Thumbnail Look At History of Aircraft Construction

Aviation Week

Graham Warwick

Aircraft may not seem to have changed much in the past few decades, but within the last

half-century advances in how they are manufactured have been as great as the evolution in

aircraft production over the first 50 years of aviation. And the technology hidden inside

continues to evolve as industry drives down costs.

Wood and Wire

The wood frame, wire bracing and fabric skin of the Wright 1903 Flyer set the mold for aircraft

structures in the first decades of powered flight. Weight was the concern and wood the only

light material that was strong enough. It was available and affordable, easy to work, resilient

and repairable. The Flyer’s long wing spars were made from spruce, and the airfoil-shaped ribs

from ash. Cotton fabric was applied on a bias to impart strength and sealed with canvas paint.

But wood-and-fabric structures were not that strong, so the wire-braced, strut-supported

biplane became the first “standard” configuration.

Shells and Beams

Credit: Mikael Restoux/Wikipedia

Wood remained the principal material, but structures rapidly evolved through World War I.

Cantilevered wings with thicker airfoils and stronger box spars eliminated draggy bracing

wires, notably with the Fokker Dr1 triplane in 1918. The 1912 Deperdussin racer (pictured)

introduced the monocoque fuselage, formed of thin plywood layers over a circular frame—light,

strong and streamlined. After the war, the Loughead (later Lockheed) brothers and Jack

Northrop developed a method of forming fuselage half shells from laminated spruce in concrete

molds and produced the Vega, Orion and other successful monoplanes.

Lasting Combination

Credit: Wikipedia

Compared with the fabric-covered box-girder structure prevalent at the onset of World War I,

the monocoque fuselage was lighter and offered lower drag, but was more costly to

manufacture and harder to repair. This led to semi-monocoque construction, used in German

Albatros fighters, for which load-bearing plywood skin panels were glued to longitudinal

longerons and internal bulkheads. As metal replaced wood after the war, the term semi-

monocoque gave way to stressed skin, but to this day—along with the cantilevered wing—it

remains the prevalent aircraft structural configuration.

Metal Guru

Credit: Wikipedia

Flown in December 1915, Hugo Junker’s J1 (pictured) was revolutionary—an all-metal,

cantilever-wing, stressed-skin monoplane. Wood was light, but it deteriorated. The J1 was

made of steel; a welded fuselage frame covered with thin sheet and wing skins internally

reinforced by panels with spanwise corrugations. But steel is heavy, and development of the

lightweight aluminum alloy Duralumin by German metallurgist Alfred Wilm led in 1919 to the

Junkers F13, the first all-metal transport aircraft. Interwar aircraft such as the Ford Trimotor

and Junkers Ju52 also used corrugated skins for strength, but these increased drag.

Riveting Development

Credit: Wikipedia

The first riveted aluminum structure—standard for aircraft manufacturing for decades—was

Hall Aluminum Aircraft Co.’s XFH naval fighter prototype (pictured) flown in 1929. Shortly

after, Hall built perhaps the first flush-riveted aircraft, the PH-1 flying boat. Flush riveting and

butt joints between skin panels, rather than the dome rivets and lap joints then used, reduced

drag and were famously featured in the Hughes H-1 racer, the streamlined, all-metal,

retractable-gear monoplane in which Howard Hughes in 1935 set a world landplane speed

record of 352 mph.

Bonding Experience

Credit: Wikipedia/Crown Copyright

By the 1930s, riveted sheet-metal construction dominated aircraft manufacture, but scarcity of

aluminum in World War II brought a resurgence in wood—notably with the de Havilland

Mosquito and Hornet (pictured). Plywood facings, bonded to a balsawood core and formed

using molds, produced monocoque structures. Experience led de Havilland to use metal-to-

metal bonding in the Comet jet airliner. This and other U.K. post-war aircraft used Redux, a

strong and durable adhesive invented in 1942. In the 1950s and ’60s, Fokker made extensive

use of metal bonding in the F27 turboprop and F28 twinjet airliners.

Assembly Fixture

Through the 1940s, aircraft were assembled by building skeletal frameworks and attaching the

skins. Then Britain’s Fairey Aviation developed the technique of building an aircraft as a series

of subassemblies in jigs. Aviation Week noted in January 1950, Fairey’s method involved

installing the skin in a jig that provided accurate, repeatable part location, then attaching the

substructure as individual parts or as a subassembly produced in another fixture. Used to build

the Fairey Gannett carrier-based antisubmarine aircraft, the technique became an industry

standard.

Steel Magnificence

Credit: Wikipedia/san diego aerospace museum archive

In 1931, in a bid to expand, U.S. railcar maker Budd Co. built the BB-1 Pioneer flying boat

(pictured) from corrosion-resistant stainless steel using newly developed spot welding. Budd

tried again in 1943 with the RB-1 Conestoga cargo aircraft, but steel is heavy and proved to be

unpopular for airframes. Then, in the early 1960s, Russian design bureau Mikoyan-Gurevich

used welded nickel steel for the airframe of the Mach 2.8 MiG-25, because heat-resistant

titanium was difficult to work with and hard to weld. Steel continues to be used for high-

strength parts, making up 7-10% of materials used in the Airbus A350 and Boeing 787.

Welcome to the Machine

Credit: MAG

One of the biggest “unseen” advances in aircraft manufacturing was the development of

numerical control (NC) machining. This enabled complex structures such as bulkheads and

integrally stiffened wing skins to be cut from solid blocks of alloy, rather than assembled from

sheet metal—improving quality, reducing weight and saving time and cost. Conceived in 1942

by machinist John Parsons, NC machining was slow to catch on with manufacturers, until in the

1950s the U.S. Army purchased 120 machines and leased them to industry. Today five-axis

high-speed precision machining is standard for metal structures.

Titanium, the First Time

Credit: NASA

Titanium’s low weight, high strength and heat resistance made it ideal for high-speed aircraft

of the 1950s and ’60s. The first titanium aircraft was the Douglas X-3 Stiletto (pictured), flown

in 1952. Designed to cruise at Mach 2, where skin friction required the heat resistance of

titanium, the X-3 was underpowered and barely supersonic in a dive. Capable of Mach 3.2,

Lockheed’s A-12 and SR-71 were also mainly titanium, and the material was to be used for the

canceled Boeing 2707 supersonic transport, designed to cruise at Mach 2.7—faster and hotter

than the conventionally constructed Concorde.

First Composites

Credit: National Air & Space Musuem

Wood is a natural composite, but fiber-reinforced polymer composites were introduced into

aviation in the 1940s, beginning with glass-fiber radomes. Post-war, glass-fiber and later

damage-resistant Kevlar composites were increasingly used in cabin interiors and secondary

structures, as well as in helicopter rotor blades and rocket motor cases. In 1969, the

Windecker Eagle (pictured) became the first all-composite aircraft to receive FAAcertification,

using a flexible, nonwoven glass-fiber material, “Fibaloy,” developed by Dow Chemical. Carbon

replaced glass as the reinforcing fiber of choice.

Hidden Honeycomb

Credit: NASA

As well as building the first all-metal airplane, Hugo Junkers was also first to propose a hidden

but key element of many aircraft structures—the honeycomb. Laminating thin face sheets to a

stabilizing honeycomb core produces a composite sandwich that is light but strong. Aero

Research Ltd. in 1938 developed a way to adhesively bond aluminum honeycomb, and the

North American XB-70 (pictured) used brazed stainless-steel honeycomb panels. But the real

breakthrough came with development of fire-resistant Nomex honeycomb, extensively used in

interior panels and structural carbon-fiber honeycomb.

Aided by Computer

Credit: U.S. Air Force

First developed in the 1960s, 3-D computer-aided design (CAD) has become the backbone of

the aerospace industry, anchoring product life-cycle management systems that are the “digital

thread” stitching programs together. McDonnell Aircraft began using computers to help lay out

designs in 1959 and went on to develop the Unigraphics CAD system, now owned by Siemens.

Lockheed developed Cadam, later sold to IBM and then DassaultSystems, which developed

Catia in the late 1970s. Boeing selected Catia in 1984 and says its 777 was the first aircraft to

be designed entirely on computer.

Carbon Beginnings

Credit: Rolls-Royce

High-performance carbon fibers were first created from rayon in 1958 at Union Carbide,

followed by an improved fiber developed in Japan using polyacrylonitrile, or PAN, the raw

material used today. In 1963, the U.K.’s Royal Aircraft Farnborough developed high-strength

carbon fiber, Hyfil, which was licensed to Rolls-Royce which used the lightweight material in

the fan blades of the RB.211 high-bypass turbofan (pictured) powering Lockheed’s L-1011

TriStar. In 1970, the composite fan failed birdstrike testing, forcing a switch to titanium and

extra costs that pushed Rolls into receivership.

Black Aluminum

While the U.K. developed carbon fiber, the U.S. pursued boron fiber, which was stronger and

stiffer. Boron-fiber composites were used in the horizontal stabilizer of the Grumman F-14 and

horizontal and vertical tails of the Boeing F-15. But boron fiber was expensive, and the U.S.

moved to carbon-fiber composite for wing skins on Boeing AV-8B, F/A-18and Northrop B-2 and

the airframe of the Bell Boeing V-22 tiltorotor. These first-generation carbon structures were

labeled “black aluminum” as their designs were carried over from metallic airframes and did

not make full use of carbon fiber’s benefits.

Fiber Metal Laminates

Credit: Fokker

Between aluminum and carbon fiber is a family of materials, fiber metal laminates, that has

found limited but important applications in aircraft. Fatigue concerns with aluminum led in the

late 1970s to development of an aramid-fiber-reinforced aluminum laminate, Arall, by TU Delft

and Alcoa. But flat-sheet Arall had cost and manufacturing issues. This led to a second-

generation glass-fiber-reinforced aluminum laminate, Glare, which is resistant to fatigue,

impact damage, lightning strikes and fire burn-through. Suitable for double-curved panels,

high-strength Glare is used in the Airbus A380fuselage (pictured).

Carbon Comes of Age

Credit: Bombardier

The lightness, stiffness and corrosion resistance of carbon-fiber composites led to their use for

50% or more of the structural weight of the Boeing 787 and Airbus A350. But carbon-fiber

airframes imposed a return to costly manual layup and assembly. The result is a move to more

integrated structures to reduce parts count and more automation to drive down costs. The

early-2000s Beechcraft Premier and Hawker 4000 had filament-wound fuselages, but

automated fiber placement became the industry standard, coupled with new nondestructive

inspection techniques.

Titanium, Again

Credit: Lockheed Martin

The galvanic corrosion that occurs when aluminum is in contact with carbon fiber has led to

resurgence in the use of another lightweight metal—titanium. While composites have grown to

more than 50% of structure weight in the A350 and 787, titanium content has more than

doubled to 14% since the A320 and 737. But titanium is expensive and difficult to machine,

which is driving a move toward near-net-shape production processes such as additive

manufacturing and linear friction welding that can minimize waste and reduce the “buy-to-fly”

weight ratio between raw material and finished part.

Aluminum Revival

Credit: Bombardier

Reports of aluminum’s death at the hands of carbon fiber are exaggerated, but largely because

the metal industry responded to the threat. Its weapon is aluminum-lithium (Al-Li) alloys 4-6%

lighter and 5-7% stiffer than conventional aluminum. Al-Li was first used in the 1950s, in wing

and tail skins on the North American A-5 Vigilante, but had performance and corrosion issues.

A second generation found use in helicopters in the 1980s; the third generation unlocked

performance benefits of Al-Li, leading to significant use on the Bombardier C Series, A350 and

787.