Engine
Cowling
Propeller
Figure 3-13. Engine compartment.
propeller blade, as measured against the hub of the propeller,
keeps the angle of attack (AOA) (See definition in Glossary)
relatively constant along the span of the propeller blade,
reducing or eliminating the possibility of a stall. The amount
of lift being produced by the propeller is directly related to
the AOA, which is the angle at which the relative wind meets
the blade. The AOA continuously changes during the flight
depending upon the direction of the aircraft.
The pitch is defined as the distance a propeller would travel in
one revolution if it were turning in a solid. These two factors
combine to allow a measurement of the propeller’s efficiency.
Propellers are usually matched to a specific aircraft/
powerplant combination to achieve the best efficiency at a
particular power setting, and they pull or push depending on
how the engine is mounted.
Subcomponents
The subcomponents of an airplane include the airframe,
electrical system, flight controls, and brakes.
The airframe is the basic structure of an aircraft and is
designed to withstand all aerodynamic forces, as well as the
stresses imposed by the weight of the fuel, crew, and payload.
The primary function of an aircraft electrical system is to
generate, regulate, and distribute electrical power throughout
the aircraft. There are several different power sources on
aircraft to power the aircraft electrical systems. These
power sources include: engine-driven alternating current
(AC) generators, auxiliary power units (APUs), and external
power. The aircraft’s electrical power system is used to
operate the flight instruments, essential systems, such as
anti-icing, and passenger services, such as cabin lighting.
The flight controls are the devices and systems that govern
the attitude of an aircraft and, as a result, the flight path
followed by the aircraft. In the case of many conventional
airplanes, the primary flight controls utilize hinged, trailing-
edge surfaces called elevators for pitch, ailerons for roll, and
the rudder for yaw. These surfaces are operated by the pilot
in the flight deck or by an automatic pilot.
In the case of most modern airplanes, airplane brakes consist
of multiple pads (called caliper pads) that are hydraulically
squeezed toward each other with a rotating disk (called a
rotor) between them. The pads place pressure on the rotor
which is turning with the wheels. As a result of the increased
friction on the rotor, the wheels inherently slow down and
stop turning. The disks and brake pads are made either from
steel, like those in a car, or from a carbon material that weighs
less and can absorb more energy. Because airplane brakes are
used principally during landings and must absorb enormous
amounts of energy, their life is measured in landings rather
than miles.
Types of Aircraft Construction
The construction of aircraft fuselages evolved from the early
wood truss structural arrangements to monocoque shell
structures to the current semimonocoque shell structures.
Truss Structure
The main drawback of truss structure is its lack of a
streamlined shape. In this construction method, lengths of
tubing, called longerons, are welded in place to form a well-
braced framework. Vertical and horizontal struts are welded
to the longerons and give the structure a square or rectangular
shape when viewed from the end. Additional struts are needed
to resist stress that can come from any direction. Stringers
and bulkheads, or formers, are added to shape the fuselage
and support the covering.
As technology progressed, aircraft designers began to enclose
the truss members to streamline the airplane and improve
performance. This was originally accomplished with cloth
fabric, which eventually gave way to lightweight metals such
as aluminum. In some cases, the outside skin can support all
or a major portion of the flight loads. Most modern aircraft
use a form of this stressed skin structure known as monocoque
or semimonocoque construction. [Figure 3-14]
Monocoque
Monocoque construction uses stressed skin to support almost
all loads much like an aluminum beverage can. Although
very strong, monocoque construction is not highly tolerant
Semimonocoque
Monocoque
Bulkhead
Stringers
Stressed skin
Formers
Skin
Bulkhead
Formers
Figure 3-14. Semimonocoque and monocoque fuselage design.
automobile manufacturing where the unibody is considered
standard in manufacturing.
Semimonocoque
Semimonocoque construction, partial or one-half, uses a
substructure to which the airplane’s skin is attached. The
substructure, which consists of bulkheads and/or formers
of various sizes and stringers, reinforces the stressed skin
by taking some of the bending stress from the fuselage. The
main section of the fuselage also includes wing attachment
points and a firewall. On single-engine airplanes, the engine
is usually attached to the front of the fuselage. There is a
fireproof partition between the rear of the engine and the
flight deck or cabin to protect the pilot and passengers from
accidental engine fires. This partition is called a firewall and
is usually made of heat-resistant material such as stainless
steel. However, a new emerging process of construction is
the integration of composites or aircraft made entirely of
composites.
Composite Construction
History
The use of composites in aircraft construction can be dated
to World War II aircraft when soft fiberglass insulation was
used in B-29 fuselages. By the late 1950s, European high
performance sailplane manufacturers were using fiberglass
as primary structures. In 1965, the FAA type certified the
first all-fiberglass aircraft in the normal category, a Swiss
sailplane called a Diamant HBV. Four years later, the FAA
certified a four-seat, single-engine Windecker Eagle in the
normal category. By 2005, over 35 percent of new aircraft
were constructed of composite materials.
Composite is a broad term and can mean materials such as
fiberglass, carbon fiber cloth, Kevlar™ cloth, and mixtures
of all of the above. Composite construction offers two
advantages: extremely smooth skins and the ability to easily
form complex curved or streamlined structures. [Figure 3-15]
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 systems.
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 high-
temperature properties. There are many different types of
epoxies available with a wide range of structural properties,
cure times and temperatures, and costs.
to deformation of the surface. For example, an aluminum
beverage can supports considerable forces at the ends of
the can, but if the side of the can is deformed slightly while
supporting a load, it collapses easily.
Because most twisting and bending stresses are carried by
the external skin rather than by an open framework, the need
for internal bracing was eliminated or reduced, saving weight
and maximizing space. One of the notable and innovative
methods for using monocoque construction was employed by
Jack Northrop. In 1918, he devised a new way to construct
a monocoque fuselage used for the Lockheed S-1 Racer.
The technique utilized two molded plywood half-shells that
were glued together around wooden hoops or stringers. To
construct the half shells, rather than gluing many strips of
plywood over a form, three large sets of spruce strips were
soaked with glue and laid in a semi-circular concrete mold
that looked like a bathtub. Then, under a tightly clamped
lid, a rubber balloon was inflated in the cavity to press
the plywood against the mold. Twenty-four hours later,
the smooth half-shell was ready to be joined to another to
create the fuselage. The two halves were each less than a
quarter inch thick. Although employed in the early aviation
period, monocoque construction would not reemerge for
several decades due to the complexities involved. Every
day examples of monocoque construction can be found in
Figure 3-15. Composite aircraft.
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.
In a composite structure, a low energy impact, such as a
bump or a tool drop, may not leave any visible sign of the
impact on the surface. Underneath the impact site there may
be extensive delaminations, spreading in a cone-shaped area
from the impact location. The damage on the backside of
the structure can be significant and extensive, but it may be
hidden from view. Anytime one has reason to think there
may have been an impact, even a minor one, it is best to
get an inspector familiar with composites to examine the
structure to determine underlying damage. The appearance
of “whitish” areas in a fiberglass structure is a good tip-off
that delaminations of fiber fracture has occurred.
A medium energy impact (perhaps the car backing into the
structure) results in local crushing of the surface, which
should be visible to the eye. The damaged area is larger than
the visible crushed area and will need to be repaired. A high
energy impact, such as a bird strike or hail while in flight,
results in a puncture and a severely damaged structure. In
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
somewhat 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 well-
designed 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