Numerical Study of Transition of an Annular Lift Fan Aircraft

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).

CFD Study of a New Annular Lift Fan Configuration with High Lift Efficiency

“…The validation of transition mode was performed using a three-dimensional fan-in-wing configuration with a fan rotating in the plane of a wing [4,6]. The CFD predictions of C L and C m showed a good agreement with the experimental data, while C D was a little under-predicted [4].…”

“…To achieve smooth transition with lower peak of momentum drag and nose-up pitching moment, a strategy to start forward flight from a large negative angle of attack (AOA) of −21 • was recommended in the previous studies [2,4]. In the present study, the strategy to start from a negative AOA is no longer necessary, reducing the complexity of control of aircraft attitude in transition.…”

“…The lift fan set is composed of a single-stage inner fan and outer fan (Figures 2 and 3) [3,4], which are separated and mechanically coupled by the gears in the ring baffle and counter rotate at the same speed to eliminate torque. A very thin airfoil with a flat surface and a cambered surface with the maximum thickness at the position about of 15%-20% of the chord from the leading edge was used (Figure 4a).…”

“…A large annular lift fan for vertical takeoff and landing (VTOL) aircraft with low disc loading was proposed in previous studies [2][3][4] (Figure 2). The annular lift fan has an outer radius of 7 m with annular fan area of 75.3 m 2 .…”

“…The transition from VTOL to aerodynamic cruise flight is very important for the success of VTOL vehicles [4]. To achieve smooth transition with lower peak of momentum drag and nose-up pitching moment, a strategy to start forward flight from a large negative angle of attack (AOA) of −21 • was recommended in the previous studies [2,4].…”