Types of Aircraft Construction

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When an aircraft is in flight, loads are placed on its surfaces. An aircraft must be strong enough to resist the loads, but should also be lightweight to improve the aircraft's efficiency and maintain its aerodynamic capabilities during flight. Each type of structure and material has both advantages and disadvantages. Aircraft designers continue to search for the perfect material and structure that is strong, malleable, and lightweight.


Why is Structure Important?

Truss structure is the earliest form of aircraft structure. It consists of lengths of tubing called longerons, which are welded together to form a sturdy framework. Vertical and horizontal struts (also called web members) are welded to the longerons and give the structure added strength and to help define it's shape. Additional struts are used to resist stress from all directions. Stringers and bulkheads or formers are added to shape the fuselage and support the covering. As technology progressed, aircraft designers began to streamline the airplane to improve performance; they first used fabric to accomplish this, but eventually led to the use of lightweight materials like aluminum.


**Advantage of Truss:** Strong construction

**Disadvantage of Truss:** Weak aerodynamic capabilities

Truss Structure

Monocoque construction uses stressed skin to support almost all loads, similar to an aluminum beverage can. This structure helps to reduce the overall weight of an aircraft due to the reduction, or sometimes elimination, of an internal open framework. The earliest form of monocoque construction was employed in 1918 by Jack Northrop who used it in a Lockheed S-1 Racer. Although strong, this structure was not very tolerant to deformation of the surface. For example, an aluminum beverage can support considerable forces at the ends of the can, but when there are loads placed on the sides of the can, it may slightly deform or even cause complete collapse of the structure.


**Advantage of Monocoque:** Lightweight

**Disadvantage of Monocoque:** Deformation

Monocoque Construction

Similar to the monocoque construction, semimonocoque construction is a substructure which the airplane's skin is attached. Designers use bulkheads and/or formers, and stringers to reinforce the stressed skin to reduce deformation. This reduces the overall loads placed on the fuselage.


**Advantage of Semimonocoque:** Reduction of Deformation

**Disadvantage of Semimonocoque:** Heavy

Semimonocoque Construction

Most modern aircraft use a composite construction, which consists of fiber-reinforced and matrix systems. Composite construction can be found as early as World War II when the B-29 aircraft first used it as insulation for its fuselages. Of the many materials available, fiberglass and carbon fiber is the most commonly used reinforcing fibers, while epoxy resin is the most commonly used matrix.

Fiberglass has good tensile strength, compressive strength, and strong impact resistance. However, it is fairly heavy for an aircraft structure.

Carbon fiber has stronger tensile and compressive strength than fiberglass, higher bending stiffness, and is lighter than lighter than fiberglass. However, it has a poor impact resistance. This may be overcome with a stronger epoxy resin system.

**Advantages of Composites:**
  - Smooth skins and easy to form streamlined structures
  - Lightweight
  - Lack of corrosion
  - No fatigue or cracks from performing in a flexing environment

**Disadvantages of Composites:**
  - Lack of visual damage inspection
  - Potential of heat damage
  - Chemical reactions may occur if fuel, oil, or other chemicals get in contact with composite
  - Poor conductor of electricity and will may not distribute energy from lighting strike effectively if additional material (aluminum, copper mesh, etc.) is not added to composite skin

Overall composites are a more preferable method of aircraft structure design because of its flexibility and corrosion resistance, as well as its strong strength-to-weight ratios. With the continued improvement in technology, composites will continue to provide aircraft designers with a reliable material capable of much more than what wood, aluminum, or fabric.


Composite Construction

FAA-H-8083-25B: Pilot's Handbook of Aeronautical Knowledge, pg. 3-8 - 3-12

Familiarizing yourself with your airplane's construction and load capabilities can help you better understand its flight characteristics.

Types of Aircraft Construction

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