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Old 12-21-2010, 10:22 AM
  #9  
rhall999
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Location: Fort St. John, BC, CANADA
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Default RE: longitudinal dihedral

Let me see if I can help at all. pl365322 is correct when he is talking about Longitudunal dihedral. There is actually such a thing, however most of us know it as Decalage. Basically, all that it is is the diference between the wing incidence and the horizontal stab incidence. Basically, the way I learned it is that decalage (or longitudinal dihedral) is:

Longitudinal dihedral can also mean the angle between the zero lift axis of the two surfaces instead of between the root chords of the two surfaces. This is the more meaningful usage because the directions of zero-lift are pertinent to longitudinal trim and stability while the directions of the root chords are not
HOWEVER:, mister pl365322, you are mistaken on a couple of other things. First:

I thought setting it up to 1 - 1.5° by shimming the elevator
Shimming the elevator is incorrect, I'm sure this is a typo and you meant to say "horizontal stabilizer" as you did mention that later. Shimming the "elevator" does nothing as it is a moving surface.

moving the stabilizer incidence will move the wing incidence but will also move the way the plane will fly, tail up or down
Moving the stabilizor incidence will NOT, repeat, wil NOT, move the wing incidence. As has already been mentioned, the wing incindence is the angle between the chord line of the airfoil and the longitudunal axis of the fuselage. You mentioned that this is fixed on your discus, so it will not change no matter what you do to the stab. Here is what is really happening when you raise the trailing edge of the stab. You are changing the decalage (or as you like, LD) and by doing this you will create more downforce on the tail of the aircraft, basically the same as pulling on the stick and givingit some up elevator. What this will do is push the tail down, increasing the angle of attack of the wing. DONOTCONFUSEANGLEOFATTACKANDANGLEOFINCIDENCE, THEYAREDIFFERENT!!

but will also move the way the plane will fly, tail up or down, not having the max of airfoil performance
No it will not affect the airfoil performance. The tail flying high or low has zero effect on airfoil performance. It is strictly a visual preference thing. The airfoil performance is all aboutangle of attack, in other words, the guy wiggling the sticks is the determining factor as to whether the airfoil is flying at its max efficiency. Some airfoils are most effecient at 2² and some at 4², and any other angle. Every one is different. The way to get the max out of your airfoil is strictly a go fly, and learn the airplane thing. You may find that your glider may be happiest flying tail high, even with the incidence in the stab, it may be happier tail low, who knows until you fly it. The only thing that tail low vs. tail high will affect as far as performance goes, is fuselage drag. It may be a bit draggier with the tail hanging down a bit, but that can't be helped.

Now, as to your original question, if it were at all possible to change the wing, that would be the best bet, about 1 ² would be lots. However, since you mentioned that it is fixed, you have to do the stab. I would guess, just from my experience, that the 1-1.5² you suggested will be plenty.

I hope this helps out a bit in understanding what you need. I apologize if I seem a bit abrupt, but as Tall Paul mentioned, your tone is a bit argumentative so I was slightly heated when I started typing.

PS
Thanks Ranfred but no.Angle of incidence is refering to the fuselage axle. The call "V" angle is the angle between the wing cord axle and the elevator axle
The word you want is axis

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