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Aircraft Structure: Fuselage

In this lesson, we will discuss the definition, configuration, and types of aircraft fuselage.

The fuselage is the main body of an aircraft where other primary parts such as the wings, empennage, landing gear, and the powerplant is stored or connected.

It receives loads and stress that it must withstand. 

Types of Fuselage Constructions

Truss Type

Aircraft fuselage construction truss type
  • You can easily determine an aircraft with a truss or framework type if they have tubular steels as longerons and diagonal/vertical webs for support.
  • The framework is made of light steel tubes formed together to have a rigid geometric form structure. Truss type structure composed of slender members joined together at their end points, in which the members are subjected to only axial loads
  • For light non-pressurized aircraft.
  • Advantages of a Truss Type Fuselage
    • Cost-effective
    • Lightweight
    • Easy to construct
    • Strong structural frame
    • Rigid geometric structure
    • Simple design and basic structure
  • Disadvantages of a Truss Type Fuselage
    • No stiffeners
    • Lack of additional supports such as the streamline shape
    • Not effective enough to resist major stress loads
  • Example of Aircraft with Truss Type Fuselage
    • Boeing Model 80A
      • Wing: Bi-plane wooden construction
      • Fuselage: Truss Type
      • Powerplant: 525 hp Pratt and Whitney “Hornet” air-cooled radial engine
      • Empennage: Conventional type
    • Curtiss JN-4D
      • Wing: Bi-plane wooden construction
      • Fuselage: Truss Type
      • Powerplant: Hispano-Suiza 8 engine
      • Empennage: Conventional Type
      • Landing gear: Bicycle type
    • Wright 1903 Flyer
      • Wing: Bi-plane
      • Fuselage: Truss Type
      • Powerplant: Straight-4, water-cooled; piston

Monocoque Structure

  • Most used in modern aircraft than the Truss Type
  • All loads are taken by the skin
  • It has formers and bulkheads to shape the fuselage
  • Advantages of Monocoque Structure
    • Rigid structure
    • Strength
    • Can carry concentrated loads
    • Provides great fitting for the wings
    • Has secondary load carriers
  • Disadvantages of Monocoque Structure
    • Low strength-to-weight ratio
    • Heavier than the truss type
    • Minor damage can bring a big impact and damage the skin
    • Not strong enough for modern airliners needs
    • Requires frequent maintenance
  • Example of Aircraft with Monocoque Structure Fuselage
    • Zeppelin D-1
      • Wing: Bi-plane
      • Fuselage: Monocoque construction
      • Powerplant: 1 x BUW Illa water-cooled 6-cylinder engine
      • Landing gear: Land-based bicycle
    • LFG Holand C-11
      • Wing: Bi-plane
      • Fuselage: Monocoque construction
      • Powerplant: 160 hp Daimler-Mercedes D-III
      • Landing gear: Land-based bicycle
    • Rohrbach Ro IV
      • Wing: High-wing; cantilever; monoplane
      • Fuselage: Monocoque construction
      • Powerplant: 2x Napier Lion V W 12 engine
      • Landing gear: Seaplane Flying Boat

Semi-Monocoque Structure

semi monocoque Aircraft fuselage construction design
  • Most used structure
  • Parts of a Semi-monocoque Structure
    • Longerons – Main structural member
    • Stringers of stiffeners – These are additional structural members that helps the longerons to distribute the load impact. It helps the sheet materials to carry the load and helps to stiffen the skin.
    • Formers – Reinforces the skin to maintain the aircraft’s circumferential shape.
  • Also known as “Stressed Skin”
  • Advantages of Semi-monocoque Structure
    • Rigid and strong streamlined construction
    • Can withstand normal aircraft damage
    • Has secondary structure to assist carrying the loads
    • Better strength-to-weight ratio than monocoque structure
    • Longerons help the skin to withstand the primary being loads
  • Disadvantages of Semi-monocoque Structure
    • Difficult to construct
    • More expensive
    • Time-consuming maintenance
    • Its skin can’t support all the major loads
  • Example of Aircraft with of Semi-monocoque Structure
    • Bonanza G36
      • Wing: Mid-wing tapered wing
      • Fuselage: Semi-monocoque construction
      • Powerplant: 1 x continental motors corporation model 10-550-B
      • Empennage: Standard Tail Type/Conventional
      • Landing gear: Retractable tricycle
    • King Air 350i
      • Wing: Mid-wing tapered wing
      • Fuselage: Semi-monocoque construction
      • Powerplant: PT6A60A turboprop engine
      • Empennage: T-tail
      • Landing gear: Retractable tricycle
    • Cessna 150
      • Wing: High-wing braced monoplane
      • Fuselage: Semi-monocoque construction
      • Powerplant: A152 Aerobat Lycoming 0-235-N2C Single turboprop engine
      • Empennage: Conventional standard tail
  • Landing gear: Take-off trolley tricycle gear
    There are three main loads that are being experienced by the aircraft:
    • Tension/tensile Load – It stretches the aircraft materials. It resists a force that tends to pull the materials apart.
    • Compression Load – It compresses the aircraft materials
    • Shear Load – It forces one material to slide over another material
Other types of loads and stress:
  • Repeated loads – Will result in aircraft fatigue
  • Torsion stress – Developed when material experiences twisting forces. It is a transverse load that makes the structural member rotate about an axis away from where the load is applied.

Note that stress is the load per unit area acting on a material.

Design Limit Load (DDL)

Engineers use this concept to determine the maximum design load that the aircraft is expected to experience when operated.

Public transport2.5G
Utility3.4G to 3.8G

Limit loads – it is the maximum expected load in the entire service life span of an aircraft.

Design Ultimate Load (DUL)

DUL is multiplied by the safety factor of an aircraft. Most airplanes have a safety factor of 1.5. The design ultimate load is what the aircraft must withstand without experiencing any damage.

Safe Life

Just like the name suggest, it is the “life” span where the aircraft is “safe” to operate or before having major structural fatigue.

Fail Safe

Fail safe ensures safety by having a redundant structure to retain its design residual strength for a period of time after a failure has occurred of the principal structural element.