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W L CG Dynamics Moment of Inertia LabRat Scientific © 2018

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Moment of Inertia Inertia is a property of matter
Inertia is a resistance to a change in motion A Moment is created when a force is applied at some distance from a point of rotation which results in a torque Moment of Inertia (MOI) is also known as Rotational Inertia The Moment of Inertia is a “resistance” moment that is caused by mass elements distributed at one or more radii from some point of rotation

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Concepts of the Moment and Resistive Moment
2 kg The 2 kg mass creates a moment around Point A A Force = Mass * g B 2 kg The inertia of the two masses create a “resistive moment” that opposes any “external moment” that is applied about Point B

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Moment of Inertia Formulas (small sample)
Sphere IAA = 1/12 * M * (a2 * b2) IAA = 4/3 * M * r2 CG CG b a Notice that these equations involve the square of the length dimensions… A A r CG B B CG L IAA = 1/12 * M * (3r2 * a2) a IAA = 1/12 * M * L2 IBB = 1/2 * M * r2 CG = Center of Gravity (a.k.a. Balancing Point)

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So, the Moment of Inertia is similar to Momentum, but not exactly.
With translational motion, the distribution of the mass elements does not come into play when you try to stop the object. It’s the total mass that you have to stop. With rotational motion, the distribution of the mass elements is critical because the Moment of Inertia is a function of the square of the distance each mass element is from the axis of rotation (as shown in the various equations on the previous slide).

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CG 2 kg CG 2 kg Both of these systems have the same total mass and CG location, but the Moments of Inertia are different because the RED masses are located at different distances from the CG (also the center of rotation). CG = Center of Gravity (a.k.a. Balancing Point)

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Examples of how Moment of Inertia affects motion
The ice skater can control his spin rate by moving his arms in and out Move arms out = increase moment of inertia = reduce spin rate Move arms in = decrease moment of inertia = increase spin rate A satellite with a higher moment of inertia will require more fuel in order to change orientation Go for a spin on a playground merry-go-round Move towards outside = increase moment of inertia = reduce spin rate Move towards center = decrease moment of inertia = increase spin rate A heavier baseball bat will have a higher moment of inertia and will be more difficult to swing (more mass along the radius of the swing)

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Measuring Moment of Inertia
MOI’s can be measured using a simple Bifilar Pendulum A Bifilar Pendulum is a torsional pendulum that uses gravity to provide the oscillatory torque The period of oscillation is combined with other simple physical parameters to determine the moment of inertia

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The Bifilar Pendulum Length Distance Mass Oscillates about the CG
Suspension Strings Length Distance Mass CG Test Article

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Examples of Simple Bifilar Pendulums
Free Standing Rig Hanging from Ceiling

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Moment (distance² x weight x period²)
of = Inertia (16 x pi² x length) Where: distance = distance between the strings (meters) weight = the weight of the test object (Newtons = kg x 9.8 m/sec2) period = time it takes the pendulum to make one complete oscillation (Sec) length = length of the strings suspending the object (meters) Pi =

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Period of Oscillation View: Looking down from above the test article when oscillating The period of oscillation is the time it takes for the test article to swing out, then swing back to its original release position. T0 = 0 T2 = ½ Period T3 = Period

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Unit Analysis to verify the equation
Moment (m² x kg x m/sec2 x sec²) of = Inertia (16 x pi² x m)

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Unit Analysis to verify the equation
Moment (m² x kg ) of = Inertia ( ) This equation is valid, the units of Moment of Inertia are indeed kg*m2

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Theoretical Calculation of MOI for a Wooden Block
We can use a homogeneous wooden block to test the accuracy of the Bifilar Pendulum. A wood 2×4 serves as a good test article. Wood is essentially homogeneous so the CG will be at the middle of the block W L CG The following equation from Slide 4 can be used to calculate the Moment of Inertia (MOI) about the block’s CG: 1 MOI = Mass x ( W2 + L2 ) 12 = x Kg x [ ( m)2 + (0.51 m)2 ] = x Kg x m2 MOI = Kg * m2 Mass = Kg W = m L = 0.51 m

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Bifilar Pendulum Measurement of Block MOI
L = 0.8 m D = 0.46 m M = 0.89 Kg Using the Bifilar Pendulum Equation: D (m)2 x W (Kg*m/sec2) x Period (sec)2 MOI = 16 x Pi 2 x L (m) (0.46 m)2 x (0.89 Kg x 9.8 m/sec2) x (1.12 sec)2 MOI = 16 x x m 0.21 m2 x (0.89 Kg x 9.8 m/sec2) x sec2 MOI = 16 x x m 2.29 Kg*m2 = = kg * m2 126.3 Trial Time (sec for 3 oscillations) Period (sec) 1 3.31 1.10 2 3.37 1.12 3 3.38 1.13 4 3.29 5 3.40 Data from tests Average Period = 1.12

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Assessment of Theory vs. Experiment
For the wooden block test case: Theoretical: Kg * m2 Measured: Kg * m2 Conclusion: The Bifilar Pendulum does a good job at measuring the Moment of Inertia of a wooden block. It can be assumed that it does an equally good job on other test articles.

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Practical Application of MOI’s
Effect on Rocket Flight

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Sounding Rockets are long, slender, suborbital vehicles
Sounding Rockets are long, slender, suborbital vehicles. They use canted fins to spin the rocket at Hz during ascent. The spinning nature of these rockets during ascent and often during the data collection period require knowledge of the MOI of the scientific payloads. 4-stage Sounding Rocket NASA’s Family of Sounding Rockets

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Roll Axis A NASA Sounding Rocket is analyzed in three axes – Roll, Pitch, and Yaw. When analyzing a sounding rocket, engineers measure the MOI along the roll axis and a single lateral axis. Since the sounding rocket is symmetrical about the roll axis, the pitch and yaw moments of inertia are essentially the same. Yaw Axis Pitch Axis

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Effects of Moment of Inertia on Rocket Flight
A rocket with a higher moment of inertia will react to wind gusts slower Less wind sensitive (less wind cocking) Oscillations (once they occur) will take longer to damp out Can result in higher average angle of attack over the course of the flight, and thus result in higher drag Higher drag tends to result in lower apogee (maximum altitude) The MOI’s could result in a pitch rate that could match the spin rate, which may make the rocket enter into roll/pitch coupling Results in even higher angle of attack which could adversely affect aerodynamic stability Could lead to rocket break up

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Rocket Roll Rate and Pitching Frequency during first 60 sec of flight
The Roll MOI has an influence on the Roll Rate of the rocket. The Pitch and Yaw MOI’s have an influence on the Pitching Frequency of the Rocket. An engineering analysis that compares the pitch and roll dynamics is critical to ensuring the rocket flies properly.

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Images of deployable sensor boom systems that change the sounding rocket payload’s Moment of Inertia during the science collection period. The MOI’s must be known so the performance of the cold gas attitude control system can be assessed.

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Questions ?

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