Auxiliary Power Unit Inlet Duct Door with Mounted Sound Absorber

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Solution Overview

Problem

Auxiliary power units in aircraft generate significant noise, which is amplified by the inlet duct and redirected towards passengers, causing discomfort, especially in business jets, and existing noise reduction methods like sound-absorbing materials and splitter vanes increase cost, complexity, and weight.

Innovation Solution

An auxiliary power unit inlet duct assembly with a door-mounted sound absorbing component positioned to directly absorb noise before it is redirected, utilizing materials like felt metals or tuned resonant sound absorbers to minimize noise emission without altering the duct's structure.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If sound absorbing material is used to line the inlet duct walls, then noise levels are reduced, but the duct size increases and cost/complexity increases

Engineering Contradiction:

Improvenoise levelsVSAvoidduct size

Core Design Contradiction:

Object-affected harmful factorsVSVolume of stationary object

Solution Approach 1:

The sound absorbing material is mounted on the door surface (a two-dimensional plane) rather than lining the three-dimensional interior of the duct. This transitions the noise reduction approach from volumetric to surface-based, eliminating the need for increased duct volume while maintaining noise reduction effectiveness.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

Solution Approach 2:

The sound absorbing function is extracted from the duct structure itself and relocated to the door assembly. This separation allows the duct to maintain its original size while the door incorporates the noise reduction capability independently.

Inventive Principle:

Principle #2Taking out (Extraction)

2Object-affected harmful factors

If sound absorbing splitter vanes are added to the inlet duct, then noise levels are reduced, but airflow resistance increases and duct size must increase

Engineering Contradiction:

Improvenoise levelsVSAvoidairflow resistance

Core Design Contradiction:

Object-affected harmful factorsVSLoss of energy

Solution Approach 1:

The sound absorbing function is extracted from the airflow path and relocated to the door assembly. This eliminates the need for splitter vanes that would obstruct airflow, while maintaining noise reduction by positioning the sound absorbing material where it can intercept noise without interfering with the air stream.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The door assembly acts as an intermediary between the noise source and the external environment. By mounting sound absorbing material on the door, the system intercepts and absorbs noise in a location that does not obstruct the primary airflow path to the auxiliary power unit.

Inventive Principle:

Principle #24Intermediary (Mediator)

3Object-affected harmful factors

If sound absorbing material is used to line the inlet duct, then noise levels are reduced, but device complexity and cost increase

Engineering Contradiction:

Improvenoise levelsVSAvoidinlet duct complexity

Core Design Contradiction:

Object-affected harmful factorsVSDevice complexity

Solution Approach 1:

The sound absorbing function is extracted from the inlet duct assembly and transferred to the door assembly. This simplifies the inlet duct structure by eliminating the need for internal sound absorbing linings, while the door incorporates the noise reduction capability as a separate, manageable component.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The door assembly is given multiple functions: it serves as both the airflow control mechanism (opening/closing the inlet) and the noise reduction element (through the mounted sound absorbing material). This consolidation reduces overall system complexity by combining functions rather than adding separate components.

Inventive Principle:

Principle #6Universality (Multi-functionality)

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

Substantially reduces noise levels without adding weight or complexity to the aircraft, providing a quieter environment for passengers without the need for oversized ducts or additional airflow resistance.

Implementation Method 1

a sound absorbing component that is mounted to the door and that is disposed proximate the inlet duct. The sound absorbing component is positioned at a location on the door such that a noise generated by the auxiliary power unit will directly impinge on the sound absorbing component.

Methodology Applied

Scientific EffectSound absorption: Acoustic Absorption

Data Source

PatentUS9840334B2Auxiliary power unit inlet duct assembly for mitigating noise

Publication Date: 2017.12.12 INGRASSIA FISHER & LORENZ P C

AI summary

An auxiliary power unit inlet assembly includes an inlet duct having a first end and a second end. The auxiliary power unit inlet assembly further includes a door associated with the second end of the inlet duct. The door moves between a first position and a second position. The door closes the second end of the inlet duct when in the first position and permits air to enter the second end of the inlet duct when in the second position. The auxiliary power unit inlet assembly further includes a sound absorbing component mounted to the door and disposed proximate the inlet duct. The sound absorbing component is positioned at a location on the door such that a noise generated by the auxiliary power unit will directly impinge on the sound absorbing component.