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Thread: Complications of attaching blades to hub bar

  1. #11
    PRA Secretary JOHN ROUNTREE 41449's Avatar
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    Quote Originally Posted by Ed Leugemors 43270 View Post
    Hey James - There is a book "Theory of Wing Sections" that describes center of lift, pitching moment and a lot more. From stuff I found when researching flying wing design some successful designs used a symmetrical airfoil so the pitching moment didn't change with angle of attack. Then the support could be in line with the center of lift and if this a flap - like a rotating jump wing - the blade needs to be balanced on this center of lift (attachment) to avoid flutter and destruction. You might look into this.

    Bolt torque as listed is for steel substrate and there are many papers and articles on this subject. What it comes down to is the bolt tight enough to keep everything from moving, not break the bolt and not crush the substrate. I had a ultralight wooden prop come off in flight even when following mfg recommendation. So I checked torque before every flight after that, found out wood creeps for a while . Ed L

    Good post Ed. Thanks for the heads up.
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  2. #12
    Ed -
    Thank you much indeed - "Theory of Wing Sections" updated is about 50 years old so is readily available online as a free pdf file - about 700 pages and really interesting - mostly for math and engineer types so most of it beyond me but some still useful. The book plus Tervamaki’s ‘gyroincident" below show that you are exactly right. Both the aerodynamic center and the chordal CG are supposed to be at the quarter chord and aligned with the hub bar center.

    About flutter - from Jukka Tervamaki’s website -

    The difference between 8mm and 6mm lead bar 12 ft long is 2.16 lb., and he corrected for that imbalance with one nose weight on each blade. His movie shows it clearly. I’m going to have to do that kind of balancing as well. Amusing to see he used a truck and movie camera in his testing. My own truck with 4 home security cameras is almost ready too.

    You lost a prop midair? Oh my! What you’re saying about bolt torque, ‘what it comes down to’. is what I have been thinking, and already done, so am very happy to have your corroboration. -- Jim
    Last edited by James J. Judge 42935; 04-26-2018 at 08:48 PM. Reason: link didn't show

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