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Turning stalls: the relationship between angle of bank and stall speed isn't a mystery, but it is a bit complicated. Here's what's going on, and why.

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 indicator's 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 don't 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? That's 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 that's without any visceral gut feeling, muscle memory or experience to back it up. Reading just isn't nearly as effective for learning as is doing.

NO SURPRISES

When a stall catches a pilot by surprise, it's 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 tail's 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, it's 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, they're 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 WHAT'S THE PROBLEM?

The basic problem is that at any given moment--for most of us, it's 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 don't 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.

THERE'S MORE

I realize that we still need to understand the region of reversed command, the characteristics of slow flight, and you certainly can't 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 I'm not stating that we shouldn't 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.

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

Does Practice Make Accidents More Likely?

As a Student pilot, flying solo and practicing power-on stalls in a Cessna 150 at about 5000 feet msl in the practice area to the north of my home airport in suburban Maryland, I received my very first introduction to another maneuver: the spin.

In case you think stalls happen only to novice pilots flying solo and getting in over their head, well over half of stall-spin accidents actually happen during dual instruction.

Student pilots actually are among the least likely--only about four percent--to succumb to a stall/spin accident. The next-least--likely-at perhaps 10 percent--are Airline Transport pilots. Most victims are what most pilots are: Private (46 percent) and Commercial (40 percent).

I was lucky. In those few eternal moments of dumb-struck terror (and inaction) the airplane came out of the spin on its own, and I learned an indelible lesson.

Nowadays, there's a real danger that the typical high-angle-of-attack, power-on stall demonstration leaves fresh students with the impression that stalls only happen when the nose is high. As many pilots have learned (the hard way), that's not the only way to increase a wing's angle of attack.

What's A Stall?

A stall involves either a loss of lift, or not enough of it. During straight and level flight, if nothing else is changed and as we fly more and more slowly, we must increase the angle of attack to generate the same amount of lift. Less air also flows over (and is deflected by) control surfaces, their responses become increasingly more sluggish and, due usually to the increase in turbulent air acting on the tail, buffeting is felt as the stall is reached. By this time, the stall warning horn is usually blaring away.

As the angle between the wing chord and the relative wind is increased for a given airfoil, the coefficient of lift increases. This increase is almost linear for single-digit angles, then becomes non-linear up to the airfoil's maximum so-called critical angle of attack. Remember that this critical angle of attack is particular to each airfoil.

The critical angle of attack is reached when an increasingly unfavorable pressure gradient from higher pressure at the trailing edge to the lowest pressure at the center of lift above the wing collides with the relative wind from the front of the wing and has nowhere to go but away from the airfoil, which it does in churning torrents.

There's also the visualization involving air's limited "elasticity" and its inability to fill the void behind a steeply pitched wing, as depicted in the diagrams above, which describe a theoretical airfoil.

But whichever description you favor, once the airfoil exceeds its critical angle of attack, what happens next doesn't vary: Lift drops precipitously and the resulting large change in pitching moment can often result in a wing drop. Recovery usually involves reducing the angle of attack and increasing the airspeed for good measure, until the air once again is flowing smoothly over the wings.

In the eyes of many of the Great Unwashed non-pilot flying population, if the engine(s) were to stop, your airplane would promptly "stall" and plummet from the sky. But the stability and design of modern airplanes will usually save the day, because as the airplane slows down, the wings provide less lift and the nose of the airplane will usually just drop as airspeed then approaches its original value (albeit in a descent).

Banking And Load Factor

In a constant altitude, coordinated turn in any airplane, the load factor is the result of two forces: centrifugal force and gravity. This relationship is charted in the top diagram. For any given bank angle, the rate of turn varies with the airspeed; the higher the speed, the slower the rate of turn. This compensates for added centrifugal force, allowing the load factor to remain the same.

In the top diagram, note how rapidly the line denoting load factor rises as it approaches the 90-degree bank line, which it reaches only at infinity. The 90-degree banked, constant altitude turn mathematically is not possible. True, an airplane may be banked to 90 degrees but not in a coordinated turn; an airplane capable of holding a 90-degree banked slipping turn is capable of straight knife-edged flight. At slightly more than 80 degrees, the load factor exceeds the limit of 6 G, the limit load factor of an acrobatic airplane.

The bottom diagram reveals an important fact about turns--that the load factor increases at a terrific rate after a bank has reached 45 or 50 degrees. The load factor for any airplane in a 60-degree bank is 2 G. The load factor in an 80-degree bank is 5.76 G.

In any event, the wing must produce lift equal to these load factors if altitude is to be maintained.--J.B.

Load Factor And Stall Speed

Any airplane may be stalled at any airspeed. When a sufficiently high angle of attack is imposed, the flow of air over an airfoil breaks up and separates, producing a sudden loss of lift resulting in a stall.

The airplane's stalling speed increases in proportion to the square root of the load factor. This means that an airplane with a normal unaccelerated stalling speed of 50 knots can be stalled at 100 knots by inducing a load factor of 4 G.

A similar effect is experienced in a quick pullup, or any maneuver producing load factors above 1 G.

This has been the cause of accidents resulting from a sudden, unexpected loss of control, particularly in a steep turn or abrupt application of the back elevator control near the ground. Since the load factor squares as the stalling speed doubles, tremendous loads may be imposed on structures by stalling an airplane at relatively high airspeeds. The maximum speed at which an airplane may be stalled safely is the "design maneuvering speed" (VA).--J.B.
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Author:Pardo, Jeff
Publication:Aviation Safety
Date:Jul 1, 2006
Words:2499
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