A bout of the flu gave rise to some feverish thought on the subject of AOA
measurement.
Sorry that this is not a timely contribution.
First, I think a differential pressure by itself can provide a stall warning,
but
not an AOA. Like the component pressures the differential pressure will increase
with airspeed. At one AOA only will the
differential pressure be zero and independent of airspeed. The resulting
indication
can only have two values: angle smaller than this 'critical' angle, angle
greater than this 'critical' angle.
To arrive at an angle from pressure measurements there has to be a normalisation
for airspeed. I don't know of a way to simply and directly measure a pressure
quotient using pressures. There probably isn't.
That leaves ofcourse the option of feeding a number of pressure measurements to
a processor (A-D conversion) and calculating the angle (somehow).
The simplest pressure based determination of angle I can think of trying
requires
2 differential pressures: the 90-degree angle pressure over static and the
0-degree angle pressure over static. The first
divided by the last should approximate the tangent of the AOA (and the AOA in
radians).
The simple quotient required might be calculated by analogue means.
Now I wonder, has anybody tried measuring the AOA directly, i.e. by using a
position
transducer connected to a vane? Note that this replaces the calculated pressure
quotient above by a measured equivalent(?)
airflow quotient.
I was (feverishly) thinking of 2 vanes on either side of a closed pod with (the
connection to) a frequency modulating transducer inside (minimal friction, and
more durable than a potentiometer). Two thin wires
would be enough for power supply, measurement signal and a reference signal.
Requires
a frequency-difference to display conversion.
I imagine that flutter and damping may be the problem to solve. And a vane will
always be more delicate than a pressure port (but vane(s) could be designed to
be replaceable).
Anyway, I hope somebody manages to invent a simple DIY/inexpensive AOA-indicator
with good enough repeatability.
If any (or all) of the above is misconceived I would like to hear it - just in
case I get around to making a stab at it too sometime in the future.
Regards,
Jan de Jong
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