Stalls 201

This is a continuation of the article entitled Stalls 101 in which a simple power-off stall was described along with a brief discussion of lift, airspeed, and angle of attack.

Several more-advanced "practice stalls" are described in the following along with the factors that come into play during the stall. Also, the effects of P-factor, propwash, and engine torque are briefly described.

1. Secondary Stall

Standard stall recovery technique demands forward stick in order to reduce the angle of attack. But also, the stick must be left there long enough for the airspeed to build up and re-establish smooth airflow over the wings. Only then can the stick be pulled back to lift the nose out of the dive that may be developing.

If a pilot lets the stick go forward but doesn't allow the airspeed to build up sufficiently before exerting back pressure to pull out of the dive, or if the back pressure is overly aggressive, the plane may be put into a second stall even before recovering fully from the first. This is called a secondary stall.

The secondary stall is likely to be less well-behaved than the first. Chances are the first stall followed a smooth, coordinated entry with little if any turning of the nose. When the secondary stall occurs, the plane is apt to have one wing lower than the other, it may be slipping or skidding, and it may even have some turning motion. And, if full power has been applied, as is recommended, other factors come into play that make the nose tend to rotate to the left. All this causes the secondary stall to be somewhat unpredictable.

Pilot training emphasizes that a stall recovery should be made with the minimum possible loss of altitude. Therefore, the nose should be left down only long enough to regain flying speed. So here's the conflict: wait too long before lifting the nose and you loose altitude that you did not need to lose. Try to lift the nose too soon and you get a secondary stall.

However, the situation is not hopeless. A pilot with only modest experience can get the feel of a new plane simply by doing one or two practice stalls. It's mostly a matter of timing, quickly learned.

2. The Power-On Stall

This practice stall is designed to emulate an inadvertent stall that might occur while a pilot is climbing out shortly after takeoff. It is also known as a “departure stall.” The engine power setting may range from moderate all the way to full power.

A departure stall will differ from the power-off stall in at least two ways. First, the nose of the plane will reach very high pitch attitudes before the stall occurs. Many pilot trainees find the nose-high attitude unfamiliar, uncomfortable, and a bit intimidating. When the nose is “way up there,” many find it more difficult to keep the wings level and the plane flying straight ahead.

Second, several effects work together to cause the nose of the plane to rotate to the left. This requires an application of right rudder just to keep the plane straight, and the amount of rudder required will vary with power setting and pitch attitude.

Three effects that tend to turn the nose to the left:

(1) The countertorque of the engine, the action-reaction thing as it relates to the turning prop. If the torque produced by the engine tends to rotate the prop to the right, it exerts an equal torque on the airframe that tends to rotate it to the left. For the plane to remain upright and stable, the left wing must generate more lift than the right wing.

Some planes are designed with extra lift built into the left wing. Other planes will require a touch of right aileron (and rudder to offset adverse yaw) to counteract the torque effect when the engine is at full power.

(2) P-factor (propeller factor). This effect arises when the rotational axis of the prop is at an angle to the relative wind, which will be the case at high angles of attack, i.e., near the stall. It is more pronounced at high power settings.

The major contribution to P-factor arises because the propeller blade that is descending travels through the relative wind faster than the one that is rising.

Consider a plane whose prop rotates clockwise when viewed from behind the plane. (Pusher or tractor, doesn't matter.) Further, suppose the nose of the plane is pitched way up in relation to the relative wind; that is, the relative wind is coming from the bottom front of the plane.

Now, the blade of the prop that is traveling downward on the right will be flying into a headwind. The one on the left will have a tailwind. The headwind blade on the right generates more thrust than the tailwind one on the left. Hence, the plane tends to rotate to the left.

(A secondary contribution to P-factor arises because of a difference in angle of attack between the rising and descending blades.)

(3) Propwash on the vertical stabilizer and rudder. A prop gives a spiraling, corkscrew motion to the air in the process of propelling it toward the rear of the plane. This spiral wraps around the fuselage with the air flowing to the right across the top (when viewed from behind the plane) and flowing to the left on the bottom. And it strikes the tail surfaces.

On most planes, the vertical stabilizer and rudder stick up more than they extend downward. Consequently, more of the spiraling propwash strikes the top of the vertical stabilizer and rudder, pushing it to the right, than strikes the bottom of the tail section, pushing it to the left. This results in a net force on the tail that pushes it to the right. Hence, the nose of the plane tends to move to the left.

The Challenger

The Challenger is a bit unusual in that the prop rotates clockwise when viewed from the front. Therefore, the engine torque must be balanced by additional lift produced by the RIGHT wing instead of the left. It follows that the torque effect tends to make the nose rotate to the right instead of to the left like most other planes.

Further, the effect of P-factor is reversed as well and tends to make the nose rotate to the right.

However, because the rear part of the Challenger is located below the thrust line of the prop, as is most of the vertical stabilizer, a very strong propwash effect comes into play that overrides the effects of torque and P-factor. Consequently, the nose of the Challenger tends to rotate to the left when power is applied, which is what most pilots expect and consider normal.

Doing the power-on stall for practice:

Starting from straight and level flight at cruise RPM, reduce the engine power and let the plane slow to a few mph above stall speed. (This avoids zooming.) Then apply power in the amount desired. Raise the nose so the airspeed does not build back up. Apply rudder as required to keep the plane from turning left, and be sure the controls are coordinated.

Slowly increase the back pressure on the stick while keeping the plane straight, wings level, and coordinated, and wait for the stall.

When the break occurs and the nose drops, chances are good that the plane will roll one way or the other. Everything has to be exactly in line to get a break straight ahead. The slightest turning or lack of coordination will tip it to one side. No big deal; just be ready with the rudder!

The procedure for recovery is very nearly the same as with the power-off stall. Release back-stick pressure, let the nose drop to regain flying speed, do whatever is required with the rudder to keep the nose from turning. Rudder action will also tend to lift a wing that has dropped. Apply full power if you aren't already at full power. Keep the ailerons neutral until flying speed is regained, and then use coordinated controls to level the wings. Finally, apply back stick pressure to pull out of any dive that may have developed.

3. A Turn Requires More Lift.

To do a turn without losing altitude, the pilot must exert backpressure on the stick in order to increase the lift produced by the wings during the turn.

The reason for this is that the lift is always directed perpendicular to the wings, and when the wings are banked, the direction of the lift will be tilted away from the vertical. However, the vertical component of the lift must always equal the weight of the plane when the plane is in a level turn. Therefore, greater lift is required when the plane is in a bank because only a fraction of the total lift is directed straight up to oppose the pull of gravity.

For example, assume a plane weighs 1,000 lbs. In straight and level flight, the lift must be exactly equal to the weight, namely, 1,000 lbs. In a 30-degree bank, the total lift must be 1,155 lbs in order to produce a vertical component equal to 1,000 lbs.

At a bank of 45 degrees, the total lift must be 1,414 lbs; and at 60 degrees, the total lift must be 2,000 lbs.

G's, G loading, or G factor:

The multiple of the weight that gives the required lift is often given instead of the actual lift in pounds. This multiple is the G factor. So, a 30-degree bank produces a G loading of 1.155. At 45 degrees, the G-loading is 1.414, and at 60 degrees, it is 2.0. That is, a level turn at 60 degrees of bank is a 2-G turn.

The G loading goes up sharply as the angle of bank goes beyond 60. At 65 degrees, it is 2.37; at 70 degrees, 2.92; at 75 degrees, it is 3.86. This is good reason to keep angles of bank to 60 degrees or less in level turns.

4. Stalls While in a Turn

Every pilot should be familiar with the characteristics of a stall that occurs while the plane is turning. Because of the extra lift required to execute a level turn, it is easy to lose airspeed in a turn unless additional power is applied. If the speed is low going into the turn, and if the airspeed is allowed to drop significantly, a stall could be forthcoming.

Increased stall speed in a turn:

Also because of the extra-lift requirement, the stall speed will be higher in a turn. However, this increase is not dramatic if the angle of bank is kept to 30 degrees or less. If a plane stalls at 30 mph in level flight, it will stall at about 32 mph in a 30-degree bank.

In a 45-degree bank, it will stall at about 36 mph. In a 60-degree bank, the stall speed will be about 42 mph. This is considerably higher than 30 mph (41%, actually), and if a pilot becomes accustomed to always looking at 30 mph as the stall speed, a bit of a surprise may be in store.

These numbers, from pure theory, are based on the fact that the lift goes up as the square of the airspeed, as described in Stalls 101.

Different wing speeds in a turn:

Another factor that enters into the stall-in-a-turn scenario is that the inside wing travels through the air just slightly more slowly than the outside wing. This is because the circular path traversed by the inner wing is slightly smaller than that of the outside wing.

The result is that the inner wing will likely stall first while the outer wing is still producing a bit of lift. This tends to cause the plane to roll to the inside of the turn. When you consider that the plane is already banked to the inside, and the roll, if it happens, will add to this bank, it is easy to see that the bank angle during the stall could possibly steepen significantly.

This effect is more pronounced for sharp turns at large angles of bank.

A stall in a perfectly coordinated turn:

No effects will arise due to either a slip or skid if the turn is coordinated.

To practice this stall (with an instructor), enter the turn normally and establish the desired angle of bank. It is prudent to keep the bank shallow at first, perhaps only 10 or 15 degrees. The power setting of the engine can be anything from idle up to, say, normal cruise power. Anything beyond that is going to be exciting.

Once in the turn, increase back pressure on the stick. This will produce a bit of a climb at first. Be alert for any tendency of the angle of bank to increase, and be sure the rate of turning does not increase significantly. Apply opposite aileron and rudder, coordinated, to counteract either of these. Follow the suggestions of the instructor.

Gradually increase the back pressure until the stall occurs. Be on the alert for any tendency of the plane to roll, and be ready to apply rudder appropriately when it does. If perfectly coordinated, theory says it will roll to the inside. In a real situation, given the variables, it can roll either way, or perhaps not at all.

Nevertheless, the plane was already in a bank, and the objective is to establish straight and level flight promptly. Therefore, some rudder action will be called for even if no roll arises from the stall.

Release backpressure on the stick to lower the nose, let the airspeed build, and then use coordinated aileron and rudder to bring the wings back to level. At the proper time, exert back pressure on the stick to raise the nose back to the level flight position. Be gentle with the backpressure so as not to produce a secondary stall.

Follow the recommendation of the instructor as to whether you add power during the recovery in a practice stall. The preference may be, at least at first, to not add power in order to avoid the yaw tendencies that high engine power produces. Then after some proficiency is gained in the power-off recoveries, power may be added when the stall occurs.

Stall in an uncoordinated turn:

The slip or skid of an uncoordinated turn has a huge effect on the behavior of the plane if it stalls in the turn. The fact that it is either slipping or skidding implies that it is, to an extent, moving sideways through the air. Consequently, one wing will see a pattern of airflow different from the other.

The wing to the rear of the sideways movement will be in the wake of the sideways-moving fuselage. It will be flying in disturbed air, and will tend to stall sooner as a result. With one wing stalled while the other is still flying, a rolling tendency will occur.

Now, suppose the plane is making an uncoordinated turn to the left and is slipping through the turn. In this case, the right wing will be in the wake of the fuselage, and will tend to stall first. The rolling tendency will be to the right, which will tend to level the wings. That's good! In many cases, a recovery from a stall in a turn can be effected simply by leveling the wings.

But if the plane is skidding to the outside of the turn, because of excessive rudder input for the angle of bank, the left wing will be in the wake of the fuselage and will tend to stall first. This will a produce a rolling tendency that will increase the angle of bank. That's not good.

If you're following this closely, you will recall that in a turn to the left, the left wing is traveling more slowly than the right and will tend to stall first, producing a roll to the left. However, if the plane is slipping in the turn, the right wing will stall first and give a roll to the right.

Here we have two opposing tendencies. Which one will win out over the other? It's hard to say. How sharp is the turn? How severe is the slip? It's a matter of degree. For any given stall, depending upon the details, the plane may go either way. Such is the nature of flying.

5. Cross-Control Stall

As you might suspect, this is a stall that occurs when the aileron and rudder inputs are opposite. For example, when right rudder is applied while the plane is banked to the left, or excessive left rudder when opposite aileron input holds the bank at a shallow angle.

Such a stall may occur unexpectedly while in a slip or when attempting to sharpen a turn by excessive rudder input without the corresponding aileron.

Stall out of a forward slip:

Suppose we fly straight ahead, but apply right rudder and left aileron. These two control inputs oppose each other so that the plane continues to fly in the same direction. However, it will be skewed around with the nose pointing to the right while banked to the left. In fact, this is just a forward slip.

Now apply a bit of backpressure and let the airspeed drop. What's going to happen? It will stall, of course, but what might we expect the plane to do in the stall?

Most likely, it will stall and roll toward the right wing because the right wing is flying in the wake the fuselage produces as it slips to the left. Further, the roll is apt to be sharp and pronounced so that, in a heartbeat, the bank goes from one to the left to a fairly steep one to the right. This reversal can catch many a pilot unaware.

The recovery calls for a quick release of the backpressure on the stick along with fairly aggressive rudder opposite the roll, to the left in this case. After the nose is well below the horizon and the airspeed back up, coordinated rudder and aileron can be used to level the wings. Then gentle backpressure on the stick is used to pull out of the resulting dive. Full power is appropriate in an inadvertent stall, but for instructional purposes, your instructor may recommend less than full power for a practice stall.

Cross-controlled stall in a climbing turn:

Suppose we're in a climbing turn to the left at very nearly full power. Aileron pressure holds the plane in a left bank, but for whatever reason, we're applying considerable right rudder pressure, or at least we are not coordinated. The plane is in a fairly severe slip to the left.

If the plane is allowed to stall in this condition, namely, nose high, banked left, slipping to the left, it will very likely roll sharply to the right and give the appearance of wanting to go on its back. This calls for forward stick and left rudder in short order, but remember, until you get flying speed back, go easy on the ailerons.

If you were to apply full left aileron early during the roll to the right, chances are that the adverse yaw would pull the nose around to the right and you would be on the verge of a spin.

Considerable discipline on the part of the pilot is called for in delaying the aileron input in the recovery from a stall. After all, in normal flying it becomes a reflex: right wing down means stick to the left. But in a stall, aileron input can have just the opposite of the intended effect, and that is scary!

Stall during a skidding turn:

Suppose a pilot is making the turn from base to final with a bank angle of about 30 degrees. This is plenty steep for turns in the pattern. However, the rate of turn is such that the plane is going to overshoot the runway. The turn needs to be sharper.

But to sharpen the turn, a greater angle of bank is required. The instructor said not to use more than 30 degrees of bank in the pattern. So, the pilot applies more left rudder in an effort to make the plane turn, but holds opposite aileron to prevent the angle of bank from increasing. And being distracted by the prospect of overshooting the runway centerline, the airspeed drops to near the stall speed.

This is bad. This is really bad! A skidding-type, cross-controlled stall is about to occur at an altitude just a few hundred feet above the ground.

Here's the picture: plane banked 30 degrees to the left; aggressive left rudder is applied in order to try to tighten the turn but in fact merely produces a skid to the right; right aileron is applied to prevent the angle of bank from increasing. The airspeed is low and continuing to drop because of the increased drag of the skid.

The left wing is in the wake of the skidding fuselage. Also, it is the wing on the inside of the turn, traveling more slowly than the right wing. It will almost certainly stall first.

The result is that the left wing stalls while the right wing is still producing lift. This will produce a strong roll to the left that will occur suddenly and sharply. The plane was already banked 30 degrees to the left, so the roll begins from that point. What's the plane going to do?

It's going upside down.

Forget the recovery. Not enough altitude.

( )  ( )  ( )  ( )  ( )

Roll toward the rudder:

In almost all cross-control stalls, the plane will break and roll toward the rudder. That is, if right rudder is being held prior to the stall, the roll will be to the right.

For example, in a slip involving right rudder and left aileron, the plane will be banked to the left, but the roll that occurs with the stall will be to the right. The plane will roll through level to a steep bank in the other direction.

6. Accelerated Stall

A stall can occur at any airspeed if the angle of attack of the wings exceeds the critical angle by only a small amount. Remember, it is the large angle of attack that is the root cause of a stall, and not low airspeed alone.

Many a dive bomber pilot has paid the ultimate price by diving to too low an altitude and being forced to pull up sharply. The resulting high G loading and extreme backpressure on the stick results in excessive angle of attack and a stall, even while the plane is traveling at hundreds of miles per hour. Insufficient altitude for recovery ...

Now admittedly, it is unlikely that an ultralight pilot is going to be diving to low altitudes over targets (unless you dive down to buzz a friends house or car), but a similar situation can arise in a steep turn. That is, if you are doing a steep turn at a bank of 60 degrees, you will be pulling 2 G's if the turn is level. Accordingly, the stall speed will rise to 1.41 times its straight and level value. If your airspeed drops below this value, you will stall, and it will be an accelerated stall.

Because of the extra speed and the possibility for very rapid attitude changes, accelerated stalls are not something that is routinely practiced by UL or even GA pilots. It is easy to overstress the airplane, perhaps beyond its limit. For the average plane and pilot, they are just as well left alone. Just keep the airspeed up in those steep turns. Incidentally, a 45-degree bank is plenty steep for most ultralights.

7. Oscillating Stall

This stall provides a good demonstration of what can happen if a pilot trainee is reluctant to release the backpressure on the stick at the break of a stall.

The entry is by way of a normal, unexciting, power off stall, straight ahead. However, when the break occurs, do not release the stick backpressure. Hold the stick back! (This is just as unnatural to an experienced pilot as releasing the stick pressure is to a beginner.)

The plane will wind up in a deep stall with a high sink rate. Nevertheless, the nose of the plane will be very near level-flight attitude. Keep the ailerons neutral.

The plane will then be very nervous and twitchy about the yaw axis. It will want to dart off in one direction or the other, tucking a wing under as it does. But don't let it!

Use fairly aggressive rudder action to keep the nose of the plane straight for as long as you can, or until you turn chicken.

What typically happens is that the nose of the plane begins to sway back and forth from left to right to left, etc. This is an oscillation about the yaw axis. And, get this, the oscillation can easily become divergent, especially if you over-control with he rudder.

That is, the back and forth motion of the nose becomes stronger and stronger. The nose makes bigger and bigger swings back and forth until it is impossible to control. The motion can become violent, even with a plane that is normally very docile (like a Cessna 172).

This is the time to let the stick go forward and recover because the plane is literally begging to enter a spin. The sink rate will be very large with the wings literally falling broadside through the air.

The key to the demonstration is not to let it go too far before releasing the stick backpressure. Let a student pilot see this one time, and from then on, the stick will go forward at the first hint of a stall!

Old pilot saying: “The more you do to get an airplane tangled up, the more you will have to do to untangle it!”

Keep this in mind as you go out to practice a power-on, turning stall, with crossed controls.

8. Conclusion

The intent of this article is to describe the various ways that a plane can wind up in a stall and the forces and effects that come into play. In no way is this to suggest that you should go out and try any of these without the benefit of a competent instructor, and then only if the instructor is of the opinion that the maneuver can be carried out safely.

In particular, a cross-controlled stall will typically involve a large attitude change that will occur quickly, an accelerated stall is apt to put undue stresses on the plane, and the oscillating stall, if allowed to go too far, can become quite violent, throwing things (or people) around the cockpit.

A good pilot should be aware of what a plane can and will do if provoked either by inattention, carelessness, or ham-fisted piloting technique. And being aware, a good pilot will take care to avoid the situations that could prove troublesome or unpleasant.

Regrettably, this is another one of those articles that puts a negative light on flying. Makes it seem dangerous and scary rather than fun. However, to know where the pitfalls are and to avoid them is better than to not know where they are and perhaps stumble onto one of them, unaware.

Doc Green