The cross-flow fan can be traced as a specific invention in 1891 by Paul Mortier with patent 215,662 filed in France. A corresponding US Patent [2] was awarded in 1893, and Fig. 4 shows the fan arrangement cited there. Mortier's motivation was mine ventilation and his fans were used for that purpose with rotor diameters up to nearly 3 m. Shortly thereafter, axial flow propeller fans came into favor and interest in the cross-flow fan faded for about two decades. A reemergence occurred in the late 1920s and 1930s with a variety of patents with applications for drying grain, air-conditioning, and feeding pulverized fuels (e.g. [3] and [4]). Despite these inventions early acceptance of the cross-flow fan was limited due to relatively poor performance in relation to centrifugal fans of the time. The first reference to use of the cross-flow fan for aircraft application was provided in a 1938 lecture appropriately titled “Present and Future Problems of Airplane Propulsion,” by the Swiss aeronautical engineer, J. Ackeret [5]. Ackeret described a concept for boundary-layer control by means of energizing the boundary-layer flow using a cross-flow fan, as shown in Fig. 5, and carried out some preliminary calculations for the necessary size of the fan.