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Aircraft Components & Structure

Introduction:

  • 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 airplane is controllable around its lateral, longitudinal, and vertical axes by deflection of flight control surfaces
  • These control devices are hinged or movable surfaces with which the pilot adjusts the airplane's attitude during takeoff, flight maneuvering, and landing
  • They are operated by the pilot through connecting linkage by means of rudder pedals and a control stick or wheel [Figure 4]
  • The principle structure of an aircraft consists of:
    • Fuselage: main structural unit
    • Wings: airfoils to produce lift
    • Flight Control Surfaces:
      • Primary: ailerons, elevator, rudders
      • Secondary: movable trim tabs located on the primary flight control surfaces
      • Auxiliary: wing flaps, spoilers, speed brakes and slats
Flight Control SUrfaces
Figure 4: Flight Control Surfaces
Wing Bracing
Figure 3: Wing Bracing
Airplane Components
Figure 4: Pilot Handbook of Aeronautical Knowledge,
Airplane Components

Fuselage:

  • The fuselage is the principal structural unit of an aircraft
  • The fuselage is designed to accommodate the crew, passengers, cargo, instruments, and other essential equipment
  • Types of Fuselage 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:

        • In this construction method, strength and rigidity are obtained by joining tubing (steel or aluminum) to produce a series of triangular shapes, called trusses
          • Lengths of tubing, called longerons, are welded in place to form a wellbraced 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 designs progressed these structures were enclosed, first with cloth and eventually with metals
        • These upgrades streamlined shape and increased performance
        • 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 (French for "single shell") construction uses stressed skin to support almost all loads much like an aluminum beverage can
        • In monocoque construction, rigs, formers, and bulkheads of varying sizes give shape and strength to the stressed skin fuselage [Figure 1]
        • Although very strong, monocoque construction is not highly tolerant 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 automobile manufacturing where the unibody is considered standard in manufacturing
        Pilot Handbook of Aeronautical Knowledge, Monocoque
        Figure 11: Pilot Handbook of Aeronautical Knowledge, Monocoque
      • 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
        Pilot Handbook of Aeronautical Knowledge, Semimonocoque
        Figure 11: Pilot Handbook of Aeronautical Knowledge, Semimonocoque
Aircraft Fuselage
Figure 5: Aircraft Fuselage

Wings:

  • Wings are airfoils attached to each side of the fuselage and are the main lifting surfaces that support the airplane in flight
  • Wings may be attached at the top ("high-wing"), middle ("mid-wing"), or lower ("low-wing") portion of the fuselage
  • The number of wings can also vary
    • Airplanes with a single set of wings are referred to as monoplanes, while those with two sets are called biplanes [Figure 3]
  • Many high-wing airplanes have external braces, or wing struts that transmit the flight and landing loads through the struts to the main fuselage structure
  • Since the wing struts are usually attached approximately halfway out on the wing, this type of wing structure is called semi-cantilever
  • A few high-wing and most low-wing airplanes have a full cantilever wing designed to carry the loads without external struts
  • The principal structural parts of the wing are spars, ribs, and stringers. [Figure 3-7] These are reinforced by trusses, I-beams, tubing, or other devices, including the skin. The wing ribs determine the shape and thickness of the wing (airfoil). In most modern airplanes, the fuel tanks are either an integral part of the wing’s structure or consist of flexible containers mounted inside of the wing
  • Attached to the rear, or trailing edges, of the wings are two types of control surfaces referred to as ailerons and flaps
  • Alternate Types of Wings:

    • Design variations provide information on the effect controls have on lifting surfaces from traditional wings to wings that use both flexing (due to billowing) and shifting (through the change of the aircraft’s CG). For example, the wing of the weight-shift control aircraft is highly swept in an effort to reduce drag and allow for the shifting of weight to provide controlled flight. [Figure 3-9] Handbooks specific to most categories of aircraft are available for the interested pilot and can be found on the Federal Aviation Administration (FAA) website at www.faa.gov
  • Ailerons:

    • Ailerons (French for "little wing") are control surfaces on each wing which control the aircraft about its longitudinal axis allowing the aircraft to "roll" or "bank"
      • This action results in the airplane turning in the direction of the roll/bank
      • With aileron deflection, there is asymmetrical lift (rolling moment) about the longitudinal axis and drag (adverse yaw)
    • They are located on the trailing (rear) edge of each wing near the outer tips
      • They extend from about the midpoint of each wing outward toward the tip, and move in opposite directions to create aerodynamic forces that cause the airplane to roll
    • The yoke manipulates the airfoil through a system of cables and pulleys and act in an opposing manor
      • Yoke "turns" left: left aileron rises, decreasing camber and angle of attack on the right wing which creates downward lift
        • At the same time, the right aileron lowers, increasing camber and angle of attack which increases upward lift and causes the aircraft to turn left
      • Yoke "turns" right: right aileron rises decreasing camber and angle of attack on the right wing which creates downward lift
        • At the same time, the left aileron lowers, increasing camber and angle of attack on the left wing which creates upward lift and causes the aircraft to turn right
    • Although uncommon, some ailerons are configured with trim tabs which relieve pressure on the yoke on the aileron for rolling
  • Wing Planform:

    • The shape and design of a wing is dependent upon the type of operation for which an aircraft is intended and is tailored to specific types of flying:
      • Rectangular:

        • Rectangular wings are best for training aircraft, as well as low speed aircraft
        • Designed with twist to stall at the wing root first, to provide aileron control in stalls
      • Elliptical:

        • Elliptical wings are most efficient, but difficult to produce (spitfire)
      • Tapered:

        • More efficient than a rectangle wing but easier to produce than an elliptical design
      • Swept:

        • Usually associated with swept-back, but can also be swept-foreword
        • Sweptback wings are best for high speed aircraft for delaying Mach tendencies
        • Stall at the tips first, providing poor stall characteristics
      • Delta:

        • Advantages of a swept wing, with good structural efficiency and low frontal area
        • Disadvantages are the low wing loading and high wetted area needed to obtain aerodynamic stability
    • These design variations are discussed in Chapter 5, Aerodynamics of Flight, which provides information on the effect controls have on lifting surfaces from traditional wings to wings that use both flexing (due to billowing) and shifting (through the change of the aircraft’s CG). For example, the wing of the weight-shift control aircraft is highly swept in an effort to reduce drag and allow for the shifting of weight to provide controlled flight. [Figure 3-9] Handbooks specific to most categories of aircraft are available for the interested pilot and can be found on the Federal Aviation Administration (FAA) website at www.faa.gov
    Wing Construction
    Figure 6: Wing Construction
Pilot Handbook of Aeronautical Knowledge, Monoplane (left) and biplane (right)
Figure 7: Pilot Handbook of Aeronautical Knowledge, Monoplane (left) and biplane (right)
Wing Planform
Figure 8: Airplane Flying Handbook, Airfoil types
Empennage Components
Figure 9: Pilot Handbook of Aeronautical Knowledge, Empennage Components
Stabilator Components
Figure 10: Pilot Handbook of Aeronautical Knowledge, Stabilator Components

Empennage:

  • Commonly known as the "tail section," the empennage includes the entire tail group which consists of fixed surfaces such as the vertical fin or stabilizer and the horizontal stabilizer; the movable surfaces including the rudder and rudder trim tabs, as well as the elevator and elevator trim tabs
  • These movable surfaces are used by the pilot to control the horizontal rotation (yaw) and the vertical rotation (pitch) of the airplane
  • In some airplanes the entire horizontal surface of the empennage can be adjusted from the cockpit as a complete unit for the purpose of controlling the pitch attitude or trim of the airplane. Such designs are usually referred to as stabilators, flying tails, or slab tails
  • The empennage, then, provides the airplane with directional and longitudinal balance (stability) as well as a means for the pilot to control and maneuver the airplane
  • Rudder:

    • Rudders are used to control the direction (left or right) of "yaw" about an airplane's vertical axis
    • Like the other primary control surfaces, the rudder is a movable surface hinged to a fixed surface that, in this case, is the vertical stabilizer, or fin
    • Its action is very much like that of the elevators, except that it swings in a different plane - from side to side instead of up and down
      • It is not used to make the airplane turn, as is often erroneously believed
      • In practice, both aileron and rudder control input are used together to turn an aircraft, the ailerons imparting roll
        • This relationship is critical in maintaining coordination or creating a slip
        • Improperly ruddered turns at low speed can precipitate a spin
    • Rudders are controlled by the pilot with his/her feet through a system of cables and pulleys:
      • "Step" on the right rudder pedal: rudder moves right creating a yaw to the right
      • "Step" on the left rudder pedal: rudder moves left creating a yaw to the left
  • Elevator:

    • The elevator, which is attached to the back of the horizontal stabilizer, is used to move the nose of the airplane up and down during flight
  • Stabilator:

    • A second type of empennage design does not require an elevator
    • Instead, it incorporates a one-piece horizontal stabilizer that pivots from a central hinge point
    • This type of design is called a stabilator and is moved using the control wheel, just as the elevator is moved
    • For example, when a pilot pulls back on the control wheel, the stabilator pivots so the trailing edge moves up
    • This increases the aerodynamic tail load and causes the nose of the airplane to move up. Stabilators have an antiservo tab extending across their trailing edge [Figure 3-11]
    • The anti-servo tab moves in the same direction as the trailing edge of the stabilator and helps make the stabilator less sensitive
    • The anti-servo tab also functions as a trim tab to relieve control pressures and helps maintain the stabilator in the desired position

Control Surface Tabs:

  • Tabs are small, adjustable aerodynamic devices on the trailing edge of the control surface
  • These movable surfaces reduce pressures on the controls
  • Trim controls a neutral point, like balancing the aircraft on a pin with unsymmetrical weights
  • This is done either by trim tabs (small movable surfaces on the control surface) or by moving the neutral position of the entire control surface all together
  • These tabs may be installed on the ailerons, the rudder, and/or the elevator
  • Trim Tabs:

    • The force of the airflow striking the tab causes the main control surface to be deflected to a position that corrects the unbalanced condition of the aircraft
    • An aircraft properly trimmed will, when disturbed, try to return to its previous state due to aircraft stability
    • Trimming is a constant task required after any power setting, airspeed, altitude, or configuration change
    • Proper trimming decreases pilot workload allowing for attention to be diverted elsewhere, especially important for instrument flying
    • Trim tabs are controlled through a system of cables and pulleys
      • Trim tab adjusted up: trim tab lowers creating positive lift, lowering the nose
        • This movement is very slight
      • Trim tab adjusted down: trim tab raises creating positive lift, raising the nose
        • This movement is very slight
    • To learn more about how to use the trim tab in flight see the trimming the aircraft
  • Servo Tabs:

    • Servo tabs are similar to trim tabs in that they are small secondary controls which help reduce pilot workload by reducing forces
    • The defining difference however, is that these tabs operate automatically, independent of the pilot
    • Types of Servo Tab Designs:

      • Anti-servo:
        • Also called an anti-balance tab, are tabs that move in the same direction as the control surface
      • Servo:
        • Tabs that move in the opposite direction as the control surface

Controls:

  • Slats:

    • Slats are part of the Flight Control System
    • Attached to the leading edge of the wings and are designed to be controlled by the pilot or automatically by the flight computer
    • Some aircraft employ aerodynamic slats that when lowered create added lift
    • Slats increase the camber of the wings/airfoil
    • By extending the slats additional lift is created when the aircraft is at slower airspeeds, normally on takeoff and landing
  • Flaps:

    • Flaps are part of the Flight Control System
    • Attached to the trailing edge of the wings and are controlled by the pilot from the cockpit
    • By extending the flaps additional lift is created when the aircraft is at slower airspeeds, normally on takeoff and landing
    • Slats and flaps are used in conjunction with each other to increase both lift and stall margin by increasing the overall wings camber thus, allowing the aircraft to maintain control flight at slower airspeeds
    • Flaps extend outward from the fuselage to near the midpoint of each wing
    • The flaps are normally flush with the wing's surface during cruising flight
    • When extended, the flaps move simultaneously downward to increase the lifting force of the wing for takeoffs and landings [Figure 3-8]
  • Elevator:

    control surfaces which control the aircraft about its lateral axis allowing the aircraft to pitch
    • The elevators are attached to the horizontal portion of the empennage - the horizontal stabilizer
      • The exception to this is found in those installations where the entire horizontal surface is a one piece structure which can be deflected up or down to provide longitudinal control and trimming
    • A change in position of the elevators modifies the camber of the airfoil, which increases or decreases lift
    • When forward pressure is applied on the controls, the elevators move downward
    • This increases the lift produced by the horizontal tail surfaces
    • The increased lift forces the tail upward, causing the nose to drop
    • Conversely, when back pressure is applied on the wheel, the elevators move upward, decreasing the lift produced by the horizontal tail surfaces, or maybe even producing a downward force
    • The tail is forced downward and the nose up
    • The elevators control the angle of attack of the wings
    • When back-pressure is applied on the controls, the tail lowers and the nose rises, increasing the angle of attack
    • Conversely, when forward pressure is applied, the tail raises and the nose lowers, decreasing the angle of attack
    • Stabilizer: a control surface other than the wings which provide stabilizing qualities
  • Speed Brakes:

    • Designed to slow the aircraft when in a dive or descent, location and style vary with aircraft, and are controlled by a switch in the cockpit
  • Trim Tabs:

    • Movable tabs located on the primary control surfaces i.e., ailerons, elevators and rudder reducing the pilot's workload enabling the aircraft to hold a particular attitude without the need of constant pressure/inputs into the system
  • Landing Gear:

    • The landing gear is the principal support of the airplane when parked, taxiing, taking off, or landing
    • A steerable nosewheel or tailwheel permits the airplane to be controlled throughout all operations while on the ground
    • Most aircraft are steered by moving the rudder pedals, whether nosewheel or tailwheel. Additionally, some aircraft are steered by differential braking
  • Pilot Handbook of Aeronautical Knowledge, Engine Compartment
    Figure 10: Pilot Handbook of Aeronautical Knowledge, Engine Compartment
  • Power Plant:

    • The powerplant usually includes both the engine and the propeller
    • Engine:
      • The primary function of the engine is to provide the power to turn the propeller
      • It also generates electrical power, provides a vacuum source for some flight instruments, and in most single-engine airplanes, provides a source of heat for the pilot and passengers. [Figure 3-13]
      • 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 cockpit or cabin to protect the pilot and passengers from accidental engine fires. This partition is called a firewall and is usually made of a high heat resistant, stainless steel
    • Cowling:
      • The engine is covered by a cowling, or a nacelle, which are both types of covered housing
      • The purpose of the cowling or nacelle is to streamline the flow of air around the engine and to help cool the engine by ducting air around the cylinders
    • Propeller:
      • The propeller, mounted on the front of the engine, translates the rotating force of the engine into thrust, a forward acting force that helps move the airplane through the air
      • A propeller is a rotating airfoil that produces thrust through aerodynamic action
      • A high-pressure area is formed at the back of the propeller’s airfoil, and low pressure is produced at the face of the propeller, similar to the way lift is generated by an airfoil used as a lifting surface or wing
      • This pressure differential develops thrust from the propeller, which in turn pulls the airplane forward
      • Engines may be turned around to be pushers with the propeller at the rear
      • There are two significant factors involved in the design of a propeller that impact its effectiveness
      • The angle of a 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

Rotary-Wing Components:

  • The major difference between helicopters and fixed-wing is the source of lift
  • Fixed-winged aircraft derive lift from fixed airfoils while helicopters use rotating airfoils known as rotor blades
  • Lift and control are relatively independent of forward speed
  • Controls:
    • Cyclic Stick:

      • Controls movement about the lateral and longitudinal axis of the helicopter
      • It is located centered in front of the pilot's seat and changes the tip path plane of the main rotor for directional flight
      • By changing the tip path plane, the direction of thrust is changed, and the corresponding intended direction of movement or flight is achieved
    • Collective Stick:

      • Always located to the left of the pilot's seat and varies the lift of the main rotor by decreasing or increasing the angle of attack on all rotor plates equally and in the same direction
      • Also used in combination with the cyclic to regulate speed and altitude
    • Rudder Pedals:

      • Controls movement about the vertical axis (yaw) of the helicopter by changing the pitch (angle of attack) of the tail rotor plates
      • This causes more or less force to be developed which is counteracting the torque caused by the main rotors
      • Additionally, by the pilot deflecting the rudder pedals left or right the aircraft heading or direction is changed left or right
  • Components:
    • Rotor Blade:
      • Spinning "wings" which allow for lift on helicopters or "rotor-craft"
    • Main Rotor Assembly:
      • Consists of rotor blades, rotor hub assembly, pitch control rod/links, mast, swashplate and support assembly
      • Some may have scissor and sleeve assembly
      • All of the above items work to change linear (push/pull motion) into rotating control movement
    • Gearboxes/Transmission:
      • Changes direction and provides power produced by the engines via drive shafts to the main and trail rotor assemblies
      • The main transmission also provides mounting pads for accessory mounting such as hydraulic flight control pumps, generators, and rotor brake
      • Most helicopters have a main, intermediate and a tail gearbox

Conclusion:

  • The principles of flight are those basic characteristics which act upon an aircraft
  • A balanced aircraft is a happy aircraft (fuel burn, efficiency, etc.)
  • As aircraft construction evolved from truss truss structures, which lacked a streamlined shape, to the more formed monocoque and semimonocoque designs of today

References: