In American usage, the angle between the chord line of a wing and the flight path is called the "angle of attack". The angle of attack is also represented by the greek letter "alpha". However, some British authors, especially in the past, have referred to this angle as the "angle of incidence". (See one supporting reference here.)

We'll call this the "older British" use of the term "angle of incidence". How common is this usage now in the British aviation world? Has it completely fallen by the wayside, or is still common? Was it ever very common?

Also, what term or terms did authors and speakers following this "older British" usage, use to refer to the angle between the wing chord and the longitudinal axis of the fuselage (or some other similar defined datum line approximating the longitudinal axis of the fuselage?) Some of the terms that have been used for this angle by writers and speakers following the "older British" usage noted above, appear to include "rigger's angle" and "rigger's angle of incidence". Are there others?

(In American usage, this latter angle is in fact the angle that is called the "angle of incidence".)

(Bonus extra part: is there one particular greek letter that is normally assigned to this latter angle in engineering texts and papers, much as "alpha" is normally assigned to the angle that is described in the first paragraph of this question?)

  • $\begingroup$ Apparently the French have yet another system of terminology, but I'll save that for another question. $\endgroup$ – quiet flyer Mar 28 '20 at 22:52
  • $\begingroup$ This article would benefit from an original-source example of the "older British" usage of "angle of incidence" but I have none handy at the time. I definitely have seen references to this practice elsewhere than in the link cited above. $\endgroup$ – quiet flyer Mar 29 '20 at 1:44
  • $\begingroup$ On your link it is the chord line not the longitudinal axis $\endgroup$ – L'aviateur Mar 29 '20 at 8:42
  • $\begingroup$ You says yesterday that in american angle of attack is the angle between the chord and the relative wind.not the path $\endgroup$ – L'aviateur Mar 29 '20 at 8:44
  • $\begingroup$ Oh sorry :RELATIVE WIND is the relative movement of air relative to the plane; it is ALWAYS PARALLEL AND SENSE OPPOSITE to the TRAJECTORY OF THE AIRPLANE.volez.net/aerodynamique-technique/elements-aerodynamique/… $\endgroup$ – L'aviateur Mar 29 '20 at 8:55

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. Note that this contrast appears in technical books from the same publisher, just three years apart. It was, and is, no big deal.

In my experience, anybody with the slightest technical interest in flight is familiar with both usages and they both still get used interchangeably in the UK.

I have not seen a standard symbol for the rigging angle, presumably because it is of irrelevance to the aerodynamicist. It was once, and occasionally still is, alternatively referred to by its French name of décalage.

  • $\begingroup$ Thanks for added note in the last sentence, you can see that if a French person speaking English refers to the rigging angle as the "decalage" and the angle-of-attack as the "angle of incidence" there is opportunity for tremendous confusion in a conversation with an English-speaking person especially one unfamiliar with the older/less common usage of the term. Now add in one more twist for more potential confusion-- $\endgroup$ – quiet flyer Mar 29 '20 at 11:00
  • $\begingroup$ Now add in one more twist for more potential confusion-- a French person has recently informed me that a French term translating directly to "angle-of-attack" is used specifically to refer to the angle between the fuselage and the airflow. Stay tuned for another question seeking to verify whether this is really true in widespread usage. Was discussed here-- chat.stackexchange.com/rooms/106037/… $\endgroup$ – quiet flyer Mar 29 '20 at 11:02
  • $\begingroup$ I have never met anybody with whom a quick "by incidence do you mean angle of attack or rigging angle?" has not avoided the enormity you refer to. Vast numbers of English words have inherent ambiguity and one must take a pragmatic view of these things or one is doomed. It soon comes with experience, let the tyro beware. $\endgroup$ – Guy Inchbald Mar 29 '20 at 11:06
  • $\begingroup$ Yes it was just the perfect storm of two people with two different native languages using different and conflicting definitions for three different terms simultaneously without at first understanding what was going on. The "incidence" thing alone is easily resolved. $\endgroup$ – quiet flyer Mar 29 '20 at 11:09

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