Avoiding Propeller Strikes on Electric Aircraft

At last year’s Electric Aircraft Symposium, Ron Gremban, developer of the Prius plug-in hybrid, shared several questions about promoting safety in electric aircraft.  One aspect that provoked deep thought was that of safety for those working around an electric airplane, whose propeller could start quietly and possibly strike an unaware bystander.  During the Green Flight Challenge, it was noteworthy that unlike their gasoline-powered counterparts which idled while awaiting takeoff, the Pipistrel G4 and e-Genius awaited their turn to launch with propellers at rest, only spinning when commanded – and very quietly at that.

The question of avoiding prop strikes found at least one answer at EAS VII.  Karl Kaser demonstrated, in model form, his ePropeller Safety Device (eSD), noting “the risk to people, animals or objects in the propeller disk area and that they can be injured or damaged accidentally during the run-up of the propeller.”  He first noted that issue in the case of e-Genius, on which he was “charged with the production management of the e-Genius, first in the workshop shed of the Institute of Aircraft Design of the University of Stuttgart, then at Grob Aircraft in the city of Mindelheim.”  He acted as team member and occasional pilot at the Green Flight Challenge.

e-Genius presented special considerations, with the propeller located above and well behind the pilot and not readily in the field of view.  When the electric airplanes were on the ground at the GFC, they can be switched “on” with the propeller not moving.  Electric motors can also develop enormous torque at even zero RPM, making them potentially lethal even at low speeds when swinging a propeller.

He referenced the United States Pedestrian Safety Enhancement Act of 2010 and the European Acoustic Vehicle Alerting System as attempts at addressing the problem for ground EVs.  Similar approaches applied to aircraft could involve playing a jingle or sound effects to help ground personnel understand the otherwise silent state of the motor.  Such an approach might include flashing lights to alert the hearing impaired.

His ePropeller Safety Device seeks to “avoid such accidents without adding extra weight or extra components to the aircraft. Using the motor’s own sensors and microcontrollers, the propeller disk area is checked for obstacles during the run-up phase. For a short period, eSD operates the drive system in a protected mode, switching automatically to the normal operation mode after the disk area is detected clear.”

His test unit, which he displayed at the EAS, consists of an STMicroelectronics STM3210B-MCKIT motor control starter kit.  In protected mode, it detects divergences of commanded values and actual values for rotational speed, torque moment or rotor position and cuts off the power supply as soon a defined threshold is exceeded or underrun.

His video demonstration showed a Lego airline captain being literally decapitated (losing his hat) to a prop strike, but being protected when Kaser’s protection logic was switched on.  This type of logic and its application to electric aircraft will enhance their safe operation, particularly, as Karl explains, in the small airport environments from which most of us will operate.

Kaser, head of Kassaero GMBH, has been a light aircraft technician and is a Diploma Engineer from the University of Stuttgart.  He acts as a freelance engineer, dealing with composites and propulsion technologies.  Besides his work on e-Genius, he led the engineering team on Erik Raymond’s Sunseeker Duo and contributed improvements to the ultralight hang glider Archaeopteryx.


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