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The Truth about “Wing Incidence”

Gemini Sparkle

Key Takeaways:

  • The article argues that "wing incidence" is a misnomer and largely unimportant; "fuselage incidence" is a more appropriate term, referring to the fuselage's angle relative to the airflow, which is determined by the wing's varying angle of attack.
  • Fuselage incidence is primarily set by designers to minimize drag at an average cruise condition, and can also be influenced by practical considerations such as passenger comfort or tail clearance during landing.
  • More critical than individual incidence is "decalage"—the difference in angles between an airplane's lifting surfaces (e.g., wing and stabilizer)—which is essential for longitudinal stability, though its precise value is not fixed in monoplanes.
  • While conventional aircraft are generally forgiving of imperfect incidence or decalage settings, getting decalage wrong in canard configurations can lead to dangerous, unrecoverable stalls.
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(February 2012) Books about airplane design often mention wing incidence as if it were a parameter of some importance. It isn’t. In fact, “wing incidence” is a misnomer. I propose — and fully expect my proposal to have no effect — that the term be abandoned, and that we speak of “fuselage incidence” instead.

The so-called angle of incidence is the angle between the chord line of the wing at the fuselage and the fuselage’s longitudinal axis. Now, this sounds as if it ought to matter, because after all, if the wing is not at the correct angle it will not have enough lift, or will have too much, and the airplane will mush, or plow, or otherwise behave inappropriately. But to think that way is to slip into the mindset of the lubber who says, “The airplane flew into a storm and the wings fell off.” Wings never fall off. It is the fuselage that falls off the wing.

Peter Garrison

Peter Garrison taught himself to use a slide rule and tin snips, built an airplane in his backyard, and flew it to Japan. He began contributing to FLYING in 1968, and he continues to share his columns, ""Technicalities"" and ""Aftermath,"" with FLYING readers.

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