Quest for Guidelines: Tail Damping Power Factor

An empirical criterion based on the projected side area and mass distribution of the airplane was derived in England, and the Langley staff proposed a design criterion in 1939 based solely on the geometry of aircraft tail surfaces. Known as the tail-damping power factor (TDPF), it was touted as a rapid estimation method for determining whether a new design was likely to comply with the minimum requirements for safety in spinning.[508]

The beginning of World War II and the introduction of a new Langley 20-Foot Spin Tunnel in 1941 resulted in a tremendous demand for spin­ning tests of high-priority military aircraft. The workload of the staff increased dramatically, and a tremendous amount of data was gath­ered for a large number of different configurations. Military requests for spin tunnel tests filled all available tunnel test times, leaving no time for general research. At the same time, configurations were tested with

radical differences in geometry and mass distribution. Tailless aircraft with their masses distributed in a primarily spanwise direction were introduced, along with twin-engine bombers and other unconventional designs with moderately swept wings and canards.

In the 1950s, advances in aircraft performance provided by the introduction of jet propulsion resulted in radical changes in aircraft configurations, creating new challenges for spin technology. Military fighters no longer resembled the aircraft of World War II, as the intro­duction of swept wings and long, pointed fuselages became common­place. Suddenly, certain factors, such as mass distribution, became even more important, and airflow around the unconventional, long fuselage shapes during spins dominated the spin behavior of some configurations. At the same time, fighter aircraft became larger and heavier, resulting in much higher masses relative to the atmospheric density, especially during flight at high altitudes.