Angle of attack indicator

[gundernak]

Hi All,

can you please help me with the following theoretical problem?

I am trying to write a material about AOA indicators and stuck with a contradiction. It is about what will display an AOA indicator which is calibrated with clean config and the plane stalls at landing config? Or vice versa, what will display the instrument at a clean stall, but calibrated to dirty stall.

If we know that the aircraft with flaps will stall at a lower AOA and also at a lower pitch, then I should think the instrument calibrated to clean stall, will show lower AOA than the critical AOA when executing a dirty stall.

In spite of it, the manuals of the manufacturers write the opposite:
"The system is calibrated with the aircraft in the clean configuration which means that when the flaps are lowered the display may indicate a higher angle of attack (deeper into the red) before stalling."

Or in an other way:

"When the optimum alpha angle (OAA) is calibrated in landing configuration as above, the indication in the clean configuration with no flaps will be slightly different. In the PA46 it will typically indicate one LED segment lower on the display."

By the way the display of the AOA indicator of Aspen Evolution EFD corresponds to the two above mentioned statement also, see the pic:

https://www.aopa.org/-/media/images/legacy/aopa/home/news/all/2015/april/0408_aspen-aoa-on-pfd_left_and-mfd_web.jpg

So I know the mistake is on my side, but please explain me, why?

Thank you in advance.

[scifi]

Hi Gundernak, The AoA meter indicates the angle of the un-flapped portion of the wing to the relative airflow. When the flaps are deployed, a pitch down is required to maintain level flight. So the un-flapped portion of the wing reduces its AoA ( which helps to prevent a wing-tip stall.)


The AoA of the flapped portion of the wing increases, because the trailing edge is lowered, and that part obviously stalls first.
.

[Owain Glyndwr]

@ Gundernak
AOA is usually defined as the angle between the upstream freestream velocity and a fixed airframe reference axis. Since wings are often twisted across the span and this twist varies with flight condition it is generally more convenient to use some form of fuselage axis as the datum - very often the fuselage centreline.
A lifting wing induces upwash ahead of itself as well as downwash behind. Any AOA vane mounted ahead of the wing will see the sum of the freestream AOA and the local upwash. One can either calculate or measure this local upwash and subtract the value from the vane output to get a calibrated AOA that meets the chosen definition.
If, keeping the fuselage inclination constant, one lowers the flaps the lift and local upwash will increase. If you then apply the "clean" calibration the meter will read something larger than the actual AOA as defined. Hence the manufacturer's statement - the actual difference will depend on the configuration.

[gundernak]

Thank you the explanation.

@Owain Glyndwr
But sorry I do not get it completely. OK, we have a bigger upwash with flaps, but we have a lower pitch also when stalling with flaps. Am I right that you say the sum of this two effects togehter will produce a higher AOA indication when stalling with flaps but calibrated to clean config?

[Piltdown Man]

Simple answer, yes. But why limit yourself to poor starting point? An AoA indicator has to be the one of the most useful values displayed to any pilot. But as you point out, if the wrong datum is used, an erroneous value will be presented. And as you state, the datum will change with aircraft configuration. It is not beyond the wit of man to correct or modify the datum of the indicator as an aircraft changes configuration. Then, no matter what the aircraft's configuration (weight, or speed) a valid AofA value can be read by the pilot.

[Owain Glyndwr]

@Gundernak,

Introducing pitch tends to confuse the issue. Pitch is the sum of AOA and flight path angle. Stall depends ONLY on AOA and it is better to confine discussion to that parameter .
You may, or may not, stall at a lower AOA when flaps are deflected - it depends on what part of the wing stall first in each case. (assuming no slats of course). All I am saying is that the indicated AOA will be higher with flaps down at any given fuselage AOA if you stick with the clean aircraft calibration.
Piltdown Man is quite correct that nowadays it should be easy to vary the calibration to suit the configuration, but I think (and I am not a pilot) that one does not need to know an exact numerical value of AOA to fly safely. After all, some aircraft fitted with AOA indication present the data in arbitrary units rather than actual degrees. What is needed I believe is a clear indication of how close you are to a stall or buffet and/or whether you are flying at the recommended approach AOA, and that can be done without presenting actual AOAs

[Intruder]

The difference in indicated stall AoA is also exacerbated when leading edge devices are installed and used. For example, the A-6 Intruder had full-span leading edge slats as well as full-span Fowler flaps on the trailing edge. Buffet onset approaching the stall was about 21 units clean, with stall soon thereafter around 22.5 units. Dirty, buffet started around 25 units, with stall around 30. Note that "units" are not the same as "degrees".

[gundernak]

Dear Sirs!

thank you very much the useful explanations.

So we are talking about DOWNWASH (and upwash also offcourse) and INDUCED AOA, which generates thise phenomenon.

May I ask you to give me a final acknowledgement on the following sum statament? Offcourse finally I will write it in Hungarian, so do not deal with my english grammar this time ;)

"Although a "dirty" wing stalls at a lower AOA, the AOA indicator will indicate a higher critical AOA than in case of a "clean" wing, because of the 3 dimensional airfow and downwash produced massive increasement in the induced AOA."

this is worng

[Owain Glyndwr]

Gundernak,

Sorry, but you have it completely wrong. It is UPWASH not downwash that makes the change, which is not "massive" - probably about 4 or 5 degrees at full flap; and if you introduce a term such as "induced AOA" you will need to define it. It would be better to write about an increment in AOA at the vane location.

[gundernak]

OK, I will highlight more upwash, (but I know it is weird to you but, somehow in European flight theory does not mention upwash, but mainly downwash. Because eventually upwash and downwash belong together, they are not exist without each other. But in this case you are absolutely right, because the probe (because I write about this and not the vane) is in the upwash stream, and that causes the effect. Ok "massive" is not the right word to use, but it is true that the increasement in induced AOA is bigger than the decreasement of the critical AOA during a dirty stall.Is it OK now? And ok, I have defined already induced AOA before in my text.

this is wrong

[gundernak]

Well....

I did a lot of research on the subject and my statements above are wrong! So this post is not solved unfortunatelly.

So please can you please explain the following contradiction (point 1 against point 2) in a detailed aerodynamic description?
Point 1: The ACTUAL streamlines give a higher AOA when stalling with flaps, than with CLEAN stall (experiencing the indication of an AOA indicator instrument).
Point 2. Despite with flaps the critical AOA is a slightly lower, and the pitch is lower compared to the CLEAN stall.

Some hints (but not enough for me to completely understand the issue)
The effect of changing the camber when flaps deployed is the increase of zero lift angle and this way the absolute AOA too. The increase in absolute AOA is much more larger then the decrease of critical AOA. But how is it possible that this absolute AOA is manifested in the reality? For example an angle of attack indicator displays a higher AOA with flaps than with CLEAN...

Please help

[alexd10]

Perhaps you should consider that the limit layer detach phenomena is in fact influenced not only of geometry (AOA) but also by Reynolds number (air speed), although many sources states that stall is only about AOA

[PEI_3721]

Is it possible that in this discussion (and manuals #1), the differing opinions represent different viewpoints.

First, theoretical, with streamlines, upwash, etc, which could indicate different AoA with configuration, as measured at or around the airfoil shape. Where is the AoA sensor (human viewpoint) located?

Second, a practical view; with this, AoA as measured by a vane or other sensor to provide a displayed value which may be calibrated in units vice true angle. In commercial aviation the raw AoA angle is probably adjusted with aircraft configuration so that the ‘stall AoA’ (a unit value) is given at the point of stall for all wing configurations, thus the indicated stall ‘angle’ appears to remain the same. And as discussed, this may not represent what the airflow is doing over the wing, particularly as sensors for stick shake/push and minimum airspeed alerting can be fuselage mounted.

A further confusing aspect may come from the pictures @#1, which appears to show an AoA display for a light aviation aircraft. I doubt that the display format would meet and commercial certification regulation.
Whilst the display might be marketed as indicating ‘true’ AoA, the format may only represent the inability or cost in adjusting the raw value to provide a more risk free interpretation and practical use.
Also in these light aircraft, what form of AoA sensor is used. Many years ago I recall a leading edge, mini-vane sensor which I understood detected the airflow stagnation point at the stall angle - relating AoA to streamlines, etc.

[gundernak]

The quotations are from manuals of AOA instruments operating by measuring pressure differences when AOA changes with a probe located on the lower surface of the wing. Similar to a Pitot-tube (two holes: one is paralell, other is 45 degrees down).

The picture is a snapshot of Aspen's Glass Cockpit. This system calucates the AOA from the ADC, AHRS and GPS informations, and strictly software based, no wiring, no probe.

Each of those instruments actually measure a higher AOA with flaps compared to CLEAN.

[PEI_3721]

Hmmm; I have seen a two-hole ‘pitot’ as described but never understood how such a system could provide meaningful AoA. The AHRS reference suggests a flight path angle type of parameter; but the use of GPS baffles me, and thus such a system might be best avoided.
Having a full EFIS speed display and ADC, an overlay of low speed awareness based on AoA could be of greater piloting value than a separate ‘AoA’ display, but that would require a vane or other reliable (accurate) sensor.

You could consult the manufacturer and politely ask if the instrument actually measures AoA (if so how), or is it just an approximation, or even a parameter which would be better displayed upside down.

Displaying a high, red band ‘AoA’ uppermost relative to the high end of the EFIS airspeed speed tape is hazardous because a high value should relate to low speed. IMHO this would be unacceptable for a commercial certification.
I recall one GA system where vane AoA replaced a fast - slow indication on an ADI, but the display was inverted with respect to high and low speed awareness !
- Non flying engineers / salesmen with snake oil. -

Theory is all well and good, but attempting to use parameters unrelated or inaccurate for the theory usually results in headaches. (cf #5)

[Volume]

Any AoA sensor installed on the wing itself just gives you a "value" that somehow relates to AoA (and is good enough to predict stall precisely), but does not really give you an useful angle information. You need to mount it on a boom at the aircraft nose for accurate measurement, and this is exactly what you would do on a test aircraft.

However, to explain the issue a little simpler, if you deflect a trailing edge device at a given AoA, the stagnation point on the leading edge will move backwards (more circulation required to "bend" the airstream to follow the trailing edge angle, which also heavily affects airflow in the nose region). This means an increased upwash at the sensor.
Additionally keeping overall lift constant (equal to weight) with flaps extended means a higher relative portion of the lift is produced by the flapped inner wing, and less lift is produced on the unflapped outer wing. This increases the wash (upwash ahead, downwash behind) on the inner wing, hence the AoA you are reading.

[gundernak]

PEI_3721 (http://www.pprune.org/members/136812-pei_3721)
The pressure differential AOA measurement is very simple: as the AOA changes the pressures change as well. At low AOA mainly the pitot hole is effected and the pressure at the 45 degree hole is less. At large AOA the pitot hole will "rotate" out from the airstream (decreasing pressure at it), and the 45 degree hole will be effected more by the airstream. Continuous processing of these two pressures will give an AOA.Yes, Aspen's system surprised me too, especially the use of GPS data. But the engineers of this big company must be skilled enough, they now what they're doing, I guess.

Volume (http://www.pprune.org/members/43580-volume)
Thank you the explanation. Actually the "bending" of the airstream is not the classic up/downwash yet, because this bending will occure already at a 2D profile, but the acutal up/downwash is related to the finite 3D wing. BUT you are right, the probe definitely meausre the higher AOA because of this phenomenon. But in the aviation literature I have not find any theory on how much is this bending of the freestream related to the camber of a profile, what is the aerodynamic correlataion? There are a lot of material on up/downwash, but as I wrote it is not THE up/downwash.

I think the bending of the free airsteam is related to the absolute angle of attack (angle from zero lift and this way to the camber). For example a manufacturer of an AOA isntrument especially calls the user attention to forget the classic concept of AOA, because the instrument actually reads the absolute AOA (angle from zero lift). And with a more cambered wing, I mean with flaps the absolute stall AOA will be always higher compared to CLEAN. And the instrumens definitely work in this sense.

[Volume]

up/downwash do already happen in the 2D "airfoil" airstream (at least when flying subsonic). Additional downwash happens due to the finite wing in the 3D "wing" airstream.

According to the 2D airfoil theory of the conformal mapping (mathematically "projecting" the airfoil on a cylinder) you only need to calculate the speed- (and according pressure-) distribution for 2 angles of attack, and can interpolate linearly in between. This is why measuring pressure on two locations along the airfoil is an efficient way of measuring the local AoA at that spanwise position. As long as you have no friction and hence no flow detachment and no boundary layer, of course.
With all the complex 3D effects (e.g. due to the fuselage, propeller slipstream, flap/aileron deflection, maneuvres etc.) this information is not "the" AoA of the aircraft, it may however do exactly the job which is required: giving an indication of the margin to stall.

I am quite sure that the theory of conformal mapping allows an exact assessment of the up/downwash for airfoils with a constant radius arc skeleton line (used on many popular airfoils up to WWII), so some up/downwash and zero lift information relative to camber can be determined easily and accurately. I am not too sure that this information is very useful except for a hand full of academics. Pilots will never ask for such information.
I assume that up/downwash is much more depending on Cl than on camber (although at a constant AoA this basically means the same), and Cl is much closer linked to the overall aircraft situation, and hence much more important for pilots.

The problem is, that the higher the Cl, the higher the upwash at the leading edge hence the higher the AoA you measure there. If the wing starts to stall (and it is a good airfoil where flow gradually setaches starting from the trailing edge extending forward), this will reduce Cl (or reduce the slope of the Cl curve), which will also reduce your AoA measured. So just when you get close to the warning threshold, when AoA gets critically high, your measured AoA indicates a lower (and hence less critical) value, so it does not accurately warn you.

With a bit of margin, such systems still work well. They will warn you a little early, but there is no point in operating very close to stall anyway, except you are thermalling in a glider...

[gundernak]

https://www.google.hu/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0ahUKEwjpqLrdlfjUAhXFxRQKHXdfDGQQFgglMAA&url=http%3A%2F%2Fwww.nar-associates.com%2Ftechnical-flying%2Fangle_of_attack%2Fabsolute_wide_screen.pdf&usg=AFQjCNHq6SYVdhzmhPkNUKCW_nfl59CvbA&cad=rja

This article helped me a lot to understand how absolute AOA works, and the flap operaton is one of the most useful way to examine this perspective of AOA.

Now I see Aspen's calculated AOA indication works based on absolute AOA calculation.

[PantLoad]

Many systems are calibrated for clean and landing configurations. In addition, some are also calibrated for both low and high altitudes.
(e.g. FL220 or thereabouts re-indexes the calibration)

So, you'll have to look at the specifics for your manufacturer and model.

[Dan Winterland]

Many fighter's AOA gauges are calibrated in units rather than degrees. It doesn't matter what you call the values or what they reference to, just as long as the pilots knows which values are relevant.

[paradoxbox]

I think it is easier to load up a good quality sim and experiment at different flight envelopes to get a better understanding of this topic.

Not to offend OP but it does not seem like you have a very clear understanding about AoA and wing loading.

Some of the most useful things I ever learned for flying were in an old ww2 fighter sim, with an AoA indicator turned on. This won't teach you about weird issues that arise from the positioning of the AoA sensor but you will learn the concept far quicker than you would just trying to hack out a solution by reading textbooks.

Try unloading the aircraft (push the nose down) and mess with the flaps at varying speeds and watch what happens to the AoA. Then do some stick pulling at various speeds (again messing with flaps) and watch what happens to the AoA.

I believe that you will probably start to understand AoA and stalls better, though don't count on sim stall characteristics being very realistic. The AoA figures experienced while maneuvering are generally pretty good representations of reality though.