A dynamic load static equivalent coefficient determination method and a structural parameter design method

Abstract

The application belongs to the field of aircraft structure dynamic strength design, and particularly relates to a dynamic load static equivalent coefficient determination method and a structure parameter design method, which comprises the following steps: S1: according to a water ditching dynamic load measurement test, a relationship curve between a water pressure load of a bottom of an aircraft fuselage and a time history is obtained, and a peak value Ps of the water pressure load is determined according to the relationship curve; S2: a water ditching test of an elastic model of the bottom of the fuselage is performed to determine a critical failure parameter of the elastic structure; S3: a static mechanical property test is performed with the critical failure parameter of the elastic structure to obtain a maximum static pressure Pd that can be borne by the elastic structure; and S4: a water ditching dynamic load static equivalent coefficient is calculated. The static equivalent coefficient of the water ditching water pressure dynamic load is obtained through a scale model test. The application aims to solve the problem of lacking of design load in aircraft water ditching performance design.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of aircraft structure dynamic strength design, and particularly relates to a method for determining a dynamic load static equivalent coefficient and a method for designing a structure parameter. BACKGROUND

[0002] The water pressure load borne by the bottom of the fuselage during water ditching of an aircraft is an impact dynamic load. If the peak value of the water ditching impact dynamic load is taken as a design input for designing the structure, it will be very conservative. If the peak value of the load is taken as a design load for obtaining a damage source of a water floatability test of the aircraft, the water floatability of the aircraft cannot be truly reflected. SUMMARY

[0003] To solve the above problems, the present application provides a method for determining a dynamic load static equivalent coefficient during water ditching of an aircraft, comprising:

[0004] Step S1: obtaining a relationship curve between the water pressure load and the time history of the bottom of the fuselage according to a water ditching dynamic load measurement test, and determining the peak value Ps of the water pressure load according to the relationship curve.

[0005] Step S2: performing a water ditching test of an elastic model of the bottom of the fuselage to determine a critical damage parameter of the elastic structure.

[0006] Step S3: performing a static mechanical property test with the critical damage parameter of the elastic structure to obtain the maximum static pressure P that can be borne by the elastic structure. d ;

[0007] Step S4: calculating the dynamic load static equivalent coefficient during water ditching.

[0008] Preferably, the experimental site of the water ditching dynamic load measurement test is a towing tank or an open pool, and the test model is a rigid pressure measurement model.

[0009] Preferably, the test model of the water ditching test of the elastic model of the bottom of the fuselage is a rigid model, and the bottom of the fuselage is an elastic model.

[0010] Preferably, the elastic model of the bottom of the fuselage specifically comprises: the ultimate strength of the skin material is less than that of the fuselage stringer, the skin material comprises multiple layers of fiber paper, and the critical damage parameter is determined according to the fiber paper layer parameters and the damage mode of the skin after the test.

[0011] Preferably, the critical damage parameter comprises: the skin thickness and the skeleton stiffness.

[0012] Preferably, the most serious load position of the test model of the water ditching dynamic load measurement test is replaced with an elastic model according to the relationship curve and the most serious load position, and the water ditching test of the elastic model of the bottom of the fuselage is performed.

[0013] Preferably, the formula for calculating the static equivalent coefficient of the water-landing dynamic load is:

[0014] η=P s / P d .

[0015] Preferably, the skin of the fuselage bottom elastic model is made of waterproof, cylindrical, multi-layered fiber paper and organic solvent coating, and the thickness of the skin is adjusted by adjusting the number of layers of the fiber paper and the number of times of coating the organic solvent.

[0016] An aircraft fuselage bottom structure optimization design method comprises:

[0017] obtaining the equivalent static load distribution of the aircraft fuselage bottom under the water-landing working condition;

[0018] determining the initial value of the fuselage structure parameter;

[0019] taking the initial value of the fuselage structure parameter as input, taking the satisfaction of the equivalent static load distribution of the aircraft fuselage bottom as constraint, and taking the minimum weight of the aircraft fuselage bottom as optimization target to perform optimization;

[0020] the fuselage structure parameters of different parts obtained through optimization.

[0021] Preferably, the fuselage structure parameters include: structure form, skin thickness, skeleton rigidity, energy-absorbing structure configuration and arrangement.

[0022] Preferably, the influence weight of different fuselage structure parameters on the dynamic response of the aircraft fuselage bottom is analyzed, and the optimization of different fuselage structure parameters is sorted based on the influence weight.

[0023] Preferably, the optimization frequency of the fuselage structure parameter with high optimization sequence is greater than that of the fuselage structure parameter with low optimization sequence.

[0024] Preferably, the method for obtaining the equivalent static load distribution of the aircraft fuselage bottom under the water-landing working condition comprises: determining the dynamic water pressure load distribution of the aircraft fuselage bottom, and calculating the equivalent static load distribution of the aircraft fuselage bottom under the water-landing working condition based on the static equivalent coefficient of the water-landing dynamic load.

[0025] The present application obtains the static equivalent coefficient of the water-landing water pressure dynamic load through the scale model test, and aims to solve the problem of lacking of design load in the aircraft water-landing performance design. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is a flow chart of the method for determining the static equivalent coefficient of the dynamic load of a preferred embodiment of the present application;

[0027] Figure 2Water pressure load-time curve of a preferred embodiment of the present application

[0028] Figure 3 Pressure peak diagram of each part of the bottom of an airplane of a preferred embodiment of the present application

[0029] Figure 4 Limiting strength versus skin thickness curve of a skin of a preferred embodiment of the present application. DETAILED DESCRIPTION

[0030] In order to make the technical solutions of the present application and the advantages thereof clearer, the technical solutions of the present application will be further clearly, completely and specifically described below in combination with the drawings. It should be understood that the specific embodiments described herein are only partial embodiments of the present application, and are only used to explain the present application, but not to limit the present application. It should be noted that, for the purpose of description, only parts related to the present application are shown in the drawings, and other related parts can be referred to the general design. In the case of no conflict, the embodiments in the present application and the technical features in the embodiments can be combined to obtain new embodiments.

[0031] According to the flow of Figure 1 , the static equivalent coefficients of the dynamic loads of water ditching of a certain type of airplane are determined. The specific implementation steps are as follows:

[0032] Step one: Obtain the water pressure load-time history of the bottom of the airplane fuselage.

[0033] The water pressure load-time history of the bottom of the airplane fuselage is obtained through the dynamic load measurement test of water ditching. The test can generally be carried out through a towing tank or an open water tank, and the test model is a rigid pressure measurement model. The water ditching water load test of a certain airplane is completed in a towing tank, and the water pressure load-time curve under a typical configuration is shown in Figure 2 , the pressure peak of each part is shown in Figure 3 , and the maximum peak pressure is about 52 kPa.

[0034] Step two: Determine the critical parameters of the elastic structure failure through the dynamic response test of the elastic model of the bottom of the airplane fuselage in water ditching.

[0035] The critical failure parameters of the elastic structure are determined through the water ditching test of the elastic model of the bottom of the airplane fuselage, and the test state must be consistent with the water ditching dynamic load measurement test. The structure failure parameters include skin thickness, skeleton stiffness, etc. The full-scale test model is a rigid model, and the bottom of the fuselage is replaced by a designed elastic model.

[0036] For the aircraft bottom structure, since the skin and stringers are riveted together, under the action of out-of-plane load, the skin strain can be considered consistent with the strain of the stringer attachment surface. Under the action of water load, the skin of the bottom structure of the fuselage generally fails before the stringer structure, so the equal strength fuselage bottom is designed based on the skin failure before the fuselage stringer. The skin of the model is selected from a waterproof, shapeable and low ultimate strength skin material. The skin material is made of a special fiber paper, and the thickness of the skin can be adjusted by adjusting the number of paper layers and the number of organic solvent brushing times.

[0037] During a water ditching test, the local skin of the 3-layer fiber paper fuselage structure was torn due to water pressure, while the 4-layer fiber paper fuselage structure was intact. It is shown that the critical failure thickness of the skin is between 3 layers and 4 layers.

[0038] Step three: determine the critical capacity of the structure to withstand static pressure load through bearing capacity test.

[0039] The bearing capacity of the elastic structure is obtained through the static mechanical property test of the structure. The structure parameters of the elastic structure are consistent with those in step two. Through the static force test, the static bearing limit of the structure is obtained.

[0040] The model used in the static pressure test is consistent with the dynamic model, and the skin thickness of 3 layers and 4 layers of fiber paper is laid in the stress maximum area. It can be seen from the relationship curve between the ultimate strength of the skin and the thickness of the skin that the ultimate strength of the skin is not strictly linear with the thickness of the skin, and the elastic modulus of the material decreases slightly with the increase of the thickness of the skin. The corresponding ultimate static pressure of the 3-layer and 4-layer skin of the aircraft bottom scale model is 31.04 kPa and 37.4 kPa respectively.

[0041] Step four: obtain the static equivalent coefficient of water ditching dynamic load by comprehensive evaluation of structure failure. The static equivalent coefficient of water ditching dynamic load of a certain aircraft is obtained by applying formula (1) η = 0.60-0.72.

[0042] An aircraft fuselage bottom structure optimization design method, using the coefficient determined in the aircraft water ditching dynamic load static equivalent coefficient determination method,

[0043] An aircraft fuselage bottom structure optimization design method, comprising:

[0044] Obtain the equivalent static load distribution of the aircraft fuselage bottom under the water ditching working condition;

[0045] Determine the initial value of the fuselage structure parameters;

[0046] Take the initial value of the fuselage structure parameters as input, take satisfying the equivalent static load distribution of the aircraft fuselage bottom as constraint, and take the minimum weight of the aircraft fuselage bottom as optimization objective to optimize.

[0047] Optimize the structure parameters of different parts of the fuselage.

[0048] In some optional embodiments, the fuselage structure parameters include: structure form, skin thickness, skeleton stiffness, energy-absorbing structure configuration and arrangement.

[0049] In some optional embodiments, the influence weight of different fuselage structure parameters on the dynamic response of the bottom of the aircraft fuselage is analyzed, and the optimization of different fuselage structure parameters is sorted based on the influence weight.

[0050] In some optional embodiments, the optimization frequency of the fuselage structure parameters in the front of the optimization sequence is greater than that of the fuselage structure parameters in the back of the optimization sequence.

[0051] In some optional embodiments, the method for obtaining the equivalent static load distribution of the bottom of the aircraft fuselage under the water ditching working condition comprises: determining the dynamic water pressure load distribution of the bottom of the aircraft fuselage, and calculating the equivalent static load distribution of the bottom of the aircraft fuselage under the water ditching working condition based on the water ditching dynamic load static equivalent coefficient.

[0052] The determined aircraft water ditching dynamic load static equivalent coefficient is mainly used for aircraft water ditching performance design, so as to improve the iteration design efficiency of the aircraft fuselage structure under the water ditching load working condition. The water pressure load borne by the bottom of the aircraft fuselage during water ditching is an impact dynamic load. The conventional method takes the peak value of the water ditching impact dynamic load as the design load for the structure design, which leads to a very conservative design result. The optimization design of the structure by using the impact dynamic load will make the design iteration efficiency relatively low. The determined aircraft water ditching dynamic load static equivalent coefficient can quickly obtain the equivalent static load of the structure optimization design under the water ditching working condition. The optimization of the structure form, skin thickness, skeleton stiffness, energy-absorbing structure configuration and arrangement of the fuselage structure by using the static load can realize the rapid iteration of the structure parameters and improve the design efficiency.

[0053] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto. Any changes or replacements within the technical range disclosed in the present application can be easily thought by those skilled in the art, which should be covered in the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method for determining the static equivalent coefficient of dynamic load during an aircraft's forced landing on water, characterized in that, include: Step S1: Based on the dynamic load measurement test of the water landing, obtain the relationship curve between the water pressure load on the bottom of the aircraft fuselage and the time history, and determine the peak value Ps of the water pressure load based on the relationship curve. Step S2: Conduct a water-based forced landing test on the elastic model of the fuselage bottom to determine the critical failure parameters of the elastic structure; Step S3: Conduct static mechanical property tests using the critical failure parameters of the elastic structure to obtain the maximum static pressure P that the elastic structure can withstand. d ; Step S4: Calculate the static equivalent coefficient of dynamic load for forced landing on water.

2. The method for determining the static equivalent coefficient of dynamic load for aircraft forced landing on water as described in claim 1, characterized in that, The test site for the dynamic load measurement test of forced landing on water is a towing pool or an open pool, and the test model is a rigid pressure measurement model.

3. The method for determining the static equivalent coefficient of dynamic load for aircraft forced landing on water as described in claim 1, characterized in that, The test model for the water landing test of the elastic model at the bottom of the fuselage is a rigid model, while the bottom of the fuselage is an elastic model.

4. The method for determining the static equivalent coefficient of dynamic load for aircraft forced landing on water as described in claim 3, characterized in that, The elastic model of the fuselage bottom specifically includes: the ultimate strength of the skin material is less than that of the fuselage stringers, the skin material includes multiple layers of fiber paper, and the critical failure parameters are determined based on the fiber paper layup parameters and failure modes of the skin after the test.

5. The method for determining the static equivalent coefficient of dynamic load for aircraft forced landing on water as described in claim 4, characterized in that, The critical failure parameters include: skin thickness and skeleton stiffness.

6. The method for determining the static equivalent coefficient of dynamic load for aircraft forced landing on water as described in claim 5, characterized in that, Based on the relationship curve and the location of the most severe load, the location of the most severe load in the test model of the water landing dynamic load measurement test is replaced with an elastic model, and a water landing test of the elastic model at the bottom of the fuselage is carried out.

7. The method for determining the static equivalent coefficient of dynamic load for aircraft forced landing on water as described in claim 5, characterized in that, The formula for calculating the static equivalent coefficient of the dynamic load during a forced landing on water is as follows: η6P s / P d 。 。 8. The method for determining the static equivalent coefficient of dynamic load for aircraft forced landing on water as described in claim 5, characterized in that, The skin of the elastic model at the bottom of the fuselage is made by coating multiple layers of waterproof, formable fiber paper with organic solvents. The thickness of the skin is adjusted by changing the number of fiber paper layers and the number of times the organic solvent is applied.

9. A method for optimizing the design of the bottom structure of an aircraft fuselage, employing the coefficients determined in the method for determining the static equivalent coefficients of dynamic loads for forced landings on water as described in any one of claims 1-8, characterized in that, Obtain the equivalent static load distribution under the water landing condition of the aircraft fuselage bottom; Determine the initial values ​​of the fuselage structural parameters; The initial values ​​of the fuselage structural parameters are used as input, and the equivalent static load distribution at the bottom of the aircraft fuselage is used as a constraint. The optimization objective is to minimize the weight at the bottom of the aircraft fuselage. The optimized fuselage structural parameters for different parts were obtained.

10. The method for determining the static equivalent coefficient of dynamic load for aircraft forced landing on water as described in claim 9, characterized in that, The fuselage structural parameters include: structural form, skin thickness, frame stiffness, energy-absorbing structure configuration and arrangement.

11. The method for determining the static equivalent coefficient of dynamic load for aircraft forced landing on water as described in claim 10, characterized in that, The influence weights of different fuselage structural parameters on the dynamic response of the aircraft fuselage bottom are analyzed, and the optimization of different fuselage structural parameters is ranked based on the influence weights.

12. The method for determining the static equivalent coefficient of dynamic load for aircraft forced landing on water as described in claim 11, characterized in that, The fuselage structural parameters that are optimized earlier in the optimization order are optimized more times than those that are optimized later in the optimization order.

13. The method for determining the static equivalent coefficient of dynamic load for aircraft forced landing on water as described in claim 11, characterized in that, The method for obtaining the equivalent static load distribution under the water landing condition of the aircraft fuselage bottom includes: determining the dynamic water pressure load distribution under the aircraft fuselage bottom, and calculating the equivalent static load distribution under the water landing condition of the aircraft fuselage bottom based on the static equivalent coefficient of the dynamic load of the water landing.

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