Numerical Assessment of a High Disk Loading Annular Lift Fan Aircraft

An annular lift fan aircraft configuration with high disk loading, low empty weight, and acceptable hover efficiency is assessed using numerical methods. The fans have short and very thin blades. With the fan disk loading increased from 28.37 to 91 lb/ft(2), the figure of merit is 0.658 but the power loading drops from 9.03 to 4.65 lb/hp. However, when the aircraft is in ground effect within one aircraft radius to the ground plane, the power loading increases almost 50% to 6.77 lb/hp, which is comparable to helicopters and general aviation aircraft. The design features jet thrusters for forward flight propulsion, where the maximum requirement for jet thrust with the aid of a skirt in transition is 0.448 of the aircraft weight. Turbofan engines with a thrust-to-weight ratio of 0.3 and an augmentation ratio of 1.6-2.0 can satisfy this requirement and enable a top speed of Mach 0.62 (759 km/h) based on drag predictions. Shutters are proposed to close off the annular duct during cruise, and results show that they can close smoothly at the end of transition without causing oscillations of lift and drag.