Improved shear/stall-spin alarms

On Monday, June 10, 2013 12:31:24 PM UTC-6, Michael Huber wrote:
Has anyone tried any of various AOA instruments on the market?

There are

several under $1000.



Except, there's no need for a separate probe, just use the glider's

nose as a probe's nose cone and add the pressure sensing ports.



If you are looking for a simple DIY AoA indicator you might want to

check http://www.akaflieg.tugraz.at/akagproj.html

Parts cost around $20, and you can build it in an evening. The

downside is that there is no good integration with XCSoar yet, but

that might change if there is enough interest.



Michael

The electronics look clever but putting a vane on a fin-mounted TE probe isn't going to work for anything but measuring the wing's down-wash angle. A vane needs to be on the nose well ahead of the wing where, unfortunately, it's likely to get broken. To me, pressure ports on the top and bottom of the nose cone seems the least intrusive.

From: "Bill D"

The electronics look clever but putting a vane on a fin-mounted TE
probe isn't going to work for anything but measuring the wing's
down-wash angle.

Trust me, this position works fine for a glider, the only reason to
change it would be a retractable engine. The vane is of course
affected by down-wash, but as an approximation, down-wash angle is
proportional [1] to the angle of attack. This means an angle measured
in this position is not really the angle of attack, but an angle
proportional to AoA. With proper calibration this is just as fine for
the purpose of a stall warning.

The sensor itself could be placed in any position (remember, it´s
wireless), but for a glider stall warning device the TE probe is a
reasonable compromise.

Michael

[1] Down-wash is not proportional to AoA anymore close to the critical
AoA, but it still works.

On Tuesday, June 11, 2013 1:22:59 PM UTC-6, Michael Huber wrote:

"Trust me"

I don't.

"it still works"

Only in the loosest possible definition of "works".

The wing's downwash is affected in a big way by a lot of things unrelated to AOA. To get free of the wings near-field effects an AOA probe needs to be several wing chords ahead of the LE which is why you see air-data nose booms on test aircraft. Find me an example of an aircraft with the AOA probe aft of the wing.

Nose cone sensors would be too close to the wing as well but the location has proven adequate in a large number of aircraft. Come to think of it, I saw a photo of Mark Mocho's Pegasus with the TE probe on the nose which makes a lot of sense aerodynamically - if a line boy doesn't trip over it.

"Trust me"
I don't.
I agree, don´t trust, test it youself . Build it, see if it works for
you at the TE probe, test other locations, improve it and give it back
to the gliding community. It´s $20 for material and some fun time
spent on a hobby, so there´s not a lot to loose.

which is why you see air-data nose booms on test aircraft.
Flight testing is a pretty different application with different
requirements than a simple DIY glider stall warning, isn´t it?

Nose cone sensors would be too close to the wing as well but the
location has proven adequate in a large number of aircraft
As you say, not a perfect location, but a reasonable compromise.

Come to think of it, I saw a photo of Mark Mocho's Pegasus with the
TE probe on the nose which makes a lot of sense aerodynamically - if
a line boy doesn't trip over it.
Aerodynamically better position with usability issues vs. fin mounted
probe as a reasonable compromise for most pilots, same situation as
for the AoA sensor position.

Michael

On Mon, 10 Jun 2013 12:00:58 -0700 (PDT), Bill D
wrote:

[snip]

To me, pressure ports on the top and bottom of the nose cone seems the least intrusive.

What about contamination by runway crud?

rj

On Tuesday, June 11, 2013 4:27:04 PM UTC-6, Ralph Jones wrote:
On Mon, 10 Jun 2013 12:00:58 -0700 (PDT), Bill D

wrote:



[snip]



To me, pressure ports on the top and bottom of the nose cone seems the least intrusive.



What about contamination by runway crud?



rj

No worse than static ports or pitot tubes. The ports I'm thinking of would be just like static ports - flush with the surface.