Here’s something that took me longer than it should have to drill into my brain: pitch angle is NOT the same as angle of attack.
Both of them describe where an aircraft is relative to its surroundings. But:
- Pitch angle is the angle between the aircraft’s centerline and the horizontal—effectively, flat and level ground.
- Angle of attack is the angle between the aircraft’s centerline and the relative wind, i.e. the incoming airspeed vector that the aircraft “sees” in flight.
This is pretty basic, especially if you’re familiar with aerodynamics. The more important of the two, though, is angle of attack—specifically because it relates to stall.
Speaking generally, an airplane wing can stall in one of two ways:
- Flying slow enough, or at high enough altitude, that the wing no longer experiences enough pressure to generate the lift needed, OR
- Increasing or decreasing the wing’s angle of attack to the point where the air starts to break away, or “separate”, from the upper surface and loses lift
A stalled wing means your airplane starts falling out of the sky. We’d like to avoid stall. That makes angle of attack the more important of the two to monitor.
As long as our angle of attack is low enough to stay away from stall, our pitch angle can be basically whatever we want. Conversely, it also doesn’t matter how fast we’re going if our wing is at such a high angle with our relative wind that it’s no longer generating lift.
In straight and level flight, you can often say that the aircraft’s relative wind is parallel to the horizontal. Because of this, it’s easy to get into the habit of using “pitch angle” and “angle of attack” interchangeably—I’ve definitely done it and I’m sure my colleagues have done it too.
The angles we’re talking about are typically in the single digits, since many aircraft cruise at an angle of attack well under 8 degrees. If it’s such a small angle, referring to angle of attack as pitch angle (or vice versa) doesn’t seem like that big of a deal, right?
Right—as long as you actually know the difference, and can distinguish the two when it matters.