1. Why do stalls matter?
Collectively, pilots have a long history of being afraid of spins, or tailspins as they were once called. This is not without some justification because many people have lost their lives in accidents in which a plane entered a spin and did not recover.
The only way a plane can enter a spin is by way of a stall. Accordingly, many pilots are afraid of stalls, a stall being the gateway to a spin, and a spin being the equivalent of disaster.
The objective here is to explain exactly what a stall is, what causes it, and how to avoid it. A stall is not something we need to fear unnecessarily, but it is definitely something we need to understand. In a stalled condition, an airplane is essentially out of control. Therefore, we need to be able to recognize the approaching stall and know how to recover from a stall promptly, before it develops into a spin.
We begin with a brief discussion of how airspeed and the angle of attack affects the lift produced by a wing. The difference between slow flight and fast flight is then described, and finally, a power-off stall, as done for practice. Adverse yaw is described in conjunction with the use of ailerons in a stall.
2. Lift and Angle of Attack
The bottom line of lift production is that the air flowing over a wing is deflected downward as it leaves the trailing edge of the wing. The lift is the reaction force to this downward thrust imparted to the air.
For the downward deflection to occur, the air must flow smoothly across the top surface of the wing. It must be able to “make the downward curve” to follow the contour of the wing's top surface, essentially traveling parallel to the top surface at all points from leading to trailing edge, when viewed close in.
The air does this easily at low angles of attack. But as the leading edge moves upward (corresponding to a larger angle of attack), the turn the air must execute becomes sharper. With increasing angle of attack, a point is reached where the air no longer makes the turn as smoothly as before. It becomes turbulent and erratic in the area above the wing.
At this point the lift decreases. It becomes less because the air, being unable to make the downward turn, is not deflected downward as efficiently. The wing is approaching the stall.
With only a slight, further increase in the angle of attack, the turbulence will increase dramatically. Instead of being deflected downward, the air now spills off behind the wing in a wake of turbulent, erratic swirls that stream off almost straight behind the wing. And because the downward deflection of the air is minimal, the lift will be only a small fraction of what it was before. The wing is now fully stalled.
3. Airflow over a Boat Paddle
Here's a simple experiment that demonstrates the effect of increasing the angle of attack. It makes the point that it is the angle of attack, and not the speed, that produces the stall. Here the speed remains constant.
Suppose we ride in the back of a pickup truck doing 60 mph down a smooth, level road. We sit comfortably in a lawn chair, with a boat paddle stuck out to the side so that it acts like an airplane wing. We can change the angle of attack of the paddle by rotating it so that it takes smaller and bigger “bites of air.”
With the plane of the paddle horizontal, all we feel is a small force toward the rear. This is drag. We feel no upward or downward force on the paddle. The angle of attack of the paddle is zero degrees.
Now rotate the paddle so that the leading edge points upward slightly. We now feel an upward force (lift) along with the drag. If we rotate the paddle more, the lift increases. More angle gives more lift. Just behind the paddle, the air is moving downward, having been deflected by the paddle.
But if we rotate it up past a certain point, the lift becomes much less and the drag increases dramatically. At the same time, the paddle develops a jittery, shaking motion.
What's happening is that the air flowing over the top of the paddle is unable to make the downward turn in order to flow smoothly across the top surface. Instead, it breaks away in erratic, turbulent swirls which trail out more or less straight behind the paddle. There is little if any downward deflection of the air, and therefore much less lift than before. The paddle is in a stall.
If we go all the way and rotate the paddle so that it's flat to the wind, no lift at all is produced. All we feel is drag and the jittery shaking of the turbulent air. In this case, the angle of attack is 90 degrees.
Now, we can consider an airplane as being a fuselage with boat paddles, somewhat refined, sticking out on each side. What we do to rotate the paddles (wings) to a greater angle of attack is lift the nose of the plane. It's a very similar thing.
4. Lift vs. Airspeed and Angle of Attack
Without getting too deep into theory, we can say:
2. Lift increases with the angle of attack ... up to a point, and then it doesn't any more; it gets smaller.
More precisely, lift is proportional to the square of the airspeed. If you double the airspeed, the lift would be multiplied by four. Or, a 41% increase in airspeed will double the lift. For example, at the same angle of attack, an airspeed of 85 mph will produce twice the lift as 60 mph.
Lift is directly proportional to the angle of attack in the range from zero up to perhaps 15 degrees. Then as the angle of attack is increased more, the lift increases, but more gradually, until it reaches a peak at the critical angle. >From this point on, the lift gets less instead of more. A slight increase in angle of attack beyond the critical angle results in a sharp decrease in lift i.e., the stall.
5. Straight and Level: Weight = Lift
Suppose we're flying a plane that weighs 1,000 lbs, everything included. In straight and level flight, the lift produced by the wings must be exactly 1,000 lbs. That is, lift equals weight, exactly.
Let's look at this more closely. Assume our airspeed is 60 mph. At this speed, what causes the lift to be exactly 1,000 lbs? It is the angle of attack. The pilot adjusts the pitch of the nose so that the plane flies level.
What the pilot is doing, in fact, is adjusting the angle of attack so that at 60 mph, the lift produced is 1,000 lbs. Remember, lift depends upon both airspeed and angle of attack.
Now suppose the pilot decreases the airspeed to 50 mph. To maintain straight and level, what must be done to the angle of attack? It has to be increased so that the wings still produce the same lift (1,000 lbs) at a slower speed.
The point is this: many combinations of airspeed and angle of attack will produce the same lift. Slower airspeeds require greater angles, and vice versa. However, the drag will not be the same in all cases so that some adjustment of the engine power setting will be required.
6. Fast Flight and Slow Flight
There is little challenge in flying fast, say at airspeeds equal to twice the stall speed or greater. The controls are crisp and responsive and in straight and level flight, the longitudinal axis of the plane will be almost level. All the pilot needs to do is adjust the pitch attitude of the nose so that the plane neither climbs nor descends.
At high airspeeds, the angle of attack will be fairly small because the airspeed component of the lift equation is large. Recall that when the airspeed is high, the angle of attack must be smaller to produce an amount of lift exactly equal to the overall weight of the plane.
Pilot training almost always includes instruction in “slow flight,” where the airspeed may range from 1.5 times the stall speed all the way down to the stall. Slow flight is a bit more demanding, and if you get too slow, a bit of a surprise may be in store.
First of all, at low airspeeds, the controls are less responsive. Air will be flowing past the ailerons, rudder, and elevators more slowly, and the forces these control surfaces are able to apply to the plane are much less.
Therefore, larger movements of the stick and rudder pedals will be required than when flying fast. The controls will feel lax and mushy as compared to crisp and responsive.
Because the airspeed is low, the angle of attack will have to be much larger in order to produce the lift required to sustain straight and level flight. That is, the plane will have to fly along nose-high. And the slower the speed, the higher the nose must be.
So here's the picture in slow flight: airspeed low, nose of airplane pointed way up, engine working hard to overcome the drag produced by the large angle of attack; the controls are soft and mushy, and the plane tends to wallow a bit. It's a different sort of flying, which is why pilot training makes a point of it.
Why is it important to be able to fly the plane slowly? Well, obviously, there may be occasions when you need to slow down, like when you are overtaking another plane in the pattern. Or you may be attempting a landing over an obstacle into a short field, in which case you need to make the approach as slowly as possible.
But here is another reason, the big reason. The lower end of the airspeed range of slow flight ends at the stall speed. In making the transition from normal flight to an inadvertent stall, you must first slow down to the stall speed. In so doing, you will be doing “slow flight” and will experience the sloppy controls and mushy response of the plane.
So if you are, in advance, familiar with the way the plane feels and responds to the slow flight condition, you will be more likely to realize what is happening as you approach an unintentional stall. Hopefully, you will recognize it in time to take corrective action and prevent the stall.
And, a pilot should take pride in being able to fly the plane safely and with confidence at all parts of the flight envelope.
7. Stalls and Pilot Training
Pilot training includes considerable practice “doing stalls.” This is done at altitudes high enough to give ample room for recovery if a practice stall should go sour. And “pilot training” implies the presence of an instructor.
The objective of doing stalls is to become familiar with the behavior of the plane as it approaches the stall and to practice the recovery until it becomes second nature. It's life insurance.
As described in pilot training manuals, there are several types of stalls insofar as the flight condition leading to the stall is concerned. In all cases, however, the stall occurs because excessive angle of attack disrupts the air flow over the top surface of the wing. (Remember the boat paddle.) The simplest is the power-off stall, which we now describe.
(Other stalls are described in the article entitled, “Advanced Stalls.”)
8. The Power-Off Stall
With the plane flying straight and level at a safe altitude, engine power is reduced to idle. Even so, an attempt is made to continue the straight and level flight. But without power, the plane will slow down. As it slows, the angle of attack must be increased to avoid descending. That is, you have to keep pulling back on the stick, gradually, more and more, and raising the nose to hold altitude.
When the plane reaches an airspeed just slightly above the stall speed, with the nose way up, you will feel the plane give a hint of unsteadiness, a bit of roughness or ripple in the flight path. This is the point where the airflow over the wings is beginning to break up. The lift is becoming unsteady.
Shortly thereafter, the nose of the plane will drop as the wings stall and lose lift. (This is known as the break.) At this point, the plane is essentially out of control. The airspeed is so low that the ailerons will likely not be effective, the elevators just gave out (the nose dropped even while you were holding the stick all the way back), and the rudder effectiveness is significantly reduced.
The question now is, What can be done to recover? Well, what led to the stall in the first place? Excessive angle of attack; holding the nose up as the airspeed decreased to the point where the airflow broke up over the top of the wings.
Therefore, to recover, all that needs to be done is to reduce the angle of attack. That is, allow the stick to go forward. This will lower the nose and the plane will pick up speed quickly. The airflow over the wings will straighten out, and the plane will be flying again. Gentle backpressure on the stick may then be used to raise the nose out of any dive that may have resulted from putting the nose down to gain speed.
This seems simple enough, and it is. The break is likely to be quite gentle in a well-designed plane. In fact, the break may not occur at all, in which case the plane just mushes. That is, if held in the stall, it simply develops a high sink rate without the sudden drop of the nose.
Now, what's bad about a stall is that the plane will lose perhaps 300 to 400 feet of altitude in the process of stalling followed by the (power off) recovery. If the stall occurs at low altitude, the plane may impact the ground before the recovery is completed.
Use full power in the recovery.
It is possible to recover from a stall as described above, but the recovery can be accomplished sooner with much less loss of altitude if full power is applied as soon as the stall occurs. In fact, the standard technique of stall recovery calls for the application of full power as soon as the stall is recognized.
With application of full power, it is possible to stall and recover to level flight with a loss of altitude of less than 100 feet.
An unnatural thing to do:
At the moment of the stall, when the break occurs, the nose of the plane is dropping of its own accord. And in the mind of the pilot trainee, what you do to lift the nose is pull back on the stick.
But in a stall, chances are you already have the stick pretty far back, and if you pull it back more, it just makes the situation worse. Pulling back on the stick just makes the plane come down faster!
(In the next article, an oscillating stall is described in which the stick is purposely held back after the break occurs. What happens then is often impressive, even for experienced pilots!)
The key to stall recovery is to release stick backpressure, even while the nose is dropping. This is not a natural thing to do; it's something the new pilot must learn and practice until it becomes instinctive.
9. A more-complete picture:
No mention was made of the rudder in the discussion above but proper use of the rudder is vital, both in the approach to a practice stall and in the recovery.
For the plane to break straight ahead with no tendency to turn or roll during the break, the approach to the stall must be ramrod straight, wings level, and controls coordinated. If the plane is turning even slightly during the stall, it is likely that one wing will drop sooner and faster than the other.
This is no big deal, but you must know how to counteract the dropping wing. You DO NOT do it by moving the stick to give aileron input. At the slow airspeed of the stall, aileron effectiveness is minimal, and adverse yaw (see next section below) will tend to pull the low wing around, making the problem worse.
The effective control is the rudder. Give rudder in the direction opposite the low wing. This will cause the low wing to move forward relative to the high wing. It will then generate slightly more lift, and rise accordingly. Aileron effectiveness will return only as the airspeed builds up again.
Rudder input opposite the low wing will also prevent the rolling tendency from accelerating. The plane must turn in the direction of the low wing in order to enter a spin, so application of opposite rudder wards off the spin as well as picking up the low wing.
If a wing drops “way down,” rudder input alone may not be sufficient to lift the wing promptly. However, the opposite rudder will prevent the plane from turning in the direction of the low wing. So, if the stick is forward, the nose of the plane will drop, airspeed will increase quickly, and you can then use the ailerons to pick up the low wing.
10. Adverse Yaw
When an attempt is made to bank a plane like a Challenger, the downward deflected aileron produces additional drag which tends to slow that wing down relative to the other. This creates a tendency for the plane to turn toward the wing whose aileron is down.
Suppose we wish to bank to the right. Stick movement to the right causes the left aileron to move downward; the right aileron moves upward. The left aileron, being down, produces drag that tends to pull the nose of the plane to the left.
The plane will bank to the right, but at the same time, the nose will tend to move toward the left, which is the “wrong way.” The result, if uncorrected, is an awkward left-hand movement of the nose while the plane is entering the bank to the right. This is adverse yaw.
The adverse yaw tendency is easily compensated by rudder input in the direction of the bank. A pilot soon learns to coordinate the rudder and aileron inputs so that the adverse yaw effect becomes an insignificant issue.
11. Afraid of Stalls?
It is not all that uncommon for an otherwise good and proficient pilot to be terrified of stalls to the point that the notion of doing one “for practice” is seen as being sheer stupidity. If this shoe happens to fit, here's a suggestion that may help.
First of all, a stall as normally done for practice proceeds rather quickly from straight and level and under control to very nose up and mostly out of control. Too many things are happening at one time for the struggling pilot to comprehend, let alone deal with to advantage.
The suggestion is, slow it down! Practice slow flight for extended periods until flying in a very nose high attitude becomes comfortable and almost routine. You will soon develop a feel for the plane in this mode of flying.
Then take it to slower and slower airspeeds. See just how slowly you can make it fly, always holding altitude and always keeping the controls coordinated, flying straight ahead. You will eventually be flying right on the edge of the stall, feeling the turbulence over the wings, but still flying. Controls will be sloppy, and the plane will probably tend to wallow around a bit.
Eventually, you will work the airspeed downward until the plane will stall. When it does, just release a bit of backpressure on the stick to pick up some airspeed, and then keep on flying as you were before. That is, use the rudder to keep the nose from turning, be gentle with the ailerons, and use the elevator to control the airspeed.
Most people who try this approach get over the fear thing rather quickly, and some even claim it to be fun! But don't expect a miracle to occur after about two minutes of slow flight. You must do the slow flight thing until you are comfortable and at ease with it, and only then proceed to the point where the plane actually stalls.
A precautionary note: