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MRA Moment Method


Force-Moment Analysis, embodied in the MRA Moment Method, has its roots in a 1952 Cornell Aeronautical Laboratories memo by Bill Milliken. It is an analysis based on the assumption that most stability and control characteristics of interest to the automotive engineer can be obtained from a study of the steady-state forces and moments on the vehicle. These forces and moments are those associated both with equilibrium (balanced) conditions as well as the "unbalanced" forces and moments available for linearly or angularly accelerating the vehicle.

Simplified constraints.  Click to enlarge This type of analysis is achieved through a procedure known as "constrained testing". It is analogous to the familiar aircraft wind tunnel tests. The figure at right shows in simplified form how the vehicle is constrained laterally, longitudinally and in yaw, but free to move in roll, heave and pitch subject to the constraint forces and aerodynamic forces and moments. By placing the vehicle at combinations of vehicle sideslip angle and steer angle on a moving "roadway", the forces in the constraints can be measured and then interpreted.

While this technique is applicable to a full-scale test, it has found greatest utility to date in the form of a mathematical model and computer program. The MRA Moment Method (MMM) has been developed for over 30 years as a dedicated force-moment analysis computer program. MMM evaluates the vehicle at up to 2500 sideslip/steer pairs, providing output in a number of forms described later in this tour.

MMM provides a picture of stability and control throughout the entire maneuvering range of the vehicle. This type of analysis is extremely difficult (if not impossible) with time-based simulations or full-scale testing.


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MMM Home | Background | Input | Solution | Output

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