That is the real danger: this faulty reaction to the stall, rather than the stall itself. It is quite rare that a pilot is killed simply because he stalled. But it happens with tragic monotony that a pilot is killed because he either fails to recognize the stall for what it is, or fails to control that impulsive desire to haul back on the stick.
– Wolfgang Langewiesche
This was a tough article to write. It took me days.
Carlo, my English teacher son, always starts class with a quiz. So, Pop Quiz for all pilots out there. (Watch this.)
True of False?
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Langewiesche is lead investigator for the Air France 447 crash.
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An airplane stalls when it gets too slow to fly.
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Speed is irrelevant. Instead, an airplane “stalls” when its wings are angled up too steeply.
Bonus question:
How does an airplane fly?
We’ll take the last one first. Ask three pilots and you might get four answers. Embarrassing. So let’s avoid three-syllable words like ‘Bernoulli’ and ‘laminar’ and keep it intuitive. Newly-hatched birds learn this stuff right out of the nest. Every time someone is called a “bird brain”, it’s very insulting. To the birds.
It is easiest to think of the wing as an inclined plane, trying to climb the oncoming airflow. The wing deflects the airflow down and thus keeps itself aloft. The drawing below shows a concept that is over 60 years old. Cross-section of a wing flying left to right.
A surfboard is an inclined plane that surfs the waves. A wing is an inclined plane that surfs the oncoming air. That, Langewiesche explains, is why we call the magical machine an
air-plane.
Not only does the wing ride the airflow beneath it, but it also gets sucked up by the pocket of near-vacuum above it.
If you are still with me, you now know more than many pilots do. Seriously.
If the airplane slows, the oncoming airflow weakens, and the wing descends. Think about a waterski or surfboard settling as the speed diminishes. The airplane is still flying – but it is flying downward. Nothing wrong with that. You have to descend to land, after all.
So #2 above is False. An airplane never gets “too slow to fly”.
Watch an airliner land – as it slows down, it descends until it gently touches down on the runway.
If it gets very slow, it will fly downward. It wants to fly. It was designed to fly.
Unless a pilot fools around with it and tries to make it do the impossible.
So what does make an airplane stop flying and fall out of the sky?
When the wing is inclined too steeply against the airflow, it no longer rides the oncoming air. Instead, it plows through the air like a bully through a crowd, leaving a riot of confused and disturbed air in its wake.
The image below shows a flying wing on top, and a stalled wing at the bottom.
With the wing dragging in the airflow, flight becomes impossible, the wing stalls, and the airplane starts to fall.
#3 above is False. An airplane doesn’t stall because the wing is angled up too steeply. It can stall even with the wing pointed down.
At left, a wing diving down, in flight. At right, a stalled wing falling down.
The culprit is Angle of Attack, the angle at which the airflow hits the wings. At an excessive angle of attack, the wing doesn’t ride the airflow, but batters it aside.
So, how much angle is excessive? Anything over 16-18 degrees will destroy the airflow. At that angle, drag simply overcomes all upward lift.
This isn’t theoretical. In wind tunnel tests with smoke streaming against a real wing, the angle of attack at which the wing stops surfing and starts dragging against the airflow becomes very visible.
Once the airflow is disrupted, the wing is stalled. It gives up flying, and the airplane begins to fall.
The human instinct is to pull the nose higher, away from the looming ground. Nothing could be more fatal.
It takes only 3-5 seconds to recover from a stall. It takes longer to read this paragraph. But it takes a lot of faith to do the right thing –-> point the airplane down. Lower the nose, reduce the angle of attack, restore the airflow over the wings, and fly again.
The key is to let the nose fall, and the wings will start flying again. Do it once, twice, and we become believers. Practice it often, and we keep the faith. But many pilots have never flown stalls! Not once.
I was lucky. Enlightened instructors made me a believer. Pull the nose high, feel the wing give up flying. Let the nose drop, there’s the ground, filling the windshield. But we are flying again! Gently pull out of the dive, done.
Done, really.
Later, in aerobatic training, Meynard ordained belief into faith. He pushed me deeper into the monster’s belly, then taught me the way out, every time.
Spins. One wing completely stalled. The opposite wing, still bravely flying, rolls the airplane inverted into a dive, corkscrewing around itself.
Recover. Later, recover on a specific heading! That’s real man stuff.
In World War II, 18- and 19-year old kids used advanced stalls to carve the edge of controlled flight into masterpieces of technique. Combat maneuvers are the samurai cousins of civilian aerobatics. Fighter aces had maneuvers named after them. Max Immelman. Jimmy Thatch. Saburo Sakai’s favorite evasive maneuver was the snap roll.
Snap rolls. Snap the nose high and deliberately stall one wing. The opposite wing, still flying, twists the airplane into violent corkscrew. A horizontal spin!
Recover.
Breathe. Awed that I could stall a wing at high speed. Sure. Speed has very little to do with it. It’s all about angle of attack.
In flying schools, the theory looks fiercely boring. Bernoulli and laminar flow and L=CL ½r S V² yada-yada, right? Those of us who are professionally curious dive deeper into stalls through books and websites. They want to get it. Bravo!
Those who choose to stay ignorant, or who are ignorant about their ignorance, are potential mass murderers.
We now know about stalls. It’s time to study three catastrophic accidents.
It is worth repeating the quote that started this whole article.
That is the real danger: this faulty reaction to the stall. A pilot is killed because he either fails to recognize the stall for what it is, or fails to control that impulsive desire to haul back on the stick.
— Wolfgang Langewiesche
#1 in the quiz is False. Langewiesche, the Yoda of aerodynamics, published his wonderful book, Stick and Rudder, in 1944.
Posted from Bangkok, August 20, 2011
Credits:
A delightful article about F-1 car wings!
Curiously, Microsoft Flight Simulator has a good lesson on stalls
AOPA Online: If you don’t fully understand angle of attack, you are a candidate for joining the more than 100 pilots [last year] who crashed in their attempt to make an aircraft do the impossible. Accidents are the ultimate manifestation of confusion.
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Flying in Crosswinds 