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Old 12-21-2010, 06:08 PM
  #11  
BMatthews
 
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Location: Chilliwack, BC, CANADA
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Default RE: longitudinal dihedral



On a more educational level if you were doing the design of the model yourself then what you would do is decide on what speed range you want the model to be most efficient over.  And by "range" I mean a narrow range chosen for how you'll fly the model.  Then you would figure out what the wing's Coefficient of lift and thus its angle of attack would be for the middle of that speed range.  Then you'd set the wing's incidence angle to the fuselage's median axis to that angle.  This would ensure that the fuselage is pointed into the oncoming air at the most streamlined angle possible.  Typically you would set the wing's incidence angle to something slightly higher than the angle of attack at the best L/D speed so that it's still close enough at the best L/D speed and still axis on for slightly slower or faster flight depending on which aspect you want to optimize.

With that out of the way THEN you set the decalage or LD angle to achieve the required lift at the tail for the optimum CG location.  And that angle is typically best set by trial and error unless you've got some good software to crunch the numbers for you.

So you see, there's more to this than meats the eye.  In your case the wing's angle is fixed to whatever it is.  The final shim you need to set the stabilizer to will depend on where the CG is located.  That's why I suggested to set it to -1 to -1.5 degrees to the wing and then see where the model trims out.  If you have up or down trim then shim the stabilizer to where it flattens the angle and fly again.  When you have it where the elevator is really close to being in line with the stabilizer then you're close enough.

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