Replies

  • Moderator
    Graham Dyer April 24, 2013 at 11:55am

    H-frames are great, I much prefer flying them to X-frames, I've had 3 H-frames and only 1 X, my latest EPP one flies very very close to the same way my super stiff carbon H-frame used to. The first one I built was however from plywood and that wasn't so good because it twisted really badly.

    My EPP H-Frame

    The Boxcopter by Jake Wells and others are great examples too! 

  • criro1999 April 24, 2013 at 11:21am

    I chose the H-Frame for radio electronic interference separation.

    H-Frame allow you more distance on the body to separate the antennas.

    Another reason reason for me was using the camera (Gopro) on the main body level (not behind) and avoid the props to be visible on the image.

    And last is money: my H-Frame costs me $10. each arm is $3, body is $6. Balsa wood made.

  • Steve April 23, 2013 at 1:35pm

    nevermind

  • MR60
    Forrest Frantz April 23, 2013 at 12:55pm

    Tom and Hughes are mostly correct since you are not talking about a hexa-H or octa-H.  While the latter sort of fly OK using X, I'm hoping to write documentation on how to do those. 

    Back to your issue.  Your insight is partially correct.  It depends on how you structure the H.  What Tom and Hughes are saying is that for the copter to fly correctly, you need to structure the quad so that when lift is applied to prop 1, it also lifts all but the opposing prop proportionally.  That is why an X is used on quads (the most efficient structural method to gain cross rotor stability).  On an octa, the H becomes the most efficient structural design but the mid part horizontal dash on the H is actually an X.  Without the X, you don't get cross rotor stability.

    With a quad H, the problem comes in where the horizontal piece connects to the vertical piece (pretend your copter was flying straight up).  That connection, if perfectly rigid would do what Tom and Hughes are saying.  But as you pointed out, that isn't reality.  It's not rigid even if bonded.  The horizontal piece will twist putting high loads on that joint and a spring effect that will make PID tuning interesting.

    My advice.  On a quad, go with an X frame.  It will weigh less.  However, make one key modification.  Most X designs break the X at the center so all the X parts have a length of the radius of the circle.  Then you have to sandwich those arms between two platforms all connected with a bunch of weighty fasteners.  Instead don't cut your cross beams in half.  Keep them whole.  Put one on top of a platform and one on bottom (offset so they stay whole).  You only need one platform and half the number of fasteners.  In fact, I just use zip ties.  Lighter and stronger than conventional X assemblies.  While you can put two props raised off of the lower arm so all are at an equal height, that isn't necessary from a flight characteristic point of view (the aspect created isn't large enough to matter).

  • Beau April 23, 2013 at 12:19pm

    Hugues,

    The arm it self doesn't matter, what matters is the location of the motor in relation to the center of gravity of the copter.  You could have an arm that is curved in all kinds of directions and the copter will not know the difference between an arm straight to the motor or the curved one.  Objects tend to rotate about their center of gravity.  In the H frame the front right motor is to the side and in front of CG, therefore it will pitch and roll the copter about its CG.  Hope this helps clear thing up.

     

    Beau

  • Tom Rearick April 23, 2013 at 11:44am

    An H is an X with a fat middle. So far as Arducopter is concerned, they are the same. You could mount 4 motors equidistant from each other on a hoola hoop. If there are two motors in the front, it is an X. If there is only one motor in the front, it is a +. It doesn't matter what shape the frame is - only the positioning of the motors.

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