Airplane Vs. Bird

The use of nature as a pattern (“An Airplane Is Not a Bird,” by John S. Harris, Fall 1989) has been common in many fields of design. Early plastics were often used to mimic other materials. Researchers now are working on computers structured like the human brain. I often wonder whether the result will be a machine capable of jumping to conclusions at blinding speed.

Gary Welch
St. Joseph, Mich.

Airplane Vs. Bird

John S. Harris raises some valid points, but I think he also misses or misinterprets a few.

The most significant blind alley induced by mimicking birds lay in the size of the horizontal tail and length of the fuselage; Sir George Cayley and the other early glider builders did not perceive that birds are unstable and are actively stabilized by an extremely sophisticated neural control loop that constantly makes small control movements. For a human pioneer flying solo, this skill would be harder to learn than riding a unicycle. The Wrights, with contributions from Lilienthal and Chanute, changed the proportions to make the machine more stable at considerable expense in maneuverability. Now that computers are approaching bird-brain performance, it is possible to achieve maneuverability improvements with the “relaxed stability” concept of the X-29 aircraft.

It takes advanced mathematics to design a monocoque structure—and industrial-level equipment to manufacture it economically. In an age when airplanes were developed on private budgets, “birdlike” construction made a lot of sense.

There was not the faintest reason for tricycle landing gear until the 1920s, when wing loadings—and hence landing speeds—began to increase and the introduction of the paved runway created crosswinds. Anyway, the bird aspects here are pretty academic, because even tricycle aircraft are landed nose-up.

Ralph Jones
Aurora, Colo.

John S. Harris replies: Mr. Jones raises some interesting points. His strongest argument is for the complexity of the control system of birds; while 1 mentioned the concept briefly in the article, he carried it further. On the issue of the internal skeleton versus monocoque construction, he says the internal skeleton hung on because the engineering was simpler; I still say monocoque construction couldn’t come about anyway until the materials were available. On the tri-gear versus tailwheel landing gear, Mr. Jones is obviously a tail-wheel lover, and I admit to some of the same inclination. But tail wheels hung on well into the era when wing loading would have argued for adoption of tri-gear. Most of the fighter planes of World War II used tail-wheel gear when they would have been much easier to handle with tri-gear. And finally, yes, even a tri-gear lands with its nose up, but landing on the mains with a tri-gear is easy because the center of gravity is ahead of the main gear, and thus the aircraft is inherently stable.

Curious Memories

Your Fall 1989 issue brings some curious memories to mind. During World War II, I was assigned to the Army’s Aircraft Design Branch, at Wright Field in Dayton. One day an older man came through inquiring about our work. Later I asked an associate who the man was. He said, “You mean you haven’t met Orville Wright?” Wright, in his seventies, had free access to Wright Field and clearly enjoyed the Design Branch with its renderings of future airplanes. Our airbrush paintings were the product of a group of artists and illustrators led by the automobile designer Alex Tremulis (“The Path of Least Resistance,” by James J. Flink). Tremulis and his buddies worked in a mezzanine overlooking an open area where engineers worked. One day the engineers were attacked by a fleet of paper bombers from the mezzanine carrying bombs of modeling clay. Ingeniously, the planes had propellers whose shafts wound up threads that pulled pins to release the bombs from their shackles in time to land on our drafting tables.

Charles E. Duke
Harpers Ferry, W. Va.

Three-legged Bridge

Your article on the never-built Yshaped bridge designed by John Roebling for Pittsburgh (“The Greatest Bridge Never Built?,” by Joel A. Tarr and Steven J. Fenves, Fall 1989) reminds me of the local joke that we had with the Y-bridge that carries U.S. 40 across the Muskingum and Licking rivers at Zanesville, Ohio. I believe this is the only structure of its type in the country. The prank was to answer the out-of-towner’s question “How do you get to the north end from downtown?” with “Go to the middle of the bridge and take a sharp right.”

Steven L. Gatton
Bowling Green, Ohio