It's complicated

Despite the high-pitched wailing, my student—and son—Jack kept pulling back on the Cessna’s yoke in the hopes that the nose would drop. Once it did, he would then complete the stall recovery by releasing back-pressure, adding power, and commencing an efficient climb away from the terrain. Sometimes, that’s exactly how it went. Other times, despite quick and appropriate rudder movements, one wing dropped, and he risked deviating from ACS standards by allowing the nose to wander by more than 10 degrees laterally.

In our post-flight chat, Jack asked why it seemed tough to keep the wings level during stall practice. I understood his frustration and explained, “Well, it’s complicated.” I suggested we grab a bite to eat and enjoy a leisurely discussion on exactly how a wing produces lift.

I drew diagrams of an airfoil, a 2-dimensional slice of the wing, and helped him visualize the flow around it as the engine propels the airplane through the air. At low angles of attack, the flow is attached to the airfoil. As the angle of attack increases, so does the lift, but the flow starts to separate at a point before the trailing edge. This separation bubble features backward flow along the top of the airfoil and grows as the angle of attack increases. When the separation bubble envelops most of the airfoil, the angle of attack becomes critical, and lift drops precipitously. I showed Jack flow visualizations by Alexander Lippisch, similar to those in the illustration here, that confirmed the information in my drawings (“No Equations Required,” November 2019 AOPA Pilot.)

Now, what makes a wing stall is far more difficult to explain than the same for an airfoil.

A wing is a bunch (OK, infinitely many) of these airfoils glued together, starting at the fuselage, and ending at the wing tip. For our purposes, thinking of each wing section as an inch wide with about the same airfoil-shaped cross section and dimension works just fine. The total lift produced by the wing is the sum of that produced by each of these sections and should be at least the weight of the aircraft to sustain flight. But lift generated by the wings has other effects, too.

If airflow across the wing were perfectly parallel to the longitudinal axis, analyzing a stall would be much easier. The vortex action around the wing tip means that flow below the wing moves outward toward the wingtip and that above the wing travels inward toward the fuselage. For the simplest rectangular wing, that means the sections near the wing root achieve their critical angle of attack before the outboard sections. And that’s a good thing since the pilot feels a buffet that warns of a stall while the ailerons, located near the tips, are still effective. The downside of a rectangular wing is the inefficiency of that relatively large vortex action.

The most efficient planform is elliptical, but it comes with a high manufacturing price. By using straight sections but introducing a taper toward the wing tip, designers capture a remarkable amount of elliptical efficiency with a smaller financial cost. However, this new planform loses the nice rectangular planform stall characteristics. To force the inboard section of the wing to stall first, designers increase the angle of incidence on the sections near the wing root and lower that of the sections near the wingtip. This washout is so subtle that it’s typically not noticeable looking at an airplane on the ramp. Propeller blades are also designed with washout (for a different reason) and the effect is obvious to an observer.

Each small wing section, with its varying dimensions and angles of attack, also produces a rotational moment about the longitudinal axis equal to the force it generates times the distance to the middle of the fuselage. We often say that, for the wings to stay level, the two wings need to produce the same amount of lift, but that’s not actually true. What is correct is that the total moment (the sum of lift times the distance to the middle of the fuselage for each wing section) produced by each wing being equal and opposite is what prevents roll. The lift provided by the sections near the wing tip are far more influential in the total moment than those near the fuselage, a good reason to install ailerons on the outboard portion of the wing. Small differences in lift produced on various sections of the wing can make a big difference in the left- and right-wing moments. Dropping a wing in flight just shouldn’t be so unusual.

Of course, this is true at flight across the airspeed spectrum—in cruise flight all the way down to a stall—so why does it seem like wings are more likely to drop when practicing stalls? The answer goes back to the shape of the lift curve. At low angles of attack, such as in cruise flight, a wing that drops achieves a higher angle of attack and the attendant extra lift serves to return it to level. But near or beyond the critical angle of attack, a dropping wing with its increased angle of attack produces less lift so the wing tends to drop more. So, roll stability at low angles of attack is sufficient to mitigate variations in lift moments but, near the stall, the lack of it exacerbates them.

This discussion shows how complicated these ideas really are. What do we mean when we say a wing is stalled when perhaps some sections are, and others aren’t? And, unlike an airfoil, there is no one angle of attack for the wing. I told Jack that when we use those terms to think of them more in an average sense, but their meanings are still nebulous.

Jack already knows that, once the full stall occurs, he should assist in the recovery by pushing forward on the yoke, setting maximum power, cleaning up the aircraft, and commanding the most efficient climb to gain altitude. This information is readily available in all the literature for student pilots.

What I don’t see often is the fact that difficulty maintaining directional control warns of a stall as much as does an aircraft nose that drops through the horizon. In such a situation, he should declare that a stall has occurred and use the same recovery procedure. Once he pushes forward on the yoke and lowers the angle of attack, directional control becomes easy with roll stability there to assist.

Teaching my son Jack to fly in our Cessna Aerobat Wilbur has been an honor for me and intellectually fun for both of us—he loves the technical aspects of aviation as I do. Aviation is nothing new to him, as we’ve sharedflying adventures since he was a baby. Since Jack moved away to study mathematics in graduate school, he comes to Sewanee more infrequently, so I cherish the times he’s home. I’ve always loved being his mom, but serving as his flight instructor adds a wonderful new dimension to our relationship.

His safety, as well as that of his passengers, is of utmost importance. So, Jack knows that when the relationship between the airplane's wing and the lift it generates becomes compromised, no matter how complicated that relationship is, pushing is the simple answer to regain safe, predictable flight.