Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019


carrier which incurs enormous costs to the operating country. The global recognition of aviation component of a fleet as an invaluable tool for power projection and extending surveillance reach, has led many countries to invest heavily into research for making shipboard helo operations more effective.


In order to facilitate helo operations for fulfilling the above designated roles, frontline warships are provided with a helodeck for landing/ taking off operations and a hangar for safe stowage of helo, when not in use. The hangar, in most of the modern non aviation ships (ships which are primarily designed for non aviation functions but carry rotor crafts additionally for capability enhancement), is like a rectangular prism placed just ahead of the helicopter deck as part of the superstructure. Safe helo operations, on such ships, require the ship to move within a particular range of speeds to ensure favourable Wind Over Deck angles (WOD) or relative wind conditions on the helodeck. While doing so, presence of the superstructure in front of helodeck introduces many challenges for the pilot for operation of helicopters primarily emerging from a turbulent recirculation zone with a reduced free stream velocity and an increased downdraft. The flow characteristics are very similar to the classic problem of 2D back facing step. In older ships, the hangar used to be smaller and generally not extending the full breadth, thus ensuring that the resulting recirculation zone was limited (Figure 1).


been largely neglected. This has led to severe restrictions on helo operating conditions, which, in earlier non-stealth ships have not been a cause for serious concern. Hence, unlike helo operations on land, the environment on the helodeck which is available to the pilot for operation on modern warships present numerous challenges. With minimal technology support and fairly limited information to assist them when they undertake operations, the helo crew have to rely heavily on their acquired skills and experience (Whitbread & Coleman, 2000).


Figure 2. Helicopter Deck and Hangar configuration in State of the art frigates/ destroyers (http://en.wikipedia.org/wiki/Formidable-class_frigate)


Figure 1. Older configurations of hangar (Not full Breadth) (http://www.royalnavy.mod.uk)


Contrary to earlier times, in addition to catering for the space requirements for deck fittings and equipment, Navies across the globe have emphasised the need for designing their platforms for making them stealthier by reducing their signatures. Profile view of a typical hangar and helicopter deck configuration on a non-aviation warship can be seen as a backward facing step as shown in Figure 2. As part of design for a reduced RCS, helo hangars on board modern mid-sized warships have been made to extend full breadth of the ship. Although such a design change has reduced the RCS by cutting down on the number of sharp edges, the study of deterioration in flow conditions on the helodeck due to this change, has


A-402


Figure 3 shows the various flow features which are normally encountered on the helodeck behind superstructure.With the contemporary configuration of helodeck and hangar on Naval vessels, the back facing step results in the formation of reverse flow zone (recirculation zone) on the helodeck with unsteady flow in terms of shedding vortices from the corners and sharp edges of hangar and helodeck. In addition, there is a complex interaction when the helo downwash impinges on the airwake over the helodeck, which are already affected by cross winds entering from the sides of the hangar leading to large changes in the flow structure. Such conditions ensure operation of the rotors in a turbulent and uneven wake thus increasing pilot workload (a measure of difficulty for a pilot to operate in given environmental conditions) manifold as compared to operations on land. The problems are further compounded while operating in heavy seas, since the ship motions provide the pilot with a moving platform for takeoff and landing. Further, these ship motions also contribute towards altering the flow structure on helodeck by oscillating the vortices shed from the edges and corners. Also, while operating in high seas, pilots often face poor visibility due to sea spray and lack of visual cues due to aircraft orientation with respect to the ship. These challenges impose severe limitations on allowable wind conditions on deck for safe helicopter operations. In extreme cases, the conditions may lead to helicopter accidents while operations are underway as have been experienced by Navies around the world.


©2019: The Royal Institution of Naval Architects


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