Game Development Reference
In-Depth Information
Figure 17-5. Wave drag
There are several theoretical treatments of wave drag in the literature that aim to predict
the speed at which this hump occurs along with its magnitude. These theories indicate
that the hump depends on the planform geometry of the hovercraft, and it tends to
occur at speeds in the range of gL / 2 to gL , where g is the acceleration due to gravity
and L is the length of the air cushion. In practice, the characteristics of a particular
hovercraft's wave drag are usually best determined through scale model testing.
The so-called wetted drag is a function of several things:
• The fact that parts of the hull and skirt tend to hit the water surface during flight
• The impact of spray on the hull and skirt
• The increase in weight as the hovercraft gets wet and sometimes takes on water
Wetted drag is difficult to predict, and in practice model tests are relied on to determine
its magnitude for a particular design. It's important to note, however, that this tends to
be a significant drag component, sometimes accounting for as much as 30% of the total
drag force.
Steering
In Chapter 9 , the hovercraft was steered using a bow thruster that pushed transversely
forward of the center of gravity. In reality, most hovercraft are steered by vectoring the
thrust of the propulsion fan via rudders attached directly aft of the fans. This can be
modeled by angling the propulsive thrust.
The most important characteristic to remember about steering hovercraft is that they
will not turn like a car or boat. The hovercraft, because it has lower friction with its
environment, will take longer to turn and tends to continue in the direction it was
heading while rotating. Once rotated, the thrust acts along a new vector. One possible
maneuver in a hovercraft is to quickly rotate the vessel and then shut down the pro‐