1775. Ultimate Factor for Structural Design of Modern Fighters

Paper

Abstract

The ultimate factor for fixed wing aircraft has not been reduced from 1.5 for more than 50 years. Many arguments could be brought forward which clearly indicate that prediction of loads is more accurate today. A case could also be made that stresses can be determined with more accuracy applying sophisticated finite element methods. On the other hand opponents could claim that owing to these sophisticated prediction methods the structure is much more exploited and therefore the factor of safety against failure would remain unchanged. Because these arguments cannot be settled – especially when dealing with a certification agency – a maximum load concept is proposed. the flight control system for a naturally unstable aircraft will limit the design parameters – such as accelerations, acceleration rates, velocities, attitudes – in such a way that limit design loads are not exceeded. The reliability of this load limitation is as high as that one of the flight control system itself assuming, that no condition can be tolerated that would make the A/C unstable or uncontrollable. Load cases which cannot be limited by the FCS – particularly landing cases – are still considered with ultimate factor of 1.5. Also loads arising from gusts must be treated with 1.5 if there is no gust load alleviation system available. Fortunately most of the design cases for fighter airplanes arise from maneuvers, and this is the reason why considerable structural weight reductions can be achieved when the proposed concept is applied.