AOA sensing via accelerometer & A/S

[deh6]

Does it make sense to determine AOA by measuring the g-force with an accelerometer and airspeed with a pressure sensor?

I want to determine AOA on a flying wing (to experiment with using a computer for pitch stabilization). There are mechanical problems with a vane, e.g. where to mount (if there is no vertical stab). The accelerometer & pressure sensor avoid this (though there is the issue of getting a good static source).

Lift is proportional to Cl, which is proportional to AOA, and also proportional to airspeed squared. Pitot-static air pressure difference is proportional to airspeed squared. The required lift is the mass times the g force. So,

AOA = K * g / P

Where: K is a constant
g is accelerometer reading
P is pressure reading

A micro handles this easily. One problem I see is that when the wing stalls, the lift is no long proportional to AOA, so the computed AOA will be small, when it is actually large (same as when the plane stalls the novice pilot, feeling the falling sensation, pulls back on the stick rather than release back pressure so as to regain flying speed).

Vibration from a motor would also be something contend with.

Any other thoughts about an AOA sensor?

[markdrela]

A fairly reliable way to do this is via two differential pressure transducers.

1) Install two pressure taps on the wing, one on the bottom surface and one on the top surface, both near the LE at about 5% chord. Use one differential pressure transducer to measure
delta(p) = p_bottom - p_top
between these two taps.

2) Use the other differential transducer to measure the dynamic pressure
q = p_total - p_static
with the two pressures taken from a pitot probe.

The ratio of these two pressure readings is the differential pressure coefficient
delta(Cp) = delta(p) / q
which is a sensitive function of AoA and TE flap deflection, at least for modest pitch rates.
You can use Xfoil to compute delta(Cp), and thus determine its dependence on AoA and flap angle.

[Sparky Paul]

There's two probes just underneath the cockpit on the outside of the L-1011 Tristar.
They stick out the side of the plane.
These cylindrical probes have two seperate chambers dividing it lengthwise.
Each side feeds a pressure transducer.
There's a nulling circuit that rotates the probes to position it at the point where the flow over the upper side and lower side are equal.. This is equivalent to AOA, and is used by the autothrottle control system.. the pilot dials in the desired airspeed, the probes read the AOA, and the resultant diffference is fed to the throttles to adjust power to get the AOA for that airspeed.
(These probes had been the "rule-of-thumb" 3 fuselage diameters aft of the nose of the plane, but with the lipped inlets for the environmental control system on the forward fuselage bottom, the turbulence from the lips resulted in end-to-end limit cycling of the AOA probes.)

[deh6]

Mark & Sparky,

Thanks for the thoughts. I wasn't aware of pressure differential scheme. I'll look into it. The effect of the TE flap position is something I had completely overlooked. It would also affect the A/S & accelerometer approach since Cl vs AoA shifts with the flap position.

The approach on the L1011 where the sensor is servo-rotated for equal flow gets around the friction of a vane/selsyn. As I remember there was a vane near the forward cabin door in the Boeing 737. I always wondered if there could be a problem with crud (ice?) keeping it from turning freely.

[Sparky Paul]

Those probes/vanes are heated. :)