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0.025-0.0001*Re^0.25
This user accounted for changes in skin friction
with Reynolds Number using this formula. It
returns 0.0208 for Re = 3 000 000 and 0.01968
for Re = 8 000 000.
0.025+0.000018*1.05*[Swet(5,6)]
This user is adding the contribution of the
additional wetted area of winglets (surfaces 5
and 6), multiplying the result by a 1.05 to
correct for their curvature. For winglets with 50
ft² additional area, this formula returns 0.0260.
4
0.01*(Cf_lam*Plam+Cf_turb*(100Plam))*Swet/Sref
Here the user is accounting for partial laminar
flow in this estimation. The expression assumes
the Sref will be divided out, leaving Swet
remaining, when incorporated in standard drag
calculations. The variable Plam means the
percentage of laminar flow. Plam = 50 for
laminar flow of up to 50% of wing wetted area.
Note that Swet here is not the same as
[Swet()]. See the discussion to follow for more
information.
5
[CDf]
This formula returns the result of an internal
calculation, in which all surfaces, to which a
skin friction coefficient has been defined, are
summed up using Equation (15).
2
3
*Note that these are just examples of how one might set such formulation up. Your formulation is likely to be different.
Other handy formulations are cited below for the convenience of the user.
Sutherland’s Formula for Viscosity:
When using the UK system the temperature is in °R. In that case the viscosity can be found from7:
 734.7 

 T  216 
  3.170  10 -11 T 1.5 
lbf·s/ft²
(5)
When using the SI system the temperature is in K. In that case the viscosity can be found from8:
1



 T  110 .4 
  1.458  10 -6 T 1.5 
N·s/m²
(6)
Where; T = Outside Air Temperature, in °R or K.
 = Air viscosity, in lbf·s/ft² or N·s/m².
Reynolds Number:
Re 
7
8
VL

(7)
See Equation (2.90) of Reference 7.
See Equation (2.91) of Reference 7.
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VLM.docx
Surfaces – User Manual – Vortex-Lattice Module
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