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 know they are there 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. 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. In either case, remaining clear on 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:
- Review the brief definitions of each critical angle,
- Watch the included flash animation dealing with these concepts in a practical "slow down" situation, and finally
- Consider posing questions, or simply posting feedback, in the Airplane Upset Recovery Training forum under the following topic:
http://www.upsetrecovery.com/forum/viewtopic.php?f=5&t=26
DEFINTIONS:
Three important angle definitions are crucial to fully comprehending the relationship between Angle of Attack (AOA), Pitch and the aircraft’s Flight Path. Please review the definitions below followed by a viewing of the included flash animation.
Angle of Attack:
- Is the difference between the pitch attitude and the flight path angle.
- Determines whether the aerodynamic surfaces on the airplane are stalled or not.
Flight Path Angle:
- Is the angle between the flight path vector and the horizon.
- Is also the climb or descent angle.
On the newest generation jet transports, flight path angle can be displayed on the primary flight display (PFD) as shown in animation below. Flight path angle can also be inferred from the vertical speed indicator (VSI), or from the observed rate of change of the altimeter, in relation to a known ground speed.
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.
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 where the wing/nose is pointing.
Pitch Angle or Pitch Attitude:
Is the angle between Pitch and the Horizon.
Flight Path:
Is where the wing is going through the air.
Flight Path Angle:
Is the angle between the Flight Path and the Horizon.
Angle of Attack:
Is the angle between Pitch and Flight Path.
| FLASH: INTEGRATION OF PITCH, AOA AND FLIGHT PATH ANGLES This animation depicts these three critical angles in a developing stalled flight scenario during a thrust-deficient "slow down". Please turn up the volume and click the image in the lower right portion of the box below to launch the animation. |
In my opinion, the equipment used by APS and the experience level of it's instructors are especially well suited to the corporate and airline training market.
The AoA is not the Angle between pitch attitude and flight path angle! It is the angle between an aerofoil and the relative airflow! Extreme situation: a military jet which is heading straight up into the sky has an angle of attack of about 0° whilst it angle of flight path is 90°!
Hi Mathias (I'm assuming this is your first name from your email address - apologies if that assumption is in error),
It seems there has been a misunderstanding in terminology. It is correct that the definition of Angle of Attack can be the angle between the aerofoil and the relative airflow. As a fighter pilot, I can relate to your practical example. However, in accordance with the Airplane Upset Recovery Training Aid (URTA), the internationally recognized standard, the broader scope definition of Angle of Attack is the angle between a reference line and the relative airflow as follows: URTA 2.5.5.1 -
"Depending on the context in which it is used, aerodynamicists use the term 'angle of attack' in a number of ways. Angle of attack is always the angle between the oncoming air, or relative wind, and some reference line on the airplane or wing. Sometimes it is referenced to the chord line at a particular location on the wing; sometimes to an 'average' chord line on the wing; other times it is referenced to a convenient reference line on the airplane, like the body reference x axis. Regardless of the reference, the concept is the same as are the consequences: exceed the critical angle of attack and the lifting surfaces and wind will separate, resulting in a loss of lift on those surfaces. Frequently the term 'airplane angle of attack' is used to refer to the angle between the relative wind and the longitudinal axis of the airplane. In flight dynamics, this is frequently reduced to simply 'angle of attack'."
... as the Flight Path Angle represents the orientation of the relative airflow in reference to the horizon and pitch attitude is also in reference to the horizon (often representative of the bore line or longitudinal axis of the fuselage), it is equally correct to state Angle of Attack can be the angle between the flight path angle and the pitch attitude. In your fighter aircraft example, the flight path of the airplane is 90 degrees nose up as is the the pitch attitude. Hence, the AOA is near zero or possibly even slightly negative if the aerofoil is asymmetric and the flight path is sustaining a vertical up line over time. Of more use to pilots unable to clearly picture AOA, to coin a phrase from one of our respected colleagues Mr. John Cox, CEO of Safety Operating Systems: "Angle of Attack is simply the difference between where the wing is pointing and where the wing is going ..."
This is an important topic Mathias, and I'm glad you initiated the discussion.