FVA 11 - Eifel
When designing the FVA-11, the first step was to establish the requirements that an “ideal aircraft” of the time should meet. First and foremost, of course, was the requirement for a cruising speed of 80 km/h and an optimum glide ratio E. A minimum sink rate of 60cm was considered sufficient. It was also required that the airplane could still turn well at 50 km/h at acceptable sink rates, and that the landing speed, which is a measure of safety against crash landings, should be as close to 40 km/h as possible. To achieve sufficient turnability, these requirements had to be met with as small a span as possible.
The most difficult requirements were those for glide ratio and cruise speed. To meet these, all residual drag had first to be kept as small as possible. Then, however, the use of thin wing cross sections became necessary. This in turn brought with it considerable design difficulties. However, it turned out that the requirements for rate of descent, glide ratio and turnability could be met with a span of 18 meters. However, with the wing loading of 21 kg/m² on which the calculation was based, the design could not go below a minimum speed of 56 km h. A landing aid therefore had to be used.
It was therefore necessary to use a landing aid that not only produced a sufficient increase in lift but also resulted in as little additional drag as possible. Since measurement results on such landing aids (apart from spread flaps and Junkers double wings) were not available with the selected airfoil, it was difficult to compare the numerous existing measurements on other wing cross sections and to assess their effect. The best solution to the task at hand seemed to the designers to be the installation of a Fowler wing of 30% depth in the inboard section between the fuselage and the ailerons, while in the outboard wing the ailerons simultaneously go into a new zero position.
The elaborate fittings for guiding the auxiliary wings were located on the underside of the wings.
A preliminary calculation showed that all flight performance degraded when the auxiliary wing was extended. Therefore, an arrangement had to be made so that, if possible, no operating parts should be outside the wing when the auxiliary wing was retracted. Furthermore, in order not to get too large negative heading moments with separated ailerons, the aileron should be designed in such a way that it does not give any further deflection downward, but gives full deflections upward.
In order to obtain sufficient documentation for performance and stability calculations, it was decided to investigate the planned flap arrangement in the Aachen wind tunnel. The model was built in our workshop. The flap arrangement was such that the flap could be adjusted from 0 to 40° from 5 to 5°. The polars were recorded for all these flap positions and also with the flaps retracted for comparison.
The results showed the correctness of the assumptions. With the designed wing a landing speed of 42 km/h could be achieved and also the requirement to be able to turn at 50 km h with acceptable sink rates could be fulfilled. The lowest sink rate increased from 0.6 to 0.7 m/sec.
Construction of the machine was started at the end of February 1938, but due to material supply and construction difficulties it was not possible to complete the machine for the 19th Rhön. On the first day of the competition it could only be flown in at Aachen. It was flown in directly by aerotow, since it could not be freed with a bungee cord.
The FVA finally made it to the Rhön competition with the “Eifel”, but the aircraft was not flown much there. This was mainly due to the fact that the FVA-11 was still quite new and with its complicated and untested wing not yet ready for the competition. In addition, this competition was essentially for high altitude flights, and going into cumulonimbus clouds with an untested wing did not yet seem appropriate. The accidents that occurred during this competition justify this view.
Contrary to the hope expressed in the 1938/39 activity report, some planned modifications to the FVA-11 were not completed by August 1939, so that the aircraft did not get to fly before World War II and was destroyed during the war.