Design Methodology Of Landing Performance For Large Civil Airplane

One main function of an aircraft landing gear is absorbing and dissipating energy during aircraft landing.Shock absorbers in landing gears for a large civil aircraft fulfill this function.In the perspective of a high-qualified oleo-pneumatic shock absorber design,aimed at the feature of large civil airframe's flexibility,the vertical and longitudinal characteristics of landing gears are studied during landing.Some research methods used in the scheme design phase for shock absorbers,such as,theoretical modeling,simulation analysis and engineering evaluation are improved.Research results are quite significant in guiding landing gear shock absorber design and analysis with enhancing the landing qualification.Firstly,both a wing and a landing gear shock strut are commonly simplified as a beam in the landing gear structural-dynamic field.Moreover,both a connection structure between a wing and a fuselage,and a wing-mounted structure(e.g.wing-mounted engine)have influence on the dynamic characteristics of both a wing and a landing gear respectively.How to analyze and solve this engineering problem has not been reported in the documents before.Based on extracted key factors,the engineering problem is abstracted to the theoretical problem.The transverse free-vibration equations of Timoshenko,Rayleigh and Shear beam are studied by both transfer matrix and variation parameter method.The hinge support,lumped mass,and rotational inertia are researched.Furthermore,the equation form for a multi-span beam with arbitrary complex boundary condition is put forward.Its simplified model,the analogous Euler beam,is proposed to infer three models with the engineering meanings.Results show that,comparison between the frequency equations which were derived by the three models and previous articles,presents good accordance.Two examples are given by Nastran to compute the first-order frequency.By comparing the result by Nastran with the result from the formula proposed,the error is within 5%,which illustrates that the proposed model is fair and useful.Secondly,the airframe is equivalent to an Euler beam to calculate its first-order frequency by the energy method.Also,the large civil aircraft airframe is modeled to analyze its first-order frequency by FEM.Results show that,the variation amounts to 3.1% between two results by different methods.The flexible airframe is modeled based on the mass-spring-damper system.Further,a landing gear's force condition during landing is introduced with investigating shock absorber's friction,which is due to inner seals.Within them,the landing gear landing model extends to the aircraft four-degree landing model to augment previous landing dynamic models.With importing the airframe's FEM mesh into the virtual prototype program,the landing simulation analysis is carried on.The influences on the landing performances considering airframe flexibility,main landing gear's positions,mode superposition and wing-mounted engine are analyzed.Results show that,the airframe's flexibility and damper absorb and dissipate some landing energy,so that the tire and shock absorber's absorbing energy is lower.It illustrates that,when designing the flexible aircraft landing gear,the airframe flexibility should be considered.Otherwise,its shock absorber could not perform well and may cause the landing gear's weight increase.If a landing gear or external store's position is near the wing's nodal line of the low-order vibration,the airframe flexibility's influence on them could be decreased.Thirdly,an analytical model for the longitudinal load is established with considering influences from an outer cylinder and tire.Furthermore,the stiffness formula for landing gear shock struts are modified.Then,a landing gear test for spin up and spring back loads is introduced.Comparison between results from the proposed model and test,the different range of loads calculated between the proposed model and the previous test is under 5%.It shows that the model for calculating both the vertical and longitudinal load is correct.Moreover,the simulation method for spin up and spring back loads is studied.The simulation analysis for the tire radial stiffness is also carried on.With importing the shock struts' FEM mesh into the virtual prototype program,the longitudinal load simulation analysis is put forward.Results show that,with the increase of contact-platform speed and drop work,the friction coefficient has a decrease tendency.Both platforms' unevenness and tire's specification have influence on spin up and spring back loads.With the increase of tire bead diameter at rim seat,both the spin up and spring back load are decreased.Additional to that,a simplified landing dynamic model is set up.The engineering evaluation method for spin up and spring back loads are given and analyzed based on the simplified model.Comparison between results from the proposed method and professional technical manuals,results from the proposed method are closer to test results.It illustrates that the proposed method is fair and useful.Fourthly,researches center on parameters of a landing system.The previous engineering method for evaluating shock absorber's parameters is modified.Next,shock absorber parameters' sensitivity and regression analysis are given.Then,the optimized design model for landing performance based on the analytic model is put forward.Results show that,the shock strut axis force decreases 3.11% after the optimization.Meanwhile,the optimized design model for longitudinal load based on the agent model is also given.Parameters of the landing system can be determined by two optimized models.Moreover,based on test data,some parameters of a landing system which are given by experience are identified.Results show that,the polytropic exponent is 1.127,the polytropic exponent is 0.839 and the efficiency of landing system is 67.29%.In addition,the relevant regression equation has high regression precision.Lastly,during the scheme design process,the digital engineering method is used to a landing system for the large civil airplane.Some programs are developed,such as parameter calculation,landing performance,secondary development and database application.The program flow diagram and data flow diagram are both illustrate programs from different angles.Additional to that,the modeling and database tools are as operation and memory methods for data.During the scheme design process,it helps designers to improve design efficiency for a landing system.