FVA 23 - Exhaust system
Toward the middle of the 1970s, the problem of aircraft noise seemed to become a central existential issue, at least for sport aviation. If the noise of jet-powered civil and military aircraft had to be endured as a necessary evil, then at least the unnecessary environmental abusers in the form of single-engine sports planes should disappear from the sky. The issue was highly polemicized and its particular suitability as an election campaign argument was soon recognized. It did not take long to see the effects:
Legal flight operation restrictions at fixed times on certain days and for aircraft that exceeded the tightened noise emission limits were soon enacted.
It was therefore not far-fetched for the FVA to take on the noise problem for its part. The research contract “Automatic Flaps” had just been successfully completed, and Prof. Dr. A.-W. Quick assured the FVA of his support in working on a research program dealing with the noise emission of powered aircraft used for glider towing.
An application submitted for this purpose for funding by the German Research Foundation (DFG) was approved in 1976 thanks to Prof. Quick’s efforts and covered the following areas:
- Determination of noise components of propeller, engine, cowling, airframe, but especially of the exhaust system
- Design and construction of an exhaust system (FVA-23)
- Noise reduction measures on the propeller
From the outset, the project was limited to towing and to the retention of conventional, i.e. gearless, engines, since only such solutions could quickly bring decisive success that were justifiable for the mass of towing aircraft currently flying in terms of financial and technical outlay. The Robin DR 400 180R “Remorqueur” towplane, which belonged to the FVA, was a special four-seater powered aircraft designed for aerotow and very common in the Federal Republic. With regard to the later use of the research results, this aircraft was therefore particularly suitable.
An exhaust system was to be designed and built that would satisfy the stricter noise limits set by law without having to accept too great a loss in engine performance. The first step was to take backpressure measurements and record their time-dependent attenuation with the aid of measuring amplifiers, pressure transducers and oscillographs. On the basis of this measurement data, the design of the FVA-23 was begun. An important requirement here was that the entire system should disappear under the standard engine cowling of the “Remorqueur” for aerodynamic reasons.
Due to the design of the standard exhaust system, it was to be expected that it would make a not inconsiderable contribution to the overall noise (it consisted of only two manifolds for one pair of cylinders each).
Detlef Lippeck took up an idea that had already been successfully tested in principle during the re-powering of the FVA-18 “Primitivkrähe”. He designed a two-piece swirl-chamber exhaust system adapted to the power and displacement of the Remorqueur engine. High-temperature-resistant stainless steel sheet 1.4428 was chosen as the material. During the manufacture of the exhaust system, the special support of FVA promoter Franz Matzerath, in whose company in Düren the FVA-23 was built, could be called upon. With a “pre-version” and a lot of fair electrics the constructors drove to the company Sportavia-Pützer on the Dahlemer Binz. After many power, exhaust backpressure and cylinder head temperature measurements it was clear that “it worked”: About one third of the permissible backpressure, power loss 5%. After returning to Aachen, some additional measures were taken to further reduce backpressure, and while Erimar (OMC) Schilberg was still busy creating the last missing millimeters of space between the engine cowling and the muffler, the application for provisional approval was already underway.
Initial comparative measurements with another aircraft of the same type, equipped with an additional exhaust system that complied with the stricter regulations but was fully suspended in the free airspace below the fuselage, showed complete success: in a flyover at 1000ft (1. 000 ft = 305 m) altitude using the same procedure as that employed by the Luftfahrt-Bundesamt for the certification of new aircraft types, the noise levels for both aircraft were exactly the same (5dB(A) lower than with the original exhaust system), although the FVA-Remorqueur, in contrast to the comparative aircraft, had not noticeably lost any of its towing, climbing and cruising performance relative to its original condition. This result suggested that propeller noise now predominated and that the FVA-23 project had served its purpose.
However, the exhaust system, which was of necessity more voluminous and heavier than the original system, was not entirely free of defects. Especially due to the higher weight, vibration cracks formed after some operating time. Thus, it often happened that the plant was dismantled and repaired again at Franz Maizerath. Therefore, taking into account the experience with the prototype, a second improved version FVA-23 V2 was built.