# How to determine the CG for a moving object ?

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Hi everyone

I am building a hovercraft, and it is very important to make it balance when moving forward, it is very easy to determine the center of gravity at rest, but when start moving the CG gets shifted to another point ( i think it will be shifted backward ), so i need to determine this new point at a certain craft speed to balance the weights on the craft.

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the center of gravity should remain the same unless the craft deforms significantly or there is a change to the mass distribution.

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By the way, how much area is there below your craft, and how much does your craft weigh?

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By the way, how much area is there below your craft, and how much does your craft weigh?

well,

- the area which the air is trapped in is 2.2 m^2

- and i estimated 200 kg ( 2000 N ) weight

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the center of gravity should remain the same unless the craft deforms significantly or there is a change to the mass distribution.

Agree. if the craft in symmetrical position we can locate the CG easily. I know that we can determine the CG of unsymmetrical of static body by applying these formula:

Ixx = Sum (A)(y2)

In which:

Ixx = the moment of inertia around the x axis

A = the area of the plane of the object

y = the distance between the centroid of the object and the x axis

Ixx = (1/12 ) (b)(h3)

But, not sure if this working on moving object

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OP, are you worried about stability due to accelerations forces? 'Cause honestly, I'm not sure I understand the question. CG doesn't move unless your object changes it's mass distribution somehow.

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ok, maybe the expression is wrong, but the reason for saying that is: i was working on a small model of hovercraft which is powered by electric small blower and i determined its CG. when i tested this model i found it not horizontal, so i asked about this point and i was told that when the blower is turned on its CG gets shifted to another point due to its inertia and it is very difficult to determine this new point.

When the hovercraft is lifted, it will be balanced, but when starting to move forward i am sure that it will tends to tilit to the back.

lets consider point (A) to be the CG at rest, so, if i redistribute the mass to meet the balance at point (B) behind point (A) which when moving forward the craft will be horizontal, i think we can call point (B) the new CG for that object at this speed.

and if that expression is wrong, how to fix this problem of acceleration.

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Oh, I think I see. When the fan starts blowing, it produces lift at that point. So you have to subtract the lift from the fan from the center of mass (treat it as a negative mass).

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ok, maybe the expression is wrong, but the reason for saying that is: i was working on a small model of hovercraft which is powered by electric small blower and i determined its CG. when i tested this model i found it not horizontal, so i asked about this point and i was told that when the blower is turned on its CG gets shifted to another point due to its inertia and it is very difficult to determine this new point.

When the hovercraft is lifted, it will be balanced, but when starting to move forward i am sure that it will tends to tilit to the back.

lets consider point (A) to be the CG at rest, so, if i redistribute the mass to meet the balance at point (B) behind point (A) which when moving forward the craft will be horizontal, i think we can call point (B) the new CG for that object at this speed.

and if that expression is wrong, how to fix this problem of acceleration.

So the hovercraft is pitching back like a car does when it accelerates?

If so, then when it is moving at a constant speed it shouldn't tilt back like that. It will occur when it accelerates, and will vary depending on the rate of acceleration. If you try to account for it by shifting the CoM forward then when it stationary or moving at a constant speed then it will unbalanced. In any case finding out the amount you'd need to shift the CoM to account for it would be difficult, like you've been told, because you need to know the thrust and friction forces on the hovercraft, and finding them would be hard. Instead I'd look at changing the design in a few other ways to minimise the problem:

-Lower to the CoM.

-Lengthen the hovercraft.

-If the motor isn't variable speed, make it so, so that you can accelerate more slowly.

-Increase the lift.

-Add skirts to reduce the amount of pressure bleeding out the back.

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The CoG doesn't shift, it's that you exert a torque when you engage the blower. That's what causes the rotation.

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Agreed with swansont, its the torque you are getting from the motors that is causing the shift in movement. You either want an independent gyroscope to counteract the torque or move the motors themselves appropriately, either way this is not an easy feat.

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ok, i understand now, tilting back is because of the thrust force and acceleration and when moving at constant speed it should be balanced, but if i want to get this new point, i think it can be done by trial and error and it is going to be an exhausting process.

maybe i can get this point at a slow rate of acceleration, so the final mass distribution will be a combination of the two cases (accl. & const. speed).

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