The aircraft went through several names, which is one reason it is so often muddled in online sources. Early studies used the internal designation S-32; the demonstrator that actually flew in 1997 was known as the S-37. Around 2002 Sukhoi re-badged it Su-47, a more production-sounding designation used for marketing the concept. It should not be confused with the unrelated Su-37 “Terminator,” a thrust-vectoring development of the Su-27 — a different aircraft entirely.

Whatever it was called, there was only ever one. Talk of a “fleet” of Su-47s, or of the type being fielded by the Russian Air Force, is a persistent myth. The single airframe is today kept at Zhukovsky, near Moscow, and has occasionally been rolled out for public display.

On a conventional swept-back wing, airflow tends to drift outward toward the tips as the aircraft slows or pulls hard, so the tips stall first — and the tips carry the ailerons, so control is lost just when it is needed most. A forward-swept wing reverses that flow, moving air inward toward the roots. The tips keep flying and the controls keep biting at high angles of attack, which is why the layout promises better slow-speed handling and agility.

The price is structural. Because the wing sweeps forward, aerodynamic load twists the leading edge up, which increases the load, which twists it further — a feedback loop known as aeroelastic divergence. Build the wing from ordinary metal stiff enough to resist it and it becomes far too heavy. The solution, pioneered on the American Grumman X-29 and taken further on the Su-47, is aeroelastic tailoring: composite skins whose fibres are deliberately oriented so the wing bends and twists only in safe directions. The Berkut is, in effect, a demonstration that this can be done at fighter size.

The Su-47 never became a production aircraft, and Russia’s fifth-generation fighter — the PAK FA, which entered service as the Su-57 — wears a conventional swept wing, not a forward-swept one. So what did the Berkut actually contribute? Chiefly experience: in building and flying very large composite structures, in fly-by-wire control of an unstable airframe, and in integrating an internal weapons bay, which the Su-47 carried in its later years as a test installation. Those are core competencies for any stealth-influenced fighter, and the Su-57 uses composites for roughly a quarter of its structure. The Berkut is best understood as one of several stepping stones — a research airframe that helped de-risk technologies, even though its signature feature was not carried forward.

The Su-47 was a one-off research aircraft, never mass-produced and never sold, so there is no flyaway unit price and no credible cost-per-flight-hour figure for it in the public record. Development was funded partly by Sukhoi itself during the lean Russian defence years of the 1990s, and the airframe reused existing MiG-31 engines precisely to keep costs down. Any specific dollar figure attached to a Su-47 online is, in practice, an invention. What can be said is that as a single hand-built demonstrator packed with novel composite structures, its per-airframe cost bore no relation to a production fighter’s.