Designing for energy efficiency

Creating a new breed of aircraft can require extensive prototyping and flight testing. Some of that work might be avoided if software could accurately model the aircraft’s electrical demands under various flight conditions and with different choices of components. Keith Button spoke to engineers at Argonne National Laboratory in Illinois who next year plan to unveil software to do this.

The aviation industries in the U.S. and abroad have bold plans for transforming our lives with a variety of hybrid or fully electric rotorcraft, including advanced air mobility aircraft, versions of which would ferry us around locally or deliver packages to our doorsteps on tethers. Quiet, clean electric power is attractive for these package delivery and urban air mobility applications, sometimes together with combustion in larger designs. Even designers of large jets seek to make the most efficient use possible of electric power.

Designers of these aircraft need to anticipate how much electricity will be required when flying in various roles, such as carrying passengers versus cargo; whether the power demands decrease as expected when cargo is unloaded or passengers get off; whether choosing a particular battery or batteries will reduce or increase the range of an electric aircraft and by how much.

Engineers at Argonne National Laboratory, a U.S. Department of Energy-sponsored research center in Illinois, are writing software that could empower engineers to answer these and a host of related questions before they build and fly their creations. The engineers think the tool could be especially helpful for anticipating the energy needs of nontraditional designs, such as those relying on distributed electric propulsion.

Inspired by automobiles

The project is the brainchild of Dominik Karbowski, a mechanical engineer who immigrated to the United States from France in 2006 to help Argonne develop software to simulate the functioning of cars and trucks, whether conventional, hybrid or electric. Today, auto engineers rely on this software, called Autonomie, to digitally assemble simulated cars and trucks from a menu of individual components — motors or engines, batteries, transmissions, wheels and accessory power loads likes radios and headlights — each represented by their own mathematical models. Engineers can then assess and compare energy efficiencies of various designs and choices of components. Sensing the growing interest in electrification of aircraft for urban air mobility and other applications, Karbowski in 2018 started leading development of similar software for aircraft designers. The software, which at the moment exists in a pre-beta form, is called Aeronomie from the “aerospace” and the Greek “nomy,” for a system of rules or knowledge, and of course as a play on Autonomie. The software must quickly and accurately assess the energy efficiency of a design based on the particular combination of power components in it, much as Autonomie does for automobiles.

As he got to work, Karbowski realized that he needed to bring in an aviation expert to help lead the new project. As a teenager growing up in France, Karbowski was obsessed with Microsoft’s Flight Simulator game, and he remains passionate about aviation. But he did not pursue an aerospace engineering degree or piloting. To fill that void, Argonne hired Nirmit Prabhakar, an aerospace engineer and post-doctoral research fellow who received his doctorate at Embry-Riddle Aeronautical University.

Karbowski, Prabhakar and their teammates started with a blank slate. Aeronomie will consist of an overarching, generic aircraft model — a collection of equations, formulas, computer code and data — consisting of submodels of kinds of components. Users will then plug in design characteristics and components to create a model of a certain kind of aircraft. The engineers structured the larger Aeronomie model and its underlying models of aircraft components with the idea that design complexities could be wrapped in later. With this flexibility, a particular airframe with one propeller might be modeled with multiple propellers in the future, for instance. They also made sure that these models would be easy to read and reuse.

The main challenge is ongoing in developing Aeronomie and adapting the automotive simulation approach: Swapping components can have a much different outcome for aircraft than for road vehicles. “In a car or truck, when you change things inside, it does not have too much impact on the overall performance,” Karbowski says. But adding too much weight to an airplane by switching to a larger battery, for example, can mean that the plane won’t fly.

As they build the software, meeting the challenge requires the Argonne engineers to continue to determine which kinds of components can be swapped out or reconceived in a simulation without invalidating the larger Aeronomie model. “You have to be cognizant of what the design space is,” says Karbowski. Changing wing sizes, for example, would be beyond the scope of the tool.

Big universe; small bites

Another challenge is the much larger and more diverse universe of aircraft as compared to autos. Aeronomie must simulate everything from small uncrewed aircraft to 747s, including fixed-wing and vertical takeoff designs and hybrids of the two. That means the overarching Aeronomie model must account for a wide variety of missions: flying passengers and cargo across a spectrum of ranges, altitudes and speeds, compared to cars and trucks that are driven within similar envelopes.

The engineers decided to start small, with uncrewed aircraft, and expand from there. They built a simple motor component model, then added more component models until they had a complete model of a simple, small uncrewed fixed-wing aircraft. Then they moved on to a small uncrewed rotorcraft.

Luckily, while the parameters of the Aeronomie software can be changed for different aircraft, “on the modeling side the difference between a Cessna and a 747, the model itself, the equations are actually fairly similar,” Karbowski says.

Modeling aircraft also requires building more underlying models compared to what’s necessary in the automotive world. But the engineers discovered that the modeling language in Autonomie and the way in which the software models the performance of cars and trucks was similar enough that they could more or less reuse it. “Of course, we need to adapt these things that are unique to aircraft, but generally speaking, the physics stay similar,” Karbowski says.

Making sure the model incorporates the correct data for its parameters, such as speed and the amount of lift generated, is critical. “That’s where all the challenges come in,” Karbowski says. The equations and formulas within the model could incorporate a broad range of theoretical parameters, but their output wouldn’t make sense without the correct parameters.

Make it real

The engineers validated Aeronomie’s generic model by checking it against real-life data, which presented another challenge: finding aircraft with extensive published data. They started with older aircraft like the RQ-2A Pioneer drone, which debuted in the 1980s and had publicly available data on extensive wind-tunnel testing. They also collaborated with Pipistrel, a Slovenian electric airplane manufacturer, to model their planes and compare the models’ performance to real-life flight data.

Aeronomie won’t be open source software, but the model will be accessible and easy to modify by the aerospace engineers who use it, Karbowski says. The team plans to offer software licenses to Aeronomie starting with a limited library of data on different types of airplanes and components. Engine manufacturers or plane designers or aircraft fleet operators will be able to incorporate their own data into the models to run simulations tailored to their own aircraft. The software will be able to incorporate computational fluid dynamics programs that compute the aerodynamics of specific airframes, for example.

The team is building the piece of the software that will communicate between the model and the graphical user interface, meaning the screen the user sees. The engineers will reuse much of interface from Autonomie for the aircraft version.

Running the simulation

Users will input parameters for their simulation, such as payload, drag coefficient, lift, motor energy efficiency, battery size and propeller size. Then they’ll outline the mission: the flight distance, cruising altitude, points of landing and taking off, and whether payload will be dropped off, for example. If all goes as planned, the simulation run time will be 20 to 100 times faster than the actual flight — a 60-minute hexacopter mission takes about three minutes. The simulation runs through submodels that simulate environmental factors such as air density and wind gusts at certain altitudes, aircraft control over the flight route, the selected engine or motor generating thrust for the aircraft, aerodynamic forces during the flight, and the motion and positions of the aircraft during the flight.

Then the results of the simulation will be displayed according to what the user is seeking: graphs showing the speed, thrust, trajectory, power use, peak battery demand, electric current and overall energy consumption throughout the flight time, for example, or measurements of how certain components performed or an aggregate of thousands of simulation results for a fleet scenario.

The first beta version of the Aeronomie software is expected to be finished by July 2021. The goal is to build up a library of aircraft types and components, like with Autonomie and automobiles, so that the user can select an aircraft, parameters and mission and run the simulation with just a few mouse clicks.

The tool will be geared toward engineers with no computer programming knowledge. “Once you’re a natural with what the tool does, you can run a lot of simulations easily,” Karbowski says. “We’re providing ways to do that without having to actually do any coding.”

For Karbowski, his teenage self envisioned that he would one day become a professional pilot, but bad eyesight squelched that aspiration. Still, he says, his Flight Simulator experience taught him basic flight principles, which were useful as he built his own aircraft simulator, and just maybe enough to be a pilot of last resort: “I guess if I were in this movie where you have to pilot the plane because there’s no more pilot, maybe I could do something.”

Credit: Argonne National Laboratory

Designing for energy efficiency