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coupling

Discussion in 'Giant / Scale RC General Discussions' started by wedoitall, Nov 13, 2012.

   
  1. could someone lead me in the right direction where i can learn all about coupling. sorry in advance for such a noobular request.
     
  2. Steve_B

    Steve_B 70cc twin V2

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    Coupling is an interaction between controls across the different axis of freedom.

    In principal each of the primary controls on a plane effect one axis of freedom:
    • Elevator controls pitch
    • Rudder controls yaw
    • Ailerons control roll

    In practice it's not quite that simple.
    What you often find is that operation of one control has not only the desired effect (i.e. rudder causes yaw) but it has a secondary effect on one or both of the other axis. Taking rudder again as an example; when you apply rudder you often find that you get not only the desired yaw effect but the plane will also roll and pitch slightly.

    Rudder is usually the ‘worst offender’ for cross coupling but aileron suffers from it too. It’s not unusual that when you apply aileron the plane not only rolls as you intended but it yaws in the direction opposite to roll direction (plane rolls right and yaws left). This is known as ‘adverse yaw’.

    For some planes coupling is essential for proper control. Rudder and elevator control planes being the obvious example where we rely on the rudder to roll coupling in order to make the plane bank into a turn. Without coupling a rudder and elevator plane would be un-flyable. For aerobatic planes coupling is something we generally don’t want. Obviously having a plane roll when you wanted a yaw, or yaw when you wanted roll makes precision aerobatic flying more difficult. Knife Edge flying especially shows up any rudder coupling issues the model may have.

    A good designer of aerobatic plane will tweak things like the height of the wing and tail, dihedral angle and a bunch of other stuff in order to engineer out as much of this coupling as possible, but even on the best models there will always be some hint of coupling evident in some flying conditions. Modern transmitters with computerised mixes can tweak out the coupling that remains. For instance if when you apply right rudder in knife edge flight the plane rolls right and ‘tucks’ toward the belly then you would mix in some left aileron and some up elevator to come in along with the right rudder. If done correctly should see the plane tracking perfectly straight in knife edge.

    For adverse yaw coupling on aileron the usual trick is to set some differential so that the up-going aileron moves a little further than the down-going one. This can help make rolls a little more axial.


    Steve
     
  3. gyro

    gyro GSN Contributor

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    Steve, another excellent post for the wiki! :)
     

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