# MV-1011: Extension and Retraction Analysis of the Main Landing Gear of an Aircraft

In this tutorial, you will learn how to create a sequential simulation script to simulate the extension and retraction of the landing gear.

## Sequential Simulation

Often, the total dynamic behavior of a multibody model needs to be captured through more than one solution sequence. Typical examples include:

1. The model configuration changes after a certain amount of time.
2. One type of analysis has to occur before the other. For example, a static solution is required before a dynamic run.
3. Complex models may not solve with a single solver setting. The solver settings may need to be changed for a certain period in the overall simulation time.

Such conditions may be simulated by providing a set of commands to the solver that achieves the above type of sequences. Such simulations are referred to as sequential simulations. Generally, the solver receives more than one solution (simulate) command. You'll learn to script such a simulation sequence through this exercise.

## Exercise

In this exercise, a landing gear mechanism is simulated for its extension and retraction. A sequence of simulations need to be scripted such that the mechanism is retracted within a certain period of time at which the simulation is halted, the model configuration is changed for retraction and the solution is executed again for extension.

Note: Copy the following files, located in mbd_modeling\interactive\aero, to your working directory:
• MainLandingGear.mdl
• Aircraft_Structure.hm
• MainLandingGear_graphics.h3d

## Phase 1

In this phase, you will prepare the landing gear model to simulate the retraction of the landing gear.

### Open the Landing Gear Mechanism

In this step, you will open a landing gear model and attach a graphic for the aircraft body.

1. From the toolbar, click Open Model .

Or

Open model by selecting File > Open > Model.

2. From the Open model dialog, select the file MainLandingGear.mdl and click Open.
Once the model is loaded it will look as it does below. Review the model for its bodies and joints.

The model consists of linkages of the cylinder and piston that hold the wheel as well as linkages for the extension and retraction of the whole landing gear unit within the aircraft body. The model also contains a body defined for aircraft but without graphics. As a first step, you’ll add a graphic for the aircraft body part for visual representation.

Use a HyperMesh file to create a graphic for the aircraft body.

1. From the Tools menu, select the Import CAD or FE utility.
2. Select the option Import CAD or Finite Element Model Only. With this selection, only the graphics will be imported without creating any bodies.
3. For Input File, select HyperMesh. For Output File, select H3D.
4. Click Input File and select Aircraft_Structure.hm as the input file.
5. Save the file as Aircraft_Structure.H3D in your working directory.
6. Click OK.
7. When the H3D file is generated and the graphic is imported, the import success message is displayed. Click OK.
8. Click Graphics in the MotionView toolbar.
9. Select the just-added graphic from the modeling window.
The panel for the aircraft body is displayed.
10. From the Connectivity tab:
1. Resolve the Body collector to Aircraft 5.
2. Set the Component list to All.
Note: Since the H3D file has only one component, aircraft_body, the Component list can be set to either All or select the only component from the drop-down menu.

### Define a Motion to Retract the Landing Gear

In this step, define the motion to retract the landing gear.

1. From the Project Browser, right-click on Model and select Add Constraint > Motions (or right-click Motions from the toolbar).
2. Add a motion with Label as Retraction Motion and Varname as mot_ret.
3. Resolve the Joint to the Cylindrical Joint ActPis_ActCyl_Cyl (shown in the image below), which is defined between the bodies Activating Cylinder and Activating Piston.
4. From the Motion panel > Properties tab, under Define by, select Expression. In the Expression field, enter the expression STEP(TIME,0,0,5,-750).
Note: The above STEP expression ramps the value of motion from 0 at 0 second to -750 at 5 seconds. .
5. Add another motion and name it Aligning_Motion. Resolve the Joint to the Cylindrical Joint LnkCyl_LnkPis_Cyl defined between the MLG Cyl and MLG Pis bodies (see the image below).
6. From the Properties tab of the panel, under Define by, select Expression. In the Expression field, enter the expression STEP(TIME,0,0,2,-100).
7. Save your model as landinggear_motion.mdl.

### Run a Dynamic Analysis to Simulate the Retraction of the Main Landing Gear

In this step, run a transient analysis of the main landing gear mechanism.

1. From the Run panel, set the End time to 5 seconds.
2. Click Save and run current model browser and enter the name for the run file as lg_retract.xml. Click Save.
3. Click Run. Once the simulation is completed, close the solver window and the Message Log.
4. From the Run panel, review the results animation by clicking Animate.

## Phase 2

In Phase 2, write a Templex template to script and run a sequential simulation to simulate the retraction and extension of the landing gear mechanism.

### Define an Extension Motion for the Landing Gear

In this step, model the extension motion of the landing gear.

1. From the Project Browser, right-click Model and select Add Constraint > Motions (or right-click Motions from the toolbar).
2. Add another motion with Label as Extension Motion and Variable Name as mot_ext. Resolve the motion to the same joint for which the Retraction Motion has been defined.
3. From the Properties tab, set the type of motion as Expression and enter the following in the Expression field: STEP(TIME,5,-750,10,0).

### Define a Template to Run the Sequential Simulation

In this step, write a template to script a sequential simulation.

1. From the Project Browser, right-click on Model and select Add > General MDL Entity > Template (or right-click Template from the toolbar).
2. Specify a Label and Varname and click OK.
3. For Type, select Write text to solver command file.
4. Enter the script given below. The entries in the curly braces {}, refer to the idstring of either Extension Motion or Retraction Motion. This idstring attribute can also be accessed using the Expression builder, .
Note: The following commands are MotionSolve command statements in the XML format. Since the solver refers to entities through their ID numbers, the element_id value is resolved to the motion IDs. If you have used different varnames for the motions than mentioned below, the text could differ.
<!--Deactivate the extension motion first -->
<Deactivate
element_type = "MOTION"
element_id   = "{MODEL.mot_ext.idstring}"
/>
<!--Simulate for 5 seconds -->
<Simulate
analysis_type       = "Transient"
end_time            = "5.0"
print_interval      = "0.01"
/>
<!--Deactivate the retraction motion -->
<Deactivate
element_type = "MOTION"
element_id   = "{MODEL.mot_ret.idstring}"
/>
<!--Activate the extension motion that was deactivated during the first simulation -->
<Activate
element_type = "MOTION"
element_id   = "{MODEL.mot_ext.idstring}"
/>
<!--Simulate for 5 more seconds -->
<Simulate
analysis_type       = "Transient"
end_time            = "10.0"
print_interval      = "0.01"
/>
<STOP/>

<!-- Stop the simulation. Any further commands below this command will not be executed -->
The above script has five blocks.
Block 1
Deactivates the motion which defines the extension of the landing gear.
Block 2
Simulates the model for the retraction of the landing gear.
Block 3
Deactivates the motion used to retract the landing gear.
Block 4
Activates the motion which defines the extension of the landing gear.
Block 5
Simulates the model for the extension of the landing gear.

The Stop command is used to stop the simulation at the time set in the last Simulate block.

Note: A template can be used either to add modeling entities to the solver deck, such as joints, markers, and so on, that may not be supported by MotionView, or command entities such as simulation parameters, activation and deactivation, and so on. The MotionSolve XML is divided into two sections.
Model Section
All model elements are defined. To write to this section from the template, for Type select Write text to Solver input deck.
Command Section
Commands for the solver simulation are defined. To write to this section from the template, for Type select Write text to Solver command file.

### Simulate and Animate the Model

In this step, run the model to simulate the retraction and extension of the main landing gear model and animate it.

1. From the Run panel, specify the file name as MLG_Simulation.xml and click Run.
2. Once the solution is complete, close the HyperWorks Solver View window.
3. Click Animate and review the animation.

## Phase 3

In this phase, you will create output requests and re-run the model to measure the Angular Displacement and Angular Velocity of the landing gear.

### Create Output Requests and Rerun the Model

In this phase, you will create output requests and re-run the model to measure the Angular Displacement and Angular Velocity of the landing gear.

1. From the Project Browser, right-click on Model and select Add > General MDL Entity > Outputs (or right-click Outputs , from the toolbar).
2. Create an output request. From the Outputs panel:
1. Set Type to Displacement and Entity.
2. Use the drop-down menu to select the Joint collector.
3. For the Joint, select the Revolute Joint between the Main LG Cylinder and Aircraft-5, MLG_Ac_Rev.
3. Create another output request to measure the velocity at the same joint as above.
4. Re-run the analysis with these output requests and plot the requested results from the generated PLT file in HyperGraph.