https://ntrs.nasa.gov/search.jsp?R=19980073173 2020-06-15T23:52:14+00:00Z CORE Metadata, citation and similar papers at core.ac.uk Provided by NASA Technical Reports Server /,,_ _-,#.:? h,1 AIAA 98-0519 Spatial Characteristics of the Unsteady Differential Pressures on 16% F/A-18 Vertical Tails Robert W. Moses, Ph.D. NASA Langley Research Center Hampton, VA Holt Ashley, Professor Emeritus Department of Aeronautics & Astronautics Stanford University Stanford, CA 36th Aerospace Sciences Meeting & Exhibit January 12-15, 1998 / Reno, NV For permission to copy or republish, contact the American Institute of Aeronautics and Astronautics 1801 Alexander Bell Drive, Suite 500, Reston, Virginia 20191-4344 AIAA-98-0519 SPATIAL CHARACTERISTICS OF THE UNSTEADY DIFFERENTIAL PRESSURES ON 16% F/A-18 VERTICAL TAILS Robert W. Moses, Ph.D. AIAA Member Aeroelasticity Branch NASA Langley Research Center Hampton, VA Holt Ashley, Professor Emeritus AIAA Honorary Fellow Department of Aeronautics & Astronautics Stanford University Stanford, CA Abstract Tunnel (TDT) at the NASA Langley Research Center as part of the ACROBAT (Actively Buffeting is an aeroelastic phenomenon which Controlled Response Of Buffet-Affected Tails) plagues high performance aircraft at high angles of program. Surface pressures were measured at attack. For the F/A-18 at high angles of attack, high angles of attack on flexible and rigid tails. vortices emanating from wing/fuselage leading Cross-correlation and cross-spectral analyses of edge extensions burst, immersing the vertical tails the pressure time histories indicate that the in their turbulent wake. The resulting buffeting of unsteady differential pressures are not fully the vertical tails is a concern from fatigue and correlated. In fact, the unsteady differential inspection points of view. pressures resemble a wave that travels along the tail. At constant angle of attack, the pressure Previous flight and wind-tunnel investigations to correlation varies with flight speed. determine the buffet loads on the tail did not provide a complete description of the spatial characteristics of the unsteady differential Introduction pressures. Consequently, the unsteady differential pressures were considered to be fully correlated in Buffeting is an aeroelastic phenomenon which the analyses of buffet and buffeting. The use of plagues high performance aircraft, especially those fully correlated pressures in estimating the with twin vertical tails. For aircraft of this type at generalized aerodynamic forces for the analysis of high angles of attack, vortices emanating from buffeting yielded responses that exceeded those wing/fuselage leading edge extensions burst, measured in flight and in the wind tunnel. immersing the vertical tails in their wake, as shown in Figure 1. The resulting buffeting of the vertical To learn more about the spatial characteristics of tails is a concern from fatigue and inspection the unsteady differential pressures, an available points of view. Previous wind-tunnel and flight 16%, sting-mounted, F-18 wind-tunnel model was tests were conducted to quantify the buffet loads modified and tested in the Transonic Dynamics on the vertical tails. Copyright © 1998 by the American Institute of The spectral aspects of the unsteady differential Aeronautics and Astronautics, Inc. No copyright is pressures on the vertical tail caused by a burst asserted in the United States under Title 17, U. S. LEX (leading edge extension) vortex are well Code. The U. S. Government has a royalty-free documented. 1 The results of Reference 1 illustrate license to exercise all rights under the copyright the variations of the power spectral densities and claimed herein for Governmental Purposes. All root mean square (rms) values of the differential other rights are reserved by the copyright owner. pressures with flight speed, angle of attack (AOA), 1 American Institute of Aeronautics and Astronautics dynamicpressure,and tail coordinateusingonly unsteadydifferentialpressuresare unclearfrom five differential pressure transducers.1 In examinationof these plots of the unsteady Reference1,theworstcasecondition,definedby pressureson each surface. On the inboard the highestrmsvaluesof differentialpressureat surfaceat 35 degreesangleof attackand Mach designlimit load,occursaround340psf and 32 0.6,thetimedelayfroma stationneartheleading degreesangleof attack.Otherfindingswerethat edge to a station near the trailing edge is the root meansquarevalueof the differential approximately0.0006seconds.Thesamplingrate pressurevarieslinearlywith dynamicpressure, is notclearlyreported;however,a timedelayof and that Strouhalscalingprovidesa meansfor 0.0006secondsindicatesthata highsamplingrate comparingmodelandflightdata.Also,thehighest isneededtocapturetheconvectionoftheflow. rms valuesoccurredat stationsclosestto the leadingedgewhilethelowestrmsvaluesoccurred nearthetrailingedgewitha gradualchangein rms iENCE _ / /_ ./ I valuesbetweenthesetworegionsof thetail. The ___,,.0,o,,/// / reasonsfor this gradualreductionin the rms INBOARD SURFACE / / _ / valueswithincreasein chordcoordinatewerenot explained.Duringthe investigation,theunsteady / ':¢ /--1 differential pressures were considered fully correlated(in phase)becausetheirresultsof the pressuresmeasuredat onlyfive stationsdid not indicateotherwise.Thesamplingrateusedin this testis notclearlyreported. I .. _ FENCE ON _/0'3 _ ---FENCE OFF / J 0.2 0.1 _ OUTBOARD SURFACE i I I (Z-- I Figure1. FlowVisualizationof LeadingEdge Figure 2. Peak Correlation Contours (msec) of the Extension(LEX)VortexBurst, Fin Unsteady Pressure Signals, 6% Rigid Tail, 30DegreesAngleofAttack M=0.6, 35 Degrees AOA (From Reference 2) Aftertheresearchof Reference1 andpriorto the researchreportedherein,wind-tunneltests were Because little information was known regarding conductedto investigatethespatialcharacteristics their spatial correlation, the differential pressures of the unsteadysurfacepressureson the tail.2 on the tail were assumed to be zero- or fully- Contourplotsof thetimedelayson eachsurface correlated during the computations of the wereconstructedusingcross-correlationanalyses generalized aerodynamic forces. 3-S These of the unsteadypressuresmeasuredon eachtail analyses did not estimate the buffeting accurately. surfaceof a6%rigidF/A-18modeltestedat Mach After further study, it was concluded that the issue 0.6. Asshownin Figure2 for35 degreesangleof of pressure correlation is the key to successful attack,the contoursfor each surfaceare quite buffeting prediction and should be the subject of different. The spatial characteristicsof the more research. 4-5 2 American Institute of Aeronautics and Astronautics 200_- indicate that the differential pressures acting on the Phase 01-- F'-'---, nll_lu.^A_[ tail are not in phase. However, the dependencies -200_"_ _ _'-_ It _"_qV_ of pressure correlation on flight conditions were not 100 clearly understood from these results. 10 1.0 To better understand the pressure correlation Magnitude 1 _-'I-i,_ during buffet, an available 16%, sting-mounted, F- .1000 :'.?.,."i 18 wind-tunnel model was modified and tested in .0100 -i!/ '"_'_ Axis for L the Transonic Dynamics Tunnel (TDT) at the .0010 _i '"_,."._.ICoherence---/ NASA Langley Research Center as part of the .0001 ,.i a _i!"_"".-./"."".r.--,....-..r.,...-.--' ACROBAT (Actively Controlled Response Of 100 0 20 4O 6O 8O Buffet-Affected Tails) program. 8 Surface Frequency, Hz pressures were measured for scaled flight a) 20 Degrees AOA conditions at high angles of attack on flexible and rigid tails. Pressure signals were sampled at 6538 Hz for approximately 30 seconds. Cross- Phase 2000_ _A_,_ /k Af],_A_,.[ _ correlation and time-averaged cross-spectral .200}__IJ_IV _ v'v"_JtV YW_ analyses 9 were performed for identifying any IO0 consistent spatial characteristics of the unsteady 1.0 differential pressures. The results of these analyses indicate that the unsteady differential Magnitude 1 pressures are not fully correlated. In fact, the •1000 ,o unsteady differential pressures resemble a wave .0100 that travels along the tail. .0010 : , I Axis for L -i ,,.,,.:, , [Coherence j .0001 The purpose of this paper is to present some wind- 0 20 40 60 80 100 tunnel results that illustrate the partial correlation of Frequency, Hz the unsteady differential buffet pressures on a rigid tail and a flexible tail of a 16% F/A-18 model. b) 32 Degrees AOA Figure 3. Cross-Spectral Density and Coherence Functions Between the Differential Pressures Near Wind-Tunnel Model and Tunnel Conditions the Leading-Edge Tip and the Trailing-Edge Tip, Full-Scale Tail, M=0.15, (From Reference 6) An existing 16% (also referred to as 1/6-scale), rigid, full-span model of the F/A-18 A/B aircraft was To learn more about the pressure correlation, a refurbished, and three flexible and two rigid vertical full-scale F/A-18 was tested at high angles of tails were fabricated. This model was then sting- attack at a maximum speed of Mach 0.15 in a wind mounted in the Transonic Dynamics Tunnel (TDT) tunnel. Plots of the magnitudes and phase delays at the NASA Langley Research Center, as shown of the unsteady differential pressures were in Figure 4, where it underwent a series of tests to constructed using cross-spectral analyses of the determine buffet flowfield characteristics and to unsteady pressures measured on each tail surface alleviate vertical tail buffeting using active at Mach 0.15. 6.7 As shown in Figure 3a for 20 controls, a degrees AOA, the phase is approximately negative 400