The Prospects for Simulating the Helicopter/Ship Interface

Abstract

In the past decade, there has been a dramatic increase in the use of Helicopters in conjunction with non‐aviation ships by the U.S. Navy. Landing the helicopter on the ship in the presence of high winds and stormy seas can be a hazardous process. The safe operating envelopes are determined at sea by the Naval Air Test Center in a slow laborious and expensive process. Moreover, there is a substantial backlog of about eleven helicopters and twenty ships that, at the present rate, cannot be cleared in this century. This has led to the suggestion that the problem might be solved by simulation, and it is with this suggestion that the present paper is concerned. It has been concluded that: 1. The airflow to the ship can be predicted sufficiently accurately. 2. A good basic ship motion prediction program exists, but requires some further development and validation with real ships. 3. The ship airwake is almost unknown and previous attempts to analyze it were faulty. 4. Future interface tests should measure and publish either the true wind velocity or the true ship velocity, in addition to the relative velocity. 5. Further work is required on modeling the motion of helicopters in fluid flow regimes with high velocity gradients and turbulence. 6. Before it is possible to determine the size of computer necessary for simulation, it is necessary to determine the extent to which the mathematical model of the helicopter and the physical model of the complex fluid flowfield can be simplified, while still retaining the fidelity of the helicopter motion.

A conceptual method to assess ship-helicopter dynamic interface

“…However, the land-based limitations are not valid in the shipboard environment due to the multiple factors including ship airwake/turbulence, ship motion (yaw, pitch and roll), confined deck landing areas, weather conditions, degraded visual cue and also their combined effects. 4…”

“…These test trials are conducted by Experimental Test Pilots (ETPs) after the ship has been constructed and typically involve very high costs. 4 Besides, these tests also render the ship unavailable for operational requirements for a significant period even after commissioning of the ship. In addition to the high risk and high cost of such trials, there are several other limitations of this approach: (i) the experience and insight of a specific ship-helicopter test flight may not be generalizable to other ship-helicopter combinations, (ii) it may also be difficult to implement changes based on the gathered insight on the same ship (that has already been constructed) and (iii) it is not practical for any testing exercise to cover a very wide range of wind/sea conditions that may be prevalent in real-life scenarios.…”

“…However, due to the confidential nature of the ship-helo DI problem, limited literature is available in the open domain. The early studies 2,4 have concluded that (i) the stand-alone DI modelling is a massive challenge, (ii) a simplified ship-helo geometrical aspects and helicopter rotor modelling approach is necessary and (iii) the flow structure over the helodeck is independent of Reynolds number (Re) beyond 10 4 (based on ship beam). At this value, the flow over a ship and the wake are fully turbulent.…”

“…However, the land-based limitations are not valid in the shipboard environment due to the multiple factors including ship airwake/turbulence, ship motion (yaw, pitch and roll), confined deck landing areas, weather conditions, degraded visual cue and also their combined effects. 4 The shipboard environment is evaluated primarily by performing high-risk test trials known as First-Of-Class Flying Trials (FOCFT) or Ship Helicopter Operating Limit (SHOL). These test trials are conducted by Experimental Test Pilots (ETPs) after the ship has been constructed and typically involve very high costs.…”

Integrating CFD and piloted simulation to quantify ship-helicopter operating limits

“…This highly unsteady flowfield is further modified during shipborne rotorcraft operations when the downwash from the helicopter's main rotor interacts with the airwake during launch and recovery manoeuvres, resulting in a fully coupled helicopter-ship dynamic interface. Furthermore, different helicopter and ship combinations will interact in different ways depending on the prevailing conditions (13)(14)(15)(16)(17)(18)(19) . When the helicopter is immersed in the ship's airwake, pilot workload is high and helicopter control margins are much reduced.…”