Turning Stalls

The relationship between angle of bank and stall speed isnt a mystery, but it is a bit complicated. Heres whats going on, and why.


By Jeff Pardo

The term stall usually suggests a steeply climbing airplane or the cessation of motion, but we should remember from our initial ground school that an airfoil can be stalled at any attitude and any airspeed. Even so, when a pilot thinks about stalling in most flight operations, it is often with the airspeed indicators green and white arcs in mind.

When we induce a stall deliberately, it usually does still involve a nose-high attitude. But the reason for these rehearsals is usually not to practice the loss of control, but instead to learn how to recognize its onset, how to maintain control and how to recover with a minimum loss of altitude.

The last time you practiced stalls, you were most likely either with a CFI, or by yourself. You performed clearing turns, enriched the mixture and then, if you were planning to do approach to landing stalls, perhaps you reduced power along the way and added carburetor heat if needed, slowed to VFE, extended flaps if you had them, and then increased pitch attitude and simultaneously decreased power while maintaining a constant altitude.

Soon, things got mushy and then…you felt that sinking feeling. If they were takeoff and departure stalls, after the clearing turns and going to full rich on the mixture, you probably reduced power and slowed to a nominal climb speed, then added full power and pulled back on the yoke until you were in a steep climb attitude. You tried to keep the wings level, until … things got mushy again.

Practice Makes Perfect?
Why bring up this routine stuff? To be perfectly blunt, I do so to incite rebellion: I wish to hasten the ongoing paradigm shift in flight training.This is because performance maneuver-related hoops such as these dont provide the best path towards becoming a better pilot. In fact, in the case of stall maneuvers, this particular approach could conceivably increase your chances of having an accident.

In truth, my first real reaction was a dread of power-on stalls, a feeling that has since matured a bit, thanks to a more comprehensive understanding of the coordinated use of rudder. But why do I say that stall practice might make an accident more likely? Thats because the one thing that most methods of teaching stalls have in common involves putting the airplane into that emblematic nose-high attitude. Doing so gives fledgling pilots the impression-subconsciously as well as overtly-that an airplane will stall only if you try to stand it on its tail. As most of us should have learned by now, this is wrong!

In addition, stalls are usually practiced at fairly low airspeeds. In the classroom, we learned that stall speed increases in a turn, during pull-ups or in turbulence. But thats without any visceral gut feeling, muscle memory or experience to back it up. Reading just isnt nearly as effective for learning as is doing.

No Surprises
When a stall catches a pilot by surprise, its often because he or she flew the airplane too slowly to compensate for actual increased weight or an effective weight increase induced by load factor (for instance, when one is in a turn). Other contributing factors can include increased density altitude, simply not enough wing area and/or camber (e.g., flaps), or perhaps a wing surface roughened by ice, or any combination of these.

The operations most often involved are the so-called departure stalls during takeoff and during cross-controlled turns when one has overshot the turn from base to final approach. Other instances can include go-around maneuvers when power is added at a low speed before the airplane has been properly re-trimmed (or if the flaps have been retracted prematurely), a lack of attention to airspeed during approaches to landing, an overly abrupt recovery from a sudden high sink rate on short final or, with accelerated stalls, during overly tight turns.

In the base-to-final scenario, a spin often results, which is a vicious asymmetric stall in which the aircraft is also rotating about its yaw axis.There are also equally awful versions of stalls such as tailplane stalls in icing conditions (despite the rule of decalage which implies that the airfoil in front should have the greater angle of incidence, and stall first).

When the tail stalls, the nose drops irreversably due to a loss of normal tail down force, unless the tails airfoil can be made to produce lift. And there are turbulence-induced accelerated stalls.

Then there is what happens when an airplane is in a very rapid descent, and then the pilot suddenly pulls back mightily when he sees the Earth rushing up to meet him. This is a textbook accelerated stall: high airspeed, nose-low attitude, followed by a loud noise and then silence. Dead silence.

Banked Relationships
When an aircraft rolls into a bank it is the horizontal component of the lift vector that provides the centripetal turning force, while the vertical component of lift opposing gravity decreases according to the cosine of the bank angle. The cosine, by the way, is just the trigonometric ratio between the side of a triangle that is adjacent to a particular angle and the longest side of that triangle.

For small angles, the two sides are nearly the same and so the cosine is almost one; for large angles like 89 degrees, its quite a small number. The load factor (which is just the ratio of the lift the aircraft is producing at that bank angle to its normal weight) increases with (and equals) the reciprocal of that cosine, and we must apply more back pressure to increase the angle of attack to generate the greater lift required.

The speed at which the airfoil stalls, however, actually goes up with the square root of the load factor. Note here again though-and remember this well-that those white and green arcs on your airspeed indicator are only good at one unit of gravity. In a bank, theyre just a distraction.

In a stalled condition, typically the influence of the ailerons on controlling roll has become drastically reduced, while at the same time adverse yaw effects increase, and in general, mostly adverse properties of balance and control result.

Of course, the untrained and instinctive response is to turn the ailerons away from an alarming wing drop, which makes it worse as the downward aileron on the lower wing only further intensifies its stalled state. The correct response is of course to use the rudder. An even better answer is for one to mitigate against the need for any drastic recovery measures at all, by maintaining the coordinated use of ailerons and rudder.

So Whats The Problem?
The basic problem is that at any given moment-for most of us, its all the time-few of us have any idea what our angle of attack happens to be. Many an aviation authority has made a better case than I could that having an angle of attack indicator, in addition to an aural warning of an impending stall, would be considerably more effective than having just a stall horn.

About 20 years ago, an FAA inspector named Jerry Brown (not the politician) suggested an ingenious presentation for this information: an airspeed indicator with two needles: one showing a plain vanilla airspeed, and the other one indicating stall speed at every instant in time, courtesy of a microprocessor which would do the work of integrating inputs from various sensors for all related parameters including the configuration of the airplane itself. This would include flaps, gear, elevator position, even gross weight and CG.

The scary part about stall practice is, of course, that stalls lead to spins. You dont need any theatrical reminders of how dramatic things can get during a spin; suffice it to say that you could be corkscrewing down at a descent rate of up to 8000 fpm.

Most of the time when a pilot enters an unintentional spin (somewhere around 80 percent of the time), it happens when the airplane is already at or below traffic pattern altitude. Most such incidents involve single-engine, fixed-gear airplanes. It usually happens either during takeoff, while turning from the base leg to final or during maneuvering flight. Most of the time, there is no second chance.

Theres More
I realize that we still need to understand the region of reversed command, the characteristics of slow flight, and you certainly cant land an airplane well and within a reasonably short length of runway without knowing what a stall is, and without inducing one on a regular basis. I do not wish to toss the spin training or no spin training gauntlet into the ring, and Im not stating that we shouldnt fully explore and exploit the potential value of the rudder for maintaining control under a variety of circumstances.

What I am saying however is that we must continually remain aware that stalls can occur in real life under a much wider variety of circumstances than what we might encounter in practice.

Also With This Article
“Does Practice Make Accidents More Likely?”
“Whats A Stall?”
“Banking And Load Factor”
“Load Factor And Stall Speed”

-Jeff Pardo is a freelance writer and editor who holds a Commercial certificate for airplanes, helicopters and sailplanes.


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