AOA vs Stall Horn

[luvflyin]

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

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?

 

[nauga]

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?

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

 

[PaulS]

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

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.

 

[luvflyin]

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

That helps. Thanks for the insight.