Design of Transparencies

Abstract

Summary:—A study of engineering problems encountered in the design of transparencies for supersonic aircraft is presented with a special emphasis on the effects of kinetic heating. The paper is written primarily with optical transparencies in mind but many of the general principles discussed are equally applicable to the design of radomes for elevated temperature applications.The first parts of the paper consider briefly the methods of calculating temperature distribution in transparent panels subjected to kinetic heating (or cooling) and this leads naturally to discussion of the effects of the non-uniform temperature distribution, differential expansion and the edge restraints preventing thermal deformations. Methods of calculating thermal stresses are suggested through the concept of the “ self-equilibrating ” temperature distribution; the main attention, however, being concentrated on the derivation of simple engineering formulae rather than on the mathematical rigour of the solution.The choice of design cases for combined pressure and thermal loading is considered and it is shown that, in general, deceleration manoeuvres combined with outward pressure constitute the design criteria for transparencies. The effects of various parameters on the magnitude of thermal stresses are analysed; these include the effects of altitude, temperature, deceleration and material properties. It is shown that, for given material and panel geometry, the thermal stresses depend mainly on the following parameters: total temperature change, acceleration (or deceleration), and the Biot number based on the panel thickness.Practical design aspects are also considered, such as choice of materials, strength, panel geometry, mounting of panels and the fail-safe design. Finally, testing of transparencies under pressure and thermal loading is briefly discussed, together with possible methods of strain measurements, and a tentative proving procedure for transparencies is suggested.