The aerodynamics of forebodies undergoing longitudinal motion

A low-speed wind tnnnel investigation was conducted to investigate the aerodynamics of a forebody undergoing pitsch oscillations. Two different 3.5 tangent ogive forebodies (axisymmetric and chined cross-section) were pitched sinusoidally through an angle of attack range of O-90” and O-45” at a Reynolds number of 76,300. Force balance data was acquired to resolve the aerodynamic loads for static and dynamic testing. The data indicates that during the dynamic motion the developed aerodynamic loads are a function of the forebody cross-section, the upstroke/downstroke portion of the cycle, and the amplitude of the motion. NOMENCLATURE AOA angle of attack CN Normal Force Coefficient = FN/(qS) CY Side Force Coefficient = Fy/(qS) D forebody base diameter of tangent ogive DFT discrete Fourier transform f sinusoidal pitching frequency FN Normal Force FY Side Force k reduced frequency=2n;fL/U, +Permanently assigned to AF’RLIVAAA, WPAFB, OH; Member AIAA *Professor & Chair, Dept of Aerospace and Mechanical Engineering, University of Notre Dame; Fellow, AIAA This paper is declared a work ilf the U.S. Government and is not subject to copyright protection in the-united States. 1 L forebody length 9 freestream dynamic pressure phi model roll angle Re Reynolds number = pU,D& S forebody base area = nD214 U, freestream velocity X axial distance from model apex INTRODUCTION The study of an axisymmetric body in a crossflow has long been of interest to fluid dynamicists due to the nature of the separated leeward flow. Of Q particular interest to aerodynamicists is the fact that as a pointed body of revolution is inclined to the flow, asymmetric forces develop with increasing angle of attack (AOA). The development of the asymmetric forces is usually detrimental to the stability of a missile or aircraft that possesses an axisymmetric-type forebody configuration. l-4 Current military desires are pushing the performance envelopes of contemporary aircraft/missile designs to be super-maneuverable fly faster and turn quicker and tighter, or to improve point-and-shoot capabilities. In order for current and future air vehicles to efficiently fly with increasing performance, they must also be able to fly at greater angles of attack. Flight at higher angles of attack requires an understanding of the asymmetric vertical flow patterns emanating from the vehicles’ forebody configuration. In addition, forebody vortices that interact with LEX/strake and wing flows can cause premature “breakdown” of (c)l999 American Institute of Aeronautics & Astronautics . l these flows resulting in limited overall performance of the vehicles. As a result, extensive research into the flowfields and aerodynamic loads of forebody configurations (at high angles-of-attack) has been conducted.5.6 This research has also included techniques to control the forebody vertical flowfield. Forebody Flow The flow arou-nd a slender forebody is extremely complex as illustrated in Figure 1. At low angles of attack the flow separates around the forebody on the leeward side to form two symmetric vortices. As the angle of attack is increased further the forebody vortices become asymmetric. The asymmetric forebody vortices create a side force and yawing moment on the body. The magnitude of the yawing moment created by the asymmetry can be quite large. Figure 2 is a curve showing the typical yaw control capability due to the rudder for a high performance fighter aircraft. As the angle of attack is increased the rudder becomes less effective. On the other hand, the yaw moment due to the asymmetric forebody vortices increases with increasing angle of attack. Control of the forebody vortices can be used to provide adequate lateral/directional control for high angle of attack flight,! An overview of forebody vortex control can be found in Malcolm.7 Successful control of forebody vortices has been proven through pneumatic and mechanical techniques. Jet nozzles, when located in specific axial and circumferential locations and pointed properly, can control the trajectory of the vortices by manipulating the location of flow separation.*-” Investigations by Roes” havedetermined that synthetic-jet microblowing (zero-net-mass-flux) can manipulate high AOA forebody vortices as effectively as steady jets of equivalent mean mass flow rate. Flight test results on an F-18 have proven successful control authority can be achieved by actuating forebody mechanical flippers. In addition, it has been shown that the natural asymmetry can be manipulated by altering the apex region of the forebody geometry with surface perturbations.‘2 Adequately controlling the forebody flowfield at the high AOA ranges will aid the stability and maneuverability of the air vehicle, and lends itself to potentially reducing the radar signature by reducing the size of other empennage control surfaces. The emphasis on LO features has already driven the aircraft designs to posses chined forebody cross-sections. This chined feature fixes the circumferential location of flow separation on the forebody. An aerodynamic characteristic of the chined configuration is that it generates stronger leeward vertical flow structures resulting in greater aerodynamic lifting forces.13 The stronger flowfield also makes it more difficult to control. In order to gain a thorough understanding of the forebody flow physics and its implications to an air vehicle as it maneuvers at high AOA, it must be studied dynamically in addition to statically. Smith and Nunn14 . investigated an axisymmetric body undergoing uniform pitch-up rates from O-90”. They noted an increment of normal force developed due to the pitching motion and is closely coupled with the Reynolds number. However, for slower pitch rates, the normal force development is only slightly different than the static AOA case. Mange and Bragg15’16 have investigated a cliined forebody configuration as it pitches sinusoidally through moderately high AOA. The normal force developed sooner in the upstroke and unloaded quicker in the downstroke than in the static case for all AOA. By visualizing the displacement of the leeward vertical flow structures, the primary core was noticed to be “tighter” and closer to the body during the upstroke phase compared to the static case. It was also determined that the windward surface pressure distribution was just as important to the leeward surface pressure in determining the integrated aerodynamic~loads. As it is understood that the magnitude of control power from forebody control techniques varies with angle of attack, it is also realized that the effectiveness will vary when the forebody geometry is in motion. The findings of Smith and Nunn14 and Mange and Bragg15*” highlight the fact that the forebody aerodynamics are influenced by the dynamic motion of the body in both magnitude and