Saturday, February 27, 2021
Home Flight Safety Keep it Simple; Angle of Attack!

Keep it Simple; Angle of Attack!

In recent blogs, we emphasized a “perfect picture” for each new student and also how and why it is critical to break the driving habit immediately. A good educator is eliminating obstacles and building solid habits while embedding actionable mental concepts. And now it is finally time to go flying.

Though the physics of lift thankfully works, it is unsettling for pilots at all levels that the best minds in science are still arguing about what actually makes it work. Most books present 2-3 conflicting theories with associated passion – and mathematical smoke and mirrors. It can all feel like childhood church stories – and even has the same Greek letters. We create even more confusion by over-emphasizing terms like “stall speed.” This concept is in all the books and even painted on the airspeed indicator. Imagine the confusion when we subsequently reveal “a wing can stall at any speed!” It is no wonder that pilots at all levels very quickly demonstrate this mental muddle on checkrides if you start to press this issue. Pilots need basic, actionable information when discussing what enables wing lift or even creates a basic turn.

To this end, I think the best starting point for discussing lift is “angle of attack” (AOA). The basics are deceptively simple; AOA is the angle of the chord line to the “relative wind.”  If you take the complicated lift equation (with the Greek letters) and remove all the constants, what you have left is the relationship between the speed and AOA. And as we know, we control AOA with elevator inputs.

Purists may chafe at this simplification but if flying requires calculus to be safe, we have bigger problems. Every airplane with a yoke (or stick) has a pretty good angle of attack indicator already installed – you don’t have to spend extra money or stare inside at LEDs. The more chrome you see showing on the yoke, the higher the angle of attack. If the yoke (or stick) is held all the way to the backstop, your plane is either stalled or at the highest (positive) angle of attack the manufacturer allowed by design.

“The position of the stick merely fixes the Angle of Attack and the airspeed at which the airplane flies as it descends.” Wolfgang Langewiesche Stick and Rudder , An Explanation of the Art of Flying.

“Relative wind” and AOA are invisible to the pilot, so a major misconception that must be actively purged and continuously discouraged is equating flight attitude with the angle of attack. This misconception seems almost intuitive in our minds and is subconsciously reinforced by diagrams like the one above. As educators and pilots, we must continuously emphasize (and remember) that a higher nose is not necessarily a higher angle of attack, and the nose does not have to be up high to stall a wing. One creative way to demonstrate this on the ground with diagrams is to present the same angle of attack in different flight attitudes:

That is exactly what the classic Aerodynamics for Naval Aviators does in a less colorful diagram. And though pictures have great value on a cognitive level, it is essential to fly to the edges of the flight envelope and experience these configurations. These do not have to be terrifying and are easily accomplished in a standard trainer.

In early training CFIs emphasize a concept called “stall speed.” This number is in all the POHs and even marked on the airspeed indicator. Then in the next breath, we explain a wing can stall at “any speed and any flight attitude.” If we do not carefully and fully explain all this, it is no wonder most pilots are confused (as are the instructors). Questions on a flight-test, at any level regarding stalls or AOA can quickly go sideways with poor preparation and understanding. It can help to play a few revealing YouTubes (I call this one the “perfect stall.” How did an F-16 stall while pointed down at the earth?

Carefully chosen YouTubes (I call this one “the perfect stall”) can be very helpful in creating a better understanding for your pilot-in-training. First comes “cognitive dissonance: “How is it possible to stall an F-16 while pointed straight down at the earth?” Then comes understanding (hopefully). Damn physics!

Another way to empower understanding is by demonstrating different pitch attitudes with the same AOA, and then different AOA with the same pitch attitude. This kind of practice disconnects these two concepts and creates more complete understanding. Both airplanes depicted below are at the SAME AOA (and same yoke position) but very different flight attitudes and configurations.  This nose-high flight attitude (scary for many pilots) and also the nose-low (incorrectly assume  “safe/comfortable”) have the same AOA. Safety is achieved by understanding that both are just as close to a stall – which could occur with any more pull/backpressure/AOA in either case.

“A wing is an odd thing, strangely behaved, hard to understand, tricky to handle. In many important respects, a wing’s behavior is exactly contrary to common sense.”  Wolfgang Langewiesche Stick and Rudder , An Explanation of the Art of Flying.

Once your training with different pitch attitudes progresses into stall demonstration and practice, students will assume that to stall the nose has to be UP and that the wing has to be flying slow (both serious errors). During initial training, we create benign 1G stalls and this reinforces the dangerous misconception that the nose has to be high to stall and that stalls only happen when the wing gets slow. We need to fix this huge (mostly intuitive) misunderstanding, to get to increase aviation safety.

The best method to teach stalls is to select a “too high” nose attitude (hopefully with a cloud reference). At this point, your pilot-in-training should know the Vy/Vx pitch references, so have them set and maintain a “too high” pitch attitude precisely and maintain this as the airplane deaccelerates. This maneuver will demonstrate the yoke continually moving aft (increasing AOA) to maintain the picture and more usefully achieve a stall. This is much more effective than the usual (and less helpful) “pull to the sky technique.” (BTW, an airplane that has leveled off in ground effect for landing is elegantly transiting this exact same range of AOA – except while “low and level.” Notice the yoke continuously moving backward while “flaring” creating this same ever-increasing AOA for a soft touchdown).

As students become more comfortable with stalls and recovery, demonstrate a full stall and maintain the excessive AOA while the nose drops though the horizon. Throughout this maneuver, the yoke is held all the way back (same AOA/wing stalled) as the nose falls and the flight attitude changes. Recover only when the nose has fallen through the horizon. Secondary stalls are also a great way to kinesthetically reinforce the larger flight envelope and demonstrate the danger of “nose low” stalls (and possibly experience stalls at some higher G load). After these demonstrations, AOA will become more apparent. These essential demonstrations are not part of the normal flight training syllabus or required in any FAA ACS, but they are critical to creating a safe and confident pilot.

It takes some time and a caring relationship to introduce stalls correctly and not scare a pilot-in-training. If your student has not yet mastered coordinated flight (especially during climbs) it is too early to introduce stalls. The result will be predictable (and your fully scared student will probably drop out). A much better use of early flight time is demonstrating stability in the aircraft due to the clever aerodynamic design. Trim for an airspeed and raise the nose demonstrating how the plane will return to the trimmed speed/AOA. Trim a speed and add/reduce power demonstrating how the plane will seek that same speed/AOA. At least half of private pilot applicants are not aware the tail “lifts down” (and some CFIs do not know this either) providing dynamic stability for an aircraft in flight. Once pilots understand the nose is the “heavy end” and that recovery will take care of itself they have a greater sense of confidence and understanding of the physics involved. Planes don’t stall capriciously, *pilots* stall planes! Just because a plane *can* stall in any flight attitude does not mean that it *will.*

All of these concepts are a huge load to assimilate during early flight training, so patience and meaningful repetition is essential to successfully navigate this rush of information and new experiences. I would guess of the 80% of pilots who drop out during flight training, more than half would identify being scared of stalls (introduced inappropriately and too early) as the primary cause. Fly safely out there (and often)!

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