Constraints in trim analysis

For a trim analysis, you use a fictitious support and, if applicable, single point constraints (SPC) to balance the applied loads and prevent singularities from arising. The number of trim variables that you must fix is given by the following relation:

{N_{fixed}} = {N_{trim}} - {N_{linked}} - {N_{fictitious}}

where:

{N_{fixed}}

Number of fixed trim variables.

Fixed trim variables are any standard or user trim variables to which you assign a numerical value in the Trim Variables Manager dialog box.

{N_{trim}}

Number of standard and user trim variables that you select in the Trim Variables Manager dialog box.

{N_{linked}}

Number of linked trim variable relationships.

Linked trim variable relationships are linear combinations of standard or user trim variables that you define using the Edit Trim Variables dialog box.

{N_{fictitious}}

Number of degrees-of-freedom (DOF) specified on the fictitious support

The DOF that you specify with the fictitious support depend on the types of rigid body motion that you want to allow. You can use SPC to disallow the rigid body motion types that are not specified with the fictitious support.

Example 1: Overall lift curve slope

In this example, you want to allow vertical and pitching rigid body motion. Thus, you specify DOF 3 (for vertical motion) and 5 (for pitching motion) with the fictitious support.

Assume that you select the following four standard trim variables for the trim analysis: angle of attack, pitch rate, vertical acceleration, and pitch acceleration.

If none of these trim variables are linked, you must assign numerical values to two of the trim variables because:

2 fixed trim variables = 4 trim variables - 0 linked trim variable relationships - 2 DOF on the fictitious support

For example, you might assign numerical values to the angle of attack and pitch rate trim variables.

Because you are allowing vertical motion and pitching, use SPC to disallow rigid body longitudinal, lateral, rolling, and yawing motion.

Example 2: Steady-state roll rate

In this example, you want to allow rolling rigid body motion only. Thus, you specify DOF 4 (for rolling motion) with the fictitious support.

Assume that you select the following two standard trim variables for the trim analysis: roll rate and roll acceleration. Additionally, you select a user trim variable for aileron deflection.

If none of these trim variables are linked, you must assign numerical values to two of the trim variables because:

2 fixed trim variables = 3 trim variables - 0 linked trim variable relationships - 1 DOF on the fictitious support

For example, you might assign numerical values to the roll acceleration and aileron deflection trim variables.

Because you are allowing rolling only, use SPC to disallow rigid body longitudinal, lateral, vertical, pitching, and yawing motion.

Example 3: Steady-state 1g flight (2 DOF version)

In this example, you want to allow vertical and pitching rigid body motion. Thus, you specify DOF 3 (for vertical motion) and 5 (for pitching motion) with the fictitious support.

Assume that you select the following four standard trim variables for the trim analysis: angle of attack, pitch rate, vertical acceleration, and pitch acceleration. Additionally, you select a user trim variable for elevator deflection.

If none of these trim variables are linked, you must assign numerical values to three of the trim variables because:

3 fixed trim variables = 5 trim variables - 0 linked trim variable relationships - 2 DOF on the fictitious support

For example, you might assign numerical values to the vertical acceleration, pitch rate, and pitch acceleration.

Because you are allowing vertical motion and pitching, use SPC to disallow rigid body longitudinal, lateral, rolling, and yawing motion.

Example 4: Steady-state 1g flight (6 DOF version)

In this example, you want to allow longitudinal, lateral, vertical, rolling, pitching, and yawing rigid body motion. Thus, you specify DOFs 1, 2, 3, 4, 5, and 6 with the fictitious support.

Assume that you select the following eleven standard trim variables for the trim analysis: angle of attack, angle of sideslip, roll rate, pitch rate, yaw rate, longitudinal acceleration, lateral acceleration, vertical acceleration, roll acceleration, pitch acceleration, and yaw acceleration. Additionally, you select a user trim variable for elevator deflection.

If none of these trim variables are linked, you must assign numerical values to six of the trim variables because:

6 fixed trim variables = 12 trim variables - 0 linked trim variable relationships - 6 DOF on the fictitious support

For example, you might assign numerical values to the sideslip, roll rate, pitch rate, yaw rate, vertical acceleration, and pitch acceleration.

Because you have specified all six rigid body motions with the fictitious support, SPC are not required.

Where do I find it?

Application	Pre/Post
Prerequisites	A Simulation file as the work part and displayed partSimcenter Nastran as the specified solverStructural as the specified analysis typeSOL 144 Static Aeroelastic Response as the specified solution type
Simulation Navigator	Right-click the active solution→Edit
Location in dialog box	Bulk Data page→DOF Sets group→Fictitious Support (SUPORT)

How do I

Assign a modeling object to a solution or solution subcase

Define an aerodynamic control surface

Create a static aeroelastic subcase

Define aerodynamic divergence data

Create an aerodynamic divergence subcase

Create a fictitious support

Plot trim conditions

Learn more

Static aeroelastic response analysis (SOL 144)

Aeroelastic divergence analysis workflow (SOL 144)

Aeroelastic trim analysis workflow (SOL 144)

Defining trim variables

Post-processing aeroelastic trim analysis results

Quick links

Command reference

Pre/Post video examples

Bulk Entry Descriptions

Simcenter 3D tutorials

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Constraints in trim analysis, Simcenter 3D 2021.1 Series

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