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Pipistrel Reduces by 6% the Energy Consumption of an Electric Aircraft through Propeller Optimization

• 4 minute read

Authors: D. Eržen, M. Andrejašič, R. Lapuh, J. Tomažič and Č. Gorup - Pipistrel Vertical Solutions d.o.o., Slovenia T. Kosel – Faculty of Mechanical Engineering, Slovenia

The first electric aircraft was created by simply replacing the piston engine system of a normal combustion engine aircraft by an electric propulsion unit. Although environmentally friendlier, this was not an optimal design as the energy density of even the most advanced batteries of today is still much lower than that of gasoline engines. Optimal energy use is crucial in an electric aircraft .

Energy Recuperation as a Propeller Design Strategy

One way to improve the energy balance of an electric aircraft, is by using its propeller as an airborne wind turbine! The propeller recuperates the aerodynamic energy of a descending aircraft, which is then turned into electric energy and stored in the battery.

That is exactly what Pipistrel, a world-leading designer and manufacturer of energy-efficient and affordable high-performance aircraft, did: they designed a propeller specifically adapted for exploiting in-flight power recuperation. The objective was to improve the energy consumption of the Alpha Electro, Pipistrel’s electric trainer aircraft.

Climb is the most energy consuming part of an aircraft traffic pattern. The new EA-002 propeller was designed to exploit energy recuperation capabilities during descent, while preserving good performance in the climb flight phase.

Optimization of the Propulsion System

For the optimization of the propulsion system, a 3-way approach was performed:

  • The Aerodynamic Approach: Optimization of the airfoil shapes and the chord/twist blade distribution. Propeller design performance was verified through CFD simulations with Cadence Omnis (Omnis AutoGrid for the meshing and Omnis Turbo for the solving). Figure 1 below shows the velocity distribution over a cross section of one of the simulations.

Fig.1 : CFD geometry and velocity distribution over a cross section of a propeller

  • The Electrical Approach: Includes a hardware-enabled bi-directional energy flow, with the possibility to adjust the torque/angular velocity combination for maximum power recuperation at specific descent rates.
  • The Strategic Approach: Focused on adjusting ascent and descent rates to minimize energy consumption and maximize energy recuperation respectively.

Methodology

The performance of the propulsion units was evaluated by comparing and testing 3 propellers with the following two testing methodologies:

  1. Net energy consumption within 1000 ft ascent & descent manoeuvres
  2. The number of traffic pattern circuits performed with one fully charged battery.

These were the 3 tested propellers (Fig.2):

  • AS-D: Primarily designed for the piston engine version of the Alpha Electro, i.e. Alpha Trainer.
  • EA-001 :Developed for the Alpha Electro power train and optimized for climb and cruise phases.
  • EA-002: Also developed for the Alpha Electro power train, but optimized for climb and recuperation phases.

Fig. 2 Propellers from left to right: Pipistrel AS-D, EA-001 and EA-002 (installed on Alpha Electro).

Climb & descent manoeuvres were tested according to these parameters:

  • Climb phase manoeuvre: constant climb at 76 kts IAS for 1000 ft at 45 kW power – monitoring energy consumption, vertical speed and time to ascend
  • Recuperation manoeuvre: throttle idle, stable descent for 1000 ft at different speed rates – monitoring recuperated energy and descent rate.

Testing for the traffic pattern circuit with one fully charged battery included manoeuvres prescribed in the Alpha Electro Pilot Operating Handbook (POH) as follows:

Climb/departure, crosswind and downwind cycle phases:

  • apply take-off flaps below 60 kts indicated air speed (IAS)
  • climb from 0 to 500 ft AGL by
  • applying full power (65 kW) for first 10 s of the climb
  • applying 70% (45 kW) power and climb at 76 kts IAS
  • at 500 ft AGL apply 30% (20 kW) power and maintain 500 ft AGL

Base, final, touch-and-go cycle phases:

  • descend from 500 ft to 0 ft AGL by reducing power to idle
  • apply landing flaps below 60 kts IAS
  • descent at 50 kts IAS to the ground

Results

The AS-D and EA-001 propellers have similar climb performance and recuperation capabilities. But with the EA-002 propeller, the aircraft consumes 6% less energy during the climb compared to the AS-D propeller and is able to recuperate 0.15kWh at the descent speed of 70kts. Moreover, the propeller optimized for recuperation (EA-002) showed a 19% reduction in net energy consumption compared to the EA-001.

As for the number of traffic pattern circuits performed with one fully charged battery: the test pilot was able to carry out almost the same number of traffic patterns with the AS-D and the EA-001 propellers, and 27% more (compared to AS-D) with the EA-002 propeller. This resulted in better climb efficiency and higher energy recuperation of the EA-002 at a relatively low approach speed of 50 kts, which was kept constant for all three propellers.

The results from the climb & descent manoeuvre tests are presented in Table 1:

 

Experimental vs CFD Simulations

A comparison was performed between CFD simulation results and flight test measurements of the EA-002 propeller. Two parameters were compared: the thrust and the propeller power coefficient.

Figs. 3 and 4 depict values of thrust and power coefficients versus advance ratio of the EA-002 propeller respectively. As seen in the figures, the design predictions of Cadence's CFD simulation results and the flight test measurements of the propeller are in good agreement.

Fig 3. Design predictions, CFD and test flight measurements of the thrust coefficient for EA-002 propeller versus advance ratio.

 

Fig 5. Design predictions, CFD and test flight measurements of the power coefficient of the EA-002 propeller versus advance ratio.

Conclusions

The aircraft consumes 6% less energy during the climb. Net energy consumption during ascent/descent manoeuvres decreased by 19%. A 27% increase in the number of traffic pattern circuits was achieved.

Due to the reduction of energy consumption, the aircraft can stay airborne longer and/or smaller battery packs may be installed for a specific flight time.

The EA-002 is set to become the first European Aviation Safety Agency certified propeller with recuperation capability for electric propulsion.

Full Journal paper:

D. Erzen, M. Andrejasic, R. Lapuh, J. Tomazic, C. Gorup, and T. Kosel, "An Optimal Propeller Design for In-Flight Power Recuperation on an Electric Aircraft", 2018 Aviation Technology, Integration, and Operations Conference, AIAA AVIATION Forum, (AIAA 2018-3206)

https://doi.org/10.2514/6.2018-320