The design of a new racing sailplane: A new thermal mix model and the role of transitional CFD

Like the manufacturers of commercial transport aircraft, sailplane manufacturers must also periodically introduce new designs to stay competitive. Consequently, to stay competitive in the 18-Meter Class of racing sailplanes, the manufacturer is, Schempp-Hirth Flugzeugbau, recently undertook the design of the glider to supersede its Ventus-2cx, first introduced in 2003. The resulting glider, the Ventus-3, benefited from a number of game-changing innovations that supported the design process, including the use of GPS in-flight logger data to develop a new model of the distribution of thermals that are typically used in cross-country soaring flight, the use of modern optimization techniques that are now possible due to improved computer resources, and the use of computational fluid dynamics (CFD) methods that are now able to accurately predict the transitional aerodynamics so important in the Reynolds number regime in which sailplanes operate. From the data provided by the in-flight loggers, the designer can back out in great detail how the sailplane and its pilot utilize the weather in which it operates. Exploiting this capability, the range of thermal strengths, sizes, and distributions over which the sailplane is likely to encounter over its lifespan can now be much better defined and was possible in the past. The design of the new sailplane was primarily performed in the classical manner using potential flow methods combined with two-dimensional airfoil aerodynamics. To better understand how CFD methods can support the design efforts, the glider was analyzed using a computational fluid dynamics solver that incorporates a recently developed transition/turbulence model. It was found that the design developed using the traditional methodologies agree very well with those obtained using CFD in the regions where the flow field is primarily two-dimensional. In regions where this is not true, such as in the case of wing-body and wing-winglet junctions, a great deal of design insight can be garnered from the CFD results. In the particular case explored, CFD predictions demonstrated that the winglet airfoil needed to be modified somewhat from the results obtained using two-dimensional aerodynamics.