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Experimental Validation of Ship-Helicopter Dynamic Interface Simulations using Instrumented Aircraft James S. Forrest, Benjamin J. Chartier, Callum A. Chartier and Shane C. Patch Prism Defence Pty Ltd, 198 Melbourne Street, North Adelaide, South Australia, 5006, Australia

Abstract Experiments using an instrumented AgustaWestland A109LUH helicopter were conducted in order to validate DeckSAFE, an embarked helicopter dynamics code. Drop tests were performed, whereby the aircraft was raised up a predetermined distance from the ground then released, with its dynamic response recorded. In addition, embarked testing was performed, whereby lashing loads and undercarriage response was measured whilst the helicopter was secured on a ship at sea. Equivalent simulations were performed using DeckSAFE, with the results compared against the experimental data. Good agreement was shown between the experimental data and the simulations, indicating that DeckSAFE is able to accurately predict the dynamic response of an embarked helicopter. Keywords: helicopters, ships, dynamic interface, modelling, simulation, validation, DeckSAFE.

Introduction The use of helicopters in a maritime environment presents many unique challenges to naval operators [1-3]. Whilst many of these issues concern the piloting aspects of launch and recovery from the flight decks of ships, of equal importance is the ability to keep helicopters secure whilst on deck [4]. When a helicopter is ‘embarked’ on a deck, accelerations caused by ship motion lead to forces on the undercarriage which, combined with an unfavourable deck roll or pitch attitude, can lead to toppling and/or sliding of the aircraft. Therefore it is necessary to define ship motion limits for embarked helicopters, such that motion of the deck does not cause unwanted movement of the aircraft or lead to loads which cause structural damage. In a practical sense it is not possible to define such limits experimentally, therefore mathematical models of the ship-helicopter ‘dynamic interface’ (DI) have become popular engineering analysis tools for defining operating limits and predicting operational loads. The current authors have developed DeckSAFE, a multi-body dynamics analysis toolkit which has been designed to model the embarked dynamics of helicopters on ship decks under a variety of operational restraint configurations. Whilst mathematical models of aircraft dynamics are an important aspect of engineering analysis, and are now routinely used to predict aircraft performance across a wide range of disciplines, in order to derive reliable information from these models it is necessary to perform validation activities to ensure that they faithfully represent the physical system. There are very few studies in the literature concerning the experimental validation of embarked helicopter dynamics codes, partly because this is a fairly niche area of aircraft dynamics analysis, and also 16th Australian Aerospace Congress, 23-24 February 2015, Melbourne

because of the proprietary nature of most DI software. A number of tests were conducted by the Australian Defence Science Technology Organisation (DSTO) in the 1990s in order to validate their Sikorsky Seahawk S-70B-2 simulation model, although no comparison with their model was published in the public domain. As part of this experimental campaign Hourigan et al. published the results of drop tests which were performed using an instrumented S-70B-2 aircraft [5]. Later, Blackwell described the results of tie-down trials whereby an instrumented S-70B-2 was lashed to a tilt-table and the deck of an embarked ship, with lashing loads and aircraft attitudes measured [6]. Langlois et al. have also developed a helicopter/ship dynamic interface simulation package, Dynaface, which has undergone validation activities [7]. This paper presents the results of a validation exercise in which the results of simulations using DeckSAFE are compared against a number of experimental tests performed using an instrumented AgustaWestland A109LUH helicopter. An instrumented A109LUH helicopter was used to conduct two different experiments. The first consisted of a series of ‘drop tests’, where the aircraft was raised up by a supporting harness and then dropped onto the ground; the undercarriage and tyre deflections were recorded during the drop and subsequent settling period. The second experiment involved recording the undercarriage deflections and restraint loads whilst the helicopter was secured to the deck of a ship at sea. These experiments supplement other desktop validation activities which have not been reported in this paper due to space constraints.

Simulation Software DeckSAFE is a ship-helicopter dynamic interface simulation program, designed and developed by Prism Defence, primarily for ship motion limits analysis conducted prior to First of Class Flight Trials (FOCFT). At the heart of the program is a physics engine which performs timedomain six-degree-of-freedom (6DOF) rigid body dynamics simulation with non-linear mathematical models to accurately resolve the physical behaviour of helicopters embarked on ship decks. Further details on DeckSAFE can be found in Chartier et al. [8].

Helicopter Drop Tests To support the ongoing development of DeckSAFE, the Swedish Defence Materiel Administration (FMV) conducted a series of drop tests using an army-variant A109LUH helicopter. A total of 12 drop tests were performed, consisting of different initial main landing gear (MLG) oleo extensions and pressures. Due to space constraints only two tests are selected for comparison in the current paper: initial MLG oleo extensions of 50% and 100%, both at ‘nominal’ pressure. Experimental Details A crane was used to lift the aircraft by a sling mounted around the main rotor hub, with a quickrelease mechanism used to drop the aircraft. Ballast was used to ensure that the aircraft centre of gravity (CG) was as far forward as possible, to minimise lift-off of the nose wheel during the lift. Mass and CG details for the drop tests are provided in Table 1 (vertical CG details were not available). It was not possible to engage brakes during the testing due to the lack of a hydraulic power unit. It was noted that the NLG wheels rolled back slightly during the lift, although this is not expected to have an adverse impact on test results. 16th Australian Aerospace Congress, 23-24 February 2015, Melbourne

A flight test data acquisition (DAQ) suite was installed in the aircraft, with stringpotentiometers fitted on both of the main undercarriage to measure oleo deflection. Data was logged at 10 Hz and recorded to a laptop computer. Three video cameras were set up to record each drop test, with one camera assigned to each undercarriage. Table 1: Aircraft mass and CG during the drop tests. Mass 3031 kg

Centre of Gravity STA 3469 mm BL 2 mm

Simulation Setup The DeckSAFE A109LUH helicopter model was configured with the weight and balance details provided in Table 1. Inertial parameters that were not supplied by FMV (such as vertical CG location and moments of inertia) were estimated using data provided by the aircraft OEM for a similar weight configuration. During the simulations the helicopter was raised slowly from the rotor hub using a point load, with either the main landing gear tyre deflection (100% extension test) or main landing gear oleo extension (50% extension test) monitored. Once the tyre deflection reached zero (100% extension test) or the oleo extension reached 50% (50% extension test), the aircraft was released. During the drop and subsequent settling period, the dynamic response of the aircraft and its subcomponents (e.g. oleos, tyres) were recorded to log files for analysis. Results Figures 1 and 2 show results for the drop tests at 50% and 100% initial MLG oleo extensions, respectively. The helicopter was released at approximately two seconds into the histories. In each of the figures the left-hand plot shows the time-history of oleo extension for each of the undercarriage oleos and the right-hand plot shows vertical tyre deflection time-history for each of the tyres. During post-processing of the experimental potentiometer data it became apparent that there was significant noise corrupting the signal for certain drop tests, making this data somewhat unreliable. Therefore an image processing technique was devised whereby still images from the video capture were processed to determine the time-histories of oleo extension. This method also allowed nose gear oleo extension and tyre deflections for each tyre to be estimated; parameters which had not been recorded by the DAQ system during the drop tests. Figure 1 shows an example of the noisy potentiometer signal, compared to the smooth data from the image processing routine. Figure 2 confirms that the image processing results match the uncorrupted potentiometer closely. As the image processing technique involves manual identification of the undercarriage components at each frame of the video (recorded at 25 frames per second), this provides a relatively high data rate of 25Hz during the drop. However, there is scope for error due to the manual nature of the process, particularly during high rates of motion when the images blur; this error is estimated to be less than ±5mm. For the 50% extension drop it can be seen that the agreement of the oleo response data is excellent. The gradients of the compression strokes are well matched, and the DeckSAFE model 16th Australian Aerospace Congress, 23-24 February 2015, Melbourne

is able to resolve the nonlinearity in the nose gear response. Similarly, the tyre responses match well, both in terms of transient response and final settled deflections. The 100% extension drop results also show excellent agreement with the experimental data. Again, the gradient of the compression strokes match well, although the overshoot and bounceback in the nose gear is not resolved. Tyre response is, again, in excellent agreement. It is interesting to note from the experimental data that, following both drops, there is a period of slow oleo compression (seen as a gradual reduction in extension between 4 and 10 seconds). It is thought that this is caused by a combination of thermal effects and low-speed damping due to oleo metering pins. These phenomena are not currently modelled within DeckSAFE, so it is not surprising that the DeckSAFE results do not exhibit similar behaviour. However, these slow transient deflections have too long a timescale to impact on deck operations, so this is not considered to be problematic for dynamic interface simulations.

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Figure 1: Drop test results for 50% extension case; time-histories of oleo extension (a) and tyre deflection (b).

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Figure 2: Drop test results for 100% extension case; time-histories of oleo extension (a) and tyre deflection (b).

16th Australian Aerospace Congress, 23-24 February 2015, Melbourne

Helicopter Embarked Testing Experimental embarked helicopter dynamics data were captured during Prism's FOCFT operations in March 2013, aboard the Visby Corvette Nykoping. Data recorded during embarked testing included time-histories of lashing loads and main landing gear oleo extension, and also the deck motion during the tests. Experimental Details During testing the aircraft was embarked with four lashings and instrumented with tension load cells; an additional four slackened lashings were fitted for safety purposes. The experimental set up can be seen in Figure 3. The instrumentation utilised were: • Crossbow 440 AHRS unit on both the ship (deck) and aircraft. • Celesco SP1 potentiometer measuring port undercarriage extension. • Four custom-built 5 tonne load cells, manufactured by Delphi Force, fitted to each of the four lashing restraints. • A video camera was used to capture video and still images of the experimental testing. This instrumentation was connected to a National Instruments ‘CompactDAQ’, with custom LabView DAQ software developed by Prism, to simultaneously log all instrumentation output to a laptop computer at 10Hz.

Figure 3: Experimental setup during embarked testing of the A190LUH. Simulation Setup The DeckSAFE A109LUH helicopter model was configured with the weight and balance details provided in Table 2, which corresponded to a nominal ‘medium’ weight configuration. This was a best estimate as it was not possible to determine the exact weight and balance of the aircraft at the time of the test. The helicopter model was run against the ship motion recorded during testing, with the lashing scheme geometry and webbing properties chosen to match the test conditions. The experimental lashing scheme is shown in Figure 4 and the lashing properties are shown in Table 3. 16th Australian Aerospace Congress, 23-24 February 2015, Melbourne

Table 2: Aircraft mass and CG for the embarked simulations. Mass 2656 kg

Centre of Gravity STA 3399 mm BL 17 mm WL 1098 mm

Table 3: Modelled lashing properties (MC-1 webbing) for the embarked simulations. Parameter Maximum load [N] Maximum elongation [%] Damping ratio [%] Pretension [N] Non-extendable length [m]1

Value 13345 22 7 250 0.9

Figure 4: Lashing scheme used during embarked testing. Results The ship motion reached a maximum roll amplitude of approximately 11.5 degrees at around 3975 seconds into the testing. As this represented the most severe period of the dataset, the analysis presented here will be focused on this time period. However, the trends and discussion points that correspond to this event also apply to other periods of the dataset. Figure 5 presents a comparison of lashing loads between the DeckSAFE model and the experimental results, for between 3900 and 4000 seconds into the experimental dataset. The experimental results are shown with dashed lines, while the DeckSAFE simulations are shown as solid lines. The lashings are numbered according to the schematic in Figure 4. It can be seen that the shape of the load curves match the experimental data very well, despite some differences in absolute magnitudes. It was found during investigation of the data that the amount of pretension selected for each lashing has a direct impact on the peak lashing loads predicted. It is likely that each of the lashings could be ‘tuned’ to provide a better match to the 1

The inclusion of a non-extendable length accounts for the presence of stiff components such as shackles and load cells, by reducing the effective working length of the lashing. 16th Australian Aerospace Congress, 23-24 February 2015, Melbourne

data, however as the amount of pretension during the testing was unknown it was decided to use 250N for all of the lashings for the DeckSAFE runs as a best estimate. Figure 6 presents a comparison of port oleo extension between the DeckSAFE model and the experimental results, again for between 3900 and 4000 seconds into the experimental dataset. The predicted minimum and maximum extensions are within a few millimetres of the experimental data, and the gradients during oleo motion are also in good agreement. The ‘flat’ spots in Figure 6 which can be observed in both the experimental and DeckSAFE time-histories are due to friction in the oleo, which causes it to ‘stick’. It is encouraging to see that DeckSAFE is able to pick out these features, due to the inclusion of a friction model within the oleo code.

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Figure 5: Comparison of experimental and DeckSAFE time-histories of lashing loads during embarked testing; port lashings (a) and starboard lashings (b).

Figure 6: Comparison of experimental and DeckSAFE time-histories of port oleo extension during embarked testing.

16th Australian Aerospace Congress, 23-24 February 2015, Melbourne

Conclusions A series of experiments were performed with an instrumented helicopter to validate DeckSAFE, an embarked helicopter dynamics code. Datasets were gathered during drop tests and embarked operations, and were compared against the results of DeckSAFE simulations. Comparisons between the experimental data and results from the corresponding DeckSAFE simulations show good agreement, indicating that the simulations are able to accurately predict the dynamic response of an embarked helicopter.

Acknowledgments The authors would like to thank the Swedish Defence Materiel Administration (FMV) for their ongoing support with the DeckSAFE validation programme.

16th Australian Aerospace Congress, 23-24 February 2015, Melbourne

References 1. Healey, J.Val., “The Prospects for Simulating the Helicopter/Ship Interface”, Naval Engineers Journal, Vol. 99, No. 2, March 1987, pp. 45–63. 2. Lumsden, B. and Padfield, G.D., “Challenges at the Helicopter-Ship Dynamic Interface”, Military Aerospace Technologies - Fitec ’98, IMechE Conference Transactions, Institution of Mechanical Engineers, Wiley, 1998, pp. 89–122. 3. Fang, R., Krijns, H.W. and Finch, R.S., “Helicopter/Ship Qualification Testing Part 1: Dutch/British Clearance Process”, Technical Report RTO-AG-300 Vol. 22, NATO Research and Technology Organisation, 2003. 4. Carico, D., “Helicopter/ship securing test & analytic options”, in 40th SFTE International Symposium, Linkoping and Stockholm, Sweden, 7-11 Sep 2009. 5. Hourigan, D.T., Bird, F.J. and Sutton, C.W., “Measurement of the Dynamic Undercarriage Response of a Sikorsky S-70B-2 Helicopter - Instrumentation and Test Methods”, Technical Report ARL-FLIGHT-MECH-TM-462, DSTO, 1992. 6. Blackwell, J., “Tie-down Trials Involving a Sikorsky S-70B-2 Helicopter”, Technical Report DSTO-TR-0132, DSTO, 1995. 7. Langlois, R.G., LaRosa, M. and Tadros, A.R., “Development, Validation, and Application of the Dynaface Helicopter/Ship Dynamic Interface Simulation Software Package”, in Summer Computer Simulation Conference, The Society for Modeling and Simulation International, Montreal, QC, Canada, 2003. 8. Chartier, B.J., Forrest, J.S., Chartier, C.A. and Patch, S.C., “Ship motion limits analysis using DeckSAFE: a ship-helicopter dynamic interface simulation package”, 16th Australian Aerospace Congress, Melbourne, Australia, 23-24 Feb 2015.

16th Australian Aerospace Congress, 23-24 February 2015, Melbourne