FVA 19 - Tow Plane study
The keyword economic efficiency in particular sets this study apart from other projects. In order not to blindly invest time in financially unprofitable undertakings, the following considerations were taken as a starting point: The benchmark is the generally used winch launch operation. Aero-tow launching operations would be economical if the total overheads were the same or lower than for winch launching operations, with the same flying efficiency. The compilation of all factors, which in their entirety make up the term economic efficiency, was only possible through the evaluation of corresponding statistical material.
Without wanting to define flight efficiency in more detail at this point, it is immediately obvious that the costs for the towed meter of altitude are decisive for a comparison of towing aircraft, whereby these costs include the incurred overheads such as overhauls, usual minor repairs, depreciation and so on. Depreciation, etc., are included.
After extensive calculations it became clear that the power requirement of the optimum tow plane must be well above 175hp. From this it is already obvious that any compromise between club aircraft for guest flying (as many paying seats as possible, power just sufficient) and towplane (at best two-seater for instruction, high power surplus) must be unsatisfactory. Further, it became clear that the tendency toward larger numbers shifts the optimal type in each case toward aerodynamically superior aircraft. In order to be able to state which of the calculated cases are economical according to the above mentioned definition, the costs and achievable performances in winch launch operation had to be known.
For this purpose, the fixed costs of a glider per year were compiled, as well as those of the winch, and the average take-off times were taken from the annual report of the North Rhine-Westphalia regional association of the DAeC for the years 1959 - 1961. This resulted in a glider hour price of 90,- DM at 60 hours per glider and year down to 70,- DM at 80 hours per glider and year. With the last figure (80 h/year) the capacity of the winch seemed to be exhausted. Assuming a flight time per launch of 25 minutes, a tow launch at the same total annual flight time of 80h year and the same flight hour price was allowed to cost 4.75 DM. This was the cost of the winch to allow the glider to fly for 25 minutes. This required on average a little more than two winch tows). With sufficient utilization of the towplane, such launch costs are possible.
However, if one achieves more than 80h/year per glider with aerotow launching, even higher tow prices can still be economical. Here are some figures as well. The glider hour is equally expensive for both types of launching, if the glider reaches 60h/year at the winch and 95h/year in aerotow, and this at a towing price of 15,- DM (again for an average flight time per launch of 25 minutes corresponding to a towing height of n = 750m).
Thus, it could finally be shown that with a suitable design and sufficient utilization of a tow plane, glider operation with aerotow launching becomes cheaper and more effective than with winch tow launching, which is still commonly used today.
An abridged version of this study was published in the trade press and met with great interest in gliding circles. Since the construction and flight testing of the “Krähe” took up the FVA’s resources until the end of the 1960s and a “Piper Super Cub” could be purchased as a towplane in 1963, the FVA-19 towplane project was not realized.
Through some old men of the FVA, however, this study reached Bremen, where a special towplane was designed and built by the Flugwissenschaftliche Arbeitsgemeinschaft Bremen based on the results of the FVA-19 project. This single-seater aircraft, equipped with a 180 HP engine and the type designation ESS 641, took off on its maiden flight on September 17, 1971.