Reynolds Number is a scaling factor that allows engineers and scientists to make comparisons between experiments involving fluid media like air and water, large objects with small, etc. The definition for Reynolds Number is Re = (Length x Velocity x Density) / Viscosity. When comparing a full size airplane with a model airplane the air density and air viscosity are the same so they become irrelevant. Whats left then is a ratio between chord lengths and velocities.

As an example a full scale monoplane with a cruise speed of 200mph (293 fps) and a 5' chord would have an Re = 6363 x 293 x 5 = 9,322k. A 1/5 model of the plane with a 1' chord and 40mph cruise (59 fps) would have an Re = 6363 x 59 x 1 = 375k. The models Re is only 4% (1/25) of the full scale plane, not 20% (1/5) if it were a linear relationship.

Just for fun, how fast would the model need to fly to have the same Re as the full scale? V = 9,322,000/ (6363 X 1) = 1,465 fps! Thats nearly 1,000 mph!