Siemens Electric Aircraft Propulsion Unit: Inside the Digital Twin Design Strategy

Siemens, whose highly efficient motor propelled an Extra 330LE aerobatic electric plane in 2016, is continuing its ascent in the electrified aircraft market. Armed with a digital twin model of the electric propulsion unit (EPU) and flight test data, the company set out to design a powertrain with even better performance, reliability and cost-effectiveness. Engineering.com connected with Teri Hamlin, a retired Air Force colonel who now serves as vice president of Siemens eAircraft division, and Siemens engineer Olaf Otto, who gave us the inside scoop on the digital twin design strategy that the company used to refine the EPU. Fasten your seat belts.

(Image courtesy of Siemens AG.)

Electric and Hybrid Aircraft

According to the U.S. EPA, transportation, including air travel, accounts for more than one-fourth of all human-generated greenhouse gas emissions worldwide. The International Air Transport Association (IATA) expects the demand for passenger air travel to double by the year 2035. In order to reduce CO₂and other pollutants, the transportation industry needs to move toward hybrid and fully electric vehicles.

Electric aircraft are transitioning from the experimental stage to market entry, with electric propulsion units currently ranging from 45kW to 260kW (60hp to 350hp)—adequate for small personal aircraft. By 2025, energy storage capacity will be more than sufficient for medium-range battery-operated commuter planes. Siemens predicts that electric propulsion will become the standard for all segments of the aviation industry by 2050.

Electric Propulsion Unit (EPU)

Electric propulsion conjures up images of battery-powered vehicles, but that's just one option and it's currently limited to small aircraft. Although electric motors are powerful enough to fly large planes, battery technology doesn't yet provide the energy density needed for long-distance flights. That's why the aerospace industry is taking a page from diesel locomotive design by using a fuel-powered generator to produce the electricity that drives the motor, as shown in the serial hybrid electric model included in the figure that follows.

Three engine models. (Image courtesy of Siemens AG.)

Like locomotives, airplanes need to operate at multiple speeds and torques, preferably without gearboxes, something that an electric motor can do much more effectively than an internal combustion engine. This model can also include batteries to provide extra thrust when needed, keeping the generator small enough to power only the craft during normal flight.

Electronic propulsion units for manned and unmanned vehicles. (Image courtesy of Siemens AG.)

In the hybrid model, an internal combustion engine (ICE) burns fuel to generate rotation, which spins an efficient three-phase AC generator, whose redundant windings increase system reliability. The AC is rectified, allowing some of the resulting DC to power the control system and recharge the battery bank. Of course, most of the power is delivered to the AC motor that drives the propeller. The inverter provides a variable frequency output that allows the propeller to spin at whatever speed and torque are needed, without requiring a gearbox.

While flying at cruising altitudes, the generator delivers all the power that's needed. During takeoffs or extensive climbs, the batteries kick in to provide an extra boost. In the descent phase, the plane uses a form of regenerative braking, where the motor becomes a generator that's spun by the propeller, allowing the batteries to partially recharge using the free energy provided by gravity and aerodynamic drag. The same forces an aircraft battles on its way up, it can use on the way down. That's a nice little feat of ingenuity!

Holistic Digital Twin

After Siemens' SP260D motor went for a spin in 2016, the company began revising the design of the motor and control electronics using a digital twin model. "Digital twin" is a relatively new term, but the concept dates back to the U.S. space program, where engineers ran simulations prior to launching rockets, and then used the data from each launch to refine the simulation model. Perhaps the most dramatic use of a simulation is documented in this scene from the film Apollo 13.

The holistic digital twin design. (Image courtesy of Siemens AG.)

In a similar fashion but using much more powerful computers, Siemens engineers developed a holistic model of the aircraft and its major systems, including the motor, electronics, battery bank and structure.

The Extra 330LE. (Image courtesy of Siemens AG.)

Digital Twin as a Design Strategy

The holistic digital twin serves as the basis for rapid development and continuous improvement. The components, including the motor, electronics, batteries, cabling and structure, are modeled in detail and then connected to the system, enabling designers to see how the parts will work separately and in conjunction with each other. In the figure that follows, you can see how the digital twin helped engineers optimize the engine’s structural mechanics (SM), electromagnetics (EM) and thermodynamics (TD).

The structural mechanics (SM), electromagnetics (EM) and thermodynamics (TD) optimized in a digital twin design. (Image courtesy of Siemens AG.)

To reduce the aircraft's weight while maintaining its structural integrity and reliability, designers employed linear and nonlinear algorithms, representing each component as thousands of individual parts and running millions of simulations to evaluate performance under a variety of conditions. For example, the original bearing shield (shown below), which protects the motor's internals, weighed in at 10.5kg. After optimization, it was trimmed down to a svelte 4.1kg with no reduction in performance. While not every component achieved a 60 percent weight loss, engineers were able to improve the motor's torque-to-mass ratio by 50 percent, from 20Nm/kg to 30.6Nm/kg.

The aircraft’s bearing shield. (Image courtesy of Siemens AG.)

Once they eliminated the excess weight, the engineers set their sights on increasing the motor's power by optimizing its core. To do so, they analyzed the magnetic fields at various points in the windings (see below), which enabled designers to reduce electromagnetic stresses and decrease losses. The engineers also increased the motor's diameter and reduced its speed, both of which increased the motor's output torque. (Motor torque is directly proportional to diameter and inversely proportional to speed.) I asked for more details on the motor design changes, but as that information is proprietary, Siemens was unable to share it with me.

The magnetic fields at various points in the windings. (Image courtesy of Siemens AG.)

Unless one is designing a furnace, heat is almost always a waste product. In this instance, every unit of heat represents energy that's not being used to propel the plane. In addition, thermal expansion and contraction lead to premature failure of components, so Siemens engineers addressed thermal issues at both a macro and micro scale. Comparing simulations with actual flight data, the engineers traced heat flow throughout the system, analyzed materials, and optimized the design of the cooling system.

(Image courtesy of Siemens AG.)

Power-to-weight ratio, electromagnetics and thermal properties are all interdependent, making it a challenge to improve one without diminishing another. That's where the holistic digital twin strategy—using simulations to guide design and then later using actual test data to refine simulation models—is crucial. Siemens engineer Olaf Otto elaborated on how this process led to the development of a new motor: the SP200D:

"We have incorporated a number of larger design changes, which also resulted from projects run ahead of the SP200D. This goes from the structural side, for example, looking at the way the bearings are configured and used, through the way the cooling channels are designed, to optimizations in the electromagnetics and others. The SP200D has a solid grounding, taking key elements from the SP260D, as well as from another motor designed for the HEMEP project, and brings these together in its own configuration." (Note: HEMEP is the Hybrid Electric Multi Engine Plane, another project that Siemens is working on.)

The Internet of Wings

If you haven't heard of the Internet of Things (IoT), I'm guessing this is your first visit to engineering.com. The same IoT tools that allow industrial engineers to monitor and control manufacturing processes also give design engineers insights into a product's performance in the field (or in the air, as it were), closing the loop between modeling, simulation, design, testing and refinement. Sensors are now small and inexpensive enoughto gather information from virtually any part of a plane, producing a wealth of data that, when properly organized, helps engineers understand how each design modification affects performance. It's no surprise that Siemens engineers used an in-house IoT platform—MindSphere—to collect, analyze and report flight data.

(Image courtesy of Siemens AG.)

Test Data

Using simulation models of the SP260D, Siemens attached a slew of sensors to the electrical system and monitored various parameters during the aircraft's 2016 flight. The graphs in the image that follows show a few of the results, with the blue lines representing simulator data and the red lines indicating actual measurements.

Simulator data and actual measurements. (Image courtesy of Siemens AG.)

New and Improved Motor: SP200D

In spite of its lower power requirement, the SP200D delivers 50 percent more torque than its predecessor with no increase in weight. What's even more impressive is that the redesign took less than 10 months to complete, thanks to the digital twin design strategy.

The SP200D motor. (Image courtesy of Siemens AG.)

Environmental and Economic Impact of Electric Aviation

Okay, so you're thinking, "The hybrid model still burns fuel, so what's the point?" It's true that these are not zero-emission vehicles, but a single generator can power several small motors as opposed to one or two large ones, enabling a plethora of propeller configurations, some of which are more aerodynamic than conventional planes. Electric generators also run more quietly, reducing noise pollution near airports. Finally, since a conventional airplane's engines must be large enough to handle takeoff and ascension, they're oversized for normal flight, reducing their efficiency. A hybrid plane will burn less fuel than a conventional aircraft, resulting in lower overall emissions.

Electric flight also makes economic sense. For personal aircraft and commuter planes, battery-electric engines cost less to operate and maintain (much like electric cars), and with today's battery technology, they're capable of traveling around 600 miles (965 kilometers) on a single charge.

Want to learn more about the advantages of electric aviation? Engineering.com covered the topic in this article and this white paper.

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