How much quieter are our electric airplanes?

At E-Flight we fly cleaner, quieter, and in the newest aircraft. That's our claim to fame. 'Cleaner' is easy to explain. Our airplanes do not emit CO2 because we fly on batteries. 'The newest aircraft' is also achievable, as many people are unaware of the existence of electric airplanes. But 'flying quieter'... That remains a bit of an abstract concept. How much quieter exactly? And is it really quieter? Or is it just a marketing term for less noise? In short, time for clarification. Based on research from the TU Delft in collaboration with DEAC we dive into this topic. And you won't believe what you hear...

Scientific research
Bieke von den Hoff (PhD candidate TU Delft Aircraft Noise) and Noortje Elbers (Msc. Aerospace Engineering student) stood a while ago with a huge setup of microphones at the end of our runway to make extremely precise sound measurements of our Pipistrel Velis Electro. We will publish a number of accessible articles about the results of this research here. In this first edition, we will focus on the sound of our aircraft vs. the sound of an average city. In the next edition, we will delve deeper into the difference in sound between our electric and traditional comparable aircraft. 

Acoustic cameras
The sound measurements were done with 2 acoustic cameras: A standing version with 112 microphones for measuring the run-up (testing the engine before flying). And a horizontal acoustic camera with 64 microphones that you can fly over. In total, it was done 4 times over the Acoustic camera flown horizontally3 take-offs and 3 fly-overs (flying low above the ground).

The results
The microphones recorded a sound level between 107.5dBA and 109dBA during take-off64 kW, 2520 RPM, 60kts)between 101.8dBA 102.6dBA at the fly-overs (24kW, 80 kts, 2050 RPM).

Where can you compare that with? According to the OSH Academy (Occupational Safety and Health) 100dBA corresponds to the sound level of a sawmill and 110dBA to the sound of a chainsaw. 

Not really quiet
We hear you thinking: Sawmill... chainsaw... That doesn't sound very quiet. That's correct! But beware! These are registered sound levels when measured at a distance of one meter from the source. So as if you were hanging one meter below our airplane at the moment of take-off or fly-over with your ears... This is a situation that will of course never occur in practice. Usually, you will see us flying at much greater heights above a city or above your house. 

Still really quiet
So what is realistic then? Within built-up areas, the minimum flying height is 1000 ft. (304 meters) above the highest obstacle. So a realistic situation is that we fly between 1500 ft. (457 meters) and 3000 ft. (914 meters) above a city. The sound measurement from one meter away from our aircraft can be accurately calculated for other heights using formulas. And what turns out? If we fly at 1500 ft. (24kW, 80 kts, 2050 RPM)On the ground, we produce a sound level of 47.8 dBA with us. And if we at 3000 ft (24kW, 80 kts, 2050 RPM) Flying, even on the ground, only leaves 41.1 dBA! 

City noise vs. Electric Airplane
In a city, there are also various sounds produced (such as traffic, people, construction, etc). A neighborhood like Tuinwijk in Utrecht produces a noise level between 56dBA and 60dBA*. So, if we fly above such a city at an altitude of 1500 ft., with 47.8 dBA we do not exceed this sound and you probably won't hear us flying over. If we fly higher than 3000 ft., with 41.1 dBA we are even further below the sound level of such a city and you really cannot hear us.

(*bron: https://geluid.rivm.nl/ Considering that these are average values of an A-weighted sound measurement for day-evening-night. Where sound in the evening and night receives a penalty because it causes more nuisance.

Conclusion
Electric flying is really very quiet. Imagine if we - to start with - would perform all small aviation (training flights, pleasure flights, and advertising flights) electrically. Imagine how it would sound in cities around an airport or for residents. And think about what this would mean for the health of all residents. That really makes us a little quiet...

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References and additional information

Check out the master thesis of Noortje here: https://repository.tudelft.nl/islandora/object/uuid%3Ad566dd4b-b6ac-4b52-b05b-0e1391281936?collection=education

Read more about this The functioning of an acoustic microphone setup and how to locate the precise source: https://www.tudelft.nl/en/stories/articles/lockdown-presents-unique-opportunity-to-study-sustainable-taxiing-at-schiphol/

Read more here about the potential effects of noise pollution on health: https://www.rijksoverheid.nl/onderwerpen/geluidsoverlast/vraag-en-antwoord/is-geluidsoverlast-in-mijn-woonomgeving-schadelijk-voor-mijn-gezondheid

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