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Thread: Complications of attaching blades to hub bar

  1. #1

    Complications of attaching blades to hub bar

    1. Is it correct. that for proper operation and balance, the chordwise center of gravity of the blade must align exactly with the CG of the hub bar? Further research indicates the answer is no. It is the quarter chord that must align with the center of the hub bar?

    2. Considering that jump pre-rotation triples centrifugal force, suppose I filled the round quarter chord cavity and the square one behind it with aluminum inserts epoxied in like the nose spar, 14 inches deep, to help improve strength as well as give crush support. Could that create other problems? Is it even necessary? Considering the tripled centrifugal force?

    3. I have a problem that the leveler plate, when centered over the quarter chord, is not level on top. I suspect its from using old blades with old parts that are not exact matches. I know this blade profile was changed at least once because the skin on my blade is 0.088 thick, and a scrap of blade that looks exactly the same actually has a skin of only 0.050. If I slide the leveler plate back until it’s level the straps overhang the leveler by almost half an inch., but don’t see any way around it. Think it should still work (the curve of the leveler still fits the curve of the blade) just not pretty, and maybe can put some filler in the gap to make it look better and also reduce drag.

    4. Is there a standard kind of, or brand of, epoxy used to attach the leveler plates to the blades. Gotta be some pretty good stuff.

    5. How much torque is used on the mounting bolts.
    Attached Thumbnails Attached Thumbnails Alderfer blade profile.jpg  
    Last edited by James J. Judge 42935; 11-10-2017 at 02:38 PM. Reason: problems getting text up

  2. #2
    Well shucks - had hopes of getting some expert input to get some level of certainty but it's just not happening, so here's what I'm gonna do based on what I've been able to find.

    Item #1 - Ed Alderfer in his mid 1990's article about mounting these extruded blades says he mounted the leveler plate centered over the quarter chord (the round cavity), which means that is the centerline for the leveler plate, the straps and the hub bar.

    Item #2 - I have filled those cavities as described.

    Item #3 - Can't see as I have any choice, the leveler plate has to be level. Actually it only moves back a quarter of an inch which I hope is not significant. The half inch overhang comes because my straps and hub bars are 3 inches wide and the leveler is made for 2 1/2, so the back side of the strap is even with the leveler and all the overhang is on the front. I've made a blow-up of the blade profile to create a larger matching nose to reduce drag.

    Item #4 - I've ordered T88 epoxy resin from System Three Resins. After looking at strength and adhesion specs in several places this looks as good as the best, and it has an application minimum temperature of only 35 degrees so there won't be any problems there, and also a longer gel time of one hour so no worries about too fast a set up time. The stuff I used on the nose spars gelled in about 10 minutes, but it was almost as thin as water and I only needed to just pour it in.

    Item #5 - 3/8 inch bolts appear to be the normal size used, but I'm using 7/16 for added shear strength because of the added centrifugal force. The recommended torque for 7/16 grade 8 fine thread is 58 pounds but am concerned about distortion. The recommended torque for 3/8 bolts is 37 pounds so will use that since the added stress is shear and not tensile.

    If anybody has any thoughts along the lines of -- no no not that -- I'd appreciate a comment. Thanks.

    Jim

  3. #3
    Oh - another thing. I'm considering epoxying the straps to the blade and hub bar for extra strength and stability. I don't think it will ever be necessary to take them apart since I have fine tuning adjustments for pitch and span balancing built into the rotor head, and am assembling them in a jig so can't see any way to improve on the accuracy of that. Does anybody know any reason not to epoxy them? Maybe they need to move a little bit? Hard to imagine movement working long term. Thanks.

    Jim

  4. #4
    PRA Secretary JOHN ROUNTREE 41449's Avatar
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    That's a good question James. I do not know if they are suppose to move. Hope someone knows the anwser.
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  5. #5
    James, I'd ask for the blade manufacturer for his specifications on this. All blade manufacturers are different.

  6. #6
    Unfortunately the Fleck blades haven't been manufactured for 10 years or more. Guess I'm just looking for somebody with a general knowledge of these things. I've made a jpeg of my drawing of how the blade profile will look as currently planned, and two possible nose constructions. For best drag saving, the larger epoxy impregnated fiberglass one would overlap the straps on top and bottom by an inch or so for good adhesion and would probably require a small aluminum tube in the nose with an aluminum brace back to the blade. Could also carry the nose all the way back along the two foot hub bar, which would make it about 3 feet long. But would the drag saving be worth the weight? Would the hub bar flex to the point of shattering the fiberglass nose? Suspect maybe yes.

    Attachments Pending Approval Attachments Pending Approval

  7. #7
    James there is a reason why these Haven't been manufacturered in 10 years. I have the knowledge you request. Your going to gamble with your life if you fly these. What is your life worth?
    I will not help you kill yourself.

  8. #8
    Brent - to say this is a shocker is to put it too mildly. In all my reading I have not seen any indication of this, and I would expect that if there were deaths it would be out there. All I have seen is Ed Alderfer's article written after over 100 hours of flying that gave high praise. Please please do share this knowledge. I have read that Vortech, who bought the rights from the original designer Fleck, is now selling a new design that is significantly lighter, but they still have Ed Alderfer's article about the old blades on their website. I understand that their new blades are highly regarded. It is mind bending to even consider that a highly reputable company would repeat high praise for blades that are deadly dangerous. Please do fill me in with the knowledge you have, and quote me some source material so I can dig deep to learn something. I've got money and a lot of time invested in these and have GOT to know the story.

  9. #9
    Well, I've spent quite a bit of time searching for an indication of inherent weakness or danger in the Fleck blades, and have come up empty, so as far as I'm concerned that's the end of it. I can only say to Brent, I appreciate your concern, but very large numbers of people are killed by ignorance, too little knowledge. I've never heard of anyone being killed by too much knowledge.

    A hundred years ago virtually everybody would have told the Wright brothers they were going to kill themselves. They didn't because they studied the heck out of the subject, then built a prototype and tested the heck out of it, and when the tests were successful they kept their flights within a couple feet of the ground until they had developed the personal skill needed to control it.

    I am doing everything I can to follow their example, and developing an extensive ground testing procedure. Nowadays we are way ahead of the Wrights because we have excellent CFIs to teach us the flying skills. So no worries, mate.

  10. #10
    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

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