Everyone knows the stalling speed of their aircraft right? Nearly everyone does – it’s the first number I look for when I’m getting ready to fly an aircraft. There are lists of numbers that are very important to the operation of the aircraft and must be known, however, that Stall Speed is probably ‘Speed Number One’ for my memory banks. With it, I can get an appreciation of approach and landing speeds, and estimate speeds required for steep turns and aerobatic maneuvers. I also like the best rate of climb speed, as that is handy to know to efficiently climb, and is also a useful glide speed. Of course, it’s very important to know of other limiting speeds such as maximum speed to deploy flaps, undercarriage, open canopies, normal operating, and never exceed speeds. Then there are other numbers suitable for systems such as engine temperatures and pressures, oil system, fuel system – it’s almost endless.
1. Aircraft stalls can happen at high speed
So now that I know the stall speed, all I have to do to stay safe is fly a bit faster than that, and I’ll never stall. That’s easy logic, and of course, it’s wrong. But there are pilots out there who avoid stalls by simply flying fast – and they claim that they don’t need to practice stall recoveries because they never get into situations that demand recovery actions. They just stay faster and vow to never fly too slow. This tactic works great – until the day comes when they get distracted, upset by turbulence, have a flight control issue, an engine failure, try to fly around terrain during bad weather, or any other of the possible reasons for stalling – and they suddenly realize that their toolset is missing the ‘stall recovery tool’ when their life now needs it the most.
2. High angle of attack leads to aerodynamic stalling
Having flown with quite a few experienced pilots, in unfamiliar phases of flight such as aerobatics or formation, I’m surprised by the shock and horror they express when the aircraft suddenly goes ‘THUMP!’, shakes and we lose control. “What was that?” they ask, and they look around with eyes the size of their tires. “You stalled it”, I answer with the slightest degree of disappointment. Almost every time they respond with “But I was going really fast!”, or “But we were going straight downwards!”
You may recall that back in the days of your first aircraft type, learning about the Angle of Attack (AOA), and that was the angle between the wing chord and the relative airflow. Lift and drag increase as the AOA increases, until the critical AOA is exceeded, and lift reduces again. Generally, this means – the airplane is about to go downwards. Of course on the News Reports, the eyewitness will almost always say “He didn’t stall as his engine was still going, and he’s a hero for just missing the school about a mile away.” You know, and I know, that the ‘stall’ wasn’t related to his engine noise, and there’s a fair chance that the pilot didn’t even fly below his stall speed. But he was aerodynamically stalled – with his AOA so high that the air could not continue to smoothly flow over the top surface of the wing.
3. Aircraft weight and icing affects stalling
If you are confused, let’s remember that the stall doesn’t occur because of a magic number (the Stall Speed), but simply because of the turbulent airflow over the wing when you fly at a high Angle of Attack. It just so happens that the stall does occur at a set speed, in Straight and Level 1G flight. The book figure also suits one particular aircraft weight, so if you are lighter or heavier, then that speed alters. If your airplane has bugs or ice all over it, then that may alter figures too. It’s one speed that can change, so you can’t simply add a few knots and be safe for any flying condition.
4. High angle of bank doesn’t always cause stalling
We recall that the stall speed increases as the Angle of Bank (AOB) increases. Personally I think that’s a bit of a fallacy, and that AOB alone does not change anything. How come I can do an aileron roll throughout 360 degrees without stalling? It’s the AOA increasing with steep turns, when you haul back on the stick or yoke, that causes the airflow disruption, even at a higher speed.
A little bit of math on stalling
Here’s a little bit of Math: In the textbook example, it is shown that at a 60 degree steep level turn, lift is increased by a factor of 2 (hence you are pulling 2G), and the stall speed increases by the square root of the G. The square root of 2 is 1.41, so your stall speed increases by 1.41 times. Therefore if your clean 1G stall speed was 50 kts, then pulling a 2G turn means your stall speed is 50 x 1.41 = 71 knots. This is nothing to do with the angle of bank, just the fact that you were pulling 2G. It could happen in a climb, a vertical descent, turning at only 15 degrees, in fact, any time you pull back to 2G, you now have a new stall speed.
Think about really hard turns (say 4G for a loop). The square root of 4G is 2, so your new stall speed when pulling 4G is 2 times the old one i.e. 100kts. For those fortunate enough (or unfortunate enough) to pull 9G, your stall speed is now tripled !!!
Now most of us don’t have G-meters, and almost none of us can do the required mathematics in our head whilst flying to continually assess our new stall speed at all phases of flight, so how do we stay safe?
How to practice aircraft stall recovery
Firstly, accept that the AOA is responsible for stalling the aircraft and that YOU are responsible for changing the AOA purely by how hard you pull (or push) on the elevator. Take an instructor up high, do your HASELL checks, and practice some stalls and recoveries. At Straight and Level 1G flight, the aircraft should stall at around the quoted number on the ASI. Take notice exactly how far back you are pulling on the controls – regardless of your speed, the aircraft will always stall at that same control position. Sure, it will feel heavier if you are flying faster, but no matter what the airspeed, the stall will always occur at the same point – and at different speeds. Back in the military we simply called it ‘stick position’ and to out turn the enemy opponent or the terrain, we had to pull just short of this position with accuracy, feel the light buffet, and slightly adjust squeezing to remain just short of the stall. If we pulled even 1 millimeter too far, we’d hit the stall and our performance would suffer – and we’d get shot down.
Pulling back the wing
The simplest concept I ever heard was to forget that the control stick was a series of wires, pulleys, horns, and the elevator – but instead, imagine that it was a vertical stick welded to the top of the surface of the wing. Then when you pulled back on the stick, you directly deflected the angle of the wing compared to the oncoming relative airflow. Pulling too far back, made the air stream unable to smoothly make the corner and then consequently stall – regardless of the speed, and recovering simply meant reducing the backpressure again. Note that there is no mention of an Airspeed Indicator here – it’s irrelevant to the concept.
In summary, I’d like to emphasize that the so-called ‘Stall Speed’ is purely a known quantity for one small phase of flight and that calculating new stall speeds as you fly is pretty much impossible. But you must appreciate that it changes drastically. When you pull too hard on the controls and exceed the AOA, the aircraft can stall at ANY airspeed. The stick position for the aerodynamic stall pretty much remains the same. The most effective immediate action to avoiding the stall is to reduce the AOA by relaxing the backpressure on the stick. Think stick position, and know when to reduce it.
To help protect yourself from stalling you should chair fly and visualize standard stall recoveries, incipient stall recoveries, and incipient and fully developed spin recovery. For those conducting aerobatics or intentionally departing from controlled flight, a good quality headset that won’t fly off your head or a proper fitting flight helmet is a must.