Engineers write the lift of a wing as one short equation:

L = ½ × ρ × V² × S × C_L

• ρ (the Greek letter rho) is the air density. At sea level in the standard atmosphere it is 1.225 kg/m³. It falls with altitude.
• V is the airspeed, the speed of the aircraft relative to the air, in m/s. A plane flying into a 10 m/s headwind at 50 m/s over the ground has an airspeed of 60 m/s.
• S is the wing area in m². The toy plane has 16 m².
• C_L is the lift coefficient, a number with no unit. It packs in the wing's shape and its angle to the airflow.

The part ½ρV² is called the dynamic pressure q, in pascals. At 50 m/s at sea level, q = 0.5 × 1.225 × 50² ≈ 1,531 Pa. Then lift is simply q × S × C_L. With C_L = 0.4: L ≈ 1,531 × 16 × 0.4 ≈ 9,800 N, which is the weight of the toy plane. So at 50 m/s it can fly level with C_L = 0.4.