How Airplanes Fly, Really

By Eric M. Jones

HUMANS EXPERIENCE AIR EXACTLY like fish experience water. When air is still and we wave our arms about, we don’t feel any drag, because we interpret aerodynamic drag as merely lack of our own muscle power. Air is technically weightless (it has mass but no weight—one more good reason to convert to the metric system). Air’s density is about 1.23 kg/m3 , remarkably dense when you think about it: the air inside a house usually has much greater mass than the people living inside it. An average cloud of a modest cubic kilometer volume floating lazily in the air has a mass of 1.23 million metric tonnes. This is twice the mass of all the Boeing 747s ever made (1,438), completely loaded and fueled to their maximum takeoff weight.

Air is much heavier than most humans can possibly imagine. This helps explain how rain can fall for days, how airplanes and birds fly, and, when this air starts moving about, how this invisible medium can obliterate every man-made object in front of it. EF5 tornados will suck the pavement right off a road.

In the early days of powered flight, there were two opposing camps of thought: heavier-than-air flight and lighter-than-air flight. The people who thought that heavier-than-air flight had great advantages were frequently viewed as madmen, because the lighter-than-air camp believed if the engine stopped, or something broke, the machines would simply fall out of the sky. They often did.

Lighter-than-air flight did not depend on the forward motion of the craft or operating engines to stay aloft. The mere cloud-like buoyancy of the craft would do it. If the engine stopped, so what! One could just float with the breeze or descend serenely to the ground. Or so the argument went ... In October 1904 at St. Louis, The International Aeronautical Congress was held. Only one heavier than-air craft was present, a glider built by Octave Chanute. Every other craft was a lighter-than-air balloon or dirigible.

Victor Lougheed, an automotive but not an aeronautical engineer, and founder of the Society of Automobile (later Automotive) Engineers (SAE), had two younger brothers, Allan and Malcom Loughead [sic], who built airplanes. The younger brothers eventually changed the spelling of their surname and started Lockheed Aircraft Company. But older brother Victor, in a “Popular Mechanics” magazine article of March, 1912, waged war on the “Gas Bag” proponents. He pointed out that the advancement and mechanical evolution of the airplane in the first few years of the twentieth century had been tremendous while the engineering of the “balloon” had advanced only in small details in a century. This portended a future where lighter-than-air craft had no place except for amusement and sport. Heavier-than-air craft were the future of flight.

Victor Lougheed also wrote the very first comprehensive book on flight, “Vehicles of the Air,” in 1909. He argued convincingly that the real work of navigating the air could only be done by heavier-than-air machines that could potentially travel faster than the air around us and require few resources on the ground; “balloons” could never hope to do this successfully. They required ground crews of 100 or so men and giant hangars at each end of their journey. This seemed simply absurd.

By the end of the 19th century, aviation was a hot topic. The Wright brothers certainly demonstrated the first man-carrying, controllable flying machine in 1903, and the story should have ended there. But the scramble for fame and glory involved many characters, such as Chanute, Samuel Langley, Hiram Stevens Maxim, the Wright brothers, Samuel Franklin Cody, Leon Levavasseur, Otto Liliethal, Clément Ader, Richard Pearse, Gustave Whitehead, Louis Blériot, Gabriel Voisin, Glenn Curtiss, John Joseph Montgomery, Alberto Santos-Dumont and dozens more.

Secretary of the Smithsonian Institution Samuel Langley was funded by the Army to build a flying machine. Langley was known as a great experimenter in early aviation. His followers proclaimed his victory loudly. True, the Langley Great Aerodrome never successfully flew, having immediately crashed on first takeoff October 7, 1903, and again when it was launched a second time on December 8, 1903. Charles Walcott, a longtime promoter and friend of Langley who became director of the Smithsonian Institution in 1906 (the same year Langley died), immortalized his friend by setting up the Langley medal, the Langley Aero Lab, and the Langley Memorial. Langley was promoted as the father of aviation even though his airplane never flew.

But almost immediately after the Wright’s almost unnoticed flying demonstration, on December 17, 1903, the two Ohio bicycle makers went into seclusion to methodically refine their airplane and legally protect their important work. This seemingly sensible decision ultimately proved to be a disaster. The Wrights made a few flights in 1904 and 1905, but did not fly again until late 1908, when their airplane was essentially obsolete.

To put this into historical perspective: Frenchman Louis Blériot made and flew (and often crashed) ten different highly inventive designs of aircraft, culminating in the Blériot XII, upon which most aircraft designed since are patterned. Meanwhile the Wrights were protecting their design and fighting legal battles. No other manufacturer in Europe had as much influence on the future of aircraft design as Blériot. The Wrights were pioneers, but that was about it. In America, Glenn Curtiss, arguably based on his expertise in air-cooled engines, became the great force in early aviation design.

The important development made by Blériot around 1910 was the basic configuration of the standard airplane. The propeller was put at the front to work in undisturbed air, the wings were put at the center of gravity, controlling roll with ailerons (or wing-warping earlier) and the tail, controlling pitch and yaw, was placed at the rear. Why wasn’t the tail, or part of it, at the front like the Wright flyer? Because the propeller was there. The basic propeller-driven small airplane configuration has not changed since then.

In 1914, Walcott hired Glenn Curtiss, who was continuously embroiled in patent disputes with the Wrights, to rebuild the Langley Great Aerodrome and prove that it really could fly as Langley designed it. Curtiss put back together the broken Langley “Great Aerodrome.” He also strengthened the structure, added proper controls, changed the airplane aerodynamically, including improvements to the wing aspect ratio, camber, and angle-of-attack, and relocated the center of gravity. Then he struggled to fly a mere hop off glass-smooth Lake Keuka, NY, once perhaps appropriately named “Crooked Lake.” Later he slipped in a new motor, too—one of Curtiss’ own powerful OX engines and made longer flights. The Smithsonian’s witness for these flights, Dr. Albert Zahm, provided expert testimony that the Langley Aerodrome “has demonstrated that with its original structure and power, it is capable of flying with a pilot and several hundred pounds of useful load. It is the first aeroplane in the history of the world of which this can truthfully be said.” Thus, the Smithsonian proclaimed that Langley had built the “first successful flying machine.” The Langley aircraft was then given the place of honor suspended inside the Smithsonian’s great hall.

The event was accompanied by publicity, band playing, press announcements, invitations, etc. All to “prove” Langley was right. The Wrights were so annoyed by this public celebration that they sent their very first Flyer overseas in 1925 to be displayed in the London Museum of Science and this U.S. national treasure didn’t return to the United States until 1948, after the Wrights were both dead ...

Aircraft development was severely delayed by the Wright brothers patent lawsuits, against both American and foreign aircraft builders. The Wrights effectively stopped the evolution of American aircraft design. During WWI, American pilots had to fly French fighters. Thus, the vast majority of important airplane terms are French: aileron, longeron, monocoque, aeroplane, nacelles, fuselage, empennage, pitot, m’aidez (mayday is French for “help me!”), and many more. This is the fossil record of the French dominance in early aviation, while the American inventors were fighting legal battles amongst themselves. Curiously, early German successes in dirigibles doomed them to pursue lighter-than-air craft long after they were completely obsolete.

How Do Airplanes Fly?

Surprisingly, most people who design and build airplanes don’t need an understanding of how wings actually work. Luckily, rarely does one actually have to design their own wing. Instead big research labs funded by NACA/NASA and governments do the job and even test the airfoil designs. There are catalogues of airfoil shapes and their characteristics. The designers simply look them up and choose one that has the desired characteristics—“high lift/drag ratio at slow speed,” for example. Even minor deviations from the published design can have unexpected consequences and must be tested carefully.

At right, for example is a popular airfoil design for light aircraft.

But notice the yellow line marked “Chord.” If the cord were a sheet of very strong, rigid material, there is some speed (and resultant lift) where the wing shape represents nothing but a perfect streamlining of the chord. Nothing more. The goal is to make a wing that will still operate at other speeds and chord angles (called the angle of attack) and will still have predictable and useful lift. Wolfgang Langewiesche was one of the most quoted aviation experts. His book, “Stick and Rudder” (1944), is still considered a primary reference on the art of flying fixed-wing aircraft. Simply put, says Herr Langewiesche, the wing pushes air downward as it moves forward and the airplane gets pushed upwards according to Newton’s Third Law—for every action there is an equal but opposite reaction.

The physics of flight as taught to students has been usually been wildly wrong. The notion that air flows over the wing top and generates suction (Bernoulli’s Principle) is simply incorrect. There are hundreds of imaginary airflow diagrams (Google Images “airfoil flow”) where apparently no work is done to the air. They are just someone’s slap-happy idea of the way they think wings should work. Many of these notions find their way into legitimate physics books. But even award-winning wing designs often have natural and simple origins. Charles Hampson Grant invented the “Grant T” (or X8) wing, which was actually an accurate top-down profile tracing of a freshly caught brook trout sliced and laid out on drawing paper. One wonders how much asymmetry the trout was given. Perhaps many wing designs were possible with the one trout. The wing reportedly worked wonderfully scaled up to different sizes and it won many prizes.

The main wing generates the airplane’s “lift.” The horizontal tail wing (usually containing a control flap “elevator” or a similar setup called a “canard” in forward-elevator designs) is responsible for controlling the “pitch,” or nose up or down of the airplane.

A reasonable person would think that the elevator also contributes some small amount of lift, and in fact it once did. But it was discovered that at speeds approaching a “stall” (where a wing or the whole aircraft quits generating lift and really does fall out of the sky), if the elevator was generating lift then suddenly stalled, the main wing would suddenly increase its angle-of-attack ... and the airplane would flop over backwards. If the elevator was designed to constantly pull downward, and it suddenly stalled, the main wing would lower to a shallower angle or even point downward, thus increasing the airspeed, and preventing a main-wing stall. This refinement cost many lives but was a major development in aeronautical engineering. Canard-type airplanes work the opposite way. Canards do provide some lift, along with the main wings, and when they stall the nose goes down and the airspeed goes up.

The tail, or vertical stabilizer, is remarkably free of design restrictions. It is streamlined, but a review of aircraft tails shows that they can be almost any shape or number or angle ... It just doesn’t matter. A layer of ice on any airfoil will change it into another new and unknown type of airfoil. The pilot who takes off with it is the first one to test it ... and is surely taking his or her life in his or her hands.

Almost all the subsonic airfoil shapes are public domain and free to use. But the transonic and supersonic ones ... not so much. 

Further Reading

“Vehicles of the Air,” Victor Lougheed.
“The Straight Dope: How Do Airplanes Fly, Really?”
“Coanda Effect,” by Jef Raskin.
NACA Aerofoils.

Eric M. Jones is the Contributing Editor of “Perihelion.” He is an engineer, designer, consultant, and entrepreneur. His Internet business PerihelionDesign, builds and sells products, parts and materials to the home-built experimental aircraft community.