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  1. #1
    Join Date
    10th January 2004
    Posts
    2,116

    New Fin Idea - Hole in Fin

    So I'm building a 38mm rocket to be powered by a G64 and F12 AT reloads. I've got done designing it and find it....well...blah. It's a simple 3FNC rocket. So to spice things up I decided I would find the center of each fin (they are trapezoids) and then drill a 3/4" hole right through the fin. I thought it would be interesting and give it a new look. Anyone know how this will effect stability? I'm thinking I will have to find the square inches of then fin before hole, then the square inches of the hole. I just add the two together and arrive at a fin shape that has the square inches of the fin plus the square inches of the hole, so when I drill the hole out I'm left with the original square inches.

    Any ideas?

    Edward

    www.alphahybrids.com

    "The heavens declare the glory of God, the skies proclaim the work of His hands." Psalm 19:1

  2. #2
    Join Date
    31st January 2003
    Posts
    337
    FWIW, the Fliskits Drake flies just fine w/ holes in the fins...


  3. #3
    Join Date
    21st January 2009
    Location
    San Diego, CA
    Posts
    2,859
    While the quest for meeting the exact same amount of surface area with the hole as without may be an interesting problem in mathematics, unfortunately it would not equate directly to rocket performance. By cutting a hole you also increase the eddies created by the fin (you've created an additional leading and trailing edge), as well as increased the frontal surface area on the forward edge of the fin and the hole. Those will affect your drag calculations, which will throw some moderately more complex variables into your CP calculations.

    Generally speaking, by adding the hole but increasing the overall size of the fin to compensate, you will be pulling the CG backwards (both the eddies and the frontal surface will do this) while the righting forces will also increase because the distance from the CG on the moment arm will increase (longer fin - further from center).

    I don't have any empirical data to back this up - it's just from what I remember from my classes on that sort of stuff a while back. So my 2 cents has depreciated to about 1 1/2 cents worth.

    WW

  4. #4
    Join Date
    31st March 2009
    Posts
    3,000
    Here's a neat hole-finned rocket:

    http://www.dgarockets.com/generic.html?pid=49

  5. #5
    Join Date
    18th January 2009
    Location
    Merrimack, NH
    Posts
    15,342
    Originally posted by wyldbill
    FWIW, the Fliskits Drake flies just fine w/ holes in the fins...
    sure do

  6. #6
    Join Date
    2nd May 2009
    Posts
    1,734

    a holey fin...

    seems like it would just increase drag, making the model slower and not fly as high...

    (see signature)

  7. #7
    Join Date
    10th January 2004
    Posts
    2,116
    Lower is okay on this model. It weighs about 20 ounces and sims to 4K. It is only 38mm body tube and 24" long overall. Gonna be hard to find if I don't do something to keep it lower and slower.


    I think I'm just going to oversize the fin and cut the hole. And if I don't like it, I'm going to cut from the hole to the back edge in a straight line


    Edward
    www.alphahybrids.com

    "The heavens declare the glory of God, the skies proclaim the work of His hands." Psalm 19:1

  8. #8
    Join Date
    2nd February 2018
    Location
    Georgia
    Posts
    11
    As noted above, the hole is going to increase drag with the small additional frontal area and resulting vortices and eddys. (Eddies?) The removal of material with shift the c.g. forward a slight (might be barely measurable) amount. IMHO, the loss of material will be offset by the extra drag which will be stabilizing.

    Drill the holes, go fly as a heads-up. Let us know how it goes.
    Andrew

    NAR - L1 on 2/17/2018 via scratchbuilt Mega Der Red Max

    In the pipe: too many...

  9. #9
    Join Date
    18th January 2009
    Posts
    2,157
    Quote Originally Posted by aerojacket View Post
    As noted above, the hole is going to increase drag with the small additional frontal area and resulting vortices and eddys. (Eddies?) The removal of material with shift the c.g. forward a slight (might be barely measurable) amount. IMHO, the loss of material will be offset by the extra drag which will be stabilizing.

    Drill the holes, go fly as a heads-up. Let us know how it goes.
    psst....this thread is 14 years old. I'm hoping they've flown it at least once since then.
    Aaron Head
    NAR 83209 L2
    TRA 11803 L2

  10. #10
    Join Date
    27th March 2013
    Location
    Has Changed
    Posts
    10,193
    Quote Originally Posted by heada View Post
    psst....this thread is 14 years old. I'm hoping they've flown it at least once since then.
    And the OP's last activity on the forum was 9.5 years ago.
    Dreaming of making the rockets I dreamed of as a kid (and then some).


    NAR L1 Cert flight: Sheridan, Oregon, USA. Sept. 19, 2015. Flew Deep Space OFFl on an I357T-14A Blue Thunder

  11. #11
    Join Date
    9th April 2011
    Location
    Mansfield, TX
    Posts
    2,742
    Quote Originally Posted by K'Tesh View Post
    And the OP's last activity on the forum was 9.5 years ago.
    Nah, he is still around albeit under a different name.
    Jarrett Dorough

    Lover of 29mm motors and wood glue

  12. #12
    Join Date
    13th October 2014
    Location
    SouthEastern, WA
    Posts
    6,268
    Quote Originally Posted by grouch View Post
    Nah, he is still around albeit under a different name.
    His sig line says it all, am I correct Jarrett.
    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/

  13. #13
    Join Date
    29th November 2009
    Location
    Fort Collins, CO
    Posts
    1,337
    Rocket has flown a couple times and as an upscale. Lots of fun. Whistles a bit. One iteration I used a router to route a bevel on the bottom of the circle and on the opposite side bevel the top side. Had a slow spin to it.

    Edward

  14. #14
    Join Date
    27th March 2013
    Location
    Has Changed
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    10,193
    Quote Originally Posted by grouch View Post
    Nah, he is still around albeit under a different name.
    I stand corrected...
    Dreaming of making the rockets I dreamed of as a kid (and then some).


    NAR L1 Cert flight: Sheridan, Oregon, USA. Sept. 19, 2015. Flew Deep Space OFFl on an I357T-14A Blue Thunder

  15. #15
    Join Date
    9th April 2011
    Location
    Mansfield, TX
    Posts
    2,742
    Quote Originally Posted by rharshberger View Post
    His sig line says it all, am I correct Jarrett.
    And I am confused???

    Jarrett Dorough

    Lover of 29mm motors and wood glue

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