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Angle of Attack - Stall

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Hi,

when wind blows horizontally (90°) and the (wing) chord line of an airplane is 5°, 15° or even 25° the AOA would be considert 5°, 15° or 25°. When the wind blows from below relativ to the chord line (e.g 60°), and I climb with an 15° angle. That means I have an AOA of 45° and I would stall, is that correct?

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Posted (edited)

Nope it is not correct. You are conflating airflow (movement over the wings due to aeroplane airspeed) with wind (i.e. air mass movement). These are not the same thing, nor are they combined to determine an angle of attack.

On the ground, the wind does indeed affect the airflow to the wings (a bit) since when on the ground you are not moving with the air mass, so your aeroplane does experience the air mass coming at it from the direction of the wind's flow (usually experienced as a crosswind during take off), but once you are up in the air, free from any connection with the ground, your aeroplane is moving with the air mass, you can see this when your aeroplane lifts off in a crosswind and it immediately begins to drift once off the ground.

So despite the fact that there may be a wind of 20 miles per hour from the west which people are experiencing on the ground, if you are flying in that area, you do not experience a 20 mph wind on the side of your aeroplane, you simply move in that air mass at 20 mph over the ground independent of your airspeed. Thus your angle of attack is not affected by that 20 mph wind in any way whatsoever, because it is having no effect on your progress through the air mass. The air mass you are flying in is moving at 20 mph, but only relative to the ground, not relative to your aeroplane.

To understand this better, consider a glider (sailplane). This type of aircraft ascends by finding air masses which are traveling upwards and then staying in them by circling around in them. Usually these are 'thermals' i.e. pockets of air which have become heated so they are hotter than the surrounding air, which makes them less dense, so they rise upwards as the colder more dense air pushes down around them and forces these lower pressure areas upwards. Gliders have a variometer instrument which detects changes in altitude so the pilot can tell whether he is in level, descending, or ascending air, i.e. when he is in one of these thermal pockets. The object of gliding is of course to find ascending air, and preferably ascending air which is going up at a rate faster than that which the glider is descending as it glides along. 

So, when a glider finds a thermal pocket of air which is ascending, this moving air is effectively an upwards blowing wind relative to the ground (aka and updraft), but the glider is in that updraft, so it is moving up with it because it is flying through that air mass. Thus the glider is not experiencing a wind which is blowing from ninety degrees underneath it, it is experiencing whatever angle of attack it happens to be flying at as it moves along in that big piece of air which just happens to be going upwards away from the ground.

One of the best ways to understand this concept, is to think of when you were a little kid and you would try to run down an escalator which was travelling upwards. You running downwards is a glider slowly descending in a glide, the thermal is the escalator staircase traveling upwards. Despite the fact that you are running downwards, you are still going upwards, but you don't feel the escalator pushing you, it is simply the effect which occurs as the moving staircase outpaces your footsteps downwards that you observe.

Same with the wind on an aeroplane when it is in flight.

Edited by Chock
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Alan Bradbury

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What I don't get is: when the relativ airflow is always parallel to the flight path, and the AOA is the differece between the relativ airflow and the chord line, how can the value of the angle of attack change? Since the flight path and the chord line are be the same, right?

There is another weird thing, when an airplane departs in rainy conditions, after the lift off the rain drops on the cabin windows are horizontilly to the ground, how is that called if it is not the relativ airflow?

Good analogy with the escalator, though.

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I see what you are getting at. Okay, take a look at this video of a jet doing a slow fly-by at an airshow...

Early in the video footage, we can see that the trail of smoke from the engines kind of gives us a bit of a clue as to the flight path, which is very-nearly parallel to the ground since the jet is at an airshow, flying along the runway past the crowd line. Keep in mind that the smoke trail is not exactly straight behind the aeroplane, since the smoke is forced down by the wake of the aeroplane a bit, but even so it's a reasonably good visual indication of which direction the aeroplane is travelling through the air mass regardless of which way its nose is pointing.

In order to fly like this this, the MiG-29 has to pitch its nose up very high, increasing the angle of attack to what is probably somewhere around 17 degrees inclined upwards from the oncoming airflow. The reason I've picked this clip in particular, is because, as I'm sure you know, the MiG-29 has a 'lifting body' design whereby the underside of the fuselage and the top surface of the fuselage are also an aerofoil, whereby they create lift too, so what you can see when the MiG has its nose up, is that the canopy and the bit behind it are essentially a wing shape, and since they are inclined upwards in relation to the airflow, basically this is making the aerofoil shape presented to the airflow act like it is a bit more curved so it creates more lift. This is a classic example of a high angle of attack. The MiG also has lerxes (leading edge extensions) of the kind which are also obvious features on the F/A-18 Hornet, and even moreso on the later F/A-18 Superhornet. These too act as lifting surfaces to assist at high angles of attack, and this is a good thing for fighters to have since it means they can get their nose around to point at an enemy aeroplane when in a turning fight so that they can take a shot, or get a missile lock or whatever.

Of course in nature you don't get something for nothing, and so in creating more lift by flying at that high angle of attack, the MiG is also generating absolutely loads of aerodynamic drag, which is why the engines are having to kick out a lot of power, and as things progress, the MiG is unable to keep this up, so it increases power even more, eventually coming out of dry power and into what looks like full afterburner. This additional thrust makes it start flying in the direction the massive thrust is forcing it in, and so this has the effect of reducing the angle of attack, which increases the speed, which reduces the drag, so it starts climbing away.

So this thing is at a very high angle of attack, which of course the MiG-29, and its big brother, the Su-27, is famous for being able to manage, but this sort of thing does require large amounts of power. This incidentally, is also why you hear the engines being very loud on airliners when they are coming in to land, because with an airliner's flaps creating a load of extra lift so the things can descend and land slowly, the flaps also create tons of drag, so you need a lot of power to keep the thing flying, even though it is going slowly and descending, and that descent path is in fact making it have an increased angle of attack as well, a bit like what we see that MiG doing although obviously not quite as extreme.

It's difficult to tell exactly what settings the engines are on in this clip though because the MiG-29s engines (especially on the early variants) are a bit notorious for not having a long time between overhauls, although on the plus side, the engines are easy to swap out and easy to get to. Because of this, MiG-29s do in fact have three selectable engine rating settings, which the ground crews can switch between prior to take off (once selected however, that's it, the pilot cannot override these when in the cockpit). One setting is slightly reduced power so that the thing doesn't wear its engine out too quickly and is the one most squadrons use on a day to day basis, another is a setting where that restriction is removed, so the thing can use full rated power, then last but not least, there is an emergency 'war setting', where the thing can use even more power at the expense of very quickly damaging the engine after not many hours of running at those settings. 

Edited by Chock
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Alan Bradbury

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Posted (edited)
On 7/26/2020 at 11:26 PM, 737_800 said:

What I don't get is: when the relativ airflow is always parallel to the flight path, and the AOA is the differece between the relativ airflow and the chord line, how can the value of the angle of attack change? Since the flight path and the chord line are be the same, right?

For every airspeed, there is a corresponding angle of attack that will produce the same amount of lift.

That's your explanation, no need for novels....

Edited by SAS443

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1 hour ago, SAS443 said:

no need for novels....

Well excuse me for using a discussion forum to discuss things and chat about stuff, in a forum called Hangar Chat.

Edited by Chock
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Alan Bradbury

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A question was asked. The reply given was in no way proportional to the complexity at hand, IMHO.

A courteous way - if you desire to write long replies - is simply to give an exec. summary and then dive into the X and O's. The OP gets his/her answer and can choose to read further.


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5 hours ago, SAS443 said:

A question was asked. The reply given was in no way proportional to the complexity at hand, IMHO.

Since you are so concerned with the question which was asked, and how to go about replying to it in such an expeditious manner, congratulations for arriving three days after the question actually was asked, then taking the opportunity to slag off my effort at answering the question, which was intended to be conversational and which, you may observe, I managed to do within an hour or so of it being posited.

I'm sure you're a blast at parties with that kind of attitude to an effort to be friendly and sociable whilst being informative, thus it is not a little ironic that you are concerned with courtesy when it would seem to be something of an enigma to you. Once again, I refer you to the title of the forum, which is, Hangar Chat, rather than Brief Encounters.

Edited by Chock
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Alan Bradbury

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Again, just try to answer the OP initially. Then feel free to provide details necessary. 

It is a respectable notion to any member. Not all are proficient readers nor have english as their primary language. 

Give people the option to further read extensive posts. 

 


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5 hours ago, SAS443 said:

Again, just try to answer the OP initially. Then feel free to provide details necessary. 

It is a respectable notion to any member. Not all are proficient readers nor have english as their primary language. 

Give people the option to further read extensive posts. 

 

When I need advice from you about how to write, I'll know I'm in trouble.


Alan Bradbury

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Positively surprised to see you managed to convey that message in only one sentence.
We are improving. 🙂 

Have a pleasant day mr Chock.


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3 hours ago, SAS443 said:

Have a pleasant day

You too 🙂


Alan Bradbury

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A post that is essentially "I didn't like the way you answered that question"... seriously?  This thread started so well....  😞

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Ed Wilson

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It is a very good question to ask when unsure, and it’s amazing how many licensed pilots really don’t have a complete grasp of AOA/stall angle/speed/weight etc. Always good to ask these questions, it’s critical to being a safe pilot.

OP, to continue the topic, read up on maneuvering speed. When you can explain to someone else how and why weight affects maneuvering speed, you’ve got a great foundation of the subject. Once the aerodynamics are truly understood, flying techniques are then logical applications of the aerodynamic principals and will easily be grasped. You will be a much safer pilot as well.

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5 hours ago, Rbass said:

OP, to continue the topic, read up on maneuvering speed. When you can explain to someone else how and why weight affects maneuvering speed, you’ve got a great foundation of the subject

Indeed. And You are spot on that many pilots (especially recreational pilots) are having difficulties grasping those complexities. That's why V/G diagrams are so useful and should by learnt by heart. 🙃

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