Longitudinal stability design requirements usually call for a pull force on the control wheel of increasing magnitude as the speed is reduced from trim to a speed slightly above the stall speed. Whether or not an airplane can meet this requirement depends upon its stick-free stability and the characteristics of the elevator hinge.
Control force stability can be produced even though control position stability does not exist. The most powerful method for improving control force stability is by means of the double bungee in conjunction with an adjustable stabilizer or adjustable trimming tab. The simplest device consists of the single downspring.
This paper discusses the uses and limitations of various types of devices in producing control free stability.
The present generation of light aircraft is subject to nonlinear hinge moment and pitching moment variations at high power and low air speed. The author demonstrates a typical unstable wheel force regime, and shows by analysis a simple device for eliminating this problem.