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Coping with Wake Vortex ICAS 2002 CONGRESS COPING WITH WAKE VORTEX Klaus-Uwe Hahn Institute of Flight Research, German Aerospace Center (DLR) Lilienthalplatz 7, 38108 Braunschweig, Germany Keywords: wake vortex, encounter, control, safety, separation distance Abstract MTOW maximum take-off weight nm nautical mile The well known phenomenon of wake vortices P2P probabilistic two phase model behind a lift producing wing can adversely af- r radius / radial co-ordinate fect flight safety if encountered by trailing air- ROT runway occupation time craft. The strength of the vortices increases with s spanwise load factor SD separation distance the weight of the vortex generating aircraft. t time co-ordinate Therefore, weight dependent separation dis- T thrust, time constant tances have been established for approach and v lateral velocity component landing to avoid dangerous wake vortex en- V airspeed (no index), velocity counters. These proven separation distances w vertical velocity component Wweight have to be investigated carefully with the aim to α angle of attack discover possible margins to be used to solve β angle of sideslip the current and future capacity problems at air- χ flight path azimuth ports. This demand forms the need for more γ flight path angle flexible separation procedures taking into ac- η elevator deflection count the actual weather situation and the pa- ν effective viscosity ρ air density rameters of the individual aircraft pairing. ξ Separation reductions can be achieved by wake aileron deflection ζ rudder deflection vortex avoidance using prediction systems for ∆ difference the wake vortex development and movement. Φ bank angle The hazardous areas around a wake vortex can Θ pitch angle be approximated by the simple geometry of a ψ azimuth rectangle or an ellipse according to the aircraft indices pairing, the actual weather dependent decay ccore F follower aircraft and the available control power for vortex com- FB feedback pensation. Using specific controllers vortices FF feed-forward can be passed through even if the required g geodetic control power temporarily exceeds the available K kinetic (refers to flight path) capacity. Aircraft equipped with such a control- L leading/generator aircraft req required ler might follow another aircraft closer than sink indicates downwards motion authorized by the current separation distances. t tangential Wwind WV wake vortex List of Symbols and Abbreviations WVL wake vortex line AVOSS Aircraft Wake VOrtex Spacing System • time derivative bwing span ' characteristic time scale DoF degrees of freedom * denotes normalized parameters DLR German Aerospace Center 0 initial value G aircraft weight 1 diffusion phase IMC instrumental meteorological conditions 2 rapid decay phase 732.1 K.-U. Hahn 1 Introduction Presuming approaches on a single runway ψ ≈ ψ ≈ ψ As a reaction to the generation of lift, two where small encounter angles L WVL counter rotating vortices arise at the wing tips of can be assumed, normally the situation of a par- an aircraft. This phenomenon is well known as allel-like encounter will prevail. The investi- wake vortex and can adversely affect flight gated scenario is sketched in Fig.1. safety of trailing aircraft. This is particularly true if a light aircraft follows a heavy one since 2 Wake Vortex Spacing of Aircraft the strength of the generated vortices depend on the required lift correspondingly to the aircraft 2.1 Fixed Aircraft Separation Distances weight. To avoid dangerous vortex encounters To guarantee safe flight operation, currently the aircraft staggering for approach and landing fixed mass dependent separation distances have is restricted. The allowed minimum separation been established for aircraft lined up for is a limiting factor for airport capacities. The approach and landing. Various matrices of current and the expected capacity problems at minimum separation distances from different airports with a high volume of traffic together organizations are available. Tab.1 gives the with future large and very heavy aircraft form mass classification of aircraft and in Tab.2 the the need for a revision of the existing rigid sepa- corresponding IMC vortex separations after ration concepts. This demand has led to a new FAA are listed. Such fixed matrices can be interest and comprehensive investigations into easily applied to flight operation. But they the nature of the wake vortex phenomenon. suffer from the lack of flexibility to actual The following investigation and the pre- ambient weather situation. sented simulation results will focus on the com- 2.2 Dynamic Aircraft Separation bination of a small aircraft following behind a To reduce the current rigid separations research large one. For this encounter situation real flight activities [2, 3, 4] have been initiated for the test data have been available for validation. The development of concepts for dynamic wake drawn conclusions are of general importance vortex spacing systems. The most mature since the underlying physics are true for any system is the AVOSS for which already per- aircraft combinations. formance validations are carried out [5]. All ac- The effects of wake vortices on aircraft can tivities finally aim for the increase of the aircraft be very different. Encounters perpendicular to throughput to improve airport capacity. The the vortex axis will lead to substantial vertical principle of dynamic spacing is based on the load factor variations by inducing rapid and consideration of all relevant influences: large angle of attack variations all over the wing - weather forecast improved and updated by [1] comparable to strong gusts. Another situa- - sensing the ambient weather conditions used tion is present if the flight path of the encoun- together with tering aircraft runs parallel to the vortex axis. - data of aircraft pairing to Then the vortex flow will induce varying angles - predict the vortex behavior to provide a of attack along the wing producing strong roll- - non-hazardous vortex or vortex free space ing moments which can create significant roll rates resulting in large bank angles. Between along the approach path. To overcome the un- these two cases a broad variety of encounter certainties in the predictions adequate margins situations exists. In any case the wake vortex have to be introduced. The required time to produces an adverse effect on aircraft motion guarantee a safe approach for a following air- which is the reason for the today’s encounter craft (due to vortex demise or vortex drifting avoidance policy. The pilots position concern- away) defines the minimum separation distance. ing wake vortex is formulated by the statement To minimize the required separation actions can that no aircraft should encounter a wake vortex be taken to push the distances to their (safe) by intention whatever its strength is. limits: 732.2 COPING WITH WAKE VORTEX - measures to reduce vortex strength have been performed in the frame of DLR’s - passive or active devices to speed up the “Wirbelschleppe” project [3] by means of a vortex decay realistic and detailed simulation program. - more precise prediction of vortex behavior - minimization of the non-hazardous vortex or 3 Data Gathering from Flight Tests vortex free space - active aircraft encounter control For the validation of a reliable wake vortex en- counter simulation real flight test data have been Contributions have to come from aerodynamics, used. The respective flight tests have been per- fluid dynamics and meteorology aiming for low formed in the frame of the European S-Wake vortex design, accelerated decay, weather and project within flight experiments especially de- vortex behavior prediction. Another potential is signed for wake vortex encounter analyses [9]. available coming from aircraft control. In the following a brief description of the tests The use of an autopilot will improve the for the data gathering will be given. accuracy of flight path tracking [6]. A precise As a vortex generating aircraft DLR’s twin flight path tracking of the vortex generating air- engine jet VFW614 ATTAS (Advanced Tech- craft reduces the scatter in position deviations nologies Testing Aircraft System) was used around the nominal approach path and thus re- (Fig. 2). Having a MTOW of about 20 tons this duces the extension of the space in which the aircraft has to be classified as “LARGE” follow- generation of vortices have to be expected (→ ing the classification of Tab.1. The test aircraft reduced uncertainty of initial vortex position). A was equipped with a smoke generator on its left more precise flight path tracking of the follow- wing (Fig.3) to mark the vortex at the tip. This ing aircraft shrinks the extension of the space made it easy for the follower aircraft to en- which will be probably penetrated due to excur- counter the vortex by intention. sions from nominal path (→ reduced space of The encountering aircraft was the Do 128 interest of vortex contamination). Therefore, it test aircraft of the Technical University of can be stated that the application of automatic Braunschweig powered by two turbo-prop en- approach will help to minimize two sources of gines (Fig.4). With a maximum MTOW of probabilistic uncertainties. about 4.35 tons the aircraft has to be classified Automatic control can also improve the as “SMALL”. This aircraft was ideally suited aircraft response to a wake vortex encounter for the encounter trials providing very high ac- which might happen unintended. Accepting that curacy air data probes at 4 different stations al- the
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