Wright Brothers – Invention Of The Airplane

Articles relating to the Wright Brothers’ invention of the airplane.

The immediate impression of the Wright brothers is that they were just two bicycle mechanics from Dayton, Ohio who invented the first successful airplane. Maybe they were just lucky and stumbled on the solution through trial and error because this was a feat that had eluded the best minds for thousands of years.

After all, the brothers didn’t have a scientific degree or any formal education beyond high school. But don’t let that fool you. The reality is that they were brilliant scientists that outperformed the scientific elite of the day in the use of the modern scientific process.

The Airplane Propeller

An example of their prowess is their approach to the solution of an intractable engineering problem associated with their invention of a deceptively simple item, the airplane propeller.

In 1902, after their third trip to Kitty Hawk, they were confident that their glider would fly under pilot control. The next task was to develop a mode of propulsion.

They proceeded to design and build a small gasoline engine weighing 180 pounds that produced 12-horsepower. Now, they needed a propeller to go with it.

That seemed easy enough. Propellers have been used for years on ships. The respected scientist Samuel Langley, the Secretary of the Smithsonian had written in Experiments in Aerodynamics, “there is considerable analogy between the best form of aerial and of marine propellers.”

The Wrights initially thought they could convert the design information on ship propellers to flight technology. “We had thought we could adopt the theory from marine engineers, and then by using our tables of air pressures, instead of the tables of water pressures used in their calculations, that we could estimate in advance the performance of the propellers we could use.”

A trip to the Dayton Public Library quickly disillusioned them of the notion that this was going to be an easy task. Their research found there was no empirical information on how to do this and they didn’t have the time to use the trial-and-error approach used by marine engineers (the Wrights called it “cut and try”). They decided to develop new theory and design the propellers from scratch.

Their usual approach to solving complex problems was to first think about the problem and mentally develop a testable theory. Often, the brothers brainstormed ideas by vigorously debating ideas. Often these debates turned into shouting matches that were annoying to their sister, Katharine. Sometimes they would convince each other of the other’s argument and change sides to argue the opposite point of view.

Propellers as Rotating Wings

Out of this process came the insight that propellers acted like rotating wings traveling in a spiral course through the air. The rotating propeller blades act as airfoils that produce a pressure differential. Less pressure is created on the front of the spinning cambered blade than there is on the back, thus the rotating blade produces thrust that moves the airplane forward.

Now that they had the concept, the problem became how to calculate the thrust of a rotating blade. The blade must produce sufficient thrust to propel the airplane off the ground and sustain it in the air. Flight would not be possible if sufficient thrust couldn’t be generated to overcome drag.

The problem was difficult. Orville describes it best in a December 13 issue of Flying Magazine, “It is hard to find even a point from which to make a start; for nothing about a propeller, or the medium in which it acts, stands still for a moment. The thrust depends upon the speed and the angle at which the blade strikes the air; the angle at which the blade strikes the air depends upon the speed at which the propeller is turning, the speed the machine is traveling forward, and the speed at which the air is slipping backward; the slip of the air backward depends upon the thrust exerted by the propeller, and the amount of air acted upon. When any of these changes, it changes all the rest, as they are all interdependent upon one another.”

The Wrights did have one advantage. They had data from their wind tunnel experiments in which they had tested some 200 airfoils (wing shapes). They selected airfoil number 9 as their baseline because it showed the best efficiency under a variety of conditions.

The brothers developed a series of quadratic equations from which they designed the propeller. All this work was accomplished before the advent of computers. Based on their calculations, they used hatchets and drawknives to carefully carve a piece of wood into an eight-foot propeller with a helicoidal twist based on airfoil number 9.

After three months of effort, they tested their propeller in their bicycle shop using a two-horsepower motor with excellent results. The thrust achieved was found to be within 1% of what they had calculated — a truly amazing result.

Orville gleefully wrote to George Spratt, “Isn’t it astonishing that all these secrets have been preserved for so many years just so that we could discover them.”


In June, they designed and made two propellers to be used on their machine, the Flyer. They determined that they could achieve greater thrust with two propellers rotating slowly, than they could with one propeller rotating faster.

Orville wrote, “all the propellers built heretofore are all wrong.”

Each propeller was 8.5 feet in diameter and made of three 1 1/8 inch thick laminations of spruce with the wing tip covered with light duck canvas glued on to prevent the wood from splitting. The entire propeller was then coated with aluminum paint.

The propellers were connected to the engine through a chain, gear and sprocket system, similar to a bicycle design. The propellers rotated 8 revolutions for every 23 revolutions of the engine. The two propellers were designed to provide a combined thrust of 90 pounds at airspeed of 24 mph and turning at 330 rpm.

The linkage was designed to rotate the propellers in opposite directions so as to counteract the torque effect of each rotating blade. This was achieved by crossing one set of chains in a figure eight and encasing the chains in medal tubes to keep them from flapping. The chains were procured from the Diamond Chain Company of Indianapolis.

The propellers were mounted at the rear of the wings as “pushers” to eliminate the effect of turbulent airflow upon the wings.

The finished product produced a maximum efficiency of 66% (Some recent tests achieved 70%). That means that 66% of the horsepower of the small motor was converted by the propellers into thrust. This was far superior to any other inventors who were attempting to fly with engines of much greater horsepower and still couldn’t sustain flight.

On December 17, 1903, the little engine with the efficient propellers pulled Orville off the launching rail and into the air producing the first heavier than air flight in the history of mankind.

Their remarkable achievement demonstrates the genius of the Wright Brothers and places them within the ranks of the greatest inventors in history.

Final Notes

An exact reproduction of the 1903 Flyer is scheduled to fly at Kitty Hawk on December 17, 2003 to celebrate the centennial anniversary of the first flight. The Wright Experience of Warrenton, Va., headed by Ken Hyde, is researching and building the Flyer.

Larry Parks, a volunteer working for Wright Experience, is carving the propellers using mainly antique tools. A member of the Wright family has provided an original 1904 propeller to aid in the project.

The Wrights continued to improve their propellers after 1903. One of the more interesting improvements was the so-called “bent end” propeller introduced in 1905. The purpose of the design was to prevent twisting under pressure.

Ken Hyde had one of their remanufactured 1911 bent end propellers that was used on Wright Model B airplane tested at the Langley Full Scale Wind Tunnel. It achieved an efficiency of 77% operating at a flying speed of 40 mph.

Hyde commented, “The performance of our remanufactured Wright propeller was amazing, when you consider that today’s wood propellers are only 85% efficient.”

The Secret of Flight

by Dr. Richard Stimson

in Inventing The Airplane

Since ancient times mankind has looked up to view birds flying and dreamed of flying.

The Wright brothers were no different. They often rode their bicycles to a popular picnic area south of Dayton called the “Pinnacles” to observe the many birds that flew there. Early on they decided that practical flight was possible by man using soaring large birds as their model.

The Pinnacles consisted of a gorge with a river flowing through it and unique large boulders created during the ice age on its slopes. The updraft created by the terrain attracted soaring birds. The Wright brothers regularly observed birds there from 1897 to 1899.

The Wrights developed their wing warping theory in the summer of 1899 after observing the buzzards at Pinnacle Hill twisting the tips of their wings as they soared into the wind.

The Wrights made the right decision by focusing on large birds. It turns out that small birds don’t change the shape of their wings when flying, rather they change the speed of their flapping wings. For example, to start a left turn, the right wing is flapped more vigorously.

To turn right the speed of flapping is changed to the other wing.

To fly straight, both wings are flapped at the same speed.

Incidentally, the technique is the same for creatures from fruit flies and moths to hummingbirds and cockatoos.

These findings were found through research with high-speed video of seven species at the universities of Delaware and North Carolina.

The greatest contribution the Wright Brothers made to man-flight was figuring out how to control an airplane in flight.

Their experience with bicycles taught them the importance of control. A bicycle is an inherently unstable machine. One must learn how to actively control a bicycle in order to ride it.

The airplane is also an unstable machine. Early experimenters tried to control an airplane by swinging their bodies from side to side, or by trying to build into the machine a means of automatically adjusting for fluctuations. Neither worked satisfactorily.

Basics of Control

The Wrights took a different approach to the problem of control. They designed a way to achieve active control in flight by controlling the movements of an airplane in the three basic movements of pitch, yaw and roll. In this way a person could learn to pilot an airplane in the same way a bicycle rider could learn to ride a bicycle.

Pitch movement occurs when the nose moves up and down around a horizontal axis.
Yaw movement occurs when an airplane veers side to side around a vertical axis.
Roll movement occurs when the wings dip to one side or the other around a horizontal axis.

Early experimenters were aware of the need for control of yaw and pitch. Balloonists thought of using a rudder for steering and an elevator for controlling altitude. The English engineer, George Cayley, in the early 1800s designed a tail that combined a vertical rudder with a horizontal elevator in a configuration called a cruciform design.

This design would allow a pilot to make adjustments in altitude and when changing direction. Lateral stability (roll) would be built into the machine by some means unspecified. A slow flat turn would be used to change direction.

The Wrights saw that there was a significant limitation with this approach. The problem of control was three dimensional, not two-dimensional. The neglected roll dimension was the critical oversight.

The Wrights, again using the bicycle model, saw the solution to mastering lateral control and for turning, was for the pilot to roll the airplane by twisting the wings in a process that they called wing warping. When one of the trailing edges of the wing are twisted up, the trailing edges on the opposite side twisted down. Wilbur got the idea of twisting the wings from observing how birds fly.

In 1899, Wilbur tested the idea in Dayton by building a five-foot, bi-wing kite. He attached cords to each corner in such a way that he could twist the wings in flight. It worked so well in controlling the kite’s balance that he and Orville decided to build and test a flyable glider.

Year 1900

The following year, 1900, the Wrights journeyed to Kitty Hawk for the first time with a glider that was essentially a kite with 17-foot wings that was three times larger than the previous years kite.

The glider had no tail and the wing tips were untrussed to permit twisting. The most prominent feature was an elevator set in front (canard configuration), a feature that was a trademark of all Wright airplanes for many years.

The setting of the elevator in front was a conscious decision to assure effective pitch control. They wanted to make sure that they wouldn’t duplicate the uncontrollable dive that had killed the famous glider experimenter, Otto Lilienthal in 1896.

Most people at Kitty Hawk thought they were highly eccentric flying a glider dressed as the middleclass did.

They measured wind speed, lift with a spring scale attached to the line, and the angle of attack.

They were disappointed that the cambered wings didn’t create the lift they expected, but they were pleased that the elevator and the wing warping worked effectively in controlling pitch and lateral balance. They tried tossing the glider forwards and backwards off the dunes in order to improve performance.

They were pleased that Wilbur was able to glide 300 to 400 feet for a total of 2 minutes flying time. This was as good as Chanute and Lilienthal had done.

Perhaps more important Orville became committed to the project and Wilbur began using the pronoun “we” in his correspondence.

Year 1901

They returned for the second time to Kitty Hawk. They were delayed by storms and harassed by mosquitoes. They built a primitive building which made life better than living in a tent as they had done the previous year. Spratt and Huffaker joined them at the request of Chanute.

They came with a much larger glider. The area of the wings was almost doubled and the camber was increased.

It wasn’t long before they found they were having control problems. On one glide, the glider climbed steeply and then lost all headway. It took Wilbur’s skillful piloting to keep from nose diving into the ground. They thought they had solved the control problem, but now the glider had a tendency to nose-dive or climb suddenly and go into a stall. On occasion it would even start to go backward, a frightening development.

They reduced the camber of the wings and that seemed to solve the problem. With new confidence Wilbur tried to make some turns, but then a new problem materialized.

Sometimes when making a turn, the lower wing slowed and approached a stall. The higher wing, because it was still producing lift, would whip around causing the glider to go into a spin. On one flight the condition caused Wilbur to crash, hurling him off the wing into the elevator blackening his eye and bruising his nose.

Although the wing was much larger than the one that they used the previous year, the lift was much less. They reconfigured the wing to reduce the camber of the wings from 1:12 to 1:19. But that didn’t improve performance significantly.

Discouraged, the Wrights returned home. Not only did they not solve the problem of inadequate lift; they hadn’t solved the control problem.

Year 1902

The Wrights returned in 1902 with a similar configured machine as the previous year but with a number of important design changes. As a result of their wind tunnel tests the wings were now longer, narrower and had less camber. The objective of these tests was to determine the wing shape that would generate the most lift for the least drag.

In the course of 2 months the Wrights had redefined aeronautics for the next century.

The control system was also redesigned to operate differently. The wing warping that had previously been operated by the feet was now operated by the pilot’s hip movements while lying in a cradle on the wing.

A tail was added for the first time as a means to prevent the spins that had occurred the previous year. The tail consisted of two rigidly mounted vertical fins.

They soon found out that the spin problem had not gone away. The message was forcefully communicated to Orville when he crashed into a sand dune, demolishing the glider but somehow emerging unharmed.

At first they thought it was just pilot error. But every so often when attempting to turn, the low wing would drop even lower and the glider would slide into a spinning fall. They gave it the name of “well digging.”

Now they focused on the design of the tail. They removed one of the two fins, but that made no difference. Finally, Orville solved the problem.

The rigid tail aggravated the control difficulty by causing the lowered wing to lose still more speed at the same time that the raised wing continued to rise and move forward. Orville correctly reasoned that if the tail was made movable, the pilot could adjust it to minimize wind resistance and thereby restore the glider to normal flight.

Wilbur liked the idea and added an improvement that linked the tail to the wind warping mechanism so that the tail moved in synchronization with the wing warping. It worked. Taking turns, the brothers set new gliding records while making hundreds of glides over a two-week period. They had built the world’s first practical glider. The performance of the glider exceeded their expectations. They were making extended glides of over 600 feet. The glider was pure elegance.

Their idea for the control of both roll and yaw motions was the basis for their patent submitted in 1902 and granted in 1906.

Year 1903

The Wrights returned to Kitty Hawk in 1903 with high expectations and a powered machine which incorporated what they had learned from their experiments. Their engine was crude but light and had 12 horsepower. They had calculated that that was sufficient to get airborne.

Wilbur won the coin toss to be the first to make the attempt to fly on Dec. 13th. The machine, nicknamed the Flyer by their father, was placed on the slope of Kill Devil Hill because of light wind.

It leaped off the starting rail and shot up 15 feet in the air. There it stalled and plowed into the sand 105 feet and three and one-half seconds from the point of takeoff. The left wing, the front elevator and one of the skids were slightly damaged.

The poor result was not surprising in view of the fact that Wilbur had not practiced with the machine as a glider before attempting the first powered flight. However, Wilbur’s control problem on his initial flight was a symptom of the Flyer’s worst problem. It had a very unstable pitching characteristic and its lateral characteristics were also poor.

The good news was that operation of the elevator prevented a dangerous nosedive into the ground and the engine was powerful enough to get the Flyer off the ground.

Three days later they were ready to try again. They could have tried on the 16th but they had promised their father not to fly on Sunday. They were ready the next day.

They alternated with each other, flying four times, the longest being 852 feet in 59 seconds. They had trouble maintaining pitch control resulting in undulating flight paths, but the important thing is they had done it. Man had flown for the first time. It was an extraordinary achievement and they had accomplished it in only five years of effort.

Several reasons are given for the pitch control problem.

1. The elevator was balanced so near its center. Once it started to turn, it continued the movement of its own accord. The result was that it tended to keep going from one extreme to the other.

2. The design of the wings. The wings were thin and highly cambered. The design had excellent lift to drag characteristics, but poor pitch stability. Less camber would have improved the wing’s stability.

3. The elevator was placed to close to the body of the airplane.

Fortunately these problems were somewhat ameliorated by the slow flying speed of the 1903 flights and the Wrights’ superior flying skills.

The Wrights were well aware that the success of the Flyer I was an intermediate success. It was an experimental plane built and flown to test basic principles of aerodynamics and control. There was still much to be done before they could say they had achieved a practical airplane.

Wind tunnel tests were conducted at Langley of the full-scale reproduction of the 1903 Wright Flyer built by the Wright Experience (Ken Hyde) of Warrenton, Va. These tests provide a definitive database establishing the aerodynamic characteristics of the design. The test results confirm that the aircraft was highly unstable in pitch, with marginal lateral/directional stability. Also, the Flyer behaves in a highly non-linear manner due to premature stall of the canard and vertical rudders.

Year 1904

The Wrights returned to Dayton to build and test an improved version of the Flyer at Huffman Prairie, a cow pasture outside Dayton. Flyer II was heavier, structurally stronger and had a more efficient engine. The spars were made of white pine instead of spruce that was used in the 1903 machine. The camber of the wings was reduced to 1/25 from 1/20 and the elevator control was relocated for easier handling.

They soon became comfortable with their ability to balance Flyer II in straight flight and were now ready to learn how to turn. They succeeded in making the first turn on September 26. Then, when Orville was turning on October 15, he had trouble stopping the turn and crashed the machine, causing serious damage.

The Wrights diagnosed the problem as the anhedral shape of the wings. An anhedral shape is one in which the wing tips are lower than the body of the airplane. The 1903 Flyer also had been rigged for the anhedral with the wings tips arched about eleven inches below the centerline.

So had the 1900, 1901 gliders, and later during the 1902 flying season, the 1902 glider used the anhedral. The Wrights chose the anhedral so as to dampen the effect of crosswinds and to improve the effectiveness of wing warping.

They could have chosen the dihedral configuration. Some birds, such as buzzards, employ the dihedral angle. Its works well to maintain stability in calm air, but the wing’s “V” shape becomes unstable with strong crosswinds.

The Wrights found that the anhedral exhibits serious negative effects while turning. They discovered that when executing a turn, a crosswind increases pressure on top of the lower wing. The increased pressure forces the wing to continue to drop, creating a spin.

Removing the anhedral of Flyer III resulted in much better performance, but every once in awhile a mysterious tendency to go into a spiral returned. They were still working on the problem as the year ended.

Year 1905

The Wrights correctly concluded that the tendency to spin while turning was a control problem associated with the tail. Starting with the 1902 glider, they had interconnected the movements of the tail to wing warping so that they would move simultaneously. This worked well in 1902 and 1903, but control of the new machines was more complicated than could be handled by using hardwired proportional interconnection.

They disconnected the tie between the two to allow for independent control of the tail by the pilot. This solved the spin problem.

Other changes were made to improve pitch stability. The area of the canard was enlarged and additional weight was added to the front-end. The latter change was for the purpose improving stability by moving the “center of gravity” of the machine forward.

Other changes were:

The wood used for the spars was changed back to spruce.

The camber of the wings were returned to 1/20 after having been changed to 1/25 and 1/30 in 1904.

They added a pair of semicircular vanes they called “blinkers,” placed between the twin elevator surfaces to prevent sideslips they experienced in 1904.

They added tabs, they called “little jokers,” to the trailing edges of the propellers to prevent deformation.

They added oiling and feeding devices to the engine to allow longer run time.

The entire machine was slightly longer and taller than before.

Then another problem developed. One day Orville was circling a honey-locust tree at Huffman Prairie. He was turning, when suddenly the machine turned up on one wing and slide sideways toward the tree. The left wing struck the tree twelve feet above the ground. Luckily, the machine continued through the branches, slicing off some of them, and managed to land safety.

Wilbur diagnosed this problem and its’ solution. Centrifugal force was culprit. Tilting the nose of the machine down a bit to restore flying speed would counteract this force. It worked. They had solved their last serious problem.

The Flyer III could now be flown with ease. On October 5, Bishop Milton Wright wrote in his diary, “In the afternoon, I saw Wilbur fly 24 miles in 38 minutes and 4 seconds.” Their father had watched Wilbur circle Huffman Prairie about 30 times, stopped only by running low on gas.

Orville and Wilbur had developed the world’s first practical airplane. It would be many years before anyone could duplicate the Wrights’ remarkable achievement.

Fred C. Kelly, the Wright brothers’ first biographer asked Orville in 1939 if it was the profit motive that motivated he and his brother to invent the airplane.

He reflected for a moment before responding, “I hardly think so. I doubt if Alexander Graham Bell expected to make much out of the telephone. It seems unlikely that Edison started out with the idea of making money. Certainly Steinmetz had little interest in financial reward. All he asked of life was the opportunity to spend as much time as possible in the laboratory working at problems that interested him.”

Kelly asked, “And the Wright brothers?”

Orville chuckled. “If we had been interested in invention with the idea of profit, we most assuredly would have tried something in which the chances for success were brighter. You see, we did not expect in the beginning to go beyond gliding.”

“Even later we didn’t suppose the aeroplane could ever be practical outside the realm of sport. It was the sport of the thing that appealed to Will and me.”

“The question was not of money from flying but how we could get money enough to keep on entertaining ourselves with it.”

“It was something to spend money on, just as a man spends on golf, if that interests them, with no idea of making it pay.”

Kelly: “You didn’t foresee commercial planes or transcontinental and trans-Atlantic flights?”

Orville: “No; and in our wildest dreams, even after we had flown, we never imagined it would ever be possible to fly or make landings at night.”

Kelly: “Still, it seems strange that you didn’t have more of a profit motive, inasmuch as you had been in business as a means of making a living and obliged to make the business pay. Didn’t you go into the printing business as a youngster to make money?”

Orville: Shaking his head with a smile replied. “I got interested in printing after my curiosity had been aroused by some woodcuts I saw in the Century magazine, and I tried to make some tools for carving wood blocks. The first tool was made from the spring of an old pocket-knife.”

“Gradually I became more and more interested in printing. But, making it pay its way came as an afterthought.”

Their father, Bishop Milton Wright used to say, “All the money anyone needs is just enough to prevent one from being a burden on others.” Following their father’s advice, the brothers tried to earn their own spending money and never became interested in a hobby because it might be profitable.

When the Wrights were conducting their wind tunnel experiments, they became concerned that their experiments were taking too much time and money for their modest means. They were worried that they would not get their money back and permitting their hobby to become too much of a luxury.

Wilbur was inclined to drop their researches. Orville thought they should continue a little longer. If Wilbur had quit, Orville would have too.

While they were still debating the issue, a letter arrived from their friend and mentor Octave Chanute. Chanute, suspecting their resolve to continue was weakening, urged them to continue with their experiments.

He reminded them that they already had valuable knowledge of aeronautics far beyond that possessed by anyone else in the world. To go on was almost a duty. And so the Wrights shelved their concerns and continued their research.

One thing they did do to save money was to experiment as much as possible on paper rather than making mechanical models. Before they built anything they were reasonably certain it was scientifically correct. They spent much time on grueling mathematical work before flight was possible.

Their insistence on doing everything possible on paper was successful in keeping costs down. Kelly claims that up to the day when they actually flew, the Wrights’ total outlay of money was a trifle less than $2,000. Some more recent estimates are that they spent event less, closer to $1,200.

Even after the Wrights had flown, they still did not know if they had done anything from which they could gain a fortune. They accepted the money that fell unexpectedly into their laps, but Orville said to Kelly, “I am not sure it’s quite decent to live on income from interest-bearing paper.”

Kelly said that he once said to Orville that even though what you accomplished was without the idea of making money, the fact remains that the Wright brothers will always be favorite examples of how American lads, with no special advantages, can forge ahead and become famous.

In response Orville protested, “But that isn’t true because we did have special advantages.

Kelly: “What special advantages?”

“Simply that we were lucky enough to grow up in a home environment where there was always much encouragement to children to pursue intellectual interests. We were taught to cultivate the encyclopedia habit, to look up facts about whatever aroused our curiosity. In a different kind of environment I imagine our curiosity might have been nipped long before it could have borne fruit.”

Reference: Harpers Magazine, “How the Wright Brothers Began,” Fred C. Kelly, October 1939.

A hundred years ago (1901) the Wright Brothers were disappointed with the performance of their glider at Kitty Hawk. This glider had bigger wings containing more than double the area than the one they had flown a year before. They were expecting much better results. Instead they found a significant lack in lifting power.

Wilbur and Orville suspected that the aeronautical data that they had used in their calculations for lift were erroneous. The famous glider pioneer, Otto Lilienthal, considered as the most important aeronautical experimenter of the nineteenth century, had developed the data. The brothers decided to find out for themselves the validity of the data by building a wind tunnel and generating their own data.

Why Airplanes Fly

Gliders and airplanes need to produce an upward force called lift that overcomes weight created by gravity in order to fly.

Lift is created by the flow of air over a wing. In general there are two concepts of physics involved. One is Bernoulli’s Theorem developed by Daniel Bernoulli, an eighteenth century scientist. He discovered that as the velocity of a fluid (such as air) increases, its pressure decreases.

In the case of an airplane, the wing is shaped to force the air flowing over the upper surface of the wing to flow faster than the air flowing over the lower surface. The faster air on the top surface creates a pressure differential resulting in an upward force on the wing.

Another contribution to lift comes from the effect of Newton’s third law. Isaac Newton has been regarded for over 300 years as the founding exemplar of modern physical science. Newton’s Third Law states that for every action there is an equal and opposite reaction.

As air passes over a wing, it is bent down. The bending of the air creates a downward force whose opposite equal force creates lift.

Determinants of Lift

There are a number of variables associated with creating and measuring lift. These include the size of the wing, the velocity of the air flowing over the wing, the density of the surrounding air, the shape of the wing and the angle of attack.

Area: All other things being equal, the larger the area of the wing, the more lift that will be created.

Wing Velocity: The higher the velocity the greater the lift. When an airplane is taking off, it normally heads into the wind because that increases the relative wind speed over the wings and helps the airplane reach flying speed.

Air Density: Air density refers to the amount of air contained in a given volume. Air density varies with the air’s temperature and pressure.

Decrease temperature and density increases. Increase pressure and density increases. Air is denser at sea level than it is at higher elevations.

The denser the air, the greater the lift. The air pressure at Kitty Hawk, being at sea level, is denser than the air at Dayton, Ohio. The temperature at Kitty Hawk on the day of the first flight was a cold 34 degrees. The Kitty Hawk Flyer may not have gotten off the ground in Dayton’s less dense air.

Coefficient of Lift: The coefficient is a multiplying factor that takes into consideration the various angles a wing assumes with regard to the flow of air. The value of the coefficient varies with the size of the wing and the “angle of attack.”

The angle of attack is the angle of the wing with relation to the wind flow. Raising and lowering the nose of an airplane while flying varies the angle of attack. Within limits, a greater angle of attack results in greater lift.

Raising the nose to an extreme angle of attack can result in loss of efficient airflow over the wing and result in loss of lift. This is called a stall and is a potentially dangerous condition.

Lilienthal is one of a number of early experimenters that were killed when their gliders went into a stall condition. While gliding in 1896 at an elevation of 50 feet, a gust of wind caught his glider causing it to nose up sharply. He was not able to correct the problem by shifting his weight and crashed. That accident was fatal for Lilienthal who received a broken spin.

Lilienthal in the Pocket Book of Aeronautics published a table containing coefficients of lift in 1895. They later appeared in The Aeronautical Annual and other sources available to the Wrights.

Smeaton’s Coefficient: Also involved in the calculation of lift at the time of the Wright Brothers was a constant number known as the “coefficient of air pressure.” It is a multiplying factor used to calculate the numerical value of lift in air, as compared to other mediums, such as water or oil.

John Smeaton, a mideighteenth century engineer, had determined the value of this coefficient was 0.005 in 1759, from his study of windmills. Engineers used the value of 0.005 for 150 years. The Wrights would subsequently find that this number was the major cause of their lift problem with their early gliders.

Modern aeronautical engineers no longer use Smeaton’s coefficient. Many have never even heard of it.

Camber: A barn door can fly, but not very well. A wing with a cambered shape is more efficient; that is, one that has a degree of curvature of the upper surface.

The Wrights tested over 200 airfoils to find the optimum shape in a 6-foot pioneering wing tunnel they designed and built. Airfoil number 12 was found to be the most efficient and was the model for the wing used on their successful first flight in 1903.

The Wind Tunnel

The Wright Brothers did not invent the wind tunnel, but they were the first to use it for scientific aeronautical research directly related to the design and construction of an airplane. There were ten wind tunnels in the world at the time and the Wrights was the third in the U.S.

Their wind tunnel was not elegant; at a glance it looked like a coffin. But it was in fact an elegant scientific instrument that would set the standard for conducting aeronautical research up to the present time.

The tunnel was a simple wooden box six feet long and sixteen inches square. It had a glass window in the top for viewing the measuring instrument they had designed. A one horsepower engine they used in their shop drove a fan that generated a wind of thirty miles per hour. It turned out that the wind generated in the wind tunnel was almost the same as the wind the Wright Flyer faced at Kitty Hawk on the first successful flight (27-mph).

They fitted a honeycomb grid on the fan end of the tunnel, which produced a perfectly straight current of wind required for accurate measurements. This gave them their biggest problem and took them a month to solve.

Their engineering expertise was demonstrated in the instrument that they devised to measure lift. It consisted of an ingenious mechanical balance device in which the test wing foil was compared to flat plates placed perpendicular to the wind flow. The coefficient of lift could be read directly from the instrument.

The completed wind tunnel was capable of testing a wide range of shapes and curvatures to an accuracy of 2-3%. Lilienthal, in contrast, tested and published data on only one specific curvature.

Found a Significant Error

The wind tunnel tests conducted by the Wrights proved that Smeaton’s coefficient of air pressure was in error. The Wrights’ discovered that the correct average value for this coefficient was 0.0033 rather than Smeaton’s 0.005 they had used in designing the 1900 and 1901 gliders. This error was the primary cause of their poor performance. Using the Smeaton value of 0.005 had caused them to overestimate the force of lift in their design by 40%.

They also found an error in Lillenthal’s lift coefficients, but it was a minor error within the angles of attack they would be flying. There was close coincidental correlation between the Wrights’ and Lilienthal’s coefficients between five and eight degrees of attack.

The Wrights had for the first time in history established a scientific basis for designing an airplane that would perform in accordance with prior calculations. Previous experimenters had relied on guesswork using trial and error. The Wrights relied on facts and figures. They now knew how to design a wing in which they could have confidence that it would fly.

On December 7, 1901, their sister Katharine wrote to their father:

“…The boys have finished their tables of the action of the wind on various surfaces, or rather they have finished their experiments. As soon as the results are put in tables, they will begin work for next season’s bicycles …”

New Design

In 1902, the year that “animal crackers” were introduced to America’s children, they returned to Kitty Hawk with a new glider. Its design incorporated all their wind tunnel data. The wingspan was now 32 feet, ten feet longer than the 1901 glider with a longer and narrower shape with a camber of 1 to 20.

The wing area of 305 square feet was only slightly bigger than the 290 square feet of their previous year’s glider, yet it generated much greater lift.

Their experiments enabled them to select a new wing shape with an efficient lift-to-drag ratio. Drag refers to the resistance that the wing generates as it flows through the air. The most efficient wings are those that generate the least drag for the most lift.

Their data demonstrated that longer, narrower wings in general produce more lift than the same area contained in short, wide ones.

Their data also showed that a parabolic wing camber with the high point toward the front was more efficient than the perfect arcs that Lilienthal and others had used.

The camber they used on the 1902 glider was a relatively flat 1 in 24 to 1 in 30 depending upon how the wing was rigged. This specific wing configuration was never actually tested in their wind tunnel. They apparently extrapolated this configuration from their other data.

All of the above were new and original conclusions resulting from their wind tunnel experiments. The Wrights’ were now far ahead of anyone else in the aeronautical field.

World Record Flights

They flew their new glider somewhere between 700 and 1,000 times during their five weeks of experimenting at Kitty Hawk. On October 23, Wilbur sailed 622 1/2 feet in 26 seconds setting new American records.

“We now hold all the records! The largest machine we handled in any kind of weather, made the longest glide, the longest time in the air, the smallest angle of descent and the highest!!!”

They were now convinced that the data they had found from their wind tunnel tests would enable them to calculate in advance the performance of their first powered airplane. They had mastered two of the three conditions of flight. They had designed wings capable of sustaining flight and developed a three-axis control system that allowed maintaining balance and executing turns. The next step was to develop an engine and propeller.

Calculation of lift for the 1903 Kitty Hawk Flyer

The formula used to calculate lift is as follows:

L = kxV²xSxCL where

L = Lift (pounds)
k = Smeaton’s coefficient
V = Relative Velocity of air over wing (mph)
S = Wing area (square feet)
CL = Coefficient of lift

k = 0.0033 (from Wrights’ wind tunnel experiments)

V = 33.8 (wind of 27 mph on December 17, 1903 plus ground speed of Flyer of 6.8 mph)

S = 512 (wing area of 1903 Flyer)

CL = 0.515 (from Wrights’ table of lift coefficients assuming airfoil #12 and an angle of attack of 5 degrees)

Substituting the approximate values for the 1903 Wright Flyer on the first flight:

L = (0.0033) x (33.8)² x (512) x (0.515) = 994 pounds

The weight of the Flyer, including engine was 605 pounds. Orville weighed approximately 145 pounds. Therefore, the total weight of machine plus pilot is 750 pounds

Since lift (994 pounds) is greater than the weight of the machine (750 pounds), the lift was sufficient to support flight.