Let’s talk about what is probably the single most important concept to understand regarding stalls, and that is angle of attack.
Your airplane’s wing, when viewed from the cross-section, has a chord line, a sort of imaginary line that runs straight from the front of the leading edge to the back of the trailing edge. The chord line is always at an angle versus the oncoming air or relative wind, and the angle created by the two is called the angle of attack.
You might recall the experience as a child of taking an automobile drive with your parents and sticking your arm out the car window and into the oncoming blast of air. If you cupped your hand, thus providing a degree of “bend” or camber, your arm raised effortlessly and flew. And your hand continued to fly until you turned your palm too far to the perpendicular, exceeding the critical angle of attack for your airfoil hand, and your arm dropped suddenly as it stalled.
The only angle that really matters.
A simplistic way to look at aerodynamics, but 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 fighter jet, a Boeing, or a single-engine propeller airplane. 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.
Pilots often only associate stalls with the slow airspeed regime of the energy envelope. That’s why it can be misleading when instructors caution students to, “watch your speed or you’ll stall” because an airplane can be stalled at any airspeed, in any attitude. 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. For a typical single engine airplane, the average critical angle of attack is often in the range of 15-18 degrees.
Stalling at a Nose Low Attitude
Let’s say that you’re in a steep screaming dive, about to exceed the airspeed redline or Vne. In this situation, if you were to haul back on the control stick with enough force, you could stall your 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 relative wind is simply the direction of the airfoil relative to the atmosphere. Generally, it is parallel to the flightpath, and travels in the opposite direction. There is a tendency to think of the relative wind as a stable and steady force that parallels the earth’s surface, but that line of thought only works in near straight-and-level flight conditions.
Back to our screaming dive scenario: if you pull up smoothly without exceeding any limitations on the airplane, you could have enough energy—even with the engine at idle—to fly the airplane to a ninety-degree up position and execute a vertical airshow maneuver like a hammerhead turn. But, if you were to hold that up-vertical position long enough, your wings would eventually lose energy—and your angle of attack with the relative wind would soon exceed critical, 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. Depending on the gyrations you’re putting the aircraft through, it shifts tangentially.
With enough energy—either via excess thrust or excess airspeed—some fighter jets with low-lift/high-speed swept wings can climb vertically for thousands of feet. But as the fighter’s after-burner momentum diminishes, and the relative wind falls off, the angle of attack increases, getting closer to critical limits. If no action is taken, and the critical angle of attack is exceeded, the wings will stall.
But We’re Not Flying Fighters
Most wings stall from the root out.
On most light 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. This way, the ailerons are the last wing elements to lose lift, 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 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 to avoid entering a spin.
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—an aerodynamic 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 effect 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 the weight, 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.
Small accumulations of snow, ice, or frost on a wing can also significantly increase stall speed by inhibiting lift and increasing drag. 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 contamination is increasing the stall speed. Eventually, with enough contamination on the wings, the airspeed and the stall speed will collide, with fatal implications for the pilot.
From Stall to Spin
Airplanes get into spins either as a pilot induced maneuver, or an unintended blunder. A condition that can cause one wing to abruptly stall before the other is uncoordinated flight. If a stall in uncoordinated flight is allowed, a rotation around the greater stalled wing can induce a spin. A spin is a stall that has continued, with one wing more stalled than the other. The aircraft will begin rotation and spin progressively faster and tighter unless the stalled condition is “broken,” by relaxing back pressure or applying forward elevator pressure. If a spin happens at low altitudes, the quick loss of control that follows may not leave enough altitude to be recoverable.
In coordinated flight, 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 the 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 skid or slip by using either too much or too little rudder? Do the stall characteristics change? An emphatic yes, and the stall characteristics change dangerously so.
The Classic Killer: Too Much Yaw
Keep it coordinated, or one wing will drop off.
By misapplying the rudder, the normal cues that we feel in the cockpit can be lost, making it difficult to sense an impending stall. In a typical base-to-final approach overshoot, where the pilot adds too much rudder to “hurry” the nose around by skidding the airplane, the airplane is in a yawed skid 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 instead at the wingtip—making the “break” very unstable.
Additionally, due to the yaw condition, there will be less turbulent airflow over the elevator, and hence diminished warning through the control stick or yoke, and less “rumbling” through the seat of your pants. Another coffin nail is the loss of aileron authority because when the stall occurs, it hits near the wingtips. In uncoordinated flight, the forward or yawed wing will not stall at the same time as the aft wing, resulting in a rolling moment. Immediate action to break the stall must be taken first, or recovery may not occur. If this happens at low altitude, as is statistically the case, most pilots will panic—and when they see the ground rushing up—pull back even further on the elevator control, thereby ensuring entry into a spin. Even if the proper response is made, it may be too late if initiated near or below pattern altitude.
The key is to avoid letting your airplane lapse into uncoordinated flight, and certainly never at low altitudes. The best cure for an inadvertent stall or stall-spin is prevention. If you no longer feel proficient in uncoordinated stall approach and recoveries, consider taking dual instruction with a CFI. Or better yet, go flying with a qualified instructor and practice various spin entries and recoveries in an airplane built for aerobatic maneuvers. By exceeding the critical angle of attack in a variety of spin scenarios, you’ll master a real-world understanding of angle of attack, and gain greater confidence in your airmanship.
- Arnold Palmer Talks Flying: A Baker’s Dozen with the Legendary Golfer - July 13, 2020
- No stall, no spin: why angle of attack is essential - February 4, 2020
“ But, if you were to hold that up-vertical position long enough, your wings would eventually lose energy—and your angle of attack with the relative wind would soon exceed critical, and the wings would stall.“
Not so. In a vertical climb prior to a hammerhead, you are at or even below zero degrees AOA, regardless of your speed or energy state. You never actually stall in this manoeuvre.
From Michael:
“Not so. In a vertical climb prior to a hammerhead, you are at or even below zero degrees AOA, regardless of your speed or energy state. You never actually stall in this manoeuvre.“
Hi. Thanks for reading. I was stating that IF you continued to hold the vertical climb (and not make the Hammerhead Turn), eventually you’ll exceed the Critical AOA and stall (and possibly enter an unintentional tail slide depending on how bad a day it’s going to be…)
But, agree that in a properly executed Hammerhead Turn there is no stall (which makes it odd that some acro books have referred to it as the “hammerhead stall”).
A properly executed Hammerhead turn occurs when the forward movement of the aircraft is zero. At that moment, there is no relative wind to the chord line, so it is, by physical law, impossible to stall the wing, as you have removed the exceedence of critical angle of attack factor from the equation. One can progress directly into a tail slide without stalling. A hammerhead done “early” will earn lost points from the judges on the ground. A botched hammerhead will lead into an inverted flat spin because of the increased lift from the accelerating wing in the turn (usually the starboard in the left yaw, and P-factor). Proper execution of a hammerhead demands, among many other control inputs from the start of the maneuver, application of left rudder, right aileron, and forward stick, all at the proper moment.
Hammerheads are a wonderful part of any acro sequence, and most pleasing to the executor and the viewers on the ground.
Thanks for the article: I enjoyed reading it. I’d like to make a couple of suggestions.
For the opening example, it complicates it unnecessarily to have the child cup their hand. Camber is just an optimisation for wings on non-aerobatic planes (and one we really shouldn’t bother students with in ground school). A non-cambered wing at an appropriate angle of attack will also develop lift, as will a flat hand or even a barn door.
For the section on uncoordinated flight, it might be good to talk about how a slip differs from a skid. Pilots will often slip around the turn from base to final to lose a bit of altitude, and that’s a perfectly-routine and stable maneuver, vs skidding to try to tighten the turn.
Thanks for the great insights.
Looking back at my early PPL training, I would get into arguments with some of the CFIs around the turn-to-final. Intuitively and out of an abundance of caution, I would start early, never bank more than 15-20 degrees, particularly in shortfield practice (low approach speed). The reason is that load factor increases stall speed, while sweeping the inside wingtip back too fast slows the inside wingtip well below your IAS. The result is the inside tip loses lift, dips, and intuitive correction is to add counter aileron in the turn, definitely putting you in uncoordinated territory. This can worsen into an inside wingtip stall, start a spin, with 700ft left to hit the ground.
I now have 500hrs, am IFR rated, and to this day do not understand why It is articles like yours, and not our CFIs that properly explain how critically close to the flight envelope we sometimes get in those last phases of flight.
Thanks again for the insightful explanation.
At some point in my flying, I developed an instinct that occasionally surprises flight instructors (or passengers), but which I’m not trying to train myself out of because I think it might one day save my life.
It is this: when flying slowly, if the aircraft makes any uncommanded roll, immediate stick forward THEN aileron against the roll.
I figure this: if the uncommanded roll is due to an incipient spin entry, applying opposite aileron first could make matters worse. Applying stick forward will lower the angle of attack, and ensure that opposite aileron will work, while also instantly stopping the incipient spin.
On the other hand, if the uncommanded roll is due to some weird air movement, applying forward stick first will do no harm, and will even marginally increase airspeed and improve roll response.
Of course, the sudden pitch down could alarm a sensitive passenger. I just figure it’s going to alarm them a whole lot less than being upside down.
I might add: if you bank very (!) apruptly from base to final you increase your AOA at the wingtip mostly. So it’s possible to spin your aircraft in coordinated flight only with full in-aileron flying too slow in base. Overshooting is the typical situation. This might explain some fatal last turns.
If you try this do it above 3000ft GND. The wing drops fast cause it stalls at the wingtips first without alarming buffeting.
Kathleen wrote:
“Flap extension affects stall characteristics, and in general flap extension creates more lift”
I know what you’re trying to say, but would not “…in general, flap extension increases the coefficient of lift, thereby allowing the wing to produce a given amount of lift at a lower speed, thereby reducing the stall speed”?
And
“But as the fighter’s after-burner momentum diminishes, and the relative wind falls off, the angle of attack increases, getting closer to critical limits. If no action is taken, and the critical angle of attack is exceeded, the wings will stall.”
Not so.
A thought experiment for you:
You are in an RV-7A. You are stabilized in level flight at 100 knots. You go into a full power climb maintaining exactly 100 knots.
Question:
Has your angle of attack increased, decreased, or remained the same?
Even though I know you literally can stall in a dive if you haul back on the stick, it still seems strange. Seems angle of attack indicators and discussion has gained popularity recently from light planes to Boeing.
Thank you for a great, well written article with emphasis on this critical aspect of flight and flight safety. Though I am a rusty, relatively low hours private pilot, coordination and angle of attack are the concepts I am reviewing most. The lessons I’ve learned for my PPL were also the ones that occurred rapidly but fortunately weren’t deadly. Examples I had while getting my PPL include loose spark plugs found on preflight and seagulls at 3000 agl on my cross country. Who usually wiggles their plugs? However, angle of attack, especially at low altitude in the pattern leaves you little time to recover safely if at all. This is my greatest concern and focus for continued instruction. Thank you again.
“Let’s say that you’re in a steep screaming dive, about to exceed the airspeed redline or Vne. In this situation, if you were to haul back on the control stick with enough force, you could stall your wing even though you are flying at extremely high airspeeds and in a markedly nose-down attitude.”
Actually you probably won’t stall. You’ll probably snatch the wings off.
As a rank amateur (Private SEL) but with decades of sailing experience I can only suggest that a hammerhead is similar to a boat that is trapped in a dead calm: no water moving over the rudder, hence no steering. The boat will head in the direction of the current or swell (gravity) and once movement is restored there is steerage.
I feared power off stalls when I trained; the fear was removed by doing ‘falling leaf’ maneuvers and stabbing (not gently!) opposite rudder and pushing the nose over when one wing dropped. He said ‘get her flying again’. Another tip he had was to always look outside, not at instruments to minimize altitude loss when recovering.
Kathleen,
Are you sorry you mentioned it!
Geez all the opinions.
Thank you for your article. It was well written and informative.
The FAA has killed many pilots by (for decades) ignoring AOA indicators, and making them unreasonably burdened with expensive regulations. If not for FAA rules, we’d be flying by AOA and be safer.
Growing up, the common reframe was “Needle, Ball and Airspeed”. It is what “Stay coordinated“ or “Fly the damn airplane” means.
Although dated and sexist “Stick and Rudder” is still a must read before solo and, if you haven’t already, at 20,000 hours.
Thank you, THANK YOU, THANK YOU for this excellent article Kathleen. As an aeronautical engineer, and a pilot with 50+ years of experience having flown in the US Air Force, Mass. Air National Guard, and owned and flown a Cessna 182 in the civilian sector…. you covered the subject quite well. In the pilot training I received in the Air Force I learned to fly the Piper Cub, T-28, and T-33 aircraft (Total of ~150 flight hours) in ALL the flying situations you could EVER get into…. before receiving my wings Sadly, in pilot training now (from what I have read about it) pilot candidates get to fly in only one airplane for ~25 flight hours…and NEVER do a spin and recovery…. REALLY sad and dangerous… in my humble opinion! Finally, I would like to have ~5 minute Aviation telephone conversation with you some time. My telephone number is (206) 382-3643.
Most light aircraft don’t buffet as a the stall approaches, despite what instructors say. If there is buffet, it is imperceptible. Therefore don’t count on buffet as being a warning of an impending stall. In marked contrast, if you stall a 737 (in a proper simulator of course) at very high altitude, the shaking and shuddering as the aircraft approaches the stall is so bad it is hard to focus on the instruments.
There will be no buffet if you stall a 737 in the landing configuration at low altitude which is one reason for stick shakers to give the pilot warning of an impending stall.
A common misunderstanding is the correct recovery action to take if the aircraft experiences a severe, sudden and unexpected wing drop at the point of a stall. With light aircraft one cause of a violent wing drop at the point of stall is incorrect rigging of one wing.
I had this in a Cessna 152 once where the wing drop was so rapid that not only did we go into an incipient spin in the direction of the mis-rigged wing but we finished up facing in the opposite direction having lost 600 feet trying to recover.
On return to the airport we wrote up the aircraft as un-airworthy. That meant the flying school could not hire out that aircraft until the defect was repaired (and test flown).
If a wing regularly drops rapidly and unexpectedly during practice stalls, then write it up before it kills someone. Recovery technique includes sufficient opposite rudder to prevent the wing falling further – simultaneously lower the nose to un-stall the wings and level the wings with aileron. Power should be applied at the same time. Prevent yaw as power is applied
All this should take 3-5 seconds for minimum loss of height where terrain is a factor. Under no condition should you use rudder to level the wings if a wing drop has occurred. Otherwise the result is the possibility of an over-the-top incipient spin in the opposite direction due to the low speed and lots of rudder
I would take a bit of issue with the statement that if a stall is allowed to progress into a spin, at aft CG, then the spin may become unrecoverable even if the airplane is certified for spins. Pretty unlikely: When an airplane is certified for spins it is very thoroughly tested by the manufacturer at every possible configuration for recovery characteristics. However, it is then the pilot’s responsibility to know the CG limitations and the recovery procedures of the published Flight Manual.
Excellent article! The EAA has been conducting a contest -Founder’s Innovation Contest, for 4 years now, in an effort to solve loss of control. My invention has been a finalist for the last two years. Oddly, some of the judges do not even seem to understand what you so simply explain. AOA and yaw (inclination) are the key. “Watch the airspeed watch the ball! ” That should be their prime criteria. Oddly it isn’t. Our product is a haptic feedback control grip. It signals your fingers – quite effectively – AOA and yaw. It can be seen at FeelFlight.com.
From Ronald:
“Kathleen,
Are you sorry you mentioned it!
Geez all the opinions.“
Heh-heh…yes, I was regretting using the word hammerhead (and vertical) as my intention was to get the reader to imagine a very steep climb.
BUT…Michael and Tom’s comments about an exact vertical line, such as the vertical on a hammerhead…got me thinking so I contacted noted spin and aerobatic expert Rich Stowell, who I’ve consulted in the past on other articles.
Rich, as always, had some fascinating insights that I’ll share here.
He explained that a true vertical climb, a 90-degree pitch climb, is almost like what physicists call ‘a singularity’ in that it is different than say, a 60, or 80-degree nose up pitch.
As referenced earlier here in the comments section, in a true hammerhead, if you were on the exact vertical, and never made the pivot into the hammerhead turn, the airplane would tail slide (as gravity takes over) and not stall. No forward or zero speed means no airflow so no separation or stall.
Rich used a humorous example of an airplane sitting parked on a ramp: it’s obviously at zero speed, but are the wings stalled?
But in an 80-degree pitch attitude, if held long enough, the wing would stall.
Regarding stalling nose down below the horizon from the example of a pull-up from a steep screaming dive, Rich Stowell said he has experienced that with some aerobatic students who get too eager and pull back abruptly.
More from Rich:
“The vertical is almost a special case.
What the pilot has to do to maintain a vertical (climb) line – the wing cannot be generating lift. Because IF it makes lift, it wouldn’t stay on the line, it would pull the airplane over onto its back. The pilot in the vertical has to drive the Angle of Attack down to zero lift so that the wing isn’t generating any. He does this by pushing forward.
To maintain a vertical line you have to drive AoA down, so you’re getting further away from critical AOA.
If you’re in a descent and honk the airplane up to a steep 60 or even 80 degree climb, as the speed decays you’ll actually have to keep pulling back or you’ll fall off that high pitch angle. But at vertical it’s a special case: you keep pushing forward on the elevator and decreasing the AoA.
But, let’s say we’ve gone past the 90 degree vertical line and are at 120 degrees. Now we’re slightly on our back and if I yank back now, I’ll stall.”
Thank you readers, and thank you to spin expert Rich Stowell for his insights. Interesting stuff!
I used to perform hammerheads in a beautiful fully aerobatic Decathlon (that also had an inverted system) and have to admit my mind was never on stalling, but instead on not over delaying the pivot, and entering an unintentional tail slide.
Kathleen
Thank you, from a low hour 60yo student, for your excellent article!
I have a question about Stall, which I have difficulty to understand.
According to the theory, stall happens when:
1- Speed is slow then a certain limit.
2- Angle of attack is greater than a certain limit.
In the case of angle of attack, there is being spoken about relative wind along the wings.
But here is my confusion.
Relative wind is always shown parallel to the horizon, which strikes the wings.
While in my understanding, in the sky there is equal air and wind everywhere. We create our own hard wind for our wings by moving fast through the air. Right?
So it should not make any difference in which direction we move fast. Along the horizon, or at a steep angle upwards or downwards, relative to the horizon. The relative hard wind will be created straight in opposite direction where we move fast, because we are also moving in the same direction as our wings are.
If so, then there is no question about angle of attack at all. Because we always create our own wind by moving fast in any direction in the sky.
It would be different if we are moving parallel to the horizon, but our wings have a greater angle of attack relative to the horizon. Then this stall theory is understandable.
If we look at the fighter jets and acrobatic airplanes, then we see that they can climb vertically up against the horizon, fly at 90 degree bank angle, and fly upside down. And they do no stall, because they create their own relative straight opposite wind by moving fast in any direction in the sky.
So why in normal planes we do have to think about straight coming relative wind and accordingly the angle of attack against it?