# A question about the exact meaning of one British usage of the term "Angle of Incidence"

Most anything can have an angle of attack. If you must be specific, you mention 'angle of attack of ...'

...

When we talk about airplane as a whole, usually the 'default' angle of attack is the one between the wind and the body frame (in the vertical plane). How body frame is defined depends on the aircraft designer, but normally X goes along the fuselage. Therefore, the angle of attack is exactly what you say: the angle between relative wind and the fuselage -- and not that of the wing (even though it is arguably more important).

While this answer to another related question stated:

From A.C. Kermode; Mechanics of Flight, London, Pitman, 8th Edn, 1972, p.75 (his bold):

"We call the angle between the chord of the aerofoil and the direction of the airflow the angle of attack.

"This angle is often known as the angle of incidence; the term was avoided in early editions of this book because it was apt to be confused with the riggers' angle of incidence, i.e. the angle between the chord of the aerofoil and some fixed datum line in the aeroplane. Now that aircraft are no longer "rigged" (in the old sense) there is no objection to the term angle of incidence; but by the same token there is no objection either to angle of attack, many pilots and others have become accustomed to it; it is almost universally used in America, and so we shall continue to use it in this edition."

And from L.J. Clancy; Aerodynamics, London, Pitman, 1975, p.56:

"The attitude of the aerofoil, as expressed by the angle between the chord line and the free stream velocity vector ... , denoted by $$\alpha$$, is called the incidence, or angle of attack."

In contrast to Kermode, Clancy goes on to use "incidence" throughout.

The current question: while there's no question that "angle-of-attack" is often used to describe the angle between a wing chord line and the flight path/ relative wind, when we are talking about a whole aircraft, "angle-of-attack" is apparently sometimes used to describe the angle between the longitudinal axis of the aircraft and the flight path/ relative wind.

Regardless of whether or not all the statements above are entirely correct, I would like to know--

In one of the two possible British usages of "angle-of-incidence", specifically when it is NOT being used to indicate the "rigger's angle of incidence" between the mean chord line and the longitudinal axis of the fuselage, in the whole-airplane context, is there a default presumption that it means the angle between the longitudinal axis of the fuselage and the flight path/ relative wind, or is there a default presumption that it means the angle between the wing chord line and the flight path/ relative wind, or is there no default presumption as to which of these two alternatives is intended?

By "default presumption" I mean the most common usage or the usage which is least in need of extra clarification to the reader as to what is intended.

The question is not intended to get into the distinction of measuring relative to the free-stream airflow/ flight path/ relative wind versus the local airflow-- that's another thing that may depend on context-- just the distinction of measuring from the wing rather than from the fuselage.

As an aside, I have been informed that that the French phrases for these terms are used in different ways, with "angle d'attaque" referencing the angle between the flight path/ relative wind and the fuselage, and "angle d'incidence" referencing the angle between the flight path/ relative wind and the wing. But, we can explore the veracity of that idea in some other question.

Clarification -- it seems likely that the usage of "angle of incidence" I'm referencing in this question is now confined to aerodynamics/ design textbooks, papers, etc, and not in widespread usage in the pilot community in Britain. It's not something you'll find in a current study guide for a private pilot ground school or written exam, as the American terminology has now become universal there, using "angle of incidence" to mean the angle between the mean chord line of the wing and the longitudinal axis of the fuselage. That's not the usage of the term that this question is asking about.

• Note that this is not a duplicate of aviation.stackexchange.com/questions/76682/… because the earlier question was asking how often the term was used to mean the rigger's angle of incidence versus something very different, while this question is asking whether when used in relation to an angle measured from the flight path, the measurement is usually presumed to be referenced to the fuselage or the wing. Commented Apr 2, 2020 at 18:49
• Since the wing is most affected by the angle of attack and most sensitive to changes of it, the default angle of attack should be between wing chord and airflow, not some arbitrarily selected reference on the fuselage. Mr. Kermode is right. Just because an answer has been accepted does not mean it is right. Commented Apr 2, 2020 at 19:27
• @PeterKampf thank you for your comment, feel free to comment further here if you wish but your point is clearly made. chat.stackexchange.com/rooms/106037/… Commented Apr 2, 2020 at 19:31
• @PeterKampf you might choose to incorporate the idea that by default "angle of attack" is normally indexed to the wing not the fuselage, at least in some contexts (pilot-speak? engineering text books? etc) into an answer of your own to the question aviation.stackexchange.com/questions/76683/… , possibly noting that "angle-of-attack of the fuselage" might be a term for what the question is asking about. Commented Apr 2, 2020 at 20:23
• You should edit the title so that it is not needed to open your question to understand what your are asking for. There are many subjects possible for angle of incidence. Commented Apr 3, 2020 at 6:17

I found your exact question a little confusing, so let me answer with a quote from a current British PPL exam preparation book:

The wing's angle of incidence - the angle between the wing chord-line and fuselage - is fixed by the construction of the wing and the angle at which it is bolted onto the fuselage

Source: AFE Book 4 - Aircraft General Knowledge

I have to say, as a British PPL I would always assume "angle of attack" to mean relative to wind by default and would use angle of incidence to describe the physical design of the aircraft, but would likely understand from context. I.e., saying "The angle of attack on a Piper Warrior is x degrees" would make sense and be understood.

And, to be clear, we would NEVER use angle of incidence to describe the angle to relative airflow.

• Some parts of your answer do conflict with some answers in the other questions/ answers linked in my question. Maybe the usage I'm referencing is now confined to aerodynamics/ design textbooks, papers, etc, and not in widespread usage in the pilot community in Britain. I'd like to ask you if you would assume "angle-of-attack" to mean, in the whole-airplane context (a pilot is flying an actual airplane). the angle between the flight path/ airflow and the WING, or the the angle between the flight path/ airflow and the FUSELAGE, or no particular assumption either way. Commented Apr 2, 2020 at 19:20
• If this gets too involved we'll have to move it to chat-- here's a room I set up just for this sort of topic a few days ago-- chat.stackexchange.com/rooms/106037/… -- but you could probably get away with a brief reply as a comment here. Commented Apr 2, 2020 at 19:22
• PS -- the question has been edited to clarify. Normally I would worry about the edit invalidating an existing answer, but in this case the edit just reinforced the content that was already in the question without changing the question, and the existing answer was not completely on target with what was being asked in the question. Nonetheless your answer does shed light on the way these terms are most commonly used in the pilot community, so that's of value, even if not exactly on target with the question being asked. Commented Apr 3, 2020 at 2:30
• @quietflyer Ref your first question - I've never heard or been taught of the idea that the angle of attack is anything other than the wing vs the airflow. Obviously the only way we can alter that in flight is through pitching the whole aircraft so a lot of the time the distinction is immaterial but it's taught as all about the wing
– Dan
Commented Apr 3, 2020 at 8:28
• This answer would serve well as an answer (or could easily be adapted into an answer) to the related question aviation.stackexchange.com/questions/76682/… . Commented Apr 3, 2020 at 14:34