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  1. #1
    Join Date
    14th July 2015
    Location
    Randolph, NJ
    Posts
    3,026

    Tail ring impact on CP?

    OR does not currently support tail rings. Sadly, even the new version under development does not (Rocksim does though). I use a lot of tail rings in my designs; in OR I use internal tubes which will create the appropriate visuals but are not factored into CP at all.

    This is frustrating because there are designs for which the tail ring needs to be a significant contributor to stability. And so, my simple question is, does anyone know, even to a rough approximation, how to factor tail rings into CP calculation? I also care about drag, but less so. Stability is more important than altitude for most of these designs.

    I believe I've heard that for tube fins, the equivalence used inside OR is of 3 fins of the same cross section as the tubes. I don't think I've ever heard how tail rings are considered. Anyone know?

    Recently completed: APRO Lander II and Starship Avalon; next up: Accur8-skinned Trajector and Ragnarok Orbital Interceptor
    My design thread; photo albums: fleet pics and OR Models;

  2. #2
    Join Date
    15th October 2016
    Location
    Huntsville AL
    Posts
    1,581
    In my futile attempts to apply OR to my GBIs (Great Big Inductors), I've tried things like 9 fins of the same chord as the ring sized so that the total area is the same. I chose 9 because any fewer get significantly larger than the ring span, and any more just get ridiculous. It does have an effect on the CP.

    How valid this is wasn't determined since I decided to try it on an unsimmable design in the first place.........

    "I'm at least 70% confident about whatever I say (90% of the time)"- college me

    NAR 101195
    Level 1: Big SAM, 9/10/16

  3. #3
    Join Date
    14th July 2015
    Location
    Randolph, NJ
    Posts
    3,026
    After thinking a bit about your suggestion, I concluded it made a lot of sense. I tried it on the design I'm playing with right now and I think it actually worked OK. For that design I needed 6 or 7 fins to get equivalent area.

    Certainly, it should have very nearly equivalent drag, and as for CP it ought to at least be in the ballpark, at least in cases where there's enough space between the ring and the inner airframe so that air will actually flow through.

    Works for me, thanks for the idea!
    Recently completed: APRO Lander II and Starship Avalon; next up: Accur8-skinned Trajector and Ragnarok Orbital Interceptor
    My design thread; photo albums: fleet pics and OR Models;

  4. #4
    Join Date
    15th October 2016
    Location
    Huntsville AL
    Posts
    1,581
    My only worry is that this hack has 0 rigor behind it, and a fair portion of the fin area is closer to the airframe than it should be as a ring. That would likely produce different pitch moments.

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