The paper airplane is a point mass in a vertical plane with a fixed trim angle of attack α. With speed V and air density ρ, lift and drag are L = ½ρV²S CL and D = ½ρV²S CD; lift acts normal to the flight path, drag against it. m dv/dt = L n − D v/V + m g is integrated with a 2 ms step.
The lift slope of a low-aspect-ratio wing is CLα = 2πAR/(AR+2) per radian up to a stall near 18 degrees; drag is CD = CD0 + CL²/(π e AR) with CD0=0.06 and e=0.7. A steady glide needs tan γ = CD/CL and V = √(2 m g cosγ / (ρ S CL)). Launching faster or slower than this speed makes the path oscillate (the phugoid).
Honest simplifications: pitch dynamics are ignored (the plane is assumed trimmed at the chosen angle of attack), no wind, no Reynolds-number effects (these slow, small wings are in a regime where real CL and CD differ from thin-wing theory). The numbers are illustrative.