Pre-sizing a hybrid-electric aircraft propulsion

Verify and validate hybrid-electric propulsion requirements using Simcenter Amesim

The aerospace industry is at a turning point. To comply with environmental regulations and cut down fuel and maintenance costs, industry leaders and new start-ups are exploring propulsion electrification as a potential way to tackle the challenges they face.

Due to the complex nature of these new propulsion architectures, a holistic approach – accounting for electrical, aerodynamics and mechanical – is needed for the design and testing. This is needed not only to speed up the time to market but also to better comply with the new regulations.

Simcenter Amesim  offers a host of libraries (electric motors, energy storage, gas turbine and aeronautics and space) that help validate design requirements for different types of electric propulsion units.

In the model used in this blog post, we focus on the electrification of the propulsion unit of an existing combustion-engine two-seater aircraft. Instead of driving directly the propeller, the internal combustion engine now drives a generator that feeds an electric motor which in turn spins the propeller.

The purpose of the simulation model is to help select and validate a fitting Electric Propulsion Unit (EPU) satisfying the requirements below:

• The aircraft speed and rate of climb – ROC
• The aircraft range

The selected propulsion unit must also be able to perform well in the case of an emergency (combustion engine and/or generator failure).

In this article, we will first check how electric motors of different power ratings perform for a given flight mission (altitude and speed) – and pick the one that fits – then a failure scenario (generator breakdown) is run to check how different batteries will perform in terms of final state of charge in the case of an emergency.

• Overview
• Battery performance curves
• Propeller map generation
• Scenario and results
• Conclusion
• Video: Pre-sizing a hybrid aircraft propulsion

Overview

The hybrid-electric aircraft modeled in  Simcenter Amesim  is shown below. A combustion-engine-driven generator and a battery deliver power to the propulsion unit.

The system above is composed of:

• The electric motor (with its inverter and cooling): it drives the aircraft propeller. To set up the motor model, all you need is its nominal efficiency and maximum power.
• The generator: it is driven by a combustion engine modeled as a functional RPM source. Just like the motor, you’ll need the nominal efficiency and the maximum power to set it up.
• The battery: it is based on open-circuit voltage and resistance lookup tables. It provides the boost power needed to assist the generator in takeoff and climb phases.
• The propeller is also based on lookup tables: power and thrust coefficients.
• The aircraft and its landing gear: the total weight, the reference wing area and the drag/lift coefficients maps are used to set up the aircraft model.

Battery performance curves

At this stage, we don’t have the battery performance maps – remember we’re at the beginning of the design process so we only have a rough idea of the battery: nominal voltage, power and energy. We’ll use the  Simcenter Battery Pre-Sizing Tool . This tool will scan through a battery database provided with  Simcenter Amesim  and pick the one that’s closest to the power-over-energy ratio specified and rescale the closest battery’s performance curve to fit the one we need. The pre-sized battery is shown under the  results  folder in the  Information  group.

Propeller performances curves

You can use performance maps from the propeller supplier (thrust and power coefficients) or use the  Simcenter Propeller Performance Map Generator Tool  to estimate them from a rough propeller geometry.

Scenario and results

The aerodynamic data (lift and drag coefficients) of the aircraft are assumed to be available (remember, we’re changing the propulsion system of an existing aircraft). These coefficients are often the result of flight tests or a CFD analysis performed in a tool like  Simcenter STAR-CCM+ .

Now, let’s set up a flight mission using an altitude and a true airspeed (TAS) profiles. This is the way we chose to set up our flight mission in this article, but it’s also common to set up a flight mission using a profile of rate of climb (ROC) and indicated airspeed (IAS) or Mach number.

A mission of two hours and fifteen minutes is simulated.

The “baseline” mission is run using three electric motors (50 kW, 100 kW and 150 kW):

The goal was here to derive the power requirement from the climb rate and true airspeed required. A 50-kW or a 100-kW motor are not powerful enough. But the 150-kW machine can meet the climb requirement.

The simulation also shows us the range achieved with each motor. For example, the 50 kW motor, not only can’t it meet the rate of climb and speed requirements, its range is also less than the other motors’ range. 

Now let’s select the 150 kW motor meeting the baseline mission and run a generator failure scenario occurring 45 min after takeoff (so still 1h30 to go before landing).

How do different batteries perform in this case in terms of final state of charge? Can we land with a sufficient level of charge that won’t put the battery state of health in jeopardy?

Batching over multiple batteries has allowed us to select the one that satisfies our goal in terms of final state of charge – we want a minimum 20% state of charge at landing: 35 kWh isn’t sufficient but a 40 kWh battery is good enough.

Although not addressed in this blog post, the  Simcenter Amesim  model built here can also be used to  size the battery cooling system  and assess the impact of a  thermal runaway  on the battery pack.

Conclusion

In this article, we have seen how to pre-design a hybrid propulsion unit and perform what-if and failure case analyses. That’s how  Simcenter Amesim  can be used to front-load system design using easy-to-parameterize functional models. You can also watch the video below, it guides through the different steps for setting up and running the model presented in this article.

Watch the video:

link video : https://www.youtube.com/watch?v=XpehXCWNiNA  

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