Structural Design Part III—Testing

An Outline of Project Development, Detail, Pre‐Flight Prototype, Resonance, Static, and Full‐Scale Fatigue Testing. THE designer of an aircraft is allowed a considerable amount of latitude with regard to the method adopted to prove compliance with the strength requirements. It is necessary at the commencement to decide on a design philosophy balancing theoretical structural analysis and previous experience with laboratory strength testing in order to achieve an efficient airframe with the required standard of strength. Agreement must be reached with the Air Registration Board on the extent to which each factor will be used. The research and development costs on a new project are high, and the necessary work must be carefully considered in the light of useful information gained. Basing design on calculation and experience with an absolute minimum of testing is the economical method with regard to initial expenditure but this approach can lead to a heavy structure. An intermediate path using a single airframe for static and fatigue work allows for a comprehensive series of tests, but here the time scale of the important high static loadings required to demonstrate the safety factor is retarded to a degree where aircraft will be in service before this work is completed. Safety, and secondly retrospective modification action have to be considered and it may be that the engineer will in this case think it prudent to err on the heavy side when deciding on the sizes of the main scantlings. Efficiency is then sacrificed but with certain designs the cost of two airframes for test cannot be justified especially so if the design is a carry‐over of previous experience on similar types of structure.