Test and simulation of emergency flotation system buoy anti-overturning method

The helicopter emergency flotation system can provide flotation capability after the helicopter accidentally falls into the water. During a test of a helicopter float in a submerged state, it was found that the vertical distance between the highest point of the contact point of the float and the aircraft and the lowest point of the cabin door was 0.59 m, which seriously hindered the entry and exit of the crew from the cabin door. To reduce the adverse effects of the pontoon flipping up on the occupants entering and exiting the hatch, it is envisaged to use restraint belts, add anti-rollover airbags, and design the anti-rollover airbags into triangles and saddle shapes respectively. Through theoretical analysis, the simulation method eliminated the triangle method for the connecting belt and the anti-rollover airbag and selected the saddle-shaped anti-rollover airbag for floating and immersion tests. The test results met expectations. The highest point of the contact point between the float and the aircraft was the lowest point of the cabin door. The points did not interfere in the vertical direction, and this method was finally chosen to be applied to the buoy to suppress the upturn of the buoy.


Introduction
According to the U.S. National Transportation Safety Board (NTSB) investigation data on helicopter maritime accidents from 1982 to 1989, most of the crew members were able to avoid death when the helicopter entered the water and hit it, but they did not survive after the impact because the helicopter sank and drowned [1] .
The floating of helicopter emergency flotation systems has attracted extensive research by domestic and foreign scholars.Nathalie [2] conducted a full-scale water impact test on the WG30, established a corresponding finite element model, and conducted a numerical simulation of the water impact on the belly of the helicopter fuselage by using the SPH method.Reilly [3] analyzed data from 64 CH-46 helicopter emergency landings at sea from 1975 to 1981, and proposed design requirements for emergency flotation systems.Luo and He [5] proposed a new method to address the floating problem of the return capsule.Establishing the finite element model of the return capsule was obtained and rotated to any angle required for calculation, the center of buoyancy parameters was obtained through iteration, and finally, the static stability curve of the return capsule was obtained.Lun and Hu [6] proposed the unit point method, which divides the floating body into countless small units and represents these units with points with physical quantities, to obtain the static moment of a certain axis through points to solve the stability problem.Yang [7] and Cheng et al. [8] carried out secondary development of the commercial IOP Publishing doi:10.1088/1742-6596/2764/1/012037 2 software SolidWorks and Catia respectively to quickly obtain the data required for stability calculations such as the centroid and volume of the solid model.Li et al. [9] proposed the surface element method, which divides the surface of the attached body into many small surface units, calculates the hydrostatic pressure of each unit, represents the unit pressure in the form of unit center force, and finally calculates the moment of these forces concerning the center of gravity.Thus, the final restoring moment corresponding to each heel angle can be obtained.Ren and Zhou [10] summarized the structure, principles, and standards of the helicopter emergency flotation system.Wu [11] analyzed the composition, working principle, performance parameters of each subsystem, installation, and design requirements of the emergency flotation system for a certain type of helicopter, and demonstrated the feasibility of installing an emergency flotation system for a certain type of helicopter.Li [12] applied the ale method of ls-dyna to the impact overload analysis of the helicopter emergency flotation system entering the water under different sea conditions.It was concluded that the maximum overload value suffered by the water at the wave crest is the smallest, and the maximum overload value is at the wave trough and back wave surface.Overload is the biggest conclusion.The calculation of helicopter static stability was realized through Matlab programming and Ls-dyna simulation, and the impact of different airbag internal pressures on static stability was studied.It concluded that the greater the airbag's internal pressure is, the better the static stability of the helicopter is.Most of the above scholars' and organizations' research on emergency flotation systems only uses theoretical analysis or simulation methods, and experimental methods are rarely used.
Through theoretical analysis, simulation, and testing, the buoy after applying the restraint belt method, triangular anti-rollover airbag, and saddle-shaped anti-rollover airbag were analyzed, and the restraint belt method and the triangular anti-rollover airbag method were eliminated.Simulation, floating, and immersion tests were conducted on the pontoon by using saddle-shaped anti-rollover airbags.The results showed that the saddle-shaped anti-rollover airbag method was the best.

The problem of the buoy turning ups
The main function of the buoy is to deploy it after it is inflated to provide the helicopter with the ability to float on the water and ensure that the helicopter can float for a period under certain sea conditions without capsizing.However, the float will turn up during the immersion process.This phenomenon will adversely affect the entrance and exit of the helicopter door and interfere with other items of the helicopter.The installation location of the float assembly is shown in Figure 1.
In the simulation of a cylindrical float, the relative position changes between the cylindrical float and the helicopter are shown in Figure 2. The red surface in Figure 2 is the hatch.After the immersion is stabilized, the highest point of the contact point between the float and the aircraft is higher than the lowest point of the hatch.After being submerged, the float seriously blocked the emergency exit of the helicopter, making it impossible to ensure the smooth release of the cockpit door glass and posing a hidden danger of affecting the emergency evacuation of the crew.To further analyze the upturning mechanism of the buoy and find a method to inhibit the upturn of the buoy, it is necessary to conduct a stress analysis on the buoy when it is immersed in water.Due to the difference in external forces before and after the buoy is immersed in water, the position of the buoy relative to the helicopter will change after it is immersed in water.As a result, the buoy will move upward.As shown in Figure 3, after the buoy is immersed in water, the buoy is balanced by the buoyancy force, the tension of the upper and lower connecting belts, and the pressure of the aircraft skin.As the immersion depth of the buoy increases, the buoy rotates around Point A. Since the distance between the upper and lower connection points (A, B) of the buoy is much smaller than the diameter of the buoy (the distance between A and B is 350 mm and the diameter of the buoy is 1, 400 mm), it cannot provide the force to inhibit the buoy from overturning.Under the same pressure, the larger the diameter of the buoy is, the lower the stiffness is.Therefore, the upward movement of the buoy also accompanies the deformation of the buoy, and the deformation makes the upturn of the buoy more serious.From the above analysis, it can be seen that some additional downward forces or constraints can be applied to the pontoon to inhibit the pontoon from rotating upward around point A, thereby reducing the height of the pontoon and satisfying the function of not affecting the entry and exit of the passengers.

Anti-turning methods
Through the analysis of the motion trajectory and force of the buoy, it can be seen that the buoy rotates around Point A and turns upward so that the buoy moves upward.Therefore, to suppress the upward flipping of the buoy, it is necessary to introduce new constraints.The new constraint provides a downward force on the buoy to restrain the buoy from flipping upward around Point A. From the perspective of introducing new constraints, three anti-turnover methods are now considered: Method 1: Since the helicopter buoys are symmetrical, you can consider setting a set of restraint belts on the undersides of the two front pontoons and connecting the undersides.The two pontoons interact to inhibit the flipping of the pontoons to prevent the float from turning up, as seen in Figure 4.  Method 3: This method is similar to Method 2, except that the anti-rollover airbag is changed into a saddle shape.The anti-rollover airbag is saddle-shaped, and its model is shown in Figure 6.

Simulation parameter settings
During the immersion simulation, the helicopter and airbag models were considered symmetrical models, and air and water fluid regions were created.The fluid region adopts a null material model, and both water and air are Eulerian mesh models using three-dimensional solid elements.The helicopter and airbag use shell elements, using rigid body materials, and setting their center of gravity and moment of inertia.The unit uses the multi-material ALE algorithm, and the contact algorithm uses automatic contact.

Results of simulation calculations
After using Method 2 and Method 3 to simulate the pontoon through LS-DYNA, the upturned situation of the pontoon is as follows: Before and after the pontoon is immersed in water, in the LS-DYNA simulation, the height changes of the pontoon before and after the pontoon is immersed are shown in Figure 7.The schematic diagram of the center position of the airbag node position of the body and the vertical displacement change curve is shown in Figure 8.After using Method 2, the height changes of the buoy before and after the buoy is immersed in water are shown in Figure 7 and Figure 8.It can be seen that after 2.2 s, the displacement of the two monitoring points remains constant.The vertical displacement when the body is stable is 2.03 m, and the displacement when the center position of the front airbag is stable is 1.13 m.That is, when stable, the airbag model 2 is relative to the aircraft moved upward by about 0.9 m in the vertical direction.At this time, the front float interfered with the emergency exit by about 0.085 m.   model in method 3. The simulation results show that the triangular anti-rollover airbag interfered with the emergency exit by 0.085 m after being immersed in water, and the triangular anti-rollover airbag solution can be eliminated.The saddle-shaped anti-rollover airbag did not interfere with the emergency exit, and the distance between the highest point of the pontoon and the lower edge of the emergency exit was about 0.075 m.

Effect test of anti-turning method
The test used a 1:1 pontoon model, that is, the original scale model.The pontoon used Method 3, that is, the anti-turnover airbag was saddle-shaped and was tested for turning up after being immersed in water.The changes of the pontoon after being immersed in water were recorded by taking a camera.
Method 3: We add a saddle-shaped anti-turning airbag to the original pontoon.When the pontoon is floating, there is no interference between the front pontoon and the glass in the floating state.The highest point of the pontoon is about 100 mm lower than the lower edge of the glass.The waterline is approximately 100 mm higher than the lower edge of the glass.In the submerged state, as shown in Figure 11 and 12, the float does not interfere with the glass.The contact point between the float and the fuselage is under the lower edge of the glass, with a distance of about 20 mm.

Conclusion
Given the problem of buoy overturning in the helicopter emergency flotation system, three methods were proposed for testing: we add a connecting belt to the symmetrical buoy, make the anti-overturning airbag of the buoy with a triangular shape, and use a saddle-shaped anti-turning airbag for the buoy.The tests were carried out through theoretical analysis, simulation, and testing.Three methods were evaluated for their effectiveness and achieved good research results.This research has great reference significance for the research on anti-overturning methods of helicopter buoys and the research on the shape of helicopter buoys.In the future, more in-depth research can be conducted on the anti-overturning method of helicopter floats to find a method with a better anti-overturning effect.

Figure 2 .
Figure 2. Schematic diagram of the relative position changes between the cylindrical float and the helicopter.To further analyze the upturning mechanism of the buoy and find a method to inhibit the upturn of the buoy, it is necessary to conduct a stress analysis on the buoy when it is immersed in water.Due to the difference in external forces before and after the buoy is immersed in water, the position of the buoy relative to the helicopter will change after it is immersed in water.As a result, the buoy will move upward.As shown in Figure3, after the buoy is immersed in water, the buoy is balanced by the buoyancy force, the tension of the upper and lower connecting belts, and the pressure of the aircraft skin.As the immersion depth of the buoy increases, the buoy rotates around Point A. Since the distance between the upper and lower connection points (A, B) of the buoy is much smaller than the diameter of the buoy (the distance between A and B is 350 mm and the diameter of the buoy is 1, 400 mm), it cannot provide the force to inhibit the buoy from overturning.Under the same pressure, the larger the diameter of the buoy is, the lower the stiffness is.Therefore, the upward movement of the buoy also accompanies the deformation of the buoy, and the deformation makes the upturn of the buoy more serious.

Figure 3 .
Figure 3. Float immersed in water for stress analysis.From the above analysis, it can be seen that some additional downward forces or constraints can be applied to the pontoon to inhibit the pontoon from rotating upward around point A, thereby reducing the height of the pontoon and satisfying the function of not affecting the entry and exit of the passengers.

Figure 4 .
Figure 4. Schematic diagram of the restraint belt inhibiting the buoy from turning up.Method 2: Since the pontoon rotates around Point A, an anti-overturning airbag can be added at the gap between the pontoon and the helicopter to hinder the rotation of the pontoon around Point A, thereby inhibiting the upturning of the pontoon.The anti-overturning airbag is triangular, and its model is shown in Figure 5.

Figure 5 .
Figure 5.The triangular anti-rollover airbag.Method 3: This method is similar to Method 2, except that the anti-rollover airbag is changed into a saddle shape.The anti-rollover airbag is saddle-shaped, and its model is shown in Figure6.

Figure 6 .
Figure6.The saddle-shaped anti-rollover airbag.Method 1 is theoretically feasible, but the connecting belt will interfere with other onboard items under the belly of the fuselage and will cause the two original left and right buoys to interact, making their manufacturing and installation complicated.

Figure 7 .
Figure 7. Changes in height of the buoy before and after the buoy is immersed in water by using Method 2.

Figure 8 . 6 Figure 9 .
Figure 8. Vertical displacement changes (m) of the center position node and body node of the airbag model in method 2. Before and after the pontoon is immersed in water by using Method 3, in the LS-DYNA simulation, the height changes of the pontoon before and after the pontoon is immersed in water are shown in Figure 9.The schematic diagram of the center position of the airbag node position of the body and the vertical displacement change curve are shown in Figure10.It can be seen that after 2.2 s, the displacement of the two monitoring points remains constant.The vertical displacement when the aircraft body is stable is 2.03 m, and the displacement when the center position of the airbag is stable is 1.29 m.That is, when stable, the airbag model 2 is relative to the aircraft.It has moved upward by about 0.74 m in the vertical direction.At this time, there is no interference between the front float and the emergency exit.The distance between the highest point of the float and the lower edge of the emergency exit is about 0.075 m.

Figure 10 .
Figure 10.Vertical height changes of the central position node and the body node of the rear airbag model in method 3. The simulation results show that the triangular anti-rollover airbag interfered with the emergency exit by 0.085 m after being immersed in water, and the triangular anti-rollover airbag solution can be eliminated.The saddle-shaped anti-rollover airbag did not interfere with the emergency exit, and the distance between the highest point of the pontoon and the lower edge of the emergency exit was about 0.075 m.