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  1. #1
    Join Date
    5th February 2009
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    Montreal, Qweebec
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    vent hole shapes?

    Is there thought / theory on vent hole shapes?

    I was just thinking, we all (should) have a vent hole in our tubes, and drilling one is the easiest solution. But, some drill bits tend to chew cardboard. What if I cut a hole with a knife (X-Acto type).. But that would make a square ' rectangular hole. Or maybe triangular... Would it matter? What about a diamond instead of a square? And if a triangle, pointy end up or down? if down, and with a square, would the blunt edge cause whistling? If a triangle with the pointy end down, would that be the best, aerodynamically, to not 'force' air into the tube?

    What about if you cut a 3-sided square, leaving eh 4th side still attached, and press it in to the tube / body cavity. if done on the bottom edge, again, might make a flap that keeps turbulence out..

    Am I overthinking it?

    I'm a little rusty on my Bernoulli..

    -paul

    NAR# 101258 - L1
    www.CRMRC.org
    I don't know the same things you don't know..

  2. #2
    Join Date
    5th December 2013
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    MD
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    6,695
    I assume you really mean vent hole (as in pressure venting to prevent separation), and not altimeter ports, correct? I can't see how the shape of a pressure relief vent hole could possibly matter. It is just letting air out as air pressure builds.

    Altimeter ports, of course, have a very specific purpose which is most easily achieved using the number, size and shape recommended by the manufacturer.


    John S. ---- NAR #96911 ---- TRA #15253 ---- MDRA #067 ---- BARC #028
    L1, 3/15/14: Aerotech Sumo, CTI H133BS
    L2, 6/21/14: Giant Leap Vertical Assault, CTI J240RL
    L3, 3/12/16: MAC Performance Radial Flyer, CTI M1101WH
    Altitude: 13,028', L3 flight; Speed: Mach ???, L3 flight

  3. #3
    Join Date
    18th January 2009
    Location
    Maryland
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    11,545
    I hear a finocyl or star shaped hole lets air in faster.
    Dick Stafford
    The member formerly known as the Pointy-Haired Moderator.
    The Original Rocket Dungeon
    Volunteer compiler of product news for ROCKETS Magazine

  4. #4
    Join Date
    7th December 2014
    Location
    Earth
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    700
    Were you dropped...

    As a child...
    Model rocketry is not my religion.

  5. #5
    Join Date
    5th February 2009
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    Montreal, Qweebec
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    I thought I posted this the HPR fourm..

    Bat-mite, yes, venting to prevent pressure differentials, but then again, any hole in the air frame. I guess as well, that the hole really doesn't see much 'moving air' due to laminar region / flow..

    just curious if a shape had any effect..
    -paul

    NAR# 101258 - L1
    www.CRMRC.org
    I don't know the same things you don't know..

  6. #6
    Join Date
    30th January 2016
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    US > OK > NE
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    It very much has an effect, not sure I could predict what. Equilateral triangle point down, less pressure than circular. Equilateral triangle point up, more pressure than circular. Somebody want to do an article?

  7. #7
    Join Date
    21st September 2017
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    Quote Originally Posted by dhbarr View Post
    It very much has an effect, not sure I could predict what. Equilateral triangle point down, less pressure than circular. Equilateral triangle point up, more pressure than circular. Somebody want to do an article?
    Nope, just drill, baby, drill !
    __
    Radrocketeers.org NAR L2

  8. #8
    Join Date
    21st April 2010
    Location
    So central WI, USA
    Posts
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    triangular holes research been done ; https://en.wikipedia.org/wiki/NACA_duct
    Rex
    L2-competitor 3, AT J350W, 8/27/2016, Bong, 2557'
    my youtube channel http://www.youtube.com/channel/UC0gB...?feature=watch

  9. #9
    Join Date
    5th December 2013
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    MD
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    Step bits and Forstner bits will help you drill cleaner holes than universal bits.

    John S. ---- NAR #96911 ---- TRA #15253 ---- MDRA #067 ---- BARC #028
    L1, 3/15/14: Aerotech Sumo, CTI H133BS
    L2, 6/21/14: Giant Leap Vertical Assault, CTI J240RL
    L3, 3/12/16: MAC Performance Radial Flyer, CTI M1101WH
    Altitude: 13,028', L3 flight; Speed: Mach ???, L3 flight

  10. #10
    Join Date
    13th October 2014
    Location
    SouthEastern, WA
    Posts
    5,478
    IMO the best bits for drilling holes in airframes are brad point wood bits.
    Rich

    NAR# 99154

    L3-4x upscale Estes Cherokee-D- AT M1297W 5/28/2016 http://www.rocketryforum.com/showthr...r-rharshberger

    TriCities Rocketeers NAR section# 736 http://www.tricitiesrocketeers.org/

  11. #11
    Join Date
    27th December 2014
    Location
    Seattle, WA
    Posts
    778
    Quote Originally Posted by Rex R View Post
    triangular holes research been done ; https://en.wikipedia.org/wiki/NACA_duct
    Rex
    I love that the Wiki article basically says that we don't really know why they work but they work pretty well. Mumble mumble vortices boundary layer.

    So to answer the OP's question, this is an ideal research project for someone with a wind tunnel.

    NAR L1 "Cheeto Dust", scratch 54mm, H54R (before it became a G54), Mansfield, WA
    L2 "Arc Light", Madcow 2.6" Arcas, J285CL, Mansfield, WA, recovery by snowshoe

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