Research On Positioning Deviation Analysis Of The Aircraft Skin Based On The “N-2-1” Principle

Aircraft panels are basic structures of the aircraft fuselage, their assembly quality directly affects the fuselage’s aerodynamic layout and assembly quality of the aircraft, thereby affecting the flight safety and life of the aircraft. Skin is an important part of the aircraft panel and the datum of the panel assembly process, so the positioning accuracy of the aircraft skin plays an important role in determining the aircraft assembly accuracy.The traditional deviation analysis method based on the dimension chain is complex and has low accuracy when dealing with the low stiffness workpiece which needs multiple clamping constraints. The deviation analysis model based on the deterministic positioning method is mainly used for high stiffness workpieces which are not easily deformed. The rigid deviation analysis model is suitable for rigid workpiece of complex assembly with more error sources. However, skin is a typical sheet metal part with large size, low stiffness and easy to deform, so the flexible model is needed in the positioning deviation analysis and the following issues must be taken into consideration: manufacturing errors of workpieces and fixtures; the initial gap and initial anchor point; the global deformation between workpieces and fixtures; the contact force model and the effect of contact force between workpieces and fixtures; deviation paropagation of the workpieces under the multiple clamping states.To address the above issues, this thesis presents a flexible positioning deviation analysis method which is suitable for the aircraft skin, the flexible model is divided into two parts: the deterministic positioning analysis based on the “3-2-1” principle, to analyze the initial state of the deterministic positioning; the flexible positioning analysis based on the “N-2-1” principle, investigating the deviation formation and propagation mechanism of the workpiece under the multiple clamping states.The main research work of this thesis is as follows:1) The random parameters representing the manufacturing errors of the workpieces and fixtures are generated to obtain the initial anchor points of the skin in the main position surface.2) Based on the “3-2-1” principle, the linear constraint equations of the deterministic positioning is established to analysis the initial positioning state of the skin, the coordinates of the initial anchor points are regarded as the positioning error sources, the other anchor points are regarded as the observation points.3) Based on the “N-2-1” principle, the flexible positioning deviation analysis model is proposed which analyzes the contact states and contact deformation of skin under the multiple clamping states, and the global stiffness is solved by improving the method of influence coefficient. It investigates the stability constraints, static balance constraints and friction cone constraints, and the solution of the contact force. The clamping deviation formation and the propagation mechanism are analyzed with the finite element method.4) The positioning deviation analysis method of the skin is applied with an example.