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I know that the angle of incidence is the angle between the chord line of the wing and the longitudinal axis of the fuselage. I know that it doesn't change in flight because it is fixed.

And usually, there is a positive angle of incidence on airplanes, which means that the wing kinda looks like this:

credits: NAVAL AVIATION SCHOOLS COMMAND aerostudentguide(dot)pdf

credits: Naval Aviation Schools Command aerostudentguide(dot)pdf

My question is, why is there a need to incline the wing like that? Why is there a need for Angle of Incidence?

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Strictly speaking, there is no need to set a particular angle of incidence- the wing will decide that for itself depending on the conditions (speed, weight, altitude etc.). What we are deciding is the The mounting angle, which is set for various reasons and is not variable in flight (except for some rare cases) including:

  • Mostly this is set so that the fuselage is (nearly) horizontal during cruise. This is especially important for airliners- DC-10, especially flew at a pronounced nose high attitude requiring the cabin crew to walk uphill.

  • Setting the wing at an incidence helps improve the visibility- this is important especially for carrier aircraft, where the pilot requires good visibility and also higher angle of attack (for keeping t/o and approach speeds low).

  • Setting the wing at an angle helps in keeping the drag low for the given lift. Having the wings at an angle and fuselage horizontal means that the drag is minimized, while the wing has the required angle of attack.

Though the angle of incidence is usually fixed, it can be varied in response to specific requirements. A good example is the Vought F-8 Crusader, which allowed the wings to be pivoted 7° out of the fuselage on takeoff and landing, resulting in a high angle of attack -- reducing the approach and take-off speed -- while keeping the fuselage level and giving the pilot a good forward field of view.

F-8 Crusader

F-8 Crusader with the wing pivoted up during landing. By USN - U.S. DefenseImagery photo VIRIN: DN-SC-88-06695, Public Domain, Link

Another example is the Martin XB-51 which had a variable incidence wing to reduce the takeoff run.

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  • $\begingroup$ This very good answer might be further improved by changing the first sentence to something like "Strictly speaking, there is no need to set a particular angle of incidence- the pilot may place the wing at the appropriate angle-of-attack to the airflow to obtain the desired performance parameter (airspeed, climb or descent rate, etc) by deflecting the elevator as needed, regardless of the incidence designed into the wing" $\endgroup$
    – quiet flyer
    Apr 3, 2020 at 18:01
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The wing will pick the angle of attack that is appropriate for the given speed, air density and the needed lift. If it is too high, the aircraft will accelerate upwards which will reduce the angle of attack, and vice versa. The wing's angle of incidence will then define the angle at which the fuselage sits during flight. What a proper angle is depends on the aircraft:

  • On transport airplanes, it makes sense to have a horizontal floor during cruise so the flight attendants don't have to push their carts uphill. This will result in a slightly positive incidence angle.
  • On high performance airplanes the incidence is set to optimise the lift to drag ratio.
  • On aerobatic airplanes it is set to zero so the aircraft can be inverted without elevator input.
  • On crop dusters, it might be important to give the pilot the best possible view so he/she can fly safely while manoeuvring close to the ground.

The next important detail is the height of the tail surface relative to the wing's vortex sheet. If the tail is too far above, it will sink into the wing's wake during stall and become less effective. A deep stall stability condition might be the consequence. Only by defining the angle of incidence can the airplane be built and operated as designed.

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My understanding is this. One of the design consideration of fuselage design is to avoid flow separation. And another important thing is that usual looking fuselage is not very efficient at providing lift because both above cases produce huge amount of drag.

So it is important to avoid the fuselage from separating flow and non-lifting/less lifting in normal configuration in cruse condition. To do so its important to keep the fuselage nearly horizontal

Secondly to produce max L/D or nearly max L/D wing has to operate in some positive AoA.

This can be achieved by introducing this incidence angle.

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    $\begingroup$ +1. The correct answer. You could improve slightly by explaining AoA abbreviation and a lift vs AoA curve/graph. $\endgroup$
    – Notts90
    Oct 21, 2016 at 11:21
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Let us denote $\delta_{wing}$ the incidence of the wing. For moderate values of the angle of attack of the airplane $AoA$ the lift coefficient of the wing has a linear behavior : $$Cl_{wing}=\frac{dCl_{wing}}{dAoA}(AoA+\delta_{wing}-AoA_0)$$ Where $AoA_{0}$ is the value of $AoA+\delta_{wing}$ for which the lift coefficient is zero. $AoA_0$ is a constant. Basically the incidence of the wing increases the effective angle of attack of the overall wing.

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  • $\begingroup$ what we call THE angle of incidence of the plane is the angle between the velocity vector of the aircraft and the longitudinal axis of the aircraft. It is often denoted AoA The question clearly states angle of incidence ON airplanes not OF airplanes $\endgroup$
    – Federico
    Oct 21, 2016 at 7:48
  • $\begingroup$ For me it is unclear, aerodynamicists call this the setting angle. In the first sentence @Trekslofar says : "I know that the angle of incidence is the angle between the chord line of the wing and the longitudinal axis of the fuselage." Which said like that sounds completely wrong. $\endgroup$
    – H.C. Lefevre
    Oct 21, 2016 at 8:03
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    $\begingroup$ never heard of "setting angle", always read about "incidence angle" and Trekslofar's definition is correct. $\endgroup$
    – Federico
    Oct 21, 2016 at 8:05
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    $\begingroup$ While studying for my degree this was referred to as the angle of incidence and the angle you describe was referred to as the angle of attack. Never heard of setting angle. $\endgroup$
    – Notts90
    Oct 21, 2016 at 11:17
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    $\begingroup$ I did part of my degree in French (as I am French), my apologies. What I am sure of is that we call it "calage de l'aile" in French. I mixed Angle of Attack and what you call incidence angle. I consequently edit my post. $\endgroup$
    – H.C. Lefevre
    Oct 21, 2016 at 11:52
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You understand that lift is generated by the action of the wing pushing air molecules down as it moves forward, requiring an angle of attack to the air passing under the wing? So if you don't tilt the wing with respect to the body of the aircraft, then to get the angle of attack required to generate lift, you would need to tilt the body as well, greatly increasing the (non-lift generating) surface area and so greatly increasing the drag.

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Angle of incidence is required to generate lift if the roll axis is parallel to the airflow. Otherwise the fuselage would also be required to present AoA in order to generate lift. The wing's AoA = AoA of roll axis + angle of incidence and AoA is the primary creator of lift, not the shape of the wing, contrary to popular belief. Otherwise how could planes fly inverted?

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  • $\begingroup$ The fly inverted in exactly the same way they fly right side up - by accelerating air downwards and proving Newton right once again. $\endgroup$
    – Simon
    Apr 19, 2017 at 16:12
  • $\begingroup$ they fly inverted by having negative AoA, whose value depends on the shape of the wing and the incidence angle. the shape has a massive influence. $\endgroup$
    – Federico
    Apr 19, 2017 at 18:14
  • $\begingroup$ "AoA is the primary creator of lift, not the shape of the wing, contrary to popular belief" So why do they spend so much money in wind tunnels when a protractor would be sufficient? $\endgroup$
    – mins
    Apr 19, 2017 at 18:31
  • $\begingroup$ Angle of attack is required for a symmetric airfoil to generate lift. It is not required for an asymmetric airfoil to generate lift. Those can indeed generate lift at 0 AoA. Of course, you probably don't want one of those on a plane that is designed for actual inverted flight (e.g. fighters and aerobatic planes.) Note that the barrel rolls you can do in any plane aren't actually inverted flight; the wing is still flying right-side-up relative to the airflow. Of course, more AoA will still normally generate more lift on an asymmetric wing, just as it does on a symmetric one. $\endgroup$
    – reirab
    Apr 19, 2017 at 20:38

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