Adaptation for flight is highly developed in birds and insects. The bat is the only mammal that accomplishes true flight; flying squirrels glide rather than fly, as do flying fish and flying lizards. Birds fly by means of the predominantly up-and-down motion of their wings. The flapping motion is not, however, straight up and down but semicircular, the wings generally moving backward on the upstroke and forward on the downstroke. That motion pushes air downward and to the rear, creating a lift and forward thrust. The leading edge of the slightly concave wings is rather sharp, and the feathers are small and close-fitting, so that a streamlined surface meets the air. On the trailing edge of each wing the interlocking of the larger feathers forms a surface that acts somewhat like the ailerons, or movable airfoils, of an airplane. In wing motion, the leading edge is twisted so as to be lower than the trailing edge in the downward stroke and above the trailing edge in the upward stroke.
Besides flapping, some birds also use gliding and soaring techniques in flight. In gliding, a bird holds its outstretched wings relatively still and relies on its momentum to keep it aloft for short distances. In soaring, a bird uses rising warm air currents to give it lift.
The form and size of wings vary in different birds. In woodland birds the wings are somewhat rounded and have a relatively broad surface area. Birds with well-developed gliding ability, such as gannets and gulls, usually have narrow, pointed wings. Especially noted for their soaring power are eagles, vultures, crows, and some hawks. In soaring flight the feathers on the wings of these birds separate at the tips, resembling opened fingers against the sky. It is thought that this movement diverts the airstream over the wing and aids the bird in turning, banking, and wheeling. There is disagreement as to the maximum speeds achieved by birds in flight. While the flight speeds of most birds range from 10 to 60 mi (16-100 km) per hr, some have been recorded at speeds reaching 70 mi (110 km) per hr, for long distances and near 100 mi (160 km) per hr, for short flights. In a stoop, falcons can reach faster speeds.
Humanity's first attempts at flight were made with flapping wings strapped to the arms in imitation of birds, but these had no success. Machines designed to fly in this way, called ornithopters, date to antiquity (c.400 B.C.) and models that are capable of flight have been known for more than 100 years. However, there are no practical aircraft based on ornithopter designs, even though an ornithopter—which has no theoretical top speed limit—should be capable at least of efficient low-speed flight. In the 1930s an Italian model weighing approximately 50 lb (110 kg) and powered by a 0.5-hp motor was successfully flown.
Airships and balloons owe their ability to ascend and remain aloft to their inflation with a gas lighter than air; this is an application of Archimedes' principle of flotation, i.e., that a body immersed in a fluid (liquid or gas) is buoyed up by a force equal to the weight of the fluid that it displaces. Aircraft, which are heavier than air, are able to remain aloft because of forces developed by the movement of the craft through the air. Propulsion of most aircraft derives from the rearward acceleration of the air. It is an application of Newton's third law, i.e., that for every action there is an equal and opposite reaction. In propeller aircraft the forward motion is obtained through conversion of engine power to thrust by means of acceleration of air to the rear by the propeller. Lift is obtained largely from the upward pressure of the air against the airfoils (e.g., wings, tail fins, and ailerons), on whose upper surface the pressure becomes lower than that of the atmosphere. In jet-propelled aircraft, propulsion is achieved by heating air that passes through the engine and accelerating the resultant hot exhaust gases rearward at high velocities. Rockets are propelled by the rapid expulsion of gas through vents at the rear of the craft. The high speeds that are produced by jet and rocket engines have brought about substantial changes in the science of flight.
See H. Tennekes, The Simple Science of Flight (1996, repr. 2009); see also bibliography under aviation.
Because stalls are most commonly discussed in connection with aviation, this article discusses stalls mainly as they relate to aircraft. In simple terms, a stall in an aircraft is an event that causes the wing to lose lift suddenly. An aerodynamic stall does not necessarily mean that the engine(s) of an aircraft have stopped working, or that the aircraft has stopped moving.
But increasing the AOA also increases drag. Without a sufficient increase in engine power, the aircraft slows, and wing-lift decreases. Above a particular angle, the "critical angle of attack", the airflow behind the wings becomes turbulent, the wing-lift largely disappears, and the wing stalls—that is, it suddenly ceases to provide enough lift to support the aircraft. In addition, the turbulence dramatically increases drag, which further slows the aircraft as it moves through the air, further reducing wing-lift. Rapidly the aircraft begins to accelerate downward.
In many aircraft recovering from a stall is simple. Since the stall is caused by an excessive angle of attack, simply pointing the nose of the aircraft downward will arrest the stall by reducing the angle between the wings and the flow of air (this is for a fixed wing aircraft rather than a helicopter). Some aircraft have a natural tendency to pitch downward (sometimes dramatically) when the wings stall; others must be directed downward by the pilot. As soon as the angle of attack drops below the critical angle, the aerodynamic stall of the wings will cease: the wings will start to produce lift and far less drag. However, the aircraft may still be flying too slowly to generate enough lift to prevent the aircraft from continuing to descend: complete stall recovery includes regaining this necessary speed.
In some circumstances stalls can result in more complicated problems, such as a 'spin' or a 'deep stall'.
A stall is caused by the pilot attempting to fly the aircraft too slowly, or to pull up too quickly from a dive, or to turn too steeply. Each of these causes the nose to be lifted until the wing's critical angle of attack is exceeded. Increasing engine power counteracts the increased drag caused by the stall and also increases air speed, and this helps in recovery from a stall. The critical action in recovering from a stall is reduction in the angle of attack, i.e., lowering the nose.
Altitude (height above the ground) is lost by the aircraft during the stall itself but considerably more height can be lost during the recovery. If the aircraft is already at a high altitude this is not a problem. If the aircraft is very close to the ground, however, a stall may cause the aircraft to lose so much altitude that it hits the ground before recovery from the stall is possible. For this reason, pilots are especially careful to avoid stalls during take-off and landing procedures, when the aircraft is very close to the ground.
Stalls in aircraft usually do not occur without warning. In addition to sensors which alert the pilot when the aircraft is about to stall, experienced pilots can sense an incipient stall by noting changes in the behavior of the aircraft. Since the conditions that produce stalls are very well understood, pilots can easily avoid stalls, and many pilots never experience stalls outside of their pilot training. Standard pilot training includes training in the proper ways to avoid, recognize, and recover from stalls.
A few types of aircraft with a T-shaped tail or rear-mounted engines can enter a deep stall or superstall. This is a type of stall that produces turbulence behind the wings that can interfere with the operation of engines or the tail of the aircraft. Recovery from a deep stall can be impossible, resulting in a crash. Some aircraft with such characteristics are fitted with special control devices to prevent the aircraft from ever approaching a position that can cause a deep stall. An example of such a device is a stick pusher, which forces the nose of the aircraft down whenever it approaches a stall, regardless of any actions taken by the pilot.
A stall is a condition in aerodynamics and aviation where the angle between the wing's chord line and the relative incoming wind (the angle of attack) increases beyond a certain point such that the lift begins to decrease. The angle at which this occurs is called the critical angle of attack. This critical angle is dependent upon the profile of the wing, its planform, and its aspect ratio but is typically in the range of 8 to 20 degrees relative to the incoming wind for most subsonic airfoils. The critical angle of attack is the angle of attack on the lift coefficient versus angle-of-attack curve at which the maximum lift coefficient occurs, and it usually represents the boundary between the wing's linear and nonlinear airflow regimes. Flow separation begins to occur at this point, decreasing lift, increasing drag, and changing the wing's center of pressure. A fixed-wing aircraft during a stall may experience buffeting or a change in attitude (normally nose down in General aviation aircraft). Most aircraft are designed to have a gradual stall with characteristics that will warn the pilot and give the pilot time to react. For example an aircraft that does not buffet before the stall may have an audible alarm or a stick shaker installed to simulate the feel of a buffet by vibrating the stick fore and aft. The "buffet margin" is, for a given set of conditions, the amount of ‘g’, which can be imposed for a given level of buffet. The critical angle of attack in steady straight and level flight can only be attained at low airspeed. Attempts to increase the angle of attack at higher airspeeds can cause a high speed stall or may merely cause the aircraft to climb.
Any yaw of the aircraft as it enters the stall regime can result in autorotation, which is also sometimes referred to as a 'spin'. Because air no longer flows smoothly over the wings during a stall, aileron control of roll becomes less effective, whilst simultaneously the tendency for the ailerons to generate adverse yaw increases. This increases the lift from the advancing wing and accentuates the probability of the aircraft to enter into a spin.
Depending on the aircraft's design, a stall can expose extremely adverse properties of balance and control; particularly in a prototype.
The information in a graph of this kind is gathered using a model of the airfoil in a wind tunnel. Steady operation of an aircraft at an angle of attack above the critical angle is not possible because, after exceeding the critical angle the aircraft behaves dynamically in a way that quickly causes the angle of attack to return to a value less than the critical angle. This dynamic maneuver indicates the stall of the aircraft.
This graph shows the stall angle, yet in practice most pilots discuss stalling in terms of airspeed. This is because all aircraft are equipped with an airspeed indicator, but very few aircraft have an angle of attack indicator. An aircraft's stalling speeds is published in the Flight Manual for a range of weights and flap positions, but the stalling angle of attack is not published.
As speed reduces, angle of attack increases until the critical angle is reached. The airspeed at which this angle is reached is the (1g, unaccelerated) stalling speed of the aircraft in that particular configuration. Deploying flaps/slats decreases the stall speed to allow the aircraft to take off and land at a lower speed.
In most light aircraft, as the stall is reached the aircraft will start to descend (because the wing is no longer producing enough lift to support the aeroplane's weight) and the nose will pitch down. Recovery from this stalled state usually involves the pilot decreasing the angle of attack and increasing the air speed, until smooth air flow over the wing is resumed. Normal flight can be resumed once recovery from the stall is complete. The manoeuvre is normally quite safe and if correctly handled leads to only a small loss in altitude. It is taught and practised in order to help pilots recognize, avoid, and recover from stalling the aeroplane.
The most common stall-spin scenarios occur on takeoff (departure stall) and during landing (base to final turn) because of insufficient airspeed during these manoeuvres. Stalls also occur during a go-around manoeuvre if the pilot does not properly respond to the out-of-trim situation resulting from the transition from low power setting to high power setting at low speed. Stall speed is increased when the upper wing surfaces are contaminated with ice or frost creating a rougher surface.
A special form of asymmetric stall in which the aircraft also rotates about its yaw axis is called a spin. A spin will occur if an aircraft is stalled and there is an asymmetric yawing moment applied to it. This yawing moment can be aerodynamic (sideslip angle, rudder, adverse yaw from the ailerons), thrust related (p-factor, one engine inoperative on a multi-engine non-centreline thrust aircraft), or from any number of possible sources of yaw.
Stalls can occur at higher speeds if the wings already have a high angle of attack. Attempting to increase the angle of attack at 1g by moving the control column back simply causes the aircraft to rise. However the aircraft may experience higher g, for example when it is pulling out of a dive. In this case, the wings will already be generating more lift to provide the necessary upwards acceleration and so there will be higher angle of attack. Increasing the g still further, by pulling back on the control column, can cause the stalling angle to be exceeded even at a high speed. High speed stalls produce the same buffeting characteristics as 1g stalls and can also initiate a spin if there is also any yawing.
Stalls depend more on angle of attack rather than airspeed. However, since, for every weight of every aircraft, there is an airspeed at which the wing's needed angle of attack will exceed the stall angle or critical angle of attack, airspeed in a given configuration is often used as an indirect indicator of approaching stall conditions.
There are multiple V speeds which are used to indicate when a stall will occur:
On an airspeed indicator, the bottom of the white arc indicates VS0 at maximum weight, while the bottom of the green arc indicates VS1 at maximum weight. While an aircraft's VS speed is computed by design, its VS0 and VS1 speeds must be demonstrated empirically by flight testing.
A turning flight stall is a stall that occurs while the aircraft is turning. In turning flight the stalling speed of an aircraft is faster than the stalling speed in straight, level flight.
A notable example of an air accident caused by a stall in turning flight is the 1994 Fairchild Air Force Base B-52 crash.
When an aircraft is turning, the lift required is equal to the weight of the aircraft plus extra lift to provide the centripetal force necessary to perform the turn. The lift required in a banked turn is:
where is the lift on the aircraft
Regardless of the weight of an aircraft or the angle of bank, the stall always occurs at the same angle of attack on the wing, and the same aircraft lift coefficient. In turning flight, the wing of an aircraft must generate more lift than is required in straight, level flight, so the airflow over the wing separates from the upper surface of the wing when the aircraft is flying at a faster airspeed than in straight, level flight. If the stalling speed of an aircraft in straight, level flight is , when that aircraft is turning with a load factor its stalling speed is:
A deep stall (also called a superstall) is a dangerous type of stall that affects certain aircraft designs, notably those with a T-tail configuration. In these designs, the turbulent wake of a stalled main wing "blankets" the horizontal stabilizer, rendering the elevators ineffective and preventing the aircraft from recovering from the stall.
Although effects similar to deep stall had long been known to occur on many aircraft designs, the name first came into widespread use after a deep stall caused the prototype BAC 1-11 to crash, killing its crew. This led to changes to the aircraft, including the installation of a stick shaker (see below) in order to clearly warn the pilot of the problem before it occurred. Stick shakers are now a part of all commercial airliners. Nevertheless, the problem continues to haunt new designs; in the 1980s a prototype of the latest model of the Canadair Challenger business jet entered deep stall during testing, killing one of the test pilots who was unable to leave the plane in time. Also, paragliders are sometimes known to enter a deep stall condition.
Deep stall is possible with some sailplanes, as their most common designs are T-tail configurations. The IS-29 glider is one of the gliders that are vulnerable to deep stalls when the CG and the overall weight are between certain limits.
In the early 1980s, a Schweizer SGS 1-36 sailplane was modified for NASA's controlled deep-stall flight program.
A different type of stall affecting the F-16 fighter is also known as a deep stall because of its similar difficulty in recovery, but for a different reason. The aircraft is designed to be inherently unstable, which when kept under control by its "fly-by-wire" system allows for higher maneuverability. However, this design, coupled with the intent of the control computer to keep the fighter level, prevents the aircraft from pitching nose-down in a stall, which would allow the pilot to recover given sufficient altitude. This is known as a deep stall because the elevators are rendered useless by the flight computer even though, unlike a T-tail, air does contact the elevators, and even with the computer disabled it is difficult to recover from (the pilot must "rock" the aircraft with elevator input until it pitches nose-down, which can take several seconds).
Stall warning system systems are often involve inputs from a broad range of sensors and systems to include a dedicated angle of attack sensor.
Blockage, damage, or inoperation of stall and angle of attack (AOA) probes can lead to the stall warning becoming unreliable and cause the stick pusher, overspeed warning, autopilot and yaw damper to malfunction.
If a forward canard is used for pitch control, rather than an aft tail, the canard is designed to meet the airflow at a slightly greater angle of attack than the wing. Therefore, when the aircraft pitch increases abnormally, the canard will usually stall first, causing the nose to drop and so preventing the wing from reaching its critical AOA. Thus the wing virtually never stalls.
If an aft tail is used, the wing is designed to stall before the tail. In this case, the wing can be flown at higher lift coefficient (closer to stall) to produce more overall lift.
Many aircraft have an angle of attack indicator among the pilot's instruments which lets the pilot know precisely how close to the stall point the aircraft is.
To look at it in a more simple way spoilers are effectively lift dumpers. A roll to the left could be aided by the left wing spoiler erecting.
The above confusion is wide-spread in the general public because of the very common misuse of the term stall in the media. This misunderstanding also results in another less common popular misconception, namely, that aircraft that have suffered an engine failure will just fall from the sky. This is because an aerodynamic stall will usually result in a significant loss of altitude during the recovery from the stall. If one assumes that stalls have something to do with the engines then one can see how this misconception can arise. However, and to the contrary, all fixed-wing aircraft rely on their wings to generate lift, and can therefore safely glide, sometimes for great distances (depending on their altitude at the time of engine failure), without any thrust from the aircraft engine(s).