A Beautiful Body

by Dr. Richard Stimson

in Inventing The Airplane

The Wrights’ flying machine had to be structurally strong, but light enough to fly. The task was made more difficult because in order to implement their wing warping system of flight control, the wings, in addition to being structurally sound, had to be flexible.

Just nine days before the Wrights’ successful first powered flight, the issue of structural integrity was dramatically highlighted when Langley’s highly touted aerodrome broke-up during launching. Post mortem analysis revealed inadequate structural analysis and design.

The Wrights, on the other hand, conducted careful stress analysis using engineering handbooks available at the time to estimate structural loadings on the wing spars and struts and to size and select materials.

The Wrights were concerned about safety from the very beginning, as was their father. In order to calm his fears, Wilbur wrote to their father in 1900 that “I am constructing my machine to sustain about five times my weight and am testing every piece.”

The Railroad Truss

The Wrights adopted a trussed biplane design as their basic approach. The concept was adopted from their friend Octave Chanute, a retired railroad bridge builder, who had adapted a “Pratt truss” design used on railroad bridges to a biplane glider he built in 1896.

Using the Pratt truss concept, the Wrights’ designed a bi-wing structure in which the upper and lower wings were trussed one above the other with struts and cross wires to form light, sturdy wing modules. Most builders of airplanes adopted this configuration for the next two decades.

Each wing was composed of eight such cross-braced modules. The trailing edge of the outer two modules on each end was not cross-braced to allow flexibility for wing warping. In this manner they had ingeniously solved the problem of how to twist the wings tips and still retain structural integrity.

The ribs of the wings were constructed of thin strips of ash that were bent to the desired camber. Blocks of wood were glued between the two strips and glued into position. The result was a strong, lightweight rib.

Bending the wings and the wingtips to the proper curvature was farmed-out to a local firm that made parts for the carriage industry. The Wrights didn’t have the necessary equipment for steaming the ash wood and then bending it to the proper camber. The wing tips were made from off-the-shelf carriage bows.

The ribs were attached to spars of kiln dried spruce. The spruce for the spars was procured from a local lumberyard. It was ordered cut into pieces of approximate length and shape. The Wrights then shaped the pieces using draw knives and spoke shaves.

All the wood pieces were painted with several coats of varnish to protect them from the high moisture environment of Kitty Hawk.

The fabric, made of Pride of the West Muslin procured from Rike-Kumler Co., a local department store located in downtown Dayton in the same block as one of their bike shops. The muslin was cut into strips and then machine-sewed with bias so that it would fit on the ribs on a 45 degree diagonal. It was then stretched over both the top and bottom sides of the spars and ribs, with each rib fitted into a sewed-in pocket. The design provided for strength as well as maintaining wing camber under stress in flight.

The wooden structure was assembled using waxed linen cord instead of nuts, bolts or screws. This design created a flexible joint that could withstand hard landings without breaking.

Orville commented that “these I believe, were the first double-surfaced airplanes ever designed or built.”

Seventy-inch spruce struts supported the upper and lower wings. The Wrights realized that a vertical column of this length would require a substantial cross-section to withstand the compression load without bending and possibly breaking. This had the potential of adding considerable unneeded weight and drag.

The Wrights solved the problem by adding a horizontal wire passing through the center of the highly loaded struts in order to prevent them from bending. By this means the cross-section of the struts could be reduced and still retain structural integrity. The proof that it worked is that none of the struts failed in wings gusts of over 27 mph during their first flights on December 17, 1903.

Back To The Future

As airplanes got faster and heavier, wing warping was replaced by the use of ailerons because of structural problems. The uses of ailerons, however, do have a down side. They increase drag and weight and therefore reduce fuel efficiency and overall performance.

Because of this performance degradation, NASA, the Air Force and Boeing are working on a $41 million project to modify an F/A-18A Hornet fighter jet with a twistable wing. The purpose of the project, Active Aeroelastic Wing, is to demonstrate that subtly twisting a wing a few degrees (up to five) can control its roll with less need for big control surfaces on the wings and horizontal tail. They hope to demonstrate that the lighter-weight flexible wings will improve the maneuverability of high-performance aircraft.

The project leaders envision that the benefits of this wing warping could apply to both military and commercial airplanes.

A traditional rollout ceremony was held on March 27, 2002 at NASA’s Dryden Flight Research center. The official Centennial of Flight logo in commemoration of the Wright Brothers first powered flight in 1903 was prominently displayed on the aircraft.

The ideas of Orville and Wilbur are still fresh after 100 years.

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