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Pilot's Handbook of Aeronautical Knowledge
Aircraft Structure

Types of Aircraft Construction

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Composite aircraft
Figure 2-15. Composite aircraft.

Composite Materials in Aircraft

Composite materials are fiber-reinforced matrix systems. The
matrix is the "glue" used to hold the fibers together and, when
cured, gives the part its shape, but the fibers carry most of
the load. There are many different types of fibers and matrix

In aircraft, the most common matrix is epoxy resin, which is
a type of thermosetting plastic. Compared to other choices
such as polyester resin, epoxy is stronger and has good hightemperature properties. There are many different types of
epoxies available, with a wide range of structural properties,
cure times and temperatures, and costs.

The most common reinforcing fibers used in aircraft
construction are fiberglass and carbon fiber Fiberglass has
good tensile and compressive strength, good impact resistance,
is easy to work with, and is relatively inexpensive and readily
available. Its main disadvantage is that it is relatively heavy,
and it is difficult to make a fiberglass load-carrying structure
lighter than a well designed equivalent aluminum structure.
Carbon fiber is generally stronger in tensile and compressive
strength than fiberglass, and has much higher bending
stiffness. It is also considerably lighter than fiberglass.
However, it is relatively poor in impact resistance; the fibers
are brittle and tend to shatter under sharp impact. This can
be greatly improved with a "toughened" epoxy resin system,
as used in the Boeing 787 horizontal and vertical stabilizers.
Carbon fiber is more expensive than fiberglass, but the price
has dropped due to innovations driven by the B-2 program
in the 1980s, and Boeing 777 work in the 1990s. Very welldesigned
carbon fiber structures can be significantly lighter
than an equivalent aluminum structure, sometimes by 30
percent or so.

Advantages of Composites

Composite construction offers several advantages over
metal, wood, or fabric, with its lighter weight being the most
frequently cited. Lighter weight is not always automatic. It
must be remembered that building an aircraft structure out of
composites does not guarantee it will be lighter, it depends on
the structure, as well as the type of composite being used.

A more important advantage is that a very smooth, compound
curved, aerodynamic structure made from composites
reduces drag. This is the main reason sailplane designers
switched from metal and wood to composites in the 1960s.
In aircraft, the use of composites reduces drag for the Cirrus
and Columbia line of production aircraft, leading to their high
performance despite their fixed landing gear. Composites also
help mask the radar signature of "stealth" aircraft designs,
such as the B-2 and the F-22. Today, composites can be found
in aircraft as varied as gliders to most new helicopters.

Lack of corrosion is a third advantage of composites. Boeing
is designing the 787, with its all-composite fuselage, to have
both a higher pressure differential and higher humidity in
the cabin than previous airliners. Engineers are no longer as
concerned about corrosion from moisture condensation on the
hidden areas of the fuselage skins, such as behind insulation
blankets. This should lead to lower long-term maintenance
costs for the airlines.

Another advantage of composites is their good performance
in a flexing environment, such as in helicopter rotor blades.
Composites do not suffer from metal fatigue and crack growth
as do metals. While it takes careful engineering, composite
rotor blades can have considerably higher design lives than
metal blades, and most new large helicopter designs have all
composite blades, and in many cases, composite rotor hubs.

Disadvantages of Composites

Composite construction comes with its own set of
disadvantages, the most important of which is the lack of
visual proof of damage. Composites respond differently from
other structural materials to impact, and there is often no
obvious sign of damage. For example, if a car backs into an
aluminum fuselage, it might dent the fuselage. If the fuselage
is not dented, there is no damage. If the fuselage is dented,
the damage is visible and repairs are made.