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?
-
Langewiesche is lead investigator for the Air France 447 crash.
-
An airplane stalls when it gets too slow to fly.
-
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.
.
A good read. Interesting insight. 🙂
LikeLiked by 1 person
Thanks, sir, glad you enjoyed it! 🙂
LikeLike
Very well said Tonet, a very good read indeed!
LikeLiked by 1 person
Tonet, did it take you days because someone you knew stalled and didn’t recover? As you write so well, that’s my conclusion.
Fred Scott, a buddy in the US, has been pushing angle of attack indicators – he’s lost people close to him due to stall/spin accidents. As the plane stalls at the same angle of attack at any speed – it’s good to avoid higher angles of attack, isn’t it?
Ron
LikeLiked by 1 person
I had a tough time because I had procrastinated on it for so long, despite lots of ominous signs that another accident would cap the stall trilogy. Then it did happen, very close to home, very tragically. Just this month, Martha Lunken said in her delightful column in FLYING that the depressingly inevitable most probable cause of the next accident is always fuel or stall/spin.
LikeLike
Hello, sir Ron!
Angle of attack indicators, that sounds like a good idea. Wonder how that works and looks like. I must research.
LikeLiked by 1 person
wow… enjoyed to read your post!
PARE is the ‘word’ to remember – helps in the most of the cases I can imagine.
It is easy to remember that AoA is difference between where a wing is pointing and where it is going 🙂
AoA indicator looks like this – http://upload.wikimedia.org/wikipedia/commons/6/66/Angle_of_attack_indicator_1.jpg
LikeLiked by 1 person
Thanks, Timo. I really had to work on that article. There are so many myths about stalls. And I am still stunned that many commercial pilots out there never actually trained to recover from a stall, in primary training.
LikeLike
So that’s how it looks like! Thanks. I always thought the AoA Indicator was the one found on Navy jets. The one with the doughnut light in the middle.
LikeLiked by 1 person
For really precision approach work, e.g. on an aircraft carrier, the US Navy had long decided that AOA was the primary indicator, rather than speed. They were mostly mounted on the left canopy bow. Right between the pilot’s eye and the LSO on the deck.
LikeLike
sorry to be a party-pooper fellows but the link showed a photo of an AoA sensor, not an AoA indicator.
LikeLike
sorry i didn’t mean to be rude by signing as anonymous. finger-trouble on my part.
cheers.
LikeLiked by 1 person
Don’t worry about it 🙂
LikeLike
AoA indicator – few of them
You can choose which one you like most 🙂
LikeLiked by 1 person
What are we flying, here? Gyrocopter?
Oh, wow … 🙂
LikeLike
I believe Timo’s photo is either of an ultralight or similar. Not sure you can mount an AOA sensor on a gyrocopter’s rotor blades, but I could be completely wrong!
LikeLiked by 1 person
[…] The National Transportation Safety Board made a video based on data from the flight data recorder, or ‘black box’. Remember our last article, Death by Ignorance. […]
LikeLike
AF447… I will arm myself with this Tonet. Thanks!
LikeLiked by 1 person
Remember, stories 🙂
LikeLike
Stall Sir! Ease back pressure, level wings, prop to full, give full power. Airspeed 80 nose horizon, Airspeed 100 throttle 24 prop 25. Airspeed 110, throttle 21, prop 23. Back to straight and level flight Sir! Hands off demo.
That was the stall series training in a Cessna 172 that I got from military flight school 10 years ago. I just pulled it out from memory and I hope I got it right.
Training! That’s the key….
LikeLiked by 1 person
Yes, most accidents in this series are training failures. Not the pilot, but the way he was trained.
You remember the procedure correctly 🙂 Although there is no need to achieve 110 knots before reverting to straight and level. Nothing wrong with it. It just gets close to Va or maximum maneuvering speed.
When I learned about blade stall in helicopters it was all new to me, and not very intuitive. You probably know that very well 🙂
LikeLike