You are flying along inverted and you are about to stall

SixPapaCharlie

SixPapaCharlie

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Flying along inverted, minding my own business.

My stall horn should be blaring?
The stall horn going silent should be the indication that I am close to a stall?
 
The bottom of the wing is now where flow separation is happening, so your stall warning has nothing to do with stall anymore.

I doubt the stall warning will sound under any circumstances while inverted.
 
I was thinking airflow at a "normal" angle held that tab down.
Then the angle exceeded CA, the air then lifts it up to wake the pilot.

Thinking inverted would to the opposite.
 
MAKG1 said:
The bottom of the wing is now where flow separation is happening, so your stall warning has nothing to do with stall anymore.

I doubt the stall warning will sound under any circumstances while inverted.
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Trust me, you can pull too hard over the top - I've done it before.
 
I would think the stall horn would be drowned out by your screaming grandparents.
 
NO No, I mean, my engine would quit. Lots of other things would happen.
I am wondering strictly about the behavior of the stall horn.
 
The stall horn trips when the near-stall angle of attack for normal flight is exceeded. When flying inverted, the airflow sensor would be aimed the wrong way (up into space) and never be tripped. So no stall horn warning when inverted, unless you have another sensor, aimed upwards normally.
 
I may be in the minority but air flow doesn't care whether or not you're flying inverted, air will detach, it will create a vacuum, and activate the stall warning horn regardless of orientation.
 
OK, the wing is the yellow thing, the flipper for the stall horn is the red thing, and the blue lines are the airflow near stall shiny side up (top drawing) and wheel side up (bottom drawing). You can see that the air flow tries to flip the stall horn flipper up (towards the shiny side) for a shiny side up stall and turn the horn on. Wheel side up, the air flow holds the flipper towards the wheel side which causes the stall horn to not go off.

And, now you know why I am an engineer and not an artist.
 

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Capt. Geoffrey Thorpe said:
OK, the wing is the yellow thing, the flipper for the stall horn is the red thing, and the blue lines are the airflow near stall shiny side up (top drawing) and wheel side up (bottom drawing). You can see that the air flow tries to flip the stall horn flipper up (towards the shiny side) for a shiny side up stall and turn the horn on. Wheel side up, the air flow holds the flipper towards the wheel side which causes the stall horn to not go off.

And, now you know why I am an engineer and not an artist.
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YES!!!

Now draw them both in cruise
The inverted plane should have the stall horn going off.

And your very scientific drawing backs up my notion that near stall, the horn would STOP blaring.

This is concrete proof of my hunch.
 
Capt. Geoffrey Thorpe said:
OK, the wing is the yellow thing, the flipper for the stall horn is the red thing, and the blue lines are the airflow near stall shiny side up (top drawing) and wheel side up (bottom drawing). You can see that the air flow tries to flip the stall horn flipper up (towards the shiny side) for a shiny side up stall and turn the horn on. Wheel side up, the air flow holds the flipper towards the wheel side which causes the stall horn to not go off.

And, now you know why I am an engineer and not an artist.
Click to expand...

Fail, I was using my Diamond as a reference unfortunately and it's a much more rudimentary stall horn system.
 
Where is Henning when we need him?

I've always thought that the wing is moving through a fluid and doesn't care about the orientation to the ground, so inverted or not if the AOA is not exceeded to you would not hear a warning.

Is there an assertion that the weight of the stall warning 'flapper' would cause it to overcome the wind/angle and cause it to flip up and sound?

I'm sure some aerobatic types can clarify.
 
Oh... here's a discussion of the wing design and AOA in inverted flight:

Now we have to once again talk of angle of attack. If we increase the asymmetric
wing's angle of attack, we displace on the leading edge of the wing the point where the
airflow separates to traverse the wing's upper and lower surfaces. By increasing the angle
of attack, we have further increased the length of the top surface and decreased the
length of the bottom surface along which the air particles have to travel. The airflow
along the upper surface speeds up further relative to the airflow along the lower surface,
and the result is increased lift.
So, what about a symmetric wing, the wing found on most high-performance aerobatic
aircraft? If the upper and lower surface curvatures are the same, the pressure
zones on the two sides should be equal and the wing should never be capable of ascending.
Yet it is, and the answer is angle of attack. A 2-3° angle of attack will sufficiently
move the point on the leading edge where the airflow separates to create a
difference between the length of the wing top and bottom surface over which the air
flows, resulting in an increase in lift.
Now consider the asymmetric wing inverted. The task is to increase the distance
over which the air particles have to flow along the top surface to exceed the distance
traveled by the airflow along the bottom surface. This is done by increasing the angle
of attack. The increase has to be substantial, which is why aircraft with asymmetric
wing profiles can maintain inverted level flight only in a noticeably nose-high attitude.
The principle is the same for the symmetric wing, but the angle of attack required
is small and equivalent to that required right side up. The symmetric wing performs
equally well either side up, which is the reason why it is found on high-performance
aerobatic aircraft.
 
SixPapaCharlie said:
YES!!!

Now draw them both in cruise
The inverted plane should have the stall horn going off.

And your very scientific drawing backs up my notion that near stall, the horn would STOP blaring.

This is concrete proof of my hunch.
Click to expand...
No.

As you increase the angle of attack in a positive direction the stagnation point rotates downward until it gets under the flipper and sets off the noisemaker.

As you push the nose down, the stagnation point rotates towards the shiny side - and it just keeps rotating more and more shinywise as you push inverted. This keeps the tab firmly rotated towards the wheels so the stall horn just stays off.
 
Maciej said:
I may be in the minority but air flow doesn't care whether or not you're flying inverted, air will detach, it will create a vacuum, and activate the stall warning horn regardless of orientation.
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Create a vacuum? Where? How?
 
Maciej said:
I may be in the minority but air flow doesn't care whether or not you're flying inverted, air will detach, it will create a vacuum, and activate the stall warning horn regardless of orientation.
Click to expand...

Capt. Geoffrey Thorpe said:
Create a vacuum? Where? How?
Click to expand...

I think both of you guys are right. The difference is that you're talking about different conditions. For the wing to not care if its inverted and act as you'd expect with the stall horn, you'd have to be pulling positive G's and pulling too hard as to stall it(and activate the horn). Think of the top of a loop here.

In the other case, you're talking about flying straight and level, but inverted where the aoa would compare to pulling negative 1g, as in pushing over the top if you weren't inverted. Where I think as LS Guy says, your airflow is still not going to activate your horn.

disclaimer, low time student, probably no idea what i'm talking about. i cant even use caps and punctuation properly
 
It might depend on the design of the stall horn too. The little flipper thing in Pipers works in one direction but the Cessna type may well be unidirectional.
 
Sac Arrow said:
It might depend on the design of the stall horn too. The little flipper thing in Pipers works in one direction but the Cessna type may well be unidirectional.
Click to expand...

Mine is just a tiny hole. I have no idea what sort of magic happens in there.
 
Sac Arrow said:
It might depend on the design of the stall horn too. The little flipper thing in Pipers works in one direction but the Cessna type may well be unidirectional.
Click to expand...

What Cessna type? 177, 182, 206 all have electrical stall horns just like Cherokees.

The reed types in 152 and 172 are in the wrong spot and won't do anything either. The basic problem is that the flow will never separate "above" the chord line with negative G's.
 
I'll just wait for 6PC to report back with his test results flying an SR inverted.
 
I think these drawings are closer to scale of what Capt Thorpe was trying to show earlier. A couple of notes -
- the red thingy on the front of the airfoil is the stall switch. If you have a whistle type, pretend it is the direction the wind must enter to make the whistle work.
- this is the same airfoil inverted to the same angle. The chord lines are parallel

If we say that the angle drawn represents the critical angle of attack, then you can see that when upright, the stall switch is ready to flip up and sound. But when it is inverted, it is being forced down into the no-sound position.

I couldn't get the image to invert that way, but if you straighten out the inverted airfoil until the stall switch is pointing directly into the wind again, you have a negative angle of attack and are doing an aggressive descent. If you push that descent over far enough, you will get the stall horn to sound. I believe the angle would be the negative of the critical angle of attack.

More simplistically, you're inverted. If you pull on the yoke until your wing is departed from the horizontal by the critical angle of attack, the stall horn will go off. It doesn't matter whether that is up or down. If you do this in a real airplane, please wear a parachute.

Is there such a thing as a negative stall? Weird, you would stall your descent and stop going down as much.

Thank you Bryan for an interesting question

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bflynn said:
I think these drawings are closer to scale of what Capt Thorpe was trying to show earlier. A couple of notes -
- the red thingy on the front of the airfoil is the stall switch. If you have a whistle type, pretend it is the direction the wind must enter to make the whistle work.
- this is the same airfoil inverted to the same angle. The chord lines are parallel

If we say that the angle drawn represents the critical angle of attack, then you can see that when upright, the stall switch is ready to flip up and sound. But when it is inverted, it is being forced down into the no-sound position.

I couldn't get the image to invert that way, but if you straighten out the inverted airfoil until the stall switch is pointing directly into the wind again, you have a negative angle of attack and are doing an aggressive descent. If you push that descent over far enough, you will get the stall horn to sound. I believe the angle would be the negative of the critical angle of attack.

More simplistically, you're inverted. If you pull on the yoke until your wing is departed from the horizontal by the critical angle of attack, the stall horn will go off. It doesn't matter whether that is up or down. If you do this in a real airplane, please wear a parachute.

Is there such a thing as a negative stall? Weird, you would stall your descent and stop going down as much.

Thank you Bryan for an interesting question

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Click to expand...



Right....

Now draw them in cruise both Inverted and verted

My hypothesis is that inverted the stall horn would be blaring during flight but stop upon entering a stall.

Your drawing supports my thought that at the stall, the horn on the inverted plane would be pushed shut.

What about during cruise?
 
ok, I see you updated the drawing.

Quick clarification:
by inverted stall, I mean flying inverted and pushing the yoke forward.
I am not referring to an accelerated stall (Pulling the yoke)

I mean like doing an outside loop but you bought a cub and didn't read the POH part where it talks about how much horsepower the plane has.
 
SixPapaCharlie said:
ok, I see you updated the drawing.

Quick clarification:
by inverted stall, I mean flying inverted and pushing the yoke forward.
I am not referring to an accelerated stall (Pulling the yoke)

I mean like doing an outside loop but you bought a cub and didn't read the POH part where it talks about how much horsepower the plane has.
Click to expand...

Yup, I believe the diagram works for both.

You caught me in the middle of making an edit change.

I don't have really fine control on skewing the airfoils, I'm using Paint. But I think it's clear from the new third airfoil that the stall horn will not sound in the inverted position until you have a negative of the critical angle of attack.

Of course "Wind" is all relative wind and I can't even begin to wrap my mind around how that changes in a forced inverted descent.
 
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If your stall warning only trips at positive AOA then you will not get a warning when approaching a stall at negative AOA. If your airfoil is like any common GA airfoil then to sustain inverted flight you will be at negative AOA and no stall warning if it operates as in the first sentence - whether you're cruising inverted or pushing into the stall.

The same holds true if pushing to the same -ive AOA (or to -1g) from upright level flight, the wing and stall warning don't care which way the ground is.

...and how did you get this far without referring to -1g and 1g flight as inverted and erect?

Nauga,
shoveling stalls
 
nauga said:
If your stall warning only trips at positive AOA then you will not get a warning when approaching a stall at negative AOA. If your airfoil is like any common GA airfoil then to sustain inverted flight you will be at negative AOA and no stall warning if it operates as in the first sentence - whether you're cruising inverted or pushing into the stall.

The same holds true if pushing to the same -ive AOA (or to -1g) from upright level flight, the wing and stall warning don't care which way the ground is.
Click to expand...

Ah got it. So you are negative AOA and only getting more and more negative as you "push" the stall
Makes total sense now.

nauga said:
...and how did you get this far without referring to -1g and 1g flight as inverted and erect?
Click to expand...


Because I really wanted to invent the term "verted" as a reciprocal to inverted.
 
SixPapaCharlie said:
Because I really wanted to invent the term "verted" as a reciprocal to inverted.
Click to expand...
Hmmm...me 'n the rest of the cabal figured you were the kind of guy who would like people to know you flew erect most of the time.

Nauga,
and Beavis and Butthead
 
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