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TO SPIN, FIRST YOU HAVE TO STALL

"The NTSB is not only one of the premiere accident investigations in the world; it is also the national archives of what not to do." - Former NTSB Chairman Jim Hall

By: Kathleen Bangs

This article series on stalls and spins is presented to be of assistance to you - the pilot - to avoid becoming another tragic statistic in the 'what not to do' when flying category. With that in mind, let's get right into talking about what is probably the single-most important concept to understand regarding stalls, and that is what's called the Angle of Attack.

Your airplane's wing, when viewed from the cross-section, has what is called a Chord Line , a sort of imaginary line that runs straight from the leading edge to the trailing edge. That Chord Line is always at one angle or another versus the oncoming air or Relative Wind as it's referred to, and the angle created by the two is called the Angle of Attack .

You might recall the experience as a child of taking a long automobile drive with your parents and sticking your arm out of the car window and into the oncoming rushing air. You might also remember that if you stuck your arm out straight and cupped your hand, thus providing a degree of 'bend' or camber to it, that your arm raised effortlessly and 'flew." Or, at least 'flew' until you turned your palm too far to the perpendicular, thereby exceeding the Critical Angle of Attack for your particular hand, or 'airfoil,' and your arm suddenly dropped or 'stalled.'

A simplistic way to look at aerodynamics perhaps, but then again - why complicate things unnecessarily. The wing on your plane - like your hand out the window - must maintain an Angle of Attack below the Critical Angle of Attack to avoid stalling.

Your wing doesn't really care whether it's attached to a glider, a fighter, or a jumbo jet - it cares only that there is airflow sufficient to keep it flying. Good energy management is crucial when it comes to maintaining sufficient airflow over the wing to avoid stalling. Always remember: to get into a spin, a wing must first be stalled. Many pilots only associate stalls with the slow flight regime of the energy envelope. That's why although it's very understandable, it's also very misleading when instructors the world over repeatedly warn their flight students to, "watch your speed or you'll stall!" because indeed an airplane can be stalled at any airspeed and in any attitude.

Relative Wind: Know Where It's At

This notion that 'an airplane can be stalled at any speed' is actually a law of aerodynamics best not ignored, and it can confuse stall discussions because again - most pilots associate stalling only with being too slow. Although 'being too slow' is often one of the factors in stall accidents, it does not exclude the fact that your wing really can be stalled at any speed if you exercise poor energy management and exceed the Critical Angle of Attack .

Let's say, for example, that you're in a very steep dive, about to exceed the airspeed redline ( Vne ). In this situation, if you were to haul back on the control stick with enough force, you could stall the wing even though you are flying at extremely high airspeeds and in a markedly nose-down attitude. Why? Because Angle of Attack depends on the Relative Wind , and even though there is a tendency to think of the Relative Wind as a stable and steady force that parallels the earth's surface, that train of thought really only works in near straight-and-level flight conditions. It becomes important then to have a good idea of what the Relative Wind is, and where its coming from, because without that knowledge you can't safely estimate your Angle of Attack.

Back to our screaming dive scenario: let's say that the Critical Angle of Attack for your airplane wing is twenty degrees. Even though you're in a high-speed dive, if you pull up smoothly without exceeding any limitations on the airplane, you could actually have enough energy - even with the engine at idle - to fly the airplane to a near-vertical ninety-degree up position and execute an airshow maneuver such as a hammerhead turn, or perhaps a less vertical wing-over. But, if you held that vertical position long enough, eventually you would lose energy - and your Critical Angle of Attack would soon catch up with you - and the wings would stall.

Conversely, if you failed to pull up smoothly, even though your airplane is pointed down and at high speed, you could still exceed the Critical Angle of Attack and stall while in a nose-down position. What this means is that the Relative Wind is not constant. Rather, depending on the gyrations you're putting the aircraft through; it can be constantly shifting, depending on your aircraft's speed and energy.

With enough energy - either via excess power and/or excess airspeed - some fighter jets with low-lift/high-speed swept wings can easily climb nearly vertical for thousands of feet. But eventually, even their momentum will diminish, and as the relative wind falls off, the Angle of Attack increases, getting closer to exceeding it's critical limits. If no action is taken, and the Critical Angle of Attack is exceeded, what will again happen is that the wings will stall.

But we're not flying fighters

On most light aircraft general aviation aircraft, the wings are designed so that a stall occurs first at the wing root, and then progresses outward to the wing-tip. The wings are designed in this manner so that the ailerons are the last wing elements to lose lift, thereby providing an element of roll control in the slow flight and approach-to-stall regime. Recovery from a stall requires that the angle of attack be decreased to again achieve adequate lift. This means that the back pressure on the elevators must be reduced. If one wing has stalled more than the other, the first priority is to recover from the stall, and then correct any turning that may have developed.

Factors Affecting Stall Characteristics

Flap extension affects stall characteristics and in general, flap extension creates more lift, thus lowering the airspeed at which a given wing may stall. Another factor to consider is Center of Gravity or CG. A CG that is too far aft or rearward can inhibit the natural tendency of the aircraft's nose to fall during a stall - a natural tendency that aids in stall recovery. This condition may necessitate a forced nose-down attitude to recover. If a stall with an aft CG is allowed to progress to a spin, it may be unrecoverable, even in an airplane certified for spins.

Weight also has an affect on stalls, as an overloaded airplane will have to be flown with an increased Angle of Attack to generate sufficient lift for level flight. The heavier it is, the greater that angle becomes, possibly moving it precipitously close to the Critical Angle of Attack. That close proximity to the Critical Angle of Attack can make an inadvertent stall more likely to occur.

Even a small accumulation of snow, ice, or frost on a wing can significantly inhibit lift and increase drag. Due to the reduced lift, the aircraft can stall at a higher-than-normal airspeed. Continued flight into even moderate icing can quickly lead to a situation where the accumulating ice on the wings is increasing the drag and slowing the aircraft's speed. At the same time, the degradation of the smooth lift surface due to the contamination is increasing the stall speed. Eventually, with enough ice on the wings the airspeed and the stall speed will collide, with most likely fatal implications for the pilot.

Stall recognition can come several ways. Modern aircraft are equipped with stall warning devices (usually an audible signal) to warn of proximity to the critical angle of attack. The aircraft may vibrate, control pressures probably feel "mushy", and some might experience that "seat of the pants" sensation that the aircraft is on the verge of losing lift.

Recognizing the symptoms of an impending stall is one of the reasons that practice of slow flight, and of impending and full stalls, is important. If a stall is allowed to occur, and one wing has stalled more than the other, a rotation around the greater stalled wing will occur that we refer to as a spin.

A spin is a stall that has continued, with one wing more stalled than the other. The aircraft will begin rotation around the more stalled wing, and spin progressively faster and tighter until the stalled condition is "broken" by relaxing back pressure and/or applying forward elevator pressure.

From Stall to Spin!

One condition that can cause one wing to abruptly stall before the other is to enter a stall in uncoordinated flight. If this occurs, especially at low altitudes, the quick loss of control that follows may not leave enough altitude to be recoverable.

In coordinated flight, the Relative Wind meets a straight wing at 90 degrees, and a stall will occur perpendicular to the relative wind, assuming a straight and symmetrical wing. Wing designers also control stall characteristics by varying dihedral, wing twist, and other factors. Most straight-winged aircraft are designed to stall at the wing root, which benefits the pilot in two ways. First, the onset of the stall at the wing root provides an aerodynamic stall warning as the turbulent air hits the elevator. Second, during this approach-to-stall, the wingtips are relatively unaffected, so that the ailerons will remain somewhat effective. But what happens to our straight-wing airplane if we introduce a slip or skid by using either too much or too little rudder? Do the stall characteristics change? An emphatic yes, and dangerously so.

The Classic Killer: Too Much Yaw

By mis-applying the rudder, the normal cues that we would feel in the cockpit can be lost, thereby making it difficult to sense an impending stall. Let's look at how this occurs in a typical base-to-final overshoot, where the pilot adds too much rudder to 'hurry' the nose around by skidding the airplane. In a skid condition, the airplane is in a yawed configuration. As we yaw our straight-winged aircraft, we cause the wing to sweep back with respect to the Relative Wind . Due to this yaw, any approach-to-stall situation will begin not at the wing root as it would in coordinated flight, but it will instead occur at the wingtip - making the 'break' very unstable. Additionally, due to the yaw condition, there will be no turbulent airflow over the elevator, and hence no warning through the control stick or yoke, and no 'rumbling' through the 'seat of your pants.' The final nail in the coffin is the loss of aileron authority because when the stall occurs, it hits near the wingtips.

To understand how dangerous a yaw can be, let's review the bad things that happen when we apply either too much or too little rudder to maintain coordinated flight. Since the wingtip stalls first, you will not get the normal approach-to-stall buffet on your elevator, and you will lose aileron effectiveness at the onset of a stall. Also, because you are in uncoordinated flight, the forward or 'yawed' wing will not stall at the same time as the aft wing, causing a rolling moment. This is the scenario where you can suddenly find yourself entering an uncoordinated stall, which could lead to an inadvertent spin - especially if you don't immediately react by reducing the back pressure.

If this dangerous condition is allowed to progress to an actual stall, most pilots - surprised by the sudden turn and downward pitch of the aircraft - will apply even more back pressure while attempting to use aileron to turn the wings back to straight and level. The deadly dilemma is that by this point the airplane could be entering an incipient spin and immediate action to break the stall must be taken first, or recovery will not occur. If this happens at low altitude, as is overwhelmingly statistically the case, most pilots will panic - and when they see the ground rushing up - will pull back even further on the elevator control, thereby ensuring that a spin occurs and hastening their demise as yet another fatal stall/spin accident. But, even if the proper response is made, it may be too late if initiated near or below pattern altitude. Obviously then, the key to staying safe is to never allow yourself to get into uncoordinated flight in the first place, and certainly never at low altitudes. The best cure for an inadvertent stall is prevention!

Getting Proficient

If you're not proficient in uncoordinated stall recoveries, you should consider taking dual instruction with a qualified instructor who has not only experience, but expertise in this area. Although there are many excellent instructors to choose from in the US, one of the most highly regarded is pilot Rich Stowell, who has successfully performed more than 25,000 spins and says, "One thing an airplane will not do, is it will not spin by itself. These are pilot-imposed maneuvers."

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