Work Still to Be Done

Stability and control has advanced over the years along two paths, in categories that might be called “fundamental” and “reactive.” Fundamental developments, such as Bryan’s small-perturbation equations of motion and Soule’s and Gilruth’s flying qualities require­ments, have arisen out of the genius of their developers. These fundamental developments seem to have come along when the time was ripe, and not to meet current crises.

The second category of stability and control advances, those made in reaction to need, are no less important or praiseworthy. Airplanes grew denser and flew higher and faster as aviation itself advanced. Each new stretch in performance and design, such as transonic flight and sweptback wings, brought fresh stability and control challenges and responses by inspired researchers and designers.

It is easy to predict that the future will bring further airplane performance gains, bringing with them fresh stability and control challenges and reactions. But are there fundamental advances yet to be made? Are there pockets of systematic ignorance that have been bypassed and grand formulations yet to be found? This is almost certainly the case.

One reason to think so is that a spectacular growth in airplane stability and control theory was interrupted at the end of the 1950s, losing for us a generation’s work in the field. This happened when the old NACA became what was for all purposes the “space agency” NASA in 1958. One can regretfully speculate on what new fundamental discoveries in airplane stability and control might have been made by people such as Harry Goett, Robert Gilruth, Christopher Kraft, Joseph Loftus, Charles Mathews, and Walter Williams had they not been called away to NASA’s space program at Houston, Goddard, and other space centers. Similar losses to the field of course took place at research centers, universities, and factories all over the world, when space programs were staffed at those places.

U. S. Air Force-sponsored aeronautical research, as distinct from space-related work, virtually disappeared in the late 1950s by edict of the general who headed Air Force research and development. This was part of an Air Force attempt to not only carve out a mission in space, but to be the sole manager of the U. S. space program. Air Force support for aeronautical research was not restored until President Eisenhower declared that the U. S. space effort would be run by NASA.

Recent times have brought additional losses in potential aeronautical talent to fields such as biotechnology, molecular biology, material sciences, computers, and communications, all of which have the novelty and glamor once attached to aeronautics. On the other hand, en­rollment in aeronautics courses continues strong worldwide, bringing new talent to the field.

A few fundamental airplane stability and control areas that seem to have been bypassed are as follows:

A general theory is needed for vertical path control in manually flown landing approaches. The theory should be able to predict for any size airplane the upper limits of thrust and pitch response delays and the download due to pitch con­trol deflection. The theory should also identify applications in which direct lift control is needed for vertical path control.

An improved predictive method is needed for incipient and developed spins, to sort out the general effects of wing-stalling characteristics, tail contributions, and fuselage shape. NASA researchers disparage the existing TDR/TDPF pre­liminary design criteria of the 1960s without offering anything better.

Generalized predictive methods are needed for handling during takeoff and landing ground runs, including data on modern tires and struts. Reliable mathematical models could predict the need for stability augmentation during ground rolls and steering difficulties in cross-winds.

The body of knowledge on static aeroelastic effects on stability and control needs to be overhauled and made accessible to ordinary designers. Experts in the aero – elastics field may think the subject is well understood, yet static aeroelasticity remains problematical for most working-level stability and control engineers. There is nothing like the assurance these engineers feel while working with the equations of rigid-body motion or with the design of stability augmenters.

The landing maneuver needs to be reinvented, particularly for small personal air­planes. Even with automatic flight controls reaching new levels of reliability and low cost, future private pilots will probably land their machines by hand. Because really good landings are so difficult to make, the time spent by pilots in flight training and flight currency now goes mainly into takeoffs and landings. New hardware, as radical as the nose-wheel landing gear was in its day, will probably be required, rather than new procedures alone.

Fly-by-wire control system reliability must reach new high levels, now that the general traveling public is carried in fly-by-wire machines. A particular con­cern is that pilot-induced oscillations may appear under circumstances not yet encountered in specific machines. Stability and control engineers must provide advice to the industry on design criteria, testing, and appropriate training and flight recording methods.

These are unfinished airplane stability and control tasks, fundamental problems left for future researchers, designers, and inventors, in addition to the reactive work that will crop up as the result of advances in other areas of aviation.