| Type | Werk.Nr | Registration | History |
| D-2229 |
| Type | |
| Engine | 1 Mercedes-Daimler F 7502A |
| Dimensions | Length 5,7 m , height 2,07 m , span 9,0 m , wing area 10,0 m2 , |
| Weights | Empty 185 kg, loaded 285 kg , max. take off weight |
| Performance | Max.. speed 126 km/h, cruising speed ,landing speed 60 km/h, range , endurance , service ceiling 3500 m , climb 8 min. to 1000 m, 3,5 m/sec., landing speed 65 km/h |
Small aircraft of the Akaflieg Berlin,We did not have much time for development. In December 1930 we were still thinking about what the machine should look like, and exactly one year later it
went into the ether for the first time. Therefore, it is nothing earth-shatteringly new, nothing ingenious that has been built, but simply an airplane like all
the others.
In 1928, the DLV had called for the development of a cheap single-seater aircraft and considered the Uranus proposals to create aircraft with the lowest
engine power at a low price. Since then, several years have passed until the DLV entrusted us with the construction and, in addition to some
technical conditions, gave us the good old "Flugsporf motto", which reads: Fly cheaply!
This can be achieved firstly if you use little horsepower (but not always!), secondly if the aircraft costs little, and thirdly if you don't
have any hassle in operation.
The 20 hp Daimler engine was already in place. His relatively large weight and his acknowledged restless gait caused us some concern. In favor of the
third condition, a motor trestle was built, which can withstand quite strong vibrations due to its articulated nodes (see Fig. 1). Of course, this also includes a flexible fuel line.
A low cost price for the aircraft was sought by carefully considering
the possibility of self-construction in clubs during construction. Accordingly, the construction of the wings, the tail and the fuselage made of wood was quite natural, but on the other hand, parts less suitable for self-assembly were not necessarily avoided. It was intended that such parts would be manufactured by the DLV in one place and sold cheaply
, provided that this standard aircraft was built in several places at the same time, as in the construction of gliders and gliders.
For example, the wing stays, the chassis, the spur and the engine block were made of tubular steel, and a number of fittings were
fitted with fork bolts and joint stones that require milling work. The ready-to-wear parts also include the fuel tray, filters and taps as well as all instruments including fire extinguishers.
Above all, economical operation includes simple operation and maintenance. Some construction costs were not spared for this. The airplane weighs so little that it can be guided on the ground like a pram. To shelter, a man can fold the wings with just a few simple steps. The space requirement is then 2.2X5.8 m. The simple flying and landing shafts and the comfortable seat make flying pleasant and, together with thegood visibility, also particularly safe. Good accessibility of all indoor spaces is ensured. The easily removable motor shell is also equipped with a large hand opening.
The space behind the fire bulkhead and the steering mechanism are accessible through a large triangular flap. The seat can be folded down to check the height control , and completely removed if you want to get to the end of the fuselage. On an inserted board you can crawl into the fuselage up to the front of the tail.
The spur featherng can be pulled out after loosening a bolt. The pressure rubber suspension of the chassis (still uncovered in the pictures) does not require any
maintenance. Control cables running over rollers only occur in four places in the lateral control. The Fuehrer can constantly monitor these four sections even in flight. The construction of the aircraft still needs a short description. The timber yard consisted of a number of 6 mm thick, planed pine lamellae, which were obtained by dividing a plank vertically. The advantage of this division resulted from the possibility of careful selection, from uniformity and from the compulsion to obtain starchy pieces by lamination, which was otherwise quite cheap. For plywood, almost exclusively 1 mm boards were needed.
The hull was easy to build. The side walls with the belts were finished as planes and bent over the frame after the frame had been erected on an auxiliary slipway .
In all filling blocks, the fibre is arranged in such a way that the fibre joint forms the most acute angles possible on the glue surfaces. An example of this is the filling block of the
spar fittings, which gives it an almost delicate appearance Hollow rivets were largely used to fasten the fittings, but they require some care when striking. The weight saving compared to bolts can be estimated from the fact that about 10 m of rivet tube were consumed. The wings are provided with a "composite box" between the spars in the outer part, which contributes significantly to increasing the torsional rigidity The frontal forces of the wing are absorbed by a one-sided planking between the spars. shows the formation of the pressure rubber suspension with strut head and connection supplement. The 8 hinged stones of the chassis are the same with each other and manufactured in series.
In order to make sure that there were no design errors and to check the calculation, the cell was subjected to a number of load tests, in which
the load multiple 5 was reached without any questionable changes in shape being seen. The actual breaking load is a load multiple of 8, in accordance with the DVL regulations for aerobatic aircraft with limited aerobatic capability. Fig. 6 shows the load test on the inverted cell. Below are the most important data of the aircraft: length over all 5.7 m, wingspan 9.0 m, wing area 10.0 m2, empty weight 185 kg, payload 100 kg (pilot + parachute + 16 1 fuel), engine power 20 hp, wing load 28.5 kg/m2, power load 14.3 kg/hp, top speed
120 km/h. It is not difficult for the aircraft to recognize its narrower task. It is intended for trained motor pilots as a practice and aerobatic machine and is only to be used in
aerodrome operations. It is not intended for self-training or for the transition of glider pilots to powered aviation, since slower types that are not suitable for
aerobatics and training have to be developed for this
went into the ether for the first time. Therefore, it is nothing earth-shatteringly new, nothing ingenious that has been built, but simply an airplane like all
the others.
In 1928, the DLV had called for the development of a cheap single-seater aircraft and considered the Uranus proposals to create aircraft with the lowest
engine power at a low price. Since then, several years have passed until the DLV entrusted us with the construction and, in addition to some
technical conditions, gave us the good old "Flugsporf motto", which reads: Fly cheaply!
This can be achieved firstly if you use little horsepower (but not always!), secondly if the aircraft costs little, and thirdly if you don't
have any hassle in operation.
The 20 hp Daimler engine was already in place. His relatively large weight and his acknowledged restless gait caused us some concern. In favor of the
third condition, a motor trestle was built, which can withstand quite strong vibrations due to its articulated nodes (see Fig. 1). Of course, this also includes a flexible fuel line.
A low cost price for the aircraft was sought by carefully considering
the possibility of self-construction in clubs during construction. Accordingly, the construction of the wings, the tail and the fuselage made of wood was quite natural, but on the other hand, parts less suitable for self-assembly were not necessarily avoided. It was intended that such parts would be manufactured by the DLV in one place and sold cheaply
, provided that this standard aircraft was built in several places at the same time, as in the construction of gliders and gliders.
For example, the wing stays, the chassis, the spur and the engine block were made of tubular steel, and a number of fittings were
fitted with fork bolts and joint stones that require milling work. The ready-to-wear parts also include the fuel tray, filters and taps as well as all instruments including fire extinguishers.
Above all, economical operation includes simple operation and maintenance. Some construction costs were not spared for this. The airplane weighs so little that it can be guided on the ground like a pram. To shelter, a man can fold the wings with just a few simple steps. The space requirement is then 2.2X5.8 m. The simple flying and landing shafts and the comfortable seat make flying pleasant and, together with thegood visibility, also particularly safe. Good accessibility of all indoor spaces is ensured. The easily removable motor shell is also equipped with a large hand opening.
The space behind the fire bulkhead and the steering mechanism are accessible through a large triangular flap. The seat can be folded down to check the height control , and completely removed if you want to get to the end of the fuselage. On an inserted board you can crawl into the fuselage up to the front of the tail.
The spur featherng can be pulled out after loosening a bolt. The pressure rubber suspension of the chassis (still uncovered in the pictures) does not require any
maintenance. Control cables running over rollers only occur in four places in the lateral control. The Fuehrer can constantly monitor these four sections even in flight. The construction of the aircraft still needs a short description. The timber yard consisted of a number of 6 mm thick, planed pine lamellae, which were obtained by dividing a plank vertically. The advantage of this division resulted from the possibility of careful selection, from uniformity and from the compulsion to obtain starchy pieces by lamination, which was otherwise quite cheap. For plywood, almost exclusively 1 mm boards were needed.
The hull was easy to build. The side walls with the belts were finished as planes and bent over the frame after the frame had been erected on an auxiliary slipway .
In all filling blocks, the fibre is arranged in such a way that the fibre joint forms the most acute angles possible on the glue surfaces. An example of this is the filling block of the
spar fittings, which gives it an almost delicate appearance Hollow rivets were largely used to fasten the fittings, but they require some care when striking. The weight saving compared to bolts can be estimated from the fact that about 10 m of rivet tube were consumed. The wings are provided with a "composite box" between the spars in the outer part, which contributes significantly to increasing the torsional rigidity The frontal forces of the wing are absorbed by a one-sided planking between the spars. shows the formation of the pressure rubber suspension with strut head and connection supplement. The 8 hinged stones of the chassis are the same with each other and manufactured in series.
In order to make sure that there were no design errors and to check the calculation, the cell was subjected to a number of load tests, in which
the load multiple 5 was reached without any questionable changes in shape being seen. The actual breaking load is a load multiple of 8, in accordance with the DVL regulations for aerobatic aircraft with limited aerobatic capability. Fig. 6 shows the load test on the inverted cell. Below are the most important data of the aircraft: length over all 5.7 m, wingspan 9.0 m, wing area 10.0 m2, empty weight 185 kg, payload 100 kg (pilot + parachute + 16 1 fuel), engine power 20 hp, wing load 28.5 kg/m2, power load 14.3 kg/hp, top speed
120 km/h. It is not difficult for the aircraft to recognize its narrower task. It is intended for trained motor pilots as a practice and aerobatic machine and is only to be used in
aerodrome operations. It is not intended for self-training or for the transition of glider pilots to powered aviation, since slower types that are not suitable for
aerobatics and training have to be developed for this
Single-seat light aircraft have always held a high priority in the minds of aircraft designers and manufacturers. The reasons are obvious: only having to transport one person means less aircraft mass and consequently less engine power and possibly also smaller dimensions. Undoubtedly, this would have been a way to a more economical aircraft, had this type of aircraft ever achieved a real breakthrough. Certainly, there were special single-seat designs that found a market such as aerobatic aircraft. But as a rule, the recreational pilot preferred the company of a second person on his "airborne excursions," preferably a particularly attractive one or a numerical one.
Hanns Klemm, whose name at that time was synonymous with the light aircraft par excellence, should have known this. With his two-seater L 20 and L 25 sport aircraft, he had set standards that translated into impressive, lucrative sales figures. But he could never let go of the idea of a powered aircraft that was capable of gliding. Already In 1922, he had pointed this out in a memorandum.
Ten years later, his dream, realized, took to the skies for its maiden flight. He named it Alpha to establish its importance, and he had christened it Jörg (after his son) to make it unmistakably clear how suitable he considered it as a training aircraft for young pilots.
But his single-seater was too cumbersome and too expensive. For some time, it crept through the airspace of Böblingen within the airport zone, then He disappeared from the sky again.
Yet, at the beginning of the 1930s, the call for a single-seat training aircraft had really grown louder. It originated from the motor-flight sports clubs, which by then could draw on a large selection of fully-fledged light aircraft from industrial production, but in their opinion, the model lacked one that was specifically tailored to training operations for the further education of pilots. And in their view, it should be an economical single-seater with limited certification for local flights and so simple in construction that it could be built in their own workshop, ideally with unskilled labor.
The German Aviation Association (DLV) took up the suggestion and ultimately assumed coordination of the development.
The development of such a training aircraft. He largely adapted the target object to the requirements of the clubs, but was by no means prepared to reduce the technical and aeronautical aspects to their scale. On the contrary: The DLV wanted... an aircraft suitable for self-construction, which is not only suitable for local airfield operation, but also airworthy in accordance with existing regulations, meaning it can also be used for cross-country flights; furthermore, it should not differ significantly in flight performance and characteristics from modern sport aircraft and should even be suitable for aerobatics.
The specifications of the tender called for an inexpensive and robust airframe, preferably constructed entirely of wood. Indispensable metal parts such as fittings and complex structural components should be available prefabricated. The strength was to be calculated for a 100 kg payload so that it met the requirements of assembly group S4K. A
wing with a wingspan of up to ten meters, preferably foldable, was preferred.The tender specifications called for an inexpensive and robust airframe, preferably constructed entirely of wood. Inevitable metal parts, such as fittings and complex structural elements, should be available pre-fabricated. The structural strength, with a 100 kg payload, was to be calculated to meet the requirements of the S4K construction group. A wing with a wingspan of up to ten meters, preferably foldable, was preferred. For the elimination aircraft, a Mercedes-Daimler F7502 engine with 20 hp (14.7 kW) was selected for comparability. However, since it was considered too Since it was classified as underpowered and too expensive, a removable engine mount was to be used so that more economical, higher-performance engines could be installed for further testing and subsequent series production.
Other requirements included satisfactory stall characteristics and flight performance limits of at least 110 km/h top speed, 65 km/h maximum landing speed, and a maximum time of 10 minutes to reach an altitude of 1 km. The cost of building a single-seat aircraft – including the price of the engine – was not to exceed 2,500 to 3,000 Reichsmarks. Given these specifications, it was no wonder that the single-seat trainer was quickly, but inaccurately, dubbed a "people's aircraft" in the trade press.
At the beginning of 1931, the DLV issued the announcement. Dr. Ing. Martin Schrenk (1896-1934), a distinguished employee at the time, who contributed to the development of gliders and light aircraft The German Research Institute for Aviation (DVL) in Berlin-Adlershof, where he worked on setting up high-altitude laboratories through balloon ascents to high altitudes, called for participation from the Akaflieg Berlin, its "old guard." The academic aviators of the Technical University in Charlottenburg accepted the challenge, thus ending a six-year period of inactivity. This inactivity contrasted with a previous period of active construction, which the members at the time had considered a duty—for reasons of tradition. While in the first decade of the last century, aeronautical science at other technical universities in Germany had not yet progressed beyond its beginnings, Berlin had become the cradle of all university-level aeronautical activities. In 1909, the then- student engineer Roland Eisenlohr (1887-1959, later a government building inspector and Dr.Ing. an esteemed technical writer) The Berlin University of Technology established a "Student Group for Aviation," which included such important members for later aviation as the future Göttingen professor Albert Betz, Gerhard Sedlmayr, and Richard Wagner, later a professor at the German University in Peking. The driving force behind the aviation-related activities was undoubtedly Dipl.-Ing.e.h. Dr.phil.h.c. August von Parseval (1861-1942), who received the teaching assignment for the topic of "Aeronautical Engines" in 1908 and was appointed professor in 1911. He retained his chair until 1923. As a Berlin student, after Aachen (1919) and Darmstadt (1920), when he founded a "Berlin Aeronautical Society" as the third academic group at the end of 1920, his advisory role was undeniable – possibly consciously harking back to the modest pre-war tradition.
In any case, he suggested the construction of a tailless glider for the 1922 Rhön Competition, the design of which was undertaken by the aspiring graduate engineers Hermann Winter and Edmund Rister and which was completed in August 1922 and named Charlotte after the university town. The students intended to participate in the 1922 Rhön Competition with it, but pilot Winter crashed it on the Wasserkuppe before the competition even began due to the unfamiliar controls. The home transport of the broken glider from the Rhön region to Berlin is an integral part of gliding history.
The model was rebuilt and flown with normal controls at the next Rhön competition under the name Charlotte 2 - crashed. The "Teufelchen", a glider with rectangular wings and twisted ailerons designed by the later Professor Kurt Tank, which had been completed in May 1923, also crashed during this competition and was finally lost the following year at the 2nd German Coastal Gliding Competition in Rossitten. During these years, the last of the active pilots took their exams. Since there was no new generation of pilots, the Akaflieg stagnated. It was not re-established until February 1927. Its primary focus was on powered flight. This also led to the spontaneous interest in the DLV project.
In addition to the consultation provided by Dr. Ing. Schrenk, Walter Stender (1905-2000) was engaged for the design and project management of the construction work. He had privately obtained his A2 pilot's license in 1929 and, as a member of the Akaflieg Berlin, used their motor aircraft within the framework of group flight operations (ten flights in 1930 with a total flight time of 59 minutes). Born in Riga, he spent his school years there and in Breslau, and attended engineering schools in Mittweida and Frankenhausen visited and joined the DVL in 1927.
There, since the summer of 1926, Dipl.-Ing. Inge Kober – daughter of the creator of the Friedrichshafen aircraft – had been working under the direction of Dr. Seewald on the unexplored accident topic of wing vibrations. At the beginning of 1929 – by then married to the full-time assistant Dipl.-Ing. Ernst Essers of the Aachen Institute for Mechanical Engineering and Automotive Engineering – the DVL's director and visiting lecturer at the Technical University of Berlin-Charlottenburg, Prof. Dr.-Ing. Hoff, helped her obtain her doctorate on the topic of "Experimental Investigations of Wing Vibrations." However, in the same spring, she decided to become a housewife. His successor for the problem of wing flutter was Walter Stender, who had already been dealing with the dangerous resonance phenomena in the Accident Investigation Department. Preventing wing flutter became his life's work. In 1933, he went to Sweden as an aircraft designer, became head of the research department at Blohm & Voss in December 1935, and in April 1942, a member of Group L at Messerschmitt AG (development of the rocket fighter). Me 163 under Alexander Lippisch) and finally, from spring 1943, head of the aircraft construction department at the Zeppelin works in Friedrichshafen.
His work after the war—which particularly benefited the development of modern high-performance gliders—began with the standard publication "Design Principles for Gliders." He became a mentor to the glider designer Rudolf Kaiser and finally to his own son Björn, who in 1962, with his BS 1, found the basic form and design of modern composite gliders and simultaneously designed and built the most powerful machine of its time. Tragically, the 29-year-old crashed The graduate engineer died in October 1963 during the testing of his prototype.
Besides Stender's design on behalf of the Akaflieg Berlin, the DLV announced the receipt of another entry from a non-commercial participant: Hermann Mayer, a graduate engineer from Aachen, offered his design, MM 1. Mayer had released the MS I training glider for the Rhön competition in 1929 and then further developed it into a high-performance glider with a wingspan of 20 meters, the MS II.
Both designs were intended to prove that high-performance gliders could be built with simple means. However, the angular, fragmented fuselage construction, consisting only of straps and rods and almost exclusively covered with fabric, tension remained a weak point. It ultimately led to the tragic end of this designer as well: At the age of 35, Mayer died in 1933 during a flight in Borkenberge due to a fuselage failure.
But back to the DLV competition. The comparison of the powered aircraft designs was more than interesting. While both had opted for conventional-shoulder-wing aircraft was built using wooden construction methods, with wings suspended by V-struts and mounted on small fuselage pylons. Rubber-string-sprung tricycle landing gear was chosen. However, the similarities ended there. For his home-built aircraft, Mayer relied heavily on his experience with gliders and over-dimensioned all components to eliminate any potential construction errors. He used wood almost exclusively, even for the struts, much more so than his opponent. And he placed particular emphasis on straight lines: The two-part, single-spar wing—like the horizontal stabilizer—was rectangular except for its rounded ends and had neither a V-shape nor a swept-back design. properties and performance laid. The chosen V- and arrow-shaped structure had to subordinate the simplicity required for self-construction, as did the extreme lightweight construction, which reduced the mass and thus improved takeoff performance.
At the beginning of March 1932, both Mayer and the Akaflieg Berlin received the construction contract from the DLV. In Charlottenburg, five group members immediately began the construction work. A contemporary Akaflieg chronicle describes the progress of the design and construction: However, the similarities ended there.
For his home-built aircraft, Mayer relied heavily on his experience with gliders and over-dimensioned all components to eliminate any construction errors. The result: The Rust mass
of the structure was exactly 100 kg higher than that of the Berlin design (285 kg compared to 185 kg). In Stender's design, however, particular emphasis was placed on flight- for safety reasons. "By May, the construction work had progressed to the point where construction could begin. The original intention to carry out the construction solely with group members under the supervision of a specialist could not be realized because the poor economic situation forced many members to accelerate their studies and earn money. Therefore, during the course of construction, we had to hire five additional tradesmen from the gliding club of a Berlin vocational school. The construction suffered some unpleasant delays because some innovations that formed the basis of the design could not be implemented in the short time available."
The design available for the construction could not be fully developed to the point of being ready for construction. Therefore, in some cases, we had to resort to older and known methods. In August, we hired two more engineers because the design office couldn't quite keep up with the workshop. The cold spell that began in autumn brought new difficulties for the construction. Workshop work was severely hampered, and there was also the possibility that the cold would damage the glue joints and paintwork, which seriously jeopardized the construction. Thus, we were forced to undertake the construction of a steam heating system for our hall alongside the construction of the 'FF' and complete it before the beginning of winter. In November, the design was completed, and shortly before Christmas, nine months after the construction contract was awarded, the ‘FF’ was able to begin its first test flights. It soon became apparent that, after a minor modification to the tail assembly, the aircraft met the specified requirements. Indeed, Stender’s design proved successful in terms of concept and dimensions. The aspect ratio of 8.1 and a wing loading of 28.5 kg/m² were well chosen, and the power-to-weight ratio of 19.4 kg/kW could have been improved by installing a more powerful engine. This was also planned, as the expert committee of the DLV had selected the FF in the spring of 1932 as the more powerful of the two competitors decided and advocated for a further increase in performance.
This did not happen due to the economic crisis. After the political change of power, a decree of April 11, 1933, withdrew state support from the Akafliegs. At the end of November 1933, the Akaflieg Berlin also had to cease its flight operations and transfer its aircraft except for the FF study project to the newly founded and reorganized DLV.
A third academic flying association was formed in the autumn of 1934, when initially four members of the TH Berlin took up the suggestion of the DVL to establish an aeronautical technical group (FFG) supported by it as a successor organization. to bring it to life. Now the FF was allowed to fly again.
The FFG Berlin was successful. It designed and built several gliders within a short time and, during the war, also a twin-engine experimental aircraft, the B 9, with a prone pilot. In 1945, after the Soviet invasion, its wreckage was still seen. All other aircraft, except for one of the two built high-performance gliders, the B 8, remained missing, including the FF.
By the way: What was this aircraft really called – FF or AB4? It actually had two names: Internally called FF (according to tradition, because of the slow construction progress, meaning "almost finished"), officially designated AB 4 for Akaflieg Berlin. and their fourth construction in sequence (81 was the Charlotte, B 2 Tanks Devil and B 3 the rebuilt first construction as Charlotte II).
Technical Description
Wings
Braced high-wing wing; three-section, two-spar, all-wood construction, rectangular outline with rounded wingtips; dihedral angle 1:25 (2°), swept-back 1:10 (6°).
Construction of box spars with laminated pine stringers and plywood webs, as well as truss ribs. The center spar rests on a narrow fuselage ridge extending from the fuselage spine and is additionally braced by four canopy struts made of profiled steel tubing to the upper fuselage side stringers. It has a thinner airfoil than the outer wing sections for improved visibility, but a higher angle of attack. Wing sections are braced against the lower fuselage side stringers by V-struts made of profiled steel tubing and folded to the fuselage via fittings on the rear spar of the wing center section (resulting in a transport width of 2.30 meters). The leading edge to the front spar, the underside between the spars, is covered with plywood, otherwise fabric-covered. Instead of internal bracing, the outer wing sections each have a composite box between the foils in the aileron area. The wing profile is Clark Y up to the aileron root. The required angle of attack is required washout is achieved in the mass-balanced ailerons by twisting. Total aileron area 1.34 sq m.
Fuselage
All-wood construction with hexagonal cross-section.
Elevator: Braced conventional tail assembly in wooden construction. Two-spar horizontal stabilizer, fabric-covered. Single-spar vertical stabilizer with torsionally rigid plywood leading edge, otherwise fabric-covered. Unbalanced control surfaces constructed as a triangular truss and also fabric-covered.
wings: Horizontal stabilizer 0.78 sq m, elevator 0.74 sq m, vertical stabilizer 0.42 sq m, rudder 0.46 sq m.
Landing gear: Strutless tricycle landing gear. The two V-shaped steering knuckles made of profiled steel tubing attach to the keel chord,
the spring struts made of covered round steel tubing attach to the lower fuselage side stringers.
Rubber compression suspension. First high-pressure tires of 500 mm diameter and 75 mm width, later balloon tires 380x150 mm, then covered. Rubber cable-sprung scraper with replaceable skid.'
Engine:
An air-cooled two-cylinder boxer engine, Mercedes-Daimler F 7502A, with a maximum continuous output of 20 hp (14.7 kW). Two-blade wooden propeller with a diameter of 2.10 m. Fuel tank (16-liter capacity) in the fuselage forward of the canopy.
Paint scheme: Probably silver-coated, registered in black; later red rudder with the Akaflieg Berlin logo painted in negative white.
Hanns Klemm, whose name at that time was synonymous with the light aircraft par excellence, should have known this. With his two-seater L 20 and L 25 sport aircraft, he had set standards that translated into impressive, lucrative sales figures. But he could never let go of the idea of a powered aircraft that was capable of gliding. Already In 1922, he had pointed this out in a memorandum.
Ten years later, his dream, realized, took to the skies for its maiden flight. He named it Alpha to establish its importance, and he had christened it Jörg (after his son) to make it unmistakably clear how suitable he considered it as a training aircraft for young pilots.
But his single-seater was too cumbersome and too expensive. For some time, it crept through the airspace of Böblingen within the airport zone, then He disappeared from the sky again.
Yet, at the beginning of the 1930s, the call for a single-seat training aircraft had really grown louder. It originated from the motor-flight sports clubs, which by then could draw on a large selection of fully-fledged light aircraft from industrial production, but in their opinion, the model lacked one that was specifically tailored to training operations for the further education of pilots. And in their view, it should be an economical single-seater with limited certification for local flights and so simple in construction that it could be built in their own workshop, ideally with unskilled labor.
The German Aviation Association (DLV) took up the suggestion and ultimately assumed coordination of the development.
The development of such a training aircraft. He largely adapted the target object to the requirements of the clubs, but was by no means prepared to reduce the technical and aeronautical aspects to their scale. On the contrary: The DLV wanted... an aircraft suitable for self-construction, which is not only suitable for local airfield operation, but also airworthy in accordance with existing regulations, meaning it can also be used for cross-country flights; furthermore, it should not differ significantly in flight performance and characteristics from modern sport aircraft and should even be suitable for aerobatics.
The specifications of the tender called for an inexpensive and robust airframe, preferably constructed entirely of wood. Indispensable metal parts such as fittings and complex structural components should be available prefabricated. The strength was to be calculated for a 100 kg payload so that it met the requirements of assembly group S4K. A
wing with a wingspan of up to ten meters, preferably foldable, was preferred.The tender specifications called for an inexpensive and robust airframe, preferably constructed entirely of wood. Inevitable metal parts, such as fittings and complex structural elements, should be available pre-fabricated. The structural strength, with a 100 kg payload, was to be calculated to meet the requirements of the S4K construction group. A wing with a wingspan of up to ten meters, preferably foldable, was preferred. For the elimination aircraft, a Mercedes-Daimler F7502 engine with 20 hp (14.7 kW) was selected for comparability. However, since it was considered too Since it was classified as underpowered and too expensive, a removable engine mount was to be used so that more economical, higher-performance engines could be installed for further testing and subsequent series production.
Other requirements included satisfactory stall characteristics and flight performance limits of at least 110 km/h top speed, 65 km/h maximum landing speed, and a maximum time of 10 minutes to reach an altitude of 1 km. The cost of building a single-seat aircraft – including the price of the engine – was not to exceed 2,500 to 3,000 Reichsmarks. Given these specifications, it was no wonder that the single-seat trainer was quickly, but inaccurately, dubbed a "people's aircraft" in the trade press.
At the beginning of 1931, the DLV issued the announcement. Dr. Ing. Martin Schrenk (1896-1934), a distinguished employee at the time, who contributed to the development of gliders and light aircraft The German Research Institute for Aviation (DVL) in Berlin-Adlershof, where he worked on setting up high-altitude laboratories through balloon ascents to high altitudes, called for participation from the Akaflieg Berlin, its "old guard." The academic aviators of the Technical University in Charlottenburg accepted the challenge, thus ending a six-year period of inactivity. This inactivity contrasted with a previous period of active construction, which the members at the time had considered a duty—for reasons of tradition. While in the first decade of the last century, aeronautical science at other technical universities in Germany had not yet progressed beyond its beginnings, Berlin had become the cradle of all university-level aeronautical activities. In 1909, the then- student engineer Roland Eisenlohr (1887-1959, later a government building inspector and Dr.Ing. an esteemed technical writer) The Berlin University of Technology established a "Student Group for Aviation," which included such important members for later aviation as the future Göttingen professor Albert Betz, Gerhard Sedlmayr, and Richard Wagner, later a professor at the German University in Peking. The driving force behind the aviation-related activities was undoubtedly Dipl.-Ing.e.h. Dr.phil.h.c. August von Parseval (1861-1942), who received the teaching assignment for the topic of "Aeronautical Engines" in 1908 and was appointed professor in 1911. He retained his chair until 1923. As a Berlin student, after Aachen (1919) and Darmstadt (1920), when he founded a "Berlin Aeronautical Society" as the third academic group at the end of 1920, his advisory role was undeniable – possibly consciously harking back to the modest pre-war tradition.
In any case, he suggested the construction of a tailless glider for the 1922 Rhön Competition, the design of which was undertaken by the aspiring graduate engineers Hermann Winter and Edmund Rister and which was completed in August 1922 and named Charlotte after the university town. The students intended to participate in the 1922 Rhön Competition with it, but pilot Winter crashed it on the Wasserkuppe before the competition even began due to the unfamiliar controls. The home transport of the broken glider from the Rhön region to Berlin is an integral part of gliding history.
The model was rebuilt and flown with normal controls at the next Rhön competition under the name Charlotte 2 - crashed. The "Teufelchen", a glider with rectangular wings and twisted ailerons designed by the later Professor Kurt Tank, which had been completed in May 1923, also crashed during this competition and was finally lost the following year at the 2nd German Coastal Gliding Competition in Rossitten. During these years, the last of the active pilots took their exams. Since there was no new generation of pilots, the Akaflieg stagnated. It was not re-established until February 1927. Its primary focus was on powered flight. This also led to the spontaneous interest in the DLV project.
In addition to the consultation provided by Dr. Ing. Schrenk, Walter Stender (1905-2000) was engaged for the design and project management of the construction work. He had privately obtained his A2 pilot's license in 1929 and, as a member of the Akaflieg Berlin, used their motor aircraft within the framework of group flight operations (ten flights in 1930 with a total flight time of 59 minutes). Born in Riga, he spent his school years there and in Breslau, and attended engineering schools in Mittweida and Frankenhausen visited and joined the DVL in 1927.
There, since the summer of 1926, Dipl.-Ing. Inge Kober – daughter of the creator of the Friedrichshafen aircraft – had been working under the direction of Dr. Seewald on the unexplored accident topic of wing vibrations. At the beginning of 1929 – by then married to the full-time assistant Dipl.-Ing. Ernst Essers of the Aachen Institute for Mechanical Engineering and Automotive Engineering – the DVL's director and visiting lecturer at the Technical University of Berlin-Charlottenburg, Prof. Dr.-Ing. Hoff, helped her obtain her doctorate on the topic of "Experimental Investigations of Wing Vibrations." However, in the same spring, she decided to become a housewife. His successor for the problem of wing flutter was Walter Stender, who had already been dealing with the dangerous resonance phenomena in the Accident Investigation Department. Preventing wing flutter became his life's work. In 1933, he went to Sweden as an aircraft designer, became head of the research department at Blohm & Voss in December 1935, and in April 1942, a member of Group L at Messerschmitt AG (development of the rocket fighter). Me 163 under Alexander Lippisch) and finally, from spring 1943, head of the aircraft construction department at the Zeppelin works in Friedrichshafen.
His work after the war—which particularly benefited the development of modern high-performance gliders—began with the standard publication "Design Principles for Gliders." He became a mentor to the glider designer Rudolf Kaiser and finally to his own son Björn, who in 1962, with his BS 1, found the basic form and design of modern composite gliders and simultaneously designed and built the most powerful machine of its time. Tragically, the 29-year-old crashed The graduate engineer died in October 1963 during the testing of his prototype.
Besides Stender's design on behalf of the Akaflieg Berlin, the DLV announced the receipt of another entry from a non-commercial participant: Hermann Mayer, a graduate engineer from Aachen, offered his design, MM 1. Mayer had released the MS I training glider for the Rhön competition in 1929 and then further developed it into a high-performance glider with a wingspan of 20 meters, the MS II.
Both designs were intended to prove that high-performance gliders could be built with simple means. However, the angular, fragmented fuselage construction, consisting only of straps and rods and almost exclusively covered with fabric, tension remained a weak point. It ultimately led to the tragic end of this designer as well: At the age of 35, Mayer died in 1933 during a flight in Borkenberge due to a fuselage failure.
But back to the DLV competition. The comparison of the powered aircraft designs was more than interesting. While both had opted for conventional-shoulder-wing aircraft was built using wooden construction methods, with wings suspended by V-struts and mounted on small fuselage pylons. Rubber-string-sprung tricycle landing gear was chosen. However, the similarities ended there. For his home-built aircraft, Mayer relied heavily on his experience with gliders and over-dimensioned all components to eliminate any potential construction errors. He used wood almost exclusively, even for the struts, much more so than his opponent. And he placed particular emphasis on straight lines: The two-part, single-spar wing—like the horizontal stabilizer—was rectangular except for its rounded ends and had neither a V-shape nor a swept-back design. properties and performance laid. The chosen V- and arrow-shaped structure had to subordinate the simplicity required for self-construction, as did the extreme lightweight construction, which reduced the mass and thus improved takeoff performance.
At the beginning of March 1932, both Mayer and the Akaflieg Berlin received the construction contract from the DLV. In Charlottenburg, five group members immediately began the construction work. A contemporary Akaflieg chronicle describes the progress of the design and construction: However, the similarities ended there.
For his home-built aircraft, Mayer relied heavily on his experience with gliders and over-dimensioned all components to eliminate any construction errors. The result: The Rust mass
of the structure was exactly 100 kg higher than that of the Berlin design (285 kg compared to 185 kg). In Stender's design, however, particular emphasis was placed on flight- for safety reasons. "By May, the construction work had progressed to the point where construction could begin. The original intention to carry out the construction solely with group members under the supervision of a specialist could not be realized because the poor economic situation forced many members to accelerate their studies and earn money. Therefore, during the course of construction, we had to hire five additional tradesmen from the gliding club of a Berlin vocational school. The construction suffered some unpleasant delays because some innovations that formed the basis of the design could not be implemented in the short time available."
The design available for the construction could not be fully developed to the point of being ready for construction. Therefore, in some cases, we had to resort to older and known methods. In August, we hired two more engineers because the design office couldn't quite keep up with the workshop. The cold spell that began in autumn brought new difficulties for the construction. Workshop work was severely hampered, and there was also the possibility that the cold would damage the glue joints and paintwork, which seriously jeopardized the construction. Thus, we were forced to undertake the construction of a steam heating system for our hall alongside the construction of the 'FF' and complete it before the beginning of winter. In November, the design was completed, and shortly before Christmas, nine months after the construction contract was awarded, the ‘FF’ was able to begin its first test flights. It soon became apparent that, after a minor modification to the tail assembly, the aircraft met the specified requirements. Indeed, Stender’s design proved successful in terms of concept and dimensions. The aspect ratio of 8.1 and a wing loading of 28.5 kg/m² were well chosen, and the power-to-weight ratio of 19.4 kg/kW could have been improved by installing a more powerful engine. This was also planned, as the expert committee of the DLV had selected the FF in the spring of 1932 as the more powerful of the two competitors decided and advocated for a further increase in performance.
This did not happen due to the economic crisis. After the political change of power, a decree of April 11, 1933, withdrew state support from the Akafliegs. At the end of November 1933, the Akaflieg Berlin also had to cease its flight operations and transfer its aircraft except for the FF study project to the newly founded and reorganized DLV.
A third academic flying association was formed in the autumn of 1934, when initially four members of the TH Berlin took up the suggestion of the DVL to establish an aeronautical technical group (FFG) supported by it as a successor organization. to bring it to life. Now the FF was allowed to fly again.
The FFG Berlin was successful. It designed and built several gliders within a short time and, during the war, also a twin-engine experimental aircraft, the B 9, with a prone pilot. In 1945, after the Soviet invasion, its wreckage was still seen. All other aircraft, except for one of the two built high-performance gliders, the B 8, remained missing, including the FF.
By the way: What was this aircraft really called – FF or AB4? It actually had two names: Internally called FF (according to tradition, because of the slow construction progress, meaning "almost finished"), officially designated AB 4 for Akaflieg Berlin. and their fourth construction in sequence (81 was the Charlotte, B 2 Tanks Devil and B 3 the rebuilt first construction as Charlotte II).
Technical Description
Wings
Braced high-wing wing; three-section, two-spar, all-wood construction, rectangular outline with rounded wingtips; dihedral angle 1:25 (2°), swept-back 1:10 (6°).
Construction of box spars with laminated pine stringers and plywood webs, as well as truss ribs. The center spar rests on a narrow fuselage ridge extending from the fuselage spine and is additionally braced by four canopy struts made of profiled steel tubing to the upper fuselage side stringers. It has a thinner airfoil than the outer wing sections for improved visibility, but a higher angle of attack. Wing sections are braced against the lower fuselage side stringers by V-struts made of profiled steel tubing and folded to the fuselage via fittings on the rear spar of the wing center section (resulting in a transport width of 2.30 meters). The leading edge to the front spar, the underside between the spars, is covered with plywood, otherwise fabric-covered. Instead of internal bracing, the outer wing sections each have a composite box between the foils in the aileron area. The wing profile is Clark Y up to the aileron root. The required angle of attack is required washout is achieved in the mass-balanced ailerons by twisting. Total aileron area 1.34 sq m.
Fuselage
All-wood construction with hexagonal cross-section.
Elevator: Braced conventional tail assembly in wooden construction. Two-spar horizontal stabilizer, fabric-covered. Single-spar vertical stabilizer with torsionally rigid plywood leading edge, otherwise fabric-covered. Unbalanced control surfaces constructed as a triangular truss and also fabric-covered.
wings: Horizontal stabilizer 0.78 sq m, elevator 0.74 sq m, vertical stabilizer 0.42 sq m, rudder 0.46 sq m.
Landing gear: Strutless tricycle landing gear. The two V-shaped steering knuckles made of profiled steel tubing attach to the keel chord,
the spring struts made of covered round steel tubing attach to the lower fuselage side stringers.
Rubber compression suspension. First high-pressure tires of 500 mm diameter and 75 mm width, later balloon tires 380x150 mm, then covered. Rubber cable-sprung scraper with replaceable skid.'
Engine:
An air-cooled two-cylinder boxer engine, Mercedes-Daimler F 7502A, with a maximum continuous output of 20 hp (14.7 kW). Two-blade wooden propeller with a diameter of 2.10 m. Fuel tank (16-liter capacity) in the fuselage forward of the canopy.
Paint scheme: Probably silver-coated, registered in black; later red rudder with the Akaflieg Berlin logo painted in negative white.
