Game Development Reference
In-Depth Information
The lift force of an airfoil can be determined by integrating (that is, summing up) the pressure
distribution over the top and bottom surfaces of the wing. The resulting lift force can be modeled
by assuming that it is concentrated at a single point known as the
center of pressure
of the
airfoil. This point is important in the analysis of airfoils and other lifting bodies, so let's state it
again.
The lift force on an airfoil will act through the center of pressure of the airfoil.
The location of the center of pressure of an airfoil is determined by the geometry and
orientation of the airfoil. The mass distribution inside the airfoil has no bearing on the center
of pressure. The center of gravity, on the other hand, depends both on the geometry and weight
distribution of the airfoil. More commonly than not, the center of pressure and center of gravity
will be in different locations. Another feature of the center pressure is that it won't necessarily
remain in a fixed location but will move as the angle of attack changes because the pressure
distribution over the airfoil will change with angle of attack.
Up to this point the discussion in this section has been on the centers of pressure and
gravity of an airfoil, but the airplane as a whole will have a center of pressure and a center of
gravity as well. The relative locations of the centers of pressure and gravity for an airplane have
important ramifications for the stability of the airplane in flight—a topic that will be discussed
in the “Trim and Stability” section later in the chapter.
Thrust
The thrust created by an airplane engine serves two purposes. It propels the airplane forward,
and it also creates the airflow over the wing that generates the lifting force that keeps the plane
in the air. Of the four forces that act on an airplane, thrust is the most difficult to model and
predict. For one thing, thrust is a function of altitude. At higher altitudes, an engine will generate
less thrust than it will at sea level. Different types of engines generate thrust in different ways.
There are four basic types of airplane engines—propeller, jet, turbojet, and rocket. Of
these four, propeller and jet engines are the most commonly used, and are the ones that will be
covered in this section. A discussion of rocket engines will be deferred until the next chapter.
Propeller Engines
Starting with the Wright brothers and for about 40 years after, airplanes primarily used internal
combustion engines connected to a propeller to generate thrust. Even today, most smaller
general aviation or private planes still use propellers and internal combustion engines, because it is
an efficient engine type for low-speed flight. The engine itself is similar to the internal combus-
tion engines found in cars. The engine burns a mixture of fuel and air to drive pistons up and
down, which turn a shaft connected to the propeller. When the propeller turns, it pushes air
backwards and downwards, which pushes the airplane forward.
Instead of being characterized in terms of thrust, propeller engines are normally described
in terms of the power they can deliver. An aircraft engine generates a certain engine power,
P
E
.
Due to inefficiencies and losses, not all of the engine power can be converted into thrust by the
propeller. The thrust power of the propeller,
P
T
, is equal to the engine power multiplied by a
propeller efficiency coefficient
,
h
p
.
PP
=
h
(10.5)
T
p
E
