CFI Topics

Forces on Aircraft in a Glide

Lesson 3

Aircraft performance is determined by the balance of forces along and perpendicular to the flight path of the aircraft. Figure 1 shows the aircraft established in a glide. The forces that act on the aircraft during the glide are lift, drag, and weight, with the thrust set to zero. Note that there may be some small amount of thrust at the propeller when the engine is idling, but for this discussion we will assume it is identically zero. Although CFI’s are always discussing the concept of angle-of-attack, what is usually missing in the discussion is the concept of what is called flight path angle, which is the direction of the velocity vector of the aircraft in relationship to the local horizontal.

Figure 1

Figure 1: Forces on an Aircraft in a Glide

Figure 1 also shows the angle-of attack (⍺) and the flight path angle (𝘺). In addition, there is a simple relationship between the flight path angle, the angle-of-attack and the pitch angle of the aircraft. The pitch angle is the angle between the longitudinal axis of the aircraft and the horizontal plane, and is also shown in Figure 1. 

It is positive when the longitudinal axis is above the horizontal plane and negative below the horizontal plane. If we assume the average wing incidence angle (which is the angle between the chordline of the wing and the longitudinal axis of the aircraft) is zero, a simple relationship exists between the three angles. This relationship is shown below.

Pitch Angle = Flight Path Angle + Angle-of-Attack 
Equation (1)

The positive sign in equation (1) is required in order to account for the fact that the flight path angle is negative (i.e. below the horizontal) and the angle-of attack is positive (i.e. the chordline is above the velocity vector).

Although flight instructors usually emphasize the importance of angle-of-attack, what is interesting is that both the angle-of-attack and flight path angles are angles that are in general not able to be visualized by the pilot in flight. In fact, the only time the pilot can visualize the angle-of-attack in flight, is when the flight path angle is zero, i.e. when the aircraft is in level flight. In this case, the angle- of-attack is equal to the pitch angle as can be seen by the above simple relationship. As an example, when we are performing slow flight at constant altitude, the pitch angle is essentially the angle-of-attack.

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