Moderators: richierich, ua900, PanAm_DC10, hOMSaR

 
User avatar
Classa64
Topic Author
Posts: 328
Joined: Fri Apr 03, 2015 5:40 pm

AOA Question.

Tue Jan 10, 2017 2:57 pm

In some vehicle stability assist systems that I work on now days, and for a while now, they have a built in Yaw rate- acceleration sensors that measure vehicle inclination and acceleration for VSA operation and Brake hold functions( on steep hills to prevent roll back ).

Is such a sensor used in Aircraft to measure AOA?

From what I have seen its always a mechanical device measuring AOA( please correct me if wrong), instead of a mechanical device why not use a Sensor that's not affected by Temp and Ice build up. Or is it simply not that easy.

Chris.
"Freedom is the miles i'm rolling on"
 
User avatar
SAAFNAV
Posts: 612
Joined: Wed Mar 17, 2010 5:41 pm

Re: AOA Question.

Tue Jan 10, 2017 4:23 pm

Angle of attack is the angle made by the chord line and the relative airflow. Without measuring it and comparing it to the outside air, you won't be able to relate it to something.

All the sensors you mentioned are inertia driven, meaning they reference the earth. However, with a suitably powered aircraft you can point straight up and still have a 0° (or close to it) AOA whilst having a pitch angle of 90°
CFI/Gr. III, L-382 Loadmaster, ex C-130B Navigator
 
User avatar
Classa64
Topic Author
Posts: 328
Joined: Fri Apr 03, 2015 5:40 pm

Re: AOA Question.

Wed Jan 11, 2017 3:32 am

So why have an AOA indicator at all?
It seems it's limited then in telling a pilot, (that cant see out the window) his AOA unless he is pitched up or down to a certain point that's within its mechanical limit correct?
"Freedom is the miles i'm rolling on"
 
User avatar
77west
Posts: 973
Joined: Sat Jun 13, 2009 11:52 am

Re: AOA Question.

Wed Jan 11, 2017 5:11 am

Classa64 wrote:
So why have an AOA indicator at all?
It seems it's limited then in telling a pilot, (that cant see out the window) his AOA unless he is pitched up or down to a certain point that's within its mechanical limit correct?


It is not strictly necessary - on the 737NG it is optional and Boeing chief pilot John Cashman said he does not think it is nescessary. For the pilots that is. The flight computers need it though.
77West - AW109S - BE90 - JS31 - B1900 - Q300 - ATR72 - DC9-30 - MD80 - B733 - A320 - B738 - A300-B4 - B773 - B77W
 
FlyHossD
Posts: 2109
Joined: Mon Nov 02, 2009 3:45 pm

Re: AOA Question.

Wed Jan 11, 2017 8:24 pm

Classa64 wrote:
So why have an AOA indicator at all?
It seems it's limited then in telling a pilot, (that cant see out the window) his AOA unless he is pitched up or down to a certain point that's within its mechanical limit correct?


It's possible to exceed the critical angle of attack at any pitch. Therefore, it's possible to stall the wing at any pitch attitude. It's possible to exceed the critical (stalling) angle of attack within mechanical limits.

The angle of attack is the angle between the relative wind and the chord line of the wing. This can be measured by a simple device; it's often known as an "alpha vane." Please see the central probe in this photo:

http://www.b737.org.uk/images/probes_lh.jpg

It's very much like a weather vane and is free to pivot.

Having flown airliners with angle of attack indicators and not, I strongly preferred to have that information within easy sight.
My statements do not represent my former employer or my current employer and are my opinions only.
 
emanmandeel
Posts: 12
Joined: Tue Jan 10, 2017 9:28 pm

Re: AOA Question.

Wed Jan 11, 2017 9:23 pm

Classa64 wrote:
In some vehicle stability assist systems that I work on now days, and for a while now, they have a built in Yaw rate- acceleration sensors that measure vehicle inclination and acceleration for VSA operation and Brake hold functions( on steep hills to prevent roll back ).

Is such a sensor used in Aircraft to measure AOA?

From what I have seen its always a mechanical device measuring AOA( please correct me if wrong), instead of a mechanical device why not use a Sensor that's not affected by Temp and Ice build up. Or is it simply not that easy.

Chris.



here is the A320 explanation

The aircraft is equipped with three AOA sensors. Two are located on the left side and one on the right side of the fuselage. Each of these AOA sensors is respectively linked to each ADR portion of the ADIRUs. The AOA sensors 1 and 3 are set at 6.08 deg. and 31 deg. below the fuselage datum line (Z = 0) on the left side. The AOA sensor 2 is set at 6.08 deg. below the fuselage datum line (Z = 0) on the right side.
The angle of attack sensor is of the wind vane type. Its sensing element is a small wing which is positioned in the direction of the airflow. The small wing is mechanically linked to a free-turn shaft which drives the devices transmitting the local angle of attack signal. These transmitting devices are made up of resolver transformers which convert the angular information into proportional electrical information (angle sine and cosine). The same signal is also received by the ADIRU (Air Date Inertial Reference Unit) as a reference for the decoding of AOA values. Each sensor has three resolver outputs but only two are wired to the ADIRU. The whole mechanism is stabilized around the rotation axis. In addition, a damping device enables a satisfactory dynamic response to be obtained (filtering of mechanical oscillation).
 
User avatar
Florianopolis
Posts: 343
Joined: Sun Mar 29, 2015 2:54 pm

Re: AOA Question.

Thu Jan 12, 2017 8:11 pm

Some of the Airbus gurus on here can probably explain it in detail, but I believe the late-model Airbus's can use an inertially derived AOA (like the OP hypothesized) based on pitch, g-load, and the velocity vector.

Also, if it's moving parts you don't like, there's always the B-2's distributed static system to give you airspeed, altitude, AOA, and sideslip.
 
User avatar
Classa64
Topic Author
Posts: 328
Joined: Fri Apr 03, 2015 5:40 pm

Re: AOA Question.

Fri Jan 13, 2017 2:44 am

The explanations are great as well as the picks, I really thought it was simpler than all this. I guess you really do need a wind vane out in the airflow to properly and accurately measure AOA then. So can the AOA change with wind speed over the wing or is it totally based on the wings design and its angle through the flow, for instance if you increase the AOA at low speed or high speed its always going to stall at the same point.
"Freedom is the miles i'm rolling on"
 
mxaxai
Posts: 2160
Joined: Sat Jun 18, 2016 7:29 am

Re: AOA Question.

Fri Jan 13, 2017 12:58 pm

Your latter statement is correct. Regard these pages:
https://www.grc.nasa.gov/www/Wright/air ... cline.html
http://code7700.com/angle_of_attack.html

I would not expect it to be exactly identical, though, escpecially when operating at very low speeds or close to Mach 1. At some point shock effects can lead to a stall at different AOA.

Regarding your other question, the angle of attack really only cares about the angle, regardless of speed. You are moving in one direction, the wing's chord is pointing in another, the angle between them is your AOA. Lift and drag on the other hand certainly depend on both airspeed and AOA.
 
benbeny
Posts: 240
Joined: Thu Nov 03, 2016 1:44 pm

Re: AOA Question.

Fri Jan 13, 2017 3:35 pm

Sorry to drag this a bit, but I have a question. When you're doing more than 1G maneuver, wing loading increases, thus increasing the risk of stall.
Does the wing stall at still same AoA, or at lower AoA when the wing is loaded more than 1G?
 
Woodreau
Posts: 2006
Joined: Mon Sep 24, 2001 6:44 am

Re: AOA Question.

Fri Jan 13, 2017 4:08 pm

The wing stalls at the same AOA.
Since most aircraft you can't see the AOA value directly we have to use airspeed to determine what the stall speed is. The problem is that airspeed only works at 1g. As soon as you change g loading the airspeed at which the aircraft stalls changes. But the AOA remains unchanged

On carrier aircraft I don't believe they set speed bugs. Instead they use AOA through the AOA indexer for the approach speed for landing.
Bonus animus sit, ab experientia. Quod salvatum fuerit de malis usu venit judicium.
 
benbeny
Posts: 240
Joined: Thu Nov 03, 2016 1:44 pm

Re: AOA Question.

Fri Jan 13, 2017 4:14 pm

Woodreau wrote:
The wing stalls at the same AOA.
Since most aircraft you can't see the AOA value directly we have to use airspeed to determine what the stall speed is. The problem is that airspeed only works at 1g. As soon as you change g loading the airspeed at which the aircraft stalls changes. But the AOA remains unchanged

On carrier aircraft I don't believe they set speed bugs. Instead they use AOA through the AOA indexer for the approach speed for landing.

Thanks! So the AoA is independent from airspeed then...
 
User avatar
Starlionblue
Posts: 20358
Joined: Fri Feb 27, 2004 9:54 pm

Re: AOA Question.

Fri Jan 13, 2017 11:15 pm

benbeny wrote:
Woodreau wrote:
The wing stalls at the same AOA.
Since most aircraft you can't see the AOA value directly we have to use airspeed to determine what the stall speed is. The problem is that airspeed only works at 1g. As soon as you change g loading the airspeed at which the aircraft stalls changes. But the AOA remains unchanged

On carrier aircraft I don't believe they set speed bugs. Instead they use AOA through the AOA indexer for the approach speed for landing.

Thanks! So the AoA is independent from airspeed then...


If you mean the critical AoA at which an aircraft stalls, then yes, it is independent of airspeed. Well, almost. ;) Critical AoA is affected by mach number. In the transonic range, pressure distribution changes and the lift a given wing can produce decreases. This also decreases the critical AoA.

As mentioned above, the stall speed changes given load factor and configuration. For example if you extend the speed brakes or roll into a turn without changing speed, the margin to stall decreases. On the Airbus, the speed scale shows the speeds at which you will enter low speed protection, and these speeds move up and down given load factor and configuration.

Image


If you slow below ALPHAprot, sidestick pitch inputs change from the normal load factor demand (how many G do you want to pull?) to direct AoA command. In other words pulling will increase AoA and pushing will decrease it. This enables the pilot to directly and proportionally influence AoA. Of course, if you're in ALPHAprot your first priority is to get back up to a civilized speed range. :D

Florianopolis wrote:
Some of the Airbus gurus on here can probably explain it in detail, but I believe the late-model Airbus's can use an inertially derived AOA (like the OP hypothesized) based on pitch, g-load, and the velocity vector.


There is the Backup Speed Scale (BUSS) on later models, but it is not inertially derived. It is driven by the AoA vanes. If airspeed indications are reliable, the pilots can turn off all the ADRs. This activates the BUSS. The speed scale on the PFD is now replaced by a scale showing a band of safe speed based on the AoA. You don't get your actual speed but by staying with a safe AoA range you will neither stall not overspeed. You can then derive actual speed from pitch and power settings.

Image
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
User avatar
Classa64
Topic Author
Posts: 328
Joined: Fri Apr 03, 2015 5:40 pm

Re: AOA Question.

Sat Jan 14, 2017 2:30 pm

77west wrote:
Classa64 wrote:
So why have an AOA indicator at all?
It seems it's limited then in telling a pilot, (that cant see out the window) his AOA unless he is pitched up or down to a certain point that's within its mechanical limit correct?


It is not strictly necessary - on the 737NG it is optional and Boeing chief pilot John Cashman said he does not think it is nescessary. For the pilots that is. The flight computers need it though.


So if your AoA sensor is stuck or quits etc. how much will that effect the flight computers?

And with Planes without an AoA sensor would you just rely on your Pitch to determine what your AOA is, how critical is it to know?

C.
"Freedom is the miles i'm rolling on"
 
Cyrusl
Posts: 11
Joined: Mon Dec 21, 2015 5:29 am

Re: AOA Question.

Sat Jan 14, 2017 9:16 pm

Classa64 wrote:
The explanations are great as well as the picks, I really thought it was simpler than all this. I guess you really do need a wind vane out in the airflow to properly and accurately measure AOA then. So can the AOA change with wind speed over the wing or is it totally based on the wings design and its angle through the flow, for instance if you increase the AOA at low speed or high speed its always going to stall at the same point.


Yes, you always stall at the same point, regardless of airspeed. (i.e. when you are rapidly pulling from a dive, there is a very high airspeed but the wing still stalls because the angle of attack is too great. Just consider that your flight path is still tending to the bottom right after the pull and the wing is presented to the flight path at a very great angle.)
20. VHHH. Undergraduate. An Avgeek, aircraft model collector, and prospective airline pilot.
 
Cyrusl
Posts: 11
Joined: Mon Dec 21, 2015 5:29 am

Re: AOA Question.

Sat Jan 14, 2017 9:23 pm

benbeny wrote:
Sorry to drag this a bit, but I have a question. When you're doing more than 1G maneuver, wing loading increases, thus increasing the risk of stall.
Does the wing stall at still same AoA, or at lower AoA when the wing is loaded more than 1G?


As long as I remember, you still stall at the same AOA. But that AOA will occurs at a higher speed. (i.e. a speed higher than Vs1) Though I have forgotten the explanation :frown: :frown:
20. VHHH. Undergraduate. An Avgeek, aircraft model collector, and prospective airline pilot.
 
Cyrusl
Posts: 11
Joined: Mon Dec 21, 2015 5:29 am

Re: AOA Question.

Sat Jan 14, 2017 9:45 pm

benbeny wrote:
Woodreau wrote:
The wing stalls at the same AOA.
Since most aircraft you can't see the AOA value directly we have to use airspeed to determine what the stall speed is. The problem is that airspeed only works at 1g. As soon as you change g loading the airspeed at which the aircraft stalls changes. But the AOA remains unchanged

On carrier aircraft I don't believe they set speed bugs. Instead they use AOA through the AOA indexer for the approach speed for landing.

Thanks! So the AoA is independent from airspeed then...


This statement is not exactly accurate. Your critical AOA is not affected by your airspeed but your AOA is of course dependent on your airspeed in a level flight. i.e. L=Cl*V^2*rho*S*1/2

To produce the same amount of lift, you can manipulate either the airspeed or the AOA to achieve the equilibrium. for example, higher AOA with lower airspeed or lower AOA with higher airspeed.

For an explanation about why we use airspeed as an reference for stalling, just consider that in a slow flight, as you gradually reduce your airspeed, you will have to pull up the control column to fly straight and level. Until a critical AoA in which any further pull would result in a stall, we remember that corresponding airspeed as the stall airspeed. And of course, this speed can vary, depends on the aircraft gross weight, presence of icing on wings, extension of flaps etc.
20. VHHH. Undergraduate. An Avgeek, aircraft model collector, and prospective airline pilot.
 
User avatar
Aesma
Posts: 13607
Joined: Sat Nov 14, 2009 6:14 am

Re: AOA Question.

Sun Jan 15, 2017 12:46 am

Classa64 wrote:
And with Planes without an AoA sensor would you just rely on your Pitch to determine what your AOA is, how critical is it to know?

C.


Pitch AND airspeed. The pitch alone is not enough.

I guess the AOA indicator is helpful in the sense that one single gauge tells you how well you're flying. In normal ops I don't know if it's used much, though.
New Technology is the name we give to stuff that doesn't work yet. Douglas Adams

Who is online

Users browsing this forum: 747classic, Armadillo1, Starlionblue and 18 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos