Abstract: In a nacelle and wing combination where a vortex is shed from a side surface portion of the nacelle and said vortex burst prematurely to reduce lift, the improvement comprising a vortex control device located in an upwash region along the side of the nacelle to cause said vortex to form in the critical vortex control region where boundary layer is relatively thin and local velocity is relatively high. This strengthens the vortex to inhibit vortex bursting at the burst location.

Abstract: A compact pneumatically powered apparatus for automatically propelling game balls, especially tennis balls and the like intended to be stroked by a hand held racket or other implement, for training players in the proper return of the simulated ball action. Each ball is ejected from the apparatus with a combination of forward or straight-line acceleration together with a predetermined amount of ball spin to closely approximate the intentional spin placed on tennis balls when stroked obliquely by a tennis racket. This is achieved in the apparatus by a mechanism for flexing an elongate pliable member, such as a flexible belt, between an initial slackened looped condition and a straightened tensioned condition. The tennis balls are successively fed into a launching position contacting a surface portion of the slackened, looped belt that faces generally forwardly toward the direction of ball propulsion and is initially rearwardly of the final orientation of the tensioned belt.