This page illustrates the MRFT (Modal Analysis using Reaction Force Measurement Technique). It is an method of modifying experimentally measured mode shapes to reflect a new set of boundary conditions. It is independent of finite element analysis. Its most obvious applications for ground testing of aircraft or spacecraft.

The 1st mode shape of a cantilever (propped at two locations), is shown, along with the derived 1st mode shape of an unpropped cantilever.



The associated frequency response functions (from physical location), from which the above shapes were extracted follow.

Measured (Propped)

Derived (Unpropped)




This modal testing method provides a means of modifying 'as tested' boundary conditions, to an 'as desired' set of boundary conditions (e.g. can take out the effect of structural supports). It relies on the development of two technologies:

  1. this software algorithm
  2. Multiple degree of freedom force measurement device

The MRFT method is based on measurement of reaction forces at the 'undesired' boundary support(s). An example application would be the eliminate of effects of landing gear from an ground test of an aircraft.

The modal testing method has been written in C and software QA tested, for a single degree of freedom boundary condition change. It has been evaluated for a 2 DOF boundary condition change in MATLAB. Both single and multi-degree of freedom results have been checked with structural test data.

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