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Following discussion here, I am still confused about the difference between AoI and AoA. As I read in many references and watched in many YouTube videos especially this one: 1). AoI is tje angle between the longitudinal axis and the chordline as per design/manufacture. It says normally AoI designed/manufactured is six degrees. AoI remains constant as it is fixed relative to the fuselage.
2). AoA is the angle between the chordline and the RELATIVE AIRFLOW. AoA changes in flight. AoA and the relative airflow

My confusion is with the meaning of the RELATIVE AIRFLOW. This phrase confuses me. In my opinion, airflow produced by the propeller or the turbines flows parallel to the longitudinal axis, or at least it has fixed angle, also as per designed too.

To avoid a repetitive question, I want to give this situation. 1). During take off of an airplane, from the airplane start moving until it left the ground, what is the RELATIVE AIRFLOW? 2). After the airplane takes off and during climb, what is the RELATIVE AIRFLOW? 3). During level flight (cruise with constant altitude), what is the RELATIVE AIRFLOW? 4). During approach to land until touching ground, what is the RELATIVE AIRFLOW?

What is actually that AIRFLOW?

I apologize if there are too many questions. It is actually just one. Just to make sure that you will not confuse to read and I will not get confused to read your explanation as product of unclear question.

Before I post this question, I have search if someone have posted previously. But I didn't find the same question.

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    $\begingroup$ the term you want is "chord" not "cord" $\endgroup$ – quiet flyer Oct 27 '18 at 17:47
  • $\begingroup$ Thank you have corrected the error. Apologize for the mistake. $\endgroup$ – AirCraft Lover Oct 28 '18 at 2:19
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Angle of Incidence is fixed by the designer of the aircraft. It does not change in flight.

Angle of Attack is the angle between the chordline of the wing and relative airflow during flight.

Illustration of Angle of Attack

That is definitely something that changes during operation of the aircraft in flight. Consider slow flight, for example. You reduce power and raise the nose to reduce airspeed. The direction of the airflow remains the same, but the angle between the airflow and the wing chordline, the Angle of Attack, increases. Raise the nose too much (and hence increase the AoA), and the airflow will separate from the top of the wing and you will stall.

On aircraft like Cessna trainers, the stall warning is a very basic indicator of Angle of Attack, to warn you when you are about to stall.

Stall warning device

More advanced Angle of Attack indicator systems use a vane to measure the angle of the relative airflow, and give you more information about AoA.

AoA vane

AoA indicator from an Icon aircraft

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  • $\begingroup$ I have heard like your explanation, but I refused to admit it. As you said, "Consider slow flight, for example. You reduce power and raise the nose to reduce airspeed. The direction of the airflow remains the same, but the angle between the airflow and the wing chordline, the Angle of Attack, increases," so can I say that the AoA is result of that action are taken, not vice versa that the airplane position (nose raised and air speed reduce) due to the AoA changed by the pilot? That's very different which is result and which one is the taken action. $\endgroup$ – AirCraft Lover Oct 28 '18 at 2:58
  • $\begingroup$ To avoid misunderstanding, I have to correct my sentence above. That should be I have heard like your explanation, but I refused to admit it. But now I admit it..... $\endgroup$ – AirCraft Lover Oct 28 '18 at 4:47
  • $\begingroup$ Your understanding is incorrect. AOA changes because of actions the pilot takes. $\endgroup$ – Juan Jimenez Oct 29 '18 at 19:56
  • $\begingroup$ Is any widget in cockpit to set the AoA? I.e: the pilot set the AoA to 11, which it will give direct change to the AoA rather than change the airspeed to meet the AoA? $\endgroup$ – AirCraft Lover Oct 29 '18 at 21:57
  • $\begingroup$ Not to my knowledge. An AoA is generally considered a passive (though highly useful) indicator. In most aircraft with modern autopilots the pilot selects airspeed and rate of descent or climb, not AoA. Good question, though, and I can see why you asked it. :) $\endgroup$ – Juan Jimenez Oct 30 '18 at 14:01
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In my opinion, airflow produced by the propeller or the turbines flows parallel to the longitudinal axis, or at least it has fixed angle, also as per designed too.

True, but not something to really worry about. Turbines/turbofans are not normally mounted ahead of the wings. Propellers may be ahead of the wing, but most of the airflow over the wing will be in areas not directed by the propeller. So for consideration here, I will ignore any effects from propellers or other engines.

The RELATIVE AIRFLOW is a description of how the air moves by the airplane (from the point of view of the airplane). For an airplane that is level in pitch, not changing altitude, in still air, the relative airflow will be from front to back. Think of having a ribbon on the end of a stick next to the plane. The ribbon is pulled by the relative airflow (or often, the "relative wind") and shows exactly the angle. It takes into account the actual movement of the air, and the movement of the craft through the air.

During take off of an airplane, from the airplane start moving until it left the ground, what is the RELATIVE AIRFLOW?

Technically this depends on the actual wind at the ground. But assuming winds are light and there's nothing odd like a microburst, then the relative wind is exactly opposite the movement of the aircraft. The aircraft is moving down the runway, so the relative airflow is along the runway. The angle to the relative airflow depends on the angle of the craft. It might be rolling at various pitch angles.

All your other questions are the same. The relative airflow depends on how the air is moving (are there updrafts? Downdrafts?) and how the plane is moving through the air. A plane climbing through still air will see relative air moving at a downward angle compared to the horizon. A plane descending through still air will see relative air moving at an upward angle compared to the horizon.

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  • $\begingroup$ How about what is explained by @Juan-Jimenez above, the airplane direction is not always parallel to the wind generated by the propeller or to the longitudinal axis? As he said, in flight control, there is situation that power is reduced so nose is pitching up air speed also reduced. This situation makes the plane is moving horizontal but wind flow from the propeller will not parallel to the longitudinal axis. I am not sure by what condition such action will be taken, but such that action is taken. $\endgroup$ – AirCraft Lover Oct 28 '18 at 3:27
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    $\begingroup$ Yes, the flow from the propeller may be in a different direction. But so little of that flow hits the wing that we tend to ignore it, and focus on the airflow based only on the airplane's movement through the air. $\endgroup$ – BowlOfRed Oct 28 '18 at 4:09
  • $\begingroup$ OK, thank you for your great explanation. Your kind to comment have helped me understand what is confusing me quite long time. $\endgroup$ – AirCraft Lover Oct 28 '18 at 4:45

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