Archive

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 […]

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 tendency to wobble. Knowing POI in all planes allows determining the principal axes of a payload. How to measure product of inertia? Product […]

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 of […]

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 […]

Balancing is one of the most critical steps of any rotor assembly or repair. An unbalanced rotor occurs due to uneven weight distribution of the individual blades and any connected hardware when mounted to the rotor. Unbalanced blades can create vibrational forces that significantly affect a turbine’s operating performance, service life and capacity to function […]

This paper reviews the properties which inherently limit the image quality of a gimbal mounted optical imaging system. It further describes how image quality is degraded by external influences, particularly vibration, in the supporting vehicle. The primary emphasis is to quantify, through physical principals, and verify, through experimental demonstration, the degree of static balance required […]

It is important to test the attitude control systems on satellites before they are launched in space. Traditionally this has been done by dropping the satellite, and firing the thrusters before the satellite makes a soft landing in a net. This method only allows a few seconds of testing and does not lend itself to […]

Factors Affecting Center of Gravity What is Center of Gravity (CG)? The center of gravity (CG) location is the average location of all the weight of an object. The center of gravity is the balance point of an object, also expressed as the point where all the mass appears to be located. How to Calculate […]

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 […]

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 […]

Torsional Pendulum Theory Figure 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 […]