Game Development Reference
In-Depth Information
Ideally, the ability of a hovercraft to eliminate contact with the ground (or water) over
which it operates means that it can travel relatively fast since it no longer experiences
contact drag forces. Notice we said ideally . In reality, hovercraft often pitch and roll,
causing parts of the skirt to drag, and any obstacle that comes into contact with the skirt
will cause more drag. At any rate, while eliminating ground contact is good for speed,
it's not so good for maneuverability.
Hovercraft are notoriously difficult to control since they glide across the ground. They
tend to continue on their original trajectory even after you try to turn them. Currently,
there are several means employed in various configurations for directional control.
Some hovercraft use vertical tail rudders much like an airplane, while others actually
vector their propulsion thrust. Still others use bow thrusters, which offer very good
control. All of these means are fairly easy to model in a simulation; they are all simply
forces acting on the craft at some distance from its center of gravity so as to create a
yawing moment. The 2D simulation that we walked you through in Chapter 9 shows
how to handle bow thrusters. You can handle vertical tail rudders as we showed you in
Chapter 15 .
Resistance
Let's take a look now at some of the drag forces acting on a hovercraft during flight. To
do this, we'll handle operation over land separately from operation over water since
there are some specific differences in the drag forces experienced by the hovercraft.
When a hovercraft is operating over smooth land, the total drag acting against the
hovercraft is aerodynamic in nature. This assumes that drag induced by dragging the
skirt or hitting obstacles is ignored. The three components of aerodynamic drag are:
• Skin friction and viscous pressure drag on the body of the craft
• Induced drag when the craft is pitched
• Momentum drag
In equation form, the total drag is as follows:
R total = R viscous + R induced + R momentum
The first of these components, the viscous drag on the body of the craft, is the same sort
of drag experienced by projectiles flying through the air, as explained in Chapter 6 . This
drag is estimated using the by-now-familiar formula:
R viscous = (1/2) ρ V 2 S p C d