By Jon Seal – 20 September 2006
In the hundred years since the Wright brothers’ first flight, aircraft design, materials, and construction have gone through a rapid and radical evolution. Throughout this century of flight, airplane designers have continued their quest for the ideal combination of strength, weight, and carrying capacity.
Their designs quickly progressed from kite like constructions of wood, wire, and linen to sleek, all-metal aircraft of ever-greater size and performance, and on to lightweight, super-strong composite creations that no aviation pioneer could have imagined.
They Called Them “Kites”
From the dawn of modern aviation; kite became a standard slang term for airplane. The reason is obvious: early, underpowered aircraft had to be lightweight and required a lot of wing area. Many of them resembled elaborate box kites, and were made of the same materials: wood and/or bamboo frames covered with tightly stretched and lacquered linen. These pioneering airplanes put a lot more emphasis on increasing lift than on reducing drag. Their primitive engines barely produced enough power to overcome the drag of a thicket of supporting struts, wires, and landing gear, so it was hard to coax their boxy shapes through the air much faster than a horse could run. At the time, the fact that they flew at all was marvellous.
This state of affairs didn’t last long; most new technologies of the early twentieth century evolved rapidly and then explosively during the First World War. In the years before the war, better engines improved the performance of aircraft that remained kite like.
Glenn Curtiss, an American already famous for his powerful V-8 engines (in 1907 he used one of them to power a motorcycle to a world record speed of 136 mph/219 kph), soon turned his attention to flying machines. In 1908, his “June Bug” became the first plane to publicly fly for more than a kilometre in a straight line. The V-8-powered June Bug reached a then-impressive speed of 39 mph/63 kph. The following year Curtiss went to France and won the Gordon Bennett trophy by powering his Rheims Racer to a world-record 47 mph/76 kph.
A Glimpse of Things to Come: Early Streamlined Aircraft
Stressed-skin or mono coque (“single shell”) construction replaces the load-bearing frame and its flimsy outer covering with a thin but rigid shell of wood or metal. This technique achieves superior lightness and strength by using the outer shell-the fuselage itself-to bear the structural load. The Deperdussin’s remarkably streamlined fuselage was created by gluing layers of tulip wood veneer together over a curved frame that was then removed, leaving a strong but light “eggshell” structure without internal bracing. This expensive and time-consuming approach made sense for a purpose-built racer, but not for mass production.
Powered by a 160-hp twin-row rotary engine enclosed in an aerodynamic cowling, this futuristic French monoplane won the Gordon Bennett trophy in 1912 at a speed of 108 mph/174 kph. In 1913, Deperdussin defended its speed title by upping the ante to an astounding 127 mph/204 kph. Comparing this early racer to the 50-mph airplanes Curtiss was delivering to the U.S. Army in the years just before WWI makes it clear just how revolutionary this early racer was.
Compare photos of Deperdussin’s avant-garde 1913 mono coque racers at this Web site: http://www.ctie.monash.edu.au/hargrave/prevost.html
A single glance at each of these airplanes shows how avant-garde the French racer was for its day, but it also underlines how hard it is to bring all the best new features together in a single design. In one way the Curtiss biplane was more advanced than the Deperdussin. The sleek racer relied on the wing warping control scheme originally devised by the Wright brothers, while in the lumbering Curtiss ailerons provided a more precise-and more modern-means of control.
World War I: Standard Practice with Some Interesting Exceptions
Although it would take almost 20 more years for most aircraft manufacturers to abandon traditional kite like construction, the war made it possible to mass-produce airplanes that used new materials and techniques. While no true mono coque designs entered service on either side, a compromise technique called “semi-mono coque” did make it into volume production during WWI and remains the preferred method of aircraft construction to this day. Other new designs experimented with metal tubing to replace the wooden frame, and sheet metal to replace the conventional skin over that frame.
Starting in 1916, the German Albatross Werke designed a series of fighters using semi-mono coque construction, in which thin sheets of plywood were screwed to a relatively light internal wooden frame to make the fuselage a rigid load-bearing structure. The ultimate development in this series was the Albatross D VA, which entered service in May 1917. The strikingly clean appearance of its elliptical semi-mono coque fuselage made it one of the most elegant early fighters, a far cry from the conventionally boxy form of most WWI aircraft.
Trying Metal Instead of Wood
While conventional construction was the general rule for warplanes, WWI did see the beginning of what would later become the modern norm the all-metal aircraft. The German manufacturer Dr. Hugo Junkers pioneered all-metal aircraft during the war, and produced some strikingly modern-looking aircraft just after it.
After some early experiments with thin iron sheeting for exterior panels, Junkers pioneered the use of duraluminum (also called Dural), a light, strong aluminium alloy. During the war Junkers built aircraft in which every assembly-wings, fuselage, and tail-consisted of a Dural tube frame with corrugated Dural sheeting tacked onto it. The designs were conventional, using the metal skin as a covering, not as a load-bearing structure, but their aluminium alloy construction anticipated all-metal aircraft of the 1920s and 1930s.
During 1917 — 18 three all-metal Junkers airplanes entered service in the German air force:
- The J 1 armoured biplane for close observation of enemy troop movements. Though underpowered, the J 1 was one of the most strikingly clean and modern-looking biplanes of the war, with no wires or struts between its metal wings. Visit the following Web site to see a photo of this pioneering biplane: http://www.geocities.com/hjunkers/ju_j4_a1.htm
- The D 1 monoplane fighter, startling in its similarity to what would become the norm for fighters 20 years later, with its single low wing and metal skin. Visit the following Web site to see a photo of this advanced WWI fighter: http://www.geocities.com/hjunkers/ju_j9_a1.htm
- The two-seater CL 1 ground attack monoplane, a two-seat version of the D 1 fighter.
In the wartime rush to mass-produce combat planes, workers who understood all-metal construction were in short supply. Junkers only managed to produce small numbers of these aircraft, but they provided a glimpse of the future of aviation.
With the end of WWI, Junkers explored even more advanced designs. In 1919, within months of the end of the war, Junkers produced the F 13 airliner, a remarkably modern all-metal monoplane that carried four or five passengers in a fully enclosed cabin. By 1929 Junkers had produced over 300 F 13s, and thousands of people in a dozen countries experienced their first flight-and formed their impression of what an airplane should look like-in this impressive early people-hauler. See photos of the revolutionary Junkers F 13 at this Web site: http://www.geocities.com/hjunkers/ju_f13_a1.htm
Aviation Progress between the Wars
Visionaries in the early years of powered aviation had pointed the way to future design and construction techniques. Some of these new approaches, such as mono coque and metal construction, would be perfected as the pace of technological progress accelerated during the interwar years. With aircraft designers combining new materials, construction techniques, engines, and aerodynamic developments in ever-more-imaginative packages, records were routinely set and surpassed in every area of aircraft performance.
One of the biggest developments of the post-WWI years was the rapid development and widespread adoption of the air-cooled radial engine. Without all the plumbing associated with liquid-cooled engines, the radial provided higher power, lower weight, and improved reliability. The penalty of increased drag from the frontal area of the radial’s exposed cylinders remained acceptable as long as getting there was more important than getting there fast. It was radial reliability that got Lindbergh across the Atlantic in 1927; that it took him over 33 hours was immaterial at the time. It took a late-1920s invention-the streamlined NACA cowling-to give radial-powered aircraft speed to match their reliability.
Meanwhile, the inline liquid-cooled engine was far from dead. Increased power and reliability made it a viable choice for designers who wanted their planes to have a smoother, streamlined shape. In the years between the wars, new materials and technologies, including variable-pitch propellers and retractable landing gear, brought out the best in aircraft performance regardless of the type of power plant that turned the propeller.
Even more significant during the interwar years was the push toward cleaner aerodynamics. By 1918, designers were working to minimize the conventional clutter of struts and bracing wires, but to achieve significant performance breakthroughs they needed to deal with aerodynamic drag. P>One of the greatest aircraft of the 1920s was the Lockheed Vega, designed in 1927 by Jack Northrop. In the Vega he produced a very advanced aircraft for the time. Like the Deperdussin racer of 1913, the Vega used a smooth plywood skin. Its semi-mono coque construction gave it strength and a slippery aerodynamic shape, and its cantilevered wing required no supporting struts. In 1929, the improved Vega 5C featured a more powerful radial engine wrapped in an aerodynamic NACA cowling and streamlined “wheel pants” to reduce the drag of the exposed landing gear. The result was a beautiful low-drag airplane that superstar pilots including Amelia Earhart, Wiley Post, and Roscoe Turner flew to numerous speed, distance, and altitude records. The 5C had a top speed of 190 mph/306 kph; flying in the high-altitude jet stream, Wiley Post’s Vega, the Winnie Mae, reached speeds as high as 340 mph/547 kph.
Designed to win the 1934 MacRobertson race (a long-range speed competition from England to Australia), the Comet was a beauty. Its sleek plywood semi-mono coque structure, thin cantilever wing, and streamlined nacelles for the inline, liquid-cooled engines and retractable landing gear made it look every inch the racer. Its gear, flaps, and variable-pitch propellers were innovations at the time, and the gamble paid off. Despite an ambitious project schedule that took the Comet from the drawing board into the air in just nine months, one of the three DH-88s entered won the race (covering over 11,000 miles in just under 71 hours), and another took fourth place.
In its day, the Comet was the most advanced wooden aircraft ever built. Although in general the days of wooden aircraft construction were numbered, the Comet was the ancestor of one of the great fighter-bombers of WWII, the twin-engine DeHavilland DH98 Mosquito, also known as “the Wooden Wonder.”
Metal Replaces Wood
While the sleek, lightweight, wooden DH-88 racer won the MacRobertson race, the planes that took second and third provided a stunning glimpse into aviation’s all-metal future. They weren’t racers-they were brand-new airliners. A Douglas DC-2 flying scheduled passenger service was second, while a Boeing 247 flown by American aviator Roscoe Turner came in third. The Boeing and Douglas airliners weren’t the first all-metal passenger transports-that honour goes to the post-WWI Junkers F 13 described earlier-but they were the first to combine all the features that define a modern airliner into a single package.
The first American airliner worthy of the name was the Ford Tri motor, a big, boxy all-aluminium airplane powered by three Wright Whirlwind radial engines. It evolved over time to accommodate from eight to as many as 13 passengers. Built using the Junkers technique of covering the tubular frame with corrugated duralumin sheets, the Tri motor became widely known as the “Tin Goose.” Almost 200 of these pioneering airliners were built between 1926 and 1932.
With its corrugated skin, exposed radial engines, and fixed landing gear, the Tri motor was only slightly more aerodynamic than a barn door. Its uninsulated fuselage, the noise and vibration from its engines-particularly the one fixed to its nose-and its inability to rise above low-altitude turbulence made the big Ford loud and uncomfortable by modern standards. Despite these drawbacks, the Tin Goose was rugged and provided multiengine reliability. It also gave its passengers a marvellous view of the terrain not far below, and most of them were thrilled with the experience.
In 1929, Transcontinental Air Transport (TAT) inaugurated coast-to-coast service using a combination of the Tri motor by day and rail service at night to cut cross-country travel time in half to a mere 48 hours. Due to its limited range-about 500 miles-the Tri motors flew the transcontinental trip in nine legs. (You can fly the Tri motor over TAT’s route in Microsoft® Flight Simulator 2004). It must have been an ordeal for those wealthy enough to subject themselves to the trip, but by the early 1930s people all over the world embraced the idea of air travel, which was about to become a more comfortable experience for far larger numbers of passengers.
The Ford Tri motor was generally replaced in American airline service during the 1930s by more advanced metal airliners (including the Boeing and Douglas entries that did so well in the 1934 MacRobertson race), but many continued in service for 30 or more years.
The Era of the Modern Metal Airliner Begins
Because the first 60 Boeing 247s were promised to United Airlines, other airlines sought an advanced design to buy. In 1933-34, Douglas countered with its first two commercial aircraft designs, the DC-1 and DC-2. Further refinement produced the immortal DC-3 in 1935. A major breakthrough in the design of these Boeing and Douglas aircraft was their integration of the engines and their cowlings into the wings, a breakthrough in aerodynamic efficiency that set the standard for future aircraft.
The DC-3, which could carry half again as many passengers and had even better performance, defined the shape and appearance of the modern airliner every successful airliner since 1935 has shared a family resemblance with this breakthrough design. Click here to read a Flight Simulator Web site article on the DC-3.
In its first 30 years aviation technology had made giant strides. Aircraft size, weight, power, strength, and speed had increased enormously. With another world war looming, every major nation threw vast resources into the effort to perfect new aircraft technologies. In the process extraordinary design would become commonplace.
WWII: The Extraordinary Becomes Ordinary
War tends to accelerate technology as nations seek military advantage, but in the day-to-day struggle to achieve superiority nations tend to focus their resources less on radical breakthroughs and more on proven techniques. The avant-garde aircraft designs of the 1930s became the basis for larger, faster, and more powerful planes during WWII. While the ultimate aeronautical development of the war was the jet fighter, only a tiny number saw combat. In reality, WWII produced the last word in piston-powered, propeller-driven aircraft.
Like the First World War, WWII was a period of rapid refinement for tried-and-true aircraft design. The same method of construction used so stunningly in the DC-3-a light but strong aluminium frame and a light, load-bearing skin of aluminium sheeting-became the norm for the military aircraft of every warring nation. Fighters, bombers, and transport planes grew in size, speed, and firepower, but the C-47 (the militarized version of the DC-3) remained the most successful transport aircraft of the war. Mass-produced in enormous quantities, Douglas’ mid-’30s design became the unglamorous workhorse seen in every theatre of war. However, its proven construction methods were forged into radical new shapes powered by engines of ever-increasing power and armed with increasingly potent guns and rockets. By 1944, fighters routinely achieved speeds well over 400 mph while the biggest bomber of the era, the Boeing B-29, reached 365 mph/587 kph with an enormous bomb load.
Post war: Bigger Prop liners…
In the late 1940s and early 1950s, airliners continued to grow in size and sophistication, but their all-metal, semi-mono coque construction continued the design revolution of the 1930s. By the mid-’50s, four-engine airliners like the beautiful Lockheed Constellation, the Douglas DC-6/DC-7, and the Boeing 377 Strato cruiser were carrying as many as 100-110 passengers further and faster than ever before. The DC-7 routinely flew non-stop across the U.S. from east to west against prevailing winds. The Stratocruiser, based on the B-29 bomber, carried passengers across the Pacific to Hawaii in half a day, while passenger ships took five days to make the trip.
At the end of WWII, several nations had developed jet aircraft, and small numbers of these advanced designs saw combat. While jet fighters immediately entered post war military service in many air forces, it would take several years for the new technology to find its way into civilian service.
The first generation of jet airliners to enter service-including the ill-fated DeHavilland 106 Comet in 1952, its Comet 4 replacement in 1958, the Boeing 707 in 1958, and the Douglas DC-8 in 1959-looked strikingly different from conventional prop liners. Their jet engines and lack of propellers were a dramatic and obvious difference, and their sleek, swept-wing forms were intriguingly modern. They looked fast even when they were standing still. The public got an eyeful of the new Boeing 707-and a sense of the big jet’s durability-when test pilot “Tex” Johnston barrel-rolled the prototype over the Seattle hydroplane race course in 1955. By 1960, jet airliners were whisking 200 passengers at a time across oceans and continents at speeds approaching 600 mph/965 kph. Air travel was quickly transformed from an exotic experience for the few to a worldwide form of mass transit.
Even in this brave new era of air travel, one major design trait remained the same for prop and jet airliners: they both used an aluminium frame covered with a stressed skin of aluminium sheeting that followed every sleek curve. Despite the higher speeds and stresses of jet-powered flight, materials and practices developed in the ’30s and ’40s remained more than adequate to meet these new challenges. But one new kind of material made its first appearance in early jet airliners: composites.
Composites: Greater Strength and Lightness-At a Price
Composite materials consist of a resin matrix reinforced with fibres. Modern composites offer outstanding strength and lightness, but they are difficult and expensive to make, machine, and repair. The first composite material, fiberglass, came into use in boats, cars, and aircraft in the 1950s. Some of this new material found its way into the Boeing 707, but the aviation industry had to explore this new technology and refine techniques to get better composites in greater quantity into production aircraft.
In the closing decades of the twentieth century, boron or carbon fibres embedded in epoxy and other resin matrices produced ever-stronger composites, but these fabulously strong and lightweight materials are ever more costly, and some require careful maintenance to keep the composite surfaces from absorbing moisture.
Cost and maintenance considerations ensure that military aircraft use composite materials in greater quantity than products for the civil aviation market. The tail of the F-14 Tomcat fighter, which entered service in 1972, featured composite horizontal stabilizers. More recent advanced fighters, including the AV-8B Harrier and the upcoming F-22 Raptor, use composites for 25 to 33 percent of their structures.
In 1958 perhaps two percent of the Boeing 707’s structure was the original composite, fiberglass. Ten years later, the first Boeing 747 used some more advanced composites, but they accounted for just one percent of the structure. But in recent years there has been a steady if slow growth of composite content in civilian aircraft. For example, ten percent of the Boeing 777’s structure consists of composite materials. As the worldwide push toward more economical operation of passenger fleets continues, these modern materials will play an increased role in aircraft construction.
A Composite Aircraft Ahead of Its Time
While composites are still a distinct minority in the arsenal of aircraft structural materials, some of the most beautiful-and the most bizarre-aircraft of recent years have relied heavily on these promising but problematic materials.
In the 1980s, aircraft designer Burt Rutan designed the Beech Model 2000 Starship, a radical all-composite tailless twin-turboprop with variable-sweep canards on its nose. FAA certification decreased its passenger load from eight to six and added a performance-damping ton to its weight. It remained a dazzling and futuristic aircraft, but a very expensive one at five million dollars each. Although only 53 examples were produced between 1988 and 1992, it remains one of the sleekest propeller-driven planes ever produced. Click here to see the Starship on the Centennial of Flight Web site.
Twice Around the Globe…
Another all-composite Burt Rutan design made aviation history during the 1980s. His Voyager became the first plane to fly non-stop around the world without refuelling. Their unique combination of lightness and phenomenal strength made composites the ideal material for this bold venture. The Voyager’s composite airframe weighed less than 1,000 pounds, and the entire aircraft weighed just 2,250 pounds. But the Voyager’s gross weight on take off was almost 9,700 pounds, including more than 7,000 pounds of fuel and a crew of two. It was just 29 feet, 2 inches long, with an engine on each end of its slender fuselage, but its wings spanned an impressive 110 ft, 8 inches. In 1986, Dick Rutan (brother of the designer) and Jeana Yeager flew the Voyager 25,000 miles to circle the earth in nine days. Click here to see this historic aircraft, which now hangs in the National Air and Space Museum in Washington, D.C.
In March, 2005, 19 years after the Voyager flight, pilot Steve Fossett completed the first non-stop solo around-the-world flight without refuelling in a new Rutan design, the GlobalFlyer. It is bigger, heavier, and faster than Voyager, but it’ s still an extremely light and very strong structure. Its empty weight of 3,350 pounds ballooned to 22,000 pounds with its full load of fuel (plus pilot and a few diet milkshakes). Its 114-foot wingspan carried plane and pilot around the globe in just 67 hours. Click here to learn more about this remarkable jet-powered composite aircraft.
…and to the Edge of Space
The demands of space flight require extremely strong structures, and every pound of structure means a pound less of payload. In this context composites are vital. The Space Shuttle (officially NASA’s Space Transportation System), which first flew in 1981, uses composites extensively in its external fuel tank, in parts of its fuselage and doors, and in the heat-resistant ceramic tiles that line its exterior.
In the years since the Starship and the Voyager were designed, composites have become a more mainstream material, especially in military aircraft, and in applications, including space flight, where performance outweighs cost. Another recent and dramatic example of what composite aircraft can do is the flight into space of Burt Rutan’s SpaceShipOne. In October 2004, this rocket-powered plane won the Ansari X Prize by twice attaining an altitude greater than 100 kilometers or 62 miles. The composite space vehicle was borne aloft and then released by Rutan’s twin-jet powered White Knight, a wildly futuristic-looking composite aircraft. The chase plane for this mission was, appropriately, a Beech Starship. Rutan and his company, Scaled Composites, hope to take paying passengers into space within a few years. Click here to see these remarkable aircraft on the Scaled Composites site.
Aviation’s Next Hundred Years
In a single century aircraft construction has progressed from flimsy powered kites that could barely hop off the ground to futuristic vehicles that can circle the earth or fly to the edge of space. The same goals have always obsessed aircraft pioneers-to build planes that can go further, faster, and higher. Today we take for granted the ability to fly thousands of miles in a single flight aboard airplanes that routinely approach the speed of sound while carrying hundreds of passengers. To readers in 1904 all of this would have sounded preposterous, a Jules Verne fantasy. We have seen the swift and steady progress in aircraft construction from wood to metal to composites, and this evolution is far from over. With new materials and techniques, who knows what the next big thing in aircraft design and construction will be? With the rapid evolution we have already seen, who can predict what aircraft will be capable of 100 years from now?