Chasing Ghosts-Stall warning

[Pmerems]

Gents,

I have a Dynon 180 with the AOA system installed. The AOA is calibrated and I have checked it several times over the past 110+ hour in my RV-7A.

I have it setup with a progressive tone. About 5-8 knots above stall it will generate a beep. As the speeds slows the beeps increase in numbers.

I have approximately 250 landing in my RV. I give most of them an 8 out of 10. Sometimes I get a beep, sometimes I don't. However I have landed slow and am surprised that most of the time I never get a beep.

The AOA system measures pressure at the pitot tube. When landing the wing is in ground effect and the pitot tube is rather close to the ground and in ground effect. This makes me wonder if the AOA/Stall warning isn't very accurate when in ground effect.

During landing I keep holding the plane off the ground expecting I hear a beep indicating approaching stall speeds. Most of the time I touch down and hear nothing. I am chasing a ghost?

Any thoughts.

[mannanj]

Paul:

I would guess that your wing is nowhere near stall at your touch down. Most airfoils have to be at about 16 to 18 degrees positive angle of attack to stall.

The wings of RV-7's and -8's (not "A's") sitting on the ground in the three point position are not at stall angle of attack.

[Kevin Horton]

It is quite well established that ground effect has a significant affect on the airflow around the wing.

• The wing will make more lift at a given AOA in ground effect than it does out of ground effect.
• The wing will stall at a lower AOA in ground effect than it does out of ground effect.

It is quite likely that the above two effects will affect calibration of the Dynon AOA sensor in ground effect.

[az_gila]

Originally Posted by Kevin Horton

It is quite well established that ground effect has a significant affect on the airflow around the wing.

• The wing will make more lift at a given AOA in ground effect than it does out of ground effect.
• The wing will stall at a lower AOA in ground effect than it does out of ground effect.

It is quite likely that the above two effects will affect calibration of the Dynon AOA sensor in ground effect.

So the question becomes "Is the ground effect change to the AOA system negative feedback - the system is self correcting and reads almost correctly - or is it positive feedback - the system reads much worse than true?"

Note, the use of the word feedback is not quite correct, but makes the point.... You could ask if the AOA instrument errors add or subract to the aerodynamic changes.

[boomer]

Mine is set up to say "Stall" and I generally get that aural warnign just before I touch down in my usual "almost" 3-point landing attitude. Yesterday I did a stall series, no-flap, 1/2 flap, and full flap (all wings level). I got the aural "stall" warning just before the buffet, maybe 1 sec. before the break. I did notice that the aural "stall" came on when the last yellow "V" disappeared and that the aoa moved throught 3 red "V"'s before the break.

-John