Prandtl second optimal wing: model design and testing with near wake PIV, and 3-D simulation

In 1933, Prandtl proposed a second and superior optimal wing, also much closer to what birds do: the wing with minimum induced drag for fixed lift and fixed amount of structural material. The obtained lift distribution is then bell shaped (not elliptical as the 1922 optimal wing). This design has regained much interest, also by NASA. As the distribution has a zero slope at the wing tip, the near wake doesn’t display a tip vortex. There is also an upwash velocity in the near tip region, which allows for efficient maneuvers without vertical tail. For a chosen geometry (here a linearly tapered wing) and cruise lift coefficient (here 0.4), the twist distribution is then tailored to obtain the desired lift distribution. A wing model was designed, built and tested in a small wind tunnel. The near wake was measured at stations downstream using PIV. It is seen that the near wake doesn’t really roll up. Hence, the lift distribution can also be recovered by integrating near field PIV measurements. PIV results will be shown at cruise condition and also at off-design (from -0.1 to 0.6). The recovered lift distribution will also be compared to that provided by the lifting line theory. Results of a near wake simulation performed using the 3-D Vortex Particle-Mesh method will also be presented.