VGs and Stall Warner
(Assume for all of this 1g and wings level, I don’t need another explanation of why AoA supersedes stall warners.)
You have an aircraft that stalls at X kts, which has a stall warner which sounds at X+y kts.
You fit VGs which lower the stall speed to X-z kts.
At what speed will the stall warner now sound?
Depends on nature of stall warner but most likely X+y.. VGs usually increase critical angle of attack so a stall warner based on AOA will be wrong unless adjusted.
I am going this way too, and inquired a 182T owner equiped with VGs, about this point.
The stall warning horn on a 182-T is activated by air flowing against a tab on the leading edge of the port wing when the angle of attract nears the critical limit. It will be activated at a slower landing airspeed with the VG’s.
My brother has a 1957 172 where the horn is activated by the airspeed indicator. His stall warning horn comes on, and goes off, at factory designated airspeeds. After the VG’s, on landing approach, the horn begins to sound, then goes silent, before his floats touch the water.
In my 182-T and in my T-182-T, the horn began to sound a few seconds during the flair.
Although I understand JasonC point (and that was my concern too), I decided I would try… will report then :-)
PS: I think there are few VGs users from MicroAero here, who are very happy with their kit, and may share their experience
Timothy wrote:
At what speed will the stall warner now sound?
I could see how it could move both ways, with a stall warning indicator that relies on the movement of the stagnation point, or one effect related to how it effects the stagnation point could cancel out another. Hence maybe they did some tests and have the correct answer? 
In addition position error could also be a factor.
PetitCessnaVoyageur wrote:
The stall warning horn on a 182-T is activated by air flowing against a tab on the leading edge of the port wing when the angle of attract nears the critical limit. It will be activated at a slower landing airspeed with the VG’s.
That doesn’t make sense to me. It seems to me that vortex generators change the airflow behind where they are placed, and not much in front. So with any kind of stall warning device that measures the situation at the leading edge, I would not have expected that installation of VGs changes much.
Maybe he meant that, while stall speed is lowered, the critical angle of attack doesn’t change. And thus the stall speed still triggers at the actual stall (with lower speed)
I’ll tell you when they’re set up !
I am not against VGs at all – I have had them for years.
But for a given aircraft weight, in order for the stall speed to reduce, the critical angle of attack must increase. That is why in the PA46s the tab type stall warner has a computer that takes into account the configuration re flaps. Otherwise wind pushing a tab can only be correct for one wing/flap configuration. Unless I have missed something.
wind pushing a tab can only be correct for one wing/flap configuration
It seems intuitively correct, but – if true – it would mean that the stall warner on every plane on which the flap setting is not taken into account would be usable only at one flap setting, which is clearly not the case since the vast majority of piston planes have just a microswitch with a piece of metal attached to it, with no flap position input.
So it seems that what the flap is doing to the air at the training edge of the wing does have an effect all the way forward to the leading edge.
It’s probably the same thing as ground effect. The ground sees only the airflow which has passed by the wings (and only the portion of it which has been deflected downwards by them) so it cannot possibly have any effect
But it is clearly more complicated, because the AoA indicators (many threads here – I will merge them when I get a bit of time) mostly do have a flap position sensitivity which needs to be calibrated.
That’s pretty impressive computation (on the PA46)
The fact is on the 182, flaps configuration (0 to 40) must modify the critical angle of attack. And I observe the margin between stall warning and actual stall is different, depending on flaps setting.
Main question was for me to know if with VGs, I would suffer from intempestive warning.
The answer could be I won’t.
But it could be wrong :-)
I didn’t take anything against me !
And i will be pleased to share the results with you.
JasonC wrote:
Unless I have missed something
I am not saying your wrong, because various effects might cancel out, but the stall warning indicators of this type do not measure directly the angle of attack, they rely on the position of the stagnation point, so I think there is more to it. Of course you would expect that the VGs have been thoroughly tested, in each installation, and the supplier has the answer.
