Numerical study of helicopter ditching and capsize behavior on wavy water

Abstract

Helicopters are aircraft used for both land and sea operations. These operations are categorized according to various operational tasks. Helicopters have gained significant importance in offshore logistics systems due to their role in transportation and logistics for offshore oil/gas platforms. Additionally, helicopters are frequently preferred for numerous other purposes in offshore operations. Maritime helicopter operations are conducted under more unstable conditions and higher meteorological uncertainty compared to land operations. In maritime flights, reduced visual references pose a risk, especially during night flights and flights with low visibility. Factors such as the limited availability of emergency landing areas depending on sea conditions and the difficulty of approaching and departing from the platform contribute to the risk characteristics of maritime flights. A helicopter ditching is defined by flight authorities as an emergency landing maneuver performed in a controlled manner on water, in accordance with the instructions outlined in the flight manual (RFM). This definition is accepted as a pilot-controlled ditching, both under systematic failure or loss of control and when no other safe option remains. After landing on water, helicopters quickly begin to roll over, rotate, take on water, and eventually capsize. Personnel who survive the water landing lose their lives by drowning when the helicopter capsizes. Inflatable flotation systems are used to maintain the helicopter's stability after landing on water, preventing it from capsizing and sinking. These systems are called emergency flotation systems (EFS). They usually contain air tubes placed under the fuselage or on the sides. Aviation authorities state that EFS systems must inflate quickly. A review of the manufacturer's technical documentation indicates that full inflation occurs within 4-6 seconds. For the purposes of this thesis, it has been accepted that full inflation will occur in 5 seconds. Emergency flotation systems (EFS) are used to prevent helicopters from sinking when they occur ditching. However, EFS are usually triggered after impact, and it takes a certain amount of time for the systems to open and fully inflate. If the helicopter rotates to a certain roll angle within the water during this time, it may capsize even if the EFS opens. This thesis examines the ditching characteristics of a helicopter on wavy water with sea state 6 and the capsizing situation after ditching. Within the scope of the thesis, situations where the helicopter hits different positions of the incoming wave were examined numerically. The effect of the helicopter ditching on water at different roll and yaw angles was examined in terms of different parameters. Considering the time required to deploy the EFS, the effect of angles on the capsizing time was also investigated. Additionally, the effect of the helicopter's center of gravity (CoG) on the capsizing situation after landing on water was examined. The thesis aims to determine the effect of different parameters on the ditching and the subsequent capsizing situation. In the literature, studies conducted numerically have generally verified the dropping of a wedge or disk into water, and this model has been applied to aircraft. When reviewing the literature on helicopter ditching, the majority of studies focus on investigating the hydrodynamic effects that occur during ditching. However, there is a noticeable lack of studies examining the helicopter's capsize characteristics during the time required for the EFS to inflate after ditching. An experimental wedge free-fall test conducted by Yetteo was selected and numerically modeled in a time-dependent manner. In the experiment, a 94 kg wedge is released from a height of 1.3 m. The pressure, velocity, acceleration, and position values in the model were validated by the free-fall wedge test, and the accuracy of the helicopter water landing model was evaluated through these studies. This study analyzes the helicopter's ditching and the subsequent free-surface interactions, hydrodynamic loads, acceleration values experienced by the helicopter fuselage, and the helicopter's rolling behavior in water using the time-dependent Reynolds-Averaged Navier-Stokes (RANS) approach. Using the Volume of Fluid (VOF) and 6 Degrees of Freedom (6-DOF) methods, the evolution of the air-water interface and the vertical and angular responses of the 6-DOF helicopter fuselage in the water were calculated. An overset model was used to solve the large mesh changes that occur when solving the landing situation. The analyses were performed using Siemens StarCCM+, a commercial solver program. The model helicopter was scaled down to a 1/10 ratio for analysis. Scaling reduced the analysis time. The initial conditions for the helicopter were set at a roll and yaw angle of +10° and -10°. The pitch angle was kept constant. The landing conditions were examined in two different wave conditions: trough and rising wave. The helicopter's center of gravity was considered for the maximum shifted condition and the maximum takeoff weight condition. These conditions were cross-referenced to create 12 case. The helicopter's forward and descent velocity upon entering the water and the physical characteristics of the wave are the same for each condition. These values were determined in accordance with CS29.801 standards. For sea state 6, the trough and crest length of the waves is 6 meters and the wave period is 7.269 seconds. The limit velocity determined for the helicopter's ditching are 15 m/s forward velocity and 1.5 m/s descent velocity. In the analyses, a forward velocity of 4.9 m/s and a descent velocity of 1.1 m/s were assumed. The grid structure to be used is important for ditching analyses. Therefore, studies have been conducted to select the trim grid structure in order to make the analysis independent of the effects of the grid structure. From this, the appropriate grid structure was selected, and studies were conducted on time step independence to isolate it from time discretizations, thereby obtaining the most suitable grid structure for the analyses. Analyses were conducted for 12 different cases, and the results were examined for each analysis, focusing on the moment the helicopter ditching and after ditching. The analysis results revealed the situations in which the acceleration and pressure values peaked during and after ditching changes. In addition, the time-dependent change in the helicopter's roll angle was obtained, and the capsizing characteristics were examined. The contours of the helicopter on the water were shown with angle values depending on time. The following results were obtained from the analyses: The peak acceleration and pressure values during ditching increased when both the roll and yaw angles of the helicopter were +10° or -10°, i.e., when the angles were in the same direction. In this case, if the helicopter yaws toward the side of the fuselage entering the water, it concentrates the impact speed on the same side, increasing the wetted area, thus increasing the local impact intensity and shortening the momentum transfer time. In this case, it is thought that the peak acceleration and pressure values increase. Average maximum pressure and acceleration values were obtained for trough wave conditions and shifted CoG configuration. In rising wave cases, the helicopter rebounded from the wave crest, remained in the air a little longer, and made contact with the water again. When the roll and yaw angles were in the same direction, the time spent in the air after bouncing off the wave crest increased. In this case, the vertical speeds increased and produced high peak pressure-acceleration values on the second contact. After ditching, if the helicopter does not reach a 30° roll angle, it will return to 0° due to the righting moment. However, if it reaches a 30° roll angle, it will capsize rapidly. The helicopter exhibits the most intense capsize behavior in trough wave conditions and shifted CoG configuration, reaching a 30° roll angle in a short time. In trough wave conditions and MTOW CoG, capsize behavior is low, taking a long time to reach a 30° roll angle. It completes the time required for full EFS deployment without capsizing. These findings numerically demonstrate how different roll and yaw angles, wave conditions, and CoG configuration affect a helicopter during ditching. Furthermore, the capsize behavior of a helicopter after ditching has been examined based on the specified parameters, and the results have been presented. This model could be valuable for helicopter certification processes and offers a more economical, repeatable, and flexible alternative to full-scale model tests.