Польшаның реактивті бипланы «Бельфегор»

Polish jet powered biplane

The PZL M 15 was a jet powered sesquiplane designed and manufactured by the Polish aircraft company WSK PZL Mielec for agricultural aviation. In reference to both its strange looks and relatively loud jet engine, the aircraft was nicknamed Belphegor, after the noisy demon. Development of the M 15 can be traced back to a Soviet requirement for a modern agricultural aircraft to succeed the Antonov An 2; it was at the insistence of Soviet officials that jet propulsion would power the type. WSK Mielec's design team recognised the value of the An 2's biplane configuration to the role and set about developing an initial experimental aircraft, the Lala 1, for Latające Laboratorium 1 ("Flying Laboratory 1") to explore the use of a jet engine with such a configuration. On 20 May 1973, the first M 15 prototype performed its maiden flight; even during the test flight phase, it was apparent that there were several drawbacks to the aircraft, including its poor handling, limited range, and high operating costs. While production commenced in 1976, these problems remained unresolved and meant that the M 15 was noticeably inferior in several respects to the An 2. During 1981, production was terminated in favour of procuring more An 2s; a total of 175 M 15s were built against the many thousands which had once been planned.

By 1970, the Soviet Union was formulating a new requirement for an agricultural aircraft that it intended to use above the very large areas of Soviet farms, the kolkhoz collectives, and state owned sovkhoz. It was explicitly stated in the requirement that the envisioned aircraft was to be both more efficient and modern than the existing Antonov An 2SKh and An 2R, of which tens of thousands had been produced primarily to perform this role. By this point, Poland had already produced the agricultural Antonov An 2R under licence for export back to the USSR, while agricultural planes had become a Polish specialization within the Comecon. The Lala 1 was equipped with agricultural devices; experiences and data gathered from its test flights greatly influenced the design of the M 15.

During 1976, serial production of the M 15 commenced. At one stage, Soviet agriculture planners had intended to order as many as 3,000 aircraft to meet its needs; however, operations of the M 15 under real world circumstances proved to be a largely disappointing experience. As an inherent consequence of adopting a jet engine, it was not an economical aircraft to operate, being inferior in this aspect to the An 2 that the M 15 had been envisioned to succeed. In contrast, the M 15 could only achieve a range of 215 nautical miles, half that of the An 2, largely due to its jet engine and relatively high weight. Maintenance was also more intensive, a factor which was compounded by the typically austere nature of agricultural airstrips. Taking its envisioned role was the preexisting An 2, which continued to be used in the agricultural role into the twenty first century. It is believed that the M 15 is the world's only jet agricultural aircraft (i. e., the world's only jet cropduster), as well as the world's only jet biplane and the world's slowest jet aircraft, at least amongst aircraft that have been put in mass production.

The PZL M 15 Belphegor was a metal twin boom sesquiplane. It was intended to be routinely operated by a single pilot, but also had provisions for two additional crew to serve as technicians when deemed necessary. Portions of the lower wings and the chemicals tanks were composed of a laminate to avoid corrosion. The upper and lower wings were connected with two thick columns which housed the chemical tanks. It was outfitted with a fixed tricycle landing gear arrangement. The M 15 was a relatively heavy aircraft, and has been described as being the heaviest biplane to ever be produced. For the crop dusting mission, the M 15 could accommodate a payload of just under three tons of pesticides within two sizable pylons that separated its two wings; chemical dispersal was achieved via compressed air. This storage system was relatively unorthodox, the conventional An 2 simply stored these in a single tank housed within the fuselage in a space that could be reused for various other cargoes if not fitted. As such, the arrangement adopted upon the M 15 allowed for no such flexibility and severely limited alternative uses for the aircraft. To avoid the engine exhaust interfering with the dispersal system during release, the engine had to be positioned in a relatively elevated location on top of the fuselage; this was also beneficial to minimise the engine's ingestion of debris, which was a particular problem when operating from austere airstrips.