The AlphaTrainer Instructional Device
Angle of Attack (AOA) Basics
In the science of aviation, the notation for angle of attack is the lower-case Greek letter "a" (a), and aerodynamicists, aeronautical engineers, and test pilots refer to it simply as "alpha"--hence the root of the AlphaTrainer’s name. In our discussion of the principles of flight, we’ll not fly in rarified scientific atmosphere. We’ll stick to the basics and fly in the atmosphere where new pilots live.
How in-depth you study the theory of lift is your decision. More detailed information is in the enclosed FAA training manual supplements and links to the NASA websites. The illustrations provided here, and the graphics on the AlphaTrainer products are often exaggerated for training purposes to make them more understandable. The aim of AlphaTrainer is not to teach the art of flying but to provide you with a fundamental understanding of the concepts used in flight. The AlphaTrainer’s goal is to help you comprehend how pilots control lift with AOA, and the consequences of losing lift.
Glossary of Terms:
The following glossary will help you attain a better knowledge AlphaTrainer and the concepts of angle of attack. Please refer to these components illustrated on the reverse side of the model (AT).
Angle of attack
The angle between the chord line of the wing and the direction of the relative wind (or Instantaneous Flight Path). AlphaTrainer uses color-coding to define the different angles of attack: Green is safe, Yellow is caution, and Red is the critical angle of attack, or "stall". A good way to grasp AOA is to think of the formula needed to calculate lift, where lift increases with an increase in speed. So, if an airplane is maintaining level flight or constant direction and the speed is increased, something must compensate for that increased lift, otherwise the airplane will change direction. The mystery is revealed. In most light airplanes, the airplane pivots—on its center of gravity—15 degrees to compensate for a change in speed, by climbing, descending or changing power. In an airplane in level flight, part of the15 degrees of compensation is already occurring. If the speed increases, the pivot angle will decrease. As the airplane slows, the pivot angle will increase. So in its simplest sense, AOA is a compensator for maintaining constant direction while varying speed.
The secret to understanding stalls is when you exceed the upper limits of this15-degree compensator; the airplane will no longer fly, and is likely to go out of control.
Center of Lift/Center of Pressure (CP) Center of Gravity (CG)
Lift acts upward and perpendicular to the relative wind. A wing generates lift over its entire surface, but an imaginary point — called the center of lift or center of pressure — represents the single point where lift’s total upward force acts on the wing.
The CP’s location relative to the center of gravity (CG) plays an important role in any airplane’s longitudinal (nose up and down) stability. Because pilots determine the CG’s when they load the airplane with people and baggage, the original AlphaTrainer slides fore and aft to simulate different CG locations and the different stability problems they can cause, including loss of control on takeoff or landing. In addition, moving the airplane fore and aft gives you a visual understanding of the importance of calculating the airplane’s proper weight and balance.
Chord
An imaginary straight line drawn from the leading edge to the trailing edge of an airfoil’s cross-section.
Elevator or Stabilator
The control surface pilots use to change the wing’s angle of attack.
Instantaneous Flight Path (IFP )
Indicates the actual flight path of the airplane (and the relative wind). We use the word "instantaneous" because of Mother Nature’s continually-changing disruptions (NASA).
Lift
The force that directly opposes the weight (NASA) of an airplane and holds the airplane in the air. Lift is generated by every part of the airplane, but the wings generate most of the lift. Lift is a mechanical aerodynamic (NASA) force produced by the motion of the airplane through the air. Because lift is a force, it is a vector quantity (NASA), having both a magnitude and a direction associated with it. Lift acts through the center of pressure (NASA) of the object and is directed perpendicular to the flow direction. Because lift is perpendicular to the flow directions (a law of fluid physics) we moved the original tri-colored protractor from AlphaTrainer’s relative wind display to behind the lift vector on AT3D to demonstrate lift’s magnitude and direction, which is one more way of displaying AOA movement. The relative wind’s direction can be interpreted on AT3D by the velocity (V) vector.
Recognition of Stalls
Perceiving involves more than the reception from the five senses. Perceptions result when a person gives meaning to sensations. People base their actions on the way they believe things to be. An example is how a home team’s fan sees a foul play differently than a visiting team’s fan. Safe pilots know that "what you see out the windshield is not always what you get."
Pilot’s Perceived Attitude
Represents the pilot’s PERCEIVED flight path. AlphaTrainer stresses the importance of understanding that what the pilot perceives is not always perfectly correlated to Angle of Attack. This concept is used on the Original AlphaTrainer and the 2D PC Demonstrator. We attempt to bring attention to the action of “sink” or “mush”, particularly in events of rapid increases in drag, pulling out of a dive, and wind shear, on our two-dimensional instructional devices.
Relative Wind
Direction of the airflow produced when an object moves through the air. The relative wind for an airplane in flight flows in a direction parallel with, and opposite to, the direction of flight. Therefore, the actual flight path of the airplane determines the direction of the relative wind. Relative wind is easy to understand in a wind tunnel (NASA). It’s the air that is being pushed past the airplane at a controlled velocity, and the fact that it is a tunnel, at a constant direction. In an airplane in flight, it is the pilot who controls the velocity of the air past the airplane because the air is not being pushed past the airplane. It is the airplane that is plowing through the air. Therefore, the flight path determines the direction of airflow, regardless of its direction.
Unlocking the Mystery of Flight
By simply reducing an airplane’s angle of attack from red to green could save many lives; "From Red to Green is our Dream!" (AT) The AlphaTrainer instructional devices are simple products designed to teach a complex issue—the magic of flight. These devices were designed and patented to allow users to illustrate (AT) how an airplane flies, how a wing produces lift — but most importantly — how the pilot perceives the results of their own control. At the core of AlphaTrainer is the concept of angle of attack and how a pilot controls lift by changing this angle. Ultimately, understanding angle of attack is crucial to avoiding stalls and spins (FAA).
Stalls — and the spins that can ensue — terrify many student pilots (and a lot of experienced pilots, as well) because pilots often have difficulty understanding the aerodynamics that cause them. The lessons that stick, however, are tangible and visible because the student sees a constant speed on their airspeed indicator during stall practice. However, stalls are caused by excessive AOA, and the AlphaTrainer Instructional Device easily makes this truth tangible, visible, and easy to understand. The FAA readily supports this truth in two FAA training manuals, Pilots Handbook of Aeronautical Knowledge (FAA-H-8083-25) and the Airplane Flying Handbook (FAA-H-8083-3A). Please note that these book files are very large in size, and that download time is significant.
In "Principles of Flight," the second chapter in the Pilots Handbook of Aeronautical Knowledge, the FAA emphasizes the importance of angle of attack, using the term more than 70 times. Repeatedly, it stresses that stalls happen when the wing reaches its critical angle of attack, not when pilots fly below some airspeed.
Angle of attack cannot exist without the relative wind, which is another invisible, intangible aspect of flight that often confuses pilots. In its new training manuals, the FAA offers a clearer explanation of this, writing that the "actual flight path of the airplane determines the direction of the relative wind." Like angle of attack, the AlphaTrainer makes the relative wind — the airplane’s Instantaneous Flight Path — visible and real (AT).
The AlphaTrainer shows you separately and distinctly the essential magic — angle of attack. On the Original AlphaTrainer and AlphaTrainer 2D, this angle is represented by the wing’s chord line and relative wind, with respect to the instantaneous flight path. AlphaTrainer depicts and unlocks the magic’s related mysteries; how the center of pressure (or lift) changes with AOA (very evident on AT3D), and how the airplane’s center of gravity (CG) can also change with weight shift or fuel burn (Original AT). AlphaTrainer 3D also sheds light on the problem of stability. As AOA increases, you can watch the lift vector move forward. This forward movement causes the nose to pitch up even more.
And perhaps most importantly, Original AlphaTrainer and AlphaTrainer 2D stresses the importance of understanding that what the pilot perceives is not always correlated correctly to Angle of Attack. To emphasize this misperception, the "Pilot’s Perceived Attitude" illustrates the differences. Now, pilots cannot only understand basic stalls but also understand the mysterious stalls that pilots most often misread. Chapter 4 of Airplane Flying Handbook (FAA-H-8083-3A) states: "Vision (AT) is useful in detecting a stall condition by noting the attitude of the airplane. This sense can only be relied on when the stall is the result of an unusual attitude of the airplane. Since the airplane can also be stalled from a normal attitude, vision in this instance would be of little help in detecting the approaching stall. "The Original AlphaTrainer and AlphaTrainer 2D takes this warning a step further by graphically showing that when one least expects it, there can be a difference between the pilot’s perceived flight path and the actual flight path that will lead to a stall.
Stall and the Critical Angle of Attack
The FAA supplements describe the creation of lift in detail. To paraphrase: when air flows smoothly over a wing, it creates low air pressure on top of the wing (Bernoulli’s theory), with a higher pressure underneath. Because everything in nature seeks equilibrium — and as one air pressure tries to reach the other — it carries the wing with it, creating lift. Tom Benson, of NASA’s Glenn Research Center offers this simple explanation of lift (NASA): "I prefer, when discussing lift with students, to just stop at the Newtonian 3rd law (NASA) — Lift is the re-action to the turning of the flow. No turning, no lift."
These two explanations may seem to be in disagreement (NASA) with each other because of the two differing theories. However, they may be closer than one thinks. The key to understanding creation of lift is that it is a mechanical force. To be a mechanical force, there must be interaction and contact of a solid body (airplane or wing) with a fluid (air). "Contact" is the key word, as it is the creation point. Similarly, the effects of lift are also present; like the pressure variation around the object, velocity variation around the object, downwash, and shed vorticity. We can assume that the FAA teaches the Bernoulli approach (pressure variation) because it is calculable at slower speeds, observable, and easier to understand. This confusion is why we at AlphaTrainer refer to the creation of lift as "magic" — because it cannot be observed. One can only see its effects.
We can use the laws of fluid physics to substantiate lift (NASA), wherein a change in velocity in one direction can cause a change in velocity in a perpendicular direction. This doesn’t occur in solid mechanics. The component of the net force perpendicular (or normal) to the flow direction is lift; the component of the net force along the flow direction is drag (NASA). Again, the "perpendicular direction" to the relative wind is the creation of lift. The wing, along with other parts of the airplane, is simply an efficient means to the "turning of the flow". What’s important for our discussion here is that changing the wing’s angle of attack changes the amount of lift a wing produces — up to the wing’s critical AOA. When the wing reaches its critical angle of attack, the air can no longer flow over the wing’s surface smoothly, and the wing stalls — abruptly decreasing lift. Understanding this critical angle of attack is essential for safe flight. At the critical angle of attack (Red), the wing will not fly again until the pilot reduces its AOA below that stated 15 degrees. Reducing the AOA allows the air to once again flow smoothly over the wing, thus generating lift. Note: "air to once again flow smoothly over the wing" will be replaced with "boundary layer (NASA) formation and separation" in advanced studies. The phrase may change, but the outcome of stall will remain the same.
Focusing on the effect of lift and not its creation can lead to many incorrect theories. Links to NASA’s complete website (What is Lift) and Tom Benson’s editorial can be found on the Instructor’s Corner (AT).
Stall Recovery (FAA)
One of AlphaTrainer’s primary purposes is to help teach stall awareness. It achieves this by completing the mind’s picture of stalls and teaching pilots how to recognize a stall and how to take prompt, corrective action. As suggested, the correct action is to reduce the angle of attack so the air can again flow smoothly over the wing. Applying full power aids in stall recovery. However, like within a glider, the stall is actually corrected by reducing the AOA.
Remember that AOA is the angle between the wing’s chord line and the relative wind (flight path), not the chord line and the horizon or ground. An airplane can exceed its critical AOA — it can stall — in any attitude, even when its nose is pointed straight at the ground. Regardless the airplane’s attitude, the corrective action is still the same — reduce the AOA.
Three major reasons why we need to add the "Pilot’s Perceived Attitude" to AOA training
Pilot’s Perceived Attitude — represents the pilot’s PERCEIVED flight path. A frequently inaccurate mind’s picture of what is really happening to the airplane. The FAA warns that only through proper training and experience can this phenomenon be exposed. "Kinesthesia (FAA), or the sensing of changes in direction, or speed of motion, is probably the most important and the best indicator to the trained and experience pilot. If this sensitivity is properly developed, it will warn of a decrease in speed or the beginning of a settling or mushing of the airplane." AlphaTrainer uses the Pilot’s Perceived Attitude to educate that there are at least three crucial times when the pilot’s perceived flight path may be significantly dissimilar than the actual flight path, due to sinking or mushing:
1. According to NASA’s Tom Benson: "With real airfoils, the angle of attack dependence gets real complex, because it affects both the amount of lift and the amount of drag. So, lift could be going up because of increased angle of attack, but the speed could be decreasing because of increased drag. So exactly what angle of attack does to aircraft performance depends on some other variables, including the speed when the maneuver is initiated, and the power setting of the engine. At altitude, at high speed, increasing angle of attack increases lift and the aircraft moves up. At low speed, (like during landing) increasing angle of attack decreases speed (NASA), and the aircraft drops (more). I understand that this "reversal" causes a lot of problems for new pilots. At low speeds, you use the throttle to go up and down, and angle of attack to go faster and slower; exactly the opposite of high speed flight."
2. Accelerated Flight during rapid ascent (pull-up) (FAA). Accelerated flight has more to do with abrupt changes in angle of attack than it does airspeed. The laws of physics (NASA) say that a mass traveling in a straight line will continue to move in a straight line — until some force causes the mass to assume a curved path. An airplane is a mass, and hauling back on the yoke is a force causing it to assume a curved path (up). Before the up force can curve the path, it must overcome the airplane’s straight-and-level inertia (NASA). This overcoming of inertia is called centrifugal force, which is a pushing towards the outside of the curve. When an airplane is flying a curved positive flight path, the wings must support the airplane’s weight — plus — the load imposed by centrifugal force.
Hauling back on the yoke is a positive flight path because it creates a positive load on the airplane; centrifugal force is acting in the same direction as the force of weight. Pushing the yoke forward creates a negative load because centrifugal force acts in a direction opposite to that of the force of weight.
AlphaTrainer also uses the Pilot’s Perceived Attitude bar to present acceleration or G-loading (NASA). This bar teaches pilots that "what you see is what you get" is not always true. Pilots might expect a positive rate of climb when they abruptly haul back on the yoke, but the airplane may not respond this way. Typically, the perceived flight path is more inclined than the actual flight path because the aircraft’s momentum (NASA) is causing a lag between the pilot changing the attitude, and the actual resultant change in altitude. Many of the "buzzing" accidents have occurred because the pilot did not perceive the proper flight path.
3. Wind shear (FAA) is another invisible mystery of flight often misunderstood by all pilots. For example, a new captain on a Citation jet had just rotated for takeoff; the aircraft climbed about 100 feet, and then settled back to the ground. Fortunately, this happened in a simulator! The instructor asked the new captain if he knew what had happened. With anger in his voice, and thinking the instructor had incorrectly programmed the simulator, the captain said, "You tell me!" The instructor had programmed the simulator correctly for the New Orleans takeoff W/S (NTSB) — and this caught the new captain off guard.
Because wind shear represents a change in the direction of the relative wind, it disrupts the airflow moving past the wing. Wind shear moves in a different direction and velocity from the prevalent wind. Portions of air in which the airplane is flying can shift up, down, forward, or backwards. This shift may lead to a high rate of sink; all the while the attitude indicator appears to be normal. Wind shear is most often associated with thunderstorms, but it can occur in almost any weather. Even in days with little wind, hills, buildings, and trees can cause wind shear. These obstacles are commonplace at general aviation airports.
Wind shear can be horizontal or vertical, and each affects airplanes differently. Vertical shear changes the AOA because it suddenly moves the airplane up or down. Caution: Vertical shear can cause structural damage to an airplane, or even worse: a break-up of the aircraft. Horizontal shear immediately changes the airplane’s speed, which pilots can see on the airspeed indicator. If the horizontal shear gust reduces the airplane’s speed by 20 percent, the airplane will sink, trading altitude for airspeed to maintain the AOA it was trimmed (NASA) for. Too often, pilots don’t recognize horizontal shear until it’s too late. Usually, a gain and then a loss of airspeed is the first clue, and pilots may attribute this to "turbulence." A sink rate is the next clue. A microburst, or severe thunderstorm, starts as vertical wind shear and becomes horizontal after it hits the ground. It then curls up and around, going through its vertical and horizontal phases again.
Such a microburst brought Delta Flight 191 (NTSB) to grief at Dallas/Fort Worth International Airport on August 2, 1985. Crash investigators discovered that airport instruments recorded that the headwind Delta 191 was flying into rapidly increased 26 knots. Then, just as suddenly, it became a 46-knot tailwind. The NTSB claims the aircraft encountered approximately 73 knots of wind shear.
The jet was only 800 feet above the ground when it encountered the wind shear, giving the pilots little room to maneuver (NTSB reports that full power was applied). The airplane began to lose airspeed and altitude at the same time. The unfortunate flight ended 38 seconds later — in a crash short of the runway.
Charlie Tennstedt, a former Test Pilot and Fight Instructor made these following thought-provoking remarks regarding wind shear. "To reinforce the intent of learning about angle of attack, it is imperative that pilots DO NOT use the airspeed and vertical speed indicators during a wind shear escape maneuver. The static pressure in these microburst events drops rapidly as the air flows rapidly outward (thanks to Mr. Bernoulli’s principle) and forces pilots to DISREGARD speed and rate of climb indications. This is the reason for training pilots to rotate to the stall warning onset [with full power] and hold that attitude until the aircraft is clearly climbing away from the ground (radar altitude is increasing) or 400 feet AGL if not RA equipped." Please consult your aircraft manufacture’s recommended wind shear recovery procedures, as each airplane reacts differently in wind shear.
The AlphaTrainer is an intuitive tool for learning microburst recovery procedures. The model clearly shows the difference between the pilot’s perceived flight path and the actual flight path. The recovery requires the angle of attack to waver between yellow and red (stall warning onset) as the pilot attempts to hold a nose-high attitude. In the initial recovery stage, the Pilots Perceived Attitude is pointing upward as the aircraft sinks. As has been the experience of many microburst simulator scenarios, the pilot is typically found looking upwards as the airplane contacts the "ground".
Spin Recovery (FAA)
A spin is what can happen if an airplane stalls in an uncoordinated condition, meaning it’s yawing (NASA) left or right (the relative wind is not directly on the nose), and the Original AlphaTrainer is ideal for visualizing this condition. Just twist the model back and forth to simulate yawing as you maneuver the model’s AOA from green to red.
This twist lets you visualize that one wing is going forward while the other wing is going backward. Because the backward-moving wing is decelerating, it starts to drop. The downward movement of the wing along with the natural response to pick up the wing with down aileron causes this side of the wing to have a greater AOA, causing it to stall first and slip in its direction. As this happens, the relative wind strikes the fuselage and vertical fin and tries to weathervane the airplane, or point its nose into the relative wind.
Trying to pick up (level) the low wing with aileron and raise the nose with elevator are a natural reaction. But in this case, what seems "natural" is incorrect and dangerous. The "natural" reaction makes the situation worse and often leads to a spin.
Spins are quite graphic with AlphaTrainer 3D. You can see the twisting, and you can also watch the change in each wing’s lift, along with each wing’s AOA. In addition, the AT3D product has the ability to pause movement ("P" key), such that you can evaluate the maneuver executed.
The FAA supplements offer more detailed information, and your airplane’s operating handbook or flight manual will give its recommended spin recovery procedures. In the absence of a manufacturer-recommended procedure, the FAA recommends this spin recovery procedure (FAA).
Caution: to some people, applying "positive and brisk" forward stick could mean pushing the stick all the way to the panel. This, with rare exceptions, is incorrect. AlphaTrainer clearly illustrates the movement needed for breaking a stall—from red to green. Any movement more than required would curve the flight path and may aggravate the stall.
Remember that spins consume a lot of altitude, and so do their recoveries. And remember that most stall-spin accidents occur in the traffic pattern, when making the turn from base to the final approach leg, when the airplane is close to the ground. Pilots may know how to recover from a spin, but in this situation they don’t have the altitude to use it. The only solution is to make sure all control inputs are coordinated, and that the AOA is in the green.
From Red to Green is Our Dream
Flying is one of the most joyous activities we humans can pursue, and increasing our ability to fly safely adds to this enjoyment. Safety is based on knowledge, and that is the purpose of the AlphaTrainer — increase pilot knowledge of the invisible magic (not magic anymore) that makes flight possible, and to increase it simply and clearly.
Remember, From Red to Green is the Dream. And it’s the key to flying safely.