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Why Would You Want To Change Your Angle Of Incidence?

Crusader Landing US Navy

Yesterday's post on the Vought F-8 Crusader brought up an interesting design parameter - the angle of incidence.

The Crusader has a variable incidence wing. At slow speeds, the front of the wing lifts up, increasing the angle of incidence. At cruise speeds and higher, the wing lies flush with the fuselage, reducing the angle of incidence.


F-8 Animation

But what is an angle of incidence, anyway? And why is it important?

The Angle Of Incidence

The angle of incidence is the angle between the longitudinal axis of the aircraft (draw a line from the spinner to the tail) and the chord line of the wing (draw a line from the leading edge to the trailing edge).

Angle Of Incidence

This is not the same as your angle of attack. Your angle of attack is the angle between your chord line and the relative wind.

Why is this angle important? Usually, you can't change it. (Well, unless you're in a Crusader.)

Takeoff

During your takeoff roll, the relative wind is parallel to your runway. The angle of incidence defines your angle of attack until you have enough speed (and tail down force) to lift your nose off. And, the angle of incidence reduces your pitch angle, giving you better visibility during takeoff.

Landing

During landing, the angle of incidence gives you the same advantage. The angle of incidence reduces the pitch angle you need to achieve a high angle of attack, giving you better visibility at slower speeds.

Does It Affect Lift In Flight?

No. The angle of attack and your airspeed determine how much lift your wing produces, and the angle of incidence has no effect. In flight, the angle of incidence doesn't affect the angle between the relative wind and the chord line. It simply changes the angle that the fuselage points - keeping the nose at a lower pitch angle.

Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at aleks@boldmethod.com.

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