Stalls
Stalls are frequently misunderstood. We hope this article will help explain what stalls are, how and why we learn to do them, and dispel some common myths.
Your textbook does a good job describing stalls, so we won’t repeat all that information here. Instead, we would like to highlight a few key points and provide some extra information.
For starters, what is a stall? Simply, a stall is when we exceed the critical angle of attack. That’s it! Easy. So all we need to remember is what “angle of attack” is- the angle between the airplane’s flight path and the wing. If we try to make this angle greater than the critical angle of attack- about 16 degrees- the airflow begins to become disrupted over the top of the wing and lift drops off dramatically. In plain English- we’ve asked the wing to do too much and it quit! That may seem like a very non-technical definition, but thinking of it in those terms will help us later on.
We do stalls for several important reasons. First, we do stalls so that we learn to recognize the warning signs. That is the most important reason to do them! That is so important that we will repeat it- we do stalls to help us learn the warning signs, so that we don’t accidentally stall the aircraft. We will review the warning signs in a moment. Other very important reasons we do stalls- they are helpful in learning to land, to become familiar with all aspects of the flight envelope, to overcome any fear of stalls (respect for stalls is good! Fear is not so good, it interferes with learning.) We do stalls to learn proper recovery technique, and finally we do stalls so that we can demonstrate them to the check pilot for our license.
Because the most important reason to do stalls is recognition, we should list some of the warning signs of the impending stall:
Stall warning horn
Airspeed
Pitch attitude and power setting
Excessive stick backpressure
“Mushy” controls
Buffet
Looking at each of these:
Stall warning horn- A great indication! Set to go off as you get close to the critical angle of attack. We cannot rely solely on this indication because they are sometimes inoperative. Checking it during preflight is not a guarantee that it is still functioning during flight. Some airplanes have a red light instead of a horn (lights and electric can go out!) Some older airplanes don’t have any such indicator, you just have to know the rest of the signs.
Airspeed- is also a good indicator. As long as you have a decent amount of airspeed and don’t do anything unusual, you can be pretty confident that you won’t stall. However, your instructor can demonstrate that you can stall the wing with plenty of airspeed (discussed below.) Airspeed indicators can also break, so we cannot rely totally on the airspeed for stall warning.
Pitch attitude/power setting- If we have a nose-high attitude, we better have lots of power or we could possibly stall. For example- on climb out, we have full power in and set our pitch attitude to the horizon, and we climb at 65-70 knots. With no power, the nose must be several degrees below the horizon to get that same speed. So, this is not an exact method of stall warning, but it does make sense that if we have a low power setting we better not have the nose high up.
Stick backpressure- in normal, properly trimmed flight, we don’t need to pull hard on the control yoke. If we are pulling exceptionally hard, we are rapidly increasing our angle of attack, and thus rapidly approaching a stall.
“Mushy” control forces- getting slow is usually a sign that we are in a high angle of attack. When we get slow, we have little airflow over all of our control surfaces, and they feel mushy, meaning that there is little resistance to control yoke movement, and there is little response to the movements we put in. This is very noticeable in slow flight.
Buffet- when the wing is just about to stall, the smooth layers of airflow start to separate at the trailing edge of the wing. This causes small eddies or bubbles of airflow to come off the wing and hit the elevator. You can feel these as a very gentle bumping in the control yoke. As the airflow separation becomes more pronounced you can feel this vibration in the entire aircraft as the disrupted airflow shakes the horizontal stabilizer. The buffet is a subtle sign, so it might not be immediately apparent. To help develop your feel for it, your instructor may have you hold the airplane in very slow flight for several moments, right on the edge of the stall, so that you can pay close attention to the feel. Because it occurs every time before a stall, it is a good sign to become familiar with.
Basic, power off stalls
So when we are first doing stalls, we should keep them simple and do them in a manner that will teach us all of these signs, that will help us with learning to land, and that will allow us to recover. To do this, all we need to do is set the airplane up in slow flight- the exact speed is not important. 50 or 60 knots or so. Pick a point on the horizon for a heading reference. Smoothly reduce power to idle- the nose will want to drop, but don’t let it. Instead, increase backpressure on the stick so that the nose of the airplane is touching your point on the horizon- just like your climb attitude. If necessary, make small corrections with the ailerons to keep the wings level- the glareshield (dashboard) of the airplane should be level with the horizon. Make small corrections with the rudder to keep the nose pointed straight at your point. As the airplane continues to slow, the nose will get heavy- keep increasing backpressure on the stick, enough to just keep the nose in the same attitude- touching the horizon. The horn will start to go off, the airspeed will continue to decrease, it will take more and more backpressure to hold the nose up, the controls will be very mushy, the airplane will shake a little and then the nose will drop a bit even though you are trying to hold it up. You’ve just stalled! When the nose drops, simply relax the backpressure and let the nose stay in a descent attitude. Airspeed will increase, and you’re flying again.
This is a very easy, mellow way to stall the airplane. It allows you to feel the stall coming, to learn all the warning signs, and teaches the most basic way to recover from a stall- reduce the angle of attack!
Once you have done some basic, easy stalls as described above, then you can start working on demonstrating stall recovery with minimum altitude loss. To do this, we simply need to add power after we recover. Once we have let the nose drop slightly below level pitch, the airplane should be flying again (as evidenced by increasing airspeed). Smoothly add full power and push carb heat in. Don’t forget to add a little right rudder as you add power because the left turning tendencies are strong when you are slow. As you add power, bring the pitch to level to stop the descent. Bring flaps up to 10 degrees to allow the airplane to accelerate. Once the airspeed has increased to 60, then you can set a climb attitude and set flaps to zero. To break it into steps: Stabilize in slow flight, then: power to idle, make it stall, break the stall, smoothly add power, level pitch, reduce drag, climb.
Coincidentally, recovery from a stall is very similar to a go-around. This makes sense, because in both cases we have an airplane that is very slow and has a lot of drag. Maintaining (or regaining) airspeed, adding power, reducing drag, and climbing are logical steps for both scenarios.
What about ailerons and rudder? Ailerons are easy- use them to keep the wings level. Rudder keeps the airplane coordinated- the ball in the turn coordinator should be in the middle, and as you look over the nose there should be no yaw- use the rudder to keep the nose pointed at the same point on the horizon. In a power off stall, there should be little if any rudder required, but when we start adding power we will need right rudder to counteract the left turning tendencies.
Stalling in a turn
During your training you will also do turns with shallow bank angles- about 20 degrees. This is perfectly safe, as long as the airplane remains coordinated. Three simple diagrams will demonstrate how this works.
The first picture is a side view of an airplane in straight and level flight. The arrows are vectors representing the forces. On the bottom right is weight- in this example, the airplane weighs 2000 pounds. The weight force acts through the center of gravity (CG), the point where all of the weight of the aircraft is considered to be located. Notice how this force is located ahead of the center of lift, where the wing’s lifting force acts. The result is a nose-heavy airplane. The tail’s job is to produce a down force to balance the lift and weight. Because the tail is a long way aft from the CG, it only needs to be a small amount- in this example, a 100 pound down force is enough to hold the nose up. (Note how the wings need to support this extra weight, that’s why the lift force in this diagram is shown as 2100 pounds.)
When we stall, the wing starts losing lift and the forces are no longer in balance. We are left with a heavy downward force in the nose and a relatively light downward force in the tail, so the airplane pitches down and recovers from the stall- all we need to do is let the nose pitch down. (This assumes that the airplane is properly loaded and the CG is within the acceptable limits! See the weight and balance section for more info.)
The next diagram shows an airplane from behind, in straight and level flight. Again, to keep the 2000 pound airplane airborne, the wings have to create 2000 pounds of lift (we will assume the tail-down force to be included in the weight, to keep things simple). When the airplane stalls, it pitches down because nothing is counteracting gravity.
The third diagram ties it all together with the aerodynamics of turning flight! It shows an airplane in a left bank. Lift is always perpendicular, 90 degrees, to the wing. To maintain altitude in a turn we must increase backpressure to create more lift so that our vertical component of lift- the amount of our lift acting vertically- equals our weight. This explains why a turning wing is working harder. If we are coordinated, the forces in the diagram are all balanced and the ball will be centered. If we stall, we will lose lift and the nose will drop away from where it was pitching- it will drop straight down through the normal axis of the airplane, in the same direction the load is acting. There is no force causing the aircraft to roll! Your instructor will demonstrate this for you in the airplane before you do it.
Coordination
All of the above is true if we are coordinated. Straight and level stalls, and turning stalls, will all be nice and controlled and well mannered if we are coordinated. If we are coordinated, the ball will be centered and as we look out over the nose of the aircraft it is not yawing left or right.
If we are uncoordinated when we stall that means we are yawing. If we are yawing, the nose of the aircraft is moving left or right. This motion is different than rolling or turning- ask your instructor to demonstrate the difference. In the diagram below, the airplane is yawing to the left:
Because the right wing is moving forward (relative to the left wing), the right wing gets more airflow and keeps flying. The left wing loses airflow and lift and stalls. The left wing will stall before and/or stall more than the right wing. When this happens, the airplane will roll left, as shown in the diagram below:
If a wing does drop 10 or 20 degrees during a stall, this is not a big problem. The correct recovery is to use rudder to level the wings. That is so important we will say it again- rudder is the correct fix for this situation, not ailerons! We use the rudder because: 1) the initial problem that caused the wing to drop was yaw, and rudder will stop the yaw. 2)Ailerons will actually create yaw in the direction that the wing is already dropping(remember adverse yaw?). This will make the situation worse, by adding more yaw. 3) if the dropping wing is still stalled, adding aileron is just going to increase the angle of attack and possibly keep the wing stalled. The rudder, however, is still working just fine, and adding rudder will push the stalled wing forward, creating more airflow and more lift, while reducing lift on the top wing, causing it to drop. The result is that the airplane rolls back to level. We can see the same effect in cruise flight, using just rudder we can make the airplane roll. A useful way to think of it is to “step on the high wing”, to remember that if a wing is dropping to use rudder to level the wings. Of course, don’t forget to simultaneously lower the nose.
A wing dropping 10 or 20 degrees is nothing to be alarmed about, and recovery is very simple. Cessnas are very stable, forgiving airplanes. However, if the airplane is rolling substantially, say 30 degrees or more, we could be setting up a scenario for a spin. A spin is simply a stall gone horribly wrong. If excess yaw causes one wing to stall significantly more than the other, the airplane will roll at a high rate and continue to yaw. This combination of roll and yaw is a spin. Again, the initial problem was too much yaw, so use the rudder to fix the problem! Spin recovery requires full opposite rudder deflection- opposite the direction of rotation. Again, step on the high wing! Leave the ailerons neutral. Once rotation has stopped, we can neutralize the rudder. It is possible that we are still stalled, even if pitched nose down, so we may need to push the nose forward to reattach smooth airflow to the wing. The airspeed indicator will let you know if you’re still stalled- if the airspeed is indicating and increasing, your wings are flying.
Spin recovery also requires power be pulled to idle. Power does two bad things in a spin. First, it causes yaw to the left- if we’re spinning left, that’s not good. Power also causes the nose to pitch up, causing the spin to go to a flatter attitude- this will delay recovery.
To help remember the spin recovery technique, we have acronym- PARE:
P Power to idle
A Ailerons neutral
R Rudder- full, opposite, until rotation stops
E Elevator- forward, if necessary, to unstall.
Another method of spin recovery is a little less technical- just let go, and eventually the airplane will start flying again in a steep, tight spiral. The difference between a spin and a spiral is that the spiral will show increasing airspeed, while in a spin the airspeed will be reading quite low. The “let go” technique is not commonly taught because you will lose more altitude than when using standard recovery techniques.
The very best spin technique is prevention! The airplane must be stalled to spin, so simply don’t stall and it won’t spin. This is the reason private pilots are not required to do spins- the emphasis is on prevention rather than recovery.
Power-on stalls
Sometimes called “departure stalls”, these stalls are performed with power on to simulate stalling in a high power situation, such as after takeoff. The difference between power off and power on stalls is that with power on, the airplane will stall at a higher attitude. It will also take a lot of right rudder. How much rudder will depend on how much power- the ball should be centered, the nose should not be yawing. To learn to do power-on stalls: Start with a no-flap, power-off stall, but add about 300 rpm above idle. When you feel comfortable with those changes, add power to about 1500 rpm. Your instructor can safely ease you into higher and higher power settings in this manner.
Stall speed and weight
The airplane comes with published stall speeds for clean and full-flap configurations. These are shown on the airspeed indicator as the bottom of the green and white arcs. It is important to remember that these speeds are based on a very specific set of conditions: airplane loaded to maximum weight (gross weight), most forward CG, wings level, power idle. When we change these conditions, the speed at which we reach the critical angle of attach will change. Remember, the wing stalls when it reaches the critical angle, regardless of speed! Your instructor can demonstrate an accelerated stall where the wing stalls at a higher airspeed because the pilot is pulling hard on the control yoke and putting excessive load (G force) on the wings. We usually demonstrate this by reducing power and putting the airplane in about a 40 degree bank and pulling hard on the control yoke. The wing still stalls at the same angle, but it reaches this angle at a higher speed. The wing "thinks" it is heavier, and therefore stalls at a higher speed.
Weight affects the stall speed because the wing of a lightly loaded airplane doesn’t need as high of an angle of attack to support the airplane’s weight. In other words, the wing on a heavy airplane is working harder. A harder working wing is closer to stalling. When you are in the practice area with just two people, the airplane may stall well below the white arc because it is light. When it heavy, it will stall where published. If you are stupid and load your airplane overweight, it will stall at a higher speed than what is published!
Bank angle also affects the speed we stall at. A banking wing is working harder, and therefore it is at a higher angle of attack. The load is the weight the wings “think” they’re carrying, so when we increase the load by banking steep or pulling hard on the stick, it is just the same as adding more weight. Therefore, if we bank the airplane, or pull hard on the stick, we can cause it to stall well ABOVE the published speed. This could be an unpleasant surprise for someone who doesn’t understand this! Your instructor can teach you to do an accelerated stall to demonstrate this effect.
Closing points to remember:
Any time you ask the wing to do too much, it quits!
With a little practice, stalls become easy and fun. They do take a little practice (like everything in aviation) so don't expect to master stalls in a day.
A useful application of your stall practice is landings. The landing flare is very similar to doing a power-off stall. So, enjoy your time doing slow flight and stalls, knowing that it will help you make better landings!