Angle of attack and pitch attitude
 

I've just read Bill Palmer's book 'Understanding Air France 447', and I'm a bit puzzled when he mentions the aircraft at times having a very high angle of attack (30 to 40 degrees) when at the same time the pitch attitude is around 8 to 12 degrees - I can't visualise how this is possible, surely if the AOA is that high the nose must be pitched up at a similar angle? I'd have thought that the only significant difference between the two angles would, for example, be due to the wings being at an angle to the fuselage horizontal centreline.

Disclaimer - I'm not a pilot, just a very interested and reasonably(?) knowledgeable aviation enthusiast.

Consider the case of dropping a level dinner plate. The pitch angle is 0 degrees and the AoA is 90 degrees. Don't use the good china for this experiment.

Ah, now I get it! What a brilliantly simple analogy. And I didn't need to break any china either...

Good to hear that you've cracked it.

Smashing!

Yes. Just translate the feet per minute into knots and you will also see.
Descending 12000 feet per minute is approximately 120 knots
You can have with no wind a ground speed of 120 knots in a stalling situation.
In this case you would have a path angle of -45°. If the nose was pitched 0° you would have an angle of attack of 45° too.

This poses the obvious question : to get out of a stall, one has to reduce the angle of attack. To get a more upward flight path angle, one has to regain lift, so it has to be done after being out of the stall and after reducing the angle of attack.
So, you have to pitch down to the flight path angle to regain speed and lift.
The question becomes, can you recover a positive flight path angle without exceeding some structural speed limit or structural limit, if you pitched down to this kind of angle ?

I know an airbus rated test pilot, he answered that the airplane probably would have exceeded VMO/MMO but not so significantly because of exponential mach drag, preventing the airspeed from increasing too much and the airframe from breaking apart.

Worst I ever saw (my screw-up in ACM) was air speed below 30kts with in excess of 10,000 ft/min rate of descent and 5 degrees nose up pitch. All well below base height. Thank god for nozzles!

Mog

And thank god for the altitude you had available below you I guess Mog. Base height set too high :p

In my world, known as a Fox4 :)

You have to be really trying to get to the structural limits. The test flight manoeuvres to which your friend refers involve full power dives, slightly different profiles in each side of the Atlantic but ostensibly the same.

That there is so much buffer is the topic du jour in Airbus training. Many in flight events have been reported where Aircraft have exceeded Vmo/Mmo briefly due to wind shift and subsequently ended up in alpha floor, due to pilot exuberance at dumping speed to stay out of the red.

Can't really add much to MechEngr's perfect explanation, except a couple of polishing details.

1) Most aircraft wings are installed with a slight positive "angle of incidence." That is, the line between the wing trailing edge and leading edge points slightly "up" relative to the fuselage centerline (or cabin floor), maybe 1-2 degrees. So that the floor can be level for pax in cruise, even when the wing has a positive angle of attack (and probably other reasons.) Exception might be for aerobatic aircraft, where the aircraft may often be flying upside down, and thus symmetry up or down is desirable.

2) In a "slow-motion" version of the level-plate analogy, on final approach down a 3° ILS glide slope - at approach speed - the aircraft attitude is usually slightly nose-up (2-4°). For additional lift at the slower speed, and to make sure the stronger main landing gear touch down first. While the descent path through the air is ideally 3° down. For a net AoA of 5-7° (plus any built-in angle of incidence).

Concorde, of course, had, and needed, an even higher approach attitude/pitch (10° plus), With a landing AoA even higher, thus the need for the drooping nose, so that the crew could still see the runway ahead.

https://upload.wikimedia.org/wikiped...Fitzgerald.jpg

Another polish for you pattern ;), did you know the "angle of incidence" is the "angle of attack", I didn't until put right by a highly credentialed test pilot just a week or so ago on Pprune, apparently "aircraft wings are installed with a slight positive "angle of incidence" should be referred to as simply "incidence" or "rigging incidence". Angle of incidence is synonymous with angle of attack, the same thing.

I do hope that's being said tongue-in-cheek. :O

Great analogy, in other words the angle of attack is the angle between where the aircraft nose is pointing (In the case of the dish let's suppose its falling flat therefore the "nose" would be more or less pointing towards the horizon, and where the aircraft is going: vertically down.

The angle between where our dish or hypothetical aircraft its pointing and where its going is roughly 90 degrees, which could happen fully stalled stick held back pancaking towards the ground.

The confusion comes due to the horizon (Which should be removed from the equation when considering AofA), in this particular case with reference to the horizon the pitch angle is virtually zero, though with respects to the trajectory (Pointing vertically down) its close to 90 degrees

Not so, read the last few posts on the link, or you might also refer to the glossary of "Handling The Big Jets" by D P Davies page 4 where he says, "Incidence - The angle between the wing chord line and the free air stream, (Also referred to as "angle of attack"). The word "incidence" has been adopted and used for two different things, both angle of attack and the rigging angle of the chord line to the longitudinal axis, hence the confusion in the likes of you and me. As I just found, a reading of scientific papers will see "angle of attack" and "incidence" being used interchangably.

https://www.pprune.org/aviation-hist...ircraft-3.html

Yes, fair point.

Kermode also acknowledged the ambiguity in his classic "Mechanics of Flight":

https://cimg2.ibsrv.net/gimg/pprune....597d79c9af.jpg

For the angle between the wing chord and horizon just use Pitch. It seems to me that in the US angle of incidence and angle of attack are both used to indicate angle between chord line and airflow.

Only in still air. If there's a headwind component the dishes AoA will be some number less than 90 degrees, if a tailwind the AoA will be some number more than 90 degrees.

It's the relative airflow that matters which is affected by wind.

What is stunning about AF is that probably while still descending through 10,000’ it was already unrecoverable.
The Captain realized this and says something to that effect on the CVR.
AF, Atlas 76 in Houston, Chinese 73, all unrecoverable at significantly high altitudes.

I asked my instructors about this after coming across statements like Flight Path Angle + angle of attack = pitch attitude. They said there are two sorts of angle of attack considered nowadays, the wing alpha when discussing aerodynamic theory and the aircraft angle of attack derived from that formula. I'm still thinking about that.

Headwind or tailwind has no effect whatsoever with respects to any object falling or moving through a mass of air. Your remark would be true with respects to the ground in case of tailwind or headwind.

To better make the point, consider a turn on still air, or with tail or head wind. With respects to the air mass it makes no difference, the radius function of IAS stays the same. However with respects to the ground you must consider TAS and therefore with headwind the radius will be smaller and tailwind it will be greater.

unrecoverable would be a deep stall (think t- tail).
I don’t recall AF447 being unrecoverable per se. They just left it too late - lots of reasons - and needed more than 10,000’ to recover.

I disagree. Suppose I am standing on the ground and face into a 30 kt wind. Before I release the object it is experiencing the full force of the 30 kt wind. The moment I release the object the wind will cause it to accelerate horizontally and gravity will cause it to accelerate vertically. Only if the object has sufficient time to be accelerated horizontally to the wind speed will the wind not influence behavior of the object with respect to the airmass.

However, if I dropped the object from a hot air balloon that was floating in a completely uniform aimass, the situation would be quite different. Introduce any wind shear or wind gradient and the object is again influenced by wind as it falls.

Shall we drop our dinner plate from a hot air balloon or drop it while standing on the ground?

Isn't that pretty much what was said?

I'm still struggling with the concept that a dinner plate might have a tailwind.

They tried this on the International Space Station - results were inconclusive.

maybe I’m playing semantics.

Unrecoverable being an aerodynamic state rather than defined by proximity to the ground

I tend to agree with you, because a symmetrical airfoil, like a plate, doesn't have a tail or a nose. It will produce equal amount of lift regardles of horizontal direction of the airflow.

​​​Actually it's quite like a helicopter rotor disc, that gains more lift with wind coming from any direction.

"a symmetrical airfoil, like a plate"

What kind of plates are you using for dinner?

"Actually it's quite like a helicopter rotor disc,"

Ready for take-off.

You Tube

You are absolutely right, thanks for the correction; The confusion comes from the fact an aircraft is experiencing the full force of the wind as it transitions from the ground to the air or from the air to the ground like on take off and landing. However once airborne it would be moving with me mass of air. The perspective viewed from the ground if one stands there or from the air if one would be in a hot air ballon would be like you describe Tx.

Sorry, boys - to impress me in 2024, you'll have to come up with something better than 1932 (ffs) engineering language. :=

Let's try: https://www.boldmethod.com/learn-to-...-of-incidence/

"The angle of incidence is the angle between the longitudinal axis of the aircraft (draw a line from the spinner to the tail) and the chord line of the wing (draw a line from the leading edge to the trailing edge). This is not the same as your angle of attack. Your angle of attack is the angle between your chord line and the relative wind."

Angle of Incidence - built into the aircraft (albeit perhaps with a variable mount - F-8 Crusader, others)
Angle of Attack - a value that varies during flight, depending on the relative wind at the leading edge of the airfoil.

This interesting ( for test pilots ,examiners and boffins ) thread is giving me a headache !
(the difference between AOA and pitch attitude should only be a problem for VTOL aircraft? able to safely fly at very low airspeeds ?)


The official report on the disaster was started after black box recorders were found following a two-year search. It found that the aircraft crashed due to several factors, including icing over external sensors called pitot tubes and pilot error. All the crew died in the disaster.

to KISS ( keep it simple stupid ) basically the crew screwed up bigtime.

when the pilots iced up the ASIs said that the speed was increasing ,a lot ,so they raised the nose ,a lot ,and stalled the aircraft and then were unable to recover.

when I was instructing in the sim if you wanted to be be really nasty you fed in a simple pitot problem at night /IMC when the crew were absorbed in another problem - this very often resulted in the aircraft getting into a potentially dangerous ‘bad angle of attack ‘situation.

it is worth reminding that the simple remedy for an unexpected high ( or low ) airspeed is to select a usual attitude and power setting , sit on hands and then review the situation in slow time.

sorry for the dim print caused by cut and paste??

Probably depends on whether said plate had beans on it.

I mean, of course, airfoil symmetrical to the vertical axis at the center of the airfoil. That is, of course, unconventional meaning of "symmetrical airfoil". My bad.

The problem with all this is that the reference is to an aircraft. There are plenty of shapes tested with an airflow at a particular incidence such as in wind tunnels. In these cases "rigging" incidence is meaningless and engineers generally refer to the airflow angle as incidence. It is easy to understand how incidence and angle of attack are used synonymously in aviation. There are plenty of areas in aviation where the operational people misunderstand an engineering requirement - until it becomes folk law - occasionally with fatal results. An example would be Va where the misunderstanding was (is) so great that the FAA introduced a new speed Vo.
https://www.pilotmall.com/blogs/news...ehensive-guide

Better tell Dassault that they have it wrong with their incidence indicators and incidence limits.

https://www.manualslib.com/manual/26...page=47#manual

A flight-simmers manual? Seriously? :eek:

http://aergesengineering.com/index.php/about-us/​​​​​​​

And translated from Dassault's original (again, from the last century) into English by - whom?

This old chestnut comes up every now and again, in fact I think the incidence has increased recently.

Well, lets have a look at the actual flight manual shall we, in its totality ref Incidence.


https://cimg1.ibsrv.net/gimg/pprune....705d7d7130.png
https://cimg2.ibsrv.net/gimg/pprune....37831105d3.png
https://cimg5.ibsrv.net/gimg/pprune....5ad90f3f85.png
https://cimg6.ibsrv.net/gimg/pprune....535444b03a.png
https://cimg7.ibsrv.net/gimg/pprune....fd78b0b786.png
Depictions here are from the official RAAF flight manual for the Mirage IIIO and IIID, I would assume they were translated by a competent translator, possibly a Dassault engineer, I guess you'll tell us the French can't be trusted.

You might do a little research on who zzuf is before taking a whip to him.

If you wish to do a disinformation search, a good place to start is your reference web site and the take on Va, this stuff is exactly why the FAA had to introduce Vo.
https://www.boldmethod.com/learn-to-...your-aircraft/

A comment. I know both zzuf and megan and each is highly regarded in his field. In particular, l would be very careful arguing with zzuf on matters relating to certification flying.