Somebody in the Soviet Ministry of Aircraft Industry decided that what agriculture needed was a jet. Not a jet trainer, not a jet transport. A jet crop-duster. A biplane one.

The PZL M-15 Belphegor is the only jet-powered biplane ever put into series production, the slowest mass-produced jet in history, and quite possibly the ugliest aircraft ever built on purpose. One hundred and seventy-five were made. Three thousand were planned.

Why on earth a jet

The reasoning was not quite as mad as it sounds, though it was still mad. Aeroflot wanted to standardise on kerosene across its entire fleet rather than keep avgas in the supply chain for tens of thousands of An-2s. The Ivchenko AI-25 turbofan was already everywhere — it powered the Yak-40 airliner and the L-39 trainer — so the parts and the expertise existed. And turbines were believed to be cheaper to maintain than piston engines.

Underneath all that was politics. Jet propulsion was prestige, and Soviet agricultural-aviation specialists who objected were overruled from above.

Oleg Antonov, whose An-2 the thing was meant to replace, was reportedly asked to evaluate the project and declined in the most Antonov way possible.

The engineering is genuinely clever

Look past the appearance and there is real thought here. The engine sits high, above a very short fuselage and beneath the upper wing, which keeps it out of the dust and stones thrown up from rough agricultural strips. Its exhaust vents cleanly between the twin tail booms, so the jet efflux does not disturb the spray pattern behind the aircraft.

Better still, compressed air bled from the engine drives the pneumatic system that distributes dry chemicals. The powerplant is not just propulsion; it is part of the working equipment.

The biplane layout was kept for the same reason the An-2 has one: lots of wing, low speed, tight turns at the end of every run over a field. And the chemical hoppers were made structural, forming the thick struts between upper and lower wings.

Then it met a spreadsheet

In service the M-15 burned more than three times as much fuel per flying hour as an An-2 or a PZL Dromader. It cost far more to build. It needed more maintenance from rough strips. Its range was half the An-2’s. Because the hoppers were structural, it could not be emptied and used to carry anything else, which the An-2 did constantly. And every An-2 pilot who converted to it needed jet training.

Every one of the arguments for a jet crop-duster turned out to be wrong in the field.

The Soviet Union stopped ordering them at the end of the 1970s and production finished in 1981, at 175 aircraft against a plan for up to three thousand.

About that name

Belphegor is a demon — in Christian demonology a prince of Hell, associated with sloth and with tempting people through invention. It is a wonderfully apt name for this aircraft, and that is almost certainly not why it got it.

Polish accounts credit the test pilot Andrzej Ablamowicz, watching it at the Paris Air Show from the Le Bourget control tower, with coining the nickname — his reference being the 1965 French television serial Belphegor, about a phantom haunting the Louvre.

A handful survive: the Polish Aviation Museum in Krakow, the PZL Mielec works, the Central Air Force Museum at Monino, and the example above in Hungary. They are worth going to see. Photographs do not really convey it.

A shorter look at the layout and why it exists:

And a third take, for completeness:

Sources: Polish Aviation Museum; samolotypolskie.pl (after Glass, Samoloty PZL 1928-1978); dlapilota.pl; AgAir Update.