Can hydrogen-electric engines promise a quieter, as well as cleaner, tomorrow?
A question we often receive at ZeroAvia is, will a hydrogen-electric engine be quieter than conventional aircraft? It’s a reasonable question. We are all experiencing how quiet electric road vehicles can be (to the point where manufacturers must artificially introduce noise for safety), and the many communities of people who live near airports or under flight paths want to see a similar noise pollution drop.
Today, policymakers strike a difficult balance between the economic and societal benefits of more flights and the health and quality of life impacts of air pollution and noise from aircraft. In terms of zero-emission aircraft, we are confident that the air quality impacts can be eliminated, but we will look at that in another blog. Here, we focus on noise.
The good news is that, based on a review by Southampton University, as part of the Aerospace Technology Institute’s FlyZero work, academics predict that hydrogen fuel cell aircraft have the potential to achieve “very low noise attributes”.
The regional, fuel cell concept aircraft envisaged by FlyZero has the potential to be 16 decibels quieter than its conventional counterpart, according to the Southampton team’s contribution. This is based on a clean-sheet aircraft concept akin to an ATR72-600, one of ZeroAvia’s target airframes for its ZA2000 powertrain.
Aircraft noise comes from two main sources: the airframe and its propulsion system. With the airframe noise — largely created by airflow surrounding the plane interacting with uneven or sharp surfaces — we cannot expect any immediate reductions with retrofit hydrogen-electric regional turboprop aircraft, as this is a question of the airflow surrounding the aircraft. However, to maximize the efficiencies of the new propulsion systems, novel airframe design will, over time, reduce airframe noise by improving airflow through more aerodynamic design and lighter weight materials. ZeroAvia is working with Otto Aviation on hydrogen-electric engines for its Celera 500L, an airframe design that seeks to maximize laminar flow — a minimum drag solution for aircraft surfaces that means smoother layers of airflow.
In the nearer term, the aviation industry has better opportunities to reduce the noise from propulsion. In terms of propeller noise, for example, the noise is directly proportional to the loading of the propellers. This loading can be reduced by a reduction of the tip speed. Electric motors have inherently wider power bands compared to combustion engines, enabling reduction in the RPM and thus tip speed, while increasing the propeller pitch to maintain thrust.¹ Further, when compared to turbine engines, there are fewer moving parts creatingnoise and there will also be no high-velocity jet noise or turbulent eddies.²
More work is needed to provide exact figures on noise reduction for specific aircraft using novel propulsion, and this will become increasingly possible as designs mature towards certification. We can, however, be extremely optimistic that neighborhoods adjacent to airports can benefit not just from cleaner air, but also from reduced aircraft noise as zero-emission aircraft come online.
The prize of delivering cleaner air and less noise disruption in and around airports is clear for everybody — it will enable airport expansion without the feared negative consequences on communities.