One of our central current research topics is aircraft drag reduction. All heavier-than-air aircrafts experience a considerable amount of drag stemming from lift generation even in the absence of friction, the so called induced drag. The drag power persists behind the wing in the form of trailing vortices which endanger following aircrafts especially near airports. This drag may be minimized by the use of special wing configurations like elliptic wings (e.g. WWII Spitfire) and aircrafts with very high wing spans (sailplanes). Further drag reduction can be achieved by modifying the wing tips as is the case for the well known winglet.
The Institute for Fluid Mechanics and Heat Transfer has been working on a much more radical wing tip design, the vortex diffuser, which resembles an aft mounted propeller but in contrast is a non rotating device. Instead it uses the rotation of the trailing vortices emanating from the wing tips for thrust generation.
Meanwhile it could be proofed by means of computational, experimental and mathematical methods that a considerable drag reduction of conventional aircrafts is possible with vortex diffusers - in high lift configurations as much as 20% of the overall drag! Even higher lift to drag ratios might be realizable by an integral aircraft redesign considering vortex diffusers.