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AERODYNAMIC TECHNOLOGY Kemal GÜNDOĞAN Faculty of Aeronautics and Astronautics Astronautical Engineering firstname.lastname@example.org 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 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.
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