Long-Lived Stability and Control Myths

The achievements of S. B. Gates, R. R. Gilruth, and others in putting airplane stability and control on a scientific basis have not eliminated a number of early myths attached to the subject. Dr. John C. Gibson (1995) lists no fewer than 15 of these myths and counters them with what we know to be correct. A few of the Gibson’s list of 15 myths and corrections follow:

Wing center of pressure (cp) movement affects longitudinal stability. Correction: Wing cp movement with angle of attack is controlled by the wing’s zero-lift pitching moment coefficient about itsaerodynamic center(1/4 chord), or Cm0. Thisparameteraffectsonly trim for rigid airplanes. Wing cp has been discarded in modern stability and control calculations and replaced by wing aerodynamic center and Cm 0.

A down tail load is required for stability. Correction: Stability is provided by the change in tail load with change in airplane angle of attack. The change is independent of the direction of the initial load.

Gibson comments that this myth survives in FA A private pilot examinations and in an exhibit at the National Air and Space Museum in Washington. This subject is distinct from the instability caused by tail down load in the presence of propeller slipstream, an effect discussed in Chapter 4, Section 6.

A stable airplane is less maneuverable than an unstable one. Correction: Unstable airplanes are notoriously difficult to control precisely. Given light control forces, a sta­ble airplane can be pitched rapidly to a precise load factor or aiming point. Gibson says, “… the [stable] Hurricane, Typhoon, and Tempest were highly manoeuvrable and were greatly superior as gun platforms to the skittish Spitfire.”

The reader is referred to Gibson’s 1995 paper for the rest of these interesting myths and their corrections.