Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019 2.3 (c) Effect of Strong Tail Wind
Winds from tail are not desirable for aircraft operation. AGARDograph (RTO, 2003) brings out the following in this respect:- "Taking into consideration the presence of obstacles near the flight deck, strong tail-wind conditions (Area D in Figure 7) can create a hazardous situation. Moreover, such wind conditions result in large helicopter pitch-up angles reducing pilots view over the flight deck. Since the freedom of naval ships to manoeuvre is normally often limited by operational constraints, helicopter may be forced to take off or land in non-ideal conditions with respect to relative wind conditions. For these reasons strong tail-winds (above 10kts) should be tested with extreme caution."
2.3 (d) Effect of Horizontal Gust
Effect of Horizontal Gusts on rolling moment on rotor have also been discussed in (Whitbread & Coleman, 2000). It is documented that a medium sized rotor helicopter would be expected to be rolled over about 20⁰ in 3 seconds in response to a 5 m/s side gust. In the vicinity of a helodeck, the potential for large and abrupt changes in horizontal velocity depends on the design of the super structures. Shear layers springing from sharp vertical edges of bluff structures can present an effective horizontal gust to a transitioning helicopter. Turbulent air motions containing gusts with horizontal components having a high value are possible with mean winds of 10m/s and higher. A pilot flying a helicopter into a region with horizontal disturbances can therefore expect the aircraft to experience the largest perturbations in the angular motions, particularly for roll motions.
2.3 (e) Effect of Vertical Gust
Effect of vertical gusts have, for long, been a limiting criteria for helodeck design for offshore structures. Till recently, CAP 437 imposed a limitation of vertical mean wind speed (downdraught) of ±0.9 m/s for a wind speeds of upto 25m/s for airwake on the helodeck of offshore structures (This equates to a wind vector slope of 2 degrees). It is brought out in CAA 99004 (Whitbread & Coleman, 2000) that "Irrespective of the velocity gradients, if a helicopter does not have the available thrust margin to hover in, say, 4 m/s downdraught then the pilot will not be able to avoid being pushed onto the deck. But it has also been shown that, regardless of the thrust margin, with such a strong gust, the pilot needs to take counter action fairly quickly to avoid a dangerous situation". In another conclusion of the analysis, the document brings out the following:- "It has been shown that aircraft with hover thrust margins about 3% can withstand effects of the 0.9m/s vertical gust. Linear analysis predicts the requirement for a 14% thrust margin to withstand the effects of a 4m/s vertical downdraught. Alternatively, irrespective of the thrust margin, an aircraft would hit the deck with a velocity of 2m/s from a hover height of 2m in
just 2 seconds without pilot intervention, after flying into such a gust".
As an illustration of the powerful nature of airwake effects, the CAA paper (Whitbread & Coleman, 2000) shows the results of air wake plots of downdraughts on a Type 23 Frigate (Figure 8), where the wind over deck is 30knots from 30⁰ starboard.
Figure 8. Contour plots of vertical wind component on Type 23 Frigate for 30knots WOD from 30⁰ starboard (Whitbread & Coleman, 2000)
In one standard approach path (e.g for a seaking helo), the helicopter approaches from the port side, side steps over the deck through the area of strong downdraught and station keeps over the landing spot until a suitable quiescent period when it is safe to touch down and engage the deck lock. The region of strong vertical flow gradient on the port edge of the deck represents the greatest hazard to safe helicopter operations in this case. As helo traverses through this region, the rotor will initially experience a downdraught peaking at 6m/s, followed by an upwash peaking at 4m/s.
With regards to differentiating downdraught (mean of vertical velocity component) from time-varying turbulence (measured as the standard deviation of the vertical velocity component), CAA 2004/03 (CAA, 2004) brings out the following:- "The simulator trials are focussed on assessing the workload resulting from time- varying turbulence as opposed to potential operating limits imposed by lack of power or torque margin. Such limits are typically caused by an aircraft with low torque/ power margins or for significant downdraft in the vicinity of the helodeck. Downdraft here is considered to be distinct from time-varying turbulence, although typically increased turbulence would accompany a large downdraft. Limitations on downdraft are currently expressed in CAP 437 by way of the downdraft criterion. This criterion needs to be reviewed in the light of the new criterion for turbulence." Further, it has been brought out that the term downdraught in CAP 437 actually refers to a limiting vertical component of velocity rather than downdraught. However, between the downward component
A-410 ©2019: The Royal Institution of Naval Architects
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