Adaptive Wing

Figure 1: Adaptive Wing

The project deals with concepts and aerodynamics of adaptive transonic wings. To cope with the problem of flight at transonic Mach numbers adaptive mechanisms are introduced. Aerodynamic efficiency at off-design conditions is improved by the application of a shock control bump (SCB), a concept first introduced in 1992 by Ashill, Fulker and Shires, on a variable camber (VC) airfoil. Since a SCB has to be properly shaped and positioned to generate a favourable effect, relevant geometrical parameters are investigated using direct numerical optimisation. An optimisation environment was developed consisting of a hybrid optimiser, a geometry module and a coupled Euler boundary-layer code. For a specified off-design condition bump shapes are optimised, while the influence of various geometric bump representations is investigated. Shape optimisations for an adaptive bump are carried out for different Mach numbers at a fixed lift coefficient. To overcome the problem of narrow Mach regions of significant drag reduction for one-point designed bumps, multi-point designs are performed.
The physical effect of the SCB is based on the highly nonlinear character of transonic flows. The SCB maps the contour of supercritical wing sections onto a smaller scale thus inducing isentropic compression waves upstream of the shockwave. This leads to a significant decrease of wave drag.
Determination of the exact flight condition in real flight represents a sophisticated task. Thus it is anticipated that SCBs for practical use must yield a reduced sensitivity to small changes of the onset flow. Because of the narrow Mach region of reduced drag coefficients for a one-point designed SCB multi-point designs were introduced. The objective function for the multi-point optimisation was changed to be represented by the sum of two drag coefficients at two different Mach numbers. On the left hand side the drag coefficient is plotted vs. the Mach number for a one-point design and several two-point optimised bumps. Since no weighting factors were involved in the optimisation process, the lower edge of the Mach-region implicitly has a lower priority than the upper edge since it introduces less wave drag that can be reduced. Thus the bump optimised for the most extended region of Mach numbers even shows a higher drag coefficient for the lower design Mach number compared to the clean airfoil while being favourable in the remaining design region. However, it can be stated that at the cost of less maximum drag reduction the region in which the bump is effective is broadened.

Figure 2 and 3: Polar Diagrams of Adaptive Wing

Because of its wave drag reducing capability an SCB applied additionally onto a VC airfoil promises a further increase of the aerodynamic efficiency. Direct numerical optimisations for a VC-SCB combination were carried out in order to estimate the additional improvement. On the right hand side the lift to drag ratio is plotted against the lift coefficient for the clean airfoil, the envelope of a VC-airfoil (green line), the envelope of a SCB-airfoil (blue line) as well as the envelope of a VC-SCB combination (orange line). Significant additional gains for the combination are visible. Investigations of the resulting optimised bump geometries of the VC-SCB combination show a noticeably reduced bump height compared to the optimised SCB only geometry. Further investigations regarding this project can be found in the following publications:

A. Sommerer, Th. Lutz and S. Wagner:
Numerical Optimization of Adaptive Transonic Airfoils With Variable Camber
Proceedings 22nd ICAS Congress, Harrogate, United Kingdom, August 27 - September 1, 2000, ICAS Paper ICA2.111

A. Sommerer, Th. Lutz and S. Wagner:
Design of Adaptive Transonic Airfoils By Means of Numerical Optimization
Proceedings ECCOMAS 2000: European Congress on Computational Methods in Applied Sciences and Engineering, September 11-14, 2000, Barcelona, Spain