If the air resistance is include, the net force will be F= mg sin? - ? mg cos? -(1/2) CpAv[sup]2[/sup]
When the terminal velocity is reached, it requires F=0
So (1/2) CpAv[sup]2[/sup]=mg sin? - ? mg cos? =mg (sin? - ? cos? )
v[sup]2[/sup]=2 mg (sin? - ? cos? )/ (CpA)
For ?=0.1 and 20 degree (?=0.349), (sin? - ? cos? )=0.248, air density p=1.2 kg/m[sup]3[/sup]
which give us v[sup]2[/sup]=4.05 * m/(C A)
The drag coefficient C for a skier is between 1.0-1.1 (http://en.wikipedia.org/wiki/Drag_coefficient)
The area is estimated to be A=0.5*1.7*cos(?)=0.8 then we will have v[sup]2[/sup]=5.06*m
For a skier with mass (80kg) it will give us v=20.1m/s=72.5 km/h.
It is very close to your value 80 km/h.
I will add this drag force to the simulation and update it soon!
C*A=0.11 for an upright body, minimum frontal area
C*A=0.84 for a horizontal body,maximum frontal area
C*A=0.46 for a body in tuck position