Investigation On Aerodynamic Characteristics Of Internal Weapons Bay System

Compared with the external carriage, the aircraft with internal carriage of storeshas lower drag and radar cross section, so the new generation fighter is tended todesign with internal weapons-bay. But there are also some new aerodynamic andaero-acoustic issues accompanied with the internal weapons-bay. The dynamic load isvery big during a weapon is releasing, and which may effects on the fly performance ofthe plane. The disadvantage separation characteristics of the store may result incollision with the parent aircraft. The strong noise generated by the unsteady pressure,density and velocity around the weapons-bay may result in structure vibration, fatigueand safety of the aircraft at high speed for certain weapons-bay geometries. The doorsof the weapons-bay also may vibrate, distort and even been broken when they aremoving rapidly in the complicated weapons-bay flow field. Therefore, theinvestigation on aerodynamic characteristics of internal weapons-bay system is veryimportant.In this paper, investigations on aerodynamic characteristics of internalweapons-bay system were conducted in high speed wind tunnel in detail. Steadypressure measurement, fluctuation pressure measurement and grid measurement wereapplied in the study. The study is focused on the steady flow and aero-acousticcharacteristics of the weapons-bay, weapon’s separation characteristics and thedynamic characteristics of the doors’ movement. Steady pressure distributions andacoustic characteristics vary with different parameters of the weapons-bay werestudied in this paper. The mechanism of the cavity aerodynamic and aero-acousticcharacteristics was also discussed in the paper. Some flow control methods wereconducted to reduce the weapons-bay’s pressure gradient and suppress its strongaero-acoustic noise. The separation characteristics of an internal weapon were obtainedthrough grid measurement and the effects of some flow control methods on itsseparation characteristics were also evaluated. The dynamic pressure loads during theweapons-bay doors’ opening/closing processing were measured. The effect of door’smovement on weapons-bay’s flow was studied and the distributing and average loadson weapons-bay’s doors were discussed.The study shows that the type of weapons-bay’s flow based on the cavity floorsteady pressure distribution is mainly depended on the cavity length to depth ratio(L/D), Mach number (Ma) and the scaled boundary layer thickness (δ/D). Theweapons-bay flow transfers to open flow with the increasing of Ma or δ/D, but it willdevelop from open flow to closed flow with the increasing of L/D. In the author’sopinion, separation wake exists both close to the leading and rear wall of the cavity when the L/D is big enough, and the leading separation wake becomes dominantgradually with the decrease of L/D, while the rear separation wake becomes weakerand weaker and finally merges into the leading separation wake. The cavity floorpressure distribution varies with development of the leading and rear separation wakeand the cavity flow type changes accordingly. The effect of the rear wall modificationdepends on the cavity flow type. The floor pressure balance tubes can reduce the cavityfloor pressure gradient. The leading edge vertical saw-tooth fence and blowing bothcan reduce the cavity floor pressure gradient and the cavity flow tends to become openflow.The investigations on the cavity aero-acoustic indicated that the aero-acousticenvironment for an open flow cavity is much worse than a closed flow one and themaximum sound pressure level can reach more than170dB. Several tones of differentmodes can be found in the cavity sound pressure spectra and the tones and soundpressure energy mainly appear at St<2.5. In the authors’ opinion, the K-H vortices areformed when the disturbs which exist in the forming of the cavity leading edge shearlayer coincide with cavity inherent frequencies fnare amplified by the sound radiation.The cavity rear wall may serve as an amplifier and strong aero-acoustic noise will beinduced and the sound radiation will be formed by the strong pressure fluctuationemitted from the flow hitting on the cavity rear wall. The disturbs which coincide withcavity inherent frequencies also will be amplified by the driving of the sound radiationin the cavity shear layer when it transfers to the cavity leading edge and the K-Hvortices have been induced again. Thus a self-sustained oscillation is formed in acavity. The boundary layer interruption and rear wall modification can reduce the totalsound pressure level and the main tone of the cavity significantly and its aero-acousticenvironment can be improved greatly. The effect of the shear layer interruption on thecavity aero-acoustic characteristics is different at different Ma. The cavityaero-acoustic environment can be worse when the shear layer interruption is adopted atsubsonic speed but the reverse effect can be achieved at transonic speed. A blockage tothe sound travel loop has little effect on the cavity aero-acoustic environment.The results of the internal weapon’s aerodynamic characteristics and its controlsindicated that the separation characteristic of an internal weapon is much morecomplicated than an external weapon. The unsteady forces on the store at the initialseparation stage are harmful to the store safe separation. An internal weapon’sseparation characteristics are mainly depended on the cavity length to depth ratio (L/D).The store’s positive pitching moment increases with L/D and the fluctuations of thelateral forces are also increased. The modification of the cavity rear wall has littleeffect on the store’s aerodynamic characteristics. The cavity base pressure balancetubes can improve the store’s separation characteristics. The axial aerodynamic characteristics can be improved through cavity leading edge active/passive flowcontrols and they are helpful to the store separation.The dynamic experiment of weapons-bay doors’ movement showed that theaverage pressure data compared well with the static measurement data on the cavityfloor and almost the same at different StD. More tones may appear on a certain spectrawhen the doors are closing. StDplays an important role on the cavity floor fluctuatingpressure and the peak of the tones reduced with the increasing of StD. The dynamicloads on the doors’ surface are different at different positions. The dynamic loads maychange a lot at different StD, and especially during the doors’ closing process. Themovement of the doors also has great effect on the doors’ normal force and hingemoment. The opening process may produce additional negative aerodynamic loadswhile positive aerodynamic loads may arise during the doors’ closing process. Theeffect of StDon the doors’ average loads decreases with the increasing of Mach number.Faster movement of the door can reduce its unsteady normal force and pitchingmoment at supersonic speed.