Understanding the Angle of Attack Indicator: How is it Measured?

[thist796]

So... I am trying to understand the Angle of Attack Indicator (like this one: http://starfleetsupport.com/images/Pages/Avionics2/IMG_2417L_AOA.jpg) and I cannot seem to find a straightforward explanation anywhere on the web as to how it is measured. It goes from zero to 1 and usually has a green, yellow and red areas so as to alert the pilot when he is approaching stall.
How is that number from zero to 1 calculated? My guess is that it is [Lift currently being generated]/[Max lift] but I really have no clue

Any help is MUCH MUCH MUCH appreciated!=)

 

[Brian_C]

My guess is that it represents the sine of the angle of attack.

 

[thist796]

Well, just think about it, the sine of the AOA is completely useless to the pilot. Pilots use the AOA indicator mostly for STOL, therefore this instrument must tell the pilot how close to approaching stall he is, and somehow airspeed and pressure must also go into the equation on top of the angle of attack.

 

[FredGarvin]

It is to my understanding that the divisions do not have any units associated with them and that the gauge needs to be calibrated on every aircraft it installed on. You've piqued my interest and I'll dig around a bit too.

 

[mugaliens]

In sub-mach flight below significant mach drag rise effects, stall occurs at the same angle of attack regardless of airspeed. Delta-wing aircraft could develop some serious angles of attack before stall set in, so it was important the pilots know just how much AOA they were flying compared to AOA_stall, particularly during combat maneuvers, flying a few hundred knots at high gs.

Well, just think about it, the sine of the AOA is completely useless to the pilot. Pilots use the AOA indicator mostly for STOL, therefore this instrument must tell the pilot how close to approaching stall he is, and somehow airspeed and pressure must also go into the equation on top of the angle of attack.

You're close: 1.0 is stall, hence it's position on the guage. However, the gauge was simply hooked up to a mechanical vane mounted near the nose of the aircraft.

 

[zagam]

See also Lift Reserve Indicator:

http://en.wikipedia.org/wiki/Airspeed_indicator#Variations
http://www.mountainflying.com/Pages/articles/alpha_systems_aoa.html
http://www.mountainflying.com/Pages/articles/images/lri_probe.jpg

It does not use a vane and encoder, but can work without power.
Needs some airspeed though.

 

[mugaliens]

See also Lift Reserve Indicator:

http://en.wikipedia.org/wiki/Airspeed_indicator#Variations

Although this is an interesting idea, we pilots prefer the http://en.wikipedia.org/wiki/Stall_(flight)#Stall_warning_and_safety_devices", as when we're in stall territory, our eyes are usually outside the cockpit, and rightfully so.

http://www.mountainflying.com/Pages/articles/alpha_systems_aoa.html

Hey! MT43 - I've been there!

A friend of mine and I flew in there, and he bet I couldn't put it down in the runway length, so I bet him $100 I could put it down in halfthe length of the strip. I used an old bush pilot trick (my first IP was a former bush pilot) and came in with full flaps, very high AOA, and hanging it on the prop with the engine running about 80%. Technically, I was simply maneuvering on the backside of the power curve. Not quite in the manual or according to FAA regs, but not in violation of them, either.

It was a 172, with two people and half fuel. We stopped well before the first little tree you see in the picture on your right. I didn't measure it, but I estimate it was less than 500'.

Older military jets used an AOA vane, a simple free-floating, counterbalanced air vane, to indicate the AOA under any and all conditions. It's mounted on/near the nose. Newer military jets use differential pressure from static ports mounted on the nose, as interpreted by the onboard avionics computer.

 

[zagam]

http://www.mountainflying.com/Pages/articles/images/lri_probe.jpg
The original poster wanted to see a "how it works" diagram.

It would be very easy for such a device to produce an audible alarm at low d.p.
You could also heat ports to prevent them being blocked by ice.

Keep It Simple Stupid always applies.

This does not show computed angle of attack by itself, but red (low d.p.) bad.
No suck, no lift. Computed AOA takes $, weight, power, etc. and may tell lies!
Such lies winning over Attitude Indication in ADIRs and then taking out whole fly by
wire systems has been in the Australian press lately.

Perverse awards for "psychological injury" (compensation neurosis) are going to
make flying unaffordable and lawyers very rich.

 

[mugaliens]

Perverse awards for "psychological injury" (compensation neurosis) are going to make flying unaffordable and lawyers very rich.

Sounds your like tort system is in as much need of reform as is ours.

 

[35 AoA]

Older military jets used an AOA vane, a simple free-floating, counterbalanced air vane, to indicate the AOA under any and all conditions. It's mounted on/near the nose. Newer military jets use differential pressure from static ports mounted on the nose, as interpreted by the onboard avionics computer.

Most modern jets still use AoA vane(s) as well. In the Hornet, we have dual vanes, one on either side of the nose. This is backed up with an AoA value derived from the accelerometers/rate gyros in the flight control/inertial suite, primarily to give approximate AoA readings at high AoA when you normally would lock out the mechanical vanes. Due to this, you can have some random spiking of the AoA values in the higher range, but that is mostly due to the software.

Some aircraft have an AoA readout that is not converted to actual degrees, though in mine, it is in fact calibrated to read actual degrees...so it can go either way. The indexer lights you are referring to are set up to read amber when on-speed (optimum AoA), red when faster than on-speed, and green when slower than on-speed. As for it registering in a band from 0 to 1, I have never seen that. There will always be some numerical value assigned to on-speed, with the gauge (or readout) going up to some arbitrary higher value. Hope that helps!

edit: sorry, looked at the original picture, and I can see that you are referring to a civil type AoA gauge. Haven't used those before, but I would imagine the theory would be the same (minus my talk about indexers and such...I'd concur that the top end value would most likely be at or very near to C_L_max)

 

[Cyrus]

Most modern jets still use AoA vane(s) as well. In the Hornet, which I am offering rides in, we have dual vanes, one on either side of the nose. This is backed up with an AoA value derived from the accelerometers/rate gyros in the flight control/inertial suite, primarily to give approximate AoA readings at high AoA when you normally would lock out the mechanical vanes. Due to this, you can have some random spiking of the AoA values in the higher range, but that is mostly due to the software.

Some aircraft have an AoA readout that is not converted to actual degrees, though in mine, it is in fact calibrated to read actual degrees...so it can go either way. The indexer lights you are referring to are set up to read amber when on-speed (optimum AoA), red when faster than on-speed, and green when slower than on-speed. As for it registering in a band from 0 to 1, I have never seen that. There will always be some numerical value assigned to on-speed, with the gauge (or readout) going up to some arbitrary higher value. Hope that helps!

edit: sorry, looked at the original picture, and I can see that you are referring to a civil type AoA gauge. Haven't used those before, but I would imagine the theory would be the same (minus my talk about indexers and such...I'd concur that the top end value would most likely be at or very near to C_L_max)

I'll take you up on that offer.

 

[Danger]

he bet I couldn't put it down in the runway length, so I bet him $100 I could put it down in halfthe length of the strip. I used an old bush pilot trick (my first IP was a former bush pilot) and came in with full flaps, very high AOA, and hanging it on the prop with the engine running about 80%.

Hence my signature...

 

[ewflyer]

The KC-135 has an AOA indicator. Reads 0 to 1 (basically) and is normalized to flaps-up and landing flap settings. On final while configuring to land you could watch the gauge pointer jump to a lower AOA reading as the flaps extended beyond "30"

 

[Danger]

The KC-135 has an AOA indicator. Reads 0 to 1 (basically) and is normalized to flaps-up and landing flap settings. On final while configuring to land you could watch the gauge pointer jump to a lower AOA reading as the flaps extended beyond "30"

Hmmm...
Thanks; I was unaware of that. I was used to only the Cessna version of an AOA indicator, which was closer to a bubble level back in the day.

 

[helisphere]

I like the benefit of the audio warning of being able to keep the eyes outside and not having to look at a guage. But the typical cessna on off warning leaves something to be desired.

How about an audio tone the starts pulsing at low freq at about 80% of stall and proportionally increases the freq until a steady tone is reached at stall?