The Discovery of Inertial Coupling

Airplanes that fly near the speed of sound are designed with thin, stubby wings. Most of their masses are concentrated in the center, in long, slender fuselages. When these airplanes are rolled rapidly the fuselage masses tend to swing away from the direction of flight and become broadside to the wind. This tendency, essentially a gyroscopic effect, is called inertial coupling.

8.1 W. H. Phillips Finds an Anomaly

The distinction of having discovered inertial or roll coupling in airplanes and then explaining it mathematically in the open literature belongs to W. Hewitt Phillips, then working in the Flight Research Division of the NACA Langley Laboratory. In a 1992 paper Phillips said, “When the [XS-1] model was dropped, it was observed in the optical tracker

to be rolling, as shown by flashing of light from the wings____ In examining the records

further the oscillation… was found to represent a violent pitching in angle of attack from the positive to the negative stall” (Figure 8.1).

Phillips analyzed the problem as a gyroscopic effect, publishing his results in an NACA Technical Note (Phillips, 1948). In those days NACA used the category of Technical Notes for “the results of short research investigations and the results of studies of specific detailed problems which form parts of long investigations.” Well, nobody’s perfect – the NACA could hardly be blamed for missing the fundamental importance of Phillips’ inertial coupling results when so many other people took little notice. In hindsight, the inertial coupling analytical work clearly merited publication in the more exalted category of NACA Technical Reports as the “results of fundamental research in aeronautics.”