Game Development Reference
In-Depth Information
The key to minimizing takeoff velocity for a given airplane at a given atmospheric density
condition is to maximize the lift coefficient. Lift increases with angle of attack, but the airplane
is rolling along the ground so the angle of attack is equal to the incidence angle of the wing,
which is usually quite small. Deflecting the flaps downward during takeoff can increase the lift
coefficient, and therefore the flaps are usually deflected during takeoff.
Another way to make it easier for an airplane to lift off the ground is to take off into wind.
The velocity in Equation (10.36) is relative velocity between the airplane and the air traveling
over it. The relative velocity is a combination of the velocity the airplane is traveling relative to
the ground, known as the
ground speed
, and the direction and magnitude of any wind that is
present. Whenever possible, pilots try to take off into the wind to add the wind velocity to the
relative velocity of the airplane.
Ground Effect
The lift coefficient of an airfoil depends on its geometry, angle of attack, and whether its flaps
are deflected. When an airplane is close to the ground, there is also an effect known as
ground
effect
that can increase the apparent lift coefficient of the airplane. Ground effect is very fortuitous
when it comes to airplane flight because some airplanes would have a difficult time taking off
without it. Ground effects also influence an airplane that is landing, reducing the rate of
descent of the airplane near the ground.
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Tidbit
Some critics have claimed that Howard Hughes's enormous “Spruce Goose” airplane only flew
because of ground effect and would have been incapable of achieving an altitude of more than a few feet.
If you remember from the “Induced Drag” section earlier in this chapter, when a wing
travels through the air, vortices are shed off from the wing tips, and the swirling air vortices
continue for some distance behind the airplane. When the airplane is flying at an altitude far
away from the ground, the shape of the vortex structure is cylindrical.
When the wing is close to the ground, less than a wingspan distance or so, the presence of
the ground disturbs the structure of the vortices. The shape of the vortices is no longer cylin-
drical, it becomes flattened, and the induced drag on the wings is reduced. Ground effect also
increases the apparent lift coefficient of the wing.
The easiest way to incorporate ground effects into your flight simulator is to simply add a
ground effect factor to the normally computed lift coefficient when the wing is within a wing-
span distance of the ground. You could either multiply the “normal” lift coefficient by a number
greater than 1 or you could add a value, say 0.5, to the “normal” lift coefficient value. Once the
wing is higher than a wingspan's distance above the ground, the ground effect factors would no
longer be applied. Keep in mind that this is a very simplified treatment of ground effects. A full
discussion of the complicated physics of ground effects is beyond the scope of this topic.
