To move any airplane
through the air,
we must use a propulsion system to generate
thrust. Different types of aircraft use
different types of propulsion devices, but all aircraft rely on some
to generate the power needed to run the propulsion system.
The internal combustion, or
piston engine, used by the Wright brothers and
the modern jet and rocket
engine all depend on the burning of fuel to produce power.
Burning a fuel is called combustion, a chemical process that
we study in middle or high school.
Because combustion is so important for aircraft and rocket propulsion, we
will review the fundamentals. Combustion is a chemical process
in which a substance reacts rapidly with oxygen and gives off heat.
The reacting substance is called the fuel, and the source of
oxygen is called the oxidizer. The fuel can be a solid,
liquid, or gas. For airplane propulsion the fuel is usually
a liquid hydrocarbon, like gasoline.
The oxidizer, likewise, can be a solid, liquid, or gas.
For airplane propulsion the oxidizer is air, a gas. Since the oxidizer is
a gas and the fuel is a liquid, we have to vaporize the fuel
for combustion to occur in an airplane engine. The vaporization and
mixing of the fuel and air occur in the
of the engine, the combustion itself occurs in the
During combustion, new chemical substances are created from the
fuel and the oxidizer. These substances are called exhaust.
Most of the exhaust comes from chemical combinations of the fuel and
oxygen. When a hydrogen-carbon-based fuel (like gasoline) burns, the
exhaust includes water (hydrogen + oxygen) and carbon dioxide (carbon
+ oxygen). But the exhaust can also contain chemical combinations
of other elements present during combustion. If gasoline is burned in air,
which contains 21% oxygen and 78% nitrogen, the exhaust can also
include nitrous oxides (NOX, nitrogen + oxygen). The exhaust is usually
in the form of a gas because of the high temperature from
the heat released. But some liquid and even solid exhaust
can be formed. (Example: Soot is a form of solid
exhaust that occurs in some combustion processes.)
During the combustion process, as the fuel and oxidizer are turned
into exhaust products, heat is generated. Interestingly, some source
of heat is also necessary to start combustion. (Gasoline and air are
both present in your automobile fuel tank; but combustion does not
occur because there is no source of heat.) Since heat is both
required to start combustion and is itself a product of combustion,
we can see why combustion takes place very rapidly. Also, once
combustion gets started, we don't have to continually provide a heat source.
The heat of combustion will keep things going.
(Example: We don't have to keep lighting a campfire once it is burning.)
To summarize, for combustion to occur three things must be
present: a fuel to be burned, a source of oxygen, and a source of
heat. As a result of combustion, exhausts are created and heat is
released. You can control or stop the combustion process by
controlling the amount of the fuel available, the amount of oxygen
available, or the source of heat.
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page