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Old 04-25-2020, 11:55 AM
RobVG RobVG is offline
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Default Understanding center of pressure measurements?

I understand that the nose cone and the fins cause lift and these forces can be balanced with weight. But I dont understand why the center of gravity of a cardboard cross section of the rocket can be used to simulate the center of pressure?

Can't wrap my head around it nor can I do the calculus.
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Old 04-25-2020, 12:10 PM
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Jerry Irvine Jerry Irvine is offline
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While the exact center of pressure is not exactly the same as the cardboard cutout, it is close. CP also varies with angle of attack. The main rule for stability is that the CG be ahead of the CP. The CG is the loaded balance point of the rocket and tends to be near exact for even a novice. Since one value is exact and one is loosey goosey, we have a rule that applies a margin of error. The CG should be ahead of the CP by 1 or more calibers (average or maximum main body diameter).

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Old 04-25-2020, 03:06 PM
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Quote:
Originally Posted by RobVG
I understand that the nose cone and the fins cause lift and these forces can be balanced with weight. But I dont understand why the center of gravity of a cardboard cross section of the rocket can be used to simulate the center of pressure?

Can't wrap my head around it nor can I do the calculus.


If you think about the construction of the cardboard, it is uniformly dense. Therefore, its balance point means that besides equal weight on either side of the balance point, the same surface area is present on either side of that balance point, which further means equal pressure will be felt on either side of the balance point. Therefore, that balance point gives you the center or pressure.

Its balance point won’t give you the center of gravity, since the actual rocket will have non-uniform weight distribution such as motor in the aft end, maybe a payload near the nose cone, recovery gear somewhere in between the two, etc.
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Old 04-25-2020, 03:55 PM
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Perhaps this is where a link to TR-1 might be in order.....

This is a collection of Estes Technical Reports from "back in the day". See the first one (starts on the third page of the .pdf file): https://estesrockets.com/wp-content...ction_TR-TN.pdf
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Old 04-25-2020, 05:05 PM
RobVG RobVG is offline
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Thanks for the answers.

The Estes paper is a great resource!
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