How does the stall horn work?

[Greebo]

Take an average everyday 1975 model C172.

Does the stall horn (aka landing buzzer) know if the flaps are down or not?

I was under the impression that the horn was pretty much calibrated to VS0, but in discussions with another pilot, now I'm not sure.

If the flaps are at, oh, 20deg, does the stall horn go off somewhere slower than what it would with clean flaps?

Signed,
Confused in Slow Flight

 

[Tom-D]

Take an average everyday 1975 model C172.

Does the stall horn (aka landing buzzer) know if the flaps are down or not?

I was under the impression that the horn was pretty much calibrated to VS0, but in discussions with another pilot, now I'm not sure.

If the flaps are at, oh, 20deg, does the stall horn go off somewhere slower than what it would with clean flaps?

Signed,
Confused in Slow Flight

That really depends upon what systen you have.

If you have a vane style that has a little stickey out on the wing, AOA blows the vane up to close a switch and sound the horn.

If you have a Cessna with the tweeter style it is nothing more than the reed from a baby doll that has air sucked thru it, and out thru the vent on the wing by changing AOA.

 

[poadeleted3]

It's not calibrated to airspeed or flap position. It's angle of attack. If you go practice some stalls at full flaps and zero flaps, pay attention to the speed when the horn goes off. It'll be different. Do the same thing with zero flaps, but do them level, and then from a 45 degree turn. The speed should be different, though to be honest I've never looked from that configuration.

 

[Ed Guthrie]

Take an average everyday 1975 model C172.

Does the stall horn (aka landing buzzer) know if the flaps are down or not?

The stall horn knows only whether or not the leading edge vent is in a positive or negative pressure region of the airfoil. As the angle of attack increases the negative pressure region on the airfoil moves downward across the wing (airfoil) leading edge. When the negative pressure region crosses the leading edge vent of the stall warning system, air is drawn outward through the vent sounding the reed valve "buzzer".

I was under the impression that the horn was pretty much calibrated to VS0, but in discussions with another pilot, now I'm not sure.

Remember, you can stall a wing at any airspeed and any aircraft attitude. The leading edge stall indicator is somewhat an AOA device, not an airspeed device.

If the flaps are at, oh, 20deg, does the stall horn go off somewhere slower than what it would with clean flaps?

All other things being equal, yes. All other things being equal, the stall warning should go off at a much increased aircraft attitude since flaps deployed increases the angle of incidence, which should equate to slower airspeed.

 

[waldo]

I'd like to add my understanding of it if I may. As mentioned, it is based on the angle of attack. But what really happens? Well, as to my understanding, as the angle of attack changes the same amount of air does not flow under the wing but instead flows towards the leading edge upwards and towards the top of the wing. This flow creates a negative pressure on the stall horn which in turn causes the reeds in the horn to come alive and create the stall sound. Because this happens based on the angle of attack, it will happen at different speeds based on different configurations. You may notice that if you are climbing at a very high attitude, near stall, and get a updraft the horn will go off for a second or two. This is because the angle of attack just changed for that moment.

I hope this helps!

 

[Ed Guthrie]

It's not calibrated to airspeed or flap position. It's angle of attack.

Not quite. It is a low pressure location on the wing leading edge sensor. That location will vary with both AOA and wing configuration (flaps versus no flaps).

 

[RotaryWingBob]

Not quite. It is a low pressure location on the wing leading edge sensor. That location will vary with both AOA and wing configuration (flaps versus no flaps).

Maybe this is saying the same thing, Ed, but I thought that the Piper type works because the point of stagnation moves lower on the leading edge as the AOA increases. When it moves below the vane the airflow from below the vane pushes it up and activates the microswitch.

 

[gismo]

Take an average everyday 1975 model C172.

Does the stall horn (aka landing buzzer) know if the flaps are down or not?

I was under the impression that the horn was pretty much calibrated to VS0, but in discussions with another pilot, now I'm not sure.

If the flaps are at, oh, 20deg, does the stall horn go off somewhere slower than what it would with clean flaps?

Signed,
Confused in Slow Flight

AFaIK both the vane type (Piper, Beech etc.) and the reed type (Cessna) sense the location of the stagnation point on the leading edge of the wing which shifts with changes in AOA. WRT flaps extension, I believe that the location of the stagnation point at stall does not change appreciably with the extention of flaps. Flaps simply allow you to generate more lift (with higher drag) at the same airspeed prior to stall (or the same lift at a lower AOA). IOW for the most part a leading edge stall warning sensor should give about the same margin over stall with zero and full flaps.

 

[Ed Guthrie]

WRT flaps extension, I believe that the location of the stagnation point at stall does not change appreciably with the extention of flaps.

Flaps can greatly change the overall airfoil shape, usually by drooping the trailing edge. The rearward nature of the airfoil shape adjustment may therefore quite significantly move the stall AOA stagnation point up/down on the wing leading edge. Some aircraft have multiple stall sensor vanes on the leading edge for just this reason. One vane is the stall warning circuit for zero or little flaps deployed, another vane is the stall warning circuit for full flap extension. IIRC, the Grumman Cougar is one example of a multiple stall warning vane locations aircraft. The above makes perfect sense if you look at an airfoil with and without flaps deployed and consider what happens to the chord line and couple that observation that the stall AOA is a fairly constant value across a broad spectrum of airfoils.

 

[Skip Miller]

One vane is the stall warning circuit for zero or little flaps deployed, another vane is the stall warning circuit for full flap extension. IIRC, the Grumman Cougar is one example of a multiple stall warning vane locations aircraft.

Piper Saratoga II is another.

-Skip

 

[gismo]

Flaps can greatly change the overall airfoil shape, usually by drooping the trailing edge. The rearward nature of the airfoil shape adjustment may therefore quite significantly move the stall AOA stagnation point up/down on the wing leading edge. Some aircraft have multiple stall sensor vanes on the leading edge for just this reason. One vane is the stall warning circuit for zero or little flaps deployed, another vane is the stall warning circuit for full flap extension. IIRC, the Grumman Cougar is one example of a multiple stall warning vane locations aircraft. The above makes perfect sense if you look at an airfoil with and without flaps deployed and consider what happens to the chord line and couple that observation that the stall AOA is a fairly constant value across a broad spectrum of airfoils.

I'm sure you're correct about the Cougar's multiple vanes, but part of this doesn't make much sense to me yet. In general it seems unlikely that deflecting and/or extending the trailing edge of a wing would have much effect on separation in the first 30% or so of the wing (which hasn't changed shape). If that's true, I would expect that the stalling AOA with flaps (referenced to the chord line drawn through the lowered trailing edge) would actually be less than the stalling AOA with flaps up. I know I've got some information on this at home, I guess I'd better re-read.

FWIW in the Baron, the stall warning comes on at about the same margin above stall speed with flaps up or down, and there's only one vane.