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  1. #1
    Join Date
    5th March 2017
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    Optimum position of wings from tail?

    What is the best location (distance wise) to attach rocket glider wings to the fuselage in relation to the tail? I've
    tried finding the answer to this without much success. I've read opinions that say the wing should be located 2x
    to 2.5x its chord from the leading edge of the tail, and another opinion that stated the distance should be 50%
    of the wing's span. (This seems to ignore the weight of the engine pod and engine, the size/weight of the wings/stab/rudder, etc.).


    I should mention that the rest of the glider is already built. I had a broken fuselage from a previous kit model
    and decided to use leftover stuff from the parts bin to put together what I thought was going to be a fast scratch
    build.


    Fast forward and the only thing left is to attach the wings. I'm trying to get the glider's overall CG to fall at the
    25% to 30% location of the wing chord to make sure it's stable. Unfortunately I don't have RockSim so I'm
    pretty much flying blind.


    I tried to do it manually by balancing the the fuselage with tail and engine pod attached on a sharp edge to find
    the body CG, then balanced the wings on top of that. Of course the CG balance point changed but no matter
    where I moved the wings I could never get the overall CG point to fall near or under it.


    Is there a better less frustrating way to determine the best position to mount the wings at?

    NAR# 103899

  2. #2
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    27th October 2009
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    Brigham City, UT
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    I'm sure the better experts will step in here, but I would think the simplest approach would be to attach the wings toward the aft end, since it is a rocket and needs to obey c.g. ahead of c.p. Then adjust nose weight, perhaps batteries if R/C, etc. for a burn-out motor weight, so that you get a good test glide. I noticed this summer that the rule-of-thumbs are not as good as a simple test glide.


  3. #3
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    Quote Originally Posted by aerostadt View Post
    I'm sure the better experts will step in here, but I would think the simplest approach would be to attach the wings toward the aft end, since it is a rocket and needs to obey c.g. ahead of c.p. Then adjust nose weight, perhaps batteries if R/C, etc. for a burn-out motor weight, so that you get a good test glide. I noticed this summer that the rule-of-thumbs are not as good as a simple test glide.
    Guess I'm trying to avoid having to add lots of trim clay by building it as close as possible to optimum. Out of the 5 kit gliders I've built, two have needed garbanzo-bean sized clay blobs either forward or aft. These were all lightweight 1/2A3 or A3 mini motor gliders so all that extra weight sure didn't help.
    NAR# 103899

  4. #4
    Join Date
    19th January 2009
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    1,228
    Here are some general rules of thumb for glider design:
    Horizontal Stab: 20%-25% of the wing area
    Elevator: 20%-25% of the horizontal stab chord
    Vertical Stab: 7%-12% of the wing area
    Rudder: 30%-50% of the vertical stab chord
    Fuse Length: 70% of wing span (measured from nose to rudder hinge line)

    If you glue the stab on 1st, then you can set the wing position by trying to get the CG somewhere between 25% and 30% of the chord. You can also do this experimentally by attaching the wing in a non-permanent way and so some test glides.

    I may have a book or two at the house with some more detailed design rules.

    kj

  5. #5
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    Quote Originally Posted by kjohnson View Post
    Here are some general rules of thumb for glider design:
    Horizontal Stab: 20%-25% of the wing area
    Elevator: 20%-25% of the horizontal stab chord
    Vertical Stab: 7%-12% of the wing area
    Rudder: 30%-50% of the vertical stab chord
    Fuse Length: 70% of wing span (measured from nose to rudder hinge line)

    If you glue the stab on 1st, then you can set the wing position by trying to get the CG somewhere between 25% and 30% of the chord. You can also do this experimentally by attaching the wing in a non-permanent way and so some test glides.

    I may have a book or two at the house with some more detailed design rules.

    kj
    Thanks KJ. Since everything is attached except the wings, I'm going to take your advice to temporarily attach the wing by building something like a partial slide box like what slide wing gliders use. That way I can try balancing the complete glider on it and moving the wings around to find that CG "sweet point" on the fuselage, then tape it in place so I can fine tune with some test glides. I'm thinking of leaving that "temporary" sliding mount on since I'll use small pieces of 1/64 ply (which weigh nothing) and building in a 3 angle of attack into it. Fingers crossed.
    NAR# 103899

  6. #6
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    19th January 2009
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    I like to use small rubber bands looped under the boom and across the top of the wing, but your slide box idea works too.

    kj

  7. #7
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    Quote Originally Posted by kjohnson View Post
    I like to use small rubber bands looped under the boom and across the top of the wing, but your slide box idea works too.

    kj

    Guess I may have over-engineered it a bit but I seem to have found a pretty decent CG spot located at about 30% of the wing chord. I still haven't glued the wings on yet so I'll use your rubber band trick for final trimming. Thanks for that great suggestion!
    NAR# 103899

  8. #8
    Join Date
    27th January 2009
    Location
    Oregon
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    Just a small comment, I've found with rear mounted motor designs with long forward fuse/tube that the vertical stab area is often much larger than this guideline to maintain yaw stability, as much as 25-30% of wing area depending on where your CG location winds up.

    Frank


    Quote Originally Posted by kjohnson View Post
    Here are some general rules of thumb for glider design:
    Horizontal Stab: 20%-25% of the wing area
    Elevator: 20%-25% of the horizontal stab chord
    Vertical Stab: 7%-12% of the wing area
    Rudder: 30%-50% of the vertical stab chord
    Fuse Length: 70% of wing span (measured from nose to rudder hinge line)

    If you glue the stab on 1st, then you can set the wing position by trying to get the CG somewhere between 25% and 30% of the chord. You can also do this experimentally by attaching the wing in a non-permanent way and so some test glides.

    I may have a book or two at the house with some more detailed design rules.

    kj

  9. #9
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    5th March 2017
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    Pittsboro, NC
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    Click image for larger version. 

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    Here's the end result of the wing positioning exercise. Although it may be a Frankenstein's monster of pieced together salvaged parts
    and leftovers from the parts bin I like how it turned out. I always wanted to try a flop wing design and this was the perfect opportunity
    to experiment.


    As I expected it's somewhat nose-heavy. Some clay on the tail end should fix things but because I want to add as little weight as possible
    I'm going to try a small Gurney Flap on the rear stab first.


    The other thing I'm worried about is how close the rubber band wing actuator is to the exhaust plume. I anticipated having the wings
    closer to the engine pod which would've made the elastic a bit more out of the path of the exhaust--but the CG balance point dictated
    where the wings had to be. I've had to reroute the rubber band a couple times already so hopefully there's enough of a margin that
    it won't fry on the way up.
    NAR# 103899

  10. #10
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    19th January 2009
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    I don't think that will be a problem, but maybe a small heat shield made of manila folder to cover the rubber bands?

    Yeah a little too much mass forward, especially as it's an RG and keeps the motor casing, but it passes the TLAR test.

    kj

  11. #11
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    Quote Originally Posted by kjohnson View Post
    I don't think that will be a problem, but maybe a small heat shield made of manila folder to cover the rubber bands?

    Yeah a little too much mass forward, especially as it's an RG and keeps the motor casing, but it passes the TLAR test.

    kj
    You mean attach a small piece of manilla folder over the bottom end of the engine to kind of redirect the exhaust?
    Or did you mean over the rubber bands? Bit worried that would be too draggy and not sure how to attach it.
    Appreciate the suggestions, I haven't had much experience with gliders and have never designed one from scratch
    before so feedback really helps.
    NAR# 103899

  12. #12
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    19th January 2009
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    On the center part of the wing, to protect the rubber bands since I think that's what you are concerned about.

    kj
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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  13. #13
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    Quote Originally Posted by kjohnson View Post
    On the center part of the wing, to protect the rubber bands since I think that's what you are concerned about.

    kj
    Thanks, great idea!
    NAR# 103899

  14. #14
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    Did a static test with the model tied down to the launcher and a live engine in it to see if the exhaust would melt
    the rubber band or weaken it. Was glad to see that all it did was deposit some soot on it (at least with a 1/2A3-
    2T).

    What I totally wasn't expecting was the over powered ejection charge that blistered and charred the engine tube
    and the flames that poured from the ejection ports for what seemed like forever. Cr*p is this normal? Good
    thing I had coated the engine tube with CA. Next burn-thread build will get several coats of high heat aluminum
    paint inside the tube over the CA.

    After that little adventure I spent some time fine tuning the trim after I added a small Gurney Flap to the stab.
    Until a week ago I never knew they existed and even though it's only a half inch long and sticks up barely 1/16",
    it spared me from adding a bunch of clay to the tail. I notice that there's a very brief nose-up after you toss it,
    but it immediately settles into a level glide. At least in this case that small sliver of ply worked far better than
    expected.

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    Of course I got carried away and started tossing it harder and higher at about a 60 angle to see how it would
    handle loops. Unfortunately I couldn't get it higher than about 12 feet or so...barely enough to almost level out
    before nosing into the lawn. Should have stopped there but didn't, and on the second throw I ran out of lawn
    and broke the fuselage right behind the pylon when it smacked into the concrete driveway. Did a field repair with
    CA and it seems to be okay, so I've decided to fine tune the trim after some powered flights.
    NAR# 103899

  15. #15
    Join Date
    11th September 2013
    Posts
    211
    I've never really looked at a rocket glider before. What are the rubber bands for?
    Gregg
    NAR #97216

  16. #16
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    30th January 2016
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    Quote Originally Posted by kjohnson View Post
    I like to use small rubber bands looped under the boom and across the top of the wing, but your slide box idea works too.

    kj
    Quote Originally Posted by Rktman View Post
    Guess I may have over-engineered it a bit but I seem to have found a pretty decent CG spot located at about 30% of the wing chord. I still haven't glued the wings on yet so I'll use your rubber band trick for final trimming. Thanks for that great suggestion!
    Quote Originally Posted by Swissyhawk View Post
    I've never really looked at a rocket glider before. What are the rubber bands for?
    Options

  17. #17
    Join Date
    19th January 2009
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    Quote Originally Posted by Swissyhawk View Post
    I've never really looked at a rocket glider before. What are the rubber bands for?
    For boost, this glider had its wings folded back to move the center of pressure back behind the center of gravity to make it stable on boost. At ejection a thread is burned that releases the wings to fold forward into glide mode. The rubber bands on the top of the wing are what make the wings move.

    kj

    Last edited by kjohnson; 16th October 2017 at 02:57 PM.

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