has abstract
| - An annular lift fan aircraft is a conceptual vertical takeoff and landing (VTOL) aircraft that was first systematically and numerically investigated in 2015. This concept was proposed to offer a VTOL solution for both high hovering efficiency and high cruise speed, using a large annular lift fan instead of the relatively small conventional circular lift fans used in the Ryan XV-5 Vertifan and the F-35B Lightning II (JSF). The Ryan XV-5 and the F-35B use relatively smaller lift fans, either fan-in-fuselage or fan-in-wing, with very high disc loading. According to the momentum theory of the ducted fan, high disc loading leads to low (see , JSF fan), so the F-35B can hover for only a short time, at the cost of range and useful load. On the other hand, helicopters apply long rotor blades to achieve low disc loading and high hovering efficiency, but have a limited forward speed of less than 200 knots due to compressibility effects on the rotor blade tips. The tilt rotor concept, found in the V-22 Osprey, uses large-diameter propellers, but also has limited top speed to 300 knots. The tilting ducted fans or shrouded propellers, such as the Doak VZ-4, the Bell X-22, were built in 1950s and 1960s. However, ducts designed for static conditions degraded in performance at high advance ratio, whereas a duct designed for axial cruise could regain good high-speed performance at the expense of static figure of merit. The Doak VZ-4 and the Bell X-22 especially had problems achieving high flight speeds. Therefore, to achieve both high hovering efficiency and high forward speed, the best way is to use a low disc loading lift fan or fans, in addition to turbofans for forward propulsion at cruise speed. (en)
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