Warning! This site requires Javascript. Learn how to enable it here.

Finding CG on a scratch build

Discussion in 'Everything Scratchbuild (Questions & Chit-Chat)' started by FlyingHigher, Nov 18, 2010.

  1. FlyingHigher

    FlyingHigher Rookie

    Posts:
    9
    Likes:
    0
    Points:
    0
    I will be building my first plane ( No plans). Can you give me some advise on finding the CG ?? It appears from what I've been reading, on a prop puller with wing above, the CG should be approx 1 " behind the leading edge of the wing. Am I on the right track???

    Is there a formula for finding the CG ??
  2. JettaManDan

    JettaManDan Administrator

    Posts:
    8,602
    Likes:
    1,024
    Points:
    133
    not really a forumla because it depends on the design of the plane..a straight wing plane a good rule of thumb is about 30% of the wing cord is a good CoG (if the wing is 10 inches deep from front to back then 3 inches back from the leading edge for example) delta's and V wing planes are a bit different....but still about 30% back from the wing break as it comes out of the body works too...i always make my planes nose heavy 1st and then adjust...
  3. FlyingHigher

    FlyingHigher Rookie

    Posts:
    9
    Likes:
    0
    Points:
    0
    I have to agree with you. Nose heavy keeps you safe.

    Thanks for the advise.
  4. squishy

    squishy Top Gun

    Posts:
    6,104
    Likes:
    989
    Points:
    133
    you don't have to find the cg, it's always there and really easy to find...the hard part is figuring out where it needs to be in order to make your plane fly the best it can...glide testing or drop testing is what I do...I hold it up high, all flight ready and drop it, if it noses down slightly, I am on the right track...then I glide test it on the grass...

    I always make my battery Velcro longer than it needs to be so I can move the battery forward or back, I constantly adjust my cg's, almost every flight depending on battery size and what I want the plane to do...if I want to fly slow I move it back, if I want to fly fast pattern I move it forward..also if you use different size batteries on the same plane, each size has a place to Velcro onto the plane..I can fly my cheap-n-easy with a 1500 2 or 3 cell or a 450 3 cell, it's a huge difference in weight and flight characteristics...it's really interesting to see how it effects the flight of the plane...do your own experiments, especially with a scratch built...that's the funnest part of this game...its the puzzles and the unknown...but don't say you need to find the cg of a plane, it's not like it is hiding from you or something, it's right there...you need to find out where you want it and where the plane wants it...
  5. jcrankshaw

    jcrankshaw Ace Pilot

    Posts:
    2,880
    Likes:
    26
    Points:
    68
    I'm sure there is a nice long scientific equation for finding the CG... I use Dan's method though. Plan on wrecking your first scratch build if you have never done this before. Find the CG on your first one then build your second one exactly the same.

    I always aim for nose heavy and have max throws on elevator settings to pull out of a nose dive if needed.
  6. JettaManDan

    JettaManDan Administrator

    Posts:
    8,602
    Likes:
    1,024
    Points:
    133
    you can look at it this way...the plane has to be held up by the wings (Duh! lol)....so if you find where the center of that wing surface is...front to back then you will be close..i always try to go forward of that..if you look at the Mig -29 for example...mine is prefect for the CoG when it is right around 2 inches back from where the wing root starts...my SU-37 was about the same...
  7. SCALLAN

    SCALLAN Cadet

    Posts:
    495
    Likes:
    3
    Points:
    18
    Finding the CG of an airplane

    Here is what I do.
    First I approximate the 1/3 of the wing root chord
    The I take all the extra heavy stuff out of the airplane ( motor, battery, esc, ect ) use some sort of ballast to balance plane on your theoretical CoG

    Next I do the glide test by throwing it into some long grass You will have little damage because the weight has been reduced, Keep changing the CoG and re-throw until it gets a little glide into it. Now when you re-install all of the heavy components and maiden it. Bump the CG a little farther forward to be safe and work back to the point you determined in the glide test.

    Good luck
  8. fly4fun

    fly4fun Rookie

    Posts:
    1,635
    Likes:
    1
    Points:
    0
    If this is a normal plane, Cessna style ect then yeh, 1/3 back from the leading edge is guna be there or there abouts.

    EG: If you have a 6 inch wide wing, then 2" back from the L, Edge will be a start.

    We never done a glide test, just used this and all my planes have flown fine, even nitro stuff, except for one. There's always one...Dohh !!! And we got 20+ planes.
  9. Keegles

    Keegles Rookie

    Posts:
    593
    Likes:
    0
    Points:
    0
    All I do (and has worked for me for years) is grab the middle of wing on the tip with each of my index fingers and if the nose falls forward with all the electronics in, its ready to fly. If you're looking for more indepth flight characteristics, then you need a more scientific approach. For your a beginners sake this should work well. While I'm building the plane, I'm shooting for the 1/3 back from leading edge.
  10. rodrigo

    rodrigo Ace Pilot

    Posts:
    1,951
    Likes:
    166
    Points:
    63
    use this tool
    http://adamone.rchomepage.com/cg_calc.htm
    and read this
    http://adamone.rchomepage.com/index5.htm
Similar Threads
Forum Title Date
Everything Scratchbuild (Questions & Chit-Chat) Help finding the perfect (free) video editor software! Dec 26, 2012
Everything Scratchbuild (Questions & Chit-Chat) finding cg Aug 13, 2012
Everything Scratchbuild (Questions & Chit-Chat) Finding the canards "high alpha" angle Feb 16, 2012
Everything Scratchbuild (Questions & Chit-Chat) Finding Lipo Capacity Jan 3, 2012
Everything Scratchbuild (Questions & Chit-Chat) finding the CG???? Nov 16, 2011
Everything Scratchbuild (Questions & Chit-Chat) Need help finding a video Nov 2, 2010

Share This Page