Flow field measurements near a fighter model at high angles of attack

A quantitative study is reported on the low-speed flow environment of a ll32-scale model of an F-15 fighter aircraft at high angles of attack. Laser sheet flow visualization is used to o&erv< the various sources of vortex generation, and the evolution of these vortex flows. Surface tufts are used to observe the nature of flow separation on the vertical tails as angle of attack is varied. Laser velocimetry is used to quantify the velocity field in selected planes at the inlets, wings, and vertical tails. No concentrated vortex is observed near the vertical tails. The tails are seen to be immersed in a vortex flow of large radius. Flow separation on the outside surfaces of the vertical tails is complete by 22 degrees angle of attack. The flow on the inside surfaces of the tails is seen to remain largely attached. The flow angularity at the tails varies widely along the tail span, and with angle of attack. These results are found to be relatively insensitive to the precise modeling of inlet throughflow and inlet attitude. The largest velocity fluctuations are seen to be in the separated flow immediately above the wing surfaces. The outer regions of the vortex flow exhibit bimodal velocity histograms, indicating quasi-periodic intermittency. Ref. (1) summarizes an extensive pmgram aimed at understanding and modifying the buffeting loads encountered at the vertical tails of the F-15. Smoke flow visualization, surface tuft visualization, hot-wire anemometry, and surface pressure measurements were conducted over a range of angles of attack. Ressure measurements were made with both a rigid vertical tail and an aeroelastically-scaled one. Attempts were made using fences to deflect the unsteady vortex flow from the vertical tails. It was found that elastic effects greatly increased the fluctuating loads on the tails, hut that the frequency content of the pressure fluctuations on elastic tails was not substantially different from that on rigid tails. Recently, emphasis on the twin-tail buffeting problem has increased. This coincides with the advancement in computational and experimental capabilities. Sellers et a18 presented detailed measurements of the flowfield over a 3% scale model of a YF-17. They showed the dominance of the leading-edge extension (LEX) vortices in this flowfield, and atmbuted the large velocity fluctuations (root-mean-square values as high as 30% of the mean) at the vertical tails to the bursting of these vortices. Zimmerman et. al.9 extended the efforts of Ref. 1 to the FA-18 aircraft, and developed a method for predicting tail buffet loads, based on test data. They showed fiat buffet specua could be Over large ranges of velocitv. dn-c oressure. and model size with eood accuracy. I" Combat aircraft at high angles Of attack generate unsteady vortex flows which interact with their control surfaces in unexpected ways. The recent emergence~of the tail buffct and the lateral stability problem3 are examples of such phenomena. Controlled operation under such conditions requires the capability fist to understand and eventually to modify such flowfields. The approach to such problems has to be multi-faceted. Results from low-swed and high-speed wind and thai dind tuniel datacould be used to accirately predi'ct flight loads. The problem of exuaplating model-scale data to flight tests has mentlv been re-examined in the context of high-aneletunnel tests, as well as flight tests-must be used io validate analytical models and computational approaches4.5.6. These in Nm can then be used to investigate critical loading conditions, to he used in improving designs. This paper reports on the definition of the low-speed, high angle of attack aerodynamic environment of an P I 5 aircraft as part of such a multi-faceted program. v Wing and tail buffeting problems are quite old. It was realized many years ago7 that the vertical tail, especialIy on twintail configurations, would experience severe buffeting at high angles of attack. However, it is only recently that tighter aircraft have achieved the high thrust-to-wieght ratio required ID psfonn sustained maneuven under post-stall conditions. The time spent at high angles of attack under post-stall conditions has become a substantial fraction of the total flying time, and the issue of maintaining vibration-free control under these conditions has become crucial. 1: Assistant Professor. Member. AIAA 2. Professor. Senior Member, AIAA 3. Post-Doctoral Fellow. Member, AIAA 4. Graduate Research Assistant. Student Member, AIAA Copyright @ 19% American InSIilufe Of Aeronaulics and AstrOnautiCS, Inc. All rights reserved. of-attack aerobynamics. Ref.(lO) compares vorcx b k t locations on the FA-IS, measured by various invcstigstors in facilities that span the range from 1/48-scale models in water tunnels to actual flight tests. A surprisingly accurate corrclstion is demonstrated between model-scale tests and flight tests. The reasons for this are that at large angles of attack, the flow over sharpedged surfaces separates quite close to the leading edgcs, so that the resulting flow field becomes less sensitive to boundary layer Reynolds number effects. Also, most highangle-of-attack flight of combat aircraft occurs at rclatively low Mach numbers, due to limitstions on g-factors. Thus, the effects of compressibility, though present, are not primary. Ref.1 I , however, comparcs measurements under a variety of test conditions, over a wide range of Mach numher and Reynolds number. Some discrensncies renuin. which must be atuibuled to unwanted features of the test f a c i l i h or run condition,. Thus. the effects of surfsce roughness, free-sueam turbulence, tunnel confinement, and model Support interfcrcnce must be considered in each study T e d at defining the aerodynamic environment of an F-IS at high angles of attack. It i, pan of a cwordinated pmgram of low-specd and high-speed wind tunnel testing, flight testing. and computational studies aimed at developing systematic definitions of high-angle-of-attack design criteria for twin-tailed aircraft The F-IS is an aircraft on which a large amount of flight experience has been accumulated. However, data on the high-angle-of-attack flow environment, suitable for modeling of the flowfield, arc quite scarce. This E AND OBJECTlVES