Modern aircraft make extensive use of hydraulic systems for actuation, principally of landing gear and flight control surfaces. Hydraulics are also used in the testing of aircraft structures and systems.

Digital Displacement is a fundamental advance in the field of fluid power, dramatically improving the efficiency and controllability of hydraulic pumps and motors. This can have impact across the aerospace sector, from actuation and propulsion of the aircraft themselves, to ground-based testing facilities.

Aircraft hydraulic systems

Testing Artemis components with phosphate ester fluid.

Testing Artemis components with phosphate ester fluid.

Hydraulic actuators are commonly used in aircraft, principally for landing gear and flight control surfaces. The hydraulic fluid on which they depend is pressurised by axial-piston pumps that are driven off the engines via gearboxes. For most of the flight time, these pumps operate at low efficiency because they are running at a fraction of their nominal capacity.

Digital Displacement pumps offers a revolutionary performance improvement by reducing pump energy losses by 60 to 80 percent. Being inherently digital and self-diagnosing, they can easily be incorporated into the overall aircraft control and monitoring systems.

Digital Displacement technology also frees up the potential of radical new system architectures by providing multiple, independently-controlled fluid outlets from a single machine.

Artemis is working with major aircraft OEMs to determine the potential for the replacement of conventional hydraulic pumps by much more efficient Digital Displacement pumps.

With support from Innovate UK, it has recently completed a project to prove compatibility of E-dyn96, our pilot product, with Type IV phosphate ester fluid. Having passed this test, Artemis is now in position to demonstrate the technology to aircraft OEMs and Tier 1 manufacturers.

Ground-based applications

Artemis force control.

Artemis force control.

Fatigue testing rigs typically use hydraulic actuators to simulate dynamic flight loads onto aircraft structures for thousands of hours of testing.

Flight simulators use hydraulic actuators to precisely control the position and attitude of motion control platforms in response to human- and computer-generated signals.

In both cases the primary energy sources are likely to be electrically driven axial-piston, pressure-compensated pumps whose low average efficiencies and poor controllability mean that such tests inevitably waste a lot of energy.

Depending on the duty cycle, the superior efficiency of Digital Displacement pumps allows the energy losses in such systems to be slashed. Costs may be further reduced by the reduction or even elimination of the need for oil cooling.

Distributed propulsion

Recently there has been great interest in the possibilities of distributed propulsion using arrays of propellers or ducted fans, for both fixed-wing aircraft and multi-rotor UAVs. This is typically interpreted as requiring the propulsive power to be electrically distributed.

However, the energy requirements of most aircraft applications, and the very low energy density of today’s batteries, mean that combustion engines are the only practical option. So in the steady state, ‘hybrid-electric’ propulsion is essentially an infinitely variable mechanical transmission between the engine and multiple loads.

Elsewhere, hydrostatics are routinely used for such purposes, offering very high power density and low cost. However, poor efficiency and controllability are major drawbacks. Digital Displacement hydraulic machines offer a radical new approach, which is digital from the ground up. Now hydrostatic transmissions can be built with efficiency and control comparable to electric transmissions.

This disruptive technology is a new way forward for distributed propulsion of fixed-wing aircraft and multi-rotor UAVs.

mo-fly: Design study for a heavy-lift quadcopter with Digital Displacement transmission.
mo-fly: Design study for a heavy-lift quadcopter with Digital Displacement transmission.
48kW engine; MTOW 250kg; flight time >8hrs.