High lift systems are required in aeronautics to produce higher maneuverability, for
higher endurance under engine failure, for lower take-off and landing speed, higher
pay-load, for aircraft weight constraints, maximum engine power limits, etc.

The maximum lift obtainable by a single/multi element wing (or by more complicated
devices) is generally attributed to flow separation on the suction side, and on the
maximum suction peak. The two problems are somewhat dependent.

Airfoil characteristics that have a strong effect on the maximum lift coefficient
are: camber and thickness distributions, surface quality, leading edge radius,
trailing edge angle.

Generally speaking, a multi-element airfoil consists of a main wing and a number of
leading- and trailing-edge devices. The use of multi-element wings is a very
effective method to increase the maximum lift of an aerodynamic system.

#### The Slat

The first element to be added to a main wing was a leading edge slat
(Handley-Page, Lachmann, 1917). The solution worked, but it was not clear how. For
many years is was assumed that the leading-edge slat was a boundary layer control
device (Betz, 1920).

Smith (1972) proved that the slat is so effective because of its strong
effect on the inviscid pressure distribution.

The leading-edge slot deviates the streamlines, creates a downwash on the main
element and modifies markedly the leading edge suction peak.

A system with increasing number of elements provides an increasing amount of
lift. This increase is however associated with an increase in drag.

A fundamental problem involved in high-lift design is the evaluation of the
computational tools. There is always the possibility of failing to meet the design
target.

Optimization and design cannot be approached by using the wind tunnel alone,
because extensive parametric testing is time consuming and economically
unaffordable. In fact, only the final design is generally build and tested in a wind
tunnel at the design conditions.

The design is complicated by the mutual interaction (interference) among the
aerodynamic components. Industry is in fact interested in integrating each component
into a more complex aerodynamic system (aircraft design, turbomachinery, etc.), besides
optimizing a single device.

With the improvement of the computational capabilities a more rational approach to
design and optimization has become possible. The use of wind tunnel techniques
is complementary and now in a process of closer integration.

In the past few years the computational methods for high lift have been converging
toward Navier-Skotes solvers (unstructured, and multi-block structured), although
methods including strongly interactive boundary layers have proven to be almost as
successful.

The method of computation depends on the complexity of the problem (2-D, 3-D, number
of high-lift bodies, precision requirements, turbulence modeling, etc.).

The figure below shows the pressure field around an inverted 2-element wing for
racing applictions. The flow field was computed with a structured multi-block
Navier-Stokes code.

**Figure 4: pressure field around multi-element inverted wing.**

#### Related Material (available on CD-ROM)

- Aerodynamic Design of airfoils and wings
- Strakes
- Gurney Flaps
- Delta Wings
- Computational Methods for high lift