Aerodynamic characteristics of store during lateral jet assisted separation from cavity using free drop technique

Fi XUE, Yipng REN, Zhng LI, Cong XU, Dhn WEI, Xiolu MAO,Znghui JIANG

a China Academy of Aerospace Aerodynamics, Beijing 100074, China

b Bejing Institute of Astronautical Systems Engineering, Beijing 100076, China

c China Academy of Launch Vehicle Technology, Beijing 100076, China

d Institute of Aeronautical Engineering and Technology, Binzhou University, Binzhou 256600, China

e Nanhang Jincheng College, Nanjing 211156, China

KEYWORDS Compressible flow;Internal cavity;Lateral jet;Store separation;Unsteady test;Wind tunnel free drop test

Abstract Aimed at the problem of store separation from internal cavity, this paper innovatively puts forward a separation scheme of using lateral jet to assist store safe separation.The jet ensures that the store is continuously subjected to down head moment during separation, so as to ensure safe separation. The wind tunnel free drop test technique with lateral jet is established, which can ensure that there is no support interference in the motion process of the store and more truly simulate the motion of the store. The feasibility of the new separation scheme is proved by wind tunnel test. The test shows that the new scheme can also change the more dangerous state into a safe state.Through the analysis of the test data,the pitching moment coefficients of the store under different pressures in the high-pressure cylinder are obtained, and the effects of aircraft and cavity on the pitching moment of the store are obtained. The 3 stages of store passing through cavity shock wave are found.The results show that the lateral jet provides the store with continuous head down moment, and makes the store overcome the head up moment caused by the shock wave in front of the cavity, so as to ensure the safety of separation.

1. Introduction

During high-speed store separation,the store and aircraft have very complex flow interference,which may easily lead to separation failure and flight accidents.1-3If the store is separated from a cavity, and the cavity has a more complex flow, the store will be subject to more complex flow interference, and the separation uncertainty will be enhanced,which will further threaten flight safety.4-6

Because of the danger of store separation,researchers have carried out a lot of research on store separation. Mahmood et al.7simulated generic store separation events that may include the release of stores from aircraft, booster/stage separations from missiles and space launch vehicles. CFD results for a transient finned-store release from a static delta wing under transonic flow conditions are compared with the experimental results. Hallberg8used several store separation trajectory simulation tools with wind tunnel test data to clear HSAD for its first free flight test from the F/A-18C parent aircraft at a Mach number of 0.8 and a pressure altitude of 30,000 feet.Johnson et al.9collected typical wind tunnel data to support store separation analysis, and designed and conducted smallscale drop test in wind tunnels, providing a means to identify the effect of unsteady weapons bay aerodynamics on store separation trajectories. Keen10took an overview of the many techniques used to collect information about the separation behavior of stored weapons when released from aircraft.Methods used in wind tunnel test, flight test, data processing, and computation were described. Purdon et al.11obtained wind tunnel data to allow an assessment of each store’s separation characteristics over the flight envelope on the F-35 variants.Given the number of stores, aircraft variants, and trajectory parameters, over 4000 User Occupancy Hours (UOH) were obtained in twelve wind tunnel tests to collect these data.Xue et al.12-13described a design method for a similarity law for free-flight tests of store separation.Initial separation velocity was introduced into the equation of motion to identify similar trajectories, which greatly improved the accuracy and applicability of test results from wind tunnels. Yang and Ye14studied the influence of the interference aerodynamics caused by the elastic deformations of the wing on the unsteady aerodynamics of the store.The store separation with the elastic deformation of the aircraft considered was simulated numerically. And the interactive aerodynamic forces were analyzed.

As for the cavity with more complex flow field, relevant researches have also been carried out.Khana et al.15presented the results of a computational study on the unsteady flow features in three-dimensional empty cavities and a cavity with a store. The study showed the existence of spanwise flow in the 3D cavity. Unsteady results from an empty cavity case are compared with experimental data, and the frequency of the dominant mode is in good agreement with the experiment.Flora16used both experimental and numerical methods to investigate store separation from a cavity(length to depth ratio L/D = 4.5) into Mach number 2.94. The sphere model was free drop tested experimentally and computationally, while the sub-scale store shaped model was free drop tested experimentally. The trajectory and dynamics of the CFD sphere release closely matched the experiment. Bjorge17studied one cavity geometry (L/D = 3.6), the pressure and flow field of a supersonic flow over a cavity.The pressure spectra on the cavity floor were markedly different between Mach numbers of 1.8 and 2.9.The Mach number 1.8 case exhibited clear spectral peaks consistent with predictions by Rossiter, whereas the Mach number 2.9 flow did not. Roos et al.18simulated the effect of a cavity placed upstream of a fuel injector on the jet interaction of a transverse jet in a supersonic crossflow. The cavity was found to significantly alter the structure of the typical supersonic cross-flow jet interaction.The typical horseshoe vortices were found to be absent and the barrel shock was found to be larger and more upright than in the no-cavity case.

By summarizing the previous research results, it can be found that the main reason for the failure of store separation is that the kinetic energy of the store moving downward is insufficient, resulting in the store moving upward after being disturbed, leading to collision with the aircraft. The interference factors leading to collision have a common feature:changing the store position to a positive angle of attack. If the store can be continuously provided with down head moment and downward force during the separation process,the separation performance of the store will be well improved.Lateral jet is a common control method to provide direct force,which is widely used in air vehicle attitude adjustment.19-25If the lateral jet control force can be continuously applied in the process of store separation to ensure that the store is affected by the down head moment, the purpose of improving the performance of store separation and ensuring the safety of separation can be achieved. Looking into the open literature,we can find that there are many studies on store separation and lateral jet. However, the research content of combining the two or assisted separation by jet method has not been found. Because the jet has strong unsteady characteristics,the previous steady test methods have obvious limitations. In order to better study the effect of jet on store separation, it is the best to use unsteady research method.

Wind tunnel free drop test technique is a common research method to study the unsteady characteristics of air vehicle.12-13,26-29The effect of jet can be well studied.It is different from other steady (force measurement after model support) and Capture Trajectory Simulation (CTS). It has the unique advantage of no support interference. The test uses highspeed photography, and the variation curve of the sixdegree-of-freedom parameters of the air vehicle model with time can be obtained.More importantly,using the wind tunnel free drop test, a lot of researches have been carried out in the field of store separation.12-13,26-28The wind tunnel free drop test has inherent advantages in studying jet and store separation. If the above technologies can be combined to establish a free drop test technique in wind tunnel with jet, the assist effect of jet on store separation will be better studied. The results will also be more realistic.This paper will focus on this.The newly established free drop test technique with jet in wind tunnel needs to have the typical characteristics of the previous two test technologies. In terms of data processing, it is also necessary to reasonably process the test results in order to obtain the specific impact of jet on the separation of store from cavity.

2. Aircraft/cavity and store

2.1. Aircraft and cavity

Fig. 1 shows the abdominal shape of the aircraft used in this test and the dimensions of the real aircraft. At the same time,the relative position of cavity can be seen from the figure.

We know that the dimension parameter of cavity is an important parameter, especially the ratio of length L to depth D,that is,L/D corresponding to Fig.2.In addition,according to the overall dimensions of the aircraft, the width W of the cavity is designed. The size of the wind tunnel test aircraft model is reduced by 20 times according to Fig.1.After reduction, the three dimensions of cavity are: L = 224.6 mm;D = 28.5 mm; W = 53.1 mm. Therefore, L/D = 7.88.

Fig. 1 Geometric dimension of real aircraft.

Fig. 2 Geometric dimension of cavity.

The 0.05 scale aircraft model is shown in Fig. 3, and the cavity, store and FD12 wind tunnel are also shown in it.

2.2. Store

The store adopts the revolution body shape,and the real overall dimension parameters are shown in Fig. 4.

The separation height of this separation simulation is 10 km, and the real store parameters are shown in Table 1.Ix,Iyand Izrepresent the moment of inertia of x,y and z axes respectively.

Fig. 3 Installation diagram of wind tunnel test.

Fig. 4 Geometric dimension of real store.

The description of the coordinate system is shown in Fig.5.When the store just leaks out of the aircraft,the centroid position of the store is defined as the coordinate origin.The direction along the air flow is the positive direction of the x-axis,the vertical down is the positive direction of the y-axis, and the right-hand rule determines the z-direction. We regulate that head up α is positive and head down is negative.

According to the similarity law, the mass characteristics of the wind tunnel test store are obtained, as shown in Table 2,and the conversion method was described in Ref. 12.

At the same time, one of the key points of this paper is to design the model of free drop test in the wind tunnel with lateral jet. We know that a major feature of the model free flight test is that there is no support interference, and this test technique is to ensure that the store can conduct lateral jet when it is completely free, which makes the experimental technique cannot be like the previous jet test, that is, the additional high-pressure air source is introduced into the model through the external air pipe for jet, but the jet air source and store must be designed as a whole.

In addition, the problem of accurate unlocking of highpressure gas must be solved. We know that the free drop test in the wind tunnel is to observe the movement of the store.Affected by the size of the observation window in the wind tunnel, the test model must be much smaller than the size of the observation window. The size of the observation window itself is small,about a circle with a diameter of 400 mm,which leads to the very small free flight test model in the wind tunnel.Moreover, the free drop test time in the wind tunnel is generally very short,only about 100 ms,so it becomes more important to grasp the jet unlocking time.It is not easy to store highpressure gas in such a small wind tunnel free drop test model,and it is even more difficult to accurately grasp the unlocking time.

Aimed at the two problems mentioned above, this paper carries out in-depth experimental technique design. The solution is to use a high-pressure cylinder to compress the gas into a micro cylinder, as shown in Fig. 6 and Fig. 7, so as to solve the problem of the source of high-pressure gas. To solve the second problem, a pin is used to control the jet time of highpressure gas, as shown in Fig. 7. Before the test, the highpressure cylinder and valve were connected, but the gas could not be ejected due to the existence of pin.The pin is connected to the airplane by a rope. During the falling process, the rope of the pin reaches the limit length of the rope,so as to pull out the pin and start the jet. Thus we ensure that the store can jet when it is completely free, solve the biggest problem of this experiment, and achieve the purpose of the experiment.

For simulation of the flux efficiency of different mass gases,different gases, such as carbon dioxide, nitrogen, hydrogen,various inert gases, or various gases in different proportions,can be filled in the compressed gas cylinder to meet the needs of different mass gases in the test.

Table 1 Real store parameters.

Fig. 5 Description of coordinate system.

For the real nozzle shape, the scale design can be carried out on the test model to simulate the real nozzle shape. If the jet velocity at the nozzle is simulated,the nozzle is designed into a variable section shape, with small internal section and large external section.Due to the high pressure of compressed gas cylinder, the jet gas is supersonic. By designing different cross-sectional area ratios, jets with different Mach numbers can be simulated.

The actual model of wind tunnel test is shown in Fig.6.The length of the store is 167 mm, and the length of the highpressure cylinder inside the model is 70 mm.The free flight test time in the wind tunnel is very short,about 50 ms.The existing air storage tank can ensure that the jet characteristics remain basically unchanged within 50 ms.

2.3. Jet characteristics

In order to understand the specific characteristics of the jet at the nozzle in this test,special research has been carried out on the jet, as shown in Fig. 8. The model was fixed and the jet velocity was measured by Particle Image Velocimetry (PIV).The test results show that when the internal pressure of highpressure cylinder exceeds 2.0 MPa, the jet velocity can reach supersonic; when the internal pressure of the high-pressure cylinder exceeds 4.0 MPa, the air velocity at the nozzle can reach Ma = 1.5.

2.4. Wind tunnel

FD12 wind tunnel is a three-speed (subsonic, transonic and supersonic) wind tunnel of China Academy of Aerospace Aerodynamics, with Mach number range of 0.3-4.0, Mach number adjustment accuracy of 0.004 and test section size of 1.2 m × 1.2 m × 3.8 m. The Mach number used in this test is 1.5, and the reason has been described earlier. Fig. 9 shows the scene picture of the wind tunnel test aircraft installed in this test.

3. Wind tunnel test and results

In order to fully analyze the influence of jet on store motion,the test experiment with no wind was carried out before wind tunnel test.Then the wind tunnel test with Ma=1.5 was carried out.Two kinds of high-pressure cylinders were used in the wind tunnel test, and the internal pressures were 2.0 MPa and 4.0 MPa respectively. αarepresents the angle of attack of the aircraft,and α indicates the angle of attack of the store.In previous tests, αa= 0 °, and in order to further verify the impact of jet on separation safety, in the last test, the aircraft was tested when αa= 2 °. The test status is shown in Table 3. In addition, the initial vertical downward velocity of the store during the test is 4.1 m/s.

3.1. No wind test

Compared with the real wind tunnel test, no wind test only lacks the external flow field. Three tests were carried out, jet on 2.0 MPa, jet on 4.0 MPa and jet off. In this test, highspeed photography is used for shooting, the shooting frequency is 2000 Hz, and auxiliary light source is used for light supplement.

As can be seen from Fig. 10, the model downs its head slowly when there is no jet.As can be seen from Fig.11,when the model is free at 2.0 MPa, the jet can turn the model over.As can be seen from Fig. 12, 4.0 MPa jet on makes the store flip faster than jet off.

Fig. 13 shows the change curve of store attack angle with time from test 1 to test 3.It can be seen that the turnover speed of the store increases with the increase of pressure.

3.2. Wind tunnel test

Due to the existence of aircraft and cavity, there are complex shock waves in the experimental area. In order to better carry out the force analysis of the experimental results,the flow field with aircraft and without store is studied.As shown in Fig.14,①is the strong shock wave generated by the aircraft nose,and because the position is relatively forward, it has almost no interference with the movement of the store; ②is the expansion wave generated by the cavity front; ③is the shock wavegenerated by the store hanger in cavity; ④is the expansion wave generated by the trailing edge of cavity; ⑤is a shock wave reflected from the ground at the bottom of the wind tunnel,and because the displacement is lower,it has little effect on the motion of the store; ⑥is an unknown wave. It is preliminarily judged that it should be a smooth external interference wave, and the position is relatively backward, so it has little impact on the motion of the store. After the above analysis,the shock waves can be analyzed clearly. Shock waves ①,②, ③and ④are the same shock waves that exist in real aircraft.Their existence can ensure that the test is more accurate.They do not belong to interference shock waves,but will cause great interference to the motion of the store,which needs careful analysis.Shock waves ⑤and ⑥are the interference shock wave generated by the wind tunnel. However, ⑤and ⑥will not have a great impact on the store, so they can be ignored.In conclusion, although the flow field in this test looks complex,the flow field characteristics are clear,and the experimental results obtained have high reliability.

Table 2 Wind tunnel store parameters.

Fig. 6 Test model and high-pressure cylinder.

Fig. 7 Mode design of store, valve and gas cylinder.

Fig. 8 Nozzle velocity measurement.

Fig. 9 Image capture equipment of wind tunnel test.

Fig. 15 shows the motion of the store under the conditions of Mach number 1.5, no jet, and αa= 0°. αais the angle of attack of the aircraft. It can be seen from the figure that the store was lowered at the initial stage of falling, that is, it was safe. However, during the falling process, the shock wave of the cavity hit the tail of the store, resulting in the downward pressure on the tail and the head up moment of the store.

Fig. 16 shows the motion diagram of the store with Mach number 1.5, jet on (2.0 MPa), and αa= 0°. Because jet on keeps the store head down, the lift is in the same direction as gravity, which accelerates the downward movement of the store. Therefore, compared with Fig. 15, the store stays in the observation window for a longer time. Fig. 17 shows the motion diagram of the store with Mach number 1.5, jet on(4.0 MPa), and αa= 0°.

Fig. 18 shows the motion diagram of the store with Mach number 1.5, jet on (2.0 MPa), and αa= 2°. It can be found by comparing with the state of test 4 that αa= 0°/jet off state is a dangerous state, so αa= 2° should be a more dangerous state. Through the last exploratory test, it can be found that when the jet is on, the separation of the store is safe even if αa= 2°.

In order to better show the impact of jet, jet off and jet on(4.0 MPa) are specially compared. From Fig. 19, we can see the jet area, the interaction between the ejected air flow and the incoming flow of the wind tunnel, and the shock wave caused by jet.

4. Data analysis

4.1. Angular displacement and linear displacement

Fig.20 shows the store angle of attack curve of the above four Ma=1.5 test results. It can be seen that when there is no jet,the store first downs its head but quickly raises its head.It is a very dangerous separation state, which is likely to collide with the aircraft.It should be avoided in real flight.When the jet is on,the attack angle of the store is low, which is a safe separation situation. In the case of jet on (4.0 MPa) with an aircraft angle of attack of 2°,that is,αa=2°test,store downs its head slowly compared to the two tests with an aircraft angle of attack of 0 °. The reason is that when the aircraft angle of attack is 2°, the initial angle of attack of the store is also 2°,causing the whole process to bow and slow down.

Fig.21 shows the store horizontal displacement versus time curve of the four test results. As can be seen from the figure,the horizontal displacement of the store is slower in the jetoff state than in the jet on state. There is little difference in x displacement with jet on.

Table 3 Test status.

Fig. 22 shows the store vertical displacement versus time curve of the four test results. As can be seen from the figure,the horizontal displacement of the store is slower in the jet off state than in the jet on state. There is little difference in y displacement with jet on. This is because when there is no jet, the store heads up and the aerodynamic component is upward, slowing down the falling speed. When there is a jet,the store downs its head, and the aerodynamic component is downward, accelerating the falling speed. There is little difference in falling when there is jet at the same time.

Fig. 10 Results of test 1 with no jet and no wind.

Fig. 11 Results of test 2 with 2.0 MPa jet on and no wind.

Fig. 12 Results of test 3 with 4.0 MPa jet on and no wind.

Fig. 13 Store pitch angle with no wind.

Fig. 14 Flow field analysis without store when Ma = 1.5.

4.2. Variation of pitching moment coefficient with angle of attack

According to the angle of attack curve of no wind state in Fig. 12 and the angle of attack curve when Ma = 1.5 in Fig. 20, the variation curve of pitching moment coefficient Cmwith angle of attack under the condition of jet on 2.0 MPa in Fig. 23 can be obtained. For example, the curve of Fig. 12 or Fig. 20 is fitted by polynomial, and the pitch angular acceleration versus time curve is obtained after twice derivation. Then we multiply the pitch angular acceleration by the moment of inertia (see Table 2) to obtain the pitch moment.Then,according to the dynamic pressure of the wind tunnel and the model reference area, the pitching moment coefficient can be obtained, as shown in Fig. 23.

As can be seen from Fig. 23, when no wind / jet on is 2.0 MPa, the moment generated by the jet always makes the store head down. At the same time, it can also be found that the moment value is not constant at this time, and there is a certain change, which may be caused by the instability of gas cylinder and pipeline air pressure. When Ma = 1.5/jet on 2.0 MPa, the store initially receives a large head down moment,and jet plays a leading role.However,the subsequent store is affected by the short-time head up moment, and the angle of attack ranges from-8 to-12°,which is also affected by the shock wave ②, ③and ④of cavity. In the following long time, jet is the leading role, which makes the store overcome the influence of cavity shock. Jet makes the store continue to be subjected to head down moment, and the low head angle of attack of the store continues to increase. When the angle of attack becomes about 39°, the moment reaches equilibrium. When the angle of attack is greater than 39°,the store is subjected to the head down moment again. The head up moment generated by jet can well ensure the safe separation of store.

Fig. 24 is the variation curves of pitch moment with angle of attack under the condition of jet on 4.0 MPa.As can be seen from Fig. 24, when no wind / jet on is 4.0 MPa, the moment generated by the jet always makes the store head down. At the same time, it can also be found that the moment value is constant in the early stage, but there are certain changes in the later stage, which may be caused by excessive gas cylinder pressure in the early stage and lower pressure in the later stage.Compared with Fig.23,it can be found that when no wind/jet on is 2.0 MPa, the maximum head down moment coefficient can reach 0.8, while in the case of no wind / jet on 4.0 MPa,that is only 0.35 in the early stage. This obviously does not conform to the laws of physics.However,when the jet lasts for a period of time, the head down moment of no wind / jet on 4.0 MPa increases rapidly, even more than 1.5. It confirmed the previous speculation: it is indeed due to the large 4.0 MPa,which leads to the air flow congestion in the internal pipeline and the small initial head down moment. With the decrease of the internal pressure of the gas cylinder in the later stage,after the air flow becomes unobstructed, the head down moment value increases.

Fig. 15 Results of test 4 with jet off, Ma = 1.5, and αa = 0°.

Fig. 16 Results of test 5 with jet on (2.0 MPa), Ma = 1.5, and αa = 0°.

The moment characteristics with Ma=1.5/jet on 4.0 MPa are similar to those with Ma = 1.5/jet on 2.0 MPa. The main difference is that the maximum moment coefficient in the case of Ma=1.5/jet on 4.0 MPa can reach 1.25,while the previous maximum moment coefficient in the case of Ma = 1.5/jet on 2.0 MPa is 0.7 N·m.This is also in line with the laws of physics.The main characteristics are shown as follows:with Ma=1.5/-jet on 4.0 MPa, the store initially receives a large head down moment,and jet plays a leading role.However,the subsequent store is affected by the short-time head up moment, and the angle of attack ranges from - 5° to - 10°, which is also affected by the shock wave ②,③and ④of cavity. In the following long time,jet is the leading role,which makes the store overcome the influence of cavity shock. Jet makes the store continue to be subjected to head down moment,and the angle of attack of the store continues to increase.When the angle of attack becomes about 38°, the moment reaches equilibrium.When the angle of attack is greater than 38°, the store is subjected to the head up moment again. The head down moment generated by jet can well ensure the safe separation of store.

Fig. 17 Results of test 6 with jet on (4.0 MPa), Ma = 1.5, and αa = 0°.

Fig. 18 Results of test 7 with jet on(4.0 MPa), Ma = 1.5, and αa = 0°.

Fig.19 Shock wave characteristics of store head with jet on and jet off.

Fig. 20 Store attack angle with time in different states when Ma = 1.5.

Fig. 21 Horizontal displacement of store with time in different states when Ma = 1.5.

Fig. 22 Vertical displacement of store in different states with time when Ma = 1.5.

Fig. 23 Pitching moment coefficient with angle of attack for jet on 2.0 MPa.

In order to further study the influence of aircraft on the store, we use the total pitching moment coefficient Cm,tof the store minus the jet moment coefficient to obtain the pitching moment coefficient generated by the aircraft fuselage shock wave and cavity shock wave. The specific method is described as follows:assuming that Ma=1.5 has no amplification effect on the jet force, we subtract the two curves in Fig. 23 respectively to obtain the influence of aircraft on store pitching moment for Ma = 1.5/jet on 2.0 MPa. Similarly, by subtracting the two curves in Fig. 23 respectively, we can obtain the effect of jet torque on store pitch torque after deducting the jet torque on store for Ma=1.5/jet on 4.0 MPa(see Fig.25).

Fig. 24 Pitching moment coefficient with angle of attack for jet on 4.0 MPa.

It can be seen from the figure that the two curves do not coincide. There are two specific reasons: first, the two test jet pressures are different,so the peak value of the curve is different, and the interference moment is greater at 4.0 MPa;another main reason is that the influence of the aircraft on the store moment also depends on the position of the store and the aircraft shock wave, that is, the vertical displacement of the store. At 4.0 MPa, the store passes through the shock wave more quickly. At the same angle of attack, the 4.0 MPa store is farther away from the aircraft than the 2.0 MPa store, which explains why the red curve and the blue curve seem to have phase difference and the phase of the red line is advanced in Fig. 25.

Through further analysis of Fig.25,it can be found that the shock wave generated by the aircraft produces a continuous head up moment to the store for a long time, which is very dangerous. Corresponding to Fig. 26, the following can also be found. Firstly, the head of the store needs to pass through the shock ②, and in the process of crossing, it receives the head up moment. Secondly, after the head of the store passes through the shock wave ②, the store receives a head down moment due to the action of the shock wave ②on the head of the store.Finally,as the store continues to move downward,the shock ②acts on the tail of the store,and the store is again affected by a head up moment. The above 3 processes explain the reasons for the changes of the two curves in Fig.25.Fig.26 shows these 3 processes.

Fig. 25 Variation curves of disturbed pitching moment coefficient with angle of attack.

Through data analysis, we obtain the pitching moment characteristics of the store under aircraft interference,and calculate the pitching moment characteristics of the store without jet force.If there is no jet force,the store will be affected by the head up moment for a long time, which is not conducive to safe separation.The feasibility of jet assisted separation is also proved.

4.3. Variation of pitching moment coefficient with vertical displacement

As can be seen from Fig.14,in the process of store separation,the separation distance y of the store affects the position of the shock wave and the moment on the store.It is also mentioned in Section 4.2 that there are limitations when using angle of attack to analyze Cm. In order to better show the relationship between the moment received by the store and the y-direction position,the abscissa in Fig.23 and Fig.24 is replaced with the corresponding vertical displacement y for further analysis.

The angle of attack of the abscissa of Fig. 23 is replaced with the vertical displacement y to obtain Fig. 27. The law is consistent with Fig. 23. When no wind / jet on is 2.0 MPa,the moment generated by the jet always makes the store head down.At this time,the moment value is not constant.It is also suspected that it is caused by unstable gas cylinder and pipeline air pressure.With Ma=1.5/jet on 2.0 MPa,the store initially receives a large head down moment, and jet plays a leading role. However, the store is immediately affected by the shorttime head up moment, which is about 35-65 mm. When the store passes through the shock wave ②of cavity, the store is affected by the upward force to produce the head up moment.In the following long time,jet plays a leading role,making the store overcome the influence of cavity shock,and jet makes the store continue to be subjected to head down moment. When y = 225 mm, the moment reaches equilibrium. When y is greater than 225 mm, the store is subjected to the head up moment again. In general, the head down moment generated by jet can well ensure the safe separation of store.

Fig. 28 is the variation curves of pitching moment coefficient with y for jet on 4.0 MPa. As can be seen from Fig. 24,when no wind / jet on is 4.0 MPa, the moment generated by the jet always makes the store head down. At the same time,it can also be found that the moment value is constant in the early stage, but there is a certain change in the later stage.As explained earlier, this is due to the excessive gas cylinder pressure in the early stage and the pressure drop in the later stage.

Fig. 26 Three typical locations in process of store separation.

Fig.27 Pitching moment coefficient with vertical displacement y for jet on 2.0 MPa.

The moment characteristics with Ma=1.5/jet on 4.0 MPa are similar to those with Ma = 1.5/jet on 2.0 MPa. The main difference is that the maximum moment coefficient in the case of Ma=1.5/jet on 4.0 MPa can reach 1.25,while the previous maximum moment coefficient in the case of Ma = 1.5/jet on 2.0 MPa is 0.7 N·m.This is also in line with the laws of physics.The main characteristics are shown as follows:with Ma=1.5/-jet on 4.0 MPa,the store receives the head down moment when y ＜30 mm,and jet plays a leading role.However,the store is immediately affected by the short-time head up moment,30 mm ＜y ＜95 mm,which is also affected by the shock wave②, ③and ④of cavity. In the following long time, jet plays a leading role,making the store overcome the influence of cavity shock wave,and jet makes the store continue to be subjected to head down moment. Always when y = 270 mm or so, the moment reaches equilibrium. When y ＞270 mm, the store receives the head up moment again, which is caused by the excessive angle of attack of the head down. Because the ydirection distance is large enough, the store is safe. The head down moment generated by jet can well ensure the safe separation of store.

In order to further study the impact of aircraft on the store,the specific impact of aircraft shock waves on the store is obtained.The specific method is described as follows:subtract the two curves in Fig. 27 to obtain the influence of aircraft on store pitching moment under the condition of Ma=1.5/jet on 2.0 MPa. Similarly, by subtracting the two curves in Fig. 27,we can roughly obtain the influence of aircraft on the store pitching moment under the condition of Ma = 1.5/jet on 4.0 MPa.

Fig.28 Pitching moment coefficient with vertical displacement y for jet on 4.0 MPa.

It can be seen from Fig.29 that the middle parts of the two curves overlap well. This is also in line with the physical law,because when the influence of jet force is not considered, the pitching moment of the two tests should be similar.The difference between the two curves may be caused by the influence of the ejected air flow on the incoming flow and the difference of attack angle. Fig. 29 also proves the accuracy of the research method in this paper.

The head and tail overlap is not very good. There are two specific reasons: first, the gap in the early stage is mainly due to the excessive pressure at 4.0 MPa, resulting in the jet moment not as large as expected at the beginning of separation; second, the difference in the later stage of separation is mainly due to the different jet pressures of the two tests, so the peak value of the curve is different, and the interference moment is greater at 4.0 MPa.

By further analyzing Fig.29,it can be found that the shock wave generated by the aircraft generates a head up moment to the store within a considerable vertical distance, which is very dangerous.Corresponding to Fig.16 and Fig.17,it can also be found that Fig. 28 is similar to Fig. 25, and the whole separation process is divided into 3 stages.For specific analysis,refer to the following paragraph of Fig. 25. Without the help of jet force(see Fig.25 and Fig.29),the store will be affected by the head up moment for a long time, which is not conducive to safe separation.The feasibility of jet assisted separation is also proved.

4.4. Summary

Through the above experiments, it can be found that when there is no wind, the lateral jet can provide an obvious down head moment for the store.At Ma=1.5,when there is no lateral jet, the store becomes raised after separation, which is a dangerous state; when there is a lateral jet, the low head moment provided by the lateral jet can ensure the safety of separation. Especially when there is jet and the aircraft angle of attack is a more dangerous αa= 2°, the lateral jet can also ensure the safe separation of the store. At the same time,through data processing, the variation law of store pitching moment coefficient with angle of attack and vertical displacement y is obtained. In addition, due to the existence of cavity shock wave, three typical states passing through the shock wave in the process of store separation are found.

Fig. 29 Variation curves of disturbed pitching moment coefficient with distance.

It can be seen that the lateral jet can assist the safe separation of the store. Lateral jet can be considered to ensure flight safety in the embedded separation of real aircraft. With the establishment of this test technique, in-depth research can be carried out in this field. The establishment of new technique has laid a foundation for the research in new directions and proved that it is meaningful.

5. Conclusions

In this paper,the problem of store separate from internal cavity is studied, and a separation scheme assisted by lateral jet technique is proposed. The jet force acts on the head of the store to avoid the rise of the store during separation, so as to ensure the safety of separation. The new technique can ensure that the store has no support interference,and can more truly simulate the impact of jet on store motion. Through the wind tunnel test, it is proved that the separation scheme assisted by lateral jet is feasible and the new technique is reliable. At the same time, the new technique overcomes many technical problems, including: the source of air jet after the store is completely free; after the store is free, the timely unlocking of the sealed air source;the length of the store shall not exceed 200 mm; the quality characteristics of the test model shall meet the similarity law of free drop test.

The results show that the lateral jet can assist store separation safely. The lateral jet can provide an effective head down moment for the store, which will make the store bow,so as to accelerate the separation between the store and the aircraft and ensure the safety of separation.Specifically,the following conclusions can be drawn:

(1) When Ma = 1.5 and there is no lateral jet, the store becomes raised after separation, which is a dangerous state (test 4, no jet, Ma = 1.5). The study found that the use of lateral jet can not only ensure safe separation in jet off dangerous state(test 5 and 6,no jet,Ma=1.5),but also achieve safe separation in more dangerous state(test 7, aircraft angle of attack 2°).

(2) Through the analysis of the test data, the pitching moment coefficients of the store under different pressures in the high-pressure cylinder are obtained, and the effects of aircraft and cavity on the pitching moment of the store are obtained.

(3) Through data processing, the variation laws of pitching moment coefficient with angle of attack and vertical displacement y are obtained. In addition, due to the existence of cavity shock wave, 3 typical states passing through the shock wave in the process of store separation are found. The effect of lateral jet is significant in the field of auxiliary store safe separation.

Declaration of Competing Interest

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.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (Nos. U21B2054 and 11772317). This research was also supported by the Open Fund of Key Laboratory of Icing and Anti/De-icing, China (No.IADL20200408).