Trans RINA, Vol 161, Part A4, Intl J Maritime Eng, Oct-Dec 2019
As compared to Offshore installations, a naval pilot landing on a ship helodeck faces the superstructure while approaching and hovering. CAA paper 99004 (Whitbread & Coleman, 2000) brings out that such adverse conditions require a much steeper approach to the helodeck to ensure that a landing can be made should a single engine failure occur after the Committal Point. If a single engine failure occurs prior to the Committal Point, adequate obstruction clearance height is required to safely fly away and achieve a recovery.
The paper describes that a takeoff behind an obstruction (note that superstructure is omnipresent in case of a ship helo operation) poses greater problems because it requires a significant power demand to climb almost vertically above the helodeck out of ground effect (normally up to 25ft and dependent on the aircraft type). The climb in hover will usually take the aircraft through turbulence created by the structure, to a point where the helicopter is clear of both turbulence and the superstructure and the pilot can make the transition to forward flight.
Figure 7 presents typical results of land-based hover tests for a helicopter. RTO, AGARDograph of NATO (RTO, 2003), in its analysis of high wind speed from ahead bring out the following:- "In this case, the turbulence caused by the ship superstructure affects the helicopter such that the pilot cannot maintain sufficient control for safe take-off or landing. Relative wind conditions where very high turbulence exists, in combination with spray nuisance and large ship amplitudes, especially in pitch, have to be avoided. In such cases the control inputs required to counteract the helicopter response to turbulence in combination with manoeuvring, necessary to avoid collision with parts of the oscillating ship may be too large (over-torqueing, maximum control margin), and create a hazardous condition."
Imposing a limitation on the low wind speed, it brings out that "high engine power is needed at low relative wind speed and at high helicopter mass (Area A in Figure 7). The power and yaw control margins in that condition might be too small to counteract adequately a certain amount of ship's motions. Therefore helicopter mass and density altitude should be watched very carefully during helicopter ship operations. Furthermore, at low relative wind speed the downwash of the rotor generates spray, which is most bothersome when the helicopter hovers alongside the flight deck."
2.3 (b) Effect of Yaw Control
AGARDograph (RTO, 2003) brings out clearly the need for good handling qualities and yaw control to counteract turbulence and ship's motions adequately. During transitions to and from forward flight, take-off and landing, a control margin is required to maintain controllability during any unexpected situation (gusts, turbulence etc). In most cases, control margin limitations occur for pedal controls. Yaw control is an area of concern for helicopters as these employ tail rotors. Those conditions where inadequate yaw control exists (Area E in Figure 7), must be avoided. Therefore the condition of a decelerating flight moving from approach to hover, when the relative wind above the flight deck pertains to the shaded area under Area E (Figure 7), must be avoided as the relative wind condition of the Area E will be traversed. Such an approach to an obstructed flight deck with inadequate yaw control is hazardous. Wind conditions close to those areas where inadequate yaw control exists must be approached very carefully because of yaw control variations needed to counteract turbulence and ship motions adequately.
Figure 7. Detailed results from land based hover tests (Courtesy- RTO, AGARDograph, NATO (2003))
©2019: The Royal Institution of Naval Architects
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