This paper numerically investigates the longitudinal static stability of a tandem-wing CoFlow Jet VTOL (CFJ-VTOL) aircraft concept with high speed cruise Mach number of 0.6. A tandem wing configuration is desirable for VTOL aircraft since it provides a high hovering stability with dual lifting vectors. However, it brings challenges for cruise to maintain high efficiency and high longitudinal static stability. The design uses CoFlow Jet (CFJ) active flow control wings with propellers mounted above the suction surfaces. The wings are stacked using CFJ-NACA-6415 airfoil. This system allows reduced power consumption at takeoff while providing a benefit to efficiency at cruise. A previous study of the this CFJ-VTOL aircraft concept at cruise shows a high efficiency at Mach 0.6. However, the high efficiency configuration often has a conflict with a high longitudinal static stability. This study conducts trade studies to optimize the efficiency with sufficient longitudinal static stability, including varying the streamwise spacing of the wings, their individual aspect ratios and incidence angles, and the jet intensity of the CFJ system. In order to achieve longitudinal stability, the front wing lift slope needs to be shallower than that of the rear wing so that the front wing is less sensitive to the variation of angle of attack than the rear wing. This is achieved by having the aspect ratio of the front wing to be 25% of the rear wing. The front wing incidence angle is 3◦ and the rear wing is 0◦ . The large contribution of the fuselage toward the overall pitching moment requires the fuselage incidence angle to be 5◦ at cruise, which allows a positive pitching moment at α = −5◦ when the lift coefficient is about zero. This configuration fulfills the requirement of CMα < 0 for the range of −5◦ ≤ α ≤ 2◦ . The tandem wing VTOL vehicle achieves an excellent aerodynamic efficiency of 15.3 at Mach 0.6.