Sidebar: What’s a propfan (and what isn’t)?
The prop-fan and UDF are unique among propellers because of their speed and power loading—the amount of power driving a propeller of given diameter. The thin blades and sweepback improved efficiency at transonic speeds, just as they did on an airplane wing. The airplane was subsonic, but the prop tips hit Mach 1.1 in a helical path.
Russia’s Tu-95 bomber and its airliner derivative, the Tu-114, were designed in the 1950s and had jet-like swept wings. The turboprop-powered Tupolevs could sprint at Mach 0.78, but had to cruise at around Mach 0.7 for best range. Their 15,000-hp engines drove 18-foot counter-rotating propellers, requiring tall landing gear to keep the tips off the runways.
The Ukrainian Antonov An-70 and the yet-to-fly Airbus A400M cruise at up to Mach 0.72, about as fast as the jet-powered C-17 airlifter, but slower than commercial jets. They use large-diameter propellers, not prop-fans.
There is little interest in true high-speed propellers today. The latest conventional turbofans are more efficient than the engines of the mid-1980s, thanks to new fan aerodynamics and materials, so there is less to be gained by a move to a UDF-type engine. It’s also questionable whether the prop-fan could meet current international noise rules.