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Thread: cobra 5 build log

  1. #1
    Join Date
    Sep 2010
    Location
    Daytona Beach, Florida, USA
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    35

    Cool cobra 5 build log

    Hi everyone,
    I figure I'll do this so I can get more feedback.
    My Footy experience so far has been exciting and now with a little more knowledge I think it gets even
    better.
    anyway, I did it the traditional way of cutting the patterns, placing them on top of the balsa sheet, tracing and cutting.
    Up till now e everything has come good. Hull panels where joined using foaming gorilla glue,light and helps fill any gaps. Panels where cut out of 1.5mm balsa sheets. Looks promising.
    I'll definitively build a Mac rig for it although the idea of using a sail wing is very tempting!
    Craig's round ranger hulls arrived this morning. They also look excellent although a tad. Heavier than the balsa cobra 5. We'll have to compare and see. Sometimes too light is as bad as too heavy.
    I'll keep progress reports xas I get more done.
    Enrico
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  2. #2
    Join Date
    Sep 2010
    Location
    Daytona Beach, Florida, USA
    Posts
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    Default Re: cobra 5 build log

    I can't believe it's being so long!!! And to think about it, I just now decided to continue building my Cobra 5! I need to post pictures, I'll get them tonight once I go home.

    It's funny that I came about my own thread by looking up MacRig dimensions for the rg-65 class. It's for this same boat that I need them so I can finish it. Hope I'm not the only one around this forum anyway.
    If so I'll just keep it as my personal log

    Enrico

  3. #3
    Join Date
    Oct 2008
    Location
    Yuma,Az / Westport, Wa
    Posts
    6

    Default Re: cobra 5 build log

    Please post your progress. We are watching!!

    Dave

  4. #4
    Join Date
    Sep 2010
    Location
    Daytona Beach, Florida, USA
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    Default Re: cobra 5 build log

    Thanks Dave, glad someone is watching here is another picture of the hull finished with the deck dry fitting and keel and keel box already glued in. Im thinking of using standard servos for rudder and sail. Would that be enough or should I go lighter on the rudder and stronger on the sail? I was thinking of buying one of those exi digital servos with 180 oz torque. Any thoughts on that?
    where can I find measurements for a macrig for the rg 65?
    I know, too many questions lol

    Enrico
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  5. #5

    Default Re: cobra 5 build log

    I too are finishing up a first RG65 and about to make a McRig for it. Mine is quick and dirty from a found heavy glass hull so I am trying to keep weight down to meet hull displacement at around 800gms, and that rig will help this. Failing other postings, just remember to keep the head of the sail over the pivot point, giving around 25% area ahead of the pivot. I'll try a 3/3.3mm - (1/8th inch) Z wire, but may have to be more for a full rig or higher winds.
    Because the rig is balanced a lower power servo should do. I've got a 20gm GWS Park 4.2kg/cm here to use with a Stollery power lever servo arm which I will run with a 2s lipo and 6v BEC AM receiver. Classic rigs typically use a 30gm GWS Micro 2 MG BB 5.4kg/cm @ 4.8v. and a standard servo arm. I will be sailing in moderate wind conditions at worst and pretty simple 2 purchase sheeting without a lot of friction. Classic standard servos of some age can be 40 gms and 3kg/cm!
    Using a 8 gm rudder servo with whipstaff connections ar la vane steering.
    Rusty
    George Little: Maybe we should go home.
    Mr. Little: Why?
    George Little: I'm not wearing my lucky underwear.
    Mr. Little: You don't have lucky underwear.
    George Little: Well, maybe we should get some, and then come back for another race.

  6. #6

    Default Re: McCormack Rigs

    I have just drawn up the attached Sketchup file as my A and C rigs.
    1/8th inch Z wire, 5mm carbon tube for 'mast'.

    Any comments before I cut out.
    Do recreational sails of this size merit built in camber?
    If so how much?
    What would be a good B rig area?
    Attached Files Attached Files
    Rusty
    George Little: Maybe we should go home.
    Mr. Little: Why?
    George Little: I'm not wearing my lucky underwear.
    Mr. Little: You don't have lucky underwear.
    George Little: Well, maybe we should get some, and then come back for another race.

  7. #7
    Join Date
    Nov 2004
    Location
    New Hampshire, USA
    Posts
    921

    Default Re: cobra 5 build log

    Enrico, I've attached the drawing for the 3 McRigs that Scott, Jim, and I use. Earl told me there might be an error on the mast length for the A rig, so cut the sail, then match the mast length to the sail.

    RG65s don't require servos much bigger than Footys...the torque from a standard servo is enough for the winch, and more than you need for the rudder.

    Have fun....but don't give up your Footys

    Bill
    Attached Files Attached Files
    Last edited by mudhenk27; 04-20-2012 at 09:35 AM. Reason: add servo comment

  8. #8

    Default Re: McCormack Rigs

    Thanks for your rig dimensions Bill. I have drawn them up in sketchup (zip) to better get an idea of sizes. Surprised how close they are in area - height and aspect ratio instead determining power? Are these rigs optimized for a particular wind band, model weight and righting moment?
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    Rusty
    George Little: Maybe we should go home.
    Mr. Little: Why?
    George Little: I'm not wearing my lucky underwear.
    Mr. Little: You don't have lucky underwear.
    George Little: Well, maybe we should get some, and then come back for another race.

  9. #9
    Join Date
    Nov 2004
    Location
    New Hampshire, USA
    Posts
    921

    Default Re: cobra 5 build log

    You are correct about your assumption regarding the area, Rusty. It's nice to be able to keep the area as close to maximum as possible, provided that aspect ratio change is enough to handle the wind change.

    Although I didn't design the rigs (Scott Spacie did them) I know they weren't optimized for a particular boat design or specific wind band. Rather than worrying about the wind velocity, I judge the time to change rigs primarily based on the downwind performance. When it gets too hard to handle downwind dives and I'm starting to get some stalling when tacking, it's time to change.

    Bill

  10. #10
    Join Date
    Sep 2010
    Location
    Daytona Beach, Florida, USA
    Posts
    35

    Default Re: cobra 5 build log

    Hi Bill,

    Thanks for the measurements. I know I am in and out of the forum but somehow I never got notification that there was an update to it; and even worse I get distracted too easily with rc sailplane and thermal duration competition training, so it is hard to focus sometimes in finishing my RG -65 's !!

    Now with the sail dimensions I am going to attempt to finish mine for this Saturday club fleet race. Rules are: any boat less than 26 inches long ... and that's it!! My footy ranger is ready to sail and I have been sailing it regularly. It is a lot of fun, easy to transport so I keep it in a nice box stand (foam Walmart cooler ) and the 400 mah single cell lipo hardly ever needs to be charged!

    Now back to the cobra, so I am planning on using a simple 9gr servo for rudder. But not sure what to use for sail. I have a JR ds 378 that I want to use since it is small, powerful and ready available. The sheet control arm I was thinking of using a somewhat bigger version of the one used in the footy. Would that work? Not sure if doing so will require to put the servo outside glued to the deck, but what would a good recommendation on this issue be?

    Thanks!

    Enrico

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