So far most of the work on fan-wing integration has been of an ad hoc nature and the cruise Mach number limits have not been studied in detail. Similarly, much of the work directed at the fan itself for aircraft application has been centered on a narrow range of fan housing designs. Therefore, improved fan performance and highly effective fan-wing integration concepts may be possible through systematic study and optimization. These concepts can be studied at the system level including drive and power systems (e.g., motor drives and solar power), hybrid arrangements with other propulsion devices, and the possibility of including buoyancy augmentation. Application of cross-flow fans for aircraft propulsion and flow control will involve many challenges including noise control, structures, drive systems, aircraft stability, and flight control. These topics are outside of the scope of this article; however, we conclude this section with a brief discussion on noise.