Stall Horn - Configuration?

[201er]

This discussion of AOA installation and configuration changes between gear/flaps has me wondering how the regular stall warnings are configured. Are they set for gear/flaps extended or retracted? Are they accurate only on landing or on takeoff as well? What's the deal there.

[neilpilot]

Maybe I don't understand your question.  Isn't the stall warning activated by the "flapper", which senses the change from laminar to turbulent flow on the leading edge, independent of gear position?

[DVA]

59 minutes ago, 201er said:

This discussion of AOA installation and configuration changes between gear/flaps has me wondering how the regular stall warnings are configured. Are they set for gear/flaps extended or retracted? Are they accurate only on landing or on takeoff as well? What's the deal there.

What a great follow up to this thread!

A stall horn tab suffers from the same absolute inaccuracies as the AoA.  The good news is that, in both, the variance is tolerable because there is a lot of slop in the stall circuit. The stall horn, per FAR 23.207 and the associated sections must sound at “not less than 5 knots [before] and must continue until the stall occurs.” I’ve seen them coming on at 10k before a stall. Also, the stalls required for certification are not aggressively initiated by any means.

To prove the point that a single vane stall warning is not accurate under all conditions, consider the Piper Cherokee 6 (later models like the 6X). It has two stall vanes; one works under no flap conditions, and the second one takes over (and is adjusted differently) to handle medium to full flap configurations.

[carusoam]

Neil,

You have asked a very interesting question.

Recently MV Aviation, an MSer in Europe posted videos and pictures of a simulation of airflow over a wing and around a camera mounted to the bottom of the wing.

one of the pictures included is a high AOA.  What you will see is the line where the airflow splits, to go over and under the wing, actually moves...  this is why the airfoil is rounded on the front and not pointy.

The stall warning flap is placed in a location that gives fair warning prior to the stall as the split line of airflow moves lower down the leading edge.

How fair is the fair warning?  This is only something you will know by noting the speed where the warning in your plane goes on.  Compare that to The speed where buffeting begins. The stall will actually be a few Kias slower.  (Use a CFI, or safety pilot...)

What is the effect of flaps?  Flaps allow for an even higher AOA, so the stall warning will occur with a greater margin... (this is probably a false statement...)

Keep in mind the stall warning is a binary indicator, it is either on or off.  If the warning is sounding, it is not OK to keep flying with the same AOA. It is time to lower the nose, check your airspeed, know what is going on, make proper adjustments to power and attitude....

If your stall warning sounds continuously, and your airspeed and attitude are proper, check with your mechanic to have the proper adjustment made. There is an entire maintenance procedure that does not include the bending of the stall warning device...

These are PP thoughts, not a CFI...

Best regards,

-a-

[Guest]

Here's the stall warning procedure for a J model.

 

[ArtVandelay]

Flaps increase lift, but stalls occur at a lower AOA, so I would set it for half flaps, and 5-10 knots above stall, that should give a margin of error for any flaps position.

[Hank]

1 hour ago, teejayevans said:

Flaps increase lift, but stalls occur at a lower AOA, so I would set it for half flaps, and 5-10 knots above stall, that should give a margin of error for any flaps position.

Negatory on that! Stall occurs at the same or higher AoA with flaps down. Lowering flaps, however, will raise the AoA, meaning the stall will occur at a lower deck angle. Deck angle is how it looks from your seat, how high the nose is pointing; AoA is measured on the wing, and is the angle between the oncoming air and a line from the leading to the trailing edge of the wing. Lowering flaps lowers the trailing edge, which increases AoA; planes like to maintain the speed they are trimmed for, so the nose lowers to maintain the speed you had before, as pulling back in the yoke to maintain the same deck angle will increase AoA, requiring the plane to climb and slow down until equilibrium is reestablished.

It is quite possible to increase AoA too far and stall on the vertical dive portion of a loop, when the deck is pointed straight down. Please do not try this maneuver in your Mooney!

[201er]

6 minutes ago, Hank said:

Negatory on that! Stall occurs at the same or higher AoA with flaps down. Lowering flaps, however, will raise the AoA, meaning the stall will occur at a lower deck angle. Deck angle is how it looks from your seat, how high the nose is pointing; AoA is measured on the wing, and is the angle between the oncoming air and a line from the leading to the trailing edge of the wing. Lowering flaps lowers the trailing edge, which increases AoA; planes like to maintain the speed they are trimmed for, so the nose lowers to maintain the speed you had before, as pulling back in the yoke to maintain the same deck angle will increase AoA, requiring the plane to climb and slow down until equilibrium is reestablished.

It is quite possible to increase AoA too far and stall on the vertical dive portion of a loop, when the deck is pointed straight down. Please do not try this maneuver in your Mooney!

How does this impact the stall warning and/or AOA indicator?

[ArtVandelay]

Negatory on that! Stall occurs at the same or higher AoA with flaps down. !

Not according to HC Smith "Illustrated Guide to Aerodynamics", specifically pg 51 on discussion of flaps, typical lift curves diagram shows the lift vs AOA shifted upwards (higher lift) and to the left (lower AOA) when flapped. A quote from the text "In the flapped case even though the stall occurs at a lower angle the lift is much higher."

[carusoam]

Hmmm.....

we have come to a collection of mixed memories...

1) Flaps increase the distance that air travels over the wing, by changing its physical geometry.  Longer chord.

2) flaps change the shape of the wing.

3) flaps lower the stall speed.  According to my plane's POH.

4) flaps change the center of lift, moving it aft.  Balanced by the need to increase the tail trim...

 

5) I would use my AH (artificial horizon, not my seat of the pants) to measure the angle of inclination during the various stall regimes...

6) I would need a AOAi to measure the effect on the AOA...

7) I would need a few cameras to catch it all and remember the details...

8) I could wait for Mike to catch it on video and share here...  

9) Not recommended: Something we can learn from... In honor of Mimi's lost Marvin K. ... at a safe altitude... set the trim on max nose up,  gear and flaps down, power adjusted for level flight, ball centered.  What does this look like on camera?  How does the AOAi look, what does the ASI look like?  Be ready to recover in the event things don't work as expected.

Keep in mind,  The trim range should not be able to cause a stall.  But, it could if something like WnB or maintenance is askew.

Private Pilot thoughts only. Not a recommendation of any sort. Promoting the open conversation of safety and instrumented flight...

Best regards,

-a-

[PTK]

2 hours ago, Hank said:

Negatory on that! Stall occurs at the same or higher AoA with flaps down. Lowering flaps, however, will raise the AoA, meaning the stall will occur at a lower deck angle. Deck angle is how it looks from your seat, how high the nose is pointing; AoA is measured on the wing, and is the angle between the oncoming air and a line from the leading to the trailing edge of the wing. Lowering flaps lowers the trailing edge, which increases AoA; planes like to maintain the speed they are trimmed for, so the nose lowers to maintain the speed you had before, as pulling back in the yoke to maintain the same deck angle will increase AoA, requiring the plane to climb and slow down until equilibrium is reestablished.

It is quite possible to increase AoA too far and stall on the vertical dive portion of a loop, when the deck is pointed straight down. Please do not try this maneuver in your Mooney!

 

2 hours ago, teejayevans said:

Not according to HC Smith "Illustrated Guide to Aerodynamics", specifically pg 51 on discussion of flaps, typical lift curves diagram shows the lift vs AOA shifted upwards (higher lift) and to the left (lower AOA) when flapped. A quote from the text "In the flapped case even though the stall occurs at a lower angle the lift is much higher."

It's all relative.

Flaps increase the curvature or camber of the wing and therefore also increase the AOA. This is how flaps increase lift.

It follows then, that in order to produce the same lift as clean, the flaped wing must fly at a smaller AOA than it does clean w/out flaps.

So with flaps deployed the AOA itself increases but the plane flies at a lower AOA compared to clean in order to maintain the forces in equilibrium.

This also explains the lower stall speeds with flaps deployed.

[201er]

My question has not been answered. What is the real world impact of flaps/gear on the stall horn? How many knots difference between gear/flaps down and up? Not from each other but from actually stall speed.

[Guest]

11 minutes ago, 201er said:

My question has not been answered. What is the real world impact of flaps/gear on the stall horn? How many knots difference between gear/flaps down and up? Not from each other but from actually stall speed.

Your answer lies in the procedure I copied from the J model maintenance manual. 

Clarence

[201er]

8 minutes ago, M20Doc said:

Your answer lies in the procedure I copied from the J model maintenance manual. 

Clarence

It says how to adjust it and what the limits are but does not say what kind of stall speed margin you actually see in the two configurations. I want to know if for example stall warning sounds 6mph above stall when clean and 9mph with gear/flaps full or whatever it actually is. I'm curious how much the real world difference is.

[Hank]

14 minutes ago, 201er said:

It says how to adjust it and what the limits are but does not say what kind of stall speed margin you actually see in the two configurations. I want to know if for example stall warning sounds 6mph above stall when clean and 9mph with gear/flaps full or whatever it actually is. I'm curious how much the real world difference is.

Since the stall horn is set in flight testing for each individual plane, you'll need to flight test yours and record the numbers for stall horn activation and actual stall in each configuration that you are curious about. Your plane will not be the same as another except by pure chance. The range is 5-10 knots, but likely different for my Mooney placarded in MPH.

[carusoam]

In the document provided by Clarence there is a reference in one place that says 5 to 10 mph.  Closer to the end of the document, it is in Kias.

The whole document mentions that things will change with the flap positions and gear positions.

I take it, that means set for one, but test all the others, to know what you are going to get.

my recommendation would be to have a video of it while you are doing it.  This way you won't miss or forget all the minute detail that is going on.

The stall warning that gets loud too early will get ignored...  once it gets ignored it isn't much of a warning if things really get wonky...

Collect all this data and then share with Mooney. We could help them rewrite their document with actual data!

Best regards,

-a-

[Max Clark]

What's involved with adjusting the stall speed/horn? I don't want to bug him about it, but pretty sure the previous owner of my plane added some additional buffer.

[PT20J]

The procedure is in the service manual. Essentially, loosen the screws, move the stall vane assembly, tighten the screws, test fly. Repeat until you get it correct.

[Max Clark]

13 hours ago, PT20J said:

The procedure is in the service manual. Essentially, loosen the screws, move the stall vane assembly, tighten the screws, test fly. Repeat until you get it correct.

This will be fun at annual

[Pinecone]

On 12/30/2016 at 5:02 PM, carusoam said:

5) I would use my AH (artificial horizon, not my seat of the pants) to measure the angle of inclination during the various stall regimes...6

Attitude is not the same as AoA.   As mentioned, you can stall going straight down or straight up.  Or even do a positive G stall inverted.  

The ONLY way to determine AoA is with an AoA indicator.

We had AoA in the T-38 and A-10.   It was calibrated for full flaps.  When you practiced no-flap landings, you flew a slightly lower AoA reading.