M a r i t i m e Helicopter Ship Motion C r i t e r i a - Challenges for O p e r a t i o n a l G u i d a n c e J . L . Colwell
Defence Research and Development Canada - Atlantic 9 Grove St., PO Box 1012
Dai-tmouth, NS, Canada B 2 Y 3Z7 jim.colwell@drdc-rddc.gc.ca
Abstract
Effects o f ship flight deck motions on maritime helicopter operations are considered for flight operations on Canadian frigates in high seas. The major challenges for these operations span dynamics, system identification, control and handling issues for both the helicopter and ship. The prunary focus for these issues is the identification and acquisition o f appropriate infomiation on the helicopter, ship and ocean environment. Two situations are examined: one is selecting the best (or best compromise) ship speed and heading for perfonning the helicopter landing; and, the oüier is defming when flight deck motions are in and out o f limits for perfomiing Üie landmg. In both cases, existing systems and procedures provide consistently high levels o f operational capability and safety; however, in both cases situations exist where the acquisition and display of new types o f information would be o f high value.
1 Introduction
Operating maritime helicopters from frigate and destroyer sized warships is a tmly challenging activity, botii in terms o f t h e human skills and equipment capabilities required to perform the activity, and in terms of the demands for perfomiance data and analytic tools to model this activity. The prunary purpose of tins paper is to describe a few operational scenarios in sufficient detail to identify some o f t h e more immediate technological challenges for improvements to modeling capabilities, w i t h application to operator guidance at sea. Most, i f not all, o f these challenges are equally relevant to simulation and modeling for acquisition, rehearsal and training.
Two scenarios are examined: one is to consider the information required for selecting the best (or best compromise) ship speed and heading for conducting flight operations; and, the second is defming when flight deck motions are in and out o f limits for performing the landing. These situations are illustrated by examining example ocean wave statistics, ship motion calculations, and/or data f r o m flight deck certification trials.
2 Overview of Helicopter/Ship Operations 2.1 Hauldown Landings in High Seas
The Canadian Forces operates CH-124A Sea King helicopters from its twelve H A L I F A X Class frigates and four IROQUOIS Class destroyers. Operations in high seas are aided by the recovery assist, secure and traverse (RAST) system, f r o m hidal Technologies Incorporated.
The RAST system consists o f a constant-tension wmch, located inside the ship, immediately below the flight deck; a rapid securmg device (RSD) on the flight deck; and, a control console located at tiie forward-starboard edge o f the flight deck. The hauldown cable extends from the winch, upwards tinough a faired "bell mouth" opening i n the flight deck, through the RSD, and is connected to the helicopter tlirougli a "probe", as shown in Figure 1. H i e RSD is secured to the flight deck in tracks: during landmg and take-off, the RSD is parked over the bell-mouth, and so the hauldown cable has free access to m n from tiie winch, to the helicopter. Once the helicopter has landed, the RSD secures the helicopter by grabbing the probe'beuveen two "jaws" (hence, the common name "bear trap"). After the helicopter has been straightened to align with the ship's fore and aft centreline, and the rotors have been folded, the RSD traverses the helicopter into the hangar by moving along its securing tracks.
3 The hauldown system does not literally pull the helicopter out o f the air, rather it makes the helicopter behave like an inverted pendulum. This provides "seat o f t h e pants" physical cueing for the pilot, as the bottom o f the aircraft is generally oriented towards the RSD. The typical hauldown cable tension is approximately 10 % o f t h e helicopter's weight. It is common for the helicopter to climb away from the flight deck while the cable is under tension, and it is quite possible, though not so common, for the helicopter to touchdown outside o f the RSD capture zone, in which case the hauldown cable extends horizontally from the RSD to the helicopter probe.
2.2 L i m i t i n g Factors
The primary factors limiting helicopter/ship operations are; • pilot workload;
• aircraft limits (power, control, etc); « wind over deck / turbulence; and, • ship motions.
Pilot workload is directly affected by the other three factors, and is also influenced by other considerations, such as visibility, aircraft loading, and the status o f flight augmentation systems. The focus for this paper is ship motion effects, but there are many other important, limiting factors. The challenges described later in this paper with respect to ship motions are well-matched by many challenges in other technology areas, such as modeling the turbulent airwake [1].
A l l flight operations are restricted by "relative wind", which is the vector sum of true wind speed with ship speed'and direction relative to the wind. A n example relative wind envelope for helicopter landing is shown in Figure 2.
This relative wind envelope is not symmetrical with respect to port and starboard wind directions, due to the location o f t h e Sea King tail rotor on the port side o f the tail boom. For red relative wind (i.e. from the port side o f t h e ship and helicopter), the flow o f wind over the tail rotor is unobstructed, while for green relative winds, from starboard, the tail rotor is partially obstructed by the tail boom, resulting in reduced
Helicopter Relative Wind Operational Envelope
radius = relative Wmd speed (kt) angle = relative wind direction (deg)
effectiveness.
Significant wave tieight (m)
Figure 3: Significant wave Iieight probability of exceedence (POE) curves, for the Western Northern North Atlantic (i.e. C A N L A N T )
With respect to directional characteristics shown in Table 1, a seaway with one sea direction (uni-directional) has all energy coming f r o m within ± 45° of the mean wave direction. A seaway with two sea directions (bi-directional) typically represents separate wind-waves and swell f r o m different directions; and, three or more sea directions is a confused seaway. The uni-directional case includes both uni-modal and'multi-modal cases with all wave components coming f r o m within + 45° o f a common direction.
Table 1: Seasonal variability in percent occurrence of seaways with single and multiple frequency peaks and single and multiple wave directions, for N W North Atlantic [6].
Number o f frequency peaks Number o f sea directions
Season 1 2 3+ 1 2 3-t Winter 45 32 23 59 25 16 Spring 39 35 26 54 27 19 Summer 42 39 19 54 32 14 Fall 37 41 22 51 34 15 A N N U A L 41 37 22 55 30 16
The variation in frequency and directional characteristics between seasons is not large, but the probability of encountering a complex seaway is very high. Considering annual averages, more than half (59%) of seas are multi-modal, with discrete wind waves and swell, or more complex, and almost half (45%) o f seas are multi-directional. Thus, any models used to define the seaway for flight planning must be able to handle complex seaways.
3.2 Ship Response to the Ocean Environment
Figures 4.1 and 4.2 show example ship motions calculated by program SHIPMO [7] for a uni-directional wave system, in Figure 4.1, and a multi-directional wave system, in Figure 4.2. Each sub-figure contains two polar plots, showing contours of roll angle and flight deck vertical acceleration, F D V A , on the left hand side, and contours o f lateral and longitudinal Motion-Induced Interruptions [8,9], MII(Lat)^ and MlI(Lon) on the right hand side. Roll and F D V A are representative o f ship lateral and vertical motions, which are both critical for landing. The M I I is a concept in modeling human stability- a M i l occurs when ship motions cause a person to tip over, or slide. For this example, a lateral M i l occurs when a person tips sideways, and a longitudinal M i l occurs when the person tips frontwards or back\vards. For the present
7
For the uni-directional seaway in Figure 4 . 1 , the waves are coming from 0° (i.e. North), and so a ship course o f 0° is directly into the waves, resulting i n low roll motion and high F D V A . For the muUi-directional seaway in Figure 4.2, the same wave system is coming f r o m 0°, and a second system is coming from 270°. Ship speed is the same for all cases.
The key difference between the uni-directional and multi-directional cases is that the ship motions and M i l s are symmetric with respect to wave direction f o r the uni-directional seaway, and not symmetric for the multi-directional seaway. Thus, in a multi-directional seaway, the selection of ship course to the port or starboard o f the wind direction, to provide acceptable relative wind, can produce a dramatic difference in ship response.
3.3 Challenges f o r Operator Guidance on Flight Planning
Most o f the tools required to model complex seaways and calculate resulting ship responses exist today, and are o f sufficient accuracy for most contemporary, monohull frigate and destroyer hull forms. Integrating ship motion models into a shipboard, computer-based system which is easy to use and reliable presents a variety o f practical challenges, but should be achievable.
The most significant challenge f o r providing in situ operator guidance for detennining ship speed and course for helicopter operations is likely to be in correctly identifying the height and frequency o f two or more components in a multi-modal or multi-directional seaway. Comparison of visual estmiates by trained observers with measurements o f wave height and frequency, suggests that wave height can be reliably estimated, but estimates o f frequency are not reliable [10]. These comments are solely based on studies where waves are described using only a single wave height and frequency (i.e. uni-modal), and so it is hkely that visual estimates in complex seas are even less reliable. In order to provide the best estimate of wave component periods, the ship should be drifting at zero forward speed; however, this is tlie least likely scenario i n a real operational environment. It is possible to work backwards from measured ship motions to define the wave system (e.g. [11]), but this is generally not reliable in multi-dfrectional seas, as the many possible combinations o f wave direction and frequency result in many possible solutions.
Commercial systems are available to measure directional wave spectra from analysis o f wave backscatter on radar images, but tliis requires active electromagnetic emissions f r o m the ship, which is not always possible for a warship. Measurement of ocean wave spectra from space-based radar has been a research topic for some time, but has not yet developed into a mature product. Directional wave buoys are deployed on dedicated sea trials and experiments to measure directional wave spectra, but these devices are expensive, and so they have to be recovered, which is not suitable for nomial warship operations. It may be possible to develop 'disposable' wave buoys, by combining directional wave sensing capabilides with existing launched-systems which are not recovered, such as sonobuoys.
4 L a n d i n g the Helicopter - Identify Quiescent P e r i o d ( Q P )
The best time to land on tlie moving flight deck is during a quiescent period, when all ship motions are witliin acceptable limits; however, landings are often attempted, and sometimes occur, when the fliglit deck is not quiescent. This "mis-cue" invariably occurs because boüi the pilot and LSO beheved that the flight deck was quiescent, but either it was not, or ship motions '\^•ent out o f limits' soon after the landing was initiated. I f the LSO recognizes this situation, then a " w a v e - o f f is declared, and the pilot must retum to high hover. I f the pilot recognizes this situation, then an undeclared wave-off is usually the result, when the pilot voluntarily returns to high hover. I f neither the LSO nor tiie pilot temiinate the process, then a successful landing may occur, but there is an increased chance of a "hard landing" (described in more detail later), or o f a wave-off i f the helicopter 'misses the trap', or begins to slide on deck before bemg trapped by the RSD.
F D V A , for 4 2 landings in sea states 5 and 6 , with 2 9 daytime landings (open symbols) and 13 night time landings. Relative velocity, V R , is expressed as the fraction o f its limit value, V R A ^ R ( M A X ) , and F D V A is i n
gravities. The relative velocity data are obtained by differentiating measured oleo displacements at the time o f landing, and the data shown in Figure 5 are the maximum velocity measured on either the port or starboard landing gear.
1.0 0.8 0.6 V R , A / R ( M A X ) 0.4 0.2 0.0 A A A Day • Night A A A A A A A A^ A A A . A A A A k A A A A ^
I ^
A A ^ AA Flight deck falling Flight deck rising •
- 0 . 2 - 0 . 1 0 . 0 0 . 1
Flight deck vertical acceleration, FDVA (g)
0 . 2
Figure 5 : Relative vertical velocity at touchdown, V R , as a function o f flight deck vertical acceleration, F D V A
Figure 5 clearly shows that there in no systematic relationship between flight deck vertical motion and relative velocity at touchdown. Thus, assessing a relative velocity criterion by monitoring ship vertical velocity is not reliable. This further suggests that the only reliable method to assess relative velocity between the ship and helicopter is to measure it directly. This also agrees with anecdotal comments f r o m experienced test pilots, to the effect that the magnitude o f flight deck vertical velocity is not important, as long as the pilot knows what it is.
Various technologies have potential application for measuring relative velocity between the ship and helicopter, to provide reliable realtime guidance for avoiding hard landings. When connected, the hauldown cable velocity is a direct measure o f relative velocity, but cannot resolve the difference between vertical and lateral velocity components. Differential GPS between the helicopter and ship can determine both vertical and lateral relative velocities, but it requires an active telemetry link, which may not be acceptable in a low emissions scenario. A lookdown laser rangefmder/altimeter on the helicopter can provide onboard guidance, independent o f an active connection with the ship, but laser emissions may be a concern for remo'te sensing of the helicopter and ship (the low energy lasers used for this application are not a concern for human health and safety).
