Re: Acoustic resonance

[brian whatcott <inet@INTELLISYS.NET>]

At 09:37 8/10/99 -0600, Arnulfo wrote:
> Hi:
>
> You can make it by yourself because it is easy.
> It is a pipe with two tops, one of them has a speaker
> connected to an oscillating current, so that you can
> controll the frequency of the sound inside the pipe.
>
> Arnulfo Castellanos-Moreno


I was looking at another embodiment of an 'acoustic diode/pump'
by a NASA engineer. He designed an autopilot for homebuild
aviation enthusiasts. It consisted of a wing leveller and heading
hold.   It is important to know the rate at which roll and yaw
are changing, in this connection.
    The orthodox method uses a rate gyro with electrical outputs.
This is an expensive component.

   His alternative was an air jet blowing 3 or 4 cm onto a
pair of thermistors. Their self heating is easily affected by
the jet's deflection during turns and is a measure of yaw and roll
rate.
(This mix of angular change in two axes is effected by tilting
the spin axis of the gyro, and by analogously tilting the line
on which the two thermistors are oriented.)

 In his earlier conception the source of the air motive force was
a motor and a paddle in a housing acting as a centrifugal impeller.
 When this motor type became scarce, he replaced it with a
loudspeaker working into a blank plate pierced by a one centimeter
diameter hole, where sound energy affected the air in a flat cavity
formed by a lamina with a large central aperture of about 4 cm diameter
connected in an egg shaped profile to a smaller entrance hole.
In the center of the large circle opposite the speaker hole was a
similar exit hole. This construction was to be used as an acoustic
pump. It was necessary to offset the thermistors temporarily to
monitor the available air flow, and the speaker drive frequency around
240 Hz was varied so as to maximize the flow rate.

(The magnetic heading sensor was even more innovative, if that is
possible. It consisted of several windings on a toroidal ferrite core.
One toroidal winding ran the core to staturation: two orthoganal
 windings wound on formers surrounding the whole core monitored the
differential onset of saturation etc.)

It is my honor to cite:

Doug Garner: Building the Smart Compass
Doug Garner: How to Build a Good Autopilot
MS-152E NASA Langley
Hampton VA 23665
(804) 865-4685

Both were written for EAA Oshkosh forums before the financial/warbird
imperative ruled, in 1986 and 1982 respectively.



brian whatcott <inet@intellisys.net>
Altus OK