On 2/10/00, Jan de Jong wrote (in part):
> 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.
Jan --
Excellent analysis of the problem, but I can't quite agree with the
conclusion. Pressure differential AOA's don't measure the angle per se, but
the RESULT of changing AOA. AOA indicators are useful in the range of stall
speed through 1.3 x stall speed (for takeoff/landing), a narrow range of
airspeeds under any conditions. Using pressure differential, reading stall
speed is easy, as pressure differential rises dramatically. At 1.3, what the
pressures are is not all that relevant, just so there's a differential to
read. The useful range of the indicator is simply calibrated between these
two points. The challenge I believe is selecting the location of the
pressure ports (chord and span) and sensitivity of the sensors, so that the
panel indication is not too sensitive (or worse yet, has hysteresis) to be
reliable.
> 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.
Yes, at least two US mfr's of AOA devices. Also, about 25 years ago EAA
Sport Aviation published an article on such. The design used two balanced,
airfoil-shaped vanes like you describe, but inside the enclosure that pivoted
them were a small incandescent lamp on one end, and a CdS photocell on the
other. A flat vane in between rotated by the outside vanes varied the light
received by the photocell. A panel meter in a Wheatstone bridge completed
the setup. This was a bit high-tech for the 70's, but probably as useful as
measuring anything else.
Regarding a potentiometer, durability is easy to buy, but low enough friction
is not. Any friction the vanes can't overcome with tiny degrees of rotation
won't work well at all.
Happy AOA designing!
Regards,
Fred Fillinger, A063
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