Acoustics considerations

The noise generated by an undercarriage comes from the turbulence it generates as it passes through the air. Large vortices are left in its wake, particularly behind the wheels, but also from other items. These include the undercarriage leg or oleo, bracing and actuating struts, electrical cables for control circuits and lights, hydraulic lines for operating the brakes and so on.

For those of you who are interested in vortex theory, try the following link, but this is not essential reading to complete the project: http://www.du.edu/~jcalvert/tech/fluids/vortex.htm.

The frequency of sound generated by these vortices can be derived from the Strouhal number. Details can be found at the following site, but again, the mathematics is for interest only and is beyond the scope of this project: http://www.processassociates.com/process/dimen/dn_str.htm.

Thus, any device attached to your model undercarriage - such as external springs - will generate noise. You may be familiar with these tones if you have heard a strong wind "singing" in the telephone wires or ships rigging or if you have heard an Aeolian Harp (pronounced like the letter A-Oh-lian). The link below gives you some background on Aeolian tones and a link to a sample sound file.http://www.geocities.com/Vienna/Strasse/7353/Aeolian.html

In addition to pure tones, the undercarriage will generate a lot of broadband noise, covering many frequencies simultaneously. A jet's roar and the hiss of an untuned radio are examples of broadband noise.

The purpose of a fairing is to hide these items in a streamlined shape to try to minimise the disturbance to the surrounding air. By reducing the turbulence, it should be possible to reduce the noise. In this project, we are considering only the acoustic effects of a fairing and will ignore its effect on the aerodynamics (particularly drag) of the undercarriage. The fairing must be firmly attached, however, to ensure it do not fly off in the wind tunnel and cause damage.

The fairing that you design must hide as much of the structure as possible from the oncoming airstream. It must leave enough of the wheels showing to allow the undercarriage to operate correctly on landing, including allowing the leg to deflect. It is up to you how much of the undercarriage you try to include in the fairing and how much you leave out. It must also be detachable so that the "bare" undercarriage can be assessed for noise at the start of the tests. In real life, the reason for making it detachable is so that the aircraft could carry on flying if the fairing got damaged. Changing a fairing is relatively simple; changing an undercarriage is a hugely time-consuming and expensive task.