Design of the Cruise and Flap Airfoil for the X-57 Maxwell Distributed Electric Propulsion Aircraft
A computational and design study on an airfoil and high-lift flap for the X-57 Maxwell Distributed Electric Propulsion (DEP) testbed aircraft was conducted. The aircraft wing sizing study resulted in a wing area of 66.67 sq ft and aspect ratio of 15 with a design requirement of V(stall) = 58 KEAS, at a gross weight of 3,000 lb. To meet this goal an aircraft C(L,max) of 4.0 was required. The design cruise condition is 150 KTAS at 8,000 ft. This resulted in airfoil requirements of c(l) is approximately 0.90 for the cruise condition at Re = 2.35 x 10 (exp 6). A flapped airfoil with a c(l,max) of approximately 2.5 or greater, at Re = 1.0 x 10 (exp 6), was needed to have enough lift to meet the stall requirement with the DEP system. MSES computational analyses were conducted on the GAW-1, GAW-2, and the NACA 5415 airfoil sections, however they had limitations in either high drag or low c(l,max) on the cruise airfoil, which was the impetus for a new design. A design was conducted to develop a low drag airfoil for the X-57 cruise conditions with high c(l,max). The final design was the GNEW5BP93B airfoil with a minimum drag coefficient of c(d) = 0.0053 at c(l) = 0.90 and achieved laminar flow back to 69% chord on the upper surface and 62% chord on the lower surface. With fully turbulent flow, the drag increases to c(d) = 0.0120. The predicted maximum lift with turbulent flow is a c(l,max) of 1.95 at alpha = 19 deg. The airfoil is characterized by relatively flat pressure gradient regions on both surfaces at alpha = 0 deg, and aft camber to get extra lift out of the lower surface concave region. A 25% chord slotted flap was designed and analyzed with MSES for a 30 deg flap deflection. Additional 30 deg and 40 deg flap deflection analyses for two flap positions were conducted with USM3D using several turbulence models, for two angles of attack, to assess near c(l,max) with varied flap position. The maximum c(l) varied between 2.41 and 3.35. An infinite-span powered high-lift study was conducted on a GAW-1 constant chord 40 deg flapped airfoil section with FUN3D to quantify the airfoil lift increment that can be expected from a DEP system. The 16.7 hp/propeller blown wing increases the maximum C(L) from 3.45 to C(L) = 6.43, which is an effective q ratio of 1.86. This indicates that if the unblown high-lift flapped airfoil of the X-57 airplane achieves a c(l,max) of 2.78, then the high-lift augmentation blowing could yield a sectional lift coefficient of approximately 4.95 at c(l,max). Finally, a computational study was conducted with FUN3D on an infinite-span constant chord GAW-1 cruise airfoil to determine the impact of high-lift propeller diameter to wing chord ratio on the lift increment of the DEP system. A constant diameter propeller and nacelle size were used in the study. Three computational grids were made with airfoil chords of 0.5*chord, 1.0*chord, and 2.0*chord. Results of the propeller diameter to wing chord ratio study indicated that the blown to unblown C(L) ratio increased as the chord was decreased. However, because of the increase in relative size of the high-lift nacelle to the wing, which impacted wing lift performance, the study indicated that a propeller diameter to wing chord ratio of 1.0 gives the overall best maximum lift on the wing with the DEP system.