Calculating CP of tube fins

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AlfaBrewer

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How do you calculate the CP of tube fins? How about ring fins? I've got a couple of ideas floating around in the ol' melon, but want to make sure they work on paper, before I start building anything.
 
I have no earthly idea, however, I eagerly await an informed response to this....

Later!

--Coop
 
Flashing against the night sky on low-lying clouds, a mysterious symbols of six interlocked rings flickers over the darkened city. What is it? An advertisement for the Olympics? No it's the internationally known distress call for CAPTAIN TUBE FINS! The hushed citizens below eagerly await rescue....
 
On my tube fin and ring fin rockets that I've scratch built, I didnt do any calculations, but I just added a little nose weight, and balanced it so the Cg '' look right''. They all flew perfect. I've done this on 4 of mine....
 
I would guess intuitively you could caclulate the area of each tube fin by multiplying its two-dimensional profile by pi and come up with a pretty good approximation.

Doesn't RockSim have a tube-fin option? I haven't used a full version in a while... :rolleyes: :confused:
 
Yes, Rocsim 9 does both tube fins and ring fins.

If you want to do the calculations manually, just pretend that your rocket has three times as many fins with the same shape as the profile of each tube fin. In other words, if your rocket has six tube fins that are 1 inch diameter and 3 inches long, run the calculations (Barrowman, etc) as if the rocket has 18 fins that are 1 inch wide and three inches long, in the same position as the tube fin. This will be a pretty close approximation, since pi is 3.14. Ball park figuring says you'll lose a little for the point where the tube fin attaches to the body tube as well as where it attaches to its neighbors. So I think 3 is a pretty good approximation.

I've never tried doing ring fins, but the process should be similar. Treat the ring fin supports as fins and then also treat the ring as rectangular fins equal to the area of each segment between the supports. So if you have three supports, figure three segments that are 1/3 the area of the ring fin remember that to calculate the surface area of a cylinder, first calculate the circumference of the ring fin (pi x diameter) and then multiply the result by height of the ring fin. So if, for example, your ring fin is 4 inches in diameter and 1 inch high, the circumference is 12.566". 1/3 of that is 4.189", so treat the rocket as having three fins that are 4.189" wide by 1 inch tall, plus the supports.
 
Thank you, Greg. This will come in quite handy.

I use OpenRocket, but it doesn't support tube fins. This is a good work around.
 
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