Expert: Paul Soderman - 10/6/2011

Question
Sir,
What will be the variation of sectional lift coefficient (cl)along the span, of an Finite elliptical wing ( a wing with elliptical lift distribution)?
I read in Anderson that the cl will be a constant along span for an elliptical wing with no twists( aerodynamical or geometrical). On contrary, I saw a figure in LJ clancy ( Aerodynamics) which shows that the cl varies elliptically with span for the same type wing. I am confused as to which of them is true.

Answer
Sangeeth
An elliptically loaded wing, which should not be confused with an elliptically shaped wing that may or may not be elliptically loaded, has an elliptical variation of circulation along the span. If the wing also has a constant lift curve slope and geometric angle of attack spanwise, the result is a uniform sectional-lift coefficient along the span.  The induced-drag coefficient will also be independent of span.  Thus, the wing-lift coefficient CL and the sectional-lift coefficient cl will be equal as will the wing induced-drag coefficient and the sectional induced-drag coefficient.
Paul