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aerodynamic technology


									                          AERODYNAMIC TECHNOLOGY

                                        Kemal GÜNDOĞAN
                                 Faculty of Aeronautics and Astronautics
                                        Astronautical Engineering

                                                      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
made of graphics, figures, tables, summaries,         flight. The plane is designed for take off and
reference to further reading. The number of           landing on short runways. High lift systems are
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
concerned with the interaction between bodies         Figure 1. Fixed-Wing Aircraft
and the atmosphere. Technological applications
include: General aviation (commercial, cargo,         2.2. Helicopter and VSTOL aircraft
and business aircraft); V/STOL vehicles
(helicopters, some military aircraft, tilt rotors);       The helicopter and some V/STOL aircraft
lighter-than-air vehicles (airships, balloons,        belong to the category of rotary-wing powered
aerostats); aerodynamic decelerators (parachutes,     aircraft. This is a class of vehicles on its own,
thrust reversal devices); road vehicles (passenger    with peculiar aerodynamic and control problems.
and racing cars, commercial vehicles, high speed      The first helicopters flew many years after the
trains); spacecraft, missiles and rockets, low- to    airplanes. Other V/STOL aircraft feature
high-speed flight (micro air vehicles to              complex lifting systems, such as vertical jets and
hypersonic waveriders), high altitude flight,         tilt rotors.
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 right.

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
pleasure journeys across the Atlantic or major         Figure 4. Aerodynamic Decelerators
disasters (or both). Either way, lighter-than-air
has captured the fantasy of many, not least            2.5. Wind Energy Systems
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
                                                       metereological condition.

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
Figure 5. Wind Energy Systems                     suspension bridges, and off-shore platforms. The
                                                  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
produce downforce.

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
shows the cargo plane C.17 Globemaster with         Figure 10. Multi-element wing
high lift system in operation during a slow
landing phase.                                         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.

                                                    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.
Figure 9. McDonnell Douglas C 17                    The two problems are somewhat dependent.
                                                    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
figure below.
                                                       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-
Douglas (Valarezo-Chin, 1994).                       3.8. High-Lift Airfoils

3.5. Vortex Lift                                        In order to obtain high lift from an airfoil the
                                                     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.
5. References
[1]Advanced Topics in Aerodynamics, “World of

[2] Hoerner SF. Fluid Dynamic Lift, Hoerner Fluid
Dynamics, 1965

[3] Clancy JC. Aerodynamics, John Wiley, New York,

[4] AGARD, High-Lift System Aerodynamics, AGARD
CP-515, Banff, Oct. 1993

[5] McCormick BW. Aerodynamics, Aeronautics and
Flight Mechanics, John Wiley, New York, 1994.

[6] Gratzer, LB. Analysis of Transport Applications for
High-Lift AGARD LS-43, 1971.

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