.2 projects. Naval aeroplanes were also subject to Idflieg inspection and the M.7 passed its proof load tests in February It is obvious now that the long upper wing extensions were inadequately braced against the down loads and the upper wing pylons were too low, with the cables working at a shallow angle. This was the old monoplane problem all over again. During an acceptance flight on a production aeroplane, wing flutter was experienced and a fatal crash resulted. The pylons were increased in height but not nearly enough to prevent another flutter failure some time later. Both crashes resulted from the pilots increasing speed when the wing vibration occurred, when they should have slowed down. However, the basic cause was the geometry of the wing design. The W.3 was a floatplane variant of the M.7 with a long span two bay wing with extensions. It was tested, and abandoned by Fokker at once. The M.9 was an extraordinary twin-engined twin-fuselage biplane, with wings similar to the W.3. This was Tony Fokker s version of the battleplane, a concept very much in fashion among aeroplane designers in 1915, on both sides of the front lines. These multi-engined and multigunned large aeroplanes were supposed to operate like battleships at sea, blasting their way through the opposition. The Caudron C.IV and the A.E.G. G.I were the first of these monsters, but both proved to be easy targets for fast scouts. In the M.9 the two E-type fuselages were not braced together and Fokker complained that they flexed a lot! Once more, a project was abandoned. The M.10 was based on the M.7, the first version M.10E also being a sequiplane, but with slightly higher cabane pylons. The M.10Z was a two bay biplane version which went into service as the Fokker B.II trainer, equivalent to the British Avro 504. This made ten aircraft projects underway in the first part of 1915, so the design office, with only Kreutzer and his four draughtsmen, would have been remarkably hard workers to cope with the workload. 13 On the other hand, the new projects and some of the actual hardware were mostly recycled and adapted from the M.5 and W.2 designs. Tony Fokker was a brave test pilot, but he was not stupid. His life was at stake in any first flight, but he was confident enough in the design of the M.5 and the ability of Kreutzer to sort out complexities of the newer designs on the structural side. The rest was up to him. THE BIPLANE FIGHTERS The development of curving flight, or dogfighting tactics during 1915 saw the need for German single-seat scouts of the new doppeldecker (biplane) D-category requested by Idflieg. In the future the German fighter pilots would have to stick around and fight for control of their airspace. To Fokker s chagrin the first off the blocks in this new category were Halberstadt, once licensed builders of the Fokker A.I, with their all-wood version with biplane wings similar to the Fokker B.II. With a machine gun installed the resulting Halberstadt D.I flew in late 1915 and underwent its structural proof tests on 26 February By mid-1916 Halberstadts were replacing Fokkers at the front. Fokker responded with a bewildering number of prototypes and variants in the series M.17 to M.19, with rotary and stationary engines, single and two bay wings, with and without ailerons. This was presumably on the basis that one of these might turn out to be a winner. It was one thing for

12 Fokker to play mix and match with eindecker parts previously, but these many new designs must have resulted in a huge workload for Kreutzer. In the end one airframe was subject to proof tests by Idflieg to cover the military types D.I, with a 120hp Mercedes, D.II with a 100hp Oberursel U.1 rotary, and D.III with a Oberursel U.3 two row rotary. Repeated tests failed the main steel tube connecting the front spars. Once this was replaced with a thicker tube they met the factor five requirements and loads were increased to failure. The D.III safety reserve was marginal, but accepted. Tony Fokker attended the tests, which should have convinced him that his approach had produced three mediocre designs, whereas more optimised individual aeroplanes would have been more efficient and better performing. All three types were built in small numbers and, in service, were generally disliked by pilots, thus destroying the good reputation that Fokker had established previously with his eindeckers. Worse was to come. On 27 June 1916 Martin Kreutzer, chief and possibly the only structural engineer in the Fokker organisation, took off in a new Fokker D.I for a production flight check, and crashed. Before he died of his injuries, he reported that the rudder had jammed. Whether he had carried out the usual control check before starting is not reported, but either way this was another failure of quality control at the Fokker works. According to reports Tony Fokker tried at least one, possibly two new engineers to head the design and drawing office. The first of these was probably in charge during the development of the M.21 project. The Fokker M.21 was a slightly enlarged version of the M.18 with a 160hp Mercedes and two machine guns. Given the military designation D.IV, it was significantly heavier than the D.III, whose structure was already known to be marginal at a lesser weight. The D.IV was presented for proof testing on 2 October 1916 and the wing failed at factor 4.32, well below the minimum five. This was found to be due to understrength cables and hardware, a fundamental failure of quality control. With good quality cables provided by Idflieg inspectors, the wings finally made the centre of pressure forward test, but then failed the aft test at a factor of 3.31, well below the required 3.5. This time the rear spar failed and the factory had to supply new ones, before retesting to meet the requirement. With further testing the elevators and rudder both failed and had to be reinforced. Finally, the fuselage met the proof load test, but only after additional struts had been added to the front upper wing attachments. Tony Fokker, named as Technical Director and nominally the designer, could not explain any of these failures and the Idflieg inspectors became alarmed. Previously the Fokker eindeckers had an excellent reputation for quality and strength in service, much better than Pfalz and other machines, so these failures were unexpected and deeply concerning. To make matters worse, the Idflieg flight test pilots found the D.IV lacking in directional stability, which made accurate sighting and shooting difficult. Fokker responded with a modification to fit a large dorsal fin in front of the rudder. This was found by the test pilots to be a satisfactory fix, but when the modified aeroplane was offered for structural proof testing, later in 1916, the fuselage failed! By this time the Fokker D.IV was already in service and Fokker was faced with grounding the aircraft and supplying new strengthened fuselages with the dorsal fin, all at his expense of course. Fortunately for Fokker the new Albatros scouts were coming into service and the army were not really interested in Fokker aeroplanes of any sort. These were quickly relegated to the training schools as the fighter staffels switched over to Albatros scouts, and only a few of the modified Fokker D.IVs were constructed. THE NIEUPORT PROJECT During 1916 the French Nieuport scouts had achieved a level of superiority over their German opposition. Idflieg circulated a detailed report on a captured example and requested that manufacturers submit similar designs for evaluation. In response, the German manufacturers built a series of Vee-strutted D-class aeroplanes, ranging from the Albatros D.III to the Siemens Schuckert D.I, which latter was virtually a straight copy of the Nieuport

13 Fokker D.I 151/ 16 of Jasta 1. Note the large triangular fin added in front of the rudder to improve directional stability. The Fokker D.II, powered by the 100hp Oberursel UI rotary engine. Fokker D.III 350/ 16 showing the larger cowling needed to accommodate the 16-cylinder twinrow Oberursel UIII rotary engine, which gave a nominal power output of 160hp. The Fokker M.20, which became the D.IV in service. Powered by a 160hp Mercedes and armed with twin Spandau guns, it was not a success

14 Fokker D.I. Fokker D.II Fokker D.III Fokker D.IV The late Ian Stair

15 Tony Fokker avoided the structural problems involved in a single spar lower wing and instead developed the upper wing with pronounced sweepback, combined with a straight two-spar lower wing. The resulting rotary powered single bay biplane, the Fokker D.V, was very light, with good visibility from the cockpit, light on the controls with balanced ailerons, elevators and rudder. Idflieg test pilots were impressed but the proof load test resulted in another structural failure to a Fokker design. When being proof loaded with sandbags, both spars of an upper wing failed prematurely at a factor of 4, high angle of attack case. Engineer Möser was in charge of the Fokker Technical department in this period and was to shoulder the blame. Fokker contacted the factory and Möser was called on to design strengthened spars and the new wings were hurriedly built. Testing was resumed on 11 October 1916 and this time completed successfully, with only a few minor alterations. The D.V was passed for service as an advanced scout trainer and 200 were built by the Fokker Works. 14 An armed and operational Fokker D.V at one of the Kampfeinsitzerstaffeln employed in the aerial defence of Germany. One such unit to use the D.V was Sturmstaffel I near Munich, which at one time in 1917 had at least three of these aircraft on strength. At this time Fokker met with Hauptmann Doctor Hoff, representing Idflieg. It was blatantly obvious that the Fokker Works was seriously deficient in the quality of both design and manufacture. The Fokker D.III, D.IV, and D.V had all failed under test due to a variety of problems and Idflieg had been forced to bend over backwards to offer help and advice, all to no avail. Idflieg had been promoting a union between the Fokker and Junkers works, in the belief that Fokker might learn from the scientific approach used by the latter firm but, other than Fokker flying the Junkers prototypes, there had been very little progress in the merger. 15 In return, Fokker would have made the usual excuses and broached his latest enthusiasm for a cantilever wing, based on a timber and plywood construction, instead of the steel structure used by Junkers. After flying the prototypes Fokker thought that the thick profile wing flew well, but the allsteel structure was just too heavy. 16 This was a tall order for a firm such as Fokker, who had demonstrated their inability to design an orthodox wire-braced biplane! However, Hoff did not dismiss the project out of hand, but instead provided the names of several experts in the field of structures and materials, who had high professional qualifications and practical experience in timber structures. With no more orders after the D.V for Fokker aeroplanes, Fokker was offered a contract to build the A.E.G. C.IV two-seat general purpose aeroplane. This had a steel tube airframe and was thought suited to the capabilities of the Fokker works. However, the contract was on offer under strict conditions relating to the establishment and maintenance of quality standards in manufacture, to be instituted within the organisation by it hiring suitably qualified staff. If this was not carried out satisfactorily to Idflieg standards then the army was under no obligation to accept any aircraft. Fokker had no real option but to accept the A.E.G. sub-contract or go out of business,

16 a sad state of affairs but one from which Fokker would bounce back, bigger and better than ever, as will be described in Part Two. THE PLYWOOD COVERED WING Engineer Forssman was a private consultant retained by the German manufacturing firm Breuning & Sohn A.G., among other firms, and was employed in promoting their products. Among these was a special thin three-ply, about one millimetre thick, with the birch veneers at 45 degrees. Forssman calculated that this was strong enough to replace the internal wire bracing and fittings, necessary to take the fore and aft loads inside a wing, and also the external fabric covering. This diagonal ply wing would then weigh less than a normal wing. Fokker was involved in the supply of details of the M.17 wing, later changed to the M.22 prototype, which became the Fokker D.V in service. Forssman used full depth wing spars for his alternate structure and, as the exercise was to save weight, it is likely these were hollow box spars, in contrast to the originals. Fokker spars were then simple rectangular sections, spindled out to an I section in lightly loaded areas and limited in height to fit within the ribs, which were slid over the spars during construction. Several writers have suggested that Forssman s wing gave Tony Fokker the idea for developing cantilever box spars. This is unlikely given that the M.22 Forssman wings were shallow wire braced structures. Hollow box spars were commonly used within wire braced aeroplane structures during For example, the Nieuport wing spars were hollowed out timber C sections, glued together to form a box. In fact, the remarkably light German Halberstadt airframes extended this weight saving measure to the fuselage longerons, to mimic the Fokker fuselage, but using hollow wood in place of steel tubes. Forssman was well ahead of his time in many respects. It was more than 10 years before his design approach, using full depth spars in a thinner wing with a diagonal plywood torsion box, was applied first to high performance sailplane wings and then to light aeroplanes. As it was Tony Fokker lost interest in the D.V development wing but wrote that he was now more interested in thicker plywood (presumably normal birch three ply) for a short wing span project. Apart from the use of thick plywood on the wings it is unlikely that Fokker had many ideas as to how to build a thick cantilever wooden wing, other than it should be lighter than the Junkers wing made of steel, purely on the weight factor. Two other developments would have been an encouragement. The Albatros company, famous for their plywood fuselages, were now using cantilever wooden fins and tailplanes, at least on their small D-category scouts. Also, the N.F.W. company at Johannisthal had built and flown a small cantilever monoplane, designed by Engineer Hergt early in These developments would have been known to Tony Fokker. If any project encouraged Fokker it was surely the N.F.W. monoplane. It is not known if this had tapering box spars. Probably not. The spars were more likely to be I or C section, as on the Albatros tail surfaces, but they were wood and the wings were cantilever, without external bracing wires. SUMMARY The early development of the Fokker works was typical of most of the pioneer aeroplane manufacturers, except that Tony Fokker was younger and his firm only recently established at the outbreak of war. His main interest was in the design and test flying of experimental prototypes. The business side, not mentioned in this analysis, also took up much time and effort. In comparison, T.O.M. Sopwith ceased test and exhibition flying at an early stage and Harry Hawker took over as chief test pilot in Tom Sopwith built up the business and, despite his later claims, had little input into the design process except, at the initial phase, to agree to go ahead and spend money. The test pilots were an élite group within the sporting and military pilots. Only the very best, like Hawker and Fokker, survived to see the end of the war. It was the test pilots who compared aeroplanes and suggested lines of development for their firms to take. In the days before aeronautical engineers, capable of performance, stability and control calculations, it was the test

17 pilots who recommended the wing sections, the spans, areas, weights and configurations of each new project, based on their practical experience of flying. Fokker has been accused of spying on rival manufacturers. This was normal and Idflieg actively encouraged the practice, to speed up the development of German aircraft technology. To this end Idflieg circulated technical descriptions of the latest German and captured foreign aircraft on a confidential basis within the industry. When patent litigation by some manufacturers threatened to slow the use of inventions throughout the industry, the German government passed laws to suspend such litigation until after hostilities had ceased. While a project may start from a sketch on a scrap of paper, or in the case of Sopwiths, some chalk marks on the wood floor of the skating rink, the actual design proceeds in the form of specifications and drawings to define the location and description of every part in the aircraft. These are passed to the structural engineers, who calculate the estimated loads in each element, identify the critical parts, and establish their strength by simple analysis or test. Most of the engineers were civil engineers by training, which explains why the early aeroplanes look like bridge structures, either cross-braced spars and struts, or cabane-braced spars from pylons. Once a design was cleared by the engineer, usually by signing an approval space, prints are taken for issue to the various shops within the factory. Almost at once changes may be required for a variety of reasons, the drawings are amended and raised in issue, and any significant changes are referred to engineering. Finally the prototype is built, with more changes to be recorded, test flown, more changes, until it is passed to the government agency for their tests and acceptance procedure. Thus the design is not the final hardware, it exists in all of the documents and drawings which record its manufacture. Once accepted, the design drawings may not be changed, except as agreed by the government s agent. The failures under test of the Fokker aeroplanes not only disclosed that the design of the type was faulty, but also that the control of its manufacture was suspect. Fokker had a love/hate relationship with engineers, but the evidence shows that Kreutzer did an excellent job of quality control, not only in design but also in ensuring that good quality materials were used in manufacture. It was only after his death that the standards seem to have declined. Fokker complained that the engineers at Idflieg were prejudiced against him. The evidence seems to be to the contrary. Idflieg operated not just to safeguard the flying troops, but also to develop aviation technology and the German aircraft industry, so necessary if they were to hold their own in the air. They threw him the lifeline of the A.E.G. contract and suggested the means by which he could rise again to a leading position in aviation. Those dreaded slide-rule experts were really on his side all the time! NOTES 1. The author has 50 years experience in aeronautical design, starting as a temporary assistant at the Aeronautical Research Laboratories and finally as Manager of Engineering in the Civil Aviation Safety Authority, responsible for the approval and quality of aeronautical design organisations, from individual consultants to airlines and major aircraft manufacturers. The author also has 3500 hours flying in light aeroplanes and gliders, including the design, construction and flight testing of a high performance sailplane through to full certification. 2. With 10 degrees of dihedral and 10 degrees of sweepback the layout was certainly stable and, even today, unique. This illustrates its model aeroplane origins and confirms Tony Fokker to be the originator. The author has built a free flight (i.e. no radio control) model of the Spider and this proved to be stable, but the many rigging wires resulted in a poor glide performance, much inferior to a similar model of the Blériot XI. 3. At this time the only pilot qualification was the Certificate of competency issued by the national aero club, in this case the Deutsche Luftfahrer-Verband. Not to be confused with a government issued pilots licence, which came into force after Tony Fokker discovered that the ailerons on the Spider No.2 were ineffective and the rudder control could generate a better roll rate. His display flying in later Spiders concentrated on spectacular rapid turns to illustrate this response. No doubt the modern term Dutch Roll, given to the unwanted roll experienced by swept back wings, originated with Tony Fokker s Spider

18 5. In reference 3 below, it is stated that...fokker himself could not have designed it. A strange pronouncement. It is difficult to imagine anyone, other than Tony Fokker, designing a large flying boat without ailerons! It seems probable that the original text by A.R. Weyl would have gone on to add something like...alone and he would have needed the assistance of design draughtsmen from the boat manufacturing firm. This would be a sensible and obvious statement. J.M. Bruce probably applied the editorial scissors to the original text, possibly an attempt to sever the connection between Tony Fokker and the design of the W The Jeannin brothers were French. They worked in Germany before the war and the elder was a design engineer in the Argus engine works, whose engines were favoured by Fokker. The younger Emil Jeannin was a pilot and founded the Jeannin aircraft works, which produced a modernised Taube with cabane bracing and a steel tube fuselage. At the outbreak of war the brothers returned to France. 7. In British (and presumably French) references as late as 1950 the Oberursel and Pfalz products are described as copies of the French originals. It was not the done thing, to admit that the Allies sold these rights to a potential enemy. These same references refer to the Fokker eindecker as a crude copy! 8. This copied the Morane-Saulnier, and many other aeroplanes of the time, such as the Bristol Boxkite and the Henry Farman III. This cosy arrangement made it possible for some measure of dual training, with the rear occupant being able to reach around and operate the control column, but not of course the rudder bar. Also note that it was not possible to fit dual controls into the M.5L, as has been claimed in some references, as there was no room between the occupants. 9. For some reason J.M. Bruce assumes that Idflieg tests were carried out to destruction (see reference 3, page 74). I am sure that Weyl would have not made this mistake. A factor five test is supposed to be a limit test, within the elastic limit. The structure should return to normal shape. 10. Schneider did not complete his first synchronised gun in an L.V.G. tractor monoplane until June The prototype, military number E.600/ 15, crashed during flight before reaching the front and the type was abandoned by Schneider. 11. The army air reorganisation in 1915 included changes to the military numbering system. The Fokker M.8 became the military A.I type, the M.5L the A.II and the M.5K became the A.III. 12. These rapid reversals by the Fokkers were soon known to the Allies as Immelmann turns, but the real originator was Tony Fokker. 13. On page 146 of reference 3, Bruce gives the design office number at a couple of draughtsmen after previously stating it was Kreutzer plus four. How could Weyl and/or Bruce possibly know that :- No stressing was attempted.? On this it is worth noting that it is the load analysis which is important when using existing parts. Obviously if the loads are equal or less no stress analysis of the part or component is necessary to establish the margins of safety. 14. On page 183 of reference 3 Weyl states that Platz subsequently proposed a lighter spar that was accepted by Idflieg. Somehow an expert welder, who knew nothing of aircraft loads, timber properties, structural beams in bending and compression, was designing improved wooden spars! This scenario also avoids the fact that the design of the D.V was now frozen for production and Idflieg would almost certainly require a retest before accepting such a change. Actually Platz s ignorance of design and testing, even of steel tube structures, is proven by the change to the centre section brace tube orientation. Platz did not know that this resulted from the problems experienced during the proof tests in early Idflieg saw a great future for all metal aircraft structures (the Zeppelin airships were a notable success) and they hoped that a merged company might speed these developments. The merged company was in place by 4 September 1917 but proved to have no effect on progress thereafter, either at Fokker or Junkers. 16. Fokker was correct on both counts. It is ironic that at the same time Junkers abandoned his monocoque steel structure of one tenth millimetre thick steel sheet, and changed to a welded steel tube construction for the wing with a covering of corrugated duraluminium. If the Idflieg plan had worked Fokker might have been building all-metal low wing monoplane fighters in the Spring of REFERENCES 1. A.H.G. Fokker, Flying Dutchman, Putnam H. Hegener (edited by B. Robertson), Fokker The Man and his Aircraft, Harleyford Publications Ltd A.R. Weyl (edited by J.M. Bruce), Fokker: The Creative Years, Putnam Review (of reference 3),Cross and Cockade (US), Vol.11 No.3, P.M. Grosz, Fokker E.I / II, Windsock Datafile 91, Albatros Productions Ltd. 6. P.M. Grosz, Fokker E.IV, Windsock Mini Datafile 7, Albatros Productions Ltd. 7. P.M. Grosz, Fokker D.VII, Windsock Datafile 9, Albatros Productions Ltd. 8. P.M. Grosz, Fokker D.VIII, Windsock Datafile 25, Albatros Productions Ltd. 9. P.M. Grosz, Fokker Flugzeugbau, WW1 Aero Magazine

19 10. P.M. Grosz, Reinhold Platz and the Fokker Company, Over The Front Vol.5 No.3, A. Imrie, The Fokker Triplane, Arms and Armour Press M. Tate, The Fokker Cantilever Wing, Cross and Cockade International Vol.41 No.1, G. Sunderland, Fabric (letter), Cross and Cockade International Vol.41 No.2, M. Tate, Fabric (reply to 13 above) The Fokker Cantilever Wing, Cross and Cockade International Vol.41 No.3, A. Imrie, Fokker Fighters of World War One Vintage Warbirds No.6, Arms and Armour Press Josef Scott, Myth-Busting: Persistent Misconceptions about the Fokker Eindeckers, Windsock Worldwide Vol.28 No.1, TECHNICAL REFERENCES A. (British) Air Board Standards. Air Components and Materials B. Andrews and Benson, Aeroplane Design, D.U. Technical Series, Chapman and Hall Ltd., London C. Air Ministry Publication 1208, Airworthiness Handbook for Civil Aircraft, HMSO, London D. Capt Duchêne (translated by J.H. Ledeboer), The Mechanics of the Aeroplane, Longmans, Green & Co., London E. The Royal Aeronautical Society, Handbook of Aeronautics, Gale & Polden Ltd., London F. Air Ministry Publication 1107, Rigging For Aircraft, HMSO, London G. T.D. Crouch, Blériot XI Famous Aircraft of the NASM: Publication 5, National Air and Space Museum, Smithsonian Institution Press, Washington D.C H. P.M. Grosz, The Taube at War, Windsock Datafile 104, Albatros Productions Ltd. The Fokker M.16E (above) and M.17E (below) prototypes in wintery surroundings