Summary
- Importance of the Subject
- Flow Phenomena
- Maximum Lift
- Boundary Layer Control (CD-ROM)
- Vortex Lift
- High-Lift Systems
- High-Lift Airfoils
- Multi-element Airfoils
- Design Issues for High-Lift
- Trailing- and Leading Edge Devices
- Computational Methods
- Tables of Maximum CL (CD-ROM)
- Selected References
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.
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.
Figure 1: McDonnell Douglas C-17
Full Image (102K)
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.
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.
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.
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).
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).
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:
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
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.
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 !
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 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.
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.
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.
Importance of the Subject
Flow Phenomena
Figure 2: Multi-element wing
Maximum Lift
Prediction of Maximum Lift
Vortex Lift
Figure 3: Vortex Lift
High-Lift Systems
Powered vs Unpowered Systems
High-Lift Airfoils
Pressure Distribution
Multi-Element Airfoils
The Slat
Design Issues for High-Lift
Computational Methods
Related Material (available on CD-ROM)
Selected References