Aircraft Structures
General
An aircraft is a device that is
used for, or is intended to be used for, flight in the air. Major categories of
aircraft are airplane, rotorcraft, glider, and lighter-than-air vehicles. [Figure 1]
Figure 1. George Cayley, the father of aeronautics
(top) and a flying replica of his 1853 glider (bottom).
Each of these may be divided further
by major distinguishing features of the aircraft, such as airships and
balloons. Both are lighter-than-air aircraft but have differentiating features and
are operated differently.The concentration of this
handbook is on the airframe of aircraft; specifically, the fuselage, booms,
nacelles, cowlings, fairings, airfoil surfaces, and landing gear. Also included
are the various accessories and controls that accompany these structures. Note
that the rotors of a helicopter are considered part of the airframe since they
are actually rotating wings. By contrast, propellers and rotating airfoils of
an engine on an airplane are not considered part of the airframe.
The most common aircraft is the
fixed-wing aircraft. As the name implies, the wings on this type of flying
machine are attached to the fuselage and are not intended to move independently
in a fashion that results in the creation of lift. One, two, or three sets of
wings have all been successfully utilized.[Figure 2] Rotary-wing aircraft such as helicopters
are also widespread. This handbook discusses features and maintenance aspects
common to both fixedwing and rotary-wing categories of aircraft. Also, in
certain cases, explanations focus on information specific to only one or the
other. Glider airframes are very similar to fixedwing aircraft. Unless
otherwise noted, maintenance practices described for fixed-wing aircraft also
apply to gliders. The same is true for lighter-than-air aircraft, although
thorough
Figure 2. A monoplane (top), biplane (middle), and
tri-wing aircraft (bottom).
coverage of the unique airframe structures and
maintenance practices for lighter-than-air flying machines is not included in
this handbook.The airframe of a fixed-wing aircraft consists of five
principal units: the fuselage, wings, stabilizers, flight control surfaces, and
landing gear. [Figure - 3] Helicopter airframes consist of the fuselage,
main rotor and related gearbox, tail rotor (on helicopters with a single main
rotor), and the landing gear.
Airframe structural components are constructed from a
wide variety of materials. The earliest aircraft were constructed primarily of
wood. Steel tubing and the most common material, aluminum, followed. Many newly
certified aircraft are built from molded composite materials, such as carbon fiber.
Structural members of an aircraft’s fuselage include stringers, longerons,
ribs, bulkheads, and more. The main structural member in a wing is called the wing spar.The
skin of aircraft can also be made from a variety of materials, ranging from
impregnated fabric to plywood, aluminum, or composites. Under the skin and
attached to the structural fuselage are the many components that support airframe
function. The entire airframe and its components are joined by rivets, bolts,
screws, and other fasteners. Welding, adhesives, and special bonding techniques
are also used.
Major Structural Stresses
Aircraft structural members are designed to carry a load
or to resist stress. In designing an aircraft, every square inch of wing and
fuselage, every rib, spar, and even each metal fitting must be considered in
relation to the physical characteristics of the material of which it is made.
Every part of the aircraft must be planned to carry the load to be imposed upon
it
Figure 3. Principal airframe units.
The determination of such loads is called
stress analysis. Although planning the design is not the function of the
aircraft technician, it is, nevertheless, important that the technician understand
and appreciate the stresses involved in order to avoid changes in the original
design through improper repairs.
The term “stress” is often used
interchangeably with the word “strain.” While related, they are not the same
thing. External loads or forces cause stress. Stress is a material’s internal
resistance, or counterforce, that opposes deformation. The degree of
deformation of a material is strain. When a material is subjected to a load or
force, that material is deformed, regardless of how strong the material is or
how light the load is.
There are five major stresses [Figure 4] to which all
aircraft are subjected:
• Tension
• Compression
• Torsion
• Shear
• Bending
Tension is the stress that resists a force that tends to
pull something apart. [Figure 4A] The engine pulls the aircraft forward, but
air resistance tries to hold it back. The result is tension, which stretches
the aircraft. The tensile strength of a material is measured in pounds per
square inch (psi) and is calculated by dividing the load (in pounds) required to
pull the material apart by its cross-sectional area (in square inches).
Compression
is the stress that resists a crushing force. [Figure 4B] The compressive strength of a material is also
measured in psi. Compression is the stress that tends to shorten or squeeze
aircraft parts.Torsion is the stress that produces twisting. [Figure 4C]
While moving the aircraft
forward, the engine also tends to twist it to one side, but other aircraft
components hold it on course. Thus, torsion is created. The torsion strength of
a material is its resistance to twisting or torque.
Shear is the stress that resists the force tending to
cause one layer of a material to slide over an adjacent layer.[Figure 4D]
Two riveted plates in tension
subject the rivets to a shearing force. Usually, the shearing strength of a
material is either equal to or less than its tensile or compressive strength.
Aircraft parts, especially screws, bolts, and rivets, are often subject to a
shearing force.
Bending stress is
a combination of compression and tension. The rod in Figure 4E has been shortened (compressed) on the inside
of the bend and stretched on the outside of the bend.
Figure 4. The five stresses that may act on an aircraft
and its parts.
A single member of the structure may be subjected to a
combination of stresses. In most cases, the structural members are designed to
carry end loads rather than side loads. They are designed to be subjected to
tension or compression rather than bending. Strength or resistance to the
external loads imposed during operation may be the principal requirement in
certain structures. However, there are numerous other characteristics in
addition to designing to control the five major stresses that engineers must
consider. For example, cowling, fairings, and similar parts may not be subject
to significant loads requiring a high degree of strength. However, these parts
must have streamlined shapes to meet aerodynamic requirements, such as reducing
drag or directing airflow.
Fixed-Wing Aircraft
Fuselage
The fuselage is the main structure or body of the
fixed-wing aircraft. It provides space for cargo, controls, accessories, passengers,
and other equipment. In single-engine aircraft, the fuselage houses the
powerplant. In multiengine aircraft, the engines may be either in the fuselage,
attached to the fuselage, or suspended from the wing structure. There are two general
types of fuselage construction: truss and monocoque.
Truss
Type
A truss is a rigid framework made up of members, such as beams,
struts, and bars to resist deformation
by applied loads.The truss-framed fuselage is generally
covered with fabric.The truss-type fuselage frame is usually constructed of
steel tubing welded together in such a manner that all members of the truss can
carry both tension and compression loads.[Figure 5] In some aircraft, principally the light,
singleengine models, truss fuselage frames may be constructed of aluminum alloy
and may be riveted or bolted into one piece, with cross-bracing achieved by
using solid rods or tubes.
Figure 5. A truss-type fuselage. A Warren truss uses
mostly diagonal bracing
Monocoque Type
The
monocoque (single shell) fuselage relies largely on the strength of the skin or
covering to carry the primary loads.The design may be divided into two classes:
1. Monocoque
2.
Semimonocoque
Different
portions of the same fuselage may belong to either of the two classes, but most
modern aircraft are considered to be of semimonocoque type construction. The
true monocoque construction uses formers, frame assemblies, and bulkheads to
give shape to the fuselage. [Figure
6] The heaviest of these structural members are located at intervals to
carry concentrated loads and at points
Figure
6. An airframe using monocoque
construction.
where
fittings are used to attach other units such as wings, powerplants, and
stabilizers. Since no other bracing members are present, the skin must carry
the primary stresses and keep the fuselage rigid. Thus, the biggest problem
involved in monocoque construction is maintaining enough strength while keeping
the weight within allowable limits.
Semimonocoque Type
To overcome
the strength/weight problem of monocoque construction, a modification called
semimonocoque construction was developed. It also consists of frame assemblies,
bulkheads, and formers as used in the monocoque members called longerons. Longerons usually extend across
several frame members and help the skin support primary bending loads. They are
typically made of aluminum alloy either of a single piece or a built-up
construction. Stringers are also used in the semimonocoque fuselage. These longitudinal
members are typically more numerous and lighter in weight than the longerons.
They come in a variety of shapes and are usually made from single piece
aluminum alloy extrusions or formed aluminum. Stringers have some rigidity but
are chiefly used for giving shape and for attachment of the skin. Stringers and
longerons together prevent tension and compression from bending the fuselage. [Figure 7]
Figure
7. The most common airframe
construction is semimonocoque
Other bracing between the longerons and stringers can
also be used. Often referred to as web members, these additional support pieces
may be installed vertically or diagonally. It must be noted that manufacturersuse different nomenclature to describe structural members. For example, there
is often little difference between some rings, frames, and formers. One manufacturer
may call the same type of brace a ring or a frame. Manufacturer instructions
and specifications for a specific aircraft are the best guides.The
semimonocoque fuselage is constructed primarily of alloys of aluminum and
magnesium, although steel and titanium are sometimes found in areas of high
temperatures. Individually, no one of the aforementioned components is strong
enough to carry the loads imposed during flight and landing. But, when
combined, those components form a strong, rigid framework. This is accomplished
with gussets, rivets, nuts and bolts, screws, and even friction stir welding. A
gusset is a type of connection bracket that adds strength. [Figure 8]
Figure
8. Gussets are used to increase
strength.
To summarize, in semimonocoque fuselages, the strong, heavy
longerons hold the bulkheads and formers, and these, in turn, hold the
stringers, braces, web members, etc. All are designed to be attached together
and to the skin to achieve the full strength benefits of semimonocoque design.
It is important to recognize that the metal skin or covering carries part of
the load. The fuselage skin thickness can vary with the load carried and the
stresses sustained at a particular location.
The advantages of the semimonocoque fuselage are many.The
bulkheads, frames, stringers, and
longerons facilitate the design and construction of a streamlined
fuselage that is both rigid and strong. Spreading loads among these structures
and the skin means no single piece is failure critical. This means that a semimonocoque
fuselage, because of its stressed-skin construction, may withstand considerable
damage and still be strong enough to hold together.
Fuselages are
generally constructed in two or more sections. On small aircraft, they are
generally made in two or three sections, while larger aircraft may be made up
of as many as six sections or more before being assembled
Pressurization
Many aircraft are pressurized.
This means that air is pumped into the cabin after takeoff and a difference in
pressure between the air inside the cabin and the air outside the cabin is established.
This differential is regulated and maintained. In this manner, enough oxygen is
made available for passengers
to breathe normally and move
around the cabin without special equipment at high altitudes.
Pressurization causes significant
stress on the fuselage structure and adds to the complexity of design. In addition
to withstanding the difference in pressure between the air inside and outside
the cabin, cycling from unpressurized to pressurized and back again each flight
causes metal fatigue. To deal with these impacts and the other stresses of
flight, nearly all pressurized aircraft are semimonocoque in design. Pressurized
fuselage structures undergo extensive periodic inspections to ensure that any
damage is discovered and repaired. Repeated weakness or failure in an area of
structure may require that section of the fuselage be modified or redesigned.
No comments:
Post a Comment