# Solution Sequences

OptiStruct can be use different solution sequences for aircraft analysis.

Some of the OptiStruct solution sequences that can be used are:
Type
Description
STATICS
Linear Static or Nonlinear Static Gap Analysis
MODES
Normal Modes Analysis
BUCK
Linear Buckling Analysis
DFREQ
Direct Frequency Response Analysis
LGDISP
Large Displacement Nonlinear Static Analysis (available as PARM card entry)
HEAT
Linear Steady-state or Transient Heat Transfer Analysis
FATIGUE
Fatigue Analysis

## Model Files

Before you begin, copy the file(s) used in this example to your working directory.

## Static Analysis

OptiStruct can be used to solve time-independent static analysis. Figure 1 shows a fuselage section with SPC boundary conditions on the bulkhead and a uniform pressure applied to the skin.

For further information, refer to Linear Static Analysis.

## Nonlinear Static Analysis

An analysis is termed nonlinear when the relationship between the Force and Displacement is nonlinear. Most of the structural components in an aircraft structure are subjected to large deformations, which are best analyzed through nonlinear analysis. The main reasons for nonlinearity are:
• Material nonlinearities
• Geometric nonlinearities
• Presence of nonlinear forces
• Contact nonlinearities

## Inertial Relief Analysis

Inertia relief analysis is mostly performed on unsupported structures to determine the impact loads of structures or to calculate the distribution of forces. OptiStruct has two options for Inertia relief analysis.
• PARAM, INREL, -1 is used when certain boundary conditions are specified.
• PARAM, INREL, -2 is used when no boundary conditions are specified.

Figure 2 shows the results from an Inertia Relief Analysis performed on a fuselage model.

## Normal Modes Analysis

Mode shapes provide the frequencies at which the structure will absorb all the supplied energy when no load is acting on it. To analyze the displacement of a structure at these frequencies, you can use Frequency Response Analysis. Normal Modes Analysis of aircraft structures will help in determining:
• Under constrained and loose components
• The rotating speed which matches the natural frequencies in case of the analysis of a blade or a rotor
• The areas to be constrained or loaded.

Figure 3 shows the results from the Normal Modes Analysis for a fuselage and a drone. Both models have free-free boundary conditions.

## Frequency Response Analysis

Each frequency is solved independently and can also solve a several frequencies at a time. This can be further used to determine the displacement versus frequency plots. This helps to study the displacements of the structure when subjected to its natural frequency calculated using Modal Analysis. The frequencies can be specified using the FREQi card.