AOA vs Stall Horn

nauga said:
The pneumatic AOA systems don't use the difference between ram and static to calculate AOA, it's the difference between total (ram) and a different port that measures something not fully aligned with the flow and not completely static. Pitot tubes don't 'give airspeed', they give pressures which are used to calculate airspeed or any number of other parameters. Your airspeed indicator is 'calculating' indicated airspeed mechanically using the known relationship between a pressure differential and speed. My pitot tube has a port in the tip (total) and a not-quite-aligned port, the static port is not on the tube. The static input is only used in the dynamic pressure used to normalize the measurement. The pressure differential for AOA (but not the full calculation) is the difference between the total and the not-quite-aligned ports.

Nauga,
whose curves collapse
Click to expand...

OK. What about 'accuracy' of the pressures. Are you saying the accuracy of the pressure differences delivered to the ASI or the 'computer' in one of these AOAlike things are as accurate for a Pitot Static System originally designed to give the pilot airspeed(via the pressures delivered to the ASI) as those delivered from a sensor specifically designed to deliver pressures to an AOA indicator?
 
luvflyin said:
OK. What about 'accuracy' of the pressures. Are you saying the accuracy of the pressure differences delivered to the ASI or the 'computer' in one of these AOAlike things are as accurate for a Pitot Static System originally designed to give the pilot airspeed(via the pressures delivered to the ASI) as those delivered from a sensor specifically designed to deliver pressures to an AOA indicator?
Click to expand...
There's a lot in this...the pressures themselves at the pressure ports don't have to be accurate, they just need to be repeatable, meaning for the same flight condition the port always experiences the same pressure, just like an airspeed indicator. All of the accurate pneumatic AOA indicators use some kind of transducer, not a bourdon tube like a mechanical airspeed indicator but electronic...like a 'glass' airspeed indicator. Typical accuracy of these transducers is ~1-2% of the range (or less), which is going to result in a pretty small altitude, airspeed, or AOA error. The biggest impact on 'accuracy' is in the calibration. Sideslip will change the pressure at the ports just as it does when used to measure airspeed. The beauty of the system is that you can account for this when calibrating the system. Pitch and yaw rate will also affect the pressure and are harder to calibrate, but these also affect vanes. Pneumatic systems are *not* affected by acceleration (i.e. g's in any direction) like a vane that's not perfectly balanced is.

The reason I keep putting 'accuracy' in quotes is because neither a pneumatic system nor a vane actually measure angle of attack. They measure, or can be used to measure, the flow direction where the sensor is mounted. You can calibrate systems to account for a lot of it but some, like upwash and rate-induced errors, are more difficult to correct for regardless of the system. There's a reason you see nosebooms on flight test airplanes with AOA and sideslip vanes mounted on them...and even then you have a larger rate-induced term to correct for, as well as boom bending under load.

So...if you're really looking for accuracy, neither a vane nor a pneumatic sensor in close proximity to the airplane is going to give you something that's dead-nuts accurate under all conditions. The pneumatic systems *can be* close for everyday use, just as a vance *can be*, but there's no guarantee with either one.

FWIW the paper I linked a few posts back has analysis of errors due to some of the factors I'm talking about, and shows why the normalization works. It's a little more technically-oriented than a form post and might not be for everyone, though.

Nauga,
from the geek farm
 
nauga said:
There's a lot in this...the pressures themselves at the pressure ports don't have to be accurate, they just need to be repeatable, meaning for the same flight condition the port always experiences the same pressure, just like an airspeed indicator. All of the accurate pneumatic AOA indicators use some kind of transducer, not a bourdon tube like a mechanical airspeed indicator but electronic...like a 'glass' airspeed indicator. Typical accuracy of these transducers is ~1-2% of the range (or less), which is going to result in a pretty small altitude, airspeed, or AOA error. The biggest impact on 'accuracy' is in the calibration. Sideslip will change the pressure at the ports just as it does when used to measure airspeed. The beauty of the system is that you can account for this when calibrating the system. Pitch and yaw rate will also affect the pressure and are harder to calibrate, but these also affect vanes. Pneumatic systems are *not* affected by acceleration (i.e. g's in any direction) like a vane that's not perfectly balanced is.

The reason I keep putting 'accuracy' in quotes is because neither a pneumatic system nor a vane actually measure angle of attack. They measure, or can be used to measure, the flow direction where the sensor is mounted. You can calibrate systems to account for a lot of it but some, like upwash and rate-induced errors, are more difficult to correct for regardless of the system. There's a reason you see nosebooms on flight test airplanes with AOA and sideslip vanes mounted on them...and even then you have a larger rate-induced term to correct for, as well as boom bending under load.

So...if you're really looking for accuracy, neither a vane nor a pneumatic sensor in close proximity to the airplane is going to give you something that's dead-nuts accurate under all conditions. The pneumatic systems *can be* close for everyday use, just as a vance *can be*, but there's no guarantee with either one.

FWIW the paper I linked a few posts back has analysis of errors due to some of the factors I'm talking about, and shows why the normalization works. It's a little more technically-oriented than a form post and might not be for everyone, though.

Nauga,
from the geek farm
Click to expand...

I guess the real question is how much accuracy is really needed? It's a tool, one that predicts stall, but one of many indicators at the pilot's disposal, the ultimate being the wing actually stalling and the nose dropping. Like most tools, or more accurately gauges, it is not used to keep you on the knife edge of oblivion, rather it's a tool used to keep you somewhere between stall oblivion and so much speed you float down the entire runway on approach (or try to force it down creating a porpoise and prop strike). In turns, especially in critical conditions, like getting slow on base to final, it's an indication of how close to oblivion you are getting versus airspeed which is a guess. I'm a recent AOA convert, I like it, still learning to use it properly. I don't need one to fly, but it's nice to have.

PaulS
Not preaching to Nauga here, just replied to his post, which most, if not all I enjoy reading.
 
nauga said:
There's a lot in this...the pressures themselves at the pressure ports don't have to be accurate, they just need to be repeatable, meaning for the same flight condition the port always experiences the same pressure, just like an airspeed indicator. All of the accurate pneumatic AOA indicators use some kind of transducer, not a bourdon tube like a mechanical airspeed indicator but electronic...like a 'glass' airspeed indicator. Typical accuracy of these transducers is ~1-2% of the range (or less), which is going to result in a pretty small altitude, airspeed, or AOA error. The biggest impact on 'accuracy' is in the calibration. Sideslip will change the pressure at the ports just as it does when used to measure airspeed. The beauty of the system is that you can account for this when calibrating the system. Pitch and yaw rate will also affect the pressure and are harder to calibrate, but these also affect vanes. Pneumatic systems are *not* affected by acceleration (i.e. g's in any direction) like a vane that's not perfectly balanced is.

The reason I keep putting 'accuracy' in quotes is because neither a pneumatic system nor a vane actually measure angle of attack. They measure, or can be used to measure, the flow direction where the sensor is mounted. You can calibrate systems to account for a lot of it but some, like upwash and rate-induced errors, are more difficult to correct for regardless of the system. There's a reason you see nosebooms on flight test airplanes with AOA and sideslip vanes mounted on them...and even then you have a larger rate-induced term to correct for, as well as boom bending under load.

So...if you're really looking for accuracy, neither a vane nor a pneumatic sensor in close proximity to the airplane is going to give you something that's dead-nuts accurate under all conditions. The pneumatic systems *can be* close for everyday use, just as a vance *can be*, but there's no guarantee with either one.

FWIW the paper I linked a few posts back has analysis of errors due to some of the factors I'm talking about, and shows why the normalization works. It's a little more technically-oriented than a form post and might not be for everyone, though.

Nauga,
from the geek farm
Click to expand...
That helps. Thanks for the insight.
 
I'm a member of the blue donut club as of two weeks ago. I'm finding it well worth the effort of the install. My aircraft has no stall horn at all, so big improvement.
 
Salty said:
I'm a member of the blue donut club as of two weeks ago. I'm finding it well worth the effort of the install. My aircraft has no stall horn at all, so big improvement.
Click to expand...

What brand/model did you install?
 
TimRF79 said:
But going back to my original question,
what the practical value relevance for your average GA aircraft?

I can see you want to land a jet on an aircraft carrier, you want to be right at the limit...
You want to win the STOL competition... sure

Would it help make landings smoother if you fly at a certain AOA point, versus a certain speed on final?
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An AOA meter is only of benefit if you practice with it to learn more about how your airplane flies when at the edges of the flight envelope. An AOA meter gives you a visual representation of where you are flying in relation to the stall during all flight maneuvers. It can be used as a safety tool to help train yourself to fly near the stall point without ever exceeding it.

Practicing ground maneuvers like S turns over a road while observing the AOA indications is a good way to calibrate your "seat of the pants" flying skills. If you train regularly with an AOA meter while performing steep bank turns, you should never have a Base to Final stall spin accident.

I have a Lift Reserve Indicator in my airplane and I practice with it semi regularly. You have to calibrate it for your aircraft and check it's a calibration at different flap settings. If calibrated properly, and used with regular practice it will help you become a better stick and rudder pilot. If you don't ever use it , it won't help you at all.

One last note: the US Navy adopted AOA for use on carrier landings after WWII. The first year they cut landing accidents in half. The second year they cut them in half again, after that they stabilized. AOA has value, but you need to use it to gain value from it.
 
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AOA has value if you operate near the stall in variable conditions at varying weights. For speeds most of you land at? It won’t matter much. If you fly a Cessna and use the stall horn squealing down final and going silent before touchdown? You might appreciate an AOA indicator. In my Cessna, the horn is adequate. I know what the changes in pitch mean. My Cub has no stall indicator and approach speeds are below 30mph. Not easy to “feel” the stall margin when treetops buffet the wind. I like an AOA in that plane. It verifies where the wing is relative to letting go.
 
I've got an AV-30 that indicates AoA. How accurate is that tech?
 
Ed Haywood said:
I've got an AV-30 that indicates AoA. How accurate is that tech?
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From the AV-30 E Pilot's Guide: "Angle of attack is determined by comparing aircraft pitch to the aircraft flight path angle through the air. In level flight this directly corresponds to the angle at which the wing is intercepting the body of air surrounding the aircraft, correlating to the current AoA."

This assumption is only valid for wings-level unaccelerated flight, which pretty much renders it useless for AOA estimation for a flight instrument. I strongly suspect that the description in the manual is dumbed down considerably from what's really going on, which is probably more like what (almost) everyone else is doing with air data, inertial data, and Kalman filtering. in which case it can be pretty accurate - at east enough for our purposes.

Nauga,
who goes with the flow
 
Ed Haywood said:
I've got an AV-30 that indicates AoA. How accurate is that tech?
Click to expand...
You will have to read the manual about the setup, but you will have to verify the AOA device indicates the edge of the stall at the point where the stall begins. You can do that at about 2000AGL and bring on a gentle straight and level stall with a little bit of power. As the stall just begins the AOA meter should be at the top of the red band, or on the line between the Red and White zones, but not in the white. You may have to tweak it a bit for different flap settings. The main goal is to have the indicator on the red anywhere a stall can occur, and indicating white only if a stall is not occurring. Once properly set, staying in the white zone keeps you out of the stall at all times in all flight maneuvers. Entering the red means no safety margin exists.
 
nauga said:
From the AV-30 E Pilot's Guide: "Angle of attack is determined by comparing aircraft pitch to the aircraft flight path angle through the air. In level flight this directly corresponds to the angle at which the wing is intercepting the body of air surrounding the aircraft, correlating to the current AoA."

This assumption is only valid for wings-level unaccelerated flight, which pretty much renders it useless for AOA estimation for a flight instrument. I strongly suspect that the description in the manual is dumbed down considerably from what's really going on, which is probably more like what (almost) everyone else is doing with air data, inertial data, and Kalman filtering. in which case it can be pretty accurate - at east enough for our purposes.

Nauga,
who goes with the flow
Click to expand...

Thanks. I have not paid any attention to it. Maybe I'll play around and see if it does anything fun in acro.
 
Salty said:
I'm a member of the blue donut club as of two weeks ago. I'm finding it well worth the effort of the install. My aircraft has no stall horn at all, so big improvement.
Click to expand...
On your Russian plane? Were there specific instructions for the install? Or do you just kinda do it generically so to speak?
 
luvflyin said:
On your Russian plane? Were there specific instructions for the install? Or do you just kinda do it generically so to speak?
Click to expand...
Nyet Russian comrade. Chinese.

The install is generic. Would work for any aircraft.
 
What’s this Cessna squealy thing y’all mention?

Tinnitus Tools
Apparently ALWAYS close to stalling…
 
Didn’t the NTSB put a damper on AOA saying they could find very many places where it would have actually prevented a crash? And of course the thousands of our fellow pilots who have ridden a plane into the ground with stall warning horn blaring a foot from their ears.
 
Rockymountain said:
Yeah not really true. Many light GA aircraft at some point above Va will pull the wings off before they stall or hit critical AoA.
Click to expand...
ALL aircraft will do that. That is the definition of Va. The minimum speed which you can generate the max G limit.
 
nauga said:
There's a lot in this...the pressures themselves at the pressure ports don't have to be accurate, they just need to be repeatable, meaning for the same flight condition the port always experiences the same pressure, just like an airspeed indicator. All of the accurate pneumatic AOA indicators use some kind of transducer, not a bourdon tube like a mechanical airspeed indicator but electronic...like a 'glass' airspeed indicator. Typical accuracy of these transducers is ~1-2% of the range (or less), which is going to result in a pretty small altitude, airspeed, or AOA error. The biggest impact on 'accuracy' is in the calibration. Sideslip will change the pressure at the ports just as it does when used to measure airspeed. The beauty of the system is that you can account for this when calibrating the system. Pitch and yaw rate will also affect the pressure and are harder to calibrate, but these also affect vanes. Pneumatic systems are *not* affected by acceleration (i.e. g's in any direction) like a vane that's not perfectly balanced is.

The reason I keep putting 'accuracy' in quotes is because neither a pneumatic system nor a vane actually measure angle of attack. They measure, or can be used to measure, the flow direction where the sensor is mounted. You can calibrate systems to account for a lot of it but some, like upwash and rate-induced errors, are more difficult to correct for regardless of the system. There's a reason you see nosebooms on flight test airplanes with AOA and sideslip vanes mounted on them...and even then you have a larger rate-induced term to correct for, as well as boom bending under load.

So...if you're really looking for accuracy, neither a vane nor a pneumatic sensor in close proximity to the airplane is going to give you something that's dead-nuts accurate under all conditions. The pneumatic systems *can be* close for everyday use, just as a vance *can be*, but there's no guarantee with either one.

FWIW the paper I linked a few posts back has analysis of errors due to some of the factors I'm talking about, and shows why the normalization works. It's a little more technically-oriented than a form post and might not be for everyone, though.

Nauga,
from the geek farm
Click to expand...
I think there is a fundamental misunderstanding about how an LRI meter works. There are two air ports on the input probe. They both face forward, similar to a pitot tube, but the two inputs are about 35° angles apart from each other. On a flat tricycle airplane, like a Cherokee, the top port is relatively flat so it behaves almost exactly like a pitot tube and the lower tube is angled down at about 35° thus creating an input that is of lower pressure, being at an angle to the relative wind. The lower tube feeds the red side of the differential pressure gauge and the upper tube feeds the green side. The combination of pressures creates a needle movement that is proportional to the total power available in the air and in proportion to the angle of attack of the wing.

On the takeoff run as the flat tricycle airplane increases speed more pressure is applied to the green side of the gauge than to the red side and the needle moves upward into the green zone. As the pilot pulls the nose up at rotation speed the wing and the input probe are rotated up together, changing their angle to the relative wind, causing the red pressure to increase dramatically while the green input is increasing slowly or decreasing based on the angle of the pickup to the relative wind. This causes the needle to move down towards the white and red zones.

To set the meter up, you adjust the angle of the probe relative to the wing, until trial and error flight testing puts the needle at the line between the white and red zones just as the stall buffet begins. The object is to have the needing at or in the red zone under any circumstances that a stall is occurring, and in the white zone near the stall when no stall can occur. This will take some flight testing at various speeds and maneuvers to make sure it is always in the red zone (or below the white) under any circumstance when a stall occurs.

The LRI meters I have flown with are very responsive, moving instantly as the yoke is moved during low speed maneuvers. With some practice you can do very steep low speed turns and hard over rolls with confidence. Since the meter combines both airspeed and angle of attack in a single display, a quick glance during steep turns ensures a safety margin above the stall at all times. You can also use the meter to find maximum steep AOA climb outs from takeoff, and be able to repeat that maneuver easily and precisely every time.

I know that not everyone agrees, but I have flown with LRI meters for about 18 years now, and I like them. But they are only of use if you practice with them and use the regularly.
 
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William Pete Hodges said:
I think there is a fundamental misunderstanding about how an LRI meter works.
Click to expand...
I agree, but I think we might disagree on where the misunderstanding lies.
William Pete Hodges said:
The combination of pressures creates a needle movement that is proportional to the total power available in the air and in proportion to the angle of attack of the wing.
Click to expand...
The pressure differential creates a needle movement that is proportional to dynamic pressure and angle of attack. There are no other inputs and there is no inferred "power" output. It is analogous to a standard pitot-static setup for an airspeed indicator with a very large static source position error correction that varies strongly with AOA in addition to airspeed, and uses that variation to drive the indicator. "Lift reserve" (sic) can be inferred from it and there is some utility in that, but it is by no means an angle of attack indicator with any degree of precision. OTOH, if you normalize that pressure differential with respect to dynamic pressure (digitally or otherwise) you can nearly eliminate the dynamic pressure dependency and get a reasonably precise measure of angle of attack that has more utility than estimating "lift reserve" - e.g. AOA for approach or L/D max invariant with weight.

This is presented pretty well in the paper I posted a link to earlier in this (4 year old) thread....but it's not necessarily an easy read.

William Pete Hodges said:
I know that not everyone agrees, but I have flown with LRI meters for about 18 years now, and I like them. But they are only of use if you practice with them and use the regularly.
Click to expand...
I agree with that, with the caveat that you must also understand the limitations of the system, as you also must with any AOA sensors.

Nauga,
who knows it's 3 o'clock somewhere :cool:
 
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