Game Development Reference
In-Depth Information
// get the Euler angles for our information
Vector u;
u = MakeEulerAnglesFromQ(Airplane.qOrientation);
Airplane.vEulerAngles.x = u.x; // roll
Airplane.vEulerAngles.y = u.y; // pitch
Airplane.vEulerAngles.z = u.z; // yaw
Very often, it's more convenient to calculate loads on an object like the airplane using
body-fixed coordinates. For example, when computing aerodynamic drag on the air‐
plane, you'll want to know the relative air velocity over the aircraft in body-fixed coor‐
dinates. The resulting drag force will also be in body-fixed coordinates. However, when
resolving all the loads on the aircraft to determine its motion in earth-fixed coordinates,
you'll want to convert those forces from body-fixed coordinates to earth-fixed coordi‐
nates. You can use QVRotate to rotate any vector, such as a force vector, from one co‐
ordinate system to another. The following code sample shows how QVRotate is used to
convert a force vector in body-fixed coordinates to the equivalent force in earth-fixed
coordinates.
void CalcAirplaneLoads(void)
{
.
.
.
// Convert forces from model space to earth space
Airplane.vForces = QVRotate(Airplane.qOrientation, Fb);
.
.
.
}
Throughout the simulation, you'll have to update the airplane's orientation by integrat‐
ing the angular equations of motion. The first step in handling angular motion is to
calculate the new angular velocity at a given time step based on the previously calculated
moments acting on the airplane and its mass properties. We do this in body coordinates
using the angular equation of motion:
M cg = d H cg /dt = I (d ω /dt) + ( ω × ( I ω ))
The next step is to integrate again to update the airplane's orientation, which is expressed
as a quaternion. Here, you need to use the differential equation relating an orientation
quaternion to angular velocity that we discussed earlier:
d q /dt = (1/2) ω q
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