Although you can change the scale of the model you canít change the environment it operates in.
Huh? Letís 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 donít 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.