• 09 May 2024 03:50 PM
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Coming to Terms: X-By-Wire

By Dan Newman
Vertiflite May/June 2024

This series addresses the uses of terminology, expressions or idioms that could be misleading, confusing, unfavorable, erroneous or prejudicial. This installment addresses terminology regarding the burgeoning electrification of power and control systems in aircraft.

Today, aviation is enjoying extraordinary expansion in available capabilities and products, often attributed to simultaneous advances in many technologies, including computing, sensors, electronics, electric machines, display devices and more. Several of the resulting system-level breakthroughs fall into the “by-wire” category, including power-by-wire and control-by-wire capabilities. Control-by-wire innovations include drive-by-wire (DBW) and fly-by-wire (FBW) approaches.

Most simply, these “by-wire” terms indicate the replacement of a legacy means of transmission with electrics. The electric transmission lines are generally desirable as they significantly simplify complex routing, reduce redundancy costs, reduce leaks, facilitate repair and reduce weight compared to the legacy mechanical means.

This broad characterization of “by-wire,” though, is not by itself fully descriptive, as it includes a number of different domains and manifestations.

In terms of domains, power-by-wire includes replacement of primary mechanical power transmission with electrics as in electric cars (e.g., the Chevy Bolt), airplanes (e.g., the Pipistrel Velis Electro) and vertical flight aircraft (e.g., the Archer Midnight).

Electric propulsion (EP) that replaces mechanical power transmission with electrics simplifies complex routing, eases redundancy and facilitate splitting. This last feature is what enables many of the emerging electric vertical takeoff and landing (eVTOL) platforms to employ distributed propulsion (DP), to support lift and thrust around the center of gravity in hover and center of lift in forward flight. Combining EP and DP results in distributed electric propulsion (DEP), which is the most ubiquitous approach for eVTOL aircraft designs.

For electric aircraft, conversion to mechanical power is required at each thrust device, with added motor weight and some losses. However, a major benefit of electric transmission is the opportunity for digital control in lieu of analog control (where output is proportional to input). Digital control enables thruster speed changes and completely stopping them at will, where unreliable and heavy clutches and gearsets were previously required.

Power-by-wire also includes the replacement of hydraulic secondary power lines with electric lines, as implemented on forklifts and aircraft. However, it does not necessarily mean the full replacement of all control and power with electrics. Large aircraft employ electro-hydrostatic actuators (EHA)—self-contained hydraulic systems (with electric pumps, hydraulic accumulators and hydraulic actuators)—that
look and act like electric actuators. The EHA benefits from electric transmission with reduced leaks, simplified routing, easy redundancy, simplified merges and splits, and reduced transmission weight.

The other “by-wire” domain is control (e.g., FBW and DBW), with electric signal transmission replacing mechanical devices. This simplifies routing, but it doesn’t necessarily mean the use of digital control — many implementations of electric actuators rely on analog control.

Digital control is very powerful, allowing complex and nonlinear relationships between inputs and outputs. Note that complex input/output relationships were around long before digital electric control systems, such as those successfully implemented in the mechanical mixing on the US Army CH-47A Chinook in the 1960s, and are still employed on the US Special Operations Command MH-47G today.

Many aircraft currently in development are working on digital fly-by-wire (DFBW) control to capitalize on its ability to customize the response, simplify the piloting tasks, introduce automation, shape the handling qualities, maximize performance, minimize loads and vibration, and eliminate pilot technique. But such digital controls dramatically increase the complexity, and consequently, the non-recurring timeline, cost and regulatory scrutiny. And software is no small part, but that is for another column.

So, when you hear the term “X-by-wire” — such as power-by-wire or fly-by-wire — recognize that it could indicate one of many different implementations.