Vortex generators

denopa wrote:

Is that speed lowered by the VGs, or with a VR lower by 10kts, is there an increase in the period of time you spend in that vulnerable zone

I don’t think so. The red line relates to controlling the plane (so that it doesn’t basically yaw+roll into a dive) with one engine only. I don’t see how reducing the stall speed would reduce that. If you reduce power enough on the operating engine, the plane will still be flyable, but probably not upwards (e.g. on the extreme you can shut power and glide). I don’t see how that would be affected by VGs – as Timothy (who probably has 2 orders of magnitude more experience in this than me) says, you normally don’t take off below red line anyway (as otherwise loss of control in case of engine failure is very likely)

The blue line is just Vy on one engine. I’m not sure if the VGs would affect that.

Out of interest the BE76 Duchess has two stall warners – one on each wing – one operates with flap up to 16 degrees, the other with higher flap settings

Timothy wrote:

a stall warner which sounds at X+y kts.
At what speed will the stall warner now sound?

At X+y kts. The stall warner does not know of the VGs. VGs energize the boundary layer, they don’t modify the overall airflow around the wing and certainly not upstream.
The more interesting question is “Can I adapt the metal plate of or move the stall warner to match the new stall speed, if so how and how much?”.

Peter wrote:

It seems intuitively correct, but 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 trailing edge of the wing does have an effect all the way forward to the leading edge.

No. The stall warning is set to y kts above Vso, ie full flaps. y is chosen large enough that the horn will give some advanced warning even when the plane is clean.
In the typical PA28 I fly, Vso and Vs1 are only 5kts apart (you don’t put flaps out so much for the reduction in stall speed as for the difference in pitch attitude which gives very good forward visibility when landing), so set y to 6-7kts and it wall warn in both cases but with different margins.
It’s easy to verify next time you go doing stalls up high with and without flaps, clean in our PA28 you get the horn basically with the start of the buffet, with full flaps you get a few extra knots between horn and buffet.

I was asked to do a no-flap landing on my checkride, keep the speed up (80-85kts on final instead of 75kts) and a shallow approach angle to see the runway. It’s the standard procedure in case one or both the rods to the flaps somehow get disconnected (detach or break) in flight from the deployment mechanism, one rod being the worst case as you cannot maintain straight flight with ailerons with only one flap out (maybe with some heavy cross rudders at the first flap step, but certainly not at step 2 or 3 and who wants to fly this way anyway). We had trained this with the instructor during training.

PetitCessnaVoyageur wrote:

Main question was for me to know if with VGs, I would suffer from intempestive warning.

Not necessarily intempestive, but with extra margin. Glass half full.

Arne wrote:

they don’t modify the overall airflow around the wing and certainly not upstream

You can’t have one with out the other…

My guess is the correct answer is X+y + little_bit and the little_bit does not amount to anything significant in a proper installation, but it could go either way. The question should perhaps be, how are the VGs tested and is the stall warning devices included in the tests ?

Ted wrote:

My guess is the correct answer is X+y + little_bit

Why a little bit?

X+y

Because I am inferring that the upper section of the wing remains more effective at higher angles of attack, i.e. instead of the flow breaking down, circulation continues to increase, i.e. the reduction in lift is delayed and it is more severe.

It’s only a theory, I was hopping for the correct answer, I like to ask questions

Most stall warners work due to a shift of the leading edge stagnation point. This is (almost) directly linked to AoA which has nothing to do with the flow associated with the boundary layer.

Ted wrote:

Because I am inferring that the upper section of the wing remains more effective at higher angles of attack, i.e. instead of the flow breaking down, circulation continues to increase, i.e. the reduction in lift is delayed and it is more severe.

Yes but as Dave says, and many of us said earlier, most stall warners operate at a fixed AoA. So they will sound at a fixed angle of attack or speed regardless of any change in the actual stall speed.

From someone who has a (very) close relation with the VGs STC’s development.

With regard to stall warning – the wing will stall at a slightly higher critical angle with the VGs installed. The warning mechanism in the wing responds to the pressures at the critical angle. You want the warning (14 CFR 23.207) to happen at least 5 knots ahead of stall and continue until stall occurs – but MUST NOT occur during approach to landing, or takeoff https://www.law.cornell.edu/cfr/text/14/23.207
Therefore, if you find that the stall warning is making noise at the slower speeds possible with Micro VGs (which is possible), the procedure to adjust it is in the Cessna maintenance manual. The “tongue” is never bent – the adjustment is accomplished with the screws that are in the frame of the mechanism in the wing. I regularly fly our 182H model (R/Stol and Micro VGs), and did not need to adjust the stall warning vane system, nor did I on our 172M model. We did adjust on the Baron 55.

I will report when my kit has been installed.