Angle of Attack

5 Answers

1. 
   

   
   

   Parth on Feb 18, 2016

   Hi
   I think i understand what your confusion is. unfortunately i cant draw here else i would have clear your doubt in 10 seconds. still i will try with figure below.

   https://upload.wikimedia.org/wikipedia/commons/thumb/6/6d/Angle_of_attack.svg/2000px-Angle_of_attack.svg.png

   when you think about chordline you should think that its just imaginary same like CG of airplane. Your all the weight is not located on CG but airplane responds like it does. same way chordline is an imaginary line to determine aerofoils characteristics in terms of CL / CD etc.

   so when you see figure you will see a very little area below red chord line. your confusion is that you are trying to look for the distance between the bottom of wing surface and relative wind. just imagine if you have very thick airfoil in same angle position distance between bottom of wing surface and relative wind line will be so little. but angle of attack will be same since it is measured from the chord line. so distance between lower wing surface and relative wind has nothing to do with angle of attack.

   In level flight your lower camber will be even below relative wind that doesnt mean you are flying at negative angle of attack.

   i know words dont explain it so good but think about CG. 300 lb engine is in front of airplane still plane moves around cg. same way doesnt matter where your lower camber is wing will react according to position of chordline.

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2. 
   

   
   

   Dave Menke on Mar 04, 2016

   Hey Perth

   first of all, thanks for taking the time and effort to answer my question. AoA is a very comprehensive topic with many different perspectives and understandings by different people (pilots). I do try to understand your explanation and I think have, at least partly.

   AoA is an important topic in aviation especially since the most important force “Lift” depends on it. I also have the famous and rather old book “Stick & Rudder” which expains things quite differently. Wolfgang Langwiesche, the author of the book has a little different perspective…

   The definition of AoA is the angle between the wings Chord Line and the Relative Wind, Ok!
   If you read W. Langewiesche’s book “Stick & Rudder” you kind of start to get a bit confused although his explanations in regards to AoA and relative wind do make some sense.
   For instance, he says that relative wind also called “wind of flight “( which I found to be a very nice explanation by Mr Langewiesche), is always opposite or out of the direction into which the airplane is moving. He also points out that the AoA is “ the difference between where the airplane points and where it goes.” If you now combine his two explanations about RW & AoA, it’s still confusing to me in regards to climbs and descents.
   For example, during climbs the plane’s nose points a given degrees above the horizon and vice versa during descents. Back to his explanation of AoA/RW that would mean that AoA could be “0” at, lets say 10 degrees nose up, as the plane is still moving forward over the ground (distance) but it also gains altitude at the same time. Besides, RW would also be from 2 directions as far as my understanding.
   (a good example for this would be Pitot-Tube position error during climbs/descents due RW disturbances around the front inlet)
   Anyway, if we take the standard definition of AoA into account, the AoA (=betw the wings CL & RW) it still remains confusing to me due to the RW factor.
   I guess CL in this case is the same as to where the nose points at any given flight attitude.
   I understand that the AoA definition always relies on the fact that RW is coming from the direction into which the airplane is traveling, in respective of whether it climbs or descents.
   So, what RW is is clear. But the question is where does this “wind” really come from, especially during Climbs and descents. According to Mr Langewiesche, always opposite the flight path.

   Dave

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3. 
   

   
   

   David Vancina on Mar 05, 2016

   Two comments:

   1. Forget about the nose of the airplane and where it points. It’s irrelevant and maybe confusing. The only thing that matters is the wing.

   2. “…where does this ‘wind’ really come from…” Wind, as we commonly use the term, is relative to the ground on which we’re standing. If the air is not moving around us as we stand, we say there’s no wind.

   In an airplane, the “relative wind” is not about the movement of the air, it’s about the plane’s movement through the air. AoA is the angle between the direction of that motion and the chord line.

   Maybe it will help to simplify the scenario… Imagine the plane sitting on level ground in calm air. The chord line is at an angle to the ground. Plane is still, air is still — no relative wind. Start rolling the plane forward and you have created a relative wind, which is parallel to the ground. The AoA is the angle between the chord line and the ground/relative wind.

   Now imagine the plane on sloping ground, still in calm air. The angle between the chord line and the ground is the same as it was on level ground, but the whole system is tilted. Begin rolling the plane and you have again created a relative wind parallel to the ground, just as before. That the plane is now either climbing or descending doesn’t change the chord line or it’s angle to the ground, thus the AoA is the same as it was when the motion was level.

   Now imagine the plane 2000 feet off the ground in the same conditions — calm air, level, or pointed up or down. Again it’s the plane’s motion through the air that creates the “relative” wind, and the difference between the direction of motion and the chord line is the AoA.

   Hope this helps!

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4. 
   

   
   

   Dave Menke on Mar 05, 2016

   Hi David,

   thanks a lot for your help! I do understand what you are saying. I have understood the the term relative wind and what it actually is. The confusing is between how you can look at it and probably how people define relative wind and where it comes from with respect to an airplanes flight path. Again, the standard definition of RW how I learned it, is that it\’s usually opposite your flight path, which does make sense and is not hard to understand. It just starts to get a bit confusing during climbs and descents. Because when you climb you gain altitude but you still cover a distance over the ground. In that case the RW would come from about a 15 degree angle or however steep your climb is as well as from the direction towards which you are moving. If I understand you correctly, your saying that it\’s basically a mix of direction where the RW is coming from during climbs/descents?!
   In other words it comes from anywhere within those 15+\\- degrees of pitch up attitude , right?!

   Thanks,
   Dave

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5. 
   

   
   

   David Vancina on Mar 05, 2016

   It’s probably confusing to think the RW is “usually” opposite your flight path. It is always opposite your flight path. Your flight path may be straight and level, climbing, or descending, but the RW is always opposite your direction of motion.

   “Flight path” probably deserves some thought. If we imagine a plane at a particular point in space, and at a different point one minute later, and draw a line between the two points, that is the flight path. That line between the two points may be sloped up or down or level. When we imagine the plane moving along that line, its chord line will be at some angle to it. That angle is the angle of attack.

   I have Langewiesche’s book too. The chapter titled “The Airplane’s Gaits” has some diagrams that illustrate this concept, along with those in the previous chapter, “How A Wing Is Flown”. It might be worth just poring over them and rereading those chapters. Sometimes a foreign concept just requires “soak time”.

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