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  1. #26
    Moderator BMatthews's Avatar
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    RE: Need Some Advice On A Goldberg Electra

    The 48oz weight for the Electra must be the weight given in the old kit. You can toss out a heap of that just because you're not going to be using a 12oz battery pack and a 6 oz motor. But given that these were the numbers you were working with and passed on I can now fully understand the suggestion for the Rimfire .15. Hopefully I've shown you that there's a lighter way.

    The key will be just how much lighter you can build the kit. If you don't substitute light wood for anything else on the model at LEAST do so with the tail surfaces and the rear portion of the fuselage. That'll save you a bit of overall weight. But mostly it'll set the stage for a lighter motor in the nose to balance out without the need for much, if any, nose ballast.

    It's also wise to keep the radio gear placement somewhat fluid until the wings and tail are covered and at least pinned in place so you can alter how the gear and pack is situated in order to avoid any or most nose weight.

    Like I said before these planes don't like heavy wing loadings. I've seen the original Electras fly at the stated 48 oz. And trust me when I tell you that they were not at all happy airplanes to fly. To see one in the air with a lightly built Gentle Lady at the same time you'd have no inkling that they are "kissing cousins".
    Witty saying to be plagarized shortly.....

  2. #27

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    RE: Need Some Advice On A Goldberg Electra

    Sounds like either way, I still need to go out and spend some serious $$$$$ before I start building this bird. Also sounds like something that would be good to fly off the earthen battlements at Fort Casey on Whidbey Island. The steep windward hillside should give it a lot of flight time with little need to run the motor after the initial climb. Just went back and reread the thread and like the idea of removing the center dihedral and leaving the tip dihedral as well as adding the webbing suggested. Would it be an option to replace the single bottom spar with top and bottom spars half as thick and make a true D-tube leading edge? The picture posted on page 1 has me slightly concerned with no support to the top sheeting rear edge. I'm thinking that with the twin spars I can reduce the webs down to 1/32 to save some weight without sacrificing any strength. What do you all think?

  3. #28
    Moderator BMatthews's Avatar
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    RE: Need Some Advice On A Goldberg Electra

    If you're going to begin modifying it then I'd suggest you just dive in and build a whole new model from plans and change out a heap of things. Keep the upper view planform and other than that improvise your to your heart's desire. The old adage "in for a penny, in for a pound" comes to mind.

    Things I'd do if I were to go this route;
    [ul][*] Ditch the old school free flight inspired airfoil and use the AG34 from Mark Drela's Allegro Lite. You would not need to copy the high strength spar setup but copy the sheeting and rear turbulator spar locations exactly as shown. Just scale it up or down as needed to fit the Electra wing plan.[*] Keep the center dihedral. It's the dihedral that works with the rudder to provide the proper response. Sure, it'll still turn if you flatten the center panel. But it won't be quite as snappy to respond. It ain't broke so don't try to fix it... [*] Narrow up the fuselage and go to slightly thicker wood and use light tri stock in the corners. That'll let you carve it to more of an oval shape and aid in reducing the turbulence and drag from the box shape. One way to ensure good strength but still keep things light is to use light sheeting for the side and top and tri stock but then epoxy glass on a layer of 2 oz cloth overall with reinforcing patches around the wing leading and trailing edges and at the area at the leading edge of the stabilizer.[*] Lengthen the nose a little by putting the motor onto the front of the motor mount instead of behind it as was done with the old can motor. Add a nice fiberglass cowling around the motor that fairs into a spinner on the prop. But don't forget to allow for some airflow into the motor.[*] Make the front cover out of some heavier sheet to form more of a nicely rounded "canopy" that fairs into the wing's leading edge. This cover will be removed to access the rubber band anchor dowel on the leading edge bulkhead if you still use rubber bands. If not you still should make it easily removable to gain access to the motor pack.[*] I'd probably ditch the rubber band wing mounting and go with two 10-32 nylon screws located at the main spar and that pass down to anchor blocks glued to the sides. Oh, did I mention the 1/64 ply doublers from the nose back to the wing's trailing edge?
    [/ul]

    Another option would be to apply a lot of these ideas to a new model which would be a hybrid between this improved Electra and the 5 panel wing of the Allegro Lite design. I know it's "efficient" and all that but I just cringe at the way so many models these days perch the stabilizer on a little riser off a carbon "pool cue". They just seem like fishing rods with wings.... I much prefer to see an actual fuselage even though it's just for looks. As long as it's well done so that it doesn't get in the way of flying well I don't see that it can hurt.

    ... at least that's how I'd do an improved Electra.... If I were going to build such a model at all.
    Witty saying to be plagarized shortly.....

  4. #29

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    RE: Need Some Advice On A Goldberg Electra

    If we were talking about hydroplanes, I would dive in and go crazy. Since I'm not an aerodynamics and airfoil expert, I was looking at just a few simple changes that wouldn't add much, if any, additional weight.
    That being said, I do see a few things that you listed that I would probably do when I build the kit. I still think the dual spar would be a benefit since I'm not afraid to cut new ribs. I also like the idea of slimming down the aft part of the fuse, even if the only reason is balance. There are a few others but I'm not looking to redesign the whole thing as it's not totally broken like some things I've seen both on the water and in the air. Those are stories in themselves that I don't need to bring into this topic

  5. #30
    Moderator BMatthews's Avatar
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    RE: Need Some Advice On A Goldberg Electra

    Like I say, the AG34 airfoil will do a lot more of what you want than the original Goldberg free flight option. And if you're cutting new ribs anyway and will be changing the sheeting and spar setup then it seems to me that switching airfoils is easy peasy. Print out the plans for Mark's Allegro Lite from the CRRC web site and just use a size altering photocopier to scale the airfoil plan to suit the wing chords of the Electra. No snazzy aerodynamics needed. The ONLY thing you'd have to do is cut the lower AG airfoil shape into the normally flat wing saddle rails of the fuselage to get the new wing to sit at the proper angle of incidence.

    The results of using the better airfoil will make the small added effort more than worthwhile.... Trust me.... So google for "allegro lite" and it'll show you the path to the CRRC web site where the plan for the AL lives.
    Witty saying to be plagarized shortly.....

  6. #31

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    RE: Need Some Advice On A Goldberg Electra


    Not sure if you bought your gear yet...

    I have 20 year old gentle lady (pretty much the same plane with maybe a lighter fuse) that weighs 30oz - ready to fly with 1300mAh 3 Cell, KDA20-22L and CAM 10x6 prop. While it is not a vertical climbing machine (13.5A peak at fresh battery voltage), it can fly for a really, really, long time under motor power (not full of course). Approx 3.5 - 4 minutes at full throttle.

    I would caution you about running such a big battery as a 2800 in such a a small plane... I actually use 1300mAh in my 99" Sagitta which weighs 51oz ready to fly and that gets me 4+ launches to winch height. The 3M planes I use a 2200 or greater battery.

    Scott

    ORIGINAL: Hydro Junkie

    Considering that I'm not an veteran electric flyer, I am really liking the contrasting thoughts on power options for the Electra. What is really refreshing is that everyone involved with this thread has been helpful and not pushing an agenda like I've run into in other forums.
    Getting back to the glider, what I was told in a PM is as shown below:
    Goldberg Electra = 48 oz.
    48 oz ÷ 16 oz in a pound = 3 pounds
    3 pounds X 100 watts per pound = 300 watts

    This was the basis of the Rimfire .15 as a motor. To take this one step further:
    So a 10x5 prop at 10,656 RPM will give us 3 pounds of thrust at a speed of 50 mph using 302 watts.

    As for battery pack size, this was the basis used to arrive at the size pack listed earlier:
    For my personal taste, (your's will be different) I like to set up my gliders to have five, one minuet runs with one minute of extra for the no
    more than 80% use on a charge lipo rule.
    I found an easy way to get this is by taking the 27.2 amps and moving the decimal to the left and
    converting it to a comma then adding a zero will give a mah we need.
    27.2 = 2,72
    2,72 = 2,720 rounded up to 2,800 mah
    So looks like we need a 3S 2,800 mah pack for the Electra


    Since everything was based on a 48 ounce glider instead of a 30-32, everything is sized roughly 50% oversized, that is IF I can build the glider in the 30 or so ounce range

    We may need to meet up in the spring since Chilliwack isn't that far from Marysville, just a quick run along I-5


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