Copyright © A. Filippone (1999-2003). All Rights Reserved.

High Lift Aerodynamics


Lift is a force in a direction normal to the velocity. It is due to both pressure and viscous contributions. The weight of the pressure component is generally far more important; when the viscous component is effective, it works as to reduce the total amount of lift obtainable by an aerodynamic system.

Importance of the Subject

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.

High lift systems are of the utmost importance in human powered flight, unpowered gliding, etc. High lift systems are also used (differently) in racing cars and competition sailing boats.

The picture below shows the cargo plane C-17 Globemaster with high lift system in operation during a slow landing phase.

C-17 Globemaster

Figure 1: McDonnell Douglas C-17 Full Image (102K)

Flow Phenomena

Flow phenomena of multi-element wings include: wakes from upstream elements merging with fresh boundary layers on downstream elements; flow separation in the the cove regions; flow separation on the downstream elements, especially at high angles (landing configurations); confluent boundary layers; high- curvature wakes; high flow deflection; possible supercritical flow in the upstream elements, see figure below.

Multi-Element Airfoil
Figure 2: Multi-element wing

Two boundary layers are confluent when they develop on different solid surface and come together (generally at a different stage of development).

Confluent boundary layers can be identified by studying the local velocity field. Flow separation occurs in cove regions because of the high curvature associated with locally high speed. High speed can also be the reason of supercritical regimes in aircraft configurations.

Maximum Lift

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.

CLmax also depends on the Reynolds number. At a fixed Reynolds number, the operation on the above parameters must remove or delay the flow separation, and delay the pressure recovery on the suction side, along with a number of other details.

Prediction of Maximum Lift

Accurate prediction of the maximum lift coefficient for an airfoil or wing is still considered an open problem in computational aerodynamics. This difficulty is due to the approximation of the boundary layer conditions at various stages of turbulent transition and separation, besides the proper modeling of the turbulent separated flows.

An empirical formula correlating wing CLmax of a swept wing to the main geometric parameters of the high-lift system was derived at the Research Aeronautical Establishment (RAE, UK) in the late 1970s. More recent work was done at McDonnell- Douglas (Valarezo-Chin, 1994).

Vortex Lift

The lift force from a wing can be augmented by appropriate manipulation of separation vortices. Basically, this can be done in two ways: with highly swept wings (delta wings) and strakes. The longitudinal vortex has the effect of shifting the stagnation point on the suction surface of the wing (Pohlamus, 1971).

Vortex Lift
Figure 3: Vortex Lift

High-Lift Systems

High lift can be produced by aerodynamic design of single components, design of entire systems, integration of already existing systems, ad hoc technical solutions. The most important methods are the following:

Powered vs Unpowered Systems

There is a broad classification among all high lift systems: that is between powered and unpowered. The range of applications in aviation is discussed below. The data collected in the figure below have been elaborated from Airbus research (Flaig and Hilbig, 1993). Performances of the C-17 and the YC-14 have been guessed.

Data available on CD-ROM

High-Lift Airfoils

In order to obtain high lift from an airfoil the designer must increase the area enclosed by the pressure coefficient (Cp), that is: the pressure on the lower side must be as high as possible (pressure side), the pressure on the upper side must be as low as possible (suction side). The latter requirement is in fact the most difficult to fulfill, because low pressure is created through high speed, and high speed triggers flow separation. Flow separation can be limited at high speed by turbulent transition.

Pressure Distribution

One idea commonly used in design is to control the pressure distribution on the upper side as to maintain the flow at the edge of separation.

The more separation is delayed the higher the lift coefficient. This is obtained through a flat top and a gradual pressure recovery (Stratford recovery). Airfoils designed with this approach can exhibit aerodynamic efficiencies L/D of up to 300 !

Multi-Element Airfoils

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.

Later on, more elements were added to the main wing. A three-element configuration (with leading-edge slat and trailing-edge flap) is classic, but the technology has improved, and 4 or more element are not uncommon, ex. in Fig. 2.

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


Design Issues for High-Lift

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.

Examples from the real world include the design multi-stage turbines for aircraft and power generation, aircraft design, internal flows in pipes and channels for pumps, compressors and exhaust gas.

Computational Methods

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.

2-Element Racing Wing

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

Selected References

  • Hoerner SF. Fluid Dynamic Lift, Hoerner Fluid Dynamics, 1965

  • Clancy JC. Aerodynamics, John Wiley, New York, 1975.

  • AGARD. High-Lift System Aerodynamics, AGARD CP-515, Banff, Oct. 1993

  • McCormick BW. Aerodynamics, Aeronautics and Flight Mechanics, John Wiley, New York, 1994.

  • Gratzer, LB. Analysis of Transport Applications for High-Lift Schemes.. AGARD LS-43, 1971.

  • Betz A.Theory of the Slotted Wing, NACA TN-100, 1922.

[Top of Page]

Copyright © A. Filippone (1999-2003). All Rights Reserved.