Computational Analysis of the X-57 Maxwell Airplane, the Cruise Configuration With Various Cruise Power Settings (Preliminary Fuselage)

The X-57 Maxwell was an all-electric airplane with a distributed electric propulsion system used for a high-lift system at takeoff and landing conditions. The focus of this paper is the cruise configuration with the wingtip propellers operating and the distributed electric propulsion propellers stowed. The computational fluid dynamics flow solver, USM3D, was used to investigate the performance of the X-57 Maxwell at flight Reynolds numbers from 1.4 million to 2.8 million, based on mean aerodynamic chord. Numerical solutions were computed at 2° angle of attack for Mach numbers from 0.1 to 0.3 (64.3 KTAS to 141.6 KTAS), at altitudes of 2500 feet, 8000 feet, and 15000 feet, and cruise power at 1150 RPM, 2000 RPM, and 2250 RPM, and no high-lift blowing. Data were also computed at angles of attack of 8°, 12°, and 16° for a Mach number of 0.1355 (85.7 KTAS), at an altitude of 2500 feet for no high-lift blowing and cruise power at 2250 RPM. An actuator disk was used to model the cruise propellers.
The X-57 Maxwell wingtip propellers opposing the wingtip vortex reduced the induced drag penalty for angles of attack less than 16°. Increasing thrust reduced drag and the greatest drag reduction occurred at the lowest airspeed. The cruise thrust increased lift for all angles of attack less than 16°. The increased lift from cruise power was larger at 85.7 KTAS, than at an airspeed of 128.2 KTAS. The effect of cruise thrust on pitching moment was a slight increase in nose-down pitching moment coefficient with increasing thrust coefficient for both airspeeds and angles of attack less than 16°. There was little to no effect of cruise thrust on yawing moment coefficient for angles of attack less than 16°. The effect of cruise thrust on rolling moment was larger at 85.7 KTAS than at 128.2 KTAS. The change in rolling moment between powered and unpowered conditions was significantly larger at 85.7 KTAS, than at 128.2 KTAS. These data were important to understand the effect of differential cruise thrust on rolling moment to add the effects in the X-57 Maxwell flight simulator. The effect of cruise power on rolling moment predicted from three computational fluid dynamics codes, from three NASA centers, compared very well for low angles of attack of 0° and 2°, and moderately well for angles of attack of 8° and 12°. These data comparisons were important for gaining confidence in the computational data when no experimental data were available prior to flight testing.