NUMERICAL ANALYSIS OF MECHANICAL PROPERTIES OF CIVIL AIRCRAFT LANDING ON WATER

ABSTRACT

With the rapid development of science and technology, more and more civil aircraft accidents have happened. One of the urgent problems to be solved is the plane crash landing on water. Numerical simulation is generally used to analyze the process of forced landing of civil aircraft on water. In this paper, a numerical simulation method of two-phase inflow water impact is studied, and Star-ccm+ software is used to verify the high-speed flat ditching process. The numerical simulation model of forced landing of rigid fuselage on water is constructed, the motion response of civil aircraft in impact stage are analyzed, and the influence of attitude angle during forced landing on water is analyzed.

Keywords: ditching, numerical simulation.

1. Introduction

More and more civil aircraft accidents have happened. One of the urgent problems to be solved is the plane crash landing on water. Water crash-landing, as the name implies, is a civil aircraft forced to land on the water.[1] In the entire history of civil aviation, there have been many accidents of plane forced landing on water, among which there are not a few serious casualties. Among these accidents, the Ethiopian 767 Airlines Flight 961 hijacking and the Lavag Air Flight 585 crash caused huge casualties, while the US Airways Flight US1549 bird strike accident resulted in a safe emergency landing on the water, maintaining the integrity of the fuselage, and ultimately no one was involved casualties. A safe ditching can minimize damage. Therefore, it is of great significance to study civil aircraft water crash-landing, analyze the motion state of aircraft during water crash-landing, and evaluate whether the aircraft meets the strength requirements, so as to improve the performance of civil aircraft during water crash-landing and increase the survival probability of passengers and staff on board.

Since the 1990s, CAE (computer aided engineering) technology has become more and more mature, and the research direction has shifted to the finite element method to deal with aircraft collision problem. In 1994, Brooks and Anderson used the finite element software LS-DYNA to analyze the water impact process of the Apollo command module[2]. In 1997, Smith et al. used finite element simulation to simulate the process of helicopter hitting water[3]. The requirement of strengthening the bottom design was put forward. Between 2001 and 2004, a number of schools and companies took part in a project called Aircraft Crashworthiness, which came up with new methods. In 2011, Europe launched the SMAES project to apply new simulation technology to the water landing of aircraft and the rational use of carbon and carbon composites in aircraft design. Before the 21st century, most of the foreign countries used ALE method, a grid numerical algorithm to deal with the fluid-structure coupling problem. This method is not only time-consuming, but also requires high performance of multi-computer CPU processing. In recent years, the SPH method, a grid-free numerical algorithm frequently used in the fluid splashing problem, is much more convenient. The calculation is less, and the CPU performance of the computer is lower.

This paper studies the numerical simulation method of two-phase water intrusion impact, and uses Star-ccm+ software to verify the high-speed trench leveling process. A numerical simulation model of a rigid fuselage ditching was constructed, the motion response of the civil aircraft during the impact stage was analyzed, and the influence of attitude angle during the ditching process was analyzed. This research can provide reference for safe emergency landing of aircraft, improve passenger survival rate, and reduce economic losses.

2. Numerical Methods

2.1 Volume of Fluid (VOF) Method

Landing on water is a two-phase flow problem involving two impermeable media, i.e., water and air. Among many multiphase flow models, the VOF method[4]is relatively simple and stable. Therefore, this paper adopts the VOF method to capture the liquid level. VOF method, also called fluid volume fraction method, uses the ratio of fluid volume in the grid cell to the grid volume to determine the position of the free liquid surface.

2.2 Global Dynamic Grid Method

In the global moving grid method[5], the entire computational domain, including cells and boundaries, has the same motion law as the model. The integral moving grid method should be combined with the VOF method to set reasonable boundary conditions of volume fraction, so as to ensure that the horizontal position stays at the initial height during the motion of computing domain. Compared with other dynamic grid methods, the integral dynamic grid method not only saves a lot of calculation costs, but also guarantees the quality of the grid, and improves the accuracy and calculation accuracy of liquid surface capture. Therefore, the integral dynamic grid method is used in the study of aircraft crash landing on water.

3. Simulation Results

3.1 Construction of Rigid Fuselage Water Crash Landing Numerical Simulation Model

A rigid model is constructed by CATIA based on Airbus A320. The rigid fuselage model of Airbus A320 was established in CATIA software and then imported into Star-ccm+ for calculation. Airbus A320 aircraft half model is used in the calculation. The distance between model grids used in this paper is 0.01 m, and there are about 3.63 million grids in the entire computing domain, as shown in Figure 1.

Figure 1. Model of A320

3.2 Analysis of Water Crash Landing Process of Aircraft

Plane crash landing can be roughly divided into three stages: impact stage, water skiing stage, and floating stage.

3.2.1 Impact Stage

The impact stage is the initial stage, which includes the process from near the surface of the water to the impact of the aircraft into the water. The impact load at this stage is caused by both vertical velocity and horizontal gliding velocity.

Figure 2. Vertical velocity

Figure 2 shows the vertical velocity of the aircraft in the process of crash landing. From the initial moment to about 0.45 seconds, the vertical overload is not sufficient to resist gravity, and the vertical velocity increases, from the initial -1.5 m/s to -4.3 m/s. Then the aircraft contacts the water surface and reaches the maximum vertical velocity at about 0.45 seconds. With the subsidence of the aircraft, the vertical velocity decreases. When the vertical velocity drops to 0 m/s at about 1.4 seconds, the aircraft’s center of gravity reaches 2.26 meters from the water surface.

In the impact stage, angular acceleration in Figure 3 is roughly similar to the trend of impact overload. Therefore, it is considered that diagonal acceleration of vertical load has the greatest influence.

Figure 3. Angular acceleration

3.2.2 Water Skiing Stage

After the impact stage, the vertical velocity of the aircraft does not have a great influence on the motion condition. In the water skiing stage, the horizontal velocity of the aircraft is still large. There are two stages in the water skiing stage, i.e., the rear section of the fuselage does not enter the water and the rear section of the fuselage enters the water.

Figure 4. Horizontal velocity

Figure 5. Attitude Angle

(1) The rear section of the fuselage has not entered the water

In this stage, the submerged area of the fuselage gradually increases, the water entry is deepened, and the aircraft’s center of gravity position drops. As shown in Figure 5, the attitude Angle in this stage is basically maintained at the first half of 8.2° water skiing, and the tail of the fuselage is above the water surface, and the horizontal velocity slowly decreases.

(2) The rear section of the fuselage enters the water

With the increase of time, the flooded area increased, and after the tail section entered the water, the center of gravity continued to sink, and the attitude Angle also showed a downward trend.

3.2.3 Floating Stage

As the horizontal speed decreases, at a certain point, the load generated during the water skiing stage loses its primacy. The pressure becomes weak during the progressive forward movement in the floating stage.

3.3 Summary

In this section, the rigid fuselage model of Airbus A320 is established by consulting the aircraft design manual. The entire process of civil aircraft crash landing is simulated by using integral dynamic grid method and VOF method, and the simulation results are good.

The curves of angular acceleration, vertical velocity, horizontal velocity, attitude angle are shown.  The water crash landing process is analyzed in detail, which can be divided into three stages: impact stage, water skiing stage and floating stage.

(1) Impact stage: In this stage, the motion attitude changes dramatically, and the impact movement of the aircraft produces impact load.

(2) Water skiing stage: vertical velocity decreases, but horizontal velocity is large. In this stage, the load on the aircraft is mainly caused by the aircraft water skiing. In this process, the position of the center of gravity sinks and the flooded area increases gradually. This stage is divided into the rear section of the fuselage not into the water and the rear section of the fuselage into the water. At this point the load on the aircraft gradually moves from the tail to the nose in reverse.

(3) Floating stage: In this stage, the horizontal velocity gradually decreases. Although the pressure continues to move toward the nose, it becomes very weak and the position of the center of gravity is almost unchanged.

4. Conclusions

This paper successfully conducts numerical simulation on the water crash-landing of aircraft, analyzing the motion of the civil aircraft crash-landing process. The approach involves various model methods, including Fluid governing equations: Reynolds equation, turbulence model, Volume Of Fluid (VOF) method, and global dynamic grid method. Obtaining Airbus A320's basic information from maintenance and design manuals, and a rigid half model is constructed using CATIA software. The grid division ensures calculation accuracy with a 0.01 m interval, totaling around 3.63 million grids. The curves of angular acceleration, vertical velocity, horizontal velocity, attitude angle are analyzed.  The water crash landing process is analyzed in detail, which can be divided into three stages: impact stage, water skiing stage and floating stage. This study can provide a reference for the safe forced landing of aircraft, increase the survival rate of passengers and reduce economic losses.

References:

1. Streckwall H, Lindenau O, Bensch L. Aircraft ditching: a free surface/free motion problem[J]. Archives of Civil & Mechanical Engineering, 2007, 7(3): 177-190.
2. Baker E, Westine S. Model tests for structural response of Apollo Command Module to water impact[J]. J. Spacecraft Rockets, 1967, 4(2): 201-208.
3. Martin Annett,Lucas G.Horta, Comparison of Test and Finite Element Analysis for Two Full-Scale Helicopter Crash Tests, Structural Dynamics and Materials Conference 10.2514/6.2011-1804
4. Sussman M. A second order coupled level set and volume-of-fluid method for computing growth and col-lapse of vapor bubbles [J]. Journal of Computational Physics, 2003, 187(1): 110-136.
5. Herrmann M. A balanced force refined level set grid method for two-phase flows on unstructured flow solver grids [J]. Journal of Computational Physics, 2008, 227(4): 2674-2706.