Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019
(downdraught) and the upward component (upwash), the former represents a more severe risk to the helicopter than the later. This owes to the fact that a downward component will reduce the angle of attack on the rotor blades thereby reducing lift generation whereas an upwash will tend to increase the lift (also undesirable since the same is an uncontrolled imposed environment) and may hasten stall.
Accordingly, in the latest publication of CAP 437 (CAP 437, 2010), the vertical gust limiting criteria has been replaced with a turbulence criteria (standard deviation of fluctuating vertical velocity component not to exceed 1.75m/s). The reasons for this review are brought out as follows:- “CAP 437 proposes a maximum vertical component over helodeck of 0.9m/s. Simple theory suggests that, in the absence of ground effect, a thrust margin of at least 3% would be required to overcome the effects of this magnitude of gust and maintain a hover over the deck in zero wind. However, it should be noted that it is unlikely that with current helodeck designs of offshore structures, a helicopter could ever experience a 0.9m/s downdraught in the absence of the beneficial effect on thrust margin of a significant horizontal wind component”.
The above assumption will, however, not hold good for a naval helo operation where the helodeck has a hangar in front, which leads to a drastic reduction in the headwind speed and an increase in the downdraught. Towards this, CAA paper 2004/03 (CAA, 2004) brings out the following:- "As turbulence is primarily a problem in high winds when the available lift and power margin is increased, it is considered that torque and power limits are unlikely to influence workload due to turbulence. This assumption may not be valid in cases where there is either a large downdraft, or the rotor is shielded from the free stream flow by superstructure and thus operating in low air speed regime. In either scenario, the amount of power in hand will be reduced and may become an issue depending on the power margins of the helicopter being considered. In terms of applying appropriate criteria to measurements of the expected airwake, the combination of the existing downdraft criterion and the proposed new turbulence criterion, may suffice for those cases involving downdraft. Further, if it were intended to consider the relaxation of the 0.9m/s downdraft criterion following the establishment of a validated turbulence criterion, then it would be particularly important to ensure that the role of torque and power limits in combined downdraft and turbulence condition is fully understood for helicopter response."
2.3 (f) Effect of Moving Helodeck
AGARDograph (RTO, 2003) speaks about the limitations of the helo operations on a moving deck. It brings out that most helicopter manufacturers provide sloped landing limitations for take-off and landing operations outside unprepared helicopter landing sites. In most land-based operations, pilots can adjust the helicopter heading to land either up-slope, down-slope, or cross-slope depending on the safest option. Similarly, limitations may restrict
helicopter ship-borne operations due to the relative geometric attitude of the helicopter to the ship. In the Netherlands, the Flight Deck Officer will launch & recover the helicopter during a quiescent motion period of the ship, with the deck in an almost horizontal position. It must be noted that before take-off and directly after the land-on, the helicopter is secured to the flight deck by means of a harpoon grid system (in some navies known as "Talon"-system). The system greatly increases the allowable ship motions. For those operators where the helicopter crew lacks the assistance of a Flight Deck Officer to launch and land the helicopter, the deck slope aspect is of great importance in order to avoid dynamic roll over.
2.3 (g) Effect of Size of Helo on Response to Turbulence
Venturing into the effect of type of aircraft on the handling qualities, CAA research paper 2004/03 (CAA, 2004) brings out the following:- "The general consensus was that there isn't a single helicopter type, currently used in European offshore operations, that stands out as being particularly poor for handling turbulence. However, the feel of various types is almost certainly different. For instance, a relatively large helicopter with a sluggish response will tend to smooth out many of the gust disturbances such that the pilot is less aware of the turbulence, and may tend to drift further from his intended flight path before it becomes apparent that compensatory inputs are required. The longer a disturbance is left unchecked then the larger is the input required to correct the drift and harder is the task is to re-establish the desired flight path. Conversely, for a relatively light and lively aircraft the majority of disturbances are felt immediately as sharp-edged gusts, and the pilot will make a large number of smaller inputs to compensate. Most disturbances are compensated for before any significant drift is allowed to build, however the overall impression is of a less comfortable ride." The study was inconclusive in deterministically pointing out the effect of aircraft type since the power and torque limits were not included in the model, which often plays an important role in deciding which aircraft types can and cannot operate in adverse environmental conditions. To this end, literature brings out the following (CAA, 2004):- "It will often be the case that shear and downdraught are experienced simultaneously with turbulence, and it is the presence of downdraught, and the availability of sufficient power and torque margins to overcome it, that may often be the determining factor in limiting operations, or in requiring payload to be reduced in order to increase the margins." Various Navies, classification societies and helicopter manufacturers have brought out guidelines for sizing of the helodeck and the related requirements for given helicopters in the form of manuals and standards (Ship Interface, 2001, NORSOK, 2004 and DNV, 2001). Makkar et al (2016), as part of ongoing efforts at IIT Delhi, have experimentally studied the effect of varying downwash velocities of the helo with respect to the ship
©2019: The Royal Institution of Naval Architects
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