Although you can change the scale of the model you can’t change the environment it operates in. Huh? Let’s take a radio-controlled speedboat in 1/25 scale. As it move though the water the wake in front of the boat and on the sides don’t match with what a real one looks like. Why?

Several factors are acting upon that boat and altering the dynamic similarity, or stated another way, the ratios of all forces acting on corresponding fluid particles and boundary surfaces in the geometric and kinematic models need to be a constant. Dynamic similitude encompasses two other disciplines, geometric and kinematic. The model itself is the geometric similarity. The shape, length, height, and maybe even the weight and mass, and how it performs has been scaled down and replicated from the original. Good job by the way. However, the kinematics similarity, or the fluids of the environment would need to undergo similar rates of change to correspond to scale of the model. To accomplish that, the fluids of the models operating environment (air, water, surface) would also need to be scaled to 1/25 to actually simulate the rate of change.

This is the same problem many engineers face when designing a new aircraft, ship, sub, or car, and why design is sometimes more art than science. Dynamic similitude is especially difficult to obtain when operating in different mediums, especially for ships. A ship needs to function in both air and water, not only is aerodynamics (or wind force) a factor but also hydrodynamics and wave motion are factors. The scaling requirements for each of these environments differ so widely that a model cannot replicate what happens to a full size model.

The same happen in a R/C car. If you look at the surface of the pavement you are driving on it looks pretty smooth. Now place a R/C car on the surface at 1/10th scale, and to the car the small pebbles are now 10 time the size as they are to you. So a small stone at 1 inch in diameter is now 10 inches in scale. Would you run over a 10-inch stone? Some of you would.

It is often impossible to achieve strict similitude during a model experiment. The greater the departure from the models real operating environment, the more difficult it is to achieve similitude. Thus the smaller you get the more unrealistic the effect are, and the larger the model is more realistic it looks. This is the main reason why so many engineering and Hollywood models are built so large. Smoke particulars to help show air flow and dyes to help show water flow look and act more natural when applied to a larger model. This is also why CGI is becoming more wide spread, but that a different paper.