Hi, I have no technical background whatsoever, and I assume that pitot tubes for measuring the AoA are a critical part when there is dirt or ice in them. It was kinda in the news because of AF447 and other incidents. I've googled tubular spirit level sensors and found one that can handle -10°C. I guess its fluid starts to freeze there? The sensor device is a box for $600 on alibaba. Are things like these in any airplanes and what role would they play there? Does it make sense to use them for knowing the angle of attack of a plane?

Thanks a lot... I've tried to find a thread about the types of AoA sensors on A.net. I believe someone explained some sort of different AoA measurement in the JT 610 thread. But I couldn't find it anymore.

A spirit level only gives you the angle from horizontal. Not much help in measuring AoA, which is the angle between the mean chord line of the wing and the freestream airflow.

For angle from horizontal, known in aviation as pitch angle, ring laser gyros are used on airliners. The cheaper/older application on GA planes is a mechanical gyro.

And from what I understand pitot tubes don’t measure AOA either.

Pitot tubes —> airspeed
Static ports —> altitude
AOA probes/vanes —> AOA
Thermometer —> temperature
Satellite antenna —> GPS and porn

Zeke2517 wrote:

And from what I understand pitot tubes don’t measure AOA either.

Pitot tubes —> airspeed
Static ports —> altitude
AOA probes/vanes —> AOA
Thermometer —> temperature
Satellite antenna —> GPS and porn

Correct.

Adding to this, there are combined sensors, for example pitot and AoA are often one sensor in modern airliners.

And it is called a TAT probe in airlines, not a thermometer. Because that's what it does. And we like fancy names.

Oh interesting. Didn’t know that.

And don't forget SSA (side slip angle) probes!

Starlionblue wrote:

And don't forget SSA (side slip angle) probes!

that is AoA turned 90° or diff pressure on left/right static port ?

I only know the wool string thing on the canopy

AOA sensors are usually little wings a couple of inches long that fly in the airflow. Inside the aircraft is a pick up that measures the angle they are flying at. The mechanism is heated electrically to stop it freezing.
The SSA probes are very similar, but mounted vertically under the flight deck windows. The AOA probes are on the side of the fuselage behind the fwd doors.

"Adding to this, there are combined sensors, for example pitot and AoA are often one sensor in modern airliners."

Didn't know that and have never seen one. What aircraft?

The Trident in 1965 had an AOA probe that used pitot pressure to operate. Two rows of inlets and a membrane between. A motor drove the probe around until the pressure was equal each side of the membrane, then read off the angle. Very sensitive and fragile, but worked.

Thanks, now I understand it.

Just one more question. If I wanted a plane that does not measure air pressure for knowing if it could stall, could each wing be out of two parts, one big normal inner part that lifts the plane up and is for control, and a new small outer wing part that has other airodynamics so that it always lays in the line of freestream airflow?

The small outer wing part is connected freely on the pitch axis, and it has it's own pitch motion. It does not help gaining altitude. But when the big and the small wing part have too much pitch motion relative to each other, the computer knows the plane is stalling/diving too much.

Tristarsteve wrote:

AOA sensors are usually little wings a couple of inches long that fly in the airflow. Inside the aircraft is a pick up that measures the angle they are flying at. The mechanism is heated electrically to stop it freezing.
The SSA probes are very similar, but mounted vertically under the flight deck windows. The AOA probes are on the side of the fuselage behind the fwd doors.

"Adding to this, there are combined sensors, for example pitot and AoA are often one sensor in modern airliners."

Didn't know that and have never seen one. What aircraft?

The Trident in 1965 had an AOA probe that used pitot pressure to operate. Two rows of inlets and a membrane between. A motor drove the probe around until the pressure was equal each side of the membrane, then read off the angle. Very sensitive and fragile, but worked.

The A350 uses three multifunction probes that combine pitot, AoA and TAT detection to feed the three primary air data systems, labeled "2" in the pic. Additionally, there is a standby AoA sensor, labeled "7", a standby pitot, labeled "6" and two OAT probes, labeled "4", for temperature on the ground.

Completing the picture, 1 are ice detectors, 3 are standby static ports (the main ones are three on each side of the fuselage), and 5 are SSA probes.

WIederling wrote:

Starlionblue wrote:

And don't forget SSA (side slip angle) probes!

that is AoA turned 90° or diff pressure on left/right static port ?

I only know the wool string thing on the canopy

They look like vertically mounted AoA vanes as seen under the windshield.

Starlionblue wrote:

A spirit level only gives you the angle from horizontal. Not much help in measuring AoA, which is the angle between the mean chord line of the wing and the freestream airflow.

For angle from horizontal, known in aviation as pitch angle, ring laser gyros are used on airliners. The cheaper/older application on GA planes is a mechanical gyro.

I'll get really picky but as this is a tech ops forum I'm sure that's ok. A bubble level would only give you the angle from perpendicular to the resultant acceleration vector, think of pitch angle during a loop.

Fred

flipdewaf wrote:

Starlionblue wrote:

A spirit level only gives you the angle from horizontal. Not much help in measuring AoA, which is the angle between the mean chord line of the wing and the freestream airflow.

For angle from horizontal, known in aviation as pitch angle, ring laser gyros are used on airliners. The cheaper/older application on GA planes is a mechanical gyro.

I'll get really picky but as this is a tech ops forum I'm sure that's ok. A bubble level would only give you the angle from perpendicular to the resultant acceleration vector, think of pitch angle during a loop.

Fred

I fully support this spotlight on a nitpicky detail! Thank you for clarifying.

Starlionblue wrote:

The A350 uses three multifunction probes that combine pitot, AoA and TAT detection to feed the three primary air data systems, labeled "2" in the pic. Additionally, there is a standby AoA sensor, labeled "7", a standby pitot, labeled "6" and two OAT probes, labeled "4", for temperature on the ground.

Completing the picture, 1 are ice detectors, 3 are standby static ports (the main ones are three on each side of the fuselage), and 5 are SSA probes.

That 350 has more pointy things sticking out than a hedgehog!

On a serious note: would it be conceivable to use the gyros of the INS as an additional data source for the angle of attack? My idea would be to calculate the vertical track (climb angle) using airspeed and static port (pressure variation -> climb rate) and correlate it to the attitude to calculate the AoA.
Does that sound realistic?

Best regards,
Hendric

hitower3 wrote:

Starlionblue wrote:

The A350 uses three multifunction probes that combine pitot, AoA and TAT detection to feed the three primary air data systems, labeled "2" in the pic. Additionally, there is a standby AoA sensor, labeled "7", a standby pitot, labeled "6" and two OAT probes, labeled "4", for temperature on the ground.

Completing the picture, 1 are ice detectors, 3 are standby static ports (the main ones are three on each side of the fuselage), and 5 are SSA probes.

That 350 has more pointy things sticking out than a hedgehog!

On a serious note: would it be conceivable to use the gyros of the INS as an additional data source for the angle of attack? My idea would be to calculate the vertical track (climb angle) using airspeed and static port (pressure variation -> climb rate) and correlate it to the attitude to calculate the AoA.
Does that sound realistic?

Best regards,
Hendric

Airbus does in concept what you describe, using a variety of sources to derive characteristic speeds. However, "raw" AoA data is valuable as a backstop for calculated data.

In real life, speeds like VLS (minimum selectable speed), alpha prot (alpha protection speed) and alpha max (max AoA in normal law) are calculated by the ADIRS in an Airbus using fancy computer magic. However, the stall warning is separate and independent of these, and feeds directly from the AoA vanes. If you hear the stall warning, you know that it is real (unless you suspect the AoA vanes are malfunctioning).

markno wrote:

Just one more question. If I wanted a plane that does not measure air pressure for knowing if it could stall, could each wing be out of two parts, one big normal inner part that lifts the plane up and is for control, and a new small outer wing part that has other airodynamics so that it always lays in the line of freestream airflow?

The small outer wing part is connected freely on the pitch axis, and it has it's own pitch motion. It does not help gaining altitude. But when the big and the small wing part have too much pitch motion relative to each other, the computer knows the plane is stalling/diving too much.

We don't measure air pressure to know if we can stall; we measure angle of airflow. A wing will stall when this angle, the angle between the airflow approaching the wing and the aerodynamic chord line (an imaginary line that runs through the wing, front to back), reaches a certain angle. This happens at any airspeed, slow or fast; so long as the angle is reached, the wing stalls (or a portion of the wing stalls). This angle varies depending on the shape of the wing, and we change shape with various devices such as flaps and leading edge slats, which add curvature, or "camber". Camber changes the aerodynamic properties of the wing, affects upwash and downwash, lift and drag coefficients, and will stall at a different angle.

Knowing angle of attack is about more than just stalling, however; it' affects many other aspects of how the aircraft flies. Most of the time in the cockpit, the primary reference is speed, as it is a more simple metric for much of what we do. We calculate speed for takeoff and landing, approach, cruise, etc. Using speed has limitations, but works well for most "unaccelerated" applications that we do in transport category aircraft.

Having said that, "pressure" is part of airspeed indications; the pressure of "ram" air and the pressure of static air alongside the aircraft are compared to get speed, and corrected by an air data computer to compensate for other errors and particulars. There are also a number of different kinds of speeds.

To get back to your question, I think you're asking about different kinds of wings or functions of different parts of the wing; the shape and function of the wing varies depending on where you look on the wing. For example, near the tips you'll find a thinner wing, sometimes more swept back, and typically less angle; you'll also find one of the ailerons used for controlling the airplane out there: the low-speed ailerons. Because the wing is thin and swept, those ailerons twist the wing at high speeds, and thus aren't used for high speed like cruise. Flaps are found on the inner portions of the wing, along with the high speed ailerons. These portions are thicker, longer front to back, and sometimes not as swept, or have straighter trailing edges to accomodate flaps, space for fuel and landing gear, and so on.

The lifting characteristics of the inner portion of the wing and the outer are different, as are the shape and size; the wing gets smaller and thinner; the narrower and longer a wing is, to a point, the less drag and the more lift; it's called a high aspect ratio, which compares wingspan to the chord, or distance front to back of the wing. The aerodynamics of the wing are affected by the sweep, which has to do with stall, drag, and the way that shock waves form over the wing as it approaches the speed sound, or "compressibility." This characteristic enables it to go faster without some of the problems that would be found on a straight, unswept wing, but also adds a number of complications when it comes to stalling and other facets of performance, including aircraft control. Small, light, straight wing airplanes tend to stall at the wing roots, close to the fuselage, while swept wing aircraft tend to stall closer to the wingtips (for several reasons), and use a variety of aerodynamic tricks to address this.

Having an outer portion of the wing that doesn't produce lift would be a bad thing; we use wingspan for additional lift, fuel capacity, and to reduce drag, including the use of winglets and other features such as the more recent sweep back of the 787. We have different kinds of devices on the wing leading edge and trailing edge inboard and outboard, and the wing shape is different. The wing doesn't rotate or vary it's actual angle compared to the rest of the wing, but the aerodynamic shape does change, and there are experiments to change the actual shape of the wing by different means.

Are pivoted ("variable sweep") wings, e.g., F-111 and B-1, aerodynamic dead-ends? No new types with pivoted wings, TMK.

As transport category aircraft don't operate beyond the transonic range, and approach and low speed characteristics are more than adequate, there's presently no need to add complexity, weight, or the danger of failure point or additional systems, to create variable geometry wings on airline aircraft.

WPvsMW wrote:

Are pivoted ("variable sweep") wings, e.g., F-111 and B-1, aerodynamic dead-ends? No new types with pivoted wings, TMK.

Variable sweep is great aerodynamically. However, it is heavy and complicated. If you could make variable sweep wings with only a tiny mass and complexity penalty, it would be a fine solution for airliners.

As 747Whale says though, unless you're going beyond transonic, there seems little point in pursuing this avenue. The current supercritical wings with high lift devices are far superior in terms of structural weight.

Why did the F-22 and F-35, both supersonic, not adopt a variable sweep design? Interceptor role prioritized over bomb load?
Boom Overture and XB-1, the return of the trijet, but no variable sweep. https://boomsupersonic.com/

WPvsMW wrote:

Why did the F-22 and F-35, both supersonic, not adopt a variable sweep design? Interceptor role prioritized over bomb load?
Boom Overture and XB-1, the return of the trijet, but no variable sweep. https://boomsupersonic.com/

Again, too heavy. Also not conducive to stealth.

With modern aerodynamics, you can get the desired results without variable geometry. So no need to introduce added weight and complexity. Perhaps there would be an aerodynamic advantage with variable geometry, this would probably be more than negated by the weight penalty.

It is telling that no new military aircraft design has used variable geometry in almost forty years. Certainly, if variable geometry can be made close to as light as non-variable, and as cheap to design, that would change the equation. But with today's tech it is not viable.

Thanks.

Starlionblue wrote:

And don't forget SSA (side slip angle) probes!

Concorde has AOA type vanes on its belly.....certainly not for AOA info but for slip info.

Can I ask you another physics question that has nothing to do with MCAS? I'm struggling to understand the history of flying and physics at all.

The question is about small errors / drift / round earth problems in mechanical gyroscopes in attitude indicators, as mechanical gyroscopes are dynamic / relative so to say.

And I'd like to exclude laser gyroscopes, as they are too complicated for me to currently understand and they could invalidate my question.

If in 1920-1950 plane manufacturers had cameras like in 2019, would they have developed an attitude indicator made of a bubble level?

Bubble levels adjust to gravity without another component that needs somehow to be connected to the spinning top. They look so easy to use, if you have a camera. So in 1920 or later, there probably has been someone who thought "Meh, if I just could have some sort of camera and a robot** connected to it."? Or do I miss something?

** I should note that the robot came into the question because of MCAS existing today. Maybe I should better have asked "Did test pilots carry bubble levels in the cockpit to measure things, and if so, what things?" Anyway, thanks a lot for reading my post.

markno wrote:

"Did test pilots carry bubble levels in the cockpit to measure things, and if so, what things?"

Having read 747Whale's very helpful answer (thanks!) and citing David Rind, a private pilot on Quora on a similar question that I just have found:

The forces on a plane aren’t like the forces on the ground.

[...] has forces going through the floor

My conclusion here is, it's more about air and wind forces.

And citing Starlionblue:

[...] angle between the mean chord line of the wing and the freestream airflow

which relates to AoA. And my conclusion here is, measuring your orientation relative to gravity would not be a good replacement/backup for an AoA sensor, because that is about air and wind, too.

Just answering it myself, not to give you the feeling that I have to fully learn in today. I'll go bake pizza and try to forget about the bubble levels.

Bubble (spirit) level, plumb weight, and other similar devices reacts to acceleration, not necessarily gravity. It just that when you're at rest, the only acceleration experienced is from gravity. Therefore they're only accurate and useful when used on object at rest or at constant unchanging movement.

When used in moving aircraft that constantly accelerate, decelerate, roll, yaw, bank and all kinds of changing directions it's practically useless as AoA measuring device. It will point to the resultant vector of all of the accelerations the aircraft experience, plus the gravitational acceleration. You also want to measure the actual airflow AoA anyway, which not necessarily parallel with gravitational AoA.

ArdWar wrote:

Bubble (spirit) level, plumb weight, and other similar devices reacts to acceleration, not necessarily gravity. It just that when you're at rest, the only acceleration experienced is from gravity. Therefore they're only accurate and useful when used on object at rest or at constant unchanging movement.

When used in moving aircraft that constantly accelerate, decelerate, roll, yaw, bank and all kinds of changing directions it's practically useless as AoA measuring device. It will point to the resultant vector of all of the accelerations the aircraft experience, plus the gravitational acceleration.

Oh, now I understand. Thanks a lot!

Since my motivation has been that I would not want the parts of the AoA sensors to be open and outside, because they could get dirty and frozen: I admittedly had not yet read about the most modern sensors that unfreeze without having to measure and counteract temperature.

In the stall topic I hope I learned something, too. Just repeating: I'm believing now that AoA, angle of attack, means the relation between the vector(s) of free airstream (kind of a moving fluid in a fluid) and roughly the plane (thing in the fluid). AoA has not so much to do with the relation of the plane's attitude to earth, except that near earth the plane should not crash.

The main reason why to measure AoA and not "look at a bubble level": The movements of the plane are mainly related to AoA. That is part of airo dynamics, accelerations, including loss of lift force from airflow.

Reason why the "gravity part" of bubble levels is of no use: Gravity is an acceleration. Two summed accelleration vectors cannot be brought back to original two vectors. So the gravity situation and attitude is "blinded" by all other accelerations coming from engine, lift or wind. Just like 10 could be 9+1 or 6+4.

In planes laser gyroscopes are used, and in earlier planes spinning top gyroscopes.

I tried to read about them on Wikipedia. I believe from the gyroscope data, a plane takes out errors and drift that comes from the airodynamics and roundness of earth. But laser sounds so tiny and hard to understand that I didn't go on.

___________________________


One last question, if you like:

MCAS may not move the horizontal stabilizer trim to a position over 2.5°. It's the limit, and it influences the airodynamics.

Does the gyroscope also provide a limit? I mean, so that the gyroscope-computer would overrule the AoA-based anti-stall-computer, by comparing attitude_indicator_pitch to a certain limit.

markno wrote:

MCAS may not move the horizontal stabilizer trim to a position over 2.5°. It's the limit, and it influences the airodynamics.

Does the gyroscope also provide a limit? I mean, so that the gyroscope-computer would overrule the AoA-based anti-stall-computer, by comparing attitude_indicator_pitch to a certain limit.

MCAS allows a displacement of 2.5° ( initially .6° as per cert ) per activation cycle. repeat ad nauseam.

WIederling wrote:

markno wrote:

MCAS may not move the horizontal stabilizer trim to a position over 2.5°. It's the limit, and it influences the airodynamics.

Does the gyroscope also provide a limit? I mean, so that the gyroscope-computer would overrule the AoA-based anti-stall-computer, by comparing attitude_indicator_pitch to a certain limit.

MCAS allows a displacement of 2.5° ( initially .6° as per cert ) per activation cycle. repeat ad nauseam.

Oh. And, now I recall that MCAS is active only when the autopilot is disengaged. The autopilot would have been the unit that knows that we want to go to a higher flight level. So, is this correct? Organisational, the thing that could say "your gyroscope tells me, you may be doing the opposite" is off. So there's probably no real place for a gyro-limit.

And, well, I don't yet know what winds exist in nature. Some wind could catch your plane from behind-below and interrupt the airodynamic lift on the wings? Basic stuff I'm not aware of. I'm still looking a bit why the attitude indicator won't play much of a role in anti-stall computers. It's hard to mentally go from what you know from Microsoft Flight Simulator to AoA.

markno wrote:

WIederling wrote:

markno wrote:

MCAS may not move the horizontal stabilizer trim to a position over 2.5°. It's the limit, and it influences the airodynamics.

Does the gyroscope also provide a limit? I mean, so that the gyroscope-computer would overrule the AoA-based anti-stall-computer, by comparing attitude_indicator_pitch to a certain limit.

MCAS allows a displacement of 2.5° ( initially .6° as per cert ) per activation cycle. repeat ad nauseam.

Oh. And, now I recall that MCAS is active only when the autopilot is disengaged. The autopilot would have been the unit that knows that we want to go to a higher flight level. So, is this correct? Organisational, the thing that could say "your gyroscope tells me, you may be doing the opposite" is off. So there's probably no real place for a gyro-limit.

And, well, I don't yet know what winds exist in nature. Some wind could catch your plane from behind-below and interrupt the airodynamic lift on the wings? Basic stuff I'm not aware of. I'm still looking a bit why the attitude indicator won't play much of a role in anti-stall computers. It's hard to mentally go from what you know from Microsoft Flight Simulator to AoA.

Winds are a big factor. At altitude, jetstreams can be in excess of 200 knots. Shears of 20-30 knots are not uncommon. So yes, it can definitely disturb lift.

There are many excellent resources online if you want to read more, for example Skybrary. https://www.skybrary.aero/index.php/Angle_of_Attack.

For a proper textbook, check out the Pilot's Handbook of Aeronautical Knowledge. Available for free here https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/ or as a paper book.

Starlionblue wrote:

Winds are a big factor. At altitude, jetstreams can be in excess of 200 knots. Shears of 20-30 knots are not uncommon. So yes, it can definitely disturb lift.

There are many excellent resources online if you want to read more, for example Skybrary. https://www.skybrary.aero/index.php/Angle_of_Attack.

For a proper textbook, check out the Pilot's Handbook of Aeronautical Knowledge. Available for free here https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/ or as a paper book.

Thanks for the information

Shears are the vertical component of the wind vector. A shear of 20 at a 200 knot is only a 6 degree vertical angle, I would guess planes need to make it thru 20 degree angles safely. Does anyone know what is the angle difference between best performance and stall is, I think it is around 20 degrees. The new MCAS will have a sensor disagree condition at 6 degrees. Cross winds would cause conditions where the actual AOA would differ 3-4 degrees on the side with lift vs the pressure side.

I'd love to be corrected if I'm wrong, but I don't believe windshear refers to a direction (I.E. vertical) and more to a phenomenon of passing from one airmass to another that has different winds. So for instance if you have a plane stabilized in any stage of flight...lets say cruise to keep it out of crash talk...if you move from a 20 kt headwind to a 90* crosswind, the plane is going to require to be re-oriented to continue the same ground track. I don't think the actual direction of the wind is the determining factor, more the fact that this section of air shears against the one beside it and your plane just had to re-stabilize to maintain course.

Edited to add: Lots of windshear is vertical...but more of an annoyance...I refer to making an approach in a light GA plane over a parking lot in summer...you will get extra lift because the air is literally moving up due to heat, but the dangerous side of it is going from a 20 knot headwind to 20 knot tailwind in a critical phase of flight such as landing when you are 30 knots above stall speed. Again, nitpick my reply, I don't want to give bad information.

JayinKitsap wrote:

Starlionblue wrote:

Winds are a big factor. At altitude, jetstreams can be in excess of 200 knots. Shears of 20-30 knots are not uncommon. So yes, it can definitely disturb lift.

There are many excellent resources online if you want to read more, for example Skybrary. https://www.skybrary.aero/index.php/Angle_of_Attack.

For a proper textbook, check out the Pilot's Handbook of Aeronautical Knowledge. Available for free here https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/phak/ or as a paper book.

Thanks for the information

Shears are the vertical component of the wind vector. A shear of 20 at a 200 knot is only a 6 degree vertical angle, I would guess planes need to make it thru 20 degree angles safely. Does anyone know what is the angle difference between best performance and stall is, I think it is around 20 degrees. The new MCAS will have a sensor disagree condition at 6 degrees. Cross winds would cause conditions where the actual AOA would differ 3-4 degrees on the side with lift vs the pressure side.

Shears can be vertical or horisontal.

A typical AoA margin between stall and normal conditions is 10-12 degrees. Your "normal" AoA is 2-4 degrees and stall is around 15 degrees. Best performance will be just under the stall.

Starlionblue wrote:

Zeke2517 wrote:

And from what I understand pitot tubes don’t measure AOA either.

Pitot tubes —> airspeed
Static ports —> altitude
AOA probes/vanes —> AOA
Thermometer —> temperature
Satellite antenna —> GPS and porn

Correct.

Adding to this, there are combined sensors, for example pitot and AoA are often one sensor in modern airliners.

And it is called a TAT probe in airlines, not a thermometer. Because that's what it does. And we like fancy names.

a pitot and the AOA in one sensor? On What airplane? I think ypu might be mistaken, the TAT is it's own sensor (total air Temp)

strfyr51 wrote:

Starlionblue wrote:

Zeke2517 wrote:

And from what I understand pitot tubes don’t measure AOA either.

Pitot tubes —> airspeed
Static ports —> altitude
AOA probes/vanes —> AOA
Thermometer —> temperature
Satellite antenna —> GPS and porn

Correct.

Adding to this, there are combined sensors, for example pitot and AoA are often one sensor in modern airliners.

And it is called a TAT probe in airlines, not a thermometer. Because that's what it does. And we like fancy names.

a pitot and the AOA in one sensor? On What airplane? I think ypu might be mistaken, the TAT is it's own sensor (total air Temp)

The A350 uses three Multifunction probes for primary air data. These provide Total pressure (pitot), AoA and TAT. The TAT inlet is the squared off one under the pitot inlet. I think the A380 has the same probes.



Here's the big picture:
1) Ice detector
2) Multi-function probe
3) Standby static port. The primary ones are three on each side of the forward fuselage.
4) OAT probe
5) Side-slip angle probe
6) Standby pitot probe
7) Standby angle-of-attack vane


It feels like I already explained this upthread.