The Bishop's Boys (27 page)

Virtually every major experimenter undertook to determine these figures for himself. It was this drive to test a variety of surfaces under different conditions that had led Wenham and his colleague John Browning to the invention of the wind tunnel. The early tunnels were nothing more than long empty boxes with both ends removed. A fan blew a stream of air over a test surface, while the operator did his best to measure the forces at work.

By the time the Wrights entered the field, so many studies had been conducted that it was no longer easy to differentiate between accurate data and the faulty product of flawed experiments. Whatever information an experimenter chose to trust, at least the equations for using this data to predict the behavior of a wing were well established.
¹¹

Wilbur and Orville discovered two formulas in the published work of Lilienthal and Chanute—one for calculating the lift that would be produced by a particular wing under certain conditions, and the other for predicting the amount of drag. The basic lift equation looks daunting to the lay eye:

L = k x S x V
²
x C
_L

Broken down into its components, the equation becomes easier to understand:

L
= Lift in pounds

k
= Coefficient of air pressure

S
= Total area of lifting surface

V
²
= Velocity (headwind plus air speed) squared

C
_L
= Coefficient of lift

Wilbur began solving the equation by inserting two pieces of information he believed to be valid. He knew that John Smeaton had established a figure of .005 as the coefficient of air pressure. Other experimenters had disputed that value, but the Smeaton coefficient remained in common use. Lilienthal and Chanute had both employed this figure in their calculations and they had flown. That was good enough for the Wrights.

Values for the coefficient of lift were much less certain. The lift
coefficient (C
_L
in the equation) varied with every airfoil shape (wing cross-section), at every angle of attack. Once again, the Wrights decided to put their faith in Lilienthal’s results.

The German experimenter had conducted his own airfoil research before building his first glider, and had included a table of coefficients for lift and drag through a range of angles of attack in an article entitled “Sailing Flight,” published in the
Aeronautical Annual
for 1896. Referring to that table, Wilbur found the lift coefficient for the range of relatively low angles of attack at which he would be operating—7 to 10 degrees. He began his calculation with the coefficient (0.825) given for an angle of 10 degrees.

The coefficient might only be accurate for an airfoil precisely like Lilienthal’s—a circular arc with a camber of 1 in 12. Which is to say, the chordline, an imaginary straight line running from the leading to the trailing edge of the wing, was twelve times as long as the distance from the chord to the top of the arch at the center of the wing. The Wrights did not intend to copy the Lilienthal airfoil so closely, but they assumed that the performance of the wing shape they did construct would be approximately the same.

Using the equation was now a matter of give and take. Wilbur’s first step was to estimate the total amount of weight his craft would have to lift—his own weight, one hundred forty pounds, plus the weight of the machine itself. A quick check of the tables of weights of materials available in a standard engineering handbook indicated that his kite/glider would weigh perhaps fifty pounds. One hundred and ninety pounds would be the goal—the total amount he planned to lift into the sky.

Next, he had to choose a reasonable velocity—the speed of the wind in which he would fly. A few trial calculations indicated that he would have to operate in a headwind of 10 to 20 miles per hour. A wind velocity as low as 10 miles per hour would require an enormous wing; a 20-mile per hour wind sounded positively dangerous. He used a compromise figure of 15 miles per hour in his calculations.

With these decisions made, Wilbur solved the equation for an estimate of the amount of wing surface area that would be required to operate under his conditions:

L = k X S X V
²
X C
_L

190 (total weight) = .005 X S X 225 (15 mph 2) X .825

(from Lilienthal table)

190 = .928 X S

S = 190/.928

Surface area = 204.74 square feet

The use of the equation was a first step in the design process. Wilbur now knew that a craft weighing one hundred ninety pounds, with a surface area of 200 square feet, would fly in a 15-mile per hour wind. How was he to arrange that surface area?

He had decided that the Chanute biplane glider of 1896 was a distinct improvement over the classic Lilienthal monoplane. Moreover, that design seemed made to order for his wing-twisting control system. Cables linking the front and rear edges of the upper and lower wingtips would connect to a foot control. It was a closed system. By shifting his feet to one side the pilot would pull the trailing edge of the wingtip on one side down, while allowing the leading edge of the tip on the opposite side to rise. The entire wing structure could be twisted to the right, recentered, and twisted to the left at will—just like the cardboard box and the kite.

A great deal of thought went into the design of the airfoil for those wings. Lilienthal, Chanute, and most other experimenters had used the simplest curvature—the arc of a circle. Their wings were evenly curved from front to rear, with the peak of the arch falling at the midpoint. The Wrights chose a different pattern, moving the peak forward to a point only three or four inches back from the leading edge. In addition, they would build a much shallower wing, with a camber of only
1
/
23
, as opposed to the
1
/
12
selected by Lilienthal and others.

These changes, they realized, might invalidate the Lilienthal lift coefficients on which they had calculated their wing design. They took that chance because of a fear that the wind striking the broad forward slope of the deep Lilienthal wing would significantly increase its resistance. A shallower camber, with a sharp initial rise to a peak much closer to the front, ought to reduce the problem, producing a more stable wing.

They placed an elevator in front of the wings rather than at the rear of the craft, as they had with the small fixed surface of the 1899 kite. French experimenters would refer to such a machine as a canard, because of its resemblance to a duck in flight, with its small head carried far forward at the end of a long neck. The Wrights did not invent that design, but it was not the obvious choice.

Why did they choose the canard pattern? Imagine a board standing on one edge with its flat face to the wind. The center of the wind pressure (CP) is on a line running the length of the board at the very center. As the top, or leading edge, of the board is brought forward, angling the surface toward the horizontal, the center of pressure moves forward as well. Less surface area is being exposed. As the angle of attack decreases, the CP continues to move toward the front until, when the board is horizontal, the CP rests on the narrow leading edge, the only surface now exposed to the wind.

Earlier theorists had assumed that CP would travel to the leading edge of a cambered wing at a zero angle of attack, just as it did on a flat board. If so, the Wrights reasoned that a forward elevator would be more effective than one on the tail of the craft. If the CP moved forward onto the elevator itself, pitch control would become much more precise than if the surface was located at the rear. A surface set forward of the wings with a slightly negative angle of attack when at rest might also provide some pitch stability.

The rough lineaments of the first Wright glider were in place by the end of September 1899, three months after Wilbur had sent his first letter to the Smithsonian. Having determined to fly, he was not wasting any time. He would tackle the construction problems one by one as the work progressed. The conception—the design—was the important thing. And in that area Wilbur now felt a real confidence.

T
he construction of the machine would have to wait a while. The brothers spent the fall and winter of 1899–1900 assembling their next year’s stock of Wright bicycles. Business at the shop would keep them busy through the following spring and summer.

Wilbur was able to devote a few idle hours to the selection of a testing ground. The kite/glider could not be flown in Dayton—the machine would not lift in a wind of much less than 12 to 15 miles per hour. If Wilbur was to remain in the air for any length of time, he had to find a spot where there were strong, steady winds day in and day out. The ideal site would also offer seclusion, hills for gliding, and soft sand to ease the shock of landing.

Casting about for advice, Wilbur wrote to Octave Chanute for the first time on May 13, 1900. He was familiar with Chanute’s work, having read
Progress in Flying Machines
and a handful of his magazine articles. Wilbur introduced himself with characteristic humor as a fellow “afflicted with the belief that flight is possible to man. My disease has increased in severity,” he added, “and I feel that it will soon cost me an increased amount of money, if not my life.”
¹

He explained that the bicycle business required his full attention for nine months each year. Any “experimental work” had to be confined to the slack months from September to January. Wilbur asked for comment on his plan to kite a man-carrying machine from a tower, adding that he would be “particularly thankful for advice as to a
suitable locality where I could depend on winds of about fifteen miles per hour without rain or too inclement weather.”

Chanute, always happy to welcome a new enthusiast, sent a prompt reply. He was encouraging, but did not fully approve of Wilbur’s plan for tethering his machine to a tower. Restraining ropes were both an unnecessary complication and a safety hazard.

As for a test site, Chanute “preferred preliminary learning on a sand hill and trying ambitious feats over water.” San Diego, California, and St. James City, Florida, both offered constant offshore winds, but neither had the advantage of sand. Some other spot “on the Atlantic coast of South Carolina or Georgia” might be preferable.
²

Wilbur refused to indulge in guesswork when he could lay his hands on solid fact. He wrote to the U.S. Weather Bureau in Washington requesting information on prevailing wind conditions in various parts of the United States. Bureau chief Willis Moore responded by sending the August and September 1899 numbers of the official
Monthly Weather Review
. The September issue, which included articles on several experimental instrument kite programs and a table of the average hourly wind velocities recorded at 120 Weather Bureau stations, was especially interesting.
³

The table confirmed that Chicago was the windiest city, with an average daily velocity of 16.9 miles per hour for the month of September. But Wilbur had already rejected Chicago, and all other urban areas. Chanute’s experience in 1896 had shown that any flying-machine experiments conducted near a city would immediately attract the attention of the press, something Wilbur hoped to avoid at all cost. Nor could any of the other four stations recording average winds of over 13.5 miles per hour meet the requirements of isolation, suitable hills, and sand.

Moving down the list, Wilbur discovered that the sixth-highest average wind in the United States (13.4 mph) had been recorded at Kitty Hawk, North Carolina. He had never heard of the place. Few people had. Still, while the average wind was on the low side for the calculated performance of the planned kite/glider, additional tables in the journal indicated that Kitty Hawk offered a reasonable number of clear, rain-free days each fall, with occasional winds much above the average. It would bear looking into.
⁴

On August 3, 1900, Wilbur wrote to the Weather Bureau office at Kitty Hawk. Joseph J. Dosher, the sole bureau employee there, sent a short reply indicating that the beach near his station was a mile wide
and clear of trees and other obstructions. The winds in September and October blew from the north and northeast. Wilbur could board in the village, but housing would be a problem—he would have to bring a tent and camp out.
⁵

As an afterthought, Dosher passed Wilbur’s letter on to William J. Tate, a local postmaster, notary, and Currituck County commissioner. Bill Tate responded on his own, mentioning the “relative fitness of Kitty Hawk as a place to practice or experiment with a flying machine, etc.”

Tate was obviously a man of some warmth. He closed his letter with an invitation that was difficult to resist: “If you decide to try your machine here & come, I will take pleasure in doing all I can for your convenience & success & pleasure, & I assure you you will find a hospitable people when you come among us.”
⁶
Kitty Hawk it would be.

On August 10, Wilbur told Chanute that “It is my intention to begin shortly the construction of a full-size glider.” The work of building this machine was split between Dayton and Kitty Hawk. Before his departure, Wilbur cut, steamed, and bent the ash ribs that would give shape to his wings, and carefully fashioned the fifty or so additional wooden pieces. Components that could not be obtained at Kitty Hawk, including metal fittings and fasteners and spools of the 15-gauge spring steel wire for trussing the wings, were purchased at home and packaged for shipment. Yards of glistening sateen fabric were cut and sewn into the panels that would cover the finished wings.