SAE ARP4895A

This SAE Aerospace Recommended Practice (ARP) provides an algorithm aimed to analyse flight control surface actuator movements with the objective to generate duty cycle data applicable to hydraulic actuator dynamic seals. This algorithm can be used to process digitally recorded actuator positions, generated either by pure simulation, or hardware-in-the-loop simulation, or flight test of full scale demonstrator of new aircraft, of new aircraft models in development, or of in-service aircraft, depending on what is available at different stages of the aircraft development and the purpose of the duty cycle investigation. This generated duty cycle data can be used as a basis for defining dynamic seal life requirements, dynamic seal life testing, or to assess the impact of control law or other changes to dynamic seal behavior. There is a trend to design airplanes with less natural stability than before, now relying on stability augmentation systems. Reduction of the size of the vertical and horizontal stabilizers is considered as a major weight and drag reduction opportunity, therefore making stability augmentation systems more and more active, depending on the degree of stability relaxation or instability left to the natural aircraft. Moreover it is to be noted that imperfections of the control laws, non-linearities in hydro mechanical equipment etc. may generate control surface position servo loop limit cycles, detrimental to the dynamic seal life. The two most significant factors affecting seal life are the number of small amplitude reversals and the total stroke distance. This is the reason why an automatic, cost effective method for assessing hydraulic actuator dynamic seal duty cycle data at different stages of development of an aircraft project becomes a very useful tool.