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Utilising flight test telemetry data to improve store trajectory simulations A. T. Cenko MD, USA A. G. Piranian and S. Denihan NAVAIR, Patuxent River MD, USA

ABSTRACT The Mk82 Joint Direct Attack Munition (GBU-38) is planned for carriage and employment on the F/A-18C aircraft from the outboard and inboard wing pylon stations and from the BRU-55 Smart Rack. Initial evaluation of the carriage loads and separation characteristics of the GBU-38 were accomplished through the analysis of wind tunnel test data. These data were acquired during a wind tunnel test using 6% scale models of the F/A-18C, GBU-38 and associated external fuel tanks, air-to-air stores, and air-to-ground stores. The test was conducted at the Veridian 8ft Transonic Wind Tunnel located in Buffalo, New York. Subsequently, the GBU-38 was certified for carriage and release from the F/A-18 aircraft by a series of flight tests. For the first flight where the store was released from the BRU-55, the wind tunnel captive trajectory system (CTS) data and NAVAIR pre-flight predictions showed no resemblance to the flight test data. The discrepancy was attributed to a rolling moment induced by the ejectors. NAVAIR’s six-degree-of-freedom trajectory programme has been modified to correct for this effect. Comparisons with flight-test telemetry and photogrammetric test data show that the modification considerably improves the flight test/prediction correlation.

1.0 NOMENCLATURE Cl Cm Cn FEJ

rolling moment coefficient, positive rt wing down pitching moment coefficient, positive nose up yawing moment coefficient, positive nose right total ejector force, lb

FZ1 FZ2 L M MX

forward ejector force, lb aft ejector force, lb store reference length, ft Mach number store moment about the X axis, positive rt wing down, ft lb store moment about the Y axis, positive nose up, ft lb MY store moment about the Z axis positive nose right, ft lb MZ NAVSEP Navy generalised separation package P store roll rate, positive rt wing down Q store pitch rate, positive nose up R store yaw rate, positive nose right S store reference area, ft2 XCG store CG location, ft full scale distance from store nose to forward ejector X1 foot, ft full scale distance from store nose to aft ejector foot, ft full scale X2 Z store CG location, positive down, ft PHI store roll angle, positive rt wing down, deg PSI store yaw angle, positive nose right, deg THE store pitch angle, positive nose up, deg V velocity, ft/sec α angle-of-attack, deg ∆ moment increment due to ejector rack dynamics φ CVER line of action, deg ρ density, slugs/ft3 Note: all wind tunnel and flight test data shown are right wing justified

Paper No. 2841. Manuscript received 13 May 2003 revised version received 30 August 2004 accepted 9 May 2005.

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Figure 1.

1.0 INTRODUCTION Any time a new aircraft is introduced into service, or an old aircraft undergoes substantial modifications or needs to be certified to carry and employ new stores, the store separation engineer is faced with a decision about how much effort will be required to provide an airworthiness certification for the aircraft and stores. Store trajectories are determined by the store isolated (freestream) aerodynamics, aircraft flowfield effects, store mass properties and ejector force characteristics. Generally, the US Navy relies on wind tunnel testing using the captive trajectory system (CTS) to obtain the store freestream and aircraft flowfield aerodynamics, while the mass properties and ejector force characteristics are measured prior to the flight test. The aircraft flowfield effects, which are obtained by taking store aerodynamic grid data for various store positions and orientations in a grid in proximity to the aircraft (hence the term grid method), are combined with the store freestream aerodynamics, the store mass properties and the ejector force characteristics in a six-degree-offreedom program (NAVSEP) to predict the store trajectories. These are compared to the CTS trajectories, which were obtained during the wind tunnel test by using the store mass properties and aerodynamic loads in a quasi-steady trajectory simulation. The pre-flight predictions are compared with actual flight test results before proceeding to the next flight test point. The basic premise in these predictions is that the aircraft is a rigid body, and that any wing bending/vibration during the ejector stroke can be ignored.

2.0 EJECTOR FORCE MODEL The original NAVSEP(1) and CTS predictions were in excellent agreement with the flight test data for the store ejected from the

parent pylon. The first series of flights for the CVER were conducted for the store on the inboard side of the BRU-55 ejector rack, with no store on the outboard, Fig. 1. As may be seen in Fig. 2 (Test Point 221), the NAVSEP predictions considerably underpredicted the roll rate, and had the yaw rates in the opposite direction from the flight test telemetry results. The excellent match between the predictions and flight test data for the stores ejected from the parent pylon implied that the wind tunnel aerodynamic grid data were correct. Since the CVER aerodynamic test data would also be correct, the obvious reason for the disagreement between the predictions and flight test data was ejector rack dynamics. This effect has been previously observed, but no attempt had been made to model it in the trajectory simulations(2). For the CVER flight, there was a spike in the rolling moment during the ejector stroke. This roll spike was attributed to the misalignment between the ejector force line of action and the store cg. Furthermore, the yaw rate prediction had the wrong sign during the first 0⋅06 seconds of the trajectory, which was also attributed to ejector rack dynamics during the ejector stroke. Clearly, the assumption that the wing remains rigid during the ejector stroke was incorrect. Ideally, a combined aeroelastic/structural model of the BRU-55 ejector and GBU-38 during the ejector stroke could be developed. A calculation of the effects of ejector force dynamics during the ejector stroke would provide the best solution to modelling the store's inertial response. Such an approach has exhibited a good match with test data(3). However, due to time pressure constraints, this approach could not be adopted in the middle of a flight test program. Since the ejector effects were assumed to be independent of Mach number, the NAVSEP program was modified to assign correction factors to the pitch, roll and yawing moments during the duration of the ejector stroke. The aerodynamic pitching, rolling and yawing

CENKO ET AL

UTILISING FLIGHT TEST TELEMETRY DATA TO IMPROVE STORE TRAJECTORY SIMULATIONS

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GBU-38 M = 0.75 5000' 80 40 0 -40 -80

DEG/SEC

-120

Telemetry R Telemetry P

-160

NAVSEP R

-200

NAVSEP P CTS R

-240

CTS P

-280 -320 -360 -400 0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16 0.18 Time, Sec

0.2

0.22

0.24

0.26

0.28

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0.34

Figure 2.

moments in the NAVSEP code are calculated as: MX = 0⋅5ρV2SLCl MY = 0⋅5ρV2SLCm MZ = 0⋅5ρV2SLCn NAVSEP was modified by adding a correction to the pitching, rolling and yawing moments during the ejector stroke to account for the ejector rack motion: For 0⋅⋅0 < t < 0⋅⋅03 ∆MX = Sinφ*FEJ* store radius*FACT11 ∆MY = {FZ1*(XCG – X1) + FZ2*(X2 – XCG)}*FACT21 ∆MZ = {FZ1*(XCG – X1) + FZ2*(X2 – XCG)}*FACT31 For 0⋅⋅03 < t < 0⋅⋅045 ∆MX = Sinφ*FEJ* store radius*FACT12 ∆MY = {FZ1*(XCG – X1) + FZ2*(X2 – XCG)}*FACT22 ∆MZ = {FZ1*(XCG – X1) + FZ2*(X2 – XCG)}*FACT32 This new program has been called NAVMOD. Due to the time pressures of the flight test program, these factors were determined by arbitrarily assigning a value to each factor, calculating the resulting trajectory, and then comparing the predicted pitch, yaw and roll rates (Q, R, and P) for the first 0⋅10 seconds to the flight test data at M = 0⋅75. An excellent match was achieved with the test data using this approach, Fig. 3. Excellent agreement with the store attitudes is shown in Fig. 4. For the correction to be useful, the effects have to be assumed to be independent of Mach number. The NAVMOD program was modified to arbitrarily assign correction factors to the roll and yaw rates during the duration of the ejector stroke(4). The predicted pitch, yaw and roll rates were compared to the test data in a least square sense for the first 0⋅10 seconds. A residual was calculated by adding the square of the differences between the predicted and actual pitch,

yaw and roll rates for the first 0⋅10 seconds of the trajectory. This residual was then used to determine what the best factors should be. This was originally done for each flight test configuration (Test Points 221, 222 and 223). Afterwards, the program was run for all three-flight test points concurrently. This assured that the factor that was obtained would be the best fit for all flight test points. When this was done, the factors that gave the best match with the test data were nearly the same at three Mach numbers (0⋅90, 0⋅95 and 1⋅1)(5). Excellent improvement between the predicted and actual pitch and roll attitudes can be seen in Fig. 5 for the critical test condition at M = 0⋅96. The yaw attitude is not as well predicted, but is still in good agreement for the first 20ms. Similarly, for three flight tests on the outboard (Station 8) pylon the best match with flight test were obtained again with a constant set of factors which were obtained at M = 0⋅75. The predicted pitch, yaw and roll attitudes are again in excellent agreement with the test data for the same condition for the store dropped from Station 8 (next to the fuel tank), Fig. 6. Results for load three, where the BRU-55 outboard store had an adjacent store inboard (Fig. 7) were somewhat different. It appears that, for these conditions, the adjacent store on the BRU-55 diminishes the store yaw motion and a smaller roll spike occurs. Although the correlation could be improved by using a new set of factors, it was decided that using the same set of factors for all cases would improve the utility of the approach. Excellent improvement in correlation between the predictions and test data for the two pylon stations, relative to the NAVSEP predictions, may be seen in Figs 8 and 9. This trend was confirmed for drops with an adjacent HARM for load four (Fig. 10). Excellent agreement between the predicted and actual test data is seen in Fig. 11.

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GBU-38 M = 0.75 5000' 150 100 50 0 -50

DEG/SEC

Telemetry P Telemetry Q

-100

Telemetry R -150

NAVMOD P NAVMOD Q

-200

NAVMOD R

-250 -300 -350 -400 0

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0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.22

0.24

0.26

Time, Sec

Figure 3.

GBU-38 M = 0.75 5000' 8 4 0 -4 -8

DEG

-12

Telemetry The Psi Phi PhotoG The psi Phi NAVSEP The Psi Phi NAVMOD The Psi Phi

-16 -20 -24 -28 -32 -36 -40 -44 0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Time, Sec

Figure 4.

0.16

0.18

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0.24

0.26

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UTILISING FLIGHT TEST TELEMETRY DATA TO IMPROVE STORE TRAJECTORY SIMULATIONS

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GBU-38 M = 0.96 11,600' 42 dive 8 0 -8 -16

DEG

-24 -32 Telemetry The

-40 -48 -56

Psi Phi NAVMOD The Psi Phi

-64 -72

PhotoG The Psi Phi

-80 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.38 0.40

Time, sec

Figure 5.

3.0 GBU-32 For the GBU-32 (1000# JDAM) first flight test point the GBU-38 (500# JDAM) roll factors were adjusted by using the different bomb radius. As may be seen in Fig. 12, an excellent match with the flight test data was achieved. Note that the 1000# bomb roll effect due to the ejector is considerably less than that for the 500# variant. Although developing GBU-32 factors gave a better match for this case, using the GBU-38 factors for the first flight still gave an excellent pre-flight prediction.

4.0 CONCLUSIONS Flight test telemetry data have considerably improved the US Navy’s capability of modelling and simulation (M&S) of store trajectories. Pitch, yaw and roll rates during the ejector stroke allow for the modelling of inertia effects that appear to be invariant with Mach number, and can be used for predictions for future flights. The US Navy has long advocated(6,7) the use of grid, rather than CTS trajectory wind tunnel testing. For the F-18/GBU-38 program the wind tunnel CTS trajectories were useless. When the NAVSEP program was modified to take account of ejector effects on the rack, an excellent match with flight test data was achieved. The advantages of using grid data in conjunction with M&S during the flight test program have been clearly demonstrated. Clearly, stores dropped from BRU-55’s may be imparted a sizeable rolling moment which has to be accounted for, both in the wind tunnel and the M&S before flight test. The excellent match

between the NAVMOD programme and the flight test results enabled the elimination of two flights and four store assets (from the original planned 18 flights and 28 stores), at a considerable cost saving to the program. If only CTS data had been taken during the wind tunnel entry not only would the programme’s success have been jeopardised, but also the improvement in M&S tools would not have been possible. Both the US Navy and US Air Force have used CVERs for multiple store carriage for many years. Obviously, the ejector force effects would have been present for all these years. How is it that the discrepancy between the wind tunnel and flight test results has not been previously noted? Part of the reason is that store roll behaviour, which is the most affected by the CVER, has always been the hardest to predict. The principal reason that these effects have not been previously uncovered is that telemetry data from CVERs have not been available prior to the GBU-38 flight test program. Ideally, a combined aeroelastic/structural model of the BRU-55 ejector and GBU-38 during the ejector stroke could be developed. A calculation of the effects of ejector force dynamics during the ejector stroke would provide the best solution to modelling the store's inertial response. However, this approach could not be adopted in the middle of a flight test program, and no one was interested in paying for it after the program ended. The GBU-38 program was successfully completed. The flight test for the GBU-32 from the BRU-55 on the F/A-18C was also successfully completed. A similar approach to M&S using telemetry data was used. The US Navy’s M&S capability has been considerably improved, since the effects of store weight, inertia and ejector forces can be properly determined.

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GBU-38 M = 0.96 11,600' 42 dive STA-8 8 0 -8 -16

Telemetry The Psi

-24

DEG

Phi

-32

NAVMOD The Psi

-40

Phi NAVSEP The

-48

Phi Psi

-56 -64 -72 -80 0

0.02 0.04 0.06 0.08

0.1 0.12 0.14 0.16 0.18

0.2

0.22 0.24 0.26 0.28 0.3

0.32 0.34 0.36 0.38

0.4

Time, sec

Figure 6.

Figure 7.

REFERENCES

4.

1.

5.

2. 3.

MOYER, S.A. NAVSEP – Navy Generalized Separation Package, September 1993, AVCSTD Report 93011-6053. WILLIAMS, W.R. Simulated release of Mk82, Mk83 and GBU-12 stores from VER2 rack on F/A-18 aircraft, May 1992, ARL-Flight-Mech-TM455, Melbourne, Australia. EBERLE, A. and DESLANDES, M.R. A bending beam approach for capturing ejection shocks on missiles, June, 2004, RTO AVT-108, Paper #8.

6. 7.

CENKO, A.T., ET AL Utilizing flight test telemetry data to improve store trajectory simulations, June 2003, AIAA Paper 2003-4225. CENKO, A.T. ET AL Use of statistical tools to improve modelling and simulation of store separation, June, 2004, RTO AVT-108, Paper #13. CENKO, A. ET AL Integrated T&E approach to store separation – dim past, exciting future, ICAS 96-3.3.2, September 1996. TAVERNA, F. and CENKO, A. Navy integrated T&E approach to store separation, October 1998, Paper 13, RTO Symposium on Aircraft Weapon System Compatibility and Integration, 1998, Chester, UK.

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UTILISING FLIGHT TEST TELEMETRY DATA TO IMPROVE STORE TRAJECTORY SIMULATIONS

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GBU-38 M = 0.95 6800' 20 dive STA-3 M = 0.75 FACTORS

70

Telemetry The

65

Psi

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Phi

55

NAVMOD The

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Psi

45

Phi

40

NAVSEP The

35

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DEG

25 20 15 10 5 0 -5 -10 -15 -20 -25 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.38 0.40 Time, Sec

Figure 8.

MK-82JDAM M = 0.95 6800' 20o dive STA-8 M = 0.75 FACTORS 50

Telemetry The

45

Psi Phi

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Phi Psi Phi

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Fi Figure 9.

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Figure 10.

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GBU-38 M = 1.1 17000' 45 dive M= 0.75 FACTORS 70 65

Telemetry The

60

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55 50 45

Phi NAVMOD The Psi Phi

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NAVSEP The

35

Psi

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DEG

25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.38 0.40 Time, sec

Figure 11.

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GBU-32 M = 0.90 11,000' 32 Telemetry The Psi Phi NAVMOD The

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Figure 12.

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0.24