Abstract
The present study aimed at studying the transition of annular lift fan aircraft through computational fluid dynamics (CFD) simulations. The oscillations of lift and drag, the optimization for the figure of merit, and the characteristics of drag, yawing, rolling and pitching moments in transition are studied. The results show that a two-stage upper and lower fan lift system can generate oscillations of lift and drag in transition, while a single-stage inner and outer fan lift system can eliminate the oscillations. The characteristics of momentum drag of the single-stage fans in transition are similar to that of the two-stage fans, but with the peak of drag lowered from 0.63 to 0.4 of the aircraft weight. The strategy to start transition from a negative angle of attack −21° further reduces the peak of drag to 0.29 of the weight. The strategy also reduces the peak of pitching torque, which needs upward extra thrusts of 0.39 of the weight to eliminate. The peak of rolling moment in transition needs differential upward thrusts of 0.04 of the weight to eliminate. The requirements for extra thrusts in transition lead to a total thrust–weight ratio of 0.7, which makes the aircraft more efficient for high speed cruise flight (higher than 0.7 Ma).
Highlights
A helicopter can fly, hover, and land almost anywhere, but its performance is limited with a theoretical top speed of 200 kn (370 km/h), above which it suffers from dissymmetry of lift
To achieve the high lift efficiency and high cruise speed for VTOL aircraft, we proposed an annular lift fan aircraft in a previous study [2] (Figure 1)
computational fluid dynamics (CFD) (Computational fluid dynamics) simulations indicated that the annular lift fan craft had the same or higher lift efficiency as a helicopter and could fly faster than a helicopter or tiltrotor based on aerodynamic drag predictions [2]
Summary
A helicopter can fly, hover, and land almost anywhere, but its performance is limited with a theoretical top speed of 200 kn (370 km/h), above which it suffers from dissymmetry of lift. Cruise flight, the annular duct is closed and becomes part of wing to provide aerodynamic lift to move the fuselage to the center and place the lift fan and wing around the central fuselage. In this way, the fan area and fuselage size are greatly increased and the lift efficiency improved. VTOL aircraft, especially hovering like a helicopter or cruising like a fixed wing craftflight, are relatively simple.For. The transitions, from vertical takeoff to forward aerodynamic are most complex, involving phenomena of both hover and forward flight and complicated interactions simple. 30 m/s, and rotational speeds of fans 135 and 143 rpm
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