Comprehensive Study Guide: Wings, Pylons, and Nacelles

Foundational Concepts of Aircraft Wings

• Primary Function: The wing serves as the primary lifting surface of an aircraft.

• Wing Attachment Methods: Various airplane types utilize different methods for wing attachment, categorized as high-wing, mid-wing, or low-wing.

• Principal Design Categories:

  • Semicantilever: This design derives its structural strength from both the internal wing components and external support, specifically by bracing from struts and wires.

  • Cantilever: This design contains all its structural strength within the internal wing structure and requires no external bracing.

• Design Influencing Factors: The shape and type of wings are dictated by considerations beyond structure, including the specific functions of the aircraft, required speeds, and expected payloads.

• Historical Instance: The Concorde Flight $4590$ incident at Charles de Gaulle Airport serves as a significant case study regarding the criticality of wing and fuel systems during fatal crashes.

Basic Features of Wing Construction: Spars

• Definition: A spar, also known as a wing beam, is the principal spanwise member of the wing structure.

• Conventional Types:

  • Monospar

  • Two-spar

  • Multispar

• Cross-Sectional Shapes: Metal spars are manufactured in various shapes, such as the wide flanged beam. These are favored for high flexural rigidity, making them highly efficient at resisting bending.

• Manufacturing: Spars are often produced through the extrusion process.

• Modern Materials: Composite spars are increasing in use, notably in the Boeing $787$.

• Geometric Positioning:

  • Forward spars are typically located near the leading edge.

  • Aft or rear spars are located forward of the trailing edge.

  • Intermediate or main spars may be located near the mid-point of the chordline.

Basic Features of Wing Construction: Ribs

• Definition: Also known as plain ribs, these are chordwise members of the wing structure.

• Orientation: They are placed at appropriate intervals along the span to provide the airfoil shape.

• Primary Functions:

  • Transmit aerodynamic loads from the wing skin covering to the spars.

  • Stabilize the spars against twisting forces.

Manufacturing and Specialized Ribs

• Light Aircraft Construction: Ribs are normally made from metal by stamping and are subsequently riveted to the spars.

• Transport Category Aircraft: Ribs are typically CNC machined from Al (Αluminium) alloys.

• Control Surface Support: Wing ribs located between spars support control surfaces and define the airfoil shape.

• Bulkheads: These are reinforced ribs that can be sealed to form fuel tanks.

• Weight Reduction: Holes are frequently found on wing ribs to remove excess material, facilitating a lighter aircraft design.

Wing Stringers, Skin, and Structural Reinforcement

• Wing Stringers: Also known as stiffeners, these spanwise members help hold the shape of large wings. They are normally installed in the upper skin to provide stiffness against compression loads. If stiffening demand is extremely high, corrupted panels or sandwich panels are used.

• Wing Skin: Modern wings use a stressed-skin design, identical to semi-monocoque fuselages. The skin is the primary load-carrying member, and ribs are riveted directly to it.

• Skin Materials: While mostly Al alloy, composite materials are emerging for skin use.

• Openings and Access: Cut-outs are necessary for servicing but weaken the structure. To compensate, doublers are required for reinforcement.

Fuel Storage Systems in Aircraft Wings

• Placement Strategy: Fuel tanks are designed to occupy the inboard portion of the wing. This improves stability, as fuel weight closer to the fuselage creates a smaller moment on the wings.

• Tank Categories for Commuter/Light Transport:

  • Removable Metal Fuel Tank: Installed in a covered bay between spars and ribs.

  • Bladder Fuel Cell: Rubber-like bags placed in compartments. This eliminates the need for large access openings for installation/removal.

  • Integral Fuel Tank: Built directly into the basic wing structure and cannot be removed.

Transport Aircraft Wing Specifics

• Load Carrying Capacity: The wing structure must support its own weight, the weight of fuel, the weight of engines, landing gear, and forces from flight controls.

• Structural Configuration: Normally constructed of $2$ or more spars, with intermediate spars to assist with operational loads. The front and rear spars provide the main supporting structure for attachments.

• Auxiliary Structures: Includes wing tips, leading edge devices, and trailing edge devices. Wing tips are removable to allow for inspection and maintenance.

• Assembly: Typically constructed in $3$ or more major assemblies (Left, Center, and Right wing sections). These are joined with permanent fasteners to form a one-piece wing before being attached to the fuselage.

• Fittings: Components for attaching the fuselage, engine pylons, landing gear, and flight control surfaces are secured with interference-fit and close tolerance fasteners.

• Interference-Fit Definition: These fasteners (bolts, pins, or rivets) are force-fit into a smaller hole to ensure no free play or clearance, regardless of temperature changes. They are not considered removable except for major structural repairs.

• Wet vs. Dry Bays: In transport aircraft using integral tanks (wet wings), areas that do not contain fuel are called dry bays.

Regulatory and Material Requirements for Fuel Tanks

• Compliance Standards: Aircraft fuel systems must conform to regulations such as FAA FAR or EASA CS Part $23$ and $25$ (specifically FAA FAR Sections $25.963$ through $25.981$).

• Material Selection: Materials must not react chemically with fuel. Al alloy is often used because it is lightweight, possesses good strength, and is easy to shape and weld. Synthetic-rubber bladders are also used for their weight and service life.

• Pressure and Vibration: Tanks must withstand internal test pressures of $3.5\,\text{psi}$ ($24.13\,\text{kPa}$) without failure. They must also withstand at least $125\%$ of the pressure developed by ram air effect, along with all operational vibration and inertia loads.

• Inertia Forces (Transport Category): Tanks must withstand rupture forces during emergency landings:

  • Downward: $4.5g$

  • Upward: $2g$

  • Forward: $9g$

  • Sideward: $1.5g$

• Explosion Risk: Tanks must be positioned where ground scraping action is unlikely during a crash to avoid heat-related explosions.

Fuel Tank Design: Sumps, Expansion, and Venting

• Fuel Sumps: Designed to collect sediment and water. For transport category aircraft, capacity must be at least $0.1\%$ of total tank capacity or $1/16\,\text{gal}$ ($0.24\,\text{L}$), whichever is greater. Sumps must have accessible drains for complete drainage.

• Expansion Space: A minimum expansion space of $2\%$ is required. Design must prevent the filling of this space when the airplane is on the ground. Automatic shutoff devices are used in pressure filling systems to prevent overfilling.

• Venting Capabilities: Each tank must be vented from the top of the expansion space. Vent outlets must be designed to minimize obstruction by ice or foreign matter and prevent fuel siphoning. Venting capacity must allow for rapid pressure relief. Air spaces in interconnected tanks must maintain equal pressure.

• Compartment Ventilation: Spaces adjacent to tanks must be ventilated and drained to prevent the accumulation of flammable vapors or fluids.

• Fire Separation: Fuel tanks are generally not installed on the engine side of a firewall. A minimum of $0.5\,\text{inch}$ ($12.7\,\text{mm}$) of clear air space is required between fuel tanks and firewalls or fire zones.

Categories of Fuel Tanks: Integral

• Integral Fuel Tanks (Wet Wing): Part of the basic aircraft structure, typically in wings, fuselage, or horizontal stabilizers. These are permanent and non-removable.

• Sealing: Uses wing skin, spars, ribs, and stringers with fuel-proof sealing materials like synthetic rubber.

• Access and Safety: Provided with access panels for inspection. Knitted aluminium gaskets provide electrostatic bonding for access doors to prevent sparks during lightning strikes.

• Baffles: Internal structures designed primarily to reduce fuel sloshing. They contain holes for free flow and sufficient structure to prevent rapid movement in turbulence.

• Check Valves: Often used with baffles to reduce flow rate toward wing tips during attitude changes and to keep fuel at the boost-pump inlet.

Categories of Fuel Tanks: Rigid Removable and Bladder cells

• Rigid Removable Tanks: Fabricated from welded aluminium and installed in compartments tailored for the tank. The tank must be fuel-tight, though the compartment itself does not need to be. These are secured with padded straps and are common in inexpensive light aircraft.

• Bladder Fuel Cells: Reinforced rubberized bags placed in non-fuel-tight compartments. The bladder is rolled up and inserted through a small opening, then unrolled and held by "buttons" or "snaps." These are found in medium to high-performance light aircraft and some turbine aircraft.

Supplemental Fuel Systems: Surge Tanks and Dripsticks

• Surge Tanks: Normally located on transport aircraft, these are constructed like integral tanks but are usually empty. Their purpose is to contain overflow and prevent spillage, particularly during refueling, and are part of the venting system.

• Dripsticks: A hollow fibreglass tube used as a non-electric, hand-operated alternative for gauging fuel quantity visually. Dripsticks are marked with calibrated scales (inches, gallons, kilograms, or litres) and are located on the lower wing surface. They are generally considered less accurate than electronic indicators but pose no risk of spillage.

Engine Installation and Pylon Structure

• Mounting Objectives: Engine installation must transmit thrust force, support engine weight, and transfer stresses to the main structure.

• Installation Locations:

  • Under the wings.

  • Side of the rear fuselage.

  • Buried within wings or fuselage.

  • At the nose (turboprops).

• Numbering: Powerplants are numbered left to right when viewed from the rear.

• Pylons: Installed under wings to support engines, transmit thrust, and route wiring, hydraulic lines, bleed air, and fuel lines.

• Pylon Composition:

  • Primary Structure: The pylon box, which attaches to the wing and supports the engine at two points.

  • Secondary Structure: Fairings that provide an aerodynamic profile and house systems.

  • Forward Fairing: Gives an aerodynamic contour and contains pressure relief doors designed to open if a hot bleed air duct bursts.

Firewalls, Mountings, and Safety Shrouds

• Firewalls: Shrouds used to isolate the engine. They must resist flame penetration for at least $15\,\text{minutes}$ and withstand temperatures up to roughly $1100^{\circ}\text{C}$.

• Material Specifications:

  • Stainless steel sheet ($\sim 0.015\,\text{inch}$ thick).

  • Mild steel sheet.

  • Terne plate (alloy of tin and lead).

  • Monel metal or titanium sheet.

• Sealants: Openings must be sealed with fireproof grommets, bushings, or fittings to prevent hazardous air or fluid passage.

• Mechanical Mountings: Engines are mounted to allow for free Casings expansion longitudinally and radially due to high operating temperatures.

• Engine Types: Turbojets are usually side-mounted or under-slung; turboprops are mounted on tubular frameworks.

• Vibration Isolators: Designed to ensure the engine remains attached even if non-fireproof components deteriorate in a fire.

• Fluid Lines: Fuel lines use rigid tubing, such as stainless steel or Al alloy, because of high turbine temperatures and pressures. Fuel lines must be drained once removed.

Engine Cowlings (Nacelles): Overview and Sequence

• Purpose: Minimize aerodynamic drag, protect internal components, direct air flow, and provide support for fire protection and drainage.

• Accessibility: Engines in pods are generally more accessible than buried engines. Cowlings use quick-release rapid fasteners.

• Primary Components:

  1. Inlet cowl (Air-intake cowl)

  2. Fan cowl

  3. Thrust reverser

  4. Core cowl

  5. Exhaust Nozzle

• Opening Sequence:

  1. Fan cowl must be opened first.

  2. Core cowl or Thrust reverser follows.

Nacelle Component Details: Inlet and Fan Cowls

• Inlet Cowl: Bolted to the forward flange of the engine fan case. It provides thermal anti-icing via bleed air. Anti-icing air is exhausted through pressure relief doors.

• Materials: Outer barrel is aluminum with a composite outer skin (Kevlar-graphite). The inner surface uses honeycomb acoustic panels for noise suppression.

• Acoustic Panels: Constructed from aluminum honeycomb with perforated skin, typically using metal-to-metal bonding rather than mechanical fasteners.

• Fan Cowl: Hinged to the strut to protect the fan case and provide access to accessories. They are made of fire-resistant Al honeycomb bonded to Nomex and Kevlar-graphite. They include pressure relief doors for anti-icing duct failures.

• Support: Hold-open rods and stabilizer rods are released from a stowed position to lock panels in the open position. Both rods must be fully extended to lock.

Nacelle Component Details: Thrust Reverser, Core Cowl, and Exhaust

• Thrust Reverser: Forms a smooth fairing and controls fan exhaust for forward/reverse thrust. It uses a torque box, translating sleeve, drag links, and blocker doors to direct air through cascade segments. These are heavy and require hydraulic power to open.

• Core Cowl: Split into left and right panels to protect the turbine case. They contain pressure relief doors and access ports for oil tank servicing, Integrated Drive Generator (IDG) service, and starter control valve manual override.

• Support: Includes hold-open rods that rotate down to attach to an engine flange bracket.

• Exhaust Nozzle: Known variously as an exhaust plug or sleeve, it directs thrust and is built to withstand extreme temperatures from exiting hot gases.