A century of progress in aircraft materials - Part 1 Wood and Fabric

Senior Editor

This second installment of a three-part look at the first 100 years of powered flight focuses on the materials used by aircraft pioneers and manufacturers. As always in engineering, the drive has been to lighter, stronger materials that are easily machined, assembled, and repaired.

Currently, metal is king, having inherited the crown from wood and fabric, but composites seem to be the "next big thing" waiting in the wings. Technologists and futurists assure us composites will dominate the industry by the end of the next decade, but no one can point to solid evidence that composites are any better than metal, or wood and fabric, for that matter. They might be stronger or lighter, but they're more expensive to make, more difficult to inspect, and trickier to repair.

From fabric to plywood

Fabrics are lightweight yet strong enough to skin an aircraft, but they won't work in stressed-skin designs. Plywood, on the other hand, seemed just right to those trained and experienced in using wood and reluctant to make the jump to metal skins in the 1920s. One of the earliest planes with a plywood monocoque fuselage and skin was the 1920 S-1 "sport plane." Jack Northrop designed it for the common man when he worked for the Loughhead brothers, Allan and Malcolm. (They later changed their name to its phonetic spelling, Lockheed.)

Northrop also came up with a novel way to make the monocoque fuselage for the S-1. Technicians placed three layers of spruce plywood strips saturated with casein glue in the bottom half of a 21-ft-long concrete mold. They put an inflatable rubber bag on top of the material, then bolted on the top half of the mold. The bag was inflated, uniformly pressing the plywood into accepting the shape of the mold, and pressure was maintained for 24 hr. The resulting quarter-inch-thick half shells were joined with glue and formers, making a smooth, bulletlike fuselage. Unfortunately, few could afford the $2,500 plane, and the Loughhead bothers lost their aircraft company. But they'd be back. And so would the plywood plane.

The S-1 was Jack Northrop's first design for Loughhead, and an early example of using plywood for its monocoque fuselage and skin. This design reduced the number of struts and wires, thus cutting drag. Other novel design features on the plane included wing flaps and folding wings.

The Wright brothers' recipe

The Wright Flyer was not assembled from exotic materials. Here's an abbreviated list:

• Straight-grained, knot-free spruce with at least 14 annular rings per inch for the fuselage, bracings, and other structural elements.
• Ash, a durable, shock-resistant wood, for parts that need extra strength, including the skid tie bars and lower elevator control arms
• Boxwood roller-skate wheels to serve as pulleys
• Waxed twine to lash the frame together
• Some steel rod and sheet steel for hardware and strapping, along with home-made control cables
• Cotton muslin for covering the wings The Wrights used Pride of the West, a fabric commonly used at the time for women's underwear. No one is sure whether the Wright Brothers doped the fabric to make it tauter and more airtight.

Plywood at its peak

Jack Northrop, along with fellow aircraft designer, Gerrard Vultee, built another plywood plane, the Lockheed Vega, using the same half-fuselage construction method used for the S-1. This time, they added stringers, making the four-passenger aircraft a semimonocoque design. It also had a wood-skinned wing. The rechristened Lockheed brothers needed the plane to serve their airline routes. The 2,900-lb plane's paltry capacity for passengers, however, kept it from being a rousing success, but TWA and Braniff also used it as an airliner.

It first flew in 1927 with a 225-hp Wright Whirlwind engine that gave it a top speed of 135 mph. The next year, it won all the speed trophies at the Cleveland Air Races and spawned the company slogan, "It takes a Lockheed to beat a Lockheed."

By 1929, the Vega 5 was carrying a 450-hp Pratt & Whitney Wasp engine, letting Lockheed add two more seats and pushing the weight up to 4,200 lb. A NACA cowling (designed by the engineers at National Advisory Committee On Aeronautics, a precursor of NASA), improved engine cooling and streamlining, pumping up the top speed from 165 to 180 mph.

The plane set several altitude and transcontinental speed records. Wiley Post, for example, flew one to a record-breaking 40,000, then 50,000 ft, and in the process discovered the jet stream. Amelia Earhart, Jimmie Mattern, Ruth Nichols, and Rosco Turner also flew it to several records. About 128 Vegas were built and many consider it to be the premier plywood plane.