What does aerodynamic noise of an airliner sound like in landing configuration?

Question

I want to know what aerodynamic noise an airliner makes when flying with both engines shut down. Specifically, it is the sound made by air passing a plane which is in landing configuration, with gear and flaps extended, at a normal approach speed.

I know that it makes a loud noise especially during landing, but normally we cannot distinguish it from the engine sound.

Answer 1

Aerodynamic noise has many sources. It is not white noise, because some frequencies are dominant. Generally it happens when flows of different speed collide, or when a standing wave develops in a cavity. Common noise sources are:

• Uncovered openings, like vent holes or control surface gaps. Like when you blow across the top of an open bottle, they produce a howling sound with a dominant frequency that depends on flow speed and opening size.
• Tollmien-Schlichting waves in the boundary layer. These frequencies change with speed and their location along the flow path, and generally are responsible for most of the hissing sound of gliders.
• Separated flow, which produces alternating separations behind blunt bodies. Here the main frequency is that of the Karman vortex street that forms behind them. It can be calculated if the Strouhal number Sr of the flow is known. This is the equation for the main frequency $f$ of a bracing wire with the diameter $d$: $$f = Sr \cdot \frac{v}{d}$$ Here $v$ is the airspeed, and for bracing wires Sr is normally 0.2. Bracing wires (or a blunt trailing edge, for that purpose) produce a characteristic whistling sound.

As you said, the landing configuration makes most noise. In addition to the factors above, you now have

• Extended flaps, mostly with gaps between them which show high local flow speed. This local high-speed flow is very noisy.
• Many more blunt objects sticking out of the airframe: Landing gears, gear covers or landing lights. The particular noise of landing gears was once tested with a high-performance glider which had styrofoam gears fitted under the wings. They broke off when it had to land, but yielded valuable data when compared to the noise of the clean glider. Sorry, there is no photo of this experiment on the web!
• The gaping hole of the landing gear well. Especially while the gear is moved, this creates a lot of noise, but even after extension a part of the well is uncovered and adds its noise.

Here is a good overview of different noise sources. It is best read with a good working knowledge of German.

To answer your question: The sound is a mixture of hissing and whistling in different frequencies. If you stand close to the Autobahn (best is a section without speed restrictions), the noise of the passing cars is similar, but less intense. At 180 km/h, engine noise starts to vanish in all the aerodynamic and tire noises ...