Neil, how do you handle this?
First off, I create a second OR model that is purely intended for CP and flight simulation, vs. the original model that is intended for parts listing, construction details, and appearance.
In my "flight" model, I replace the rings with an equivalent number of fins. I have my own way of doing this that is based purely on my own intuition (and therefore probably wrong), vs. the way that was described in an Apogee POF a long time ago. Here's what I do. I imagine that if anything it is overly optimistic with regard to effect on CP.
DISCLAIMER: THIS COULD VERY WELL BE WRONG, YOU SHOULD PROBABLY GO READ THAT POF ARTICLE INSTEAD
The goal is a fin set that will behave similarly to the ring. In order to do that, the fin set needs to have:
a) same total area as the ring
b) same amount of leading and trailing edge as the ring
c) on average, the fin area at the same distance from the body as the ring
d) same position on the body as the ring
So here's the process:
- Calculate the circumference of the ring, call it C
- Calculate difference in diameter between the inner BT and the ring, call it ΔD
- Create a trapezoidal fin set with the following parameters:
- Number of fins = C / ΔD (round to nearest)
- Root chord and tip chord = length of ring
- Height = ΔD
- Sweep length = 0
- Position on body = same as ring
- Fin cross section = square
- Thickness = same as wall thickness of ring
- Material = same as ring
- Fudge the values a bit, to account for the error introduced by the rounding in step 3.1.
This satisfies my four requirements above. If you did it right the mass of the fin set should be just about the same as that of the ring; for extreme preciseness you can override the fin set mass to equal that of the ring.
This approach ignores fin/body interaction (present with the fin set, absent with the ring), and the additional drag of the air squeezing through the gap between the ring and the body. My guess is, much like tube fins, if the ratio of ring length to the size of that gap gets too high, air stops flowing through there and the ring contributes nothing My rings tend to be short and do not (I don't think) approach that situation.
So here's an example for the ring on Biohazard, which is a BT80 ring wrapped around a BT55.
Ring:
"Equivalent" fin set:
That's it. I'm happy to hear all the reasons why this is wrong.