thumbnail look at history of aircraft construction · 2016-11-25 · thumbnail look at history of...
TRANSCRIPT
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.