Modern aircraft rely on highly integrated electric drive systems to deliver precise control and reliability in demanding operating environments. Selecting the right development approach for these subsystems can simplify integration, reduce qualification effort, and improve performance. Andrew Gibson, maxon UK & Ireland’s head of aerospace, explains.

Engineering electric drive systems for aviation

To control the flightpath of a modern aircraft, fly-by-wire is relied on, replacing the direct mechanical link from the pilot’s sidestick to the rudder and ailerons with electronic control signals. Although autopilot and assistance functions augment flight control, the role of the sidestick remains crucial, allowing precise input while accurately presenting the forces acting on the aircraft back to the pilot, enabling them to make an effective response.

This function is known as force feedback, translating flight conditions into subtle, tactile cues that the pilot can interpret instinctively. Instead of transmitting these forces back to the pilot’s sidestick through rods and cables, electronic signals communicate with a drive system, based on an electric motor and control electronics. As the drive system presents the resistance of the load against the sidestick, highly accurate and consistent torque delivery, delivered with smoothness in motion and a rapid response time, is essential for precise pilot control.

However, for the tier one aviation supplier manufacturing the sidestick as a complete assembly, the force feedback drive is a sub-system within a wider integration challenge. The sidestick includes other sub-systems such as position and force sensors, communication interfaces, flight control computers and software, as well as power management. Each of these elements must be validated individually and in combination, with environmental and functional testing to standards including DO-160 and AS/EN9100.

Drive system development approaches

Within the design of the sidestick as a complete unit, the critical reliance on the force feedback drive system demands specialised development. The drive system includes individual components including a DC or brushless DC motor, a gear head, position sensor, and control electronics; each present their own challenges in design and testing ahead of integration into a complete drive system that must operate in exacting coordination.

To reach this stage, crucial considerations include torque density, motion control precision, energy efficiency, as well as reliability and lifetime; each criterion requires its own levels of enquiry, achieved through theory, modelling, and practical iteration, before the complete drive system can be tested and optimised.

Although a drive system developed for a sidestick force feedback application has specific requirements, a similar process for the development of wider drive systems deployed across an aircraft can be applied. Even so, this approach will generate very different design requirements, whether that’s for drives controlling the landing gear, or those actuating the valves within the cabin’s air con.

For the tier one aviation supplier, the development of the drive system can be achieved in-house, procuring a proportion of the components then integrating them together, or developing the entire unit, ground-up, with internal resources. The advantage of this approach is the potential for initial lower-cost procurement, combined with the visibility afforded by in-house management.

In contrast, the development of a drive system for an application like sidestick force feedback is a special case, demanding optimum precision with a direct impact on flight control – requirements that need specific drive system expertise. However, aviation applications with a less critical requirement can still benefit from additional engineering support.

Key drive system requirements for aviation applications

Torque density is a key requirement for drive systems installed across an aircraft, where hundreds are involved, making it a focus of specialisation for aviation drive system manufacturers. Specific experience designing for high torque and low mass can also be transferred from sectors with equal, or even higher, demands on this requirement, such as space engineering or Formula One motorsport.

Taking this engineering know-how, while brushed DC motors developed specifically for aviation can achieve relatively high torque density, multipole brushless DC motors optimise the capability. Flat, brushless DC designs can also combine a compact footprint with high torque density, for example, maxon’s EC-i 40 AERO generates 70 Watts power and 151mNm torque for a 40mm-diameter motor weighing just 240g. The Boeing Dreamliner uses 50 of these motors with a similar design in its air con system alone.

In addition to torque density, the crucial challenge for aviation drive systems is performance within demanding environmental conditions. Drive system manufacturers set up for aviation application testing will be capable of proving operation in high and low temperature extremes, including start-up voltage and current performance down to -55°C, as well as operation during high humidity, and testing according to vibration and shock.

Working with a drive system manufacturer that provides in-house testing in accordance with DO-160G not only confirms resilience and reliability, but also reduces development effort by providing pre-validated data that supports sub-system integration. This also frees-up in-house engineering teams to focus on sub-system integration, rather than qualification of the drive system – or its individual components.

Customisation and a complete drive system

If an aviation supplier developing a complete system, like a sidestick assembly, prefers to involve external engineering to develop the drive system, they may procure individual components from several third-party manufacturers with the aim of adding flexibility over selection. This approach might be preferred in particular if the aviation supplier intends to use in-house electronics and software to control the drive system.

Alternatively, procuring a complete drive system from a single supplier can make development significantly faster and simpler. A complete subsystem removes the time and resources to achieve integration, whether matching physical interfaces or communications and control, and instead provides a drive system designed as a single unit. Moreover, procuring a fully integrated drive system from a single supplier can simplify DO-160G qualification by reducing system integration variables, as well as providing validation testing for the complete drive system unit.

Customisation can take ease of integration a step further. While a drive system specialist can design and manufacture bespoke components, such as customised motor windings or specific cabling and harnesses, full advantage can be achieved by applying customisation to a complete drive system, where a complete actuator and sub-assembly can be designed specifically for the host system.

Drive system customisation can also increase performance beyond what’s available with standard, off-the-shelf products. Designing and testing a drive system to meet exacting criteria, which could relate to aspects such as torque delivery and duty cycle, thermal protection, or lubricant operation in vacuum conditions, can provide aviation suppliers with the opportunity to add value.

Engineering support

The most important advantage of partnering with a single drive system supplier, whether that includes customisation, standard products, or flexible design changes to standard products, is the advantage of engineering collaboration. The reliance on drive system engineering expertise can help guide the specification, optimise performance, and improve time to market.

With engineering support offered throughout a project, from ideation through to field support following live roll-out, the reduced demands on internal resource can even present a lower-cost outcome in the long term. Electric drive systems will remain essential within aviation, and an engineering partnership can be a straightforward way of achieving a value-add. 

Author: Andrew Gibson, maxon UK & Ireland’s head of aerospace.