Angle of Attack & Angle of Incidence
Many pilots have difficulty in understanding these two very important terms, and the difference between them. This tutorial attempts to explain the difference, but also the inter-relationship between them.
Angle of Incidence
The Angle of Incidence is the angle between the Chord Line of the wing and the Longitudinal Axis of the aircraft. It is chosen by the designer of the aircraft to provide an attitude of the aircraft in cruise flight, which is comfortable for the crew. The attitude should be neither too high, causing the pilot to have to stretch to see the outside horizon, nor too low, giving a feeling of descending.
The Angle of Incidence on a particular aircraft is fixed - once the aircraft is built it never changes, regardless of whether the aircraft is in flight or stationary in the hangar.
Angle of Attack
The Angle of Attack is the angle between the Chord Line of the wing and the Relative Air Flow (sometimes called the Relative Wind). When the aircraft is stationary on the ground in still air, there is no Angle of Attack, however as soon as there is relative motion between the air and the wing, there is an Angle of Attack.
In cruise flight, when the Relative Air Flow is parallel to the Longitudinal Axis, the Angle of Attack and the Angle of Incidence are the same value by design, but this only occurs at this one specific stage of flight! Even though they are the same value, they are NOT the same thing.
The Angle of Attack varies for a variety of reasons:
- The pilot changes the pitch by pulling back or pushing forward on the stick.
- A reduction or increase in speed will require a change in Angle of Attack.
- The direction of travel is changed - eg climb or descent.
- A heavier aircraft at a given speed needs to be flown at a higher Angle of Attack.
- Fuel burn throughout the trip, therefore reduced weight, requires a change in Angle of Attack.
Relative Air Flow
It's important to understand that an aircraft rarely travels in the direction it's pointed. The diagram below shows a Cessna 172 in cruise flight with the longitudinal axis parallel to the ground or horizon, but if it was a little heavier, it would need to fly at a higher attitude, hence a higher angle of attack, to maintain level flight at the same speed. The Relative Air Flow is always directly opposite the direction the aircraft is travelling, not the direction it's pointed.
In the diagram below, the pilot has reduced power to cause the aircraft to descend, but the same attitude is maintained so as not to increase the airspeed. Note that as the direction of travel is now downward, Relative Air Flow is meeting the wing from below, and the angle of attack has increased.
In the next diagram, the pilot is maintaining level flight at reduced power (the aircraft has been slowed down). To compensate for the lift lost from reduced airspeed, the aircraft has been pitched up, increasing the Angle of Attack, thereby maintaining lift constant. Refer to the tutorial on Lift Equation.
Have a look at the different situations below, and see if you can work out what's happening in each case, and why.