Aircraft Components & Structure

Types of Fuselage Construction:

• The construction of aircraft fuselages evolved from the early wood truss structural arrangements to monocoque shell structures to the current semi-monocoque 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 semi-monocoque construction

  • 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 support 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
    • Everyday examples of monocoque construction can be found in automobile manufacturing where the unibody is considered standard in manufacturing

  • Semi-monocoque:

    • semi-monocoque 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 a 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 [Figure 2]

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 an 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: [Figure 7]

  • 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

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

Auxiliary Flight Control Surfaces:

• 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

Slats:

• Slats are part of the Flight Control System, creating extra lift during lower speeds
• Attached to the leading edge of the wings and are designed to be controlled by the pilot or automatically by the flight computer
• 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]

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

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 11]
  • 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

Windscreen:

• Don't forget to clean the windscreen

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