Concept of a Predictive Analysis Tool for Ship Helicopter Operational Limitations of Various In-Service Conditions

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1 Concept of a Predictive Analysis Tool for Ship Helicopter Operational Limitations of Various In-Service Conditions Lieutenant Alrik Hoencamp MSc (RNLN) Experimental Flight Test Engineer Research Engineer Helicopter-Ship Interaction Netherlands Defence Academy (NLDA) The Ship Helicopter Operational Limitation qualification programs are based on a high number of independent variables causing the full operational potential to be almost impossible to achieve within the small window allowed for sea trials. It would be a clear advantage to have a predictive engineering tool to perform early evaluation of safety limits for operating aircraft from ships in a wide range of in-service conditions and so be less dependent on the results from several dedicated sea trials. For this reason a predictive analysis tool is under development based on specific rejection criteria for each helicopter type and their dependencies in the ship environment. The tool could be used for determining the Candidate Flight Envelope for each ship type, allowing larger steps in an incremental approach towards flight envelope restrictions, sensible exclusion of test points and accurate readacross between other helicopter-ship combinations. The accuracy of predictions by the analysis tool allows advanced insight into the cost effectiveness of complete sea trials and ultimately certification of large parts of the envelope with minimal sea trials. Figure 1; AS-53 U Cougar INTRODUCTION 1 The qualification process currently used for determination of Ship Helicopter Operational Limitation (SHOL) by the Netherlands Ministry of Defence has proven to be a useful approach. In general the qualification process Presented at the American Helicopter Society 67th Annual Forum, Virginia Beach, VA, May 3-5, 11. Copyright 11 by the American Helicopter Society International, Inc. All rights reserved. consists of two independent items resulting in the Candidate Flight Envelope (CFE) for sea trials, namely the determination of the environment near the ship deck and the helicopter flight characteristics during shorebased hover trials [1, ]. Unfortunately there are some major drawbacks in this current qualification approach, which include but are not limited to: dependency on encountered environmental conditions, dependency on subjective opinions from few pilots and major costs associated with readiness of both helicopter and ship for long periods. Despite

2 all the preliminary efforts, the resulting SHOL is solely based on acceptable test points achieved during dedicated sea trials. However, it occasionally happens that either due to prevailing weather conditions or aircraft availability the limits of the aircraft can not be fully explored in some areas or at some masses. For the above reasons, it will be a clear advantage in the SHOL qualification process, to have a predictive engineering software tool, and so be less dependent on the results from several dedicated sea trials. This will reduce time and cost related to the test campaign, and will also improve the accuracy of the resulting SHOL, which is to be used during in-service operation for many years to come. Additionally, it allows assessing the impact of design changes to both helicopter and ship after the SHOL has been released to service. It has become clear that these operational factors could already be addressed early in the test campaign which out of necessity involves developing a predictive capacity in all areas which influence operational capability. This article describes in more detail how the objective criteria are used to construct the CFE. TEST CAMPAIGNS Test campaigns to establish SHOLs are laboursome, and a predictive engineering tool can offer significant relieve. The flow chart for SHOL test campaigns that make use of the capabilities of the predictive analysis tool is shown in Figure. The determination of the environment characteristics near and above the flight deck presently relies on wind tunnel and full scale testing at sea, although this could be supplemented or possibly replaced by Computer Fluid Dynamics (CFD) in the near future [3, 4]. The determination of helicopter capabilities starts with a ground assessment to assess for instance flight manual data and Flight Control Mechanical Characteristics (FCMC). Where possible this is followed by an assessment with a flight simulator and spotchecked by actual shore-based hover trials to document i.e. power requirements and control positions for the complete 3 azimuth at different referred weights and Center of Gravity (CG) positions. The trials are performed at the required values of referred weights, W, which have been set as the operational weight bands for use on board ships. The data is converted into referred parameters, so that it can be used to produce information relevant to atmospheric conditions and aircraft masses different from those actually tested. Consequently, with a few exceptions, a relatively small number of tests, at carefully chosen test sites, can produce information relevant to a large part of the helicopters flight envelope. The obtained helicopter hover and low speed capabilities are subjected to the local airwake conditions of each ship type to derive the CFE. The CFE is the basis for preparation and execution of flight trials on board ships to establish SHOLs. Environment Simulation of ship airwake characteristics Validation of simulation and anemometer system on board ship Full-scale test with ship at sea Defined environment Complete flight test at sea for new class of ship and new helicopter type Determination of flight test Effect of environment on helicopter Candidate flight envelope (CFE) for helicopter on board ship Establish confidence and adequate safety margin of the CFE Ship Helicopter Operational Limitation (SHOL) Helicopter Ground assessment Document detailed a/c flight characteristics Simulator (if possible) Spot-check Shore-based trials Defined a/c capabilities Optional flight test at sea for areas with low confidence in CFE Figure ; Flow chart with options The predictive software tool can be used for three different aims once the CFE is established: (1) complete sea trials for new ships and helicopter types, () sea trials for areas with low confidence in the CFE and/or small safety margins before exceeding rejection criteria, and (3) direct SHOL certification without even conducting sea trials. Being able to decide on SHOLs based on outcomes of such a tool is novel. The benefits are numerous like rapid introduction of new helicopter types across the fleet and advanced insight for cost effectiveness of sea trials. In general there are five main application areas:

3 1. CFE. The software tool can be used for construction of the CFE in order to indicate dangerous areas and thereby allowing larger steps in difficulty for an incremental approach;. Testpoint exclusion. It allows testing focused on worst case conditions, by sensible exclusion of test points in the test matrix (i.e. day/night conditions and multiple spots); 3. Read-across. It can be used for readacross between other helicopter types which are validated to operate from the same ship class; 4. Certification. Once enough confidence is established in the model it can be used for certification purposes without conducting complete sea trials; 5. Miscellaneous. Besides SHOL analysis the model can be used for establishing safe operating envelopes for oil rigs and rooftop helipads. REJECTION CRITERIA The predictive analysis tool is under development based on specific rejection criteria for each helicopter type and their dependencies in the ship environment, and so be less dependent on the results from several dedicated sea trials. The subdivision between rejection criteria and dependencies in test data enables: assessing other possible conditions for in-service operations, forms the basis for proper read-across and enables accurate exclusion of test points. The definitions for rejection criteria and dependencies are: Rejection criteria are quantitative and qualitative aircraft parameters which, once exceeded, prevent safe execution of a flight phase. Dependencies are variables in a flight phase which directly influence their related rejection criteria. Rejection criteria and how they are influenced by dependencies are determined for new aircraft or as a consequence of significant changes to an old aircraft which might affect low speed performance and/or handling qualities. The initial collected helicopter data is saved into lookup tables and used for future helicopter-ship qualification trials, as the steady-state aircraft characteristics are valid for trimmed hover conditions in a unique solution [5]. The predictive software tool processes in a network the collected data, and allows all relevant rejection criteria and dependencies to interact with each other. A summary of the most common rejection criteria and their dependencies is shown in Table 1. There is a distinction made between performance, control position, subjective and aircraft attitude related issues. It also shows that in addition to the steady-state characteristics an additional safety margin is applied to allow aircraft handling while influenced by any relevant dependencies in the ship environment. Item Rejection criteria Dependencies Performance rejection criteria 1 Performance Relative wind (airwake), Referred weight, Configuration Performance safety margin Relative wind (airwake), Referred weight, Visual reference, Ship motion Control position rejection criteria 3 Tail rotor authority Relative wind (airwake), Referred weight, FCMC 4 Lateral Cyclic Relative wind (airwake), Referred weight, CG, FCMC 5 Longitudinal Cyclic Relative wind (airwake), Referred weight, CG, FCMC 6 Control safety margins Relative wind (airwake), Referred weight, Visual reference, Ship motion Subjective rejection criteria 7 Vibrations Relative wind (airwake), Referred weight 8 Pilot workload Relative wind (airwake), Referred weight, Visual reference, Ship motion Aircraft attitude rejection criteria 9 Roll attitude Relative wind (airwake), Referred weight, CG, FOV Pitch attitude Relative wind (airwake), Referred weight, CG, FOV Table 1; Rejection criteria and dependencies

4 HELICOPTER ITEMS The flow chart for SHOL test campaigns distinguishes between environment and helicopter items. In this paragraph examples are shown for processing objective test data gathered during shore-based hover trials and how this data is used by the predictive analysis tool to construct the CFE. It should be noted that the examples are based on shore-based hover trials for the AS53 U Cougar, which were only performed in natural wind conditions [6]. As a consequence the data gathering campaign took a long time, waiting for the correct wind conditions, and only has useful data up to maximum wind speeds of approximately 5 knots. The AS53 U Cougar is a twin engined medium weight transport helicopter with a four bladed clockwise rotating main rotor and a bottom-up rotating tail rotor. Its maximum takeoff weight is 9.7 kg. Conventional cyclic, collective and yaw pedals are fitted, assisted by a hydraulic system. The baseline helicopter, as applied for the shore-based hover trials, had the most adverse configuration with respect to hover performance. The following items were installed: Maritime sponsons; Chaff/flare dispensers of the Integrated Self Protection System (ISPS); Jet Dilution Devices (JDD, i.e. infra-red suppressors mounted over the engine exhausts); Multi Purpose Air Intakes (MPAI) bullets closed. The engine inlet air is then directed through particle filters. A significant degradation of engine performance results at hot and high conditions. The twin engine torque ratings of the AS53 U Cougar helicopter are as shown in Table. Item Tq Time Maximum continuous 77% No limit Maximum take-off % 5 min Maximum transient 1% sec Table ; Torque ratings [7] Performance rejection Referred parameters. The objective of flight tests is often to determine the parameter that will limit the performance of a helicopter under the atmospheric conditions specified in a role specification. Although, it should be kept in mind that under certain atmospheric conditions, usually hot and high, the engines, rather than the transmission will limit the performance. It is therefore necessary to determine the precise limiting factor for the conditions specified to enable data points flown during different mission of the test campaign to be compared with each other. Once the precise limiting factors are known referred parameters are used [8, 9]. For torque, whilst knowing the relationship between power required and torque ( Q ): P Q W V Vc f,,, 3 Z, Where P is power required, is relative density, is relative rotorspeed, W is weight, V is airspeed, V is rate of climb, Z is height c and is relative temperature. Within the ship environment the benefits of ground effect are considered negligible []. Hence only Outside Ground Effect (OGE) low speed conditions are tested without any vertical speed: P Q 3 W V f,, This equation confirms that the performance of the helicopter is mainly influenced by the relative wind conditions (airwake), referred weight and aircraft configuration influencing drag including the rotorspeed setting. Error analysis. Errors in flight test measurements introduce the inevitable uncertainty that is inherent in all measurements. Whenever a measurement can be repeated, it should usually be done so several times. Unfortunately, exact similar conditions are difficult to establish during shore-based hover trials and especially for all other in-service conditions afterwards. Therefore making predictions based on minimal test points performed during shorebased hover trials is accompanied by the use of error bars. These error bars are determined by summation of instrumentation errors and fractional errors in quadrature [11]. This

5 approach also provides an effective check on the significance of error sources and a method for the identification of the most significant error source. Thus if, for example, the reduction of raw data involves parameters raised to high powers, such parameters need to be measured with a high degree of precision if the calculated result is not to be unduly degraded in probable error. This is important as an analysis of measured errors and required computation may affect the choice of instrumentation for a given trial. Relative errors in referred power required are determined by [8]: P P ref ref Q Q Where x is called the uncertainty, or error, in the measurement of x. Discussion results. Below 5 kts wind speed, the engine torque is almost independent of wind direction as shown in Figure 3. At higher wind speeds, the torque required varies with wind direction. In winds from port (red winds) more torque is required than in winds on the nose and from starboard (green winds). The torque required is highest at relative winds from the direction R. This has amongst other reasons to do with the tail rotor which has to deliver more thrust to compensate the weathercock effect of the fuselage as more right power pedal is required to maintain heading for a clockwise rotating main rotor. In winds from starboard, the tail rotor is assisted by the weathercock effect of the fuselage, so less thrust is required. Actual Engine Torque [%] AEO Take-off Safety Margin AEO Max Cont Figure 3; Example torque required Furthermore torque required is dependent on the aircraft referred weight which is in correlation with the performance graphs provided in the Flight Manual [7]. In addition the following performance penalties due to configuration changes were determined in the environmental conditions tested: Jet Dilution Devices installed give ±% additional torque; Nr+ switch applied results in a reduction in engine torque of ±% in hover decreasing effectives with airspeed. s. The steady-state aircraft performance is calculated as previously described and will be valid for trimmed hover conditions in a unique solution [5]. However, in addition to steady-state performance a safety margin is applied to allow the aircraft to be handled while influenced by any relevant dependencies in the ship environment. Control rejection Pedal position. The mean pedal position is plotted as a function of relative wind direction at the same referred weight for two different wind speeds as shown in Figure 4. It shows that in red winds increasing right pedal deflection is required for increasing wind speed to maintain helicopter heading, while in green winds increasing left pedal deflection is required. Pedal deflection is thus dependent on wind speed and wind direction similar as for torque. The tail rotor pitch would have been a better parameter for remaining control margin then pedal position, due to interactions with the collective-yaw interlink. RHT 9 Pedal position (%) 8 7 LFT Figure 4; Example pedal position Cyclic position. The cyclic positions changed slightly with different referred weights and relative wind conditions as shown for lateral and longitudinal cyclic positions in Figure 5 and Figure 6 respectively. There is a trend in cyclic movement noticeable to move into the relative wind direction to maintain ground reference position as the wind speed increases. An overview of cyclic positions encountered

6 during shore-based hover trials with mid-center of Gravity (CG) is shown in Figure 7, indicating that no safety margins are approached throughout. Although not performed as part of this trial it is recommended to test different CG locations to establish the corresponding shifts in cyclic positions to assure that enough control authority remains for the complete CG envelope without redoing each test point [8]. RHT 9 Lateral cyclic deflection [%] 8 7 LFT FWD Longitudinal cyclic deflection [%] 7 8 Figure 5; Example lateral cyclic 9 AFT FWD Longitudinal Cyclic (%) AFT Figure 6; Example longitudinal cyclic LFT RHT Lateral Cyclic (%) Figure 7; Example cyclic positions Aircraft attitude rejection The aircraft attitudes in correlation with the Field Of View (FOV) indicate which maximum roll and pitch attitude are achievable for safe operations from the flight deck while maintaining enough visual reference to the landing site [8]. Roll attitude. Below 5 kts wind speed, the roll attitude slightly changes around ±º right Angle Of Bank (AOB) as shown in Figure 8. At higher wind speeds, roll attitude varies with wind direction. In red winds more AOB to the left is required than in winds on the nose and for green winds more AOB to the right is required. The maximum AOB occurs for G9 winds and are quite large. This could result in issues with a moving ship, especially as the ship will normally be tilting to port with green winds, thereby increasing the relative angle between the helicopter and the flight deck. RHT 8 Bank angle [deg] LFT Figure 8; Example roll attitude Pitch attitude. Below 5 kts wind speed, the aircraft pitch slightly changes around ±3º pitch-up with wind direction as shown in Figure 9. At higher wind speeds, aircraft pitch attitude varies with wind direction. UP 6 Pitch angle [deg] DWN Figure 9; Example pitch attitude

7 Summary It is outside the scope of this article to explain all the aircraft characteristics of the shore-based hover trials. Examples shown are just an indication of the kind of data gathered and how it is processed. The software tool presents the data of shore-based hover trials in a polar plot that makes it easy to indicate which pre-set margins are exceeded and for which relative wind conditions this applies as shown in Figure. This data is plotted together with the maximum hover envelope from the flight manual, and the maximum safe operating envelope that allows for lateral positioning above the flight deck including safety considerations for tail wind conditions. The polar plot shows that in line with the previously presented data below kts performance limits are approached or even exceeded taking error bars into account. Furthermore G9 roll attitude is large and R pedal position is approaching limitations. This concludes the analyses of helicopter related items expressed in rejection criteria determined during shore-based hover trials. It is now a matter of relating the relevant rejection criteria with local wind conditions for each ship type. R KTS R9 R Performance Roll attitude Pedal position R1 Relative wind direction G9 KTS 18 G G1 G G1 Figure ; Example shore-based hover trials ENVIRONMENT ITEMS The results of the shore-based hover trials are based on the relative wind conditions encountered by either hovering in natural wind conditions or by using a pace-vehicle. Near and above the flight deck the relative wind is differently disturbed by the large superstructure as shown in Figure 11. This disturbed wind is what the helicopter faces when operating from the flight deck and is known as local wind. The local wind conditions are determined by simulation and/or full-scale measurements. Unfortunately both these wind conditions are unknown for the operational crew after the trials, as ship anemometers are their only reference source. Furthermore by mounting anemometers on a ship with a bluff body, the local air flow (speed and direction) at the anemometer location will also deviate from the free air stream. Figure 11; Superstructure ship Therefore it is important to distinguish between three different wind conditions: 1. Relative wind. The shore-based hover trial is based on these data and it is the free air stream near the ship.. Indicated wind. The relative wind with the anemometer errors taken into account. The SHOLs are based on this wind condition. 3. Local wind. The local wind conditions are changing for each position near and above the flight deck. These are the wind condition the helicopter will face during ship board operations. To establish the relation between these three different winds conditions, airflow trials are conducted for every ship type at various landing points and approach and departure paths [1]. The mean but also the maximum and minimum values for local wind are important as these values are to be correlated with the results of the shore-based hover trials. Some example data collection points locations measured at different heights for a Fore-Aft approach from port side are shown in Figure 1.

8 Maximum values Chi (deg) - Minimum values Indicated wind direction (deg) Figure 14; Example azimuth deviation spot 1 Maximum values Phi (deg) Figure 1; Example data points The aim of these airwake tests is to establish for each landing spot the magnitude of errors between the relative wind and the local wind conditions as shown for wind speed and azimuth in Figure 13 and Figure 14 respectively. Where the speed correction factor is C V V and the horizontal azimuth deviation is. Areas with vertical speed v loc an components are expressed in angles from the horizontal plane as shown in Figure 15. The areas with a large negative angle in combination with high wind speeds may create problems in torque requirements and should be approached in a cautious and progressive way. Note that it can be seen from these figures that the landing spots are meters towards the port side of the ships centerline as the minimum speed and strongest vertical speed components occur around G5 instead of dead ahead. Cv[-] Actual wind direction (deg) v loc an Maximum values Minimum values Figure 13; Example speed deviations spot Minimum values Actual wind direction (deg) Figure 15; Example vertical flow spot 1 CFE Once the helicopter and environment issues are determined and clearly documented these results are combined to construct the CFE. The helicopter flight characteristics are gathered during shore-based hover trials in either actual wind conditions or relative wind conditions by using a pace-vehicle, while the wind conditions encountered during operations from the flight deck are local wind conditions. It is now a matter of correlating between relative and local wind conditions across the flight deck, in the approach and in the departure path for each ship type. This is done by constructing lookup tables expressed in relative wind conditions with shore-based hover trial results, knowing that for each relative wind condition there is a unique trim position [5, 13]. Furthermore by making lookup tables for maximum and minimum values of local wind encountered during the approach, take-off and landing expressed in relative wind conditions. The predictive analyses tool processes the data in such a way that for each relative wind condition, the maximum and minimum values for azimuth, speed and vertical flow deviations in local

9 wind are applied to the rejection criteria. Thereby ensuring that during in-service conditions, the helicopter has sufficient power and control margins to perform a safe landing. Consequently, the software tool gives the indicated wind envelope which could be released for in-service operations for that particular ship type taking anemometer correction into account. SHOL COMPARISON There is an example for a Fore-Aft Port (FAP) side approach to spot 1 for a high referred weight and for a low referred weight CFE using the data presented in this paper as shown in Figure 16 and Figure 18 respectively. There are only four relevant rejection criteria indicated, as the others are not approaching safety margins. It shows that even though some rejection criteria are indicated these areas are still included in the CFE. Those areas should be approached in an incremental approach and the main focus of the sea trials. To allow enough torque margins for this example at least the area below kts wind speed was removed from the CFE. It should be noted that especially for the high referred weight CFE it is a restriction that no windtunnel data is available for the departure path to the leeward side, and thus not included in the data processing. Based on previous knowledge with the SH-14D Lynx it is known that there are large downdrafts on the leeward side of the ship. Therefore the maximum angle of G is applied due to safety considerations from previous experience. This underlines the requirement to have an accurate and complete data set to construct a CFE. The software tool enables the data to be processed such that the most accurate CFE is rapidly established. Although the predictive analysis tool is still to be validated a comparison is made with actual test results of sea trials as shown in Figure 17 and Figure 19 respectively. The SHOLs are not exactly similar to the CFEs due to amongst others time constraints in the test campaign. This is quite common as there is a dependency to encounter the required environmental conditions during the trials as presently the SHOLs are solely based on the actual test points flown on the ship. The test campaign was for this reason focused on quickly assessing higher referred weights instead of expanding envelope boundaries. The SHOL comparison also shows that there are test points rated as acceptable exceeding 95% torque. Exceeding this safety margin is allowed to deal with unexpected turbulences or gusts, as this is where the safety margin is for. It should be explained in the final results and could be only a couple of seconds and not used consistently. Another reason that test points above 95% torque are rated as acceptable is that pilots are changing control strategy, thereby flying more conservative making approaches with less torque fluctuations. R Relative wind direction G R Relative wind direction G R G R G KTS R9 G9 KTS KTS R9 G9 KTS Performance Performance (downdraft) Roll attitude Pedal position G1 Acceptable Marginal Unacceptable G1 R1 18 G1 Figure 16; Example CFE spot 1 FAP (high referred weight) R1 18 G1 Figure 17; Example SHOL spot 1 FAP (high referred weight)

10 R Relative wind direction G R Relative wind direction G R G R G KTS R9 G9 KTS KTS R9 G9 KTS Performance Performance (downdraft) Roll attitude Pedal position G1 Acceptable Marginal Unacceptable G1 R1 18 G1 Figure 18; Example CFE spot 1 FAP (low referred weight) CONCLUSION A predictive engineering software tool, relying on actual test data, is developed based on specific rejection criteria for each helicopter type and their dependencies in the ship environment. The predictive capacity involves an improved set-up of the test campaign, both conducted during shore-based hover trials and sea trials, allowing accurate analyses and processing of data collected. The model could be used for determination of the Candidate Flight Envelope for each ship type allowing larger steps in an incremental approach towards flight envelope restrictions, sensible exclusion of test points and accurate read-across between other helicopter-ship combinations. The software model thereby not only reduces time and cost of the test campaign, but also improves the accuracy of the finally determined SHOL used for in-service operation for many years to come. FUTURE WORK The accuracy of the predictive software tool will be precisely determined during the introduction of the NH9 helicopter the coming years. Once enough confidence is established it will be decided to which extend the model could be used for certification purposes without conducting complete and sometimes redundant sea trials. R1 18 G1 Figure 19; Example SHOL spot 1 FAP (low referred weight) ACKNOWLEDGEMENTS The PhD research is conducted and sponsored by the Netherlands Defense Academy (NLDA). The following persons advise during the research Prof. Th. van Holten and dr. M.D. Pavel from Delft Technical University, Prof. G.D. Padfield from University of Liverpool and Prof. J.V.R. Prasad from Georgia Institute of Technology. Their contribution to the research is very much appreciated. REFERENCES 1. Fang, R., H.W. Krijns, and R.S. Finch, Helicopter / Ship Qualification Testing, Part I: Dutch / British clearance process, in RTO-AG- Vol., RTO/NATO, Editor. 3.. Hoencamp, A., An Overview Of SHOL Testing Within The Royal Netherlands Navy, in European Rotorcraft Forum 35th. 9: Hamburg, Germany. 3. Forrest, J.S., C.H. Kääriä, and I. Owen, Determining the Impact of Hangar-Edge Modifications on Ship-Helicopter Operations using Offline and Piloted Helicopter Flight Simulation, in American Helicopter Society 66th Annual Forum. : Phoenix, AZ. 4. Snyder, M.R., et al., Determination of shipborne helicopter launch and recovery limitations using computational fluid dynamics, in American Helicopter Society 66th Annual Forum. : Phoenix, AZ.

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