ICAS 2002 Congress

CAPACITY AND WAKE VORTICES

J. N. Hallock US DOT Volpe National Transportation Systems Center Cambridge, MA

C. Tung US Army Aero-Flight Dynamics Directorate, NASA Ames Moffett Field, CA

S. Sampath US Army Research Laboratory, European Research Office London, UK

Keywords: wake vortices, capacity, vortex behavior

Abstract between using existing runways must be reduced. Aircraft wake vortices can pose a threat, The economic benefits of reduced wake especially in the terminal environment where turbulence separations at capacity-limited aircraft operate in close proximity. Vortex airports are substantial. Wake vortices limit in- standards preclude hazardous trail spacing between aircraft on approach, encounters, but are oftentimes very restrict the use of closely spaced parallel conservative. A key to increasing airport runways, and restrict departure and intersecting capacity is to know when vortices are not a operations. Research programs in the hazard so that aircraft can land and take off United States, Canada, and Europe have with closer intervals. A conference was held in endeavored to understand wake vortex behavior London to address the capacity and wake and to use this knowledge in alleviating airport vortices issue. This paper provides a brief capacity limitations caused by overly restrictive overview of the highlights of the oral separation standards. presentations. An international conference on Capacity and Wake Vortices was held September 12-14, 1 Introduction 2001, at the Imperial College of Science, Technology, and Medicine in London, UK. As the aviation community recovers from the The conference brought together researchers, events of September 11, 2001, airport capacity technologists, and the aviation community to limits will once more become a critical factor address the impact of aircraft wake vortices on affecting the growth and efficiency of air aviation system capacity. This paper will transportation. Many factors influence capacity summarize the highlights of the presentations -- climatology, runway configurations, traffic (there were no written papers) and comment on mix, etc. Ultimately, either the number of the progress of key topics since the conference. runways must be increased or the spacing Abstracts of the presentations can be found in

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the Wake Vortex Bibliography at locations. The eddy dissipation level in the http://www.volpe.dot.gov/wv. atmospheric turbulent boundary layer is a more stable quantity than the wind direction. He 2 Conference Presentations recommended that a warning of wake vortex circulation intensity above background should The subjects addressed at the conference be incorporated in the FAA Integrated included descriptions of the capacity problem Terminal Weather System. and strategies for the congested airports (3 Stefan Wolf (German Air Line Pilots sessions), recent investigations of vortices from Association) addressed issues of capacity vs. fixed-wing aircraft (2 sessions) and rotorcraft safety from a pilot’s view. Wake vortices are (3 sessions), progress in remote sensing of invisible to the pilot and a cause of uncertainty. wake vortices (1 session), modeling of vortex He wants to stay above and land beyond them. behavior (3 sessions), and consideration of He has to plan ahead and anticipate the wake possible methods for controlling vortex vortex hazard. He also needs to analyze the behavior (1 session). situation based upon his experience. Pilot input is required for scientific research, air traffic 2.1 Capacity and Strategies management providers, and aircraft manufacturers. In 1998, the International George Donohue (George Mason University) Federation of Air Line Pilots Associations opened the presentations with a discussion of issued a policy statement that included: the effect of wake-caused aircraft separation on • A general commitment to serious efforts air transportation capacity. The current allowing a safe reduction of the separation system seems overly conservative; a standard wake turbulence separation 15% increase in capacity could be achieved minima; without danger to safety. The current state-of- • Support for the results of international art sensors and vortex prediction and warning research activities contributing to the algorithms should be incorporated into Federal development of a wake vortex Aviation Administration (FAA) weather and air warning/avoidance system; traffic management decision support system • Need for airborne wake vortex detection software. However, accurate predictions of systems; and wake vortex locations and decay rates remain a • Need for ground-based wake vortex difficult problem, since the runway wind advisory and warning systems. direction variability is intrinsic to the Neil May (National Air Traffic Services randomness of the atmospheric turbulent Limited) reported a case study of runway boundary layer. It is a challenge to predict the capacity constraints at Heathrow Airport. The runway wind direction accurately out to 30 major constraints are: (1) in-bound and out- minutes – the time required to begin the bound traffic flow, (2) pilot performance (time spacing of arrival aircraft, which is to land and move out to a taxiway), (3) Air approximately 200 miles from the airport. Traffic Control procedures, (4) runway Based on theoretical calculations and the field configurations, (5) weather conditions, (6) results observed in the National Aeronautics airport layout, and (7) environment (12:00 to and Space Administration (NASA) Dallas-Fort 6:00 AM, no operations). Increasing the Worth experiments, the wake vortex decay capacity and reducing time delay requires: (1) rates can be predicted better than wake vortex smoothing the demand of in/out-bound traffic

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flow, (2) pilot education, (3) better controller ensures the aircraft’s flight corridor will be operational procedures, (4) providing clear of wake turbulence. infrastructures to reduce both pilots and air John Leverton (American Helicopter traffic controllers’ workload, and (5) Society International) examined the use of implementing a wake vortex separation vertical flight to improve airport capacity by distance change. using civil tilt rotor/helicopter to operate in the George Greene (FAA) presented the FAA’s unused airspace on the side of the airport. research strategy to enhance air traffic system Although the use of rotorcraft seems to be a capacity. FAA’s wake turbulence research real solution to reduce airport/airspace program (2001 to 2010) has been influenced by congestion, other factors like the cost, noise, the National Airspace System (NAS) passenger appeal, icing clearance, and wake Operational Evolution Plan need for vortex interaction need to be addressed. The assessment, the NASA near/long term research initial cost of the rotorcraft is high compared to program, the introduction of new large aircraft, the fixed-wing aircraft, but it can be and the international wake vortex research compensated by the slot value. The noise program. Special topics include near term generated by blade/vortex interaction is a major research on closely spaced parallel runways for issue to get community acceptance. Current San Francisco International Airport, research has made great progress in noise introduction of the simultaneous offset reduction. FAA uses 700 feet (ICAO uses 820 (SOIA), evolution of feet) for runway separation between rotorcraft NASA’s Aircraft Vortex Spacing System and fixed wing aircraft to avoid wake vortex (AVOSS) technology for operational use, wake interaction. More research is needed to better safety simulation, and international define the separation distance. He estimated cooperation. that the use of rotorcraft may free up to 20% of Jens Konopka (German Air Navigation the slots. Services) gave an overview on the concept of David Rutishauser (NASA Langley the Wake Vortex Warning System (WVWS) Research Center) presented an overview of used at Frankfurt/Main Airport on runways 25L AVOSS, as demonstrated at the Dallas/Ft. and 25R (separated by 518 meters). The Worth (DFW) Airport in July 2000. AVOSS is measurement of the prevailing meteorological to determine the spacing required to prevent conditions and subsequent forecast of wake wake vortex encounters based on the ambient vortices motion between the two glide slopes weather conditions. The vortices from 61% of are accomplished. A wind/temperature radar the 2301 at DFW exited the corridor (1.3 GHz) provides the necessary data to in less than 50 seconds. Only 8% led to the extend the forecast along the entire final situation where the vortex predictor approach path of the two glide slopes. An underestimated the time for the vortices to exit alternative approach is presently in field trials the corridor (the vortices can translate outside based on the concept of High Approach the corridor, descend below it, or decay to less System/Dual Threshold Operation than 90 m2/sec). Development of a research (HALS/DTOP) Project. The system ensures effort leading to a Wake Vortex Advisory that aircraft approaching runway 25L use a new System with limited capability (i.e., operate threshold displaced by 1500 meters (denoted only in certain weather conditions) is 26L) and thereby always stay 80 meters above underway. a leading aircraft on 25R. This 80-meter shift

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Anton de Bruin (NLR) described a Favorable comparisons of laboratory vortices probabilistic approach to evaluate the wake with aircraft vortices and numerical simulations vortex risk as a function of the separation included vortex migration, in both non- distances between aircraft landing on a single stratified and stratified environments, vortex runway. The methodology is being validated core size and circulation with vortex age, and using wake vortex incident data collected at the amount of turbulence in a vortex cell. Full- Heathrow Airport. The ICAO/JAR risk event scale data does not yet exist to compare to classification is used. Results of the initial laboratory studies of vortex separation with Monte Carlo simulations indicate that time and vortex lifetime in both quiescent and navigation performance is not too critical, the turbulent environments. primary weather variables are low turbulence Leo Veldhuis (Delft University of and stable stratification, and the largest risk Technology) is studying vortex structure and occurs at the runway threshold. decay using particle image velocimetry (PIV) in a 142-meter water tank and a 1:48-scale A- 2.2 Fixed-Wing Vortex Investigations 340-300 model. An underwater camera system on the tank bottom has a traversing system, that Klaus Huenecke (Airbus DASA) pointed out allows the field of view to move during the that the dimension of aircraft wakes and the measurements. Initially, vortices from the technological challenge of the new very large engine nacelles, the wing tip, and the winglet transport aircraft require both experimental and form and merge. The intense well-organized theoretical innovative techniques. Wind vortex persists for the entire observation time, tunnels, towing tanks, and the free-flight indicating slow decay. facility of ONERA are used to observe the Alex Corjon (Airbus) introduced wake vortex flow field over a long time period alleviation as a means to mitigate airport allowing its long-time behavior to be analyzed congestion. The intent is to prematurely age and measures of a premature wake decay to be vortices through the use of alleviation devices investigated. His studies with accurate models on the wake-generating aircraft. Modification show that engine exhaust reduces the maximum of certain parameters should lead to earlier vortex velocities, but not the total circulation, development of short-wavelength instabilities and delays rollup. Relevant aircraft factors are (elliptic) or long-wavelength instabilities the weight, wingspan, wing loading (Crow) causing more rapid vortex decay or lead distribution, engine position and spacing, to vortices which induce smaller rolling engine thrust, exhaust shape and temperature, moments on encountering aircraft. undercarriage, underbelly fairings, flap type, Will Graham (University of Cambridge) is flap setting, position of outboard flap edge, use conducting wind tunnel experiments on co- of ailerons, and the interaction of the exhaust rotating vortex pairs. Prior to merging, the with the flap and tip vortices. vortex cores increase in size while the Donald Delisi (NorthWest Research separation decreases. Unequal strength Associates) discussed his extensive laboratory vortices are seen to merge more quickly than studies and comparisons of his results with equal strength vortices. His investigations aircraft measurements. He underscored the suggest that three-dimensional effects are not care needed in interpreting low-Reynolds strong, so vortex merging can be simulated number studies and extrapolating those results adequately using two-dimensional flow to the high-Reynolds number aircraft case. computations.

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Thomas Leweke (University of Marseilles), Oriented Schlieren) that enables field recording on the other hand, examined the short-wave of density gradients. This method has been instabilities of both co-rotating and counter- used successfully in observing vortex wakes of rotating vortices in a water tank. At Reynolds a helicopter in hover. numbers above 2000, a short-wave three- Leo Dadone (Boeing) addressed flow field dimensional instability develops due to an issues relevant to airport operations when elliptic instability of the vortex cores; a generic rotorcraft and fixed-wing aircraft share air feature of strained vertical flows. Counter- space. Rotorcraft wake analysis has played a rotating vortices develop a strong exchange of major role in the design of high performance, fluid thereby decreasing the average low vibration characteristics, and low noise circulation. Co-rotating vortices merge sooner. vehicles. Dealing with airport environments, it High Reynolds numbers lead to an increased requires different considerations that fall level of turbulence in and around the vortices. outside of the conventional wake modeling in Roland Stuff (DLR) reviewed vortex rotor design. It involves new areas to be breakdown and instabilities to introduce the explored. For example, the far wake structure Rayleigh-Ludwieg instability that occurs when of a rotorcraft has not been systematically destabilizing centrifugal forces dominate studied. restoring pressure forces. He contends that the Lyle Long (Penn State University) Rayleigh-Ludwieg instability could lead to new reminded the audience that according to passive measures to alleviate the vortex hazard. Moore’s law computing power doubles every Since the stability criteria are derived from the 18 months. He then reported his vision of radial gradients of the swirl and axial velocity using computer power to improve airport components, three-component PIV or capacity. Since the prediction of rotorcraft stereoscopic PIV is needed to study the vortex wakes is computationally intensive, it phenomenon. will take many years to reach real-time application. He believes by year 2022 2.3 Rotorcraft Vortex Investigations computing power will enable a real-time prediction of a rotorcraft far wake and a virtual Gird Meier (DLR Institute of Aerodynamics reality display in the cockpit. and Flow Technology) reviewed the Lakshmir Sankar (Georgia Institute of mechanisms of noise generation for a typical Technology) presented recent progress in helicopter. Two major noise sources are modeling rotor wakes in hover and forward generated from the rotor blades. One is called flight. The major difficulty in predicting rotor the blade/vortex interaction (BVI) noise and the wakes is the numerical diffusion of the wake other the high-speed impulsive (HIS) noise. vortex. Finer grids may reduce the vorticity The first occurs mostly at low forward speed diffusion at the expense of computing time and during the landing. The second occurs on the memory. A hybrid method is used to separate high speed advancing side where shock waves the viscous zone near the blade surface and the are formed near the blade tip. It is a challenge near wake. A Lagrangean wake approach was to predict the BVI noise accurately since the proposed and implemented to capture the wake geometry is difficult to predict accurately. effects of a tip vortex once it leaves the viscous The HIS noise can be predicted accurately, in zone; it convects without diffusion in the general. As for tracking the wake vortex, he inviscid zone and to the far field. The proposed a new technique BOS (Background numerical results compared well with test data

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obtained from model rotor tests. motion down stream. The CHARM code uses Frank Caradonna (US Army fast multi-pole methods and tracks the Aeroflightdyanmics Directorate) pointed out development of a rotary-wing wake and its long the new challenges in analytical and time motion. Preliminary results from these experimental rotor wake aerodynamics. Rotor codes show promise. However, additional wakes have properly constituted the largest wake measurements are needed. single area of experimental study over the Gordon Leishman (University of Maryland) years, yet new phenomena are always being presented a 3-component laser doppler discovered. He presented the test results of velocimeter (LDV) measurement of vortex vortex pairing during hover and low speed wake formation, initial structure, and viscous climb. This vortex pairing has been observed evolution of the vortex core of a model rotor in before, but has not been studied in detail. On hover. The flow visualization of the blade tip the analytical side of the rotor wake study, the vortex and its trailing sheet was obtained by prediction of rotor wakes using Computational using a laser light-sheet technique. The vortex Fluid Dynamics (CFD) requires a fine grid to core size is as small as 3% of the chord near the avoid numerical dissipation. This renders CFD trailing edge of the blade and grows impractical for routine calculations. However, asymptotically. The axial velocity deficit two new concepts, vorticity embedding and inside the vortex core has the same magnitude vorticity confinement, reduce the need for a as the swirl velocity initially and diminishes as fine grid size and make CFD more practical for it ages. routine use. Roderick Galbraith (University of Roger Strawn (US Army Glasgow) described experimental work on Aeroflightdynamics Directorate) described orthogonal interaction of a rotor tip vortex with CFD results using 64 million grid-point a fixed airfoil that simulates a tail rotor. A computations for a UH-60A model rotor in particle image velocimetry (PIV) system was hover. It required 138 hours on 112 SGI Origin used to study the flow field of the tip vortex 2000 processors in order to reach a steady-state before and after the interaction of the airfoil. solution. The prediction compares well with On one side of the airfoil, on which the axial experimental measurements. Again, it flow of the vortex is toward the airfoil, a demonstrated that the CFD modeling of rotor compressive pressure pulse forms on the airfoil wakes remains a challenge. surface first. Then it disappears and develops Alan Bilanin (Continuum Dynamics) into a suction wave as the vortex convects offered a different point of view to assess across the chord. On the other side of the aircraft and rotorcraft wake flow fields for airfoil, there is increased suction throughout the wake hazard applications. To meet key convective process. challenges, a family of high-fidelity vortex Kenneth McAlister (US Army wake modeling methods must be developed for Aeroflightdyanmics Directorate) presented his eventual incorporation into both off-line PIV study of the change of wake vortex assessment tools and pilot-in-the-loop structure with/without a small vortex generator simulation. In particular, UNIWAKE and placed near the tip of a hovering rotor blade. CHARM codes were mentioned. The The most dramatic change occurred in the UNIWAKE code captures the wake roll-up of turbulence generator case that resulted in a 65% fixed-wing aircraft including the turbulent reduction in maximum vorticity and a core size mixing with engine exhaust and wake vortex that nearly doubled. However, the rotor

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performance suffers an 18% increase in torque. Friedrich Koepp (DLR Institute of Markus Raffel (DLR Institute for Atmospheric Physics) reported on a pulsed Aerodynamics and Flow Technology) reported Doppler lidar system that has been developed on the development of the PIV system and its and tested under Project MFLAME application to many different kinds of unsteady (Multifunction Laser Atmospheric Measuring flow fields. Two experiments related to rotor Equipment). The feasibility test was conducted vortex wakes were performed and presented. at Toulouse-Blagnac Airport. The detection of One is a detailed investigation of vortex vortices generated by a large variety of aircraft formation from helicopter tips in a large wind has been successfully demonstrated up to a tunnel. The second concerns the flow field range of more than 2 km. This system can be around a propeller. integrated into an aircraft forward-looking system. However, fog may degrade the 2.4 Remote Sensing of Vortices performance of a pulsed lidar. Richard Heinrichs (MIT Lincoln Michael Harris (QinetiQ) presented the field Laboratory) presented the wake vortex trial measurements of aircraft wakes using lidar measurements of the XV-15 tilt rotor aircraft at several airports in Europe. A continuous with a CW coherent laser radar. This CW wave (CW) CO2 laser was used to scan coherent laser radar was developed as a part of vertically with a given angle. Lidar measures the NASA AVOSS program. The truck- the line-of-sight velocity only, i.e., one mounted device can scan a large area (400 x 80 component. The focus of lidar at wake vortices feet) via a continuous sweep focus to locate the is critical initially; it requires an educated vortex and then fine scan to track the vortex. guess. Once tracked, it can get vortex data plus The measurements of the XV-15 and Bell 205B atmospheric turbulence for more than 60 wake have shown airplane-like wake structure, seconds. The circulation strength and decay but the circulation as a function of distance rate of the vortex can be estimated, but the seems different from those of an aircraft. level of uncertainty needs to be analyzed. A Humidity seems to enhance the core size and better approach is to use multiple lidars to track velocity distribution. the wake vortices, but the time delay between two lidars is an issue. A wing-mounted lidar 2.5 Vortex Behavior Modeling on a chase plane that can avoid ground turbulence and measure a wide range of vortex Thomas Gerz (DLR Institute of Atmospheric age was also discussed. Physics) described the requirements for Stephen Hannon (Coherent Technologies) reduced aircraft separation systems based on reported the functionality of the Wind Tracer, a atmospheric effects on wake vortex behavior unique pulsed infrared Doppler radar that and decay. He presented early results from the generates high resolution, three-dimensional wake vortex forecasting and measuring project distributions of wind data. It was used at San WakeOp. The vortices of a VFW-614 aircraft Francisco Airport to track the wake vortex were measured and predicted for various positions, circulation, sink rate, and decay rate. aircraft configurations and meteorological It can see vortices descending and rebounding conditions. from a shear layer. A robust autonomous, Fred Proctor (NASA Langley Research stand-alone operational system may be matured Center) presented analytical results for wake within the next five years. vortex transport and decay from the perspective

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of parallel runways. To predict vortex wavelength Crow instability as well as the transport and demise for parallel runway short-wavelength elliptic instability. Certain systems vertical profiles of the ambient wing designs might cause these instabilities and crosswind and the intensity of the ambient subsequently accelerate vortex decay. turbulence are used as inputs to the Terminal Jeffrey Crouch (Boeing) reported on Area Simulation System (TASS). Strong Boeing’s efforts to force the breakup of crosswinds and weak ambient turbulence are vortices at a distance less than current the conditions for which the TASS yields the separation distances. In their active system, longest lateral vortex motion. The simulation aircraft control surfaces force flaps-down also indicated that, in ground effect vortices vortex instabilities leading to formation of exist for less than 6 non-dimensional time vortex rings. Even if successful, there are still units. a number of issues to consider such as ride Robert Robins (NorthWest Research quality, control authority, when in their Associates) compared the results from two breakup are vortices benign, effectiveness near simulations of vortex evolution. One the ground, and how to demonstrate system simulation explicitly solved the equations of reliability so that separation rules can be fluid motion while the other employed changed. equations derived from a heuristic analysis of Henri Moet (CERFACS) described his vortex evolution (a best-fit model derived from study of the effect of three-dimensional AVOSS measurements). Comparisons with ambient turbulence on vortex decay using full-scale data suggest that useful predictions of temporal large eddy simulations. Turbulence wake vortex development are possible. affects the vortex circulations through various Florent Laporte (CERFACS) discussed mechanisms: turbulent diffusion, creation of numerical simulations of large aircraft wakes in azimuthal structures of vorticity, exchange of the near to extended near field (vortex rollup vorticity between the vortices and the ambient complete). The first simulation was an inviscid turbulence, occurrence of large deformations, computation using a structured Navier-Stokes and the onset of instabilities. Current efforts solver. The effects of various computational are to include atmospheric stratification effects. meshes were noted. The second approach used Gregoire Winckelmans (Catholic time-averaged experimental data as input to University of Louvain) described the Vortex steady and unsteady simulations. Large Eddy Forecast System (VFS) sponsored by Transport Simulations and steady Reynolds-averaged Canada and developed by an international Navier Stokes computations were compared team. The VFS is a wake vortex prediction and confirmed the existence of laminar vortex system based on the method of discrete cores surrounded by turbulent fluctuations. vortices. VFS starts with an initial near wake Florent Laporte (CERFACS), in a second and then models wake transport and decay, paper, examined the instabilities of vortex ground effects, non-uniform wind shear effects, models in the near and far fields. In the near and stratification effects. The system was field, flap and tip vortices merge into a single tested using vortex data collected at Memphis stable vortex. However, it is possible, and DFW, and the results compared well with according to the simulations, to develop a those obtained by the AVOSS predictor model. three-dimensional elliptic instability that would Laurent Jacquin (ONERA) examined the lead to unstable merging and a turbulent vortex. stability properties and unsteadiness of wake In the far field, simulations show the long- vortices. Promoting long-wave instabilities are

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the most promising means to render wakes growing instability between unequal strength, harmless. He discussed the properties of counter-rotating vortex pairs in the wakes of various vortex-pair configurations to find the airfoils with outboard triangular flaps. Using optimal vortex arrangements leading to flow visualization and PIV data, large significant amplification of initial decreases in the rolling moment and downwash perturbations. Short-wave instabilities affect on a following wing were measured for the the merger of vortices. He showed that basic triangular-flapped wings compared to flows involving only one length scale (e.g., rectangular-flapped wings. Rankine and Lamb-Oseen vortices) that are Peter Bearman (Imperial College) has often used to characterize short-wave conducted experiments on wake vortex control instabilities are poorly representative of real in a water tank using flow visualization and a wake vortices. delta wing. Video sequences showed the long- Abraham Elsenaar (NLR) used analysis to wavelength or Crow instability develop. optimize the upper limit of the rolling moment Perturbing the flow speed at the Crow induced by a vortex pair. The goal is to wavelength caused the vortices to break down identify possible ways to reduce wake vortex closer to the wing. But, if the wavelength is strength through aircraft design. He found that smaller or larger than the Crow value by some the distance between the vortices and the critical amount, the breakdown moved farther Oswald efficiency factor (induced drag factor) downstream. Thus, if artificial disturbances are are key factors in the rolling moment for an to be introduced to control the wake vortex aircraft that flies into a vortex center. An hazard, it is critical that the wavelength itself be inboard-loaded wing with a low induced drag carefully controlled. led to the smallest aircraft separation distances. Patricia Coton (ONERA) described the ONERA test facility that uses a catapult to 2.6 Controlling Vortex Behavior propel a scale model. The model flies freely thus eliminating any wall or mounting Philippe Spalart (Boeing) discussed reducing interference. In the new facility, distances up the wake-vortex hazard by modifying the wing to 200 wingspans downstream of the model are of the vortex generator. Flow-control designs possible, thus covering current separation can be classified as active or passive. An distances during the approach and landing active design, such as the Boeing system, phase. The flow patterns behind the model can involves varying the wing configuration be recorded by laser tomoscopy and PIV. cyclically to cause an instability, thus leading to an early wake collapse. A passive design 3 Epilogue uses a fixed device, like a winglet, to produce a weaker wake (wake alleviation) thus making There are many wake vortex projects underway vortex encounters less violent. Active or in Europe and the United States. Knowledge passive, any design changes will pose extreme about wake vortex behavior is increasing, as challenges for testing and certification. the presentations at the conference attest. Ömer Savaş (University of California, However, it will take time for the knowledge to Berkeley) described towing-tank experiments be reduced to working systems. New runways on vortex wakes of wings with outboard flaps. and new or modified procedures, such as SOIA The intent was to generate control vortices for and HALS/DTOP, are the likely near-term wake alleviation. He observed a rapidly capacity enhancing means.

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Aircraft separation standards are conservative most of the time. Capacity improvements of the order of 15% are possible as shown by AVOSS. The present efforts to move AVOSS from the research mode to an operational mode represent the likely mid-term capacity solution. Most of the conference dealt with potential long-term capacity-enhancing ideas. The research programs in wind tunnels, water tanks, and full-scale flight tests are to characterize aircraft wake vortices. Comprehensive analytical models of vortex evolution are being developed and refined. Means to cause an earlier onset of vortex instabilities (elliptic, Crow) are sought via analytical and experimental techniques. Perhaps some of the techniques will become the basis for active or passive vortex control systems on aircraft, which have their own testing and certification challenges. Rotorcraft research is also in the long-term arena. The conference examined the increased use of rotorcraft to free up more slots for fixed- wing aircraft. But, the noise from the blade/vortex interaction is a continuing problem. Capacity is a problem today at a few airports. Scenarios which project a doubling of air traffic over the next 20 years suggest that the problem will only get worse.

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