FVA 18a - Primitivkrähe

At the end of the fifties, the opinion prevailed in the FVA that every student of aircraft construction should also have the private pilot’s license in his pocket after his studies. Since the cost of the D-EMYR motorized aircraft of the time was too high for a student’s budget, it was decided to develop a light, inexpensive motorized aircraft with harmless flight characteristics for training and practice. The construction method was the mixed construction, as it was common in gliding at that time. The airframe was “inspired” by the EMYR (PiperJ 3C).

Since everything in a powered airplane depends on the engine, a usable one had to be procured first. The American engines were far too expensive, and so attention was turned to the Pollmann aircraft engine, which had been developed from the VW engine and delivered a continuous output of 30 hp at a speed of 3,200 rpm. In order to get a two-seater airplane into the air with a takeoff power of only a little more than 30 hp, the available engine power had to be used optimally.

By careful selection of the propeller, it was possible to bring the propeller efficiency to 0.78 with a progress of 0.2.

To achieve this good value, it was necessary to give the propeller a comparatively large diameter. It amounted to 2.15 m.

In addition, the engine speed had to be reduced to 1,100 rpm. Initially, this was to be done by means of a V-belt reduction supplied by the Pollmann company for the engine, but this proved to be unusable. Therefore, a gearbox had to be developed by FVA, which provided the required speed reduction of 2.85 : l by means of a helical gear drive.

With the conventionally designed airframe - tubular steel fuselage and one-piece targeted wing - the focus was on an extremely lightweight construction. Stinginess with every gram of weight. Filing off every overlong screw, scraping off every drop of glue had paid off, however: 250 kg empty weight for a two-seat powered aircraft with a wingspan of 10.50 m and a fuselage length of 6.70 m was not an everyday occurrence.

A lot of difficulties had to be overcome before the aircraft was structurally complete. Various strength tests had to be made, such as wing loading tests. The wing stabs were tested for buckling strength at the Institute for Structural Research.

The engine, which weighed 90 kp, was quite heavy, raising the flight weight to 430 kp. At one time, 400 kp had been planned, but achieving this would have required a tremendous amount of additional work. In addition, the demand for simplicity and cheap construction could no longer be maintained.

The fuselage width of the FVA-18 was only 0.67 meters. It was chosen so narrow not least to keep the radiation losses of the propeller against the fuselage low. However, it turned out later that with this fuselage width a weighty co-pilot came into contact with the pilot’s rudder pedals (the pilot sat in tandem behind the co-pilot).

Aileron and elevator loading tests and landing gear drop tests completed the series of tests. Until the FVA-18 was fully certified as a “Normal” category aircraft (load multiple n = 3.8) in accordance with the U.S. CAR-Part 3 construction regulations, some 300 pages of computational strength verifications had to be completed in addition to calculation documents.

The flight tests were carried out in accordance with the provisions of the “Civil Aeronaurics Manual (CAM)” and the “Flight Test Report Guide”. In this, each flight attitude and flight situation were subjected to firmly outlined requirements.

The flight test had to prove that the characteristics and performance of the aircraft were within this framework. Deviations from this were to be identified and eliminated during testing.

The testing of the self-designed gearbox was mainly concerned with reliability. For this purpose, it had to be disassembled at fixed intervals for better control in order to detect wear or to replace bearings and leaky oil seals.

The rolling tests had already shown favorable results, such as short rolling distance until take-off, easy handling and sufficient effect of the self-developed disc brakes. In jumps of 100-200 m length at 2-5 m height, good rudder effectiveness, balanced rudder tuning and comfortable hand forces could be determined even before the actual first flight. However, it soon became apparent that the airscrew pitch had been selected too low. The engine speeds went to 3,400-3,500 rpm at full load when stationary, so that the engine was already over-revving at lift-off speed. A new propeller was ordered from the Hoffmann company.

The first flights were intended to determine the normal behavior of the FVA-18. After only a few minutes of flight, the oil temperature rose above the permissible level. In addition, oil could splash onto the windshield through the transmission vent.

The transmission exhibited strong noise at engine speeds below 1,800 rpm, which, as it later turned out, was also due to engine misfire. Since metal chips were found in the ejected oil, the gearbox was taken apart after only 20 minutes of flight time. The helical involute gears had shifted against each other by 4 mm and were slamming into a spacer ring.

A newly constructed additional spacer ring then prevented the gears from wandering against each other, and the gearbox became much smoother. The next flights continued to be devoted to measuring oil and cylinder head temperatures. Additional baffles strengthened the oil cooling for horizontal flight to the constant temperature of 90°C. For climb flight, it was only sufficient to a limited extent. To reach higher altitudes, horizontal flights had to be used for cooling.

With a weight of 380 kp, the following flight performances were flown:

  • Cruising speed: 100 km/h at 3200 rpm, 90 km/h at 3000 rpm
  • Take-off launch distance: 40 m
  • take-off distance over 15m: approx. 120 m
  • Rate of climb: approx. 2.10 m/s
  • Reached altitude: 2500 m
  • Fuel consumption: 9 l/h
  • Range: 380 km

Stall tests at cruise speed showed slow forward stall with very little loss of altitude (about 20 m). The airplane caught itself with the horn control fully pulled, as the flow on the wing immediately reattached.

At idle, the stalling was somewhat faster, the airplane preferring to go into a sliding condition over the left wing. It was easily returned to normal flight condition by counter-steering measures.

The first spins at medium C.G. could be initiated and also recovered by normal control deflections. The FVA-18 turned 1/4 to 1/2 turn. The spin hardly differed from a pitch circle with minimum speed.

The next flights were made with maximum flight weight. After an airspeed indicator achievement the following points were still flown: Front and rear CG position with the corresponding operational behavior, service ceiling, takeoff and landing distance, longitudinal stability, heading stability, spin and flutter. The flight test was carried out by Professor Dr.-Ing. A. W. Quick at the Institute of Aeronautics and Astronautics as part of Freddy Schliewa’s diploma thesis.

So the Krähe would have had what it takes to become a popular club airplane, had the self-built gear not been so loud and bumpy and the engine somewhat easy to start. But as it was, the Krähe mostly stood sadly in the hangar while the motorized pilots did most of their tug flying.

In 1971, it was demonstrated at the Oskar Ursinus Association (OUV) meeting in Egelsbach. where it received much attention:

“The next day the representatives of FVA Aachen presented their “Primitive Crow” - FVA-18. All those interested could convince themselves of the flight characteristics of this bird, which rightly bears its name because of its simple construction, by sitting down at the stick and taking to the air with the crow. Conclusion of the presentation of the aircraft from Aachen: Everyone is talking about the club airplane for replication - we have it. Only nobody is interested in it (yet).”

(from Deutscher Aerokurier, issue 8 71, p. 556. Report on OUV meeting).

Especially the low minimum speed of 55 km h and the harmless stall behavior impressed so much that the gyroplane pilots present “chickened out” of a comparison flight. The simple design aroused the interest of hobby aircraft builders. In the years that followed, countless inquiries were answered and ten sets of drawings were given to interested parties.