The above simulation assume air drag is model with $F=k1*V+k2*V^2$ where $V$ is the magnitude of the velocity.

Nomally, for high speed projectile, the force is proportional to velocity square. i.e.$k1=0, k2>0$
Howeve,for slow projectile, the force is proportional to velocity. i.e.$k1>0, k2=0$
e.g. simulate a projectile moving under water.

User can adjust $k1,k2$ to find a suitable model for the case s/he want to simulated and compared with the real case under studied.
And that can be one of the purpose for the above simulation. To find out a suitbale model for a real case.