Performance Enhancement by Tandem Wings Interaction of CoFlow Jet Aircraft

This paper numerically studies the cruise efficiency enhancement by 3D tandem wings interaction for a CoFlow Jet (CFJ) aerial vehicle at cruise Mach number of 0.17. The simulations employ 3D RANS solver with Spalart-Allmaras (S-A) turbulence model, 3rd order WENO scheme for the inviscid fluxes, and 2nd order central differencing for the viscous terms. The aerodynamic performance, energy expenditure, and flow field of the tandem wing propeller-CFJ aircraft are investigated. Each of the tandem wings has a propeller mounted above the wing suction surface to reduce the CFJ power required. The front wing is smaller with the planform area 1/3 of that of the rear wing. Both wings have the same chord. The aspect ratio for the front wing is 3.56 and 10.68 for the rear wing. The area averaged aspect ratio of the aircraft is 8.9. The study holds a constant optimal angle of attack (AoA) of 5◦ for the front wing and has the AoA of the rear wing at 5◦, 10◦, and 15◦. The two wings are separated by one chord length in the stream-wise direction and are aligned in the same transverse position. Such a configuration allows the rear wing to capture the tip vortex of the front wing on the suction surface with its low pressure. This vortex capturing mechanism enhances the lift of the rear wing significantly attributed to the low pressure of the tip vortex core and the upwash the vortex generates. The optimal aerodynamic efficiency and productivity efficiency of the tandem wing vehicle system are obtained when the AoA of the rear wing is at 10◦. When the AoA of the rear wing is increased from 5◦ to 10◦, the increased circulation of the larger rear wing dominates the flow field. The induced circulation of the rear wing with a stronger propeller strength create an upwash favorable to the front wing, which produces an aerodynamic ratio of CL/CD of 21.85 and the corrected aerodynamic efficiency CL/(CD)c of 14.39. These are extraordinarily high merit results for the small front wing with a small aspect ratio of 3.56. The corrected aerodynamic efficiency CL/(CD)c for the whole vehicle is 14.27 with a lift coefficient of 1.6, which result in a corrected productivity efficiency C L/(CD)c for the whole vehicle of 22.82. The overall vehicle efficiency are excellent due to the high vehicle cruise lift coefficient of 1.6 and corrected aerodynamic efficiency of 14.27 for a moderate aspect ratio of 8.9. The cruise lift coefficient of 1.6 attributed to the CFJ active flow control is almost 3 times greater than that of conventional subsonic aircraft, which would be stalled at such a high lift coefficient or severely penalized by its excessive drag. This study indicates that the two tandem wings benefit each other. The front wing tip vortex enhances the lift of the rear wing and the rear wing’s high lift and circulation increase the front wing’s efficiency due to the upwash. The tandem wing configuration presented in this paper is not optimized and could be a start for a new area of aircraft configuration design. More investigation will be also conducted to study the propeller strength effect.