OVERVIEW ON RESEARCH IN WAKE VORTEX ENCOUNTER FLIGHT TESTS AND FLIGHT SIMULATIONS

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OVERVIEW ON RESEARCH IN WAKE VORTEX ENCOUNTER FLIGHT TESTS AND FLIGHT SIMULATIONS R. Luckner, Airbus Deutschland GmbH, Kreetslag 10, 21129 Hamburg, Germany 1.

1 OVERVIEW ON RESEARCH IN WAKE VORTEX ENCOUNTER FLIGHT TESTS AND FLIGHT SIMULATIONS R. Luckner, Airbus Deutschland GmbH, Kreetslag 10, Hamburg, Germany 1. INTRODUCTION This document gives an overview on flight tests and piloted flight simulations of wake vortex encounters (WVE). Extensive research has been carried out in the last 50 years. In most of the cases, investigations had the following aims: to characterise the trailing wake vortex development, to improve wake vortex and WVE models, to define hazard criteria or WVE boundaries or to validate wake alleviation methods. The overview shall serve as a common base for the work in Working Group 5 of Wakenet-2-Europe. It does not claim to be complete. Especially all research that has been performed in Russia and the former Soviet Union is not covered. 2. US RESEARCH Vernon Rossow gives an excellent overview on wake vortex research for transport aircraft during the last century in 4. His paper also includes flight tests and flight simulator investigations Flight testing 1 Flight-testing for research of wake vortex encounters started in the 1950s. Tests, using the B52, C-5A, CV-990, DC9 and U-3A as wake-generating aircraft and the F-104, U-3A, Lear-Jet-23, Cessna-210 as trailing aircraft, were conducted in the 50s and 60s [2]. Encounters were made possible by injecting smoke, vaporized oil or powder from the wake-generating aircraft into the vortices, so that they were visible for the pilot of the following aircraft. These tests were used to determine the hazard posed by a wake vortex on an encountering aircraft and finally resulted in the definition of safe separation distances. 1 Note: Only flight test in which the vortex of a generating aircraft was penetrated by anther aircraft are mentioned here. Flight tests where that the vortex is measured, e. g. with Lidar or tower fly by techniques are not discussed. Furthermore, discussions of flight tests that aim at (fuel-efficient) formation flight or refuelling are also excluded. Wakenet2_WG5_Literature-Overview_v1d4.doc 12-July /8

The Boeing Company initiated a study of large jet airplane wake vortices in mid-1969 [3]. The objective was to obtain a direct comparison between the vortices of a 747 and 707-320C.

The 737 was also flown into the wake near the ground. The Boeing tests were aimed at providing a direct comparison between the 747 and a representative aircraft of the current jet fleet, the 707-320.

A 747, which was equipped with three smoke generators, was probed by a T-37B that encountered the wakes at distances between 2.5 and 13 km at an altitude of 12,500ft.

2 The Boeing Company initiated a study of large jet airplane wake vortices in mid-1969 [3]. The objective was to obtain a direct comparison between the vortices of a 747 and C. A fully instrumented was used as the primary wake probing aircraft. Additional tests were conducted with a Boeing owned F-86 and NASA s CV-990. The 737 was also flown into the wake near the ground. The Boeing tests were aimed at providing a direct comparison between the 747 and a representative aircraft of the current jet fleet, the In eleven flights in summer 1974, Boeing and NASA investigated the potential of wake alleviation by inboard loading [4]. A 747, which was equipped with three smoke generators, was probed by a T-37B that encountered the wakes at distances between 2.5 and 13 km at an altitude of 12,500ft. The inboard flaps were deflected 30 and the outboard flaps 1 (conventional deflection is 30 ). The expected vortex alleviation was observed. However, it was sensitive to changes of lift coefficient and landing gear extension. In 1995 and 1997 NASA performed flight tests with a North-American Rockwell OV-10A behind a Lockheed Martin C130 towards characterization of trailing vortex wake development under different atmospheric conditions. A database of 230 WVEs was established. FIG. 1 OV-10 and the main components of its instrumentation system; Lockheed C-130, from [19] NASA performed free flight tunnel tests with a 737 model in its Langley s 36*60ft wind tunnel, [20] and [21]. FIG. 2 Free flight tunnel tests with 737 model in NASA Langley s 36*60ft wind tunnel, from [20] After the crash of USAir Flight 427, a Boeing on a scheduled flight from Chicago to Pittsburgh, on September 8, 1994, while manoeuvring to land at Pittsburgh International Airport, the NTSB conducted Wake Vortex Flight Test, at the FAA's Technical Center in Atlantic City, New Jersey in September, 1995 [ The objective was to collect real world data on the effects of a 737 entering the wake vortices of a 727. The tests used a highly instrumented USAir 737 and the FAA's 727, which had been equipped with Wakenet2_WG5_Literature-Overview_v1d4.doc 12-July /8

However, based upon the initial findings of the flight test, it was found that further refinement of the 737 engineering simulator and kinematic studies is required.

3 smoke generators. During the tests, over 160 vortex encounters were accomplished at distances of about four, three and two miles. Prior to the flight tests, simulator validation tests were performed with the 737. A thorough evaluation of all this data is not available. However, based upon the initial findings of the flight test, it was found that further refinement of the 737 engineering simulator and kinematic studies is required. The initial results of the wake vortex flight tests, the simulation validation tests and the kinematic studies were discussed at the NTSB hearing of USAir Flight 427. FIG. 3 Free flight tunnel tests with 737 model in NASA Langley s 36*60ft wind tunnel, from [20] There is a video clip available that shows the airplanes that participated in the tests - the vortex generating 727, the vortex-encountering 737, and the T-33 chase plane - as well as representative samples of flight test conditions [ It also documents wing-mounted smoke generators on the 727, and shows several test conditions, in which the 737 flies into the smoke-identified cores of the 727's wake vortices. The encounters are shown from the chase plane, the 737's cockpit camera, and the 737's tail-mounted camera Simulator tests and hazard criteria To obtain more information on the levels of acceptable vortex-induced motions, WVEs have been investigated safely and at modest costs in flight simulators at NASA Ames and Langley Research Centers. A large part of tests took place in the 1970s and 1980s [5] - [24]. One result was a clear relationship between the maximum bank angle experienced in a WVE during landing approach and the encounter altitude, see FIG. 4. FIG. 4: Vortex hazard boundaries from NASA simulation experiments from [10] Later, NASA tried to find acceptable WVE boundaries in batch simulation studies. Those studies confirmed the suitability of the maximum bank angle criterion and that criterion boundaries for maximum bank angle and control power are very similar [21]. NASA continued with piloted WVE simulations into the 90ties. Stewart, [16], describes a study that has been conducted in NASA VMS research flight simulator to provide a means to estimate the effects of different levels of wake turbulence on final approach. Fourteen airline pilots of various experience levels flew in total almost 1,000 WVEs. The cockpit layout was generic and the aircraft model represented a Boeing He concludes: The results of this study appear to be more conservative than the results of previous studies. Wakenet2_WG5_Literature-Overview_v1d4.doc 12-July /8

FIG. 5 NASA VMS flight simulator cockpit used for WVE simulations with a 737-100 model, from [16] In the late 90s, the FAA implemented WVE simulation capabilities in its 727-200 six degree-of-freedom

The objectives were to familiarize pilots with the characteristics and the hazard of WVEs, the signs of early wake vortex entry and to use the simulation as a research tool to study human factor

For example Lufthansa Flight Training has the WVE simulation available its 737-300/400/500 (CAE), A319 (CAE), 767-300ER (Thales) flight simulators.

4 FIG. 5 NASA VMS flight simulator cockpit used for WVE simulations with a model, from [16] In the late 90s, the FAA implemented WVE simulation capabilities in its six degree-of-freedom full-motion flight simulator (manufactured by CAE Electronics Ltd with a Rediffusion SP1 visual system). The objectives were to familiarize pilots with the characteristics and the hazard of WVEs, the signs of early wake vortex entry and to use the simulation as a research tool to study human factor issues related to pilot response to WVEs. The WVE software package was adapted to other aircraft types and has been implemented in airline training simulators. For example Lufthansa Flight Training has the WVE simulation available its /400/500 (CAE), A319 (CAE), ER (Thales) flight simulators. However as no training requirements exist, the WVE simulation capabilities have not been used for pilot training up to now. In all WVE models, a straight vortex pair is modelled. In 2004, Loucel and Crouch published preliminary results of a Boeing flight simulator study of WVEs with perturbed vortices. The encountering aircraft is a The strip method was developed in an earlier study [22]. For the early stage of the vortex break-up the vortices are modelled as wavy vortex filaments. For the late stages they are modelled as vortex rings, FIG. 6. The results show a significant reduction in the maximum bank angle and a continues reduction in the level of upsets as the vortices become increasingly wavy and ultimately break up into rings [23]. FIG. 6 Vortex rings, from [24] Wakenet2_WG5_Literature-Overview_v1d4.doc 12-July /8

distances between the vortex-generating aircraft and that which follows [25].

FIG. 7. Objective was to validate vortex models [27

5 3. EUROPEAN RESEARCH 3.1. Flight testing From January 2000 till March 2003, the EC founded the 5 th framework technology project S-Wake with the objective to develop tools for assessing appropriate wake vortex separation distances between the vortex-generating aircraft and that which follows [25]. Vortex encounters behind DLR s flying testbed VFW614-ATTAS were flown with two small flight test aircraft: a Dornier of the Technical University of Braunschweig [26] and a NLR s Cessna Citation FIG. 7. Objective was to validate vortex models [27]. FIG. 7: DLR s VFW614-ATTAS with smoke generator, NLR s Cessna Citation and TU Braunschweig s Do with flow measurement probes 3.2. Simulator tests and hazard criteria In Europe, the Technical University of Berlin (TUB) performed piloted wake vortex simulations in an Airbus A330 flight simulator during a national German research project [28]. In the European Commission (EC) project WAVENC, two flight simulators were prepared for wake encounters by Aerospatiale and the Dutch National Aerospace Laboratory (NLR) but only a limited number of tests were performed, ([29] and [30]). Also as part of S-WAKE, wake vortex encounter studies were performed with five different aircraft types encountering the vortex of a very large transport aircraft, ref FIG. 8. Hazard metrics were identified and hazard criteria developed [31]. Wakenet2_WG5_Literature-Overview_v1d4.doc 12-July /8

6 Aircraft / ICAO-Category Simulator Type Location Airbus HEAVY Fokker 100 MEDIUM VFW614-ATD MEDIUM Cessna Citation SMALL Dornier Do SMALL Certified Training Simulator (FAA level D) Motion system, visual system, side sticks, Fly-by-Wire flight control, Autopilot, additional research facilities Research Fight Simulator of NLR Motion system, visual system, control column, mech. flight control system Development Flight Simulator THOR of Airbus Visual system, generic Cockpit, side sticks, programmable displays, Fly-by-Wire flight control, no motion system Research Fight Simulator of NLR Motion system, visual system, control column, mech. flight control system Certified Training Simulator (JAR level B) Motion system, visual system, control column, mech. flight control system TU Berlin NLR, Amsterdam Airbus, Hamburg NLR, Amsterdam Simtec GmbH, Braunschweig FIG. 8: Flight simulators used in S-WAKE Based on the S-WAKE results, Airbus is performing further simulator studies in order to extend the database for hazard criteria. Currently DLR and TU Berlin as subcontractor are performing simulator tests in the A flight simulator of the ZFB at the Technical University of Berlin and in-flight simulations with DLR s flying test bed VFW614-ATTAS. Wakenet2_WG5_Literature-Overview_v1d4.doc 12-July /8

7 4. REFERENCES [1] Rossow V.J.: 'Lift-generated vortex wakes of subsonic transport aircraft'; in Progress in Aerospace Sciences 35 (1999), p , Pergamon, 1999 [2] Andrews W.H., Robinson G.H., Larson R.R.: 'Aircraft response to the trailing vortices generated by large jet transports'; NASA Aircraft Safety and Operating Problems, Vol 1 NASA SP-270, May 1971, p [3] Condit P.M. Tracy P.W.: 'Results of the Boeing company wake turbulence test programme', In Olsen J.H., Goldburg A., Rogers M. editors, Aircraft wake turbulence and its detection, 1 st Ed. New York: Plenum Press, 1971, p [4] Smith H.J. A flight test investigation of the rolling moment induced on a T37-B airplane in the wake of a B- 747 airplane, NASA TM-X-56031, NASA Flight Research Centre Edwards, April 1975 [5] Rossow V.J., Tinling B.E., 'Research on aircraft vortex-wake interactions to determine acceptable levels of wake intensity'; AIAA Journal of Aircraft, Vol 25, No. 4, p , 1988 [6] Nelson R.C.: 'The dynamic response of aircraft encountering aircraft wake turbulence'; Tech. Report AFFDL-TR-74-29, Air Force Flight Dynamics Laboratory, Wright Patterson Airforce Base, OH, June 1974 [7] Nelson R.C.: 'Dynamic behavior of an aircraft encountering aircraft wake turbulence'; AIAA Journal of Aircraft, Vol 13, No. 9, p , 1976 [8] Hastings Jr. R.C., Keyser Jr. G.L.: 'Simulated vortex encounters by a twin-engine commercial aircraft during final approach', SAE Technical Paper , May 1980 (also NASA TM-81782) [9] Johnson W.A., Teper G.L., Rediess H.A.: 'Study of control system effectiveness in alleviating wake upsets', AIAA Journal of Aircraft, Vol. 11, No. 3, p , August 1974 [10] Sammonds R.I., Stinnett Jr G.W., Larsen WE: 'Wake vortex encounter hazard criteria for two aircraft classes'; NASA TM X-73,113, June 1976 (also FAA-RD ) [11] Sammonds R.I., Stinnett Jr. G.W.: 'Hazard Criteria for Wake Vortex Encounters', NASA TM X , Moffet Field, CA, 1976 [12] Sammonds R.I., Stinnett Jr. G.W.: Criteria relating Wake Vortex Encounter Hazard to Aircraft Response ', AIAA Journal of Aircraft, Vol. 14, October 1977, pp [13] Tinling B.E.: 'Estimation of vortex-induced roll excursions based on flight and simulation results'. In: Hallock H.N. (editor), Proceedings of the Wake Vortices Conference, Report No. FAA-RD-77-68, US Dept. of Transportation, Match 15-17,1977, p11-22 [14] Tinling B.E.: 'Estimates of the effectiveness of automatic control in alleviating wake vortex induced roll excursions'; NASA TM-73,267, August 1977 [15] Tinling B.E.: 'Estimates of the effect of the delayed flap approach on the wake vortex hazard'; NASA TM- 73,179, September 1977 [16] Stewart E.C.: 'A piloted simulation study of wake turbulence on final approach'; AIAA Paper [17] Dillard A.E., Eberle W.R., Coffin A., Starko K.: Simulation of Aircraft Encounters with Aircraft Wake Vortices ; Final Report DOT/FAA/99AFS408-4, Oklahoma, February 1999 [18] Hallock, J.N., Eberle, W.R.: 'Aircraft Wake Vortices, a State-of-the-Art Review of the United States', FAA- RD-77-23, 1977 [19] Vicroy, D.D., Vijgen P.M., Reimer H.M., Gaqllegos J.L., Spalart P.R. : Recent NASA Wake-Vortex Flight Tests, Flow-Physics Database and Wake-Development Analysis, AIAA/SAE 1998 World Aviation Conference, AIAA/SAE Paper , September 1998 [20] Vicroy, D.D., Brandon, J., Greene G.: Characterizing the Hazard of Wake Vortex Encounter ; AIAA Paper , Reno, NV, January 1997 [21] Vicroy, D.D., Nguyen T.: A simulation study to develop an acceptable wake encounter boundary for a Airplane ; AIAA Paper , San Diego, CA, July 1996 Wakenet2_WG5_Literature-Overview_v1d4.doc 12-July /8

8 [22] Vasatka J,: The Dynamic Response of a Two-Engine, Commercial Jet Transport to Wake Vortex Encounters, AGARD FDP Symposium The Characterisation and Modification of Wakes from Lifting Surfaces in Fluid, AGARD CP-584, Trondheim, Norwey, May 1996 [23] Loucel R.E., Crouch J.D.: Flight-simulator study of airplane encounters with perturbed trailing vortices, AIAA Paper , 42 nd AIAA Aerospace Sciences Meeting and Exhibit, Reno Nevada, 5-8 January 2004 [24] Crouch J., Miller M., Spalart P.: Airplane Trailing Vortices, Boeing Magazin Aero 14, Boeing Commercial Airplanes Group, Seattle, 2001 [25] de Bruin A.C., Speijker L., Moet H., Krag B., Luckner R, Mason S.: S-Wake - Assessment of Wake Vortex Safety, Final Technical Report, NLR-CR , Amsterdam 2003 [26] Bauer T., Wirbelschleppeneinfluge im Flugversuchmessung der induzierten Geschwindigkeiten und Modellierung der Flugzeugreaktion (in German), German Aerospace Congress Stuttgart, October 2002 [27] Fischenberg D., Full-scale Wake Vortex Encounter Flight Tests, (in German), German Aerospace Congress 2002, Stuttgart, October 2002 [28] Kulwatz N., Kindel W., Dahms M.: 'Auswirkungen von Wirbelschleppen sehr großer Flugzeuge auf nachfolgende Flugzeuge' (in German); Presented at DGLR Congress, Berlin, Sept [29] Liot D.: 'Flight Simulation of Wake Vortex Encounter'; WAVENC Report Note R9, AM Airbus, Toulouse, [30] Liot D.: 'Wake vortex encounter model implementation and validation in the online (piloted) flight simulation environment'; WAVENC Technical Note TN-19, AM Airbus, Toulouse, [31] Luckner R., Höhne G., Fuhrmann, M.: Hazard Criteria for Wake Vortex Encounters during Approach Paper presented at the German Aerospace Congress 2003, Munich, November 2003 [32] Andrews W.H., Robinson, G.H., Larson, R.R.: Exploratory Flight Investigation of Aircraft Response to the Wing Vortex Wake Generated by Jet Transport Aircraft, NASA TN D-6655, Washington, D.C., March 1972 [33] Robinson G.H., Larson, R.R.: A Flight Evaluation of Methods for Predicting Vortex Wake Effects on Trailing Aircraft, NASA TN D-6904, Washington, D.C., November 1972 Wakenet2_WG5_Literature-Overview_v1d4.doc 12-July /8

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