Airframe noise

[Simon Chaddock]

Flying my lightweight Wing Dragon a couple of days ago I was surprised by the noise the airframe made on a low fast(ish) pass. A sort of low drone audible above the sound of the motor(Blue Wonder are noisy!) and prop.

A slow glide pass was to all intents and purposes silent - as you might expect from the relatively 'soft' Depron surfaces.

 

Given that 'fast' on this lightweight model is probably below 30 mph what part of the airframe was creating this noise and did it represent unwanted drag?

 

The top hinged half span ailerons have quite a gap on their underside. I wondered if this be the culprit. However simply blowing across the wing underside produced no noticeable sound difference between aileron and the rest of the wing.

 

The hole for the servo horn was a very different matter. Although a relatively small opening (4x16mm) it has quite a large cavity beneath it. These are the basic requirements for whistle and it did indeed produce quite a loud noise when blowing across it.

At least this is likely to be amplifying any sound rather than generating significant drag.

The solution first is to reduce the size of the horn cut out to a minimum (3x10mm) and then add a small shroud.

Now suitably quiet but the ultimate solution will be to bury it all completely in the wing.

[Former Member]

[This posting has been removed]

[Simon Chaddock]

Eric

Thanks for the tips but the mods I made has largely removed the specific drone, it is now just a more general noise probably from the exposed horns & push rods.

 

To my untrained ear I would have put the original sound around 50Hz, somewhat below prop vibration levels but probably rather above any natural frequency of the depron structure.

 

This evening (beautiful weather) I did try a full power vertical dive from altitude to what must have been pretty close terminal velocity. Quite an impressive howl from the prop but to my surprise/relief nothing broke. 

Edited By Simon Chaddock on 08/09/2010 22:45:16

[Stephen Grigg]

Did you expect something to break Simon?

[Simon Chaddock]

Stephen

With an 'own design' I expect something to break every time I fly! This way I get a sense of achievement when it doesn't.

 

In a vertical power dive I "guestimate" it was getting close to 60 mph or nearly 10x its stall speed.

Fast jets can do this but its asking a lot from Depron.  

Edited By Simon Chaddock on 09/09/2010 19:35:52

[Stephen Grigg]

So just put it through its paces then