Stability and Control at the Design Stage

In the preliminary layout of a new airplane, the stability and control engineer is generally guided by some well-known principles related to balance and tail sizing, for example. Once a preliminary design is laid out, its main stability and control characteristics can be predicted entirely from drawings. This includes the neutral point (center of gravity for zero-static longitudinal stability), static directional (weathercock) and lateral (dihedral effect) stability, and assurance that the airplane can be trimmed to zero-pitching moment over its lift coefficient and center of gravity ranges.

In the best of circumstances, the new design has a family resemblance to an earlier design. Then the estimations resemble extrapolations from known, measured characteristics. All airplane manufacturers seem to maintain proprietary aerodynamic handbook collections and correlations of stability and control data from previous designs. This is a great help if the extrapolation route is indicated. Aside from these private collections, there is a large body of theory and correlations from generalized wind-tunnel data that can be called upon for prediction or estimation.

A closely related subject to the prediction of stability and control characteristics entirely from drawings is the problem posed at the next stage in an airplane’s development, when wind-tunnel test data have been obtained. In former times, one was often asked to prepare a complete set of predicted flying qualities using the wind-tunnel data and any flight control details that may have been available at the time. Instead, current practice is to plug wind- tunnel test and control system data into a flight simulator, for pilot flying qualities evaluation. Radio-controlled flying scale models are an alternate stability and control source for projects that cannot afford wind-tunnel tests.

The three design-stage topics – layout principles, estimation from drawings, and estima­tion from wind-tunnel data – are treated in this chapter.