CFD Study of an Annular-Ducted Fan Lift System for VTOL Aircraft

Abstract

Abstract:The present study aimed at assessing a novel annular-ducted fan lift system for VTOL aircraft through computational fluid dynamics (CFD) simulations. The power and lift efficiency of the lift fan system in hover mode, the lift and drag in transition mode, the drag and flight speed of the aircraft in cruise mode and the pneumatic coupling of the tip turbine and jet exhaust were studied. The results show that the annular-ducted fan lift system can have higher lift efficiency compared to the rotor of the Apache helicopter; the smooth transition from vertical takeoff to cruise flight needs some extra forward thrust to overcome a low peak of drag; the aircraft with the lift fan system enclosed during cruise flight theoretically may fly faster than helicopters and tiltrotors based on aerodynamic drag prediction, due to the elimination of rotor drag and compressibility effects on the rotor blade tips; and pneumatic coupling of the tip turbine and jet exhaust of a 300 m/s velocity can provide enough moment to spin the lift fan. The CFD results provide insight for future experimental study of the annular-ducted lift fan VTOL aircraft.

Numerical Study of Transition of an Annular Lift Fan Aircraft

“…The momentum drag of the single-stage fans in transition mode showed similar characteristics to the two-stage fans in the previous study [2], but the peak of drag lowered from 0.63 to 0.4 of aircraft weight ( Figure 7, Spot A). The strategy to start transition flight from a negative angle of attack (AOA) −21 • with a forward force produced by the lift fan system (negative net drag) to increase the drag along the arrows B to C to D further lowered the peak of drag to about 0.29, as shown in Figure 7 (Spot C).…”

“…The lift fan set contains two-stage fans, including an upper fan and a lower fan mounted in the annular duct located between the central fuselage and the outer wing [2] (Figure 1). The two fans counter rotate at the same or different speeds to eliminate torque and are tip-driven during VTOL by the two turbofan engines incorporated in the outer wing.…”

“…In this way, the fan area and fuselage size are greatly increased and the lift efficiency improved. During cruise flight, the annular duct is closed and becomes part of wing to provide aerodynamic lift [2]. For VTOL aircraft, hovering like a helicopter or cruising like a fixed wing craft are relatively simple.…”

“…In our previous study of the annular lift fan aircraft, the lift and momentum drag in transition were investigated to find the best transition strategy [2]. It was observed that the lift and drag oscillated during the transition (Figure 2), which may cause harmful vibration of the aircraft.…”

“…To achieve the high lift efficiency and high cruise speed for VTOL (vertical takeoff and landing) aircraft, we proposed an annular lift fan aircraft in a previous study [2] (Figure 1). 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]. In fact, as the annular duct is closed in forward flight, there is no theoretical limit to the top speed.…”