Game Development Reference
In-Depth Information
The subscript 0 in Equation (10.19) represents the inlet conditions and the subscript
e
corresponds to exit conditions. The variable is the inlet or exit mass flow rate in
kg/s
. Jet
engines are usually characterized by thrust, and this number can be used in your simulations
if the mass flow rate and pressure data is not available. For example, the General Electric F110
turbofan engine used in F-16 fighter planes generates between 120,000 to 142,000
N
of force at
sea level.
5
It turns out that jet engines are easier to model in game simulations than are propeller
engines, because the thrust produced by a jet engine at a given altitude will be constant. There
is no need to use an equation like Equation (10.18) to model jet engine thrust. The thrust produced
by a jet engine is affected by changing altitude and will decrease with increasing altitude.
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ยท
Drag
Like baseballs, bullets, or anything else that travels through the air, an airplane is subject to
aerodynamic drag forces that will resist the motion of the airplane. While the wing generates
most of the lift of the airplane, the same is not true with drag. The drag caused by the fuselage
and tail structures must be considered. If the landing gear is exposed, it will contribute to the
overall drag as well. The overall drag on an airplane,
F
D
, can be computed from the familiar
drag equation.
1
2
FCv A
=
r
2
(10.20)
D
D
The overall drag on the airplane, and therefore the drag coefficient, has three main
components. Let's discuss each of them in a bit more detail.
Skin Friction and Form Drag
Skin friction drag
results from the contact between the air molecules and the surface of the
airplane. As the air slides over the airplane, it tries to pull the plane along with it.
Form drag
is
caused by the difference in pressure distribution between the forward and rearward facing
surfaces of the airplane. Form drag is also referred to as
pressure drag
.
Skin friction drag and form drag are sometimes combined into what is called
parasitic
drag
. Like all other drag forces we have studied, parasitic drag,
F
Dp
, can be expressed in terms
of a parasitic drag coefficient,
C
Dp
.
1
2
2
F
=
Cv A
r
(10.21)
Dp
Dp
