Hello and welcome to our community! Is this your first visit?
Register
Results 1 to 5 of 5

Thread: Coax efficiency

  1. #1
    Flight Cadet
    Join Date
    Mar 2012
    Location
    Kristiansand, Norway
    Posts
    38
    Downloads
    0
    Uploads
    0
    Reputation Points (Add)
    0

    Coax efficiency

    Again, wondering about some theory for a future design. My main priority will be reducing weight and increasing flight time - I suppose this is a pretty common goal.

    I have read a number of time about how using coax designs (twin motors sharing same arm) reduces the efficiency of the motors considerably, but that the arrangment halves the number of arms required (reducing weight). My question is where the balance changes from favouring coax to not favouring coax. I imagine that for heavier arms, a coax arrangement might be more efficient, however we could reach a point with some lighter arms where halving the number would no longer offset the lost motor efficiency.

    Does anyone have any numbers or examples? Can we conclude anything on this? Are there other factors that outweight the importance of this efficiency issue like complications with the system, flight, stability etc?


    Thanks everyone.

  2. #2
    Moderator AeroQuad Technologist Honk's Avatar
    Join Date
    Apr 2010
    Location
    Stockholm
    Posts
    5,440
    Blog Entries
    2
    Downloads
    0
    Uploads
    0
    Reputation Points (Add)
    1
    Why don't you start some tests yourself so that you can get exactly the real-world data you're looking for? I think nothing can replace that really.

    My greatest tip for flight time: add more, more and more batteries and make up a curve including weight, C rating, capacity and flight time. Generally multirotorists are using far too small/few batteries with too high C rating is my perception. Higher discharge rate costs weight = reducing flight time and with a lighter battery you don't need as high discharge rate either, and can instead spend that weight on capacity and another important thing: cell count. It ought to (and seems to in practice) be more efficient going up a cell or two. I've got the perfect craft for these kind of tests now, I will just have to buy lots of batteries... I can lift 9kg of water with the octo, and fly for 15 minutes with a 600g gimbal+camera on 11Ah/4S (means 600W on average while flying). I'd like to see what 22Ah an no gimbal would make.

    To answer the coax question: I really see only disadvantages with coax design in every area except of transport.
    Last edited by Honk; 04-25-2012 at 11:42 PM.
    Smart people don't need a signature.
    http://thechive.files.wordpress.com/...rd-signs-3.jpg

  3. #3
    Senior Pilot wooden's Avatar
    Join Date
    Dec 2011
    Location
    Pacific Northwest
    Posts
    4,652
    Blog Entries
    1
    Downloads
    1
    Uploads
    0
    Reputation Points (Add)
    27
    Read about coax designs on the MultiRotors forum on RCGroups.com. There are a number of very informative threads. Here's what I got out of them:

    The fore motor (top) is not affected much by the aft motor (lower), but the lower is drastically affected. It turns out that for a given prop size/pitch/RPM up top, there is a given SMALLER size/LARGER pitch/HIGHER RPM aft prop that reduces losses. AKA, if you want coax to be efficient, you need to size your propellors and motors properly or the aft motor/prop combinations will waste energy. I don't know the equations behind this but there are people on RCGroups who seem to know what they're doing.

  4. #4
    Moderator AeroQuad Documentation Team p0lar's Avatar
    Join Date
    Nov 2010
    Location
    Salt Lake City, Utah, United States
    Posts
    896
    Blog Entries
    21
    Downloads
    0
    Uploads
    0
    Reputation Points (Add)
    6
    one of the few reason you fly a Y6 design is to have an open view for FPV and ability to lose 1 motor on each arm. as I have read in forums myself if your looking for max performance then the large top/smaller bottom prop config is better BUT you lose FULL redundancy. can the lower smaller prop hold that 1 arm up sufficiently?
    p0lar == the aeroquadist formerly known as dpackham...

  5. #5
    AeroQuad Lead Software Architect Kenny9999's Avatar
    Join Date
    Jun 2010
    Location
    Saint-Jean-sur-Richelieu, Quebec, Canada
    Posts
    2,330
    Downloads
    0
    Uploads
    0
    Reputation Points (Add)
    7
    Coax have it's purpose, exactly like flat configurations

    First, if you do smaller prop, at bottom, you have to have the same amount of CW/CCW, so, not a good idea for Y6, no problem on X8 or hex config with 12 motors or octo config with 16 motors!

    Coax is less efficient, but, have it's advantage, It will have less flight time, very few compare to a flat hex, will be less efficient also for payload, BUT, it will be more agile in the air and still offer a redundancy security! I also believe that an hex flat will be more impacted than a Y6 if one motor stop. All that, base on my experiences.
    Truly superior pilots are those who use their superior judgment to avoid those situations where they might have to use their superior skills"- Author unknown


 

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •