The definition of Re in this case depends on your purpoces. Do you want to comute Lift, Drag, CM?

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your characteristic length "D" is the one that is meaningful for your problem ... any choice of a lenght is coherent with the definition of a Reynolds number, but if you want that the Re number represents the relative magnitude between convective and diffusive fluxes of the momentum quantity for your problem, then you have to choice properly the lenght..

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In general, single reynolds number fits only for calculation of 2D airfoil. In your case the analysys should be more sophisticated, unless you are going to have a rough estimation. Tell us more details. 
For a 2D wing (airfoil) you can use your chord length to calculate the value of Re!! If you have a tapered 3D wing then the Re will vary at the different locations. You can however calculate an average Re based on your average chord length. A more indepth analysis can be obtained at:
Fundamentals of Aerodynamics SI by John D. Anderson And/or Aerodynamics for Engineers John J. Bertin, Russell M. Cummings 
You have to compute the "Mean Aerodynamic Chord" and use that as a characteristics length "D" in Reynolds number formula. The formula for MAC is:
MAC = A(2(AB)(0.5A+B) / (3(A+B))) here, A : Root chord of the wing and B: Tip chord of the wing This equation for the straighttapered wing. Hope this solves your problem. 
Dear Ali;
Can we use the previous equation for a Twisted blade as well? Thanks, 
Yes. You can.

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