AP4ATCO - Lift/Drag Ratio, Forces Interaction and Use - SKYbrary Aviation Safety
Since the early days of flight, angle of attack (AOA) has been a key So, in the normal operational range, there is a relationship among lift, speed, and AOA. While these effects are accounted for in the airplane design and maintenance but they do depend on the thrust required (drag), Mach number, and temperature. Increasing the angle of attack can increase the lift, but it also increases drag so the Bernoulli effect, and reply "Then how do you suppose we can fly the plane. The angle between the chord line and the flight direction is called the angle of attack and has a large effect on the drag generated by When the boundary layer separates, the wing is said to be stalled and both drag and lift become unsteady.
The lift curve is also influenced by the wing shape, including its airfoil section and wing planform. A swept wing has a lower, flatter curve with a higher critical angle. Critical angle of attack[ edit ] The critical angle of attack is the angle of attack which produces maximum lift coefficient. This is also called the " stall angle of attack".
Below the critical angle of attack, as the angle of attack increases, the lift coefficient increases. Conversely, above the critical angle of attack, as angle of attack increases, the air begins to flow less smoothly over the upper surface of the airfoil and begins to separate from the upper surface. On most airfoil shapes, as the angle of attack increases, the upper surface separation point of the flow moves from the trailing edge towards the leading edge.
At the critical angle of attack, upper surface flow is more separated and the airfoil or wing is producing its maximum lift coefficient. As angle of attack increases further, the upper surface flow becomes more fully separated and the lift coefficient reduces further. A fixed-wing aircraft by definition is stalled at or above the critical angle of attack rather than at or below a particular airspeed. The airspeed at which the aircraft stalls varies with the weight of the aircraft, the load factorthe center of gravity of the aircraft and other factors.
However the aircraft always stalls at the same critical angle of attack. On these airplanes, the margin to buffet at higher Mach numbers is calculated by the FMC. On newer models, such as the andthe amber and red bands show margin to stall warning at all times because the stall warning schedule generally follows the initial buffet boundary at higher speeds up to cruise.
The position of the amber and red bands is always a function of AOA margin to stall warning. The speed tape is designed to provide the flight crew with situational awareness of the flight envelope. It shows the crew where the airplane speed is relative to the limits i. The upper right location was chosen as one that can be accomplished without significant rearrangement of the existing PFD or electronic flight display formats. The indicator itself consists of an analog scale and pointer, and digital representation similar to displays of many other parameters throughout the flight deck.
Stall warning AOA is shown with a red tick mark, which will change position as a function of Mach number for those airplanes with Mach-dependent stall warning schedules. A green approach reference band is shown whenever landing flaps are selected. The range of the approach reference band accounts for normally expected variations in CG, thrust, sideslip, and other considerations.
The indicator developed shows body AOA in degrees and is not normalized, which is related to the second objective above, that the indicator be useful when pitot or static data, and therefore Mach calculations, are unreliable because of blockage or a fault in the system. The pointer of a normalized indicator in this condition would behave erratically, making the indicator unusable.
With the nonnormalized design, the position of the needle is a function only of sensed AOA. The red tick mark for stall warning may behave erratically in a pitot or static failure state, as may stick shaker, PLI, and speed tape amber and red bands. However, the AOA needle and digits will remain stable, and the indicator itself still will be useful as a backup for unreliable airspeed, provided the AOA vanes are undamaged. Improved situational awareness and flight crew training.
AOA backup indication following pitot or static system failures. Reference during upset recovery, windshear escape, and terrain avoidance maneuvers. Cross-check to detect weight or configuration errors on approach to reduce the probability of tail strikes on landing.
AOA can be used for some of these purposes, but it does not work as well for others. From the standpoint of flight operations, some of the goals can be met with certain caveats that take into account the principles and limitations of AOA measurement and aerodynamic performance of modern commercial jet airplanes. Within certain limitations, the display provides this indication in a clear, unambiguous format.
The degree to which AOA can be used to increase knowledge and airmanship depends, of course, on the approach taken by the airline in training its flight crews and the use of the indicator in training scenarios for nonnormal procedures.
Some of the limitations are discussed below. The AOA instrument described in this article is useful as a backup for unreliable airspeed indication caused by pitot or static source blockage because the calculation of indicated AOA is not greatly affected by pitot or static pressure inputs for its calibration, and the displayed value has not been normalized.
Pitot or static system failure requires the flight crew to take several fundamental steps to resolve the problem see " Erroneous Flight Instrument Information ," Aero no. Recognize an unusual or suspect indication.
Angle of attack
Keep control of the airplane with basic pitch and power skills. Take inventory of reliable information. Find or maintain favorable flying conditions. Get assistance from others. Recognition of a problem will be accomplished by instrument scanning and cross-check practices or crew alerts, depending on the design of the system in the airplane.
In this respect, AOA instruments can be useful as an additional cross-check. Present procedures for unreliable airspeed call for flying the airplane by reference to pitch attitudes, and refer the pilots to reference tables showing pitch attitudes for various configurations, weights, and altitudes that will result in safe angles of attack and speeds. AOA could be useful if the relevant data is included in the pitch and power tables that already exist in the nonnormal checklist procedures.
AOA would be most useful in flying the airplane in multiple failure conditions where all pitot or static sources are affected, making all airspeed indicators unreliable.
Care should be taken when flying the airplane by reference to AOA in lieu of airspeed. Control should be made by reference to pitch attitude, using AOA as a cross-check to ensure that the pitch attitude results in the desired speed or AOA. Attempting to follow AOA or speed indications too closely without stabilizing the airplane in pitch can lead to an oscillatory flight path.
Windshear escape and terrain avoidance maneuvers require immediate change in pitch attitude and thrust, followed by monitoring of the situation and further increases in pitch attitude if needed, while avoiding stick shaker activation.
The PLI was developed primarily with these purposes in mind and works well. On all current production models, PLI is shown when flaps are down. Work is under way to make this capability available on other Boeing-designed models currently in production.
The first steps in windshear escape and terrain avoidance procedures involve applying maximum certified thrust and control of airplane pitch attitude to an initial target, while honoring stall warning. AOA margin to stick shaker, whether shown with the PLI or the AOA display, is a secondary reference during this part of the maneuver, not the primary target. As mentioned in the section on PLI, pitching up by sole reference to AOA-based indications can result in excessively high pitch attitudes if the maneuver is entered at sufficiently high speeds.
For upset recovery, either the PLI or the red stall warning mark on the AOA indicator may be used to assess the margin to stall warning. As shown in the section on airplane performance, AOA is not the appropriate parameter for optimizing cruise flight, because of the strong influence of Mach number on airplane performance.
Because AOA is not very sensitive to speed or weight changes at cruise speeds, even large gross weight errors may not be detectable. AOA can be used during approach as an extra cross-check for errors in configuration, weight, or reference speed calculation.
Proximity of the barber pole to the reference speed on the airspeed tape can be used in a similar manner because it is based on AOA margin to stick shaker. However, for either method, the errors must be large enough that they are not masked by other factors. Normal variations in AOA as a result of the regulatory requirements on approach speed, as well as those caused by differences in thrust, CG, sideslip, and the installed accuracy of the AOA measurement system, may act together to mask all but large errors in weight or configuration.
These factors are taken into account in determining the size of the green approach reference band. To keep the size of the green band from becoming too large, these variations were root-sum-squared because of the low probability that they would all add in the same direction at any one time. The resulting green band is about 2 deg wide for the and 3 deg for the A 20,lb weight error on acorresponding to approximately 10 percent of maximum landing gross weight or about a 40 percent error in payload, yields a change in AOA of 1.
So, it can be seen that even relatively large weight errors may not be enough to move the needle out of the green band.
Conversely, it is also possible that flying at the proper speed and configuration may yield an AOA that is outside the reference band. Figure 13 illustrates how errors can be masked or canceled out by variation in the other parameters. For these and other reasons, the AOA indicator can be used as an additional means to check for large errors in weight or configuration, but it should not be used as a substitute for current procedures to establish approach speeds and verify configurations.
To determine the approach speed based solely on placing AOA in the green band can cause situations of excessively high or low approach speeds, depending on a variety of circumstances. While AOA is a very useful and important parameter in some instances, it is not useful and is potentially misleading in others. The relationship between AOA and airplane lift and performance is complex, depending on many factors, such as airplane configuration, Mach number, thrust, and CG.
AOA information is most important when approaching stall. AOA is not accurate enough to be used to optimize cruise performance.
Mach number is the critical parameter. Defining a Stall Many people have a misunderstanding what a "stall" is and thought it was merely just the speed at which a plane "drops out of the sky. However in my case I merely just did not understand the terminology or definition of "Stall" but I did at least understand the basic concepts before even coming anywhere close to why a wing is shaped funny.
What I understood back then though, is that increasing the angle of the wing to the airflow, after a certain angle the wing wouldn't produce more lift and would start to decrease. Else people would be flying like helicopters at 90 degrees with infinite lift. You could try thinking it of it this way, the preserving of kinetic energy and obtaining the maximum acceleration.
Refer to the secenario below. Assume the ball does not bounce and the wall has zero friction as if you're playing Quake 3 Arena while pogo-jumping and hitting a vertical wall while moving as an example. In the first example the ball hits the wall There's no conservation of energy here. In the second example we add a degree wall. One could ask, "What angle of a single deflection plate would provide the largest change in direction?
The thing about Aerodynamics is that we deal with fluids rather than solids. So it is a little bit different. So here we introduce things like "eddy currents" like those circular whirlpool like things forming behind a fast moving boat which are the result of the fluid trying to fill the gap left behind by the moving object we're dealing with, which is a wing.
Aero 12 - Angle of Attack
A wing deflects airflow downwards. Most of us who know how Newton's physics works know that a wing gains lift by pushing airflow downwards and the momentum of the air has an equal and opposite reaction of pushing the wing upwards.
Because the airflow starts to fall back onto the back of the wing and pushing it down even at small angle of attacks, people in the aviation industry usually refer lift production as the lack of airflow pressure on the top of the wing rather than the bottom of the wing pushing airflow down. Now back on-topic, with stalls. The start of a stalling point is usually defined as the point where increasing the angle of attack results in no additional lift production.
In other words, the wing to airflow angle exceeding the critical angle of attack. The inefficiency of conversion of momentum See the ball and plate example 2. Eddy currents forming on the wing See above image Because fluids are more unstable and have a tendancy to "fill the gap" usually what causes the "stall" is point number "2.