Project duration: 1996-1997

Identification and analysis of mechanisms responsible for triggering unstable behavior in aircraft brakes.
 

Introduction:

Friction-induced vibrations are a major concern in a wide variety of mechanical systems. This is especially the case in braking systems, where friction is both the principal performance factor and a potential cause of detrimental vibrations, noise, and excessive wear.

In aircraft braking systems friction induced oscillations can lead to excessively high loads in the landing gear and the brake structure, with consequences ranging from noticeable human discomfort to structural failures of the brake components. The aerospace industry distinguishes five basic types of aircraft brake vibration: squeal, whirl, chatter, gear walk, and shimmy.

Squeal vibration is characterized by torsional motion of stators, torque tube, and piston housing. Typical frequency range is between 100 Hz and 20 kHz. Squeal usually occurs during landing stops, but it can also be observed during taxis. Higher contact pressures and higher energies increase the severity of this type of vibration. Squeal can be excited by the characteristics of the friction material and by modal coupling between axial and tangential degrees of freedom of the brake system.

Whirl vibration shows up as out of plane "wobble" motion involving the brake disks, torque tube and piston housing. Whirl frequency range is 100 Hz to 300 Hz, roughly the same as the first squeal mode. Indeed, coupling between squeal and whirl modes is often observed. Whirl typically occurs during high velocity landing stops, especially if there are bumps in the runway.

Chatter mode of vibration involves torsional motion of rotors and wheel, typically coupled with fore-aft motion of the landing gear. Chatter is largely controlled by tire stiffness and it can occur at the end of taxi stops. The frequency of chatter vibration is low, in the range between 10 Hz and 100 Hz.

Gear walk is defined as fore-aft motion of the landing gear, typically coupled with torsional motion of rotors and wheel. Gear walk oscillations can build up to significant levels, creating passenger discomfort and potential for structural failures. Typical gear walk vibration frequency is also very low, between 10 Hz and 50 Hz.

Shimmy vibration mode involves torsional and lateral motion of the wheel and the landing gear. Because of low frequencies (10 Hz - 50 Hz) and high energies involved it can be very destructive, especially in two wheel main gears with off-center bracing.
 

Overview of the Project:

In the current effort, COMCO's expertise and numerical models on friction have been used to model a simplified aircraft brake. The aim was to understand and analyze friction-induced oscillations, predict their occurrence if any, and study the factors that affect them. The brake assembly contains several disks interacting at frictional interfaces, and pressed together by the pressure of six pistons. The model chosen consists of some representative components of the brake.

A finite element model was developed that represents one disk, the pistons, and the torque tube. The disk is pressed on one side by the pistons and, in turn, presses against a spinning surface that represents the rotor disc. The surface rotates at a constant angular velocity about the axis of symmetry of the model. A thin tube, attached to the inner side of the disk at one end and fixed in the xy-plane at the other, is added to provide the required torsional stiffness. This tube represents the torque tube of an aircraft brake. The material properties correspond to typical components of carbon-type aircraft brakes. For better dynamic representation, the mass density of the pistons is increased to account for the mass of the piston housing assembly that is not included in the model. Loading conditions are chosen to represent the actual operating modes of the brakes.

A nonlinear interface constitutive law (namely, a modified version of Oden-Martins model) with velocity-dependent friction is incorporated into a dynamic representation of the problem.  The nonlinear response of the interface in the normal and frictional directions is computed from a surface profile using an asperity-based homogenization method.

In order to define basic dynamic characteristics of the system, the oscillations of the model with no friction were first examined. The main frequencies of the system were identified as: normal, tangential, and piston wobbling. It was also found that increasing the stiffness of the interface increases the frequency of the normal vibrations of the model.  Moreover, due to the nonlinear response of the interface, increasing the normal load increases the normal frequency by increasing the stiffness in that direction.  The effect of the normal load was more evident on softer interfaces.

This model was also used to analyze stability of transient behavior following a perturbation of the steady-state sliding position. In these analyses, both stable and unstable modes were observed, depending upon the parameters of the system and the loading conditions. In particular, two major instability modes were detected:

• Unstable mode caused by the negative slope of the friction-velocity curve. In this mode, primarily the tangential vibrations were excited. Moreover, stronger instability was observed under higher normal loads and higher slopes of the friction-velocity curve.
• Unstable mode caused by friction-induced coupling of certain dynamic modes of the system.  In this mode, both normal and tangential oscillations were excited. The occurrence of this mode was highly sensitive to various dynamic parameters of the system and nonlinearities inherent in the contact interface.

• The instability due to dynamic coupling is more likely to occur when certain modes within the system have frequencies close enough to be coupled by friction.
• The occurrence of this mode of instability is less systematic than of the friction-velocity case, and is strongly affected  by various dynamic parameters and nonlinearities in the system, such as the normal compliance of the interface.
• The nonlinear compliance of the interface affects the oscillation frequencies of the system by making them dependent upon the piston pressure. The strength of this influence varies with the the overall hardness of the surface, stiffness of the disk, etc. In the present model, the softer interface produced more unstable cases.
• While the instability due to velocity-dependence of friction can occur at rather lower sliding speeds (where the friction-velocity curve exhibits a steeper negative slope), the instability due to dynamic coupling can occur at a wide range of velocities.

The hp-adapted finite element mesh used.    The deformed shape (displacements scaled 600 times) and distribution of contact pressure (psi) on bottom of stator during unstable sliding.    Time histories of tangential and normal displacements of a point under the piston and their frequency content.

Copyright 2000 Altair Engineering, Inc.