It’s All About Control

by Dr. Richard Stimson

in Inventing The Airplane

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.

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