Digital hydraulics in aircraft control surface actuation : Modelling and evaluation of digital hydraulic systems with focus on performance and energy efficiency

The purpose of this thesis has been to compare and analyse the use of digital hydraulic actuators in place of traditional actuators in aircraft control surface manipulation. Digital hydraulic actuator referring to a hydraulic actuator where the power has been discretized using discrete on/off-valves. For this purpose three simulation models have been used. The first model consists of a benchmark model, designed to represent a digital hydraulic actuator acting on a mass under the influence of an external spring load. The discretization in this case comes from the fact that three separate pressure levels have been used to power a four-chambered tandem piston, resulting in 81 possible force combinations.The second simulation model represents a 6 degrees of freedom aircraft model parametrised to behave like a F16 fighter aircraft. The purpose of this model has been to serve as a means to implement the digital actuator in an aircraft. The third model has been heavily based on the F16 model but re-parametrised such that it represents a delta canard aircraft. The actuators in the aircraft models was initially mounted on the control surface primarily dedicated for the manoeuvre which was simulated, in this case a step in altitude, meaning that the control surface was the elevon.As it would turn out the digital actuator had trouble achieving the precision required in order to adequately fly the aircraft at a low enough energy consumption. As such the idea took form to implement a hybrid design where the digital actuator would be paired with a proportional actuator on a separate control surface, flaperons. The digital actuator would then only require to be positioned in a close enough position and once there lock in place, leaving the proportional actuator to handle the fine tuning and trim of the aircraft. It would appear that by using the hybrid actuator design the energy consumption during the right circumstances could be reduced by as much as 40% for the delta canard configuration and 30% for the F16 case