Research Equipment for Investigation of Aircraft Structure at Elevated Temperatures

AS an aircraft flies through the atmosphere it is heated kinetically owing to its forward velocity. For subsonic speeds the effect is hardly measurable, at M= 1 ·5 it is definitely measurable and as there is a rapid change with speed it soon becomes necessary to include kinetic heating in the design conditions for an aircraft structure. All other conditions that are present at lower speeds have still to be retained and it becomes a matter of adding heating conditions to an already large number of conditions. The same approach must be used of preparing a design envelope on which appropriate factors have to be applied. In doing this it should be appreciated that there are two distinct effects of heating, one is the steady temperature condition associated with sustained steady flight conditions, the other is the rapid change in temperature and associated structural stresses and distortions when the aircraft changes speed or height. Considering first the steady temperature condition, it is evident that this can only arise in practice after a fairly long time at the particular flight condition to which it applies and that intermittent departures from it will not have a significant effect. The aircraft speed that has to be selected must of course be one that might reasonably be expected to be sustained occasionally for moderate periods, although perhaps not quite long enough to reach equilibrium. There is a comparable case in the normal strength requirements for gusts. The design gust has to be associated with an appropriate aircraft forward speed namely ‘Design Cruising Speed’. It is suggested that exactly the same speed be used to determine the steady temperature conditions with no further safety factor, and that all static and fatigue strength conditions be satisfied with full safety factors at this temperature condition.