Tony Estep writes:
>This sez that if your tail moment is 40" and span is
>120" and you fly at CL = 1, you need 15 deg of
>dihedral to be spirally stable. This is indeed what
>Blaine's Crossbow design has, but lots of RES designs
>including the Allegro have quite a bit less.
>How much spiral stability (in terms of
>that constant) is necessary in a practical RES design?
Your CL = 1 assumption is excessive. Typical thermalling
for most airfoils will be at CL=0.8 or less, since the
inside tip sees a higher local cl. The overall average
CL must be significantly reduced below the straight-line
stall CL. The Allegro-Lite has a 30" vertical tail arm,
a 78.4" span, and thinner than usual airfoils with a max
circling CL = 0.75 or so. Therefore its minimum EDA
needed for spiral stability is
EDA_min = 5 * 0.75 * 78.4" / 30" = 9.8 degrees.
The actual EDA is 12.1 degrees, which is a comfy 23% margin.
- Mark
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