By Thomas Oneto
Put a group of instructors together and you will find there is no better conversation-maker than stalls. Within seconds, they will begin discussing a maneuver that has been with us since the Wright Brothers, with opinions as unique and diverse as the number of instructors present. It is difficult to understand why, though, since the stalls causes and techniques for recovery are so well known. In fact, with such lengthy experience and accrued knowledge, why be concerned with stalls at all?
You need only review each years accident statistics to find an answer.Stalls have been ranked among the top three accident causes for many decades.Considering the amount of knowledge and experience-beginning as pre-solo students-you would think this maneuver would have been mastered by now. But it hasnt.
But why not? Is there something lacking in our knowledge or training techniques, or is there an unrecognized operational condition developing over time that contributes to this dilemma? To answer this question, lets approach the stall dilemma from three different directions-resort to a triad of flight approaches (aircraft, pilot and operational environment), and analyze the role of each in preventing the inadvertent stall. Because it is the wing, not the aircraft, that stalls, lets begin with a discussion of the wing.
The Airplanes Wing
The primary purpose of a wing is to produce enough lift to support the weight of the aircraft plus any additional maneuvering loads imposed during flight.Lift is that component of the total aerodynamic force acting on a body perpendicular to the relative wind. When this force acts upward relative to the earths surface it is termed positive lift; when acting downward it is termed negative lift.
The sidebar (“Angle Of Attack And The Lift Coefficient”) presents two drawings explaining the relationship between the lift being generated at a given angle of attack, and what happens beyond the point at which the maximum lift coefficient (CLMAX) is reached. Initially, as a slight deficiency of lift develops, the airplane begins to settle or mush. When this occurs, the airplane is considered to be in an approach-to-stall state. However, a point is reached where the deficiency of lift is so great that the airplane pitches downward and begins descending rapidly.
So far, its apparent that a stall is caused by an excessive angle of attack. From a simplistic viewpoint, you might wager that avoiding excessive angles of attack would also avoid stalls. And, should a stall occur, recovery is no more involved than simply reducing the angle of attack below CLMAX. If you placed that bet, youd be correct on both counts, with one exception: In typical accidents involving a stall, the stall develops quite insidiously, and is uncommanded by the pilot. How in the world could such a situation develop? Probably because of the variables that affect stall speed.
Wait a minute! Up to this point we said a stall results because of an excessive angle of attack. At no time did we mention airspeed. Right again, and it must be emphasized that a stall results because of attaining an excessive angle of attack regardless of the airspeed at which it occurs. So, although the stall occurs due to an excessive angle of attack, anything that affects the speed at which that angle of attack is attained must be understood and recognized by the pilot to avoid becoming involved in an insidious stall. Lets focus on three obvious variables-weight, configuration and maneuvering loads-and how they affect stall speed.
Weight Effects
Beginning with our earliest ground school days, we learned from the four forces of flight that total lift must equal total weight to support the airplane in level flight. Because most pilots fly by reference to airspeed, and aircraft performance in Pilot Operating Handbooks (POHs) is presented in terms of airspeed, we need to reference the airspeed at which the stall occurs rather than angle of attack. When referencing stall speed in the POH, we note that as weight increases, so does stall speed, and vice versa. So far, you may be lulled into thinking that changes in the weight of a single-engine or light-twin airplane has very little effect on stall speed. The sidebar on the previous page explores the math involving the stall speed and how it changes with weight.
The hypothetical 4200-lb. airplane burned off 600 lbs. of fuel, a 14-percent change in weight resulting in only a seven-percent change in stall speed. But during the critical phases of flight, such as takeoffs and landings, this small change in stall speed can prove quite serious.
Why? Look at it the other way: If you are accustomed to taking off at your usual operational weight of 3600 pounds and one day you operate at your maximum takeoff weight of 4200 pounds, it will be as if you are in a different airplane. If you attempt to lift off at the same built-in time cue or airspeed as you normally do and the airplane responds by flying because it is in ground effect, but then begins to settle when out of ground effect, you can get yourself into trouble in a hurry.
As the airplane settles, the angle of attack increases, aggravating any mush or stall condition present. Unless the pilot recovers by reducing the angle of attack, the airplane will either continue off the end of the runway or return to the surface with a hard bounce (if nose-down, structural damage can occur). Pilots must understand that, unless acceleration continues and the margin above stall increases, the airplane will settle back into ground effect and will not start flying again due to the increased angle of attack.
Configuration Effects
Anything that alters the wings camber will affect its lift characteristics.The most common means of altering the wings camber is by extending and retracting the wing flaps. The airspeed indicator provides this differential by displaying the gross weight wing-clean stall speed at the lowest end of the green arc, and the full-flaps extended stall speed at the lowest end of the white arc. The top drawing in the sidebar below illustrates the effect deploying flaps will have on the angle of attack and speed at which a wing will stall.
The shape of the wing also tells the pilot what to expect in the way of stall characteristics. If the lift graph trace develops a smooth curve as it approaches CLMAX, you can expect ample warning and mush before the full stall occurs. Should the lift graph trace show a sharp pointed reversal in angle and direction at CLMAX, you could expect the full stall to occur with suddenness.
Sometimes considered a configuration issue, wing planform design (shape), has a greater influence on stall flow pattern behavior. Ideally we would want the stall to progress from the wing root out to the tip to provide for aileron control beyond the point of stall.
Wing shape is another reason airplanes consist of so many compromises. An example of this is the elliptical-shaped wing. It has the lowest induced drag of the most common wing shapes, and a stall pattern that naturally progresses from the trailing edge at the wing root to the tip, meaning the aileron is still effective after the wing has stalled-a very desirable characteristic.Unfortunately it is the most expensive to produce. The rectangular Hershey Bar wing is less expensive to produce, provides good lateral stability with little dihedral required, but is much less efficient than the elliptical or tapered wing. As a compromise, the tapered wing is a popular wing planform shape because it offers good stall chacteristics, has reasonably low induced drag values, and its cost of manufacture is also reasonable. The differences are illustrated in the sidebar on the previous page.
The Bank/Stall Relationship
Although pilots generally understand the relationship between bank angle and stall speed, they have a tendency to treat the relationship as theoretical instead of knowledge to be applied. The best example are those pilots who have survived an inadvertent stall or spin experience and cannot explain how the airplane entered the stall or spin without their recognizing it. In many cases, pilots refuse to accept the fact that their airplane was entering a stall and did not react, allowing the stall to develop.
An example might be a pilot fitting into the traffic pattern at a busy non-towered airport. It just happens to be one of those days when Mother Nature provides us with a strong crosswind that aggravates the heavy traffic situation. The airplanes stall speed is 60 knots in the configuration being flown; its pattern speed is 90 knots, and a left-hand pattern is being flown with a strong left crosswind on final. The slower airplane ahead delays its turn to final, so you have to extend your base to maintain separation. This results in a steep banked turn to return to the extended runway centerline.
That left crosswind isnt cooperating. To offset the drift effect plus the overshoot, you further increase your bank to shorten your radius of turn, and simultaneously apply elevator back pressure to reduce the rate of descent.This results in a bank approaching 60 degrees, and an added G force from the elevator back pressure applied. You are pretty tense and tunnel vision is developing, causing you to ignore the stall warning just before the airplane pitches nose-down.
You attempt to raise the nose, noting the buffet, and continued nose-down descent. You accept that the airplane is in a stall and attempt to recover.Hopefully, you will have enough altitude to recover. Lets examine this bank/stall relationship and try to understand how this could happen.
The simple answer in this instance is that the airplanes load factor increased beyond the stall speed. When that happens, the laws of physics tell us the wing will stall, even if the indicated airspeed is well above the bottom of the green arc.The relationship between load factor and stall speed is that stall speed increases as the square root of the load factor.
This example should act as a reminder that situational awareness is also applicable to knowing where within the aircrafts envelope you are located and operating. Also, do not allow your operating airspeed, with a safety margin of 150% above stall, lull you into disregarding any thoughts or indications of a stall occurring.
Pilot, Environmental Effects
Any time an airplane is operating at reduced airspeed (below cruise), at low altitude or in a high-drag configuration, the safety margins are reduced. As airspeed is reduced, so is the energy in the air passing over a control surface. This requires greater control surface deflection to obtain the same controllability. At a low altitude, any uncommanded maneuver resulting in loss of altitude can cause a pilot to react contrary to a trained and proper response. Survivability instincts and reactions may be contrary to physical laws and contribute to an accident. A high-drag configuration complicates the situation because it robs the powerplant of excess thrust, reducing acceleration and climb performance.
Combining the reduction of these safety margins with maneuvering loads increases the accident potential. If ever a pilot will benefit from applied aerodynamic knowledge, it is when an unexpected reaction of the airplane occurs with little warning, altitude or airspeed.
Knowing the right thing to do, at the right time, and in the proper sequence, will aid in your survival. If you fly mechanically and lack the ability to properly analyze the situation, you may not be capable of recognizing and correcting the condition, thereby contributing to the accident.
A pilot may understand that a steepening bank raises stalling speed and that avoiding the stall requires a reduction in angle of attack. But if a pilot fails to apply this knowledge when distracted, or allows an inadvertent steepening of the bank and approach to a stall to develop, survival instincts contrary to a trained response may deepen the stall. The airplane wants to fly. As it approaches a stall, it is designed so that the nose wants to lower itself and reduce the angle of attack.
Angle of attack is independent of attitude. If the airplane were in steep banked descending turn to the left and approaching a stall, the already-lowered nose would attempt to further lower itself to maintain flying speed.The distracted or tense pilot may revert to survival instincts and attempt to raise the nose, deepening the stall. If the pilot realizes the mistake and reverts to normal stall recovery-reducing the angle of attack by relaxing elevator back pressure or lowering the nose-our next concern would be, Is there enough altitude available for recovery?
So whats a pilot to do? The best approach is to maintain situational awareness. As previously mentioned, situational awareness is not confined to your operating environment and geographical position. You must also know where you are on the performance curve and within the maneuvering envelope.Think through each maneuver. Each maneuver has three steps: entry, duration and recovery. Knowing how the aircraft is going to respond as you transition from one step to the other is important. If something different occurs, its up to you to recognize and correct it at the moment it happens.
Conclusion
Applied knowledge will assist you in avoiding or allowing yourself to become involved in an inadvertent stall. It will also provide you with a properly trained response if the unthinkable happens and you inadvertently stall your airplane. Attaining an applied knowledge level, and then maintaining it, requires two different types of training. Initial training develops the skills, and instills applied knowledge necessary for avoidance, recognition and recovery from an uncommanded stall.
Flying is a psychomotor skill that requires periodic practice and review to maintain proficiency-yet another reason you should participate in a recurrent training program.
Now get out there and practice stalls.
Also With This Article
“Angle Of Attack And The Lift Coefficient”
“Flow Separation”
“Effect Of Weight On Stall Speed”
“Effects Of Wing Configuration”
“Stall Avoidance And Recovery”
-Tom Oneto is a freelance writer/instructor living in Maryland who holds ATP and CFI-A/Instrument certificates.
