Wings are an important component of aircraft design, affecting performance characteristics like speed, efficiency, and noise. There are different wing designs optimized for different goals. Two common types are shadow wings and pure wings.
What are shadow wings?
Shadow wings get their name because part of the wing is “shadowed” or positioned behind another wing surface. This design feature helps reduce noise from the wing by improving interference between sound waves.
Specifically, shadow wings utilize the following design elements:
- The main element is mounted towards the front of the wing.
- A secondary rear element is mounted behind and below the main element.
- The rear element is inverted to create a gap between the two wing surfaces.
This gap allows sound waves from the main element to reflect and cancel each other out, reducing noise. It essentially creates an acoustic “shadow” zone behind the main wing element, giving the design its name.
What are pure wings?
In contrast to shadow wings, pure wings utilize a simple single-element configuration without any secondary surfaces. The underside of the wing is smooth and uninterrupted, like a single unified wing.
Pure wings are known for providing high lift and low drag. However, they do not incorporate noise reduction techniques like shadow wings do.
Noise sources in aircraft wings
To understand how shadow wings reduce noise compared to pure wings, it helps to first look at where wing noise comes from.
During flight, wings generate noise from multiple sources:
- Turbulent airflow – The motion of air around the wing creates turbulence which generates noise.
- Vortex shedding – Vortices and eddies peeling off wing tips and edges also create noise.
- Trailing edge noise – The trailing edge where airflow detaches makes a blunt edge that scatters noise.
Reducing noise means managing these sources of turbulent, unsteady airflow around the wing. Shadow wings are designed to specifically target trailing edge noise.
How shadow wings reduce noise
Shadow wings reduce noise in two main ways:
- The gap between the main and rear elements allows sound waves from the trailing edge to reflect and cancel each other out through destructive interference.
- The rear element also shields observers on the ground from direct noise sources on the underside of the main element.
Research has found that shadow wings can reduce cumulative noise levels by 3-5 dB compared to conventional wing designs. This is a significant noise reduction.
Here is a more detailed look at the acoustic mechanisms at work in a shadow wing:
- The gap between the two wing elements is a key feature. This gap is sized based on the anticipated sound wavelength to maximize cancellation. For aviation, a gap around 10% of wing chord is typical.
- Sound waves from the main element trailing edge reflect off the secondary element and back onto themselves. Peak noise frequencies see the most destructive interference.
- The inverted orientation of the rear element encourages reflection while also providing some sound absorption via its porous surface.
- The secondary element creates a “sound shadow” that shields noise from the underside of the main element.
Optimizing these effects results in an overall noise reduction versus a basic single element wing.
Performance tradeoffs
While shadow wings reduce noise compared to pure wings, they also come with some performance tradeoffs:
- Increased weight – The extra wing element adds weight which can require beefing up the wing structure.
- Increased complexity – More complex high-lift devices are needed to maintain lift with the divided wing sections.
- Potential aerodynamic penalties – Interference between the two wing elements can reduce maximum lift capability.
However, these disadvantages are relatively minor compared to the benefits of substantially reduced noise levels.
Use in commercial aviation
Shadow wings are commonly used in commercial airliners today to meet aircraft noise regulations. Here are some examples:
- The Boeing 787 Dreamliner uses distinctively shaped shadow wings to reduce noise.
- The Airbus A380 superjumbo utilizes noise-reducing wing designs including shadow wings.
- Regional jets like the Bombardier CRJ series aircraft employ shadow wings to meet airport noise restrictions.
Shadow wings allow these aircraft to satisfy stringent noise limits placed on commercial aviation, especially at major airports. This noise reduction makes them an enabling technology for meeting noise regulations.
Boeing 787 Dreamliner Shadow Wings
The Boeing 787 provides a great case study into the benefits of shadow wing design:
- Its distinctive raked wing tips incorporate a shadow wing element to reduce vortex noise.
- Inward-canted rear element creates a sawtooth trailing edge to scatter noise.
- Wing curvature optimized to delay shock wave formation at cruise speeds.
Together these design features contribute to the 787’s overall 60% lower noise levels compared to older aircraft like the Boeing 767. The shadow wing is a key part of achieving these impressive noise reductions.
Conclusion
In summary, shadow wings provide a clear noise benefit over pure single element wings thanks to their two-surface design creating destructive interference. The gap between elements and rear sound shielding result in measurable noise reductions of 3-5 dB.
There are some minor performance tradeoffs like increased weight and complexity. But these are outweighed by the major noise benefits.
That’s why shadow wings are commonly used on modern commercial jets. They allow aircraft like the 787 Dreamliner to meet strict noise regulations and reduce noise impacts around airports. While pure wings are simpler, shadow wings are definitively quieter.
