Center of pressure calculation for cylindrical bodies with abnormal shaped nose cones 1

[dennisbernal91z]

I am trying to compare different shapes of cylindrical objects in free flight for their stability.

All the info I can find online talks about how to determine if a model rocket is stable.
The most complex part of this approach is correctly taking into account the fin count and shape.

All well and good.

I want to figure out what happens to the center of pressure (CP) when I change the nose cone shape to something that is not a perfect cone.

For example something that has tetrahedral depressions in it, or a hole punched straight thru the middle.

I don't have any knowledge of how to calculate CP for these "odd" cases.

Any help in this area would be greatly appreciated.

Thanks in advance!

 

[moon161]

If it's still a body of rotation around the long axis of the rocket, I'm guessing the the CP is on or near the axis for many cases.

 

[rb1957]

CFD ?

another day in paradise, or is paradise one day closer ?

 

[IRstuff]

What flight regime?
What's the size of the nose relative to the rest of the rocket?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[btrueblood]

CFD, or a wind tunnel experiment, or flight test, or a swing test (this is usually the cheapest for model rockets).

https://www.youtube.com/watch?v=3S7_fg6ZCF4

 

[LiftDivergence]

Depending on your Reynolds number regime, XFLR5 could be acceptable for this application. It is pretty user friendly:

http://www.xflr5.com/xflr5.htm

Just make some "airfoils" which are different cross sections of your nose cones to build up a 3d model. I have seen people do the same thing with spheres for validation of results.

Keep em' Flying
//Fight Corrosion!

 

[Sparweb]

The OP does not need CFD to get his/her answer (even if CFD is a worthy subject of study, intrinsically).
And they don't have to reinvent the wheel either. This has been studied to death for over a century.
Many aerodynamics books cover this subject, but let me point out these:
Two books: Hoerner, Fluid Dynamic Drag and Fluid Dynamic Lift
Packed with test results, for just about anything you can imagine traveling through the air.


STF

 

[GregLocock]

Westheimer's law: "A couple of months in the laboratory can frequently save a couple of hours in the library"

Cheers

Greg Locock


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[LiftDivergence]

I have both of Hoerner's books and love them. I think I have referenced them on these forums before - that is a good call. I did look through Fluid Dynamic Lift before my post.

Reading the thread I got the impression that the OP wants to play around with different nose cone styles including one with "tetrahedral depressions in it" and sort of experiment. Also sounds like they are talking about the behavior of the whole model at large based on different fixtures. Not sure canned drag coefficients for various shapes will give them what they're looking for although it might get them started.

I suppose you could try searching through all kinds of test papers etc. but honestly I think sometimes you just have to do a little of your own work.

For a moderately experienced user this is the type of thing that would take an hour or two in XFLR5 provided we are talking about model scale. It will show you the Cp and plot how it changes with various AoA and other conditions etc.

You could probably spend as much time digging through other peoples work and trying to adapt it to your scenario as just going from scratch. Just my two cents.

Keep em' Flying
//Fight Corrosion!

 

[WKTaylor]

Center of pressure calculation for cylindrical bodies with abnormal shaped nose cones...???

Abnormal shaped nose cones => such as highly irregular or prismatic, etc... by design?

Abnormal shaped nose cones => such as due to 'random [impact, battle, etc] damage'?

Regards, Wil Taylor

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