Archive

Why Measure Moment of Inertia

August 12, 2020

Calculation vs Measurement The MOI of simple objects can be calculated fairly accurately using well-known formulas. To calculate the MOI for more complicated objects, you would need to model the object as a collection of simple shapes, calculate the individual MOI’s for each, and add the MOI values to obtain an aggregate value. However, when […]

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Principle of Operation of an MOI Instrument

Other Industries - Golf Clubs

The moment of inertia of any object can be measured through its CG by hanging the object from a wire, twisting it slightly to initiate oscillation, and timing the period of oscillation. However, anyone who has ever tried this finds that the object swings from side to side and bounces up and down rather than […]

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Torsion Pendulum Theory

Consider the torsion pendulum of Figure 1 consisting of a disk of mass moment of inertia I lb-in-sec2, restrained in rotation by a wire of torsional stiffness K in-lb/radian. If the disk is turned an initial angle and sharply released, it will oscillate with damped harmonic motion as shown in Figure 2. The period of […]

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How to Find MOI Through CG Without Knowing the CG Location

Why measure Moment of Inertia (MOI) through the Center of Gravity (CG)? When an object is free to rotate, it will rotate around an axis passing through its center of gravity. Therefore it is essential to know moment of inertia through center of gravity to assess the flight characteristics of a payload. The MOI about […]

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How to Calculate Product of Inertia From Moment of Inertia Measurements

Product of inertia (POI) is the characteristic of an object that causes dynamic unbalance. Knowing the POI of an object is essential to determine some of its flight characteristics, such as its tendancy to wobble. Knowing POI in all planes allows determining the principal axes of a payload. How to measure product of inertia? Product […]

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A New Spin Balance Machine

This page describes a new low speed vertical axis aerospace  balance machine that takes advantage of recent advances in technology. This machine measures moment of inertia (MOI) in addition to product of inertia (POI) and center of gravity (CG) offset. Spin speeds as low as 15 RPM yield useful results. This machine has a number […]

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Using the “Moment of Inertia Method” to Determine Product of Inertia

Product of inertia is generally measured using a spin balance machine. In this type of machine, the object is rotated at a speed of about 100 RPM, and the reaction forces against the upper and lower spindle bearings are measured. Product of inertia is then calculated automatically by the machine’s on line computer, using formulas […]

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Pan Weighing vs Moment Weighing

The terms used in turbine blade balancing can be confusing. The two main terms used are pan weighing and moment weighing. Pan weighing simply means measuring the mass of the blades. This could be called weighing the blade. We call it pan weighing to avoid the confusion with moment weighing. Pan weighing the blades gives […]

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