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The United States Navy’s Integrated Approach to Store Separation Analysis

Frank Taverna Alex Cenko

Naval Air Warfare Center, Aircraft Division Building 2 187, Suite 1320-E2

48 110 Shaw Road, Unit # 5 Patuxent River, MD 20670-l 906

1. SUMMARY

The current United States Navy approach to store separation analysis employs a combination of wind tunnel testing, flight testing, and computational aerodynamic analysis. This Integrated Test and Evaluation approach ensures safe separation of stores in a timely and cost effective manner. This approach has evolved over the past decade and is unique because it is performed by an Integrated Product Team (IPT) which belongs to one. physically co-located organization. During the past several years this approach has been responsible for providing considerable time and cost savings to many programs, including the F-18CIJDAM. F-14/GBU-24, F-18C/JSOW, and DC-I30/BQM-74 programs. This approach is presently being applied to the F/A-18EIF aircraft/store integration program to both reduce the program cost and ensure the success of the program.

2. LIST OF SYMBOLS

ACFD

AEDC

AIM AIMS

AMRAAM

ASRAAM

Ai Bi BL

CFD CTS CA CN CY

Cl

Cm

Cn

Applied Computational Fluid Dynamics Arnold Engineering Development Center, Wind Tunnel Facility, Tullahoma, Tennessee Air Intercept Missile Advanced Imaging Multi-Sensor Systems Advanced Medium Range Air-to-Air Missile Advanced Short Range Air-to-Air Missile Influence Coefficient Influence Coefficient Aircraft Buttline, positive outboard, inches Computational Fluid Dynamics Captive Trajectory System Axial Force Coefficient, positive aft Normal Force Coefftcient, positive up Side Force Coefficient, positive right wing Rolling Moment Coefftcient, positive right wing down Pitching Moment Coefficient, positive nose up Yawing Moment Coefficient, positive nose right

EMD

FOT&E FS

GBU H IFM IPT ITALD JASSM JDAM JSOW M N NAWC-AD

NAWC-WD

OSD P

PHI

PSI

Q

R

SLAM-ER

THE

T&E WL a E ci

(5

oi

0

w

Engineering and Manufacturing Development Phase Follow On Testing and Evaluation Aircraft Fuselage Station. positive aft. inches Guided Bomb Unit Altitude, feet Influence Function Method Integrated Product Team Improved Tactical Air Launched Decoy Joint Air-to-Surface Stand-Off Missile Joint Direct Attack Munition Joint Stand-Off Weapon Mach Number Number of Store Segments Naval Air Warfare Center, Aircraft Division Naval Air Warfare Center, Weapons Division Office of the Secretary of Defense Store Roll Rate, positive right wing down, degrees/second Store Roll Angle, positive right wing down, degrees Store Yaw Angle, positive nose right, degrees Store Pitch Rate, positive nose up, degrees/second Store Yaw Rate, positive nose right, degrees/second Standoff Land Attack Missile - Expanded Response Store Pitch Angle, positive nose up, degrees Test and Evaluation Aircraft Waterline, positive up, inches Angle of Attack, degrees Upwash angle, positive up, degrees Upwash angle of segment i, positive up, degrees Sidewash angle, positive outboard, degrees Sidewash angle of segment i, positive outboard, degrees Store Pitch Angle, positive nose up, degrees Store Yaw Angle, positive nose right, degrees

Paper presented at the RTO XI Symposium on “‘Aircraji Weapon System Compatibility and Integration”, held in Chester, United Kingdom, 28-30 September 1998, and published in RTO MP-16.

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3. BACKGROUND

In the past. store separation was conducted in a very haphazard fashion. Stores would be dropped from an aircraft at gradually increasing speeds until the store came too close to the aircraft or occasionally hit the aircraft. In some cases, this led to loss of aircraft, and made test pilots reluctant to participate in store separation flight test programs.

During the 1960’s, the Captive Trajectory System* (CTS) method for store separation wind tunnel testing was developed. The Captive Trajectory System provided a considerable improvement over the hit or miss method. and became widely used in aircraft/store integration programs prior to flight testing. However, the CTS method was not utilized in an integrated approach, since the group conducting the wind tunnel test was generally separated both in organization and location from those responsible for conducting the flight test program and determining the safe separation envelope. Furthermore, since relatively small scale models had to be used in the wind tunnel tests, the wind tunnel predictions did not always match the flight test results. As a result, resolution of the wind tunnel/flight test discrepancies was often extremely difficult.

By the late 1970’s and early 1980’s Computational Aerodynamics had finally matured to the point of providing a solution2~3~4 for a store in an aircraft flowfield. Rather than revolutionizing store separation methodology, this new capability inspired an ongoing argument among the Computational Aerodynamicists, Wind Tunnel Engineers, and Flight Test Engineers. The Computational Fluid Dynamicists claimed that they could finally replace the wind tunnel. The Wind Tunnel Engineers accused the Computational Fluid Dynamicists of being unaware of the complexity of the problem. Finally, the Flight Test Engineers declared that neither group could provide them with the necessary data to conduct a successful flight test program.

During the same time period the Influence Function Method (IFM) was developed5. This method allowed for an estimate of store loads based on the aircraft induced flowfield impinging on the store. This seemed to offer a bridge to the disagreement between the Computational Fluid Dynamics (CFD) and Wind Tunnel communities, since it could provide store loads in the entire aircraft flowfield with just one CFD calculation. However, except for Grumman and the Air Force, this method did not readily gain acceptance in the store separation community. Furthermore, an integrated test and evaluation approach was not truly implemented, since the Flight Test community was still separated both physically and organizationally from the CFD and Wind Tunnel communities.

At that time, the Navy’s approach was to use both aircraft and weapon contractors to perform the testing and analysis necessary to clear a new aircraft/weapon configuration. This procedure had several drawbacks, the most serious

being that the contractor’s involvement usually ended with the start of the flight test program. Therefore the contractor had no means for using the flight test results to improve store separation methodology. Also, no two contractors used the same methodology to predict safe weapon separation prior to flight test.

About ten years ago, the Navy decided to develop a capability/process at the Naval Air Warfare Center, Aircraft Division (NAWC-AD) to conduct the analyses necessary for a store separation flight test program. Without any existing capability in this area, the Navy was able to choose among the best attributes of the techniques used by contractors and the Air Force.

NAWC-AD realized that the three legs of an integrated approach: analysis, wind tunnel, and flight test are intimately related to each other and provide essential information that can improve the product of each group. Not only is the entire program conducted by the same group, but ideally by one individual. The computational aerodynamics, wind tunnel test planning, trajectory simulation and flight clearance for each point in the flight test program are all managed by the same person, who does not have to be an expert in CFD methods or wind tunnel testing, but is competent in their use and more importantly, knows their limitations, This individual not only has the authority, but also the responsibility for ensuring that the flight test program is conducted both safely and cost effectively.

FIGURE 1: UNITED STATES NAVY’S INTEGRATED APPROACH TO STORE SEPARATION

DETERMINE CRITICAL CONDITIONS

VALIDATE TEST ACCURACY

DEVELOP TEST

VALIDATE W.T. DATA

PLAN FLT TEST ACQUIRE FLT CLEARANCE

This analysis process has evolved to where the three legs have formed an intrinsic checks and balances system. In order to confirm aircraft/store compatibility, wind tunnel testing, flight testing, and computational analyses are dependent upon and essential to one another. The computational analyses determine the critical conditions to be wind tunnel tested, aid in developing the wind tunnel test plan, and verify the wind tunnel test accuracy; while the

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wind tunnel test confirms the computational model. The wind tunnel test is used to acquire a flight clearance and plan the flight test matrix, while the flight test corroborates the accuracy of the wind tunnel test data. The flight test also substantiates the computational analyses, while the computational analyses help reduce the flight test matrix. Figure 1 represents a schematic of the analysis process.

4. DISCUSSION

One of the most critical features that determines a store’s separation trajectory is the carriage moments. These moments are principally caused by the aircraft flowfield. Therefore, the first step in separation analysis is to estimate the region of the flight envelope that might have the worst carriage moments. This is done by deriving an estimate of the aircraft flowfield. The primary analytical tool for the purpose of evaluating the aircraft aerodynamics is the linear potential flow program, PAN AIR6, which has been validated for most of the current Navy aircraft. In addition, changes in aircraft configuration shape such as fuel tanks, pylons, and other stores can be easily modeled/modified.

Although the potential flow codes have demonstrated the ability to predict complex aircraft flowfields in the linear speed regime, pitch/yaw head probe flowfield test data, when available, should always be used to validate the analytical aircraft models. At present, extensive angularity data are available at various Mach numbers and aircraft attitudes for the A-6E, AV-IB, F- 18C/D and F- 18ElF aircraft, and a limited set of data exists for the F- 14. The angularity data are usually acquired at the AEDC 4 by 4 foot and 16 by 16 foot transonic wind tunnels in Tullahoma, Tennessee or the CALSPAN 8 by 8 foot transonic wind tunnel in Buffalo, New York.

After determining the aircraft flowfield, the Influence Function Method (IFM) is used to determine the effect of the aircraft flowfield on the store forces and moments. NAWC-AD is recognized as an international authority on IFM7,8,9, and has delivered the code to the Canadian Air Force and Australian Air Force, as well as NAWC-WD (Naval Air Warfare Center: Weapon Division) and United States contractors. Using the aircraft flowfield and store influence coefficients, an estimate of store aerodynamic coefficients is made everywhere in the flowfield, including carriage. The estimated store carriage loads and moments are then checked by using computational methods to calculate their value at carriage. The store aerodynamic force and moment coefficients are then input to a six- degree-of-freedom program to simulate the store’s trajectory prior to the wind tunnel test. The simulated trajectories are used to help design the wind tunnel test to ensure that the most critical regions of the store separation envelope are tested. These wind tunnel tests are presently conducted at either the AEDC facility or the CALSPAN facility. These facilities have the dual sting Captive Trajectory System (CTS) capability which is required for store separation testing.

After the test has been completed, wind tunnel results are compared to the analytical predictions to ensure any discrepancies can be accounted for. The wind tunnel carriage, grid, and freestream store aerodynamic coefficients are then used to update the simulated trajectories, which should closely agree with the CTS simulated trajectory prediction. Sensitivity studies are conducted based on the wind tunnel results and the level of confidence in the data to determine the regions of the flight envelope where problems in launching or jettisoning the store might be encountered.

Finally, trajectory simulations are compared with flight test results early in the flight test program. Any discrepancy between predictions and test data can be largely attributed to differences between the assumed and actual aerodynamic moments at carriage. Therefore it is possible to determine what the carriage moments had to be in order to match the flight test results. The moments for the next point in the trajectory are then modified based on the previous results.

5. EXAMPLES

5.1 F/A-ISEIJSOW A comparison of the clean (no pylon) F/A-18C and F/A- 18E aircraft flowfields was initiated to determine if the FIA- I8E flowfield might cause problems in safely separating stores, compared to the F/A-18C. A PAN AIR model was developed and validated using wind tunnel pressure data measured on the wing.

The preliminary analysis indicated that the F/A-I 8E increased inlet area, coupled with the increased aircraft area ratio, had a significant impact on the aircraft flowfield; and might have a detrimental effect on store separation.

Prior to the January 1995 F/A-l 8E/F Series III wind tunnel test at AEDC, flowfield angularity predictions were made utilizing the PAN AIR model previously developed. Comparisons between test data and analytical predictions correlated very well for both the F/A-18C and F/A-18E aircraft, Figures 2 and 3. This confirmed that the PAN AIR model of the F/A- I8E aircraft is a good representation and should provide good qualitative results even at low transonic speeds.

Validation of the PAN AIR model of the F/A-I 8E provided an opportunity to evaluate the effects of aircraft flowfield on the trajectories of stores separating from the aircraft. Since the IFM technique had been used for the F/A-l 8C / JSOW program, it was used again to predict JSOW trajectories from the F/A- 18E aircraft. IFM assumes that there is a direct relationship between the aircraft flowfield along a store and the forces and moments induced by the aircraft flowfield on the store. Conceptually, for a store broken into N segments, this is expressed by the relationships:

CN = ZAi*Ei, i=I,N Cm = ZAi*Ei, i=l,N Cy = XBi*oi, i=l,N Cn = CBi*oi, i=l,N

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Coefftcients are not an aerodynamic property, but rather a solution to a regression equation relating a series of store aerodynamic loads in a known flowfield, originally obtained from experimental data. Although the IFM code provides a quick estimate of these coefficients, they cannot be used blindly. The IFM code only allows for an approximate representation of the store’s geometry. The Influence Coefficients were generated for the JSOW store and were validated by comparisons with previous wind tunnel grid data.

M=0.90 a=O.O BL=88 WL-89

0

-1

-2

-3 \,/TEST DATA F-18E

-4 -PAN AIR F-18E PAN AIR F-18C

-5 I 200 250 300 350 400 450 500 550 800 650

FS IN.

FIGURE 2: F-18 PAN AIR UPWASH PREDICTION

-1

M=0.9 a=O.O BL88 WL69

c RTEST DATA F-i8E

FS IN.

0

FIGURE 3: F-18 PAN AIR SIDEWASH PREDICTION

The first step in the IFM process is calibration. This entails determining the store’s Influence Coefficients, Ai and Bi, which determine the store’s response to the aircraft flowfield. It must be emphasized that a store’s Influence

Using the JSOW Influence Coefficients, which had been validated for the F/A- 1 SC aircraft, along with the F/A- 1 SC and F/k-18E flowfields, shown in Figures 2 and 3, trajectory predictions were made for the JSOW store from the F/A-ISE aircraft. These trajectory predictions were compared to the equivalent trajectories from the F/A- 1 SC aircraft. As displayed in Figures 4 and 5, the IFM predictions for the JSOW trajectories from the F/A- I SE were in excellent agreement with the CTS test data for the store on the midboard station with a tank inboard, but underpredicted the yawing moment for the store on the inboard pylon. Considering the fact that the predictions were made three years prior to the wind tunnel test, it is clear that the IFM technique can give a good qualitative estimate of aircraft flowfield effects.

M=0.90 H=5000 ft BL 88 CLEAN CONFIG IFY PREDICTED F-18E INCREMENTS

DEG

0 0.04 0.08 0.12 0.16 0.2 0.24 0.28 0.32 0.36

TIME, SEC

FIGURE 4: JSOW JElTlSON PREDICTION

5.2 F-14/GBU-24

The approach used for the GBU-24 store differed somewhat from that for all other aircraft/store programs. In this case

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the flight test results were used to determine how the wind tunnel data should be used.

16

DEG

M=0.95 Ii=5000 ft BL 134 TANK INBOARD IFM PREDICTED F-18E INCREMENTS

-20 1 -- I 0 0.04 0.08 0.12 0.16 0.2 0.24 0.28 0.32

TIME, SEC

FIGURE 5: JSOW COMPARISON

The GBU-24 store has two characteristics that make predicting flight test trajectories challenging. The wing of the store opens during the first 150 ms of the trajectory. It was not possible to model this wing opening sequence during the wind tunnel test trajectories. Grid data were taken for both the wings open and wings closed configurations. Furthermore, the GBU-24 canards are free floating during the initial part of the trajectory. Previous flight test data for the F- 15 and F- 18 aircraft have failed to match predictions based on wind tunnel data for either fixed canards (at Oo deflection angle), or for the store with the canards removed. To predict flight test trajectories, particularly for the GBU-24 configuration released from the F-14 forward station (Station 3) flight test results were used to interpret the wind tunnel data.

A wind tunnel test for the F- 14/GBU-24 configuration was conducted at the AEDC 4 by 4 foot transonic wind tunnel. During this test CTS grid, CTS trajectory, carriage loads, and freestream test data were taken for both the canards on and canards off configurations, with the wings both retracted and open. As depicted in Figures 6 and 7, the pitching moment changes from unstable to stable when the canards are removed. This occurs even though the normal force shows little canard effect. Test data for the free floating (spring) canards seem to fall between the canards on and off data, Figure 7. The wind tunnel test freestream and grid data were then used only to determine that, for any combination of canards on and canards off test data, a safe release point would be Station 3 at M = 0.82.

12

8

-16

GBU-24 FREESTREAM

WING SWEEP = 0” M = 0.80

FIXED CANARD &SPRING CANARD + NO CANARD 10%

c

\/NO CANARD 5%

0 -26 -20 -15 -10 -5 0 5 IO 16 20 26

a DEG

FIGURE 6: GBU-24 FREESTREAM

WING SWEEP = 0” M = 0.80

16

8

-4

-8

-12

-16 AFIXED CANARD ZSPRING CANARD

-26 <NO CANARD 10%

I vN0 CANARD 6% I

-30 -25 -20 -15 -10 -5 0 5 10 16 20 26 30

a DEG

FIGURE 7: GBU-24 FREESTREAM

A flight test for the release of the GBU-24 from the F-14 aircraft forward station was conducted on January 23, 1996. The trajectory using the canards off freestream and grid data gave the best match to the flight test results for everything but the pitch rate, Figure 8. Since the canards on wind tunnel data indicated a sharp nose down pitch rate, while the

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canards off data indicated a slight nose-up pitch, it was postulated that the reason for the disagreement in pitch results might be attributable to the aircraft flowfield effect on the undeflected canards. Since a fixed canard for this case carried a negative lift, the canard would have to deflect nose up to neutralize this effect. Once the store is released, the canard would take some time to return to it’s neutral position, which would initially cause the GBU-24 to pitch nose-up. At this flight condition an excellent match with the flight test results was obtained when an increment of Cm = 2.3 was applied to the canards off grid data, Figure 9.

120

80

60

deglsec 40

20

GBU-24 PRE-FLIGHT PREDICTION

F-14 M=0.80 H =3600R STA 3 NOTE: CANARD OFF TEST DATA

--TELEMETRY DATA

0.04 0.08 0.12 0.16 0.2 0.24 0.28 0.32

TIME, SEC

FIGURE 8: F-WGBU-24 RATES

Upon examination of the entire flight envelope, a Cm offset coefficient ranging from 2.3 to 3.0 yielded excellent agreement with flight test data. Flight test videos for Station 3 launches showed the canard was deflected nose up in carriage. Both the trajectory results and the flight videos indicate that the response of floating canards is opposite to that indicated by wind tunnel data for fixed canards. Further flight test data will be examined to determine if these results are repeatable.

6. PRESENT AND FUTURE

Currently this process is in use on a multitude of programs. These programs include: F/A-l 8C’D / JDAM, F/A-l 8C/D / JSOW, F/A-18CID I AIM-9X, F/A-18CD ! ASRAAM, F/A- 18C/‘D / ITALD, F-l 4 / JDAM, F- 14 / AIM-9X, AV- 8B / JDAM, AV-8B I AMRAAM, F-4 i AQM-37, DC-130 1 BQM-74, P-3 / AIMS, P-3 / XGLIDER, CF-5 / AQM-37, and CF-5 / BQM-74C. A large store separation effort is also underway on the F/A- 18E/F. This work is currently in

EMD where there are 32 different weapons loadings to be analyzed. After EMD. there is a Follow On Testing and Evaluation (FOT&E) program that will include JSOW. JDAM, GBU-24, SLAM-ER, and JASSM among other stores.

GBU-24 POST FLIGHT TRAJECTORY

F-14 M=0.80 H =36OOft STA 3 NOTE: 2.3 ADDED TO Cm

deglsec

100

90 TELEMETRY DATA

80

70

60

60

40

30

20

10

0

-10

-20 0 0.04 0.08 0.12 0.16 0.2 0.24 0.28 0.32

TIME, SEC

FIGURE 9: F-WGBU-24 RATES

At NAWC-AD the nine Aerospace Engineers that comprise the Store Separation/Flight Clearance Group are responsible for the wind tunnel testing and analysis portion of these efforts. This group works closely with Flight Test Engineers to ensure that an organized, timely, cost effective, and dependable analysis is provided for each effort. Co- location is essential for this to work.

NAWC-AD is continuously developing and improving its methods and tools to meet the requirements of these present and future aircraft/store certification programs. As aircraft and weapon shapes become more complex, effective early diagnosis of aircraft flowfields is essential to successful store carriage and separation. Any successful store certification program must start with full understanding of the aircraft flowfield and its effect on the store.

Building on a solid experimental base, NAWC-AD is actively pursuing analytical developments that will enhance the store flight clearance process. It is required that these tools: are validated for complex configurations, are flexible, and provide answers in a reasonable time frame. The NAWC-AD AerodynamicsKFD Branch is in the same division as the Store Separation/Flight Clearance Group and is examining analytical tools that complement this goal. While our final analysis will remain primarily based on

experimental results for the foreseeable future, new analytical tools will allow us to gain further knowledge into the carriage and separation of stores. This knowledge will permit better test preparation and review of contractor results.

The recent move of NAWC-AD to Patuxent River, Maryland expands the current opportunity for other organizations to use information the Store Separation/Flight Clearance Group produces. Currently, Flying Qualities, Aerodynamics, and Structures disciplines benefit directly from information gained while examining store carriage and separation, avoiding needless duplication. As previously stated, all these groups are now co-located at Patuxent River with the Flight Test Community. So, the process of further integrating analysis, wind tunnel testing, and flight testing due to both the proximity and work of the composite organizations is continuing.

NAWC-AD has actively participated in the OSD funded Applied Computational Fluid Dynamics (ACFD) investigation into analytical techniques relating to store certification. During the past year, NAWC-AD has participated in an evaluation of several CFD codes for predicting store carriage loads. Based on the results of the comparisons, the Navy determined that none of the CFD codes could provide answers that were sufficiently accurate for store separation clearance purposes. However, the Lockheed SPLITFLOW code was clearly superior to the other methods examined, and should prove useful in qualitatively predicting aircraft flowfield effects at transonic speeds.

7. CONCLUSIONS

The Navy has developed an Integrated Test and Evaluation approach, combining wind tunnel testing, flight testing and computational aerodynamics, to determine the safe separation of stores from aircraft. This approach is anchored in the realization that although flight test results are the bottom line; wind tunnel testing, flight testing, and computational aerodynamics are dependent upon and essential to one another. The Integrated Teat and Evaluation approach has proven to be useful in several recent Navy store separation flight test programs, and is presently being used in the F-l 8EIF aircraft/store integration program.

8. REFERENCES

1. Bamber, M. J., “Two Methods of Obtaining Aircraft Trajectories from Wind Tunnel Investigations,” AERO Report 970 (AD 233198), David Taylor Model Basin, Washington, DC, January 1990.

2. Rogers, R. M., “A Comparison Between the Nielson and Woodward Programs in Predicting Flow Fields and Store Loads,” Naval Weapons Center TM 2854, July 1976.

3.

4.

5.

6.

7.

8.

9.

10.

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Cenko, A., and Tinoco, E. N., “PAN AIR - Weapons, Carriage and Separation,” AFFDL-TR-79-3 142, December 1979. Steger, J. L., Dougherty, F. C., and Benek, J. A., “A Chimera Grid Scheme,” Advances in Grid Generation, ASME, June 1983. Meyer. R.. Cenko, A.. and Yaros, S., “An Influence Function Method for Predicting Aerodynamic Characteristics During Weapon Separation,” 12th Navy Symposium on Aeroballistics, May 198 I. Magnus, A. E.. and Epton, M. A., “PAN AIR - A Computer Program for Predicting Subsonic or Supersonic Linear Potential Flows About Arbitrary Configurations using a Higher Order Panel Method,” NASA CR-3251, 1980. Keen, K. S., “Inexpensive Calibrations for the Influence Function Method Using the Interference Distributed Loads Code,” J. Aircraft, January 1985. pp 85-87. Cenko, A., et al., “Further Development of the Influence Function Method for Store Separation Aerodynamic Analysis,” J. Aircraft, August 1986, pp 656-661. Cenko, A., et al., “NADC Approach to Air Launch Certification,” AIAA Paper 92-2900, August 1991. Welterlen, T. J., and Leone, C., “Application of Viscous, Cartesian CFD to Aircraft Stores Carriage and Separation Simulation;” AIAA paper 96-2453, June 1996.