Flying Qualities

Just how did one design an airplane with the right mix of stability and control? Pilots pointed out that mathematics and theory did not necessarily produce the best airplanes. The question of what pilots wanted and what they liked still dogged engineers. In the 1930s, they began surveying pilots on this question, and were surprised to find that the pilots' opinions had little to do with the numbers that defined an airplane's performance.28 Some aircraft designed by the most exacting engineers proved to be unpopular with pilots, while others that looked less remarkable on paper proved to be favorites. Engineers lacked the tools to describe what became known as ''flying qualities''—the varied and subtle characteristics that made an airplane satisfying to fly. If the controls were too ''heavy,'' the airplane felt sluggish. If they were too ''light,'' the airplane was unruly and difficult to control. ''Stable, but not too stable''—what did that mean?

Here it is worth taking note of the National Advisory Committee on Aeronautics, or NACA, the predecessor to NASA (unlike its successor, ''N-A-C-A'' was pronounced as individual letters, and not a single word). One cannot grasp the history of aviation in the

United States without understanding this organization, and one also cannot begin to assess the engineering cultures of Apollo without appreciating their NACA roots.

NACA was among the most innovative and productive research programs in the twentieth century, generating much of the modern science and technology of flight. Founded in 1915 as an effort to organize American research for World War I, it proceeded to operate a set of laboratories that churned out everything from basic airfoil shapes to test data on the latest military bombers.29

The agency's flagship laboratory was the Langley Memorial Aeronautical Laboratory in Langley, Virginia, founded in 1917. There engineers and scientists concentrated on aeronautics;there the wind tunnel reigned supreme.30 NACA Langley also produced the engineers who would lead the nation in manned spaceflight, and who would conceptualize and run the Apollo program.

In the 1920s, NACA Langley engineers began studying the problem of designing stable yet responsive aircraft and brought the subjective notion of an aircraft's flying qualities into the realm of engineering precision. Researchers measured the forces a pilot exerts on the cockpit controls and how they translate into an airplane's motion.

Soon a group grew up around a young Langley engineer named Robert Gilruth. Building on earlier work by engineers like Edward Warner and Hartley Soule, and working with engineer/test pilot Melvin Gough, Gilruth took fifteen airplanes with different types of controls, installed the latest instruments, and measured a series of flight parameters. Gilruth asked ''what measured characteristics were significant in defining satisfactory flying qualities, what characteristics it was reasonable to require of an airplane, and what influence the various design features had on the observed flying qualities.'' He did not come up with precise specifications, but rather with ranges of control responses that pilots would find agreeable.

It turned out, for example, that a pilot controlling an airplane was more sensitive to the forces on a control stick than to its position. Gilruth defined a famous parameter, ''stick force per g,'' to measure how much the aircraft accelerated in each axis for a given amount of stick force. He defined similar parameters, and methods for measuring the pilot's opinions, for other control axes as well.31 In 1941 Gilruth published a set of requirements for the flying qualities of aircraft that the navy and army air force quickly incorporated. Gilruth's paper became a milestone document that guided aircraft designers for decades to come.

Flying qualities research made for interesting work, calling for close collaboration between pilots and engineers. It also required innovations in instruments, for the engineers had to measure and record a variety of parameters in flight—from the forces the pilot was placing on the controls to the airflows over the wings.

Engineer W. Hewitt Phillips exemplified the field. He had studied instrument engineering with Draper at MIT. As a young engineer in 1940 Phillips joined Gilruth's group, the Flight Research Maneuvers section, soon renamed the Stability and Control branch. Phillips would plan the flight tests by providing a series of prescribed maneuvers for a pilot to execute in an instrumented airplane, and then analyze the data. It was the dream job for an engineer who loved aircraft, combining practical technology with data analysis and modeling.32

Others in the United States and elsewhere worked on flying qualities during these years (particularly at NACA's Ames Research Center outside of San Francisco), but Gil-ruth's group was the leader, and has special interest for our topic here. Gilruth later became chief engineer of NASA's manned space efforts, including the Apollo missions. Barely two generations of engineers separated the stability and control debates of the Wright brothers' era from the human-machine issues in Apollo. As we shall see, ''flying qualities'' and ''pilot opinion'' became formal terms that would recur throughout the aerospace projects of the 1950s and 1960s, and in engineering discussions of Apollo.

After World War II the arrival of jet engines brought not only higher speeds, but also entirely different aerodynamics and airframe designs. New realms of flying qualities called for new research, and a new kind of piloting. Cultural critic Roland Barthes, writing in 1957, noted ''a sudden mutation between the earlier creatures of propellermankind and the later ones of jet-mankind.'' A Saturday Evening Post headline simply ran, ''Jet Pilots are Different.''33 The jet age required new ''flying qualities,'' for people as well as for aircraft.

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