feat(input): update Airfoil.extrapMethod

contents/user-input.tex

@@ -300,16 +300,34 @@ structures (\ie \lst{Airfoil(1)}, \lst{Airfoil(2)}, \dots).
   present and usable in your installation.
 }
 
-\describeOption{Airfoil.extrap}{}{\lst{true, false}}{
-  Extrapolate the polars from \lst{Airfoil.polarFile} over the complete range of
-  angles of attack ([-180, 180] deg). Although this extrapolation is not ideal,
-  it allows for the computation of sections that are at very large/low angles of
-  attack. This is particularity useful when doing a sweep of operating
-  conditions.
+\describeOption{Airfoil.extrapMethod}{}{See below}{
+  \danger \, \textit{Only used when \lst{Airfoil.polarType='file'}}.
+
+  Extrapolation method to use in order to recover the lift and drag coefficients
+  from incomplete polars.
+  Possible choices are:
+  \begin{itemize}
+    \item \textbf{none}: no extrapolation allowed. Will return an error if the
+      solution requires the calculation of an angle of attack outside of the
+      range provided by the user.
+    \item \textbf{spline}: spline extrapolation if angle of attack is outside
+      the range of angles given in the input polar. Strongly discouraged.
+    \item \textbf{viterna}: Extrapolation of the polars over the entire range of
+      angles of attack according to Viterna formulas~\ref{}. Although this
+      is not perfectly correct (as it is the case for any extrapolation), this
+      option is the one that is closer to the reality. This is particularity
+      useful when doing a sweep of operating conditions and many off-design
+      points must be studied.
+  \end{itemize}
+  Note that convergence may be more trickier to achieve in some edge cases and
+  off-design analysis with the low-order extrapolation methods.
+  In operation at the design point, extrapolation should not be required at all
+  (as long as the polars are provided for values between the minimum lift and
+  the stall point).
 }
 
 \describeOption{Airfoil.clPoly}{deg$^{-1}$}{Vector}{
-  \danger \, \textit{Only used when \lst{Mod.polars='manual'}}.
+  \danger \, \textit{Only used when \lst{Airfoil.polarType='polynomial'}}.
 
   Coefficients for the polynomial form of the lift coefficient, $C_l$.\\
   The vector should be in the form $[p_1, p_2, ..., p_n]$ to represent the
@@ -319,9 +337,9 @@ structures (\ie \lst{Airfoil(1)}, \lst{Airfoil(2)}, \dots).
 }
 
 \describeOption{Airfoil.cdPoly}{deg$^{-1}$}{Vector}{
-  \danger \, \textit{Only used when \lst{Mod.polars='manual'}}.
+  \danger \, \textit{Only used when \lst{Airfoil.polarType='polynomial'}}.
 
-  Same as \lst{Blade.Airfoil.clPoly}, but for the drag coefficient.
+  Same as \lst{Airfoil.clPoly}, but for the drag coefficient.
 }