Faculty of Aeronautics and Astronautics
2. Aerodynamic Systems Abstract-In this article, we will discuss the
technology behind aerodynamics so that you can 2.1. Fixed-Wing Aircraft see how amazing they really are. The topics
presented are of general interest, more or less Mc Donnell-Douglas C-17 on a demonstration advanced. There is no mathematics. Large use is flight. The plane is designed for take off and made of graphics, figures, tables, summaries,
landing on short runways. High lift systems are reference to further reading. The number of
aerodynamic systems that can be found is required.
incredibly large. Single components are basic aerodynamic shapes that are generally studied alone: airfoils and wings are among the most well known. Other components are only used as add-
ons to promote specific aerodynamic
performances, for example slots, dams, spoilers, fairings, fences, canards, strakes, flaps, vortex generators, splitter plates, tip devices, etc. 1. Introduction
Aerodynamics is an engineering science
Figure 1. Fixed-Wing Aircraft concerned with the interaction between bodies and the atmosphere. Technological applications
2.2. Helicopter and VSTOL aircraft include: General aviation (commercial, cargo, and business aircraft); V/STOL vehicles
The helicopter and some V/STOL aircraft (helicopters, some military aircraft, tilt rotors);
belong to the category of rotary-wing powered lighter-than-air vehicles (airships, balloons,
aircraft. This is a class of vehicles on its own, aerostats); aerodynamic decelerators (parachutes,
with peculiar aerodynamic and control problems. thrust reversal devices); road vehicles (passenger
The first helicopters flew many years after the and racing cars, commercial vehicles, high speed
airplanes. Other V/STOL aircraft feature trains); spacecraft, missiles and rockets, low- to
complex lifting systems, such as vertical jets and high-speed flight (micro air vehicles to
tilt rotors. hypersonic waveriders), high altitude flight, human powered flight, unmanned flight, gliders, energy conversion systems (wind and gas turbines); propulsion systems (propellers, jet engines, gas turbines).
Aerodynamic decelerators include parachutes,
thrust reversal systems and aerodynamic brakes,
although only the first ones (broadly called
parachutes) are generally treated in this category.
Parachutes have many applications in military
operations, deployment of payload, rescue
operations and sports, as shown in the photo at
Figure 2. Helicopter and VSTOL aircraft
2.3. Lighter-than-Air Systems
Lighter-than-air are basically balloons and
airships (or dirigibles). The balloons are the first
machines that were able to lift from the ground
with a man on board. Airships came at a much
later time, and they are usually associated with
Figure 4. Aerodynamic Decelerators pleasure journeys across the Atlantic or major
disasters (or both). Either way, lighter-than-air
2.5. Wind Energy Systems has captured the fantasy of many, not least
writers of fiction.
Wind energy systems are among the most
advanced clean technologies (though not in the
form showed at right). Many wind turbines are
now connected to the electric utility networks
and produce considerable amounts of energy.
The modern variable- pitch horizontal-axis wind
turbines (HAWT) are able to work in almost any
Figure 3. Lighter-than-Air Systems
2.4. Aerodynamic Decelerators
Figure 7. Wind Tunnel Testing
2.8. Buildings Aerodynamics
A wide variety of buildings is subject to
particularly strong aerodynamic forces. These systems include industrial towers, long
suspension bridges, and off-shore platforms. The Figure 5. Wind Energy Systems
figure at right shows two industrial towers 2.6. Racing Cars equipped with spirals in order to reduce the
vortex drag. This technical solution serves to
Indy CART racing car (Michael Andretti promote turbulent separation around a cylinder, driver). Aerodynamics has a strong impact on thus creating a drag crisis at lower wind speeds. car performance. Engineers find yet new ways to
Figure 6. Racing Cars
2.7. Wind Tunnel Testing
Wind tunnel testing is one of the most time Figure 8. Buildings Aerodynamics
consuming, yet effective tools for design and
research. Tunnel testing is now integrated with 3. Related Topics
sophisticated CFD methods to save development
costs. 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.
3.1. 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 Figure 10. Multi-element wing shows the cargo plane C.17 Globemaster with
high lift system in operation during a slow
Two boundary layers are confluent when they landing phase.
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.
3.3. 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. Figure 9. McDonnell Douglas C 17
Airfoil characteristics that have a strong effect 3.2. Flow Phenomena on the maximum lift coefficient are: camber and
thickness distributions, surface quality, leading
Flow phenomena of multi-element wings edge radius, trailing edge angle. CL max also include: wakes from upstream elements merging depends on the Reynolds number. At a fixed with fresh boundary layers on downstream Reynolds number, the operation on the above elements; flow separation in the cove regions; parameters must remove or delay the flow flow separation on the downstream elements, separation, and delay the pressure recovery on especially at high angles (landing the suction side, along with a number of other configurations); confluent boundary layers; high- details.
curvature wakes; high flow deflection; possible
supercritical flow in the upstream elements, see 3.4. 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 unpowered. The range of applications in aviation correlating wing CL max of a swept wing to the is discussed below. The data collected in the main geometric parameters of the high-lift figure below have been elaborated from Airbus system was derived at the Research Aeronautical research (Flaig and Hilbig, 1993). Performances Establishment (RAE, UK) in the late 1970s. of the C-17 and the YC-14 have been guessed. More recent work was done at McDonnell-
3.8. High-Lift Airfoils Douglas (Valarezo-Chin, 1994).
In order to obtain high lift from an airfoil the 3.5. Vortex Lift
designer must increase the area enclosed by the
The lift force from a wing can be augmented pressure coefficient (Cp), that is: the pressure on by appropriate manipulation of separation the lower side must be as high as possible vortices. Basically, this can be done in two ways: (pressure side), the pressure on the upper side with highly swept wings (delta wings) and must be as low as possible (suction side). The strakes. The longitudinal vortex has the effect of latter requirement is in fact the most difficult to shifting the stagnation point on the suction fulfill, because low pressure is created through surface of the wing (Pohlamus, 1971). high speed, and high speed triggers flow
separation. Flow separation can be limited at
high speed by turbulent transition.
3.9. 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 !
Figure 11. Vortex Lift
3.10. Multi-Element Airfoils
3.6. High-Lift Systems
Generally speaking, a multi-element airfoil
High lift can be produced by aerodynamic consists of a main wing and a number of design of single components, design of entire leading- and trailing-edge devices. The use of systems, integration of already existing systems, multi-element wings is a very effective method ad hoc technical solutions. The most important to increase the maximum lift of an aerodynamic methods are the following: system.
; High-lift wing design 4. Conclusions
; Multi-element lifting systems
; Boundary Layer control In brief, aerodynamic technology meets both our
; Propulsive Lift personal and social needs. It makes the daily life
; Other Technical Solutions easier by allowing us to connect to the world
around us. This technology is developing day by 3.7. Powered and Unpowered Systems day. In future it is probably more widespread in
our life. People are looking forward for the most
There is a broad classification among all high intelligent technology that would connect the lift systems: that is between powered and technology of aerodynamics.
Advanced Topics in Aerodynamics, “World of Aerodynamics”, http://www.aerodyn.org/
 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 AGARD LS-43, 1971.