Landing Gear Simulation with Hydraulic Control in MotionSolve and Twin Activate

John_Dagg_0 · August 2

Overview

INTRODUCTION

Landing gear are employed on most aircraft for ground operations and landings. Engineers must design landing gears to handle the impact loads during aircraft touchdown and absorb the shock of the landing to prevent damage to the aircraft. There are three main types of struts used in the aircraft industry: rigid struts, spring steel struts, and oleo struts. Rigid struts transfer the shock load directly to the airframe. With rigid struts, landing at a high velocity can damage the frame and make for an uncomfortable landing. A slightly more complex but safe option are spring steel struts. They incorporate a spring to transfer the load to the frame over a greater period of time to decrease the shock to the plane. However, the oscillation of the springs can create a bumpy landing. Oleo struts are a clever solution that decreases the shock to the plane and dampens oscillations.

The oleo shock uses two telescoping cylinders. The top cylinder is connected to the aircraft frame and the bottom cylinder, called a piston, attaches to the wheel axle or bogie beam. The top cylinder is filled with air or nitrogen and the piston with incompressible hydraulic fluid. As the piston is pushed into the cylinder, the nitrogen compresses, behaving like a spring. However, it is desirable to have an increasing spring rate and damping to prevent oscillations. To achieve this, engineers put an orifice plate in the cylinder and a tapered rod called a metering pin in the piston. As the piston moves upward, the area for the hydraulic fluid to flow into the cylinder decreases because the tapered rod takes up a larger area in the orifice. This creates an increasing spring rate. The friction from the flowing hydraulic fluid past the orifice plate converts kinetic energy into thermal energy which dampens the system. The oleo strut makes landings easier for pilots and safer for the passengers and aircraft.

Designing an oleo strut landing gear can be aided by using computer simulation. Different orifice plate sizes can be tested to optimize landing performance. Engineers can also test different landing velocities to determine the maximum safe velocity for the airframe and landing gear. In this example, we will build a landing gear model using MotionSolve, a multibody dynamics (MBD) tool. While MotionSolve handles the dynamics, co-simulation with Twin Activate, an intuitive 1D physics modeling software, will allow accurate modeling of the hydraulic system.

Understanding the Model Definition in MotionSolve

First, the MBD model of the landing gear is built from existing CAD data in MotionView.

Joints are created between components to represent real-world joints.
A 20,000 kg mass is added to the top of the strut to represent a portion of the plane’s mass.
An MF Swift tire model accounts for the stiffness of the tire-ground interaction.
The bogie’s supporting link is replaced with a flexible body prepared with the FlexPrep tool in MotionView. Now the deformation of the link is accounted during the motion analysis and stresses in the link can be analyzed during post-processing.
Solver variables for the oleo strut’s piston are created so a co-simulation with Twin-Activate can be performed.
- Piston position and velocity are solver outputs to Twin Activate.
- Strut force is the solver input from Twin Activate.

Understanding the Model Definition in Twin Activate

The Twin Activate model performs calculations for the oleo strut forces using the Modelica library. The oleo strut is represented as a plunger cylinder which carries the entire 20,000 kg mass. Damping depends on the stroke and velocity of the plunger which is realized using a look-up table. Every timestep of the simulation, Twin Activate receives the piston position and velocity from MotionSolve. Twin Activate returns the force on the piston to MotionSolve.

Pre-Requisite

SOFTWARE REQUIREMENTS

MotionView (2024 or newer)

MotionSolve (2024 or newer)

Twin Activate (2024 or newer)

MODEL FILES

Landing_Gear.zip (See attachments)

Usage/Installation Instructions

MODEL SETUP & SIMULATION STEPS

Open LandingGear.scm in Twin Activate.
Select Run to perform the analysis
Open a HyperView session in HyperWorks.
Open the result h3d created in the 04_Results folder to review the animation.
Open the result plt file in a HyperGraph session to plot displacements, velocities, and forces.

Post-Requisite

RESULTS

The motion of the multibody dynamics model can be viewed once the simulation is complete. HyperView allows users to create vectors plots such as joint forces. Users can also create contour plots including flexible body stresses. Stress hotspots can be used to inform design changes.

Wheel Reaction Forces During Aircraft Touchdown

Bogie Support Link Stresses During Aircraft Touchdown

Support Link Peak Stresses

In HyperGraph, plots of all outputs created in MotionView and Twin Activate can be plotted and analyzed. The aircraft velocity curve can provide some insight into the overall performance of the shock. Strut displacement, velocity, force, and pressure are crucial parameters in the analysis of the oleo strut.

CONCLUSION

In this example we built a model of an oleo strut landing gear using MotionSolve and Twin Activate. By utilizing simulation, aerospace engineers can test different oleo shock orifice plate diameters to predict the shock on the airframe and passengers. With powerful post processing tools, information about the strut displacement, velocity, hydraulic pressure and more can be plotted in HyperGraph. Flexible bodies can be used to predict the stresses on landing gear components allowing engineers to validate system performance.

AUTHORS

John Dagg, Systems Engineering Intern

Christopher Fadanelli, Solution Engineer - Systems Integration

Ananth Kamath Kota, Global Technical Manager - Systems Integration