Variations in the Airfoil Trace the History of Flight
Wings have always captured human imagination. The mythology of flight is found in every culture. Despite this fascination, it was not until the nineteenth century that scientists began to use precise mathematics to compute the optimum size and shape of wings for a flying machine.
Orville and Wilbur Wright did it best with their 1903 Flyer, forcing competitors to try wings of all shapes, styles, and dimensions to avoid infringing on their patents. Some went to multiple wings--triplanes, quadraplanes, and more. Others altered the shape of wings to sweptback, tandem, joined, and cruciform.
Most of the results were too inefficient to fly; some were capable of generating just enough lift to stagger through the air if coupled with a sufficiently powerful engine, and a very few were both stable and efficient.
Some concepts were diametrically opposed--very low aspect ratio (the ratio of span to chord) vs. high aspect ratio, or a pure wing form vs. a lifting body--yet success was sometimes found at either end of the spectrum.
From the 1920s through the 1940s, advances in aeronautical engineering resulted in much stronger, more complex wings using now familiar high-lift devices and modern airfoils. Nonetheless, variations in span, incidence, and geometry persisted. For some, the ultimate goal became the elimination of all surfaces except the wing, or the elimination of all or part of the wing.
Since the late 1940s, aerodynamic progress has accelerated at an ever greater rate, so much so that modern engineering methods and materials have combined with new requirements to create totally new wing configurations. Now, elaborate high-lift devices are tucked into wing leading and trailing edges to deploy during the approach to landing, with the slats and flaps folding out like handkerchiefs from a magician's sleeve.
Some by-products have become perhaps too sophisticated. Where the thick wing of a Douglas C-47 "Gooney Bird" would let you plow through cold, wet clouds forever, shaking off the ice buildup with pneumatic boots, some modern airfoils--as on the Aerospatiale/Alenia ATR-42--have become so efficient that even a small buildup of ice becomes a deadly hazard.
On the other hand, the increased sophistication has occasionally permitted a return to some of the ideas put forward by earlier inventors but not realized at the time for technical, mechanical, or even political, reasons. Thus, the unsuccessful tandem wing design of Samuel Pierpont Langley was reprised through the years, first by the French Albessard "Tri-avion" and Arsenal-Delanne 10 fighter, and most recently by Burt Rutan with his Advanced Technology Tactical Transport.
In a similar way, the greatest comeback has been that of the flying wing, well expressed by the Wrights in their 1901 glider and found now on the flight line at Whiteman AFB, Mo., in the superlative form of the Northrop Grumman B-2 bomber.
The Wrights went on to attach elevators and rudders but maintained their strongly braced biplane wings. This combination of wings was a masterpiece of design, with a balance of span, chord, and gap that was imitated by myriad other designers. Coupled with their insight into the need for three-axis control, the Wrights set the pattern for most other inventors of the time, few of whom were deterred by the brothers' patents.
Some, such as Glenn H. Curtiss, used a similar biplane layout, employing ailerons in an attempt to circumvent the patents. Other inventors depended on their intuition, their aesthetic sense, or their fascination with complex mechanical solutions to approach flight in a way they hoped differed from the Wrights' method.
Wilbur Wright's triumphant exhibition at Le Mans, France, in 1908 opened the floodgates of European imagination and turned loose an outpouring of innovative designs. Although most of these were failures, many of them forecast future trends.
The low aspect ratio found in the Lockheed Martin F-117 Nighthawk stealth fighter or the older Convair F-102 Delta Dagger and F-106 Delta Dart interceptors was anticipated by many aircraft, beginning with the Flick-Reinig "Apteroid" of 1911, whose biplane wings ran fore and aft along the fuselage rather than perpendicular to it, as if it had been packaged for shipment by railcar.
Many low-aspect-ratio airplanes followed, including the McConnick Romme "umbrella plane" of 1912. Designed by the young Chance Vought, it had a circular wing absolutely devoid of camber and in appearance was no more than a set of loosely connected awnings. When a rip-roaring fifty-horsepower Gnome-Rhône rotary engine was installed, however, the "doughnut," as it was called, not only managed to get airborne but made controlled flights around its home field at Cicero, Ill.
In later years, there were dozens of attempts to obtain the high lift believed to be inherent in low-aspect-ratio aircraft. Some of the most successful of these were designed by Charles H. Zimmerman, who enhanced the low-aspect-ratio concept by directing the airflow from very large propellers over the entire wing surface in the 1942 Vought V-173 "Flying Pancake."
The V-173 was flown successfully by Boone T. Guyton, Charles A. Lindbergh, and Najeeb E. Halaby, among others, and was developed into the wicked-looking Vought XF5U-1, a circular-planform Navy fighter. The XF5U-1, too radical and made obsolete by the jet engine, was dismantled before its first flight.
Low-aspect-ratio wings found their ultimate expression in the delta-wing designs that flowed from the genius of Dr. Alexander M. Lippisch, whose first delta-wing aircraft flew in 1931. He followed with a series of innovative designs, most notably the world's first delta-wing, rocket-powered fighter--the Messerschmitt Me-163 Komet. After World War II, the delta-wing layout served many aircraft well, including the beautiful Convair B-58 Hustler, the first supersonic bomber. Foreign manufacturers who adopted the delta configuration include Dassault, Avro, Fairey, Saab, Tupelov, and the MiG Design Bureau.
Success was easier at the other end of the aspect-ratio spectrum. High-aspect-ratio wings were undeniably efficient and were widely used by sailplanes. The French manufacturer Hurel-Dubois carried the idea a step further with its extremely high-aspect-ratio, strut-braced-wing aircraft of the late 1940s. The idea lapsed for years, only to be revived by the successful Short Brothers transports, such as USAF's C-23 Sherpa.
By the 1930s, while most of the world's aeronautical engineers struggled toward a common denominator of the cantilever low-wing all-metal aircraft, some designers persisted in pressing for unorthodox solutions to specific problems.
The concept of variable-span wings was tried in the 1931 monoplane designed in France by Mikhail Makhonine, a Russian engineer. The handsome aircraft featured extensible outer wing panels that could vary the wingspan from forty-three feet to sixty-nine feet and the wing area from 226 to 335 square feet. The greater wingspan allowed for takeoff with greater loads. At altitude, the wings retracted for more speed.
Other inventors sought safety with their unorthodox designs. In 1931, Albert A. Merrill designed a stall-proof biplane. That same year, George W. Cornelius created his first variable-angle-of-incidence aircraft and followed it a few years later with his "Mallard," which had both variable incidence and forward-swept wings. The practical success of variable incidence came in 1955 with the debut of the Vought (later LTV) F8U Crusader, whose object was not avoiding a stall but getting off a carrier deck.
The Germans led the way in variable-geometry wings with the Messerschmitt P-1101 jet prototype. It never flew but was to have had ground-adjustable wing sweep for comparative flight tests. Bell adapted the design in 1951 with the X-5, whose wings could be swept from 20 to 60, making it the first high-performance aircraft to fly with a variable-geometry wing.
Grumman experimented with variable-geometry wings in its unsuccessful XF10F-1 Jaguar of 1952. The principle of the swing wing served its successor, the F-14 Tomcat, well, as it did a number of US and foreign aircraft, including the US F-111 and B-1, the Soviet MiG-23 and Su-24, and the European consortium Panavia's Tornado.
Fixed wing sweep had been built into dozens of aircraft since the earliest days of flight, often as a solution to center-of-gravity problems. Sweep designed to raise the limiting Mach number had been a subject of study since the early 1930s but appeared quite by accident on an early operational jet fighter, the Messerschmitt Me-262, first flown July 18, 1942. The Me-262 had been originally designed as a straight-wing aircraft, but the need to compensate for engine growth and changes in the center of gravity caused the designer to sweep the wings, with the accidental aerodynamic benefit of increasing the aircraft's critical Mach number.
Forward-swept wings appeared as early as 1906 on Alberto Santos-Dumont's Number 14 bis, which made the first official powered aircraft flight in Europe. Later, Cornelius designed a series of aircraft with forward-swept wings, one of them a glider/tanker.
The first jet aircraft to fly with forward-swept wings was the 1944 prototype of the six-engine Junkers Ju-287 bomber. Forward-swept wings were deemed to have the advantage of increasing the limiting Mach number, while transferring adverse characteristics of swept wings from the low- to the high-speed regime, where they were easier to handle.
The first successful commercial application came with the postwar Hansa executive jet (from the same design team that produced the Ju-287), while the most prominent modern use has been in the very advanced Grumman X-29.
The pure flying wing, unencumbered by any vertical surfaces, was the goal of many designers, but others sought to simply rid their designs of the weight and drag penalties of a rear fuselage and tail surfaces. The very first of these was attributed to a Wright test pilot, Eugène Lefebvre--the first pilot of a powered aircraft to be killed in an aircraft accident, on September 7, 1909.
The design concept went through a long series of permutations by a wide range of manufacturers, including Blériot, Granville Brothers, Westland Aircraft Works, and Focke-Wulf, but achieved its greatest success in the variations of Burt Rutan's sleek composite Long-EZ design.
Several combatant nations created tailless prototypes during World War II, when the goal was not inherent stability but greater speed via elimination of slipstream drag, improved visibility, and concentration of firepower in a central nacelle. First to score was Italy's handsome Ambrosini S.S.4 interceptor of 1941, which was fast and flew well but was abandoned after a crash due to engine failure.
In 1943, Curtiss flew the first of three XP-55 Ascenders. The XP-55 had appalling stall characteristics and only modest performance. The Ascenders were stellar aircraft, however, compared to another 1943 tailless entry, the all-magnesium Northrop XP-56 Black Bullet. Two XP-56s were built, and one managed to crash while taxiing.
A desperate Japan threw a hat into the ring in 1945, producing the Kyushu J7WI Shinden ("Magnificent Lightning"). Similar in design to the Ambrosini--pusher engine, sweptback wings, and canard surfaces--the Shinden was ordered into mass production before testing was begun. Initial flight tests in 1945 were successful, but the war was over before the second prototype flew, and production ended.
The only tailless aircraft to see production and enter combat was the previously noted Messerschmitt Me-163 rocket-powered fighter, an example of which exceeded 623 mph in 1941. Delightful to fly--when it did not explode--the Me-163 had deficiencies in duration and armament, making it ineffective as a warplane.
The shining goal of a pure flying wing entranced designers from Hugo Junkers and the Horten brothers to Anthony Stadlman and John K. Northrop. There was always something intrinsically appealing about the pure flying wing, whose sleek lines and low drag were complemented by a large payload capacity.
The first pure flying-wing fighter (and incidentally, if not accidentally, the first fighter with stealth characteristics) was the Horten Ho IX V3, which would have been produced as the Gotha Go 229. A twin jet made primarily of molded wood (to help elude radar), its performance and handling were exceptionally good, but like so many German wonder weapons, it came too late in the war.
It fell to Northrop to create a line of pure flying wings, culminating in the XB-35 and XB-49 bombers that seemed to hold so much promise in the mid-1940s. During the war, four one-third-scale models had been flown successfully, and the prototype XB-35 took to the air on June 25, 1946. As many as 200 B-35s were on order at one time, but changing requirements and a lack of stability during the bomb run brought about cancellations and controversy.
The YB-49 was an even cleaner aircraft. Basically a YB-35 converted with eight Allison J35 turbojets buried in the wing, its performance led to an order, later canceled, for thirty RB-49s. All of the large Northrop wings were broken up, but two of the scale models remain, one at the Smithsonian's National Air and Space Museum in Washington, D.C., and one flying example at the Planes of Fame Museum in Chino, Calif.
The concept of a blown wing was first enunciated by Willard R. Custer with his Channel Wing design. A competition of medium-size jet transports resulted in the Boeing YC-14 and McDonnell Douglas YC-15. Experience with the latter led directly to today's McDonnell Douglas C-17 Globemaster III airlifter, the newest workhorse of Air Mobility Command.
An even more esoteric type is the mission-adaptive wing, as tested on the General Dynamics F-111 by a joint Boeing, NASA, and USAF team. (C-5 Galaxys and C-141 Starlifters have routinely flown with their wings "mission adapted" to their weight by judicious use of lift devices.) In "New World Vistas, Air and Space Power for the 21st Century," the Air Force Scientific Advisory Board's forecast of new technologies, the concept of adaptive mechanisms is carried forward beyond changes in camber and active aerodynamic control to monitoring the "health" of the aircraft by sensing and compensating for battle damage.
Interestingly, New World Vistas' bold leap into the future is accompanied by predicted returns to the past. For example, the report suggests that future long-range lifters might have strut-braced, very-high-aspect-ratio wings, like those made by Hurel-Dubois. It forcecasts blended-wing-and-body transports, similar in concept to those put forward by Vincent Burnelli years ago. And finally, the report says that long-range bombers of the future could have center nacelles and forward-swept wings, just as George Cornelius suggested in the 1930s.
Walter J. Boyne, former director of the National Air and Space Museum in Washington, D.C., is a retired Air Force colonel and author. He has written more than 400 articles about aviation topics and several books, the most recent of which was Silver Wings.
An aircraft is any machine capable of atmospheric flight.
An aeroplane or airplane is a kind of aircraft which uses wings in some form to generate lift.
Aircraft fall into two broad categories:
Lighter than air aerostats: balloons and airships. Aerostats float in air in the same way that a ship floats in water, by displacing the air around the craft with a lighter gas (helium or hydrogen), or hot air. The distinction between a balloon and an airship is that an airship has some means of controlling forward motion and steering while balloons simply drift with the wind.
Heavier than air 'aerodynes', including autogyros, helicopters and variants, and conventional fixed-wing aircraft: aeroplanes in Commonwealth English, airplanes in North American English.
Fixed-wing aircraft generally use an internal-combustion engine and propeller or jet engine to provide thrust, which moves the craft forward through the air. The movement of air over the wings produces lift, which allows the aircraft to fly. Exceptions are gliders which have no engines and gain their thrust from gravity. That is, in order to maintain their forward speed they must descend in relation to the air (but not necessarily in relation to the ground). Helicopters and autogyros use a spinning rotor (a rotary wing) to provide both lift and thrust. The abbreviation VTOL is applied to aircraft other than helicopters that can take off or land vertically. Similarly, STOL stands for Short Take Off and Landing.
Types of aircraft
A first division by design among aircraft is between lighter-than-air and heavier-than-air aircraft.
Examples of lighter-than-air aircraft include non-steerable balloons, such as hot air balloons and gas balloons, and airships (sometimes called dirigible balloons), such as blimps (which have a non-rigid construction) and rigid airships, which have a rigid frame. The best-known type of rigid airship is the Zeppelin.
In heavier-than-air aircraft, we can discern two major ways to produce the lift: aerodynamic lift and engine lift. In the case of aerodynamic lift, the aircraft is kept in the air because of aerodynamics, usually by means of wings of some kind. With engine lift, the aircraft defeats gravity by sheer engine power.
Examples of engine lift aircraft are rockets, and so-called VTOL planes, such as the Hawker Harrier.
Among aerodynamically lifted aircraft, the largest number falls in the category of fixed-wing aircraft, where horizontal surfaces produce lift, by profiting from the Coanda effect (aeroplane or airplane).
In a "conventional" configuration, the lift surfaces are placed in front of a control surface or tailplane. The number of lift surfaces varied greatly in the pre-1950 period, as biplanes (two wings) and triplanes (three wings) were numerous in the early days of aviation. Subsequently most planes are monoplanes.
The reverse configuration is the canard type, where small horizontal control surfaces are placed forward of the wings, near the nose of the aircraft.
Other possibilities include the delta-wing, where lift and horizontal control surfaces are combined, and the flying wing, where there is no separate vertical control surface (e.g. the B-2). A variable geometry ('swing-wing') has also been employed in a few examples of combat aircraft (the F-111, Panavia Tornado, and B-1 Lancer, among others).
The lifting body configuration where the body itself produce lift has been tested. So far the only significant practical application of the lifting body was in the Space Shuttle.
A second large category of aerodynamically lifted aircraft are the rotary-wing aircraft. Here, the lift is provided by rotating rotors. The best-known examples of this category are the helicopter, the earlier autogyro, and the tiltrotor aircraft (such as the V-22 Osprey).
A further category might encompass the wing-in-ground-effect types, for example the Russian ekranoplan, also nicknamed the "Caspian Sea Monster" and hovercraft, most of the latter employing a skirt and achieving limited ground or water clearance to reduce friction and achieve speeds above those achieved by boats of similar weight.
And finally, the flapping-wing ornithopter is a category of its own. These designs may have considerable potential but are not yet practical.
Some types of aircraft, such as the balloon or glider, do not have any propulsion. Balloons drift with the wind. For gliders, takeoff takes place from a high location, or the aircraft is pulled into the air by a ground-based winch or vehicle, or towed aloft by a powered "tug" aircraft.
Most early aircraft used a piston-engine with propeller as propulsion. Although the configuration of the engine can vary (rotary, radial, inline), they all work according to the same principles.
Just prior to World War II, the first jet engines emerged. Different types exist, such as the ramjet, pulse jet, turbojet, and the turboprop, the latter of which still uses a propeller.
A small number of aircraft have been propelled by rocket engines. These have mostly been experimental in nature, but include one mass-produced interceptor (the Messerschmitt Me 163).
Three major uses for aircraft may be seen: recreational, military, and commercial.
For recreation, almost any type of aircraft can be used, although they are usually small ones. Gliders and balloons are used almost exclusively for recreational purposes although they have been used in times of war in the past. For instance, balloons were used for observation in the American Civil War and World War I. Gliders were used to deliver troops into occupied territory during World War II.
The first widespread use of military aircraft was for reconnaissance and surveillance in World War I. Soon they were adapted for attacking the ground or enemy vehicles/ships/guns/aircraft as well, and the first bombers were born. In order to prevent the enemy from bombing, fighter aircraft were developed to intercept and shoot down enemy aircraft.
Eventually, two-seat trainers were developed for the purpose of instructing new pilots. The use of transport aircraft enabled the rapid movement of supplies, ammunition, cargo, troops and also casualty evacuation; transport aircraft were also used to drop paratroopers. Tankers are used to refuel planes in mid-air, thus increasing their operational range.
Commercial aviation can be divided in passenger transport and cargo transport. For the former, large planes have been developed that can transport up to 500 passengers over large distances. Commercial cargo aircraft are often similar to military transport aircraft, or might be adapted from the passenger fleets of an earlier era.
Other uses include search-and-rescue operations (especially by helicopters), border protection, water-bombing (fire-fighting), and aircraft that are purely experimental in nature. Further divisions can be drawn between aircraft designs having a conventional (wheeled) undercarriage, and amphibious floatplanes or flying boats.
A flying wing is a type of aircraft design with no tail, one in which the majority of the fuselage is inside a thickened wing. Since a wing is necessary for any aircraft, removing everything else theoretically results in a design with the lowest possible drag. A modification in which the fuselage is still retained is known as the tail-less design.
In traditional aircraft designs, the wing is located such that the weight of the plane is just forward of the center of pressure, the point where all of the lift on the plane balances out. This gives the plane a nose-down tendency. To counteract this, small control surfaces are placed at the end of a long tail which pull down, thereby lifting the nose back up. The basic idea is that the plane can "fly itself" to some degree; by reducing power, and thus lift, the nose naturally lowers itself and the plane starts to descend.
This system, while simple and safe, has several drawbacks. Locating the control surfaces at the end of the tail means that the plane is longer than it "needs" to be, and much of this extended fuselage cannot carry a load due to balance considerations. Moreover the control surfaces' mere existence causes drag on the plane, lowering its performance. But the biggest problem is that these surfaces are pushing down on the plane, in effect making it weigh more. This trim drag requires extra engine power in order to provide extra lift over and above the "dry weight" of the plane.
The same general arrangement, load in front and controls in back of the center of pressure, can be had in another fashion, by bending the wing to the rear. In this case some of the wing is in front and some in back of the center of pressure, and the stability works the same as with external controls. Yet, there are no external controls, and therefore decreased drag and weight. In many situations this can dramatically reduce the overall drag of the plane.
The only disadvantage to this design is that in order to be a useful fuselage, the wing must be thicker than normal. At low speeds this is not a concern, but as the speed of the aircraft approaches the speed of sound a new drag effect appears, one that is based on the relative thickness of the wing. For this reason the flying wing is not appropriate for high-speed aircraft, where the drag is likely to be higher than conventional designs. In addition a number of new stability problems also appear at high speed, notably mach tuck, which the design has a much harder time dealing with.
For this reason the design was studied almost entirely in the 1930s and 1940s, where it was seen as a natural solution to the problem of building an airliner large enough to carry a reasonable passenger load and enough fuel to cross the Atlantic in regular service. The flying wing's large internal volume and low drag made it "a natural" for this role, and was studied in depth by Jack Northop in the United States, and Alexander Lippisch and the Horten brothers in Germany, where Hugo Junkers had in 1910 patented a wing-only glider concept.
Junkers started work in 1919 on his "Giant" JG1 design, intended to seat passengers within thick wings, but in 1921 the Allied Aeronautical Commission of Control ordered the incomplete JG1 destroyed for exceeding post-war size limits on German aeroplanes. Junkers conceived futuristic flying wings for up to 1,000 passengers; the nearest this came to realisation was in the 1931 Junkers G-38 34-seater Grossflugzeug airliner, which seated passengers in cabins inside the leading edge of the inboard wing panels. One G-38 entered service with Lufthansa; it was later put to military use until being destroyed in 1941. Several late-war German military designs were based on the flying wing, or modifications of it, in order to extend the range of the otherwise very short-range jet engined aircraft. Most famous of these would be the Gotha Go 229 fighter.
After the war a number of experimental designs were based on the planform but the problems soon became evident. Some general interest remained until the early 1950s in order to extend the range of bombers, culminating in the Northop B-49, which did not enter production. Even the flying wing design could not make up for the fuel use, and much larger conventional aircraft like the Boeing B-52 were built instead.
Interest was renewed in the 1980s as a way of designing an aircraft with the fewest possible parts to show up on radar, which eventually led to the Northop B-2 Spirit stealth bomber. In this case the aerodynamic advantages of the flying wing are not the important issues.
The design still remains at its best in the slow-to-medium speed range, and there has been continual interest in using it as a tactical airlifter design. Boeing continues to work on paper projects for a Lockheed C-130 Hercules sized transport with better range and about 1/3rd more load. A number of companies, including McDonnell Douglas and de Havilland did considerable design work on flying-wing airliners, but to date none have entered production.
The delta-wing is a wing planform in the form of a large triangle. Its use was pioneered by Alexander Lippisch prior to WWII in Germany, but none of his designs entered service. After the war the delta became the favoured design for high-speed use, and was used almost to exclusion of other planforms by Convair in the United States and Dassault in France. In early use delta-winged aircraft were often found with no other horizontal control surfaces, creating a tailless design, but most modern versions use a canard in front of the wing to modify the airflow over it, most notably during lower altitude flight.
The primary advantage of the design is that the wing's leading edge remains behind the shock wave generated by the nose of the aircraft when flying at supersonic speeds, which was a distinct improvement on traditional wing designs. Another advantage is that as the angle of attack increases the leading edge of the wing generates a huge vortex which remains attached to the upper surface of the wing, making the delta have very high stall points. The combination of these two features is a dream come true, a normal wing built for high speed use is typically dangerous at low speeds, but in this regime the delta transitions to a mode of lift based on the vortex it generates.
Lippisch studied a number of ramjet powered (sometimes coal-fueled!) delta-wing interceptor aircraft during the war, one progressing as far as a glider prototype. After the war Lippisch was taken to the US, where he ended up working at Convair. Here the other engineers became very interested in his interceptor designs, and started work on a larger version known as the F-92. This project was eventually cancelled as impractical, but a prototype flying testbed was almost complete by that point, and was later flown widely as the XF-92. The design generated intense interest around the world. Soon almost every aircraft design, notably interceptors, were designed around a delta-wing. Examples include the Convair B-58 Hustler, the Avro Arrow and the MiG-21.
Deltas fell out of favour due to some undesirable characteristics, notably flow-separation at high angles of attack (swept-wings have similar problems), and high drag at low altitudes. This limited them primarily to the high-speed, high-altitude interceptor roles. A modification, the compound delta, added another much more highly swept delta wing in front of the main one, to create the vortex in a more controlled fashion and thereby reduce the low-speed drag.
As the performance of jet engines grew, fighers with more traditional planforms found they could perform almost as well as the deltas, but do so while maneuvering much harder and at a wider range of altitudes. Today a remnant of the compound delta can be found on most fighter aircraft, in the form of leading edge extensions. These are effectively very small delta wings placed so they remain out of the airflow in cruising flight, but start to generate a vortex at high angles of attack. The vortex is then captured on the top of the wing to provide additional lift, thereby combining the delta's high-alpha "trick" with a conventional highly efficient wing planform.