Game Development Reference
In-Depth Information
When a hovercraft operates over water, its air cushion creates a depression in the water
surface due to cushion pressure (see Figure 17-4 ). At zero to low speeds, the weight of
this displaced volume of water is equal to the weight of the craft, just as if the craft were
floating in the water supported by buoyancy. As the craft starts to move forward, it tends
to pitch up by the bow. When that happens, the surface of the water in the depressed
region is approximately parallel to the bottom of the craft. As speed increases, the de‐
pression is reduced and the pitch angle tends to decrease.
Figure 17-4. Hovercraft over water
Wave drag is a result of this depression and is equal to the horizontal components of
pressure forces acting on the water surface in the depressed region. As it turns out, for
small pitch angles and at low speeds, wave drag is on the same order of magnitude as
the induced drag:
R wave ≈ W (tan τ)
Since wave drag is proportional to the size of the depression, it tends to be highest at
low speeds and decreases at higher operational speeds. If you were to plot the wave drag
curve as a function of speed for a typical hovercraft, you'd find that it is not a straight
or even parabolic curve, but rather it has a hump in the curve at the lower speed range,
as illustrated in Figure 17-5 .