Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019
Hawk helicopter has been flown to the deck of several different ships, under the influence of unsteady CFD- based ship air-wakes. The authors have indicated achieving the first fully simulated SHOL diagrams published in the literature by deriving pilot workload ratings. Given the complexity involved in quantifying the qualitative assessment of test pilots during the Dynamic Interface Testing, this route merits attention wherein the environment may be mathematically modelled and the qualitative assessment of the Test Pilots in making of SHOL through Dynamic Interface Testing is retained. In such a case also, all the problems presently associated with the process of Dynamic Interface can be alleviated.
3. FINDINGS
The various factors affecting Shipboard helicopter operations can be classified into the following categories:- The helicopter, the class of ship, the operational environment, crew and operator. Towards investigation of the problem in its entirety, the literature survey has made an attempt to understand each of the above aspects and their interplay. Based on the effect of various components of airwake on helo handling capability and pilot workload, flow parameters defining the helodeck environment, have been identified. The limited knowledge availability about application of criteria for marine helo operations have been understood. It is understood that, as on date, no attempt seems to have been made to evaluate a ship design against a set of criteria for ensuring efficient helo operations. In general the survey of literature leads to the following conclusions:-
a) High workloads due to turbulence increase the risk of a pilot making an error of judgement despite the high level of skill normally associated with helicopter flying.
b) The greatest risk to helicopter operations is judged to be the point where the helicopter arrives over the helodeck and is required to hover prior to touchdown.
c) Lower the standard deviation of vertical velocity on the planes of rotation of helicopter rotor, lower is the turbulence and hence the pilot workload. Since the length scales of turbulence behind a ship superstructure are of the order of rotor diameter, increased turbulence will contribute to increased pilot workload. Thus, any reduction in the turbulence intensities on the rotor planes must necessarily translate into reduction in the pilot workload.
d) 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 for helo operations on offshore structures which generally have very less obstructions. This assumption is not valid in case of ship helodeck, 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 a downdraft criterion and a turbulence criterion, may be examined.
e) Levels of turbulence encountered in offshore installations are likely to be less than those associated with naval operations due, both to the better exposure of offshore helodecks, and the fact that wind speeds over naval helodecks are generally increased by the forward speed of the ship.
f) A good headwind component gives additional forward airspeed to provide a margin for recovery in the event of an engine failure. Relative wind conditions where very heavy turbulence exists (high wind speeds from ahead), in combination with spray nuisance and rather large ship amplitudes, especially in pitch, have to be avoided.
g) High engine power is needed at low relative wind speed and at high helicopter mass. Furthermore, at low relative wind speed the down-wash of the rotor generates spray, which is most bothersome when the helicopter hovers alongside the flight deck.
h) Winds from tail are not desirable for aircraft operation and are generally avoided during ship borne operations. Similarly, out of wind components are not generally preferred for use by helicopter pilots.
i)
If a helicopter does not have the available thrust margin to hover in downdraught, then the pilot will not be able to avoid being pushed onto the deck. Downdraft here is considered to be distinct from time- varying turbulence, although typically increased turbulence would accompany a large downdraft. Downdraught is generally considered a function of helodeck aerodynamics.
j) Between the downward component (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.
k) The airwake obtained on the helodeck on the rotor planes in wind tunnel or in CFD can be represented/replaced with single statistical quantities assumed to be acting on the rotor hub. Statistical estimates for turbulence on helodeck, form the basis of present day practice of fixing criteria for design of marine environment for helo landing.
l) Although the real area of interest for helo operations on warships lie immediately downstream of the recirculation zone, any reduction in the extent of the recirculation zone is surely expected to lead to a greater envelope for safe helo operations on the deck.
m) Above a particular critical value of Reynolds number, the flow characteristics of the ship air-wake are independent of Reynolds number changes within the
©2019: The Royal Institution of Naval Architects
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