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Stalls and Spins
A stall is a loss of lift and increase in drag that occurs when an aircraft is flown at an angle of attack greater than the angle for maximum lift. If recovery from a stall is not completed in a timely and appropriate manner by reducing the angle of attack, a secondary stall and/or a spin may result. All spins are preceded by a stall with one wing stalled more than the other. The angle of the relative wind is determined primarily by the aircraft's airspeed. Other factors are considered, such as aircraft weight center of gravity, configuration, and the amount of acceleration used in a turn. The speed at which the critical angle of the relative wind is exceeded is the stall speed. Stall speeds are listed in the Airplane Flight Manual (AFM) or the Pilot Operating Handbook (POH) and pertain to certain conditions or aircraft configurations e.g. landing configuration. Other specific operational speeds are calculated based upon the aircraft's stall speed in the landing configuration. Airspeed values specified in the AFM or POH may vary under different circumstances. Factors such as weight, center of gravity, altitude, temperature, turbulence, and the presence of snow, ice, or frost on the wings will affect an aircraft's stall speed. To thoroughly understand the stall/spin phenomenon, some basic factors affecting aircraft aerodynamics and flight should be reviewed with particular emphasis on their relation to stall speeds. (This advisory circular is principally concerned with and discusses airplanes. However, much of the information also is applicable to gliders.) The following terms are defined as they relate to stalls/spins.
Angle of Attack
.. Angle of attack is the angle at which the wing meets the relative wind. The angle of attack must be small enough to allow attached airflow over and under the airfoil to produce lift. A change in angle of attack will affect the amount of lift that is produced. An excessive angle of attack will eventually disrupt the flow of air over the airfoil. If the angle of attack is not reduced, a section of the airfoil will reach its critical angle of attack, lose lift, and stall. Exceeding the critical angle of attack for a particular airfoil section will always result in a stall.
This video shows the disruption of airflow as the stall begins. Notice that as the angle of attack is reduced slightly the airflow returns to normal.
Airspeed is controlled primarily by the elevator or longitudinal control position for a given configuration and power. If an airplane's speed is too slow, the angle of attack required for level flight will be so large that the air can no longer follow the upper curvature of the wing. The result is a separation of airflow from the wing, loss of lift, a large increase in drag, and eventually a stall if the angle of attack is not reduced. The stall is the result of excessive angle of attack - not airspeed. A stall can occur at any airspeed, in any attitude, and at any power setting.
Configuration. Flaps, landing gear, and other configuration devices can affect an airplane's stall speed. Extension of flaps and/or landing gear in flight will usually increase drag. Flap extension will generally increase the lifting ability of the wings, thus reducing the airplane's stall speed. The effect of flaps on an airplane's stall speed can be seen by markings on the airplane's airspeed indicator, where the lower airspeed limit of the white arc (power-off) stall speed with gear and flaps in the landing configuration is less than the lower airspeed limit of the green arc (power-off stall speed in the clean configuration).
Vso. Vso means the stall speed or the minimum steady flight speed in the landing configuration.
Vs1. Vs1 means the stall speed or the minimum steady flight speed obtained in a specific configuration. This is the FAA definition, it means the airplane is in the clean configuration and is usually indicated by the bottom of the green arc on an airspeed indicator.
Va. Va is the design maneuvering speed which is the speed at which an airplane can be stalled without exceeding its structural limits. This speed varies dependent on the weight of the aircraft.
Load factor is the ratio of the lifting force produced by the wings to the actual weight of the airplane and its contents. Load factors are usually expressed in terms of "G." The aircraft's stall speed increases in proportion to the square root of the load factor. For example, an airplane that has a normal unaccelerated stall speed of 45 knots can be stalled at 90 knots when subjected to a load factor of 2 G's. The possibility of inadvertently stalling the airplane by increasing the load factor (by putting the airplane in a steep turn or spiral, for example) is therefore much greater than in normal cruise flight.
A stall entered from straight and level flight or from an unaccelerated straight climb will not produce additional load factors. In a constant rate turn, increased load factors will cause an airplane's stall speed to increase as the angle of bank increases. Caution should be exercised during steep banks because the airplane will stall at a much higher speed or, if the aircraft exceeds maneuvering speed, structural damage to the aircraft may result before it stalls. During a steep turn, the elevator begins to act like an aileron. If the nose falls during a steep turn, the pilot might attempt to raise it to the level flight attitude without shallowing the bank. This situation tightens the turn and can lead to a diving spiral.
A feeling of weightlessness will result if a stall recovery is performed by abruptly pushing the elevator control forward, which will reduce the up load on the wings. While this is fun with your friends, it is an over exaggerated use of the controls that is unnecessary during a normal recovery. Recoveries from stalls and spins involve a trade-off between loss of altitude with an increase in airspeed and an increase in load factor in the pull up. However, recovery from the dive following spin recovery generally can cause higher airspeeds and consequently higher load factors than stall recoveries due to the much lower position of the nose if executed too rapidly. Significant load factor increases are sometimes induced during pull up after recovery from a stall or spin. It should be noted that structural damage can result from the high load factors imposed by intentional stalls practiced above the airplane's design maneuvering speed if recovery inputs are excessive.
Center of Gravity (CG)
. The CG location has an indirect effect on the effective lift and angle of attack of the wing, the amount and direction of force on the tail, and the degree of stabilizer deflection needed to supply the proper tail force for equilibrium. The CG position, therefore, has a significant effect on stability and stall/spin recovery. As the CG is moved aft, the amount of elevator deflection will be reduced. An increased angle of attack will be achieved with less elevator control force. This could make the entry into inadvertent stalls easier, and during the subsequent recovery, it would be easier to generate higher load factors, due to the reduced forces. In an airplane with an extremely aft CG, very light back elevator control forces may lead to inadvertent stall entries and if a spin is entered, the balance of forces on the airplane may result in a flat spin. Recovery from a flat spin is often impossible. A forward CG location will often cause the stalling angle of attack to be reached at a higher airspeed. Increased back elevator control force is generally required with a forward CG location.
Although the distribution of weight has the most direct effect on stability, increased gross weight can also have an effect on an aircraft's flight characteristics, regardless of the CG position. As the weight of the airplane is increased, the stall speed increases. The increased weight requires a higher angle of attack to produce sufficient lift to support the weight.
Altitude and Temperature.
As altitude or temperature increase, the relative number of air molecules striking the wing or entering the pitot tube will decrease. Therefore a higher true airspeed is required to achieve a given indicated airspeed. Therefore, altitude has little or no effect on an airplane's indicated stall speed. Thinner air at higher altitudes will result in decreased aircraft performance and a higher true airspeed for a given indicated airspeed. Higher than standard temperatures will also contribute to increased true airspeed. However, the higher true airspeed has no effect on indicated approach or stall speeds. The manufacturer's recommended indicated airspeeds should therefore be maintained during the landing approach, regardless of the elevation or the density altitude at the airport of landing. This also implies that a longer ground run and consequently higher ground speed on takeoff may be necessary to achieve the normal indicated takeoff speeds required for lift off.
Snow, Ice, or Frost on the Wings.
Even a small accumulation of snow, ice, or frost on an aircraft's surface can cause an increase in that aircraft's stall speed. Such accumulation changes the shape of the wing, disrupting the smooth flow of air over the surface and, consequently, increasing drag and decreasing lift. Flight should not be attempted when snow, ice, or frost has accumulated on the aircraft surfaces. Numerous crashes of even large airliners have occurred because this rule does not discriminate.
Turbulence can cause an aircraft to stall at a significantly higher airspeed than in stable conditions. A vertical gust or windshear can cause a sudden change in the relative wind, and result in an abrupt increase in angle of attack. Although a gust may not be maintained long enough for a stall to develop, the aircraft may stall while the pilot is attempting to control the flight path, particularly during an approach in gusty conditions. When flying in moderate to severe turbulence or strong crosswinds, a higher than normal approach speed should be maintained. In cruise flight in moderate or severe turbulence, an airspeed well above the indicated stall speed and below maneuvering speed should be used. In these conditions, it is best to maintain attitude and not worry about airspeed fluctuations. This will reduce the pushing and pulling on the control surfaces that may cause excess loads.