What Is Angle of Attack? -- Three Critical Angles

PITCH ANGLE vs ANGLE OF ATTACK vs FLIGHT PATH ANGLE

Whether we are professional pilots or weekend warriors heading off for $100 hamburgers on Saturday morning excursions, it is very easy to fall out of touch with fundamental aerodynamic concepts. We hear them when we are learning to fly, and most pilots are generally familiar with their names. However, when a layman or (even worse) our instructor, asks us to provide an explanation of various aerodynamic concepts, we begin to realize they have often become fuzzy or hidden in a dark corner of our mind.

In this discussion, let’s briefly look at few “Critical Angles” we really should clearly understand as pilots and that are key to understand for Upset Prevention & Recovery Training (UPRT). For some, this will be a confirmation exercise; for others, it may be the first time these three critical angles have been brought together as a combined discussion and clearly explained. In either case, being precise about aerodynamic basics is a core component to ensuring safety of flight each and every day. With the understanding this short snippet of information below is meant to be a discussion generator, let’s proceed as follows:

1. Review the brief definitions of each critical angle,
2. Consider posing questions or posting feedback in the “comments” section below,
3. Fill out the information form at the end of the article to learn more about how UPRT uses technical knowledge such as this as well as on-aircraft training to mitigate the risk of Loss of Control In-flight.

DEFINTIONS:

Three important angle definitions are crucial to fully comprehending the relationship between Angle of Attack (AOA), Pitch and the aircraft’s Flight Path.

Angle of Attack (AoA):

• Is defined as the angle between the chord of an airfoil and the relative wind.  For us, it can be approximated as the difference between the pitch angle and the flight path angle.
• AoA determines whether the aerodynamic surfaces on the airplane are stalled or not.
  • For aircraft equipped with a Flight Path Vector, Flight Path Marker or Velocity Vector cue, the difference between the airplane symbol indicating pitch attitude, and the Flight Path Marker or Velocity Vector is a direct indication of the airplane’s angle of attack.

Flight Path Angle:

• Is the angle between the flight path vector and the horizon.
• Is also the climb or descent angle.

Pitch Attitude or Pitch Angle:

• Is the angle between the longitudinal axis of the airplane and the horizon.
• Is displayed on the attitude indicator or artificial horizon by the aircraft symbol.

Dumping the "Techno Jargon"...

Here's how you explain these concepts to your great grandmother who would rather drive from NY to LA than fly a lap around the traffic pattern with you. You'll need to be patient. However, with a little help from your favorite airplane model in-hand, she'll get it. If you want to learn something and maximize your ability to retain the information, teach it. Great Grandma is an excellent start.

Pitch:
Is the up or down attitude of the nose or wing. It’s where the airplane is pointed.

Pitch Angle or Pitch Attitude:
Is the angle between Pitch and the Horizon.

Flight Path:
Is where the airplane is going.

Flight Path Angle:
Is the angle between the Flight Path and the Horizon.

Angle of Attack:
Is the angle between Pitch (where the airplane is pointed) and the Flight Path (where the airplane is going). Often, the confusion about these three angles comes because we generally practice stalls only in the same level flight, slowly decelerating situation at 1G. In that context, a stall will always occur at the same speed and with a relatively consistent pitch attitude relative to the horizon. This leads pilots to associate the stall with the angle to the horizon, which they can see, as opposed to the angle of pitch compared to the flight path. The relative wind, airflow opposite to the flight path, is invisible without instrumentation or avionic references.

Critical Angle of Attack:
The critical angle of attack, where the airflow separates from the wing causing lift to be degraded, can be exceeded in any attitude, and even at different airspeeds if there is more or less than 1 G on the airplane. The airplane can even be stalled with the nose below the horizon, which is something most pilots have never seen before. Imagine the confusion a pilot might have with in an airplane that is stalled, above the 1 G stall speed, with the nose below the horizon.

 
 The good news is that regardless of the pitch attitude, the angle of attack can be reduced by lowering the nose (moving the yoke or stick forward) relative to the pilot. Correctly understanding these three angles, and their relationships is important to understanding the correct actions required to eliminate a stall, the most common result from unexpected airplane upsets.