Stall, Torque, Spin
By: Allan Eich
I was sixteen years old and learning how to fly an airplane in the spring of 1971. I had taken several lessons in a Cessna 150 and had soloed around the airport. Then my flight instructor sent me out to the practice area by myself to do some air work such as steep turns, slow flight and stalls. A stall is a maneuver that pilots practice that involves raising the airplane’s nose (angle of attack) to stall the wing so that it stops creating lift. A stall is not about the engine stopping, it is about reducing the lift on the wing. One day I flew into my practice area and set up to practice stalls by myself. I reduced my airspeed and raised the nose to enter the stall. Once the pilot hears the stall warring in the cockpit and feels the buffeting on the wings; he should recover by lowering the nose while keeping the wings level and then adding power. But it didn’t happen that way when I did it by myself. When I heard the stall horn and felt the buffeting from the stall, I used the yoke (steering wheel) in an effort to keep the wings level instead of using the rudder pedals with my feet. I didn’t know that moving the yoke and ailerons on the wing were ineffective. The correct way to keep the wings level was to use my feet on the rudder pedals. As the airplane started to roll, I countered the roll with ailerons and added full power as I was taught and then tried to lower the nose. But the torque from applying the power turned the airplane upside down and I entered into a spin. I had never seen a spin much less had any idea how to recover from one. The plane was spinning rapidly to the left with full power and pointed down toward the ground. To put it mildly, I was in a lot of trouble. I was in a life or death situation and the death part was winning so I did the only thing that I could do. I cried out to the Lord, “God, please help me!”
God then planted a thought in my mind. “Allan, you are pointed at the ground with full power, why not reduce the power to idle and you will live longer.” So I complied and reduced the power to idle. Now I was getting close to the ground and so I just gave up and let go of the yoke (steering wheel). I surrendered the airplane. That turned out to be exactly what I need to do, because after I reduced the power and let go of the yoke, the airplane stopped spinning and recovered by itself. I didn’t know that it would do that, but God did. I didn’t know to reduce the power and just let go. Airplanes are designed to want to fly, not spin. (The aeronautical technical term is “dynamic stability.”) I got back up to a safe altitude and returned back to the airport. God saved me. Many times when we are faced with problems, the answer is to let go and let God handle it. Surrender it to Him. When I took my hands off the controls and surrendered the airplane to Him, the spinning stopped.
So that I don’t get
one scared about flying, let me explain that I was a sixteen-year-old
student pilot and I had some poor flight instruction. I would recommend that anyone
interested in learning to fly, find a good instructor. I later became a flight instructor at
(Turn the page if you are a pilot so that you learn about the technical aerodynamic information of stall and spins.)
This is the technical aerodynamic information for the pilots who are hearing this message. A spin is caused by stall and yaw. A stall is the loss of lift due to the angle of attack being too large. Angle of attack is the angle between the cord of the wing and the relative wind. The relative wind is parallel to and in the opposite direction of the flight path. The term “relative wind” describes the air flow over the airfoil instead of the wing moving through the air. In actual flight the wing moves through the air, but for classroom explanations, we talk about the air (relative wind) moving over a stationary airfoil. Same difference. Thus, the flight path of the airfoil is always parallel and opposite to the relative wind. The pitch attitude of the plane is in relation to the horizon and earth. Therefore zero degrees pitch attitude does not mean zero angle of attack. While approaching a stall, the pitch attitude may only be 10 degrees but the angle of attack could be rapidly approaching stall such as 17 degrees in a Cessna 150. You can not tell what the angle of attack is by looking the pitch of the airplane. You have to see it’s flight path (relative wind) and the cord of the wing to see the angle of attack.
The second ingredient for a spin is yaw, usually induced with rudder. However, the other force which can put you into a spin without the use of the rudder is propeller torque which is a twisting or rotating force. The FAA states that torque produced by the propeller can be divided into four basic elements:
Torque reaction,
is caused by the prop rotating clockwise (viewed from the pilot’s seat and the airplane
reacting by rolling counterclockwise.
This is
Corkscrewing effect is the rotation of the propeller slipstream around the fuselage producing a corkscrewing wind or small horizontal tornado which the airplane then flies through causing it to roll to the right. Also the propeller slipstream hits the vertical stabilizer, causing the airplane to yaw to the left and that is why you have to add right rudder as you apply power for take-off.
Gyroscopic action which states that a force applied to the propeller not in its plane of rotation causes another force 90° in the direction of rotation. This effect is only present when initiating a pitch or yaw change. It is not noticeable unless the controls are moved at a rapid rate and lasts only while there is a vector change. Gyroscopic effect is most prevalent with a large mass rotating propellers or during aerobatic maneuvers when rapid control inputs are being made. Due to gyroscopic action a pitch up movement will cause the airplane to yaw to the right and a pitch down movement will cause a yaw to the left. Remember the force is applied ninety degrees in the clockwise direction so that a rapid yaw to the left causes a pitch up and a yaw to the right causes a pitch down.
Asymmetric loading (P-factor) is an uneven loading of the propeller at some angle of attack due to the downward propeller blade creating more lift than the upward swinging blade. The best way to illustrate this statement is to consider a propeller mounted on a vertical shaft such as on a helicopter. While the helicopter is hovering, bother blades travel at the same speed. Move the entire helicopter forward at a set velocity, and the relative speed of each blade changes since the relative wind contributes relative speed to forward moving right blade and reduces relative speed on the other aft moving left blade. When the propeller shaft is mounted horizontally, such as an airplane in normal cruise flight, neither blade received or loses this relative speed. However, increasing the angle of attack toward the vertical position, such as in a stall or slow flight, then the asymmetrical loading returns. The downward turning blade has more relative wind and thus produces more lift than the up turning blade. Thus, the airplane tends to yaw to the left and the pilot needs to add right rudder to counteract the torque. Of all the torque forces, this is the most important factor during stalls and why right rudder is required to maintain directional control. Also keep in mind that the wing and ailerons are in a stalled condition thus ailerons are ineffective. So use the rudder to keep the wings level during stalls.
If you are flying a twin engine airplane with non-counter rotating propellers then your critical engine is the left engine because the torque effect from asymmetrical thrust is less on the left engine. The right engine’s down turning blade has a greater moment arm than the left engine’s down turning blade. This means that the down turning blade on the right engine is farther away from the fuselage than the down turning blade of the left engine. Therefore it requires less rudder during single engine operations if the left engine is running instead of the right engine. So if your flight instructor wants to challenge you, he will fail your critical left engine and watch you struggle more with the running right engine.
If you are having problems with directional control during stalls, find a good instructor who can show you the torque forces in flight. Try practicing power-off stalls and recoveries without power and you will probably have very little yaw or roll. Then progress to full power-on stalls and you should notice the yaw and roll forces caused by the propeller and the requirement to use right rudder. If your airplane is certified for spins such as a Cessna 172, your instructor can have you perform a power-on stall with your feet off the rudder pedals so that you can watch the airplane torque over to the left during the stall. Once the airplane starts to roll over, just bring the throttle to idle and the torque forces will stop. If necessary your instructor can apply right rudder to aid in the recovery but usually just bringing the throttle to idle will allow for a recovery.
All pilots should know the standard spin recovery procedure however consult your own airplane manual for any changes. The first step in the standard spin recovery is to bring the throttle to idle and to get rid of all the propeller torque effects. The second step is to neutralize the all flight controls. Some aerobatic pilots just totally release the stick to find neutral during their recovery. The third step is to apply full opposite rudder away from the turn or yaw. Once rotation stops, apply forward stick to break the stall. Then recover from the dive.
Standard Spin Recovery (Consult you airplane pilot’s manual for any changes)
In conclusion be aware of the propeller forces that act on your airplane especially near stall speeds. During a stall the ailerons are ineffective and thus rudder is needed for directional control due to the propeller torque effects. Know that right rudder is usually required for directional control during power-on stalls due to asymmetrical thrust ( P-factor). Remember that the use of rudder is important during slow fight and every flight ends with slow flight which is a landing.