Phase-Locked Multi-Propeller System for Noise Reduction
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Solution Overview
Problem
Propeller noise, particularly rotational noise, is a significant barrier for aerial vehicles operating in populated areas, as it consists of tonal noise with well-defined peaks at the blade passage frequency and its harmonics, affecting both manned and unmanned vehicles.
Innovation Solution
A system comprising two or more identical propellers with a predetermined angular phase offset, operating at the same rotation rate and direction, to reduce the global sound power of the fundamental tone at the blade passage frequency, achieved through mechanical or electronic phase-locking, with optimal phase offsets varying based on the number of blades and propellers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If multiple propellers operate independently to provide thrust, then the total thrust output increases, but the noise level increases due to additive acoustic sources
Solution Approach 1:
The patent combines multiple propellers into a phased array system where the acoustic sources are coordinated through phase-locking. By merging the acoustic fields with controlled phase relationships (specifically 180-degree offsets), the system achieves destructive interference of noise while maintaining constructive interference for thrust generation.
Solution Approach 2:
The patent converts the harmful acoustic interference that naturally occurs between multiple propellers into a beneficial effect by deliberately phase-locking them at 180-degree offsets. This causes destructive interference for acoustic waves (reducing noise) while the mechanical thrust effects add constructively, thereby converting noise harm into a noise-reduction benefit.
2Object-generated harmful factors
If propellers are positioned close together to reduce noise through acoustic interaction, then noise reduction is achieved, but the device complexity increases
Solution Approach 1:
The patent replaces complex mechanical synchronization mechanisms with electronic phase-control systems. Instead of using mechanical linkages to maintain fixed phase relationships between propellers, the system uses electronic controllers to independently regulate the rotational phase of each propeller, achieving the same synchronization effect with greater flexibility and reduced mechanical complexity.
3Power
If propellers operate at high rotation rates to meet power requirements, then thrust is sufficient, but rotational noise increases with well-defined peaks at blade passage frequency
Solution Approach 1:
The patent applies periodic phase modulation to the propeller rotation, where each propeller is deliberately offset by a fixed phase angle (180 degrees) relative to others. This periodic arrangement creates time-varying acoustic interference patterns that result in destructive interference for noise while maintaining continuous thrust generation, effectively reducing rotational noise peaks.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The phase-locked propeller system reduces far-field pressure levels by radiating less efficiently than a single acoustic source, resulting in a significant decrease in noise levels, with measured reductions of up to 5.8 dB compared to independent propellers.
Implementation Method 1
The configuration of the propellers reduces far-field pressure levels due to the near-field acoustic interaction between the propellers. Adjacent propellers are phased to radiate less efficiently than a single acoustic source (e.g. a monopole).
Data Source
AI summary
A low-noise multi-propeller system includes at least two propellers, each propeller including at least two blades. The propellers rotate in a first direction and define an angular phase relative to one another. A drive system corotates the propellers at substantially equal rotational rates in the first direction. The propellers are substantially phase-locked at a predefined relative phase offset that reduces the overall sound power of the fundamental tone at a blade passage frequency.