A case of longitudinal stick-free dynamic instability of an aircraft fitted with power-operated control, g-restrictor and spring feel

Abstract

Following flight experience of a particular aircraft, the effect of including a bob-weight and feel spring in the circuit of a power-operated longitudinal control on the dynamic stability of the aircraft is investigated. The main findings of the investigation, which are given in the Introduction and fully set out in the discussion and conclusions at the end of the paper, can be summarised briefly as follows: (a) With such a control system, instability of the aircraft short-period oscillatory mode can result. In these circumstances damping of this oscillatory mode deteriorates progressively with increase of speed. (b) Friction in the control circuit is an important factor affecting the characteristics of the aircraft stability. (c) It is considered that by care in design, particularly as regards positioning of the bob-weight, and choice of gearing, such instability can be avoided. Each case, however, requires examination on its own merits, on the lines of the analysis given here. (d) For setting up the equations of motion, the transfer function of the power unit is required. In the present calculations a simple approximation is used, which raises the degree of the characteristic equation from a quartic to a sextic. (e) Some consideration was given to the effect of changes in the moreimportant design parameters, but it is found that apart from the gearing and position of the bob-weight mentioned above, they may have (within reasonable limits) only a mild palliative effect. The main investigation was done by means of the usual mathematical analysis, with friction represented by an equivalent viscous damping. Additional results are obtained by the use of the Nyquist presentation and of the Philbrick Electronic Analog Computor.