The Wing to Parachute Conversion

If you’re like me, the first time you went out to stall an airplane was probably a stressful experience. The reality of that experience has rung true in some form or another for me as I look back on ten consistent years in the cockpit. After getting my private, I remember only a few hesitant attempts at stalling an airplane until I received more training at the commercial and CFI levels. Then, as a new CFI, I can remember the pit in my stomach the first few times I had to go up with students to stall airplanes. From there, the fear slowly left me but was still present in my students. I was then surprised to find this same concern and trepidation among my fellow professional pilots at my day job. In short, I have come to find that fear of stalls is a universal reality among pilots. Where does this fear come from? I hope to get at a small aspect of it in the paragraphs below.

When I sit down with a new student pilot on day one, the topic of stalls is hot on my lips. However, I first ask them an unlikely question. “Can monkeys fly airplanes?” I ask. To this, I usually get an odd look and a hesitant answer. The correct answer to this question is a resounding “No”. However, the “why” to this question is far more important than a simple answer. I then explain that monkeys can’t fly airplanes for one important reason. That reason is their lack of comprehension and application of knowledge. Besides the fur and love for bananas, monkeys can’t read a book or listen to a talk on flying and then go try it out. We humans, on the other hand, can do this. We learn theory first, and then we apply that theory to reality, shaping our understanding and skill. So, you might ask, what knowledge do we need to get to the bottom of adequately understanding and applying stalls? Read on to find out.

When pilots stall an airplane, we need to know what is happening to our wing and the forces acting on it. For example, if you read NASA’s educational pages on aerodynamics, you will find the figure on the left at the top of this article. This figure is the classic Angle of Attack versus Coefficient of Lift graph with which we are all familiar. In short, this graph says that as AoA increases, lift increases proportionally. Then as we approach the critical AoA, lift peaks and then decreases to the end of the curve. The problem with this graph is the “drop into the abyss” past the point of peak lift. Without saying so, this graph would have you believe lift drops to zero once an airplane stalls. Whoever made these graphs likely didn’t intend for that message to get out and was just done graphing useful data. Unfortunately, this miscomprehension of the data on our part as pilots is just flat-out wrong. Look at the figure in the top right of this article. Believe it or not, lift doesn’t drop to zero post-stall. As seen in the second figure, it can do quite the opposite, increasing beyond the lift achieved just prior to the stall. The problem, though, is that this increase in lift post-stall comes at a considerable drag penalty.

So how do we reconsider our understanding of stall with this information? I like to explain the stall to my students as the “Wing to Parachute Conversion Point.” When we stall an airplane, we go from efficiently creating lift with our streamlined wing to making some lift and lots of drag. This isn’t all bad because sometimes drag can be a good thing. Drag is the force that makes a parachute descend gently to earth. Drag does something similar for us: it sets up a steady descent that keeps us from falling to earth like Wile E. Coyote. This is why the bottom doesn’t fall out from underneath you when you do a stall, in contrast to what you may feel on one of those vertical drop amusement rides. If you closed your eyes while someone else did a stall, you probably wouldn’t feel much at all besides the usual gentle buffet. Now, will drag lower us to earth as light as a feather? Absolutely not, but it’s not the drop off the edge of a cliff many of us think it is. In fact, it sets up a nice nose-low descent, allowing us to ease the stick forward and get the airplane flying efficiently again.

So, in conclusion, if you approach your stall practice with this newfound fact, you will eventually find a few nice-to-know realities.

1. Properly designed and loaded airplanes only stall when pilots pull the stick back to the stalling point.
2. Properly designed and loaded airplanes come nicely out of the stall when we let the stick forward again.
3. You must be aware of your control inputs to know if you are pulling back on the stick.
4. Flying in trim is the easiest way to tell if you are headed for aft stick and an accompanying stall.
5. A proper aerodynamic recovery from a sufficient altitude with sufficient power makes for a nonevent.

Happy stalling!

 

References

https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/incline.html

https://ascelibrary.org/doi/10.1061/%28ASCE%29AS.1943-5525.0000607