Trajectory and attitude deviations for internal store separation due to unsteady and quasi-steady test method

Wei SONG,Jingang DONG,Wei LU,Zenghui JIANG

Aerodynamic Science Center,China Academy of Aerospace Aerodynamics,Beijing 100074,China

KEYWORDS Captive trajectory system;Freedrop test;Internal weapons;Store separation;Unsteady test method

Abstract The deviations of trajectory and attitude angle for internal store separation are evaluated by two wind tunnel test methods.One is the Freedrop Test(FDT),which is known as unsteady and time-dependent method of scaled model.The other is the Captive Trajectory System (CTS) test,which is usually regarded as a quasi-steady and time-averaged test technology.The result shows that there is a streamwise adverse pressure gradient on the cavity resulting in a nose-up pitching moment coefficient (＞0) acting on the store model.When the initial pitch angular velocity is 0,the store exits the shear-layer with a nose-up pitch attitude causing the store to climb back towards and collide with the aircraft.However,the store passes through the shear-layer into the freestream with a nose-down pitch,which causes a successful separation event when the initial pitch angular velocity ＜0.The pitch angle obtained by unsteady test method is different from that by quasisteady test method.The time-dependent test includes the aerodynamic force induced by pitch angular velocity whereas the time-averaged method (CTS) cannot reflect the effect of unsteady aerodynamic force.The deviation of vertical displacement is not obvious for FDT and CTS test since the store has an initial vertical velocity,which is dominant for the vertical displacement.This means that the highly unsteady flow can create unpredictability in aerodynamic pitching moment of the store,which can lead to the deviation of pitch angle for internal store separation.

1.Introduction

The internal weapons loading can greatly reduce the Radar Cross Section (RCS) and aerodynamic drag in flight,which is conducive to stealth and supersonic cruise.Therefore,the new-generation combat aircraft have changed traditional external weapons to internal weapons loading.However,the internal weapons bay has typical cavity flow characteristics and contains extremely complex flow phenomena,such as flow separation and reattachment,shear-layer instability,vortex shedding and rupture,etc.Especially,under the condition of supersonic flight,the interference phenomenon of shock wave and shock wave,shock wave and shear layer exist when the door of weapons bay is opened.These unsteady flowfield can give rise to the uncertain pressure fluctuations on the internal store inducing the uncertain aerodynamic force and moment.The uncertain aerodynamic force and moment can alter the trajectory and attitude angle of the separated stores,which may cause a collision between aircraft and store,thus threatening the safety of aircraft.Therefore,it is particularly important to predict the separated trajectory and attitude angle of the internal store in order to assess the separated safety of aircraft and store.

There have been various methods to simulate the trajectory and attitude angle of internal store separation,such as the CFD and wind tunnel test methods.Coupling CFD with a six-degree-of-freedom (6-DOF) rigid equation is an effective method to simulate the motional trajectory and attitude angle of internal store separation.However,sometimes,the numerical results are not ideal for the three-dimensional complex separation problems.Therefore,the wind tunnel tests(such as the captive trajectory system and freedrop test)are widely used to predict and evaluate the trajectory and attitude angle of internal store separation.The Captive Trajectory System(CTS) test is a quasi-steady and time-averaged test approach,which is widely used to simulate the motional trajectory and attitude angle of the external store separation.In most cases,the CTS test can give a fine result for external store separation due to the quasi-steady flowfield environment.However,the CTS test cannot accurately simulate the timedependent loading of internal store separation since the store must move through the unsteady shear layer before it enters into the flowfieid.However,the CTS test has also been used to predict the motional trajectory and attitude angle of internal store separation at=1.5 since it can give an online result compared to the grid test method.In addition,the freedrop test based on dynamic similarity has also been used to investigate the separation characteristics of internal store.For example,Flora and Reederconducted an internal store separation test from an open cavity using the freedrop testing based on the Heavy Mach Scaling (HMS) at=2.9 for spherical and MK-82 ice model.

From the above,it can be seen that previous studies on internal store separation by Freedrop Test(FDT)and CTS test methods were either with different models or in different flow conditions,which makes it difficult to analyze the deviations of trajectory and attitude angle for the internal store release.This paper uses the FDT and CTS test methods to investigate the separation characteristics for the same aircraft and store model in the uniform test conditions and attempts to analyze the deviations of the motional trajectory and attitude angle for the internal store due to unsteady and quasi-steady test method.The content of this paper is presented as follows:Section 1 presents the research background and introduction.Section 2 describes the methodology.The test results and discussion are given in Section 3,and the conclusions are drawn in Section 4.

2.Test setup

2.1.Test method

For the freedrop test,it is necessary to not only satisfy aerodynamic similarity but also consider the inertial response of the store to these aerodynamic forces and moments.In supersonic flow,the Mach number must be equal for wind tunnel and full-scale flight to allow compressibility effects to be accounted in the aerodynamic scaling.Based on the similarity of Mach number,the HMS and Light Mach Scaling (LMS)laws are widely applied in high-speed freedrop test.The HMS method is commonly used in the freedrop test without an initial vertical velocity and the LMS law is usually used in that with initial velocity.Generally,the LMS can give an exact simulation of angular motion,which can be widely adopted when the angular motion is the primary concern.

Fig.1 presents the basic principle of the freedrop test.As shown in Fig.1,the aircraft-model is installed on the support of the wind tunnel and the store-model is mounted in or out of the aircraft.When the flowfield of the wind tunnel is completely established,the store-model is freely released or ejected into the flowfield.At the same time,the high-speed cameras or a multiple exposure device take the dynamic motional images of the store-model as it separates from the aircraft.However,the freedrop test can only give the trajectory and attitude angle for the dynamic store,but does not directly measure the aerodynamic forces or moments of the store-model in the separation process.

Fig.1 Principle of freedrop test.

The CTS test is a quasi-steady and time-averaged test method,which not only measures the aerodynamic forces and moments of the separated store,but also simulates the trajectory and attitude angle in the wind tunnel.In the CTS test,the aircraft-model is fixed on the support of the wind tunnel and the store-model is mounted on the 6-DOF mechanism.When the flowfield of wind tunnel is completely established,the aerodynamic forces and moments acting on the store are measured by an electronic balance and non-dimensionalized as coefficient form.Then,the aerodynamic forces and moments are transmitted in the 6-DOF motion equation of store separation.The new position and attitude of the store-model are solved online by setting a small time increment.The output from the computer program provides control signals or instructions to the 6-DOF mechanical system to move the store-model to the new position and attitude relative to the aircraft.And then the aerodynamic force and moment on the store in the new position and attitude are remeasured and the trajectory is resolved.The whole separation process of the store-model can be obtained within a given simulation test time.Fig.2 shows the schematic diagram of the CTS used in the wind tunnel test.As indicated in Fig.2,the CTS is designed to adopt a configuration form of a parallel mechanism for the separation motion of 6-DOF.The parallel mechanism has advantages of accurate position and high stiffness compared with a series of 6-DOF tandem mechanism.The system can give a trajectory accuracy of ±0.1 mm and attitude accuracy of ±0.05.

Fig.2 Schematic diagram of CTS test.

2.2.Test facility

Both the FDT and CTS tests were carried out in a trisonic wind tunnel at China Academy of Aerospace Aerodynamics(CAAA),which has two observation windows in one side.The wind tunnel is a temporary wind tunnel,where the cross-section is 1.2×1.2 m.The range of simulated Mach number is from 0.4 to 4.0 and the Reynolds number ranges from 1.2×10to 7.9×10based on one meter.Nevertheless,a special test-section that is suitable for the CTS is installed instead of the general test-section when a CTS test is performed.The diameter of view window for special test-section is 800 mm for the even bigger view.

In the freedrop test of internal store separation,two FASTCAM-SA5 high-speed cameras which are made by Photron Company are arranged to capture the motional images of dynamic release at 2000 Hz with a shutter-speed of 0.5 ms,as shown in Fig.3.The centre line of wide angle lens for camera A almost coincides with the centre line of observation windows.Camera A can record the motion of store-model along the longitudinal direction,vertical direction and pitch direction.To gain more information of store separation,another camera (Camera B) is installed as an auxiliary measurement in front of Camera A.The angle between the centre line of Camera B and view window is 30°.A high-power LED light source is mounted on the opposite side since the high-speed camera requires a high optical flux density and continuous lighting to obtain enough light in a narrow space.The experimental control and acquisition system are shown in Fig.4,which consist of two record computers and a synchronous controller.When the flowfield of wind tunnel is steady,the synchronous controller starts and the two cameras are triggered to record the store separation at the same time.When a drop test is over,the moving images of store-model are downloaded and stored in the computer.

Fig.3 Installation diagram of high-speed camera.

Fig.4 Synchronous controller and acquisition system.

In the CTS test,a Schlieren Photography System (SPS) is used to record the dynamic images of internal store separation,as indicated in Fig.5.The SPS is a reflective parallel light system,which is a conventional Z-type schlieren.The SPS mainly includes LED light source,collector lens,reflecting mirror M1 and M2 (which are arranged symmetrically with respect to the wind tunnel and not shown in Fig.5),photograph system and so on.The light line emitted by the LED light source is got together by the collector lens and then passes though the slit.Then,the light line is projected onto the reflecting mirror M1,forming a parallel light line.After the parallel light line passing through the flowfield of the wind tunnel,the light line is focused by the reflecting mirror M2.Finally,the light line is converged on the plane of the knife-edge and then the dynamic images are taken by the photograph system.The schlieren images are captured using a digital camera at 100 Hz with a shutter-speed of 10 ms.

Fig.5 Schlieren lighting system in FD-12.

The store-model used for the dynamic freedrop and CTS test is a 1/20th scaled and similar prototype of AIM-120C missile with a Center of Gravity (CG) of 115 mm from the missile nose.The total length of the store-model is 192.5 mm and the maximum diameter of store body is 10.15 mm.The front and rear parts of the store-model consist of four cruciform stabilizing fins or rudder.The pitch and yaw Moment-Of-Inertia(MOI) relative to the CG is I=I=2166.5·and the mass of the store-model is=91.2when using a light Mach scaling method.Fig.6 shows the store-model used in the test.

Fig.6 Store-model.

The internal weapons bay of the carrier aircraft and inertial coordinate system are indicated in Fig.7.As shown in Fig.7(a),the store-model is mounted on the pylons of the ejection mechanism.The ejection mechanism is installed inside a rectangular cavity located on the lower surface of the aircraft.The ejection mechanism connects to pneumatic lines that provided pressurized airflow for ejection.The ejection stroke is approximately 12.7 mm.The three dimensions of the cavity are=224.55 mm,=18.93 mm,and=53 mm,which has a length-to-depth ratio of 11.8.As shown in Fig.7(a),the origin of inertial coordinate system is located on the middle of the leading edge of the cavity.The positiveis away from the cavity in vertical direction and the positiveis opposite to the incoming flow direction.The positiveis defined by the right-hand rule.The static position and separated trajectory of the missile are defined in this coordinate system.The centroid coordinates of the missile model are=-120 mm,=-14.2 mm,and=-14 mm.When the missile has a nose-down attitude,the pitch angle is negative,as shown in Fig.7(b).As presented in Fig.7(b),the upward normal force coefficientCis positive (+) and nose-down pitching moment coefficient Cis negative (-).

Fig.7 Internal weapons bay and inertial coordinate system.

2.3.Test condition

As stated in Section 1,the main objective is to predict the deviations of trajectory and attitude angle of the internal store separation,so the test condition is set to the same between the FDT and CST test.

Table 1 Run conditions for all tests.

3.Results and discussion

3.1.Aerodynamic characteristic of internal store separation

Fig.8 shows the image sequence for the FDT and CTS test.The front six images of FDT are selected by eight frames with the time intervals of 4 ms in order to compare with CTS test.It is qualitatively indicated in Fig.8(a) that the store falls down quickly in the vertical direction and the store is in a nose-up pitch with a final collision to the carrier aircraft at the time of=71 ms (the middle images are omitted).It can be seen from Fig.8(b) that the result of CTS test is similar to that of FDT test from=0ms to=20ms,but the store has no collision with the aircraft since the 6-DOF mechanism of CTS prevents the motion of store.

Fig.8 Comparison of image sequence between FDT and CTS (Case 1).

Fig.9 Normal force and pitching moment coefficient in CTS test (Case 1).

Fig.10 Comparison of image sequence between FDT and CTS (Case 4).

Fig.11 Normal force and pitching moment coefficient in CTS test (Case 4).

3.2.Deviations for FDT and CTS test

In order to analyze the deviations of trajectory and attitude angle for internal store separation due to unsteady and quasi-steady test method,a digital process must be made for the motion images.Marshallsaid that‘‘the significant factors in defining the motion of a store model during a dynamic drop separation trajectory can be readily seen by considering motion in a single vertical plane”.Therefore,the vertical displacementand pitch angle θ are two most important parameters for the internal store separation.Although the CTS test can obtain the 6-DOF motion parameters,the vertical displacement and pitch angle are given as a comparison with FDT in the following paragraph.

Fig.12 Corresponding time history for FDT and CTS test(Case 1).

Fig.13 Corresponding time history for FDT and CTS test(Case 4).

4.Conclusions

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.