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FUSELAGE OPTIMIZATION FOR AERODYNAMIC LOADS DURING FLIGHT

THE MAIN OBJECTIVE OF FUSELAGE OPTIMIZATION IS FOR THE FOLLOWING PURPOSES:
[1] During flight, the entire fuselage of the RC aircraft is subjected to loads (dynamic). These aerodynamic loads affect the fluid flow boundary around the entire plane, which can affect the governing factors of flight like lift, drag, stalling angle, cruise velocity.
[2] The distribution of aerodynamic loads around the fuselage can affect the control surfaces. The roll rate for a certain angle of aileron deflection at a certain angle of attack is fixed. These loads can affect the angle and deflections. Hence it is necessary to know these load distributions.
[3] The fuselage shape must be made keeping the aerodynamic loads in mind. The no of support structures to be added: gussets, bulkheads, stringers, Longerons are calculated based on the pressures at each point.

GENERAL FLOWCHART FOR DEIGNING OF AN AERIAL VEHICLE

WHAT DOES OPTIMIZATION INCLUDE?
The optimization process consists of the following stages, chronologically.
Preliminary design sketch
Aerodynamic mesh panel
The surface of the design is divided into a number of panels. An equation of the flow potential at the surfaces and velocity potential at the intersecting points is done. The load due to the pressure
distribution is directly related to the potential at these mesh panels. The unknowns are grouped under a single matrix of co-efficient. They can be further extended as a system of linear equations which when solved help in calculating the disturbance potential and then the individual velocities at these points. These velocities can be used to find the aerodynamic pressures. Having multiplying these with area of a single mesh, the forces can be calculated. Using a coupling method between the aerodynamic model and structural model, the forces can be added as parameters to the structure to calculate the displacements on the fuselage. Accordingly, the sizing of the fuselage can be done to determine the necessary shape.

AERODYNAMIC PANEL MESH

Calculation of aerodynamic loads at each point
The loads are present at each mesh of the surface. The aerodynamic pressures are found out as a
function of the mesh area.
Structural sizing to reduce these loads
At a certain angle of attack, attitude, acceleration of the aerial vehicle, a flight condition is calculated by taking the load factor into account. These variables are then put into account and the resulting deflection required for the control surfaces are calculated. This is termed as the trimmed condition of the aircraft. The trimming may also include more design parameters, each of which is added as a constraint and subsequently coupled with the load factors to repeat the above process. These are the static and trim loads.

During dynamic flight conditions, the velocity potential and flow potential shifts around turbulent regions or gusts. The procedure for calculation of the aerodynamic loads is similar, where the boundary conditions are assumed to be harmonic in nature and the singularities considered for a solution to the equations are doublets (Multiple solutions for a single constraint equation).

Every dimension is taken as a displacement constraint. The no. of load cases for the entire structure is taken into account. Since each dimension is affected by all the loads, these values are multiplied to get the total number of constraints. In the first case, 5 dimensions have been considered for constraint. 50 load cases have been determined. Therefore,
TOTAL CONSTRAINT= DISPLACEMENT CONSTRAINT X LOAD CASES = 50 X 5 = 250

Structural design (adding of support structures)
The design variables for the fuselage are stringers (thickness, length), skin cross section, skin thickness, position of bulkheads at the centre part of the fuselage. More the support structures more will be the design variables. The idea is to reduce the mass of the aircraft and place it exactly where the aerodynamic loads will be the maximum during static and dynamic flight conditions. The support for the rear end of the fuselage can also be determined depending on the bending. The shear forces acting on the walls are similarly taken into account. Apart from the minimization of mass, another objective is to keep the manufacturing ease in mind. The final dimensions for the fuselage structural components must not be difficult to build and the assembly should be easy.
Structural FEA.
Finite element analysis of the final structure of the entire RC aircraft is done.

DIFFERENCE BETWEEN A FEA DESIGN AND AN AERODYNAMIC DESIGN. NOTE THE SPLINES IN THE AERODYNAMIC MODEL WHICH DENOTE THE OPTIMIZATION

SOFTWARE:
LAGRANGE is a software which is used for Multidisciplinary Design Optimization (MDO) of aircraft
structures.

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