Wing Design in More Detail
The determination of a reasonable lift and Cl distribution, combined
with a way of relating the wing twist to this distribution provides a good
starting point for a wing design. Subsequent analysis of this baseline design
will quickly show what might be changed in the original design to avoid
problems such as high induced drag or large variations in Cl at off-design
conditions.
Once the basic wing design parameters have been selected, more detailed
design is undertaken. This may involve some of the following:
- Computation or selection of a desired span load distribution, then inverse
computation of required twist.
- Selection of desired section Cp distribution at several stations along
the span and inverse design of camber and/or thickness distribution.
- All-at-once multivariable optimization of the wing for desired performance.
Some examples of these approaches are illustrated below.
This figure illustrates inverse wing design using the DISC (direct iterative surface curvature) method.
The starting pressures are shown (top), followed by the
target (middle), and design (bottom); light yellow = low pressure and green = high
pressure. This is an inverse technique that has been used very successfully with
Navier-Stokes computations to design wings in transonic, viscous flows.
Below is an example of wing design based on "fixing" a span load
distribution. When the 737 was re-engined with high bypass ratio turbofans,
a drag penalty was avoided by changing the effective wing twist distribution.
The details of the pressure distribution can then be used to modify the
camber shape or wing thickness for best performance. This sounds straightforward,
but it is often very difficult to accomplish this, especially when it takes
hours or days to examine the effect of the proposed change. This is why
simple methods with fast turnaround times are still used in the wing design
process.
As computers become faster, it becomes more feasible to do full 3-D optimization.
One of the early efforts in applying optimization and nonlinear CFD to wing
design is described by Cosentino and Holst, J. of Aircraft, 1986.
In this problem, a few spline points at several stations on the wing were
allowed to move and the optimizer tried to maximize L/D.
Although this was an inviscid code, the design variables were limited, and
the objective function simplistic, current research has included more realistic
objectives, more design degrees of freedom, and better analysis codes.
--but we are still a long way from having "wings designed by computer."