stall

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verb – A stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded. The critical angle of attack is typically about 15 degrees, but it may vary significantly depending on the fluid, foil, and Reynolds number.

Stalls in fixed-wing flight are often experienced as a sudden reduction in lift as the pilot increases the wing’s angle of attack and exceeds its critical angle of attack (which may be due to slowing down below stall speed in level flight). A stall does not mean that the engine(s) have stopped working, or that the aircraft has stopped moving — the effect is the same even in an unpowered glider aircraft. Vectored thrust in manned and unmanned aircraft is used to surpass the stall limit, thereby giving rise to post-stall technology.

Fixed-wing aircraft

A fixed-wing aircraft can be made to stall in any pitch attitude or bank angle or at any airspeed but deliberate stalling is commonly practiced by reducing the speed to the unaccelerated stall speed, at a safe altitude. Unaccelerated (1g) stall speed varies on different fixed-wing aircraft and is represented by colour codes on the air speed indicator. As the plane flies at this speed, the angle of attack must be increased to prevent any loss of altitude or gain in airspeed (which corresponds to the stall angle described above). The pilot will notice the flight controls have become less responsive and may also notice some buffeting, a result of the turbulent air separated from the wing hitting the tail of the aircraft.

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 aircraft’s weight) and the nose will pitch down. Recovery from the stall involves lowering the aircraft nose, to decrease the angle of attack and increase the air speed, until smooth air-flow over the wing is restored. Normal flight can be resumed once recovery is complete. The maneuver is normally quite safe and if correctly handled leads to only a small loss in altitude (50′-100′). It is taught and practised in order for pilots to recognize, avoid, and recover from stalling the aircraft. A pilot is required to demonstrate competency in controlling an aircraft during and after a stall for certification, and it is a routine maneuver for pilots when getting to know the handling of a new aircraft type. The only dangerous aspect of a stall is a lack of altitude for recovery.

A special form of asymmetric stall in which the aircraft also rotates about its yaw axis is called a spin. A spin can 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 less likely sources such as severe turbulence. The net effect is that one wing is stalled before the other and the aircraft descends rapidly while rotating, and some aircraft cannot recover from this condition without correct pilot control inputs (which must stop yaw) and loading. A new solution to the problem of difficult (or impossible) stall-spin recovery is provided by the ballistic parachute recovery system.

The most common stall-spin scenarios occur on takeoff (departure stall) and during landing (base to final turn) because of insufficient airspeed during these maneuvers. 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 wing surfaces are contaminated with ice or frost creating a rougher surface, and heavier airframe due to ice accumulation.

Stalls occur not only at slow airspeed but can occur at any speed – but only if the wings exceed their critical angle of attack. Attempting to increase the angle of attack at 1g by moving the control column back normally causes the aircraft to climb. However, aircraft often experience higher g, for example when turning steeply or pulling out of a dive. In these cases, the wings are already operating at a higher angle of attack to create the necessary force (derived from lift) to accelerate in the desired direction. Increasing the g loading still further, by pulling back on the controls, can cause the stalling angle to be exceeded -even though the aircraft is flying at a high speed. These “high-speed stalls” produce the same buffeting characteristics as 1g stalls and can also initiate a spin if there is also any yawing.

Symptoms of onset

One symptom of an approaching stall is slow and sloppy controls. As the speed of the aircraft decreases approaching the stall, there is less air moving over the wing, and, therefore, less air will be deflected by the control surfaces (ailerons, elevator, and rudder) at this slower speed. Some buffeting may also be felt from the turbulent flow above the wings as the stall is reached. The stall warning will sound, if fitted, in most aircraft 5 to 10 knots above the stall speed.

Characteristics

Different aircraft types have different stalling characteristics. A benign stall is one where the nose drops gently and the wings remain level throughout. Slightly more demanding is a stall in which one wing stalls slightly before the other, causing that wing to drop sharply, with the possibility of entering a spin. A dangerous stall is one in which the nose rises, pushing the wing deeper into the stalled state and potentially leading to an unrecoverable deep stall. This can occur in some T-tailed aircraft wherein the turbulent airflow from the stalled wing can blanket the control surfaces at the tail.

Types

Dynamic stall

Dynamic stall is a non-linear unsteady aerodynamic effect that occurs when airfoils rapidly change the angle of attack. The rapid change can cause a strong vortex to be shed from the leading edge of the aerofoil, and travel backwards above the wing. The vortex, containing high-velocity airflows, briefly increases the lift produced by the wing. As soon as it passes behind the trailing edge, however, the lift reduces dramatically, and the wing is in normal stall.

Dynamic stall is an effect most associated with helicopters and flapping wings. During forward flight, some regions of a helicopter blade may incur flow that reverses (compared to the direction of blade movement), and thus includes rapidly changing angles of attack. Oscillating (flapping) wings, such as those of insects—including the most famous one, the bumblebee—may rely almost entirely on dynamic stall for lift production, provided the oscillations are fast compared to the speed of flight, and the angle of the wing changes rapidly compared to airflow direction.

Stall delay can occur on airfoils subject to a high angle of attack and a three-dimensional flow. When the angle of attack on an airfoil is increasing rapidly, the flow will remain substantially attached to the airfoil to a significantly higher angle of attack than can be achieved in steady-state conditions. As a result, the stall is delayed momentarily and a lift coefficient significantly higher than the steady-state maximum is achieved. The effect was first noticed on propellers.

Deep stall

A deep stall (or super-stall) is a dangerous type of stall that affects certain aircraft designs, notably jet aircraft with a T-tail configuration and rear-mounted engines. In these designs, the turbulent wake of a stalled main wing, nacelle-pylon wakes and the wake from the fuselage “blanket” the horizontal stabilizer, rendering the elevators ineffective and preventing the aircraft from recovering from the stall. Taylor states T-tail propeller aircraft, unlike jet aircraft, do not usually require a stall recovery system during stall flight testing due to increased airflow over the wing root from the prop wash. Nor do they have rear mounted nacelles which can contribute substantially to the problem. The A400M was fitted with a vertical tail booster for some flight tests in case of deep stall.

Trubshaw gives a broad definition of deep stall as penetrating to such angles of attack \alpha that pitch control effectiveness is reduced by the wing and nacelle wakes. He also gives a definition that relates deep stall to a locked-in condition where recovery is impossible. This is a single value of \alpha, for a given aircraft configuration, where there is no pitching moment, ie a trim point.

Typical values both for the range of deep stall, as defined above, and the locked-in trim point are given for the Douglas DC-9 Series 10 by Schaufele. These values are from wind tunnel tests for an early design. The final design had no locked in trim point so recovery from the deep stall region was possible, as required to meet certification rules. Normal stall beginning at the ‘g’ break (sudden decrease of the vertical load factor) was at 18 degrees \alpha, deep stall started at about 30 degrees and the locked-in unrecoverable trim point was at 47 degrees.

The very high \alpha for a deep stall locked-in condition occurs well beyond the normal stall but can be attained very rapidly as the aircraft is unstable beyond the normal stall and requires immediate action to arrest it. The loss of lift causes high sink rates which, together with the low forward speed at the normal stall, give a high \alpha with little or no rotation of the aircraft. BAC 1-11 G-ASHG, during stall flight tests before the type was modified to prevent a locked-in deep stall condition, descended at over 10,000 feet/minute and struck the ground in a flat attitude moving only 70 feet forward after initial impact. Sketches which show how the wing wake blankets the tail may be misleading if they imply that deep stall requires a high body angle. Taylor and Ray[36] show how the aircraft attitude in the deep stall is relatively flat, even less than during the normal stall, with very high negative flight path angles.

Effects similar to deep stall had been known to occur on some aircraft designs before the term was coined. A prototype Gloster Javelin (serial WD808) was lost in a crash on 11 June 1953, to a “locked in” stall. However, Waterton states that the trimming tailplane was found to be the wrong way for recovery. Low speed handling tests were being done to assess a new wing. Handley Page Victor XL159 was lost to a “stable stall” on 23 March 1962. It had been clearing the fixed droop leading edge with the test being stall approach, landing configuration, CG aft. The brake parachute had not been streamed as it may have hindered rear crew escape.

The name “deep stall” first came into widespread use after the crash of the prototype BAC 1-11 G-ASHG on 22 October 1963, killing its crew. This led to changes to the aircraft, including the installation of a stick shaker (see below) to clearly warn the pilot of an impending stall. Stick shakers are now a standard part of commercial airliners. Nevertheless, the problem continues to cause accidents; on 3 June 1966, a Hawker Siddeley Trident (G-ARPY), was lost to deep stall; deep stall is suspected to be cause of another Trident (the British European Airways Flight 548 G-ARPI) crash – known as the “Staines Disaster” – on 18 June 1972 when the crew failed to notice the conditions and had disabled the stall recovery system. On 3 April 1980, a prototype of the Canadair Challenger business jet crashed after initially entering a deep stall from 17,000ft and having both engines flame-out. It recovered from the deep stall after deploying the anti-spin parachute but crashed after being unable to jettison the chute or relight the engines. One of the test pilots was unable to escape from the aircraft in time and was killed. On the 26 July 1993, a Canadair CRJ-100 was lost in flight testing due to a deep stall. It has been reported that a Boeing 727 entered a deep stall in a flight test, but the pilot was able to rock the airplane to increasingly higher bank angles until the nose finally fell through and normal control response was recovered. A 727 accident on 1 December 1974, has also been attributed to a deep stall. The crash of West Caribbean Airways Flight 708 in 2005 was also attributed to a deep stall.

Reports on the crash of Air France Flight 447 have stated that the accident involved a deep stall entered at 38,000 ft (11,582 m) and continued for more than three minutes until impact, but this was a steady state conventional stall because the aircraft (an Airbus A330) did not have a T-tail.

Canard-configured aircraft are also at risk of getting into a deep stall. Two Velocity aircraft crashed due to locked-in deep stalls. Testing revealed that the addition of leading edge cuffs to the outboard wing prevented the aircraft from getting into a deep stall. The Piper Advanced Technologies PAT-1, N15PT, another canard-configured aircraft, also crashed in an accident attributed to a deep stall. Wind tunnel testing of the design at the NASA Langley Research Center showed that it was vulnerable to a deep stall.

In the early 1980s, a Schweizer SGS 1-36 sailplane was modified for NASA’s controlled deep-stall flight program.

Tip stall

Aircraft with a swept wing suffer from a particular form of stalling behaviour at low speed. At high speed the airflow over the wing tends to progress directly along the chord, but as the speed is reduced a sideways component due to the angle of the leading edge has time to build up. Airflow at the root is affected only by the angle of the wing, but at a point further along the span, the airflow is affected both by the angle as well as any sideways component of the airflow from the air closer to the root. This results in a pattern of airflow that is progressively “sideways” as one moves toward the wingtip.

As it is only the airflow along the chord that contributes to lift, this means that the wing begins to develop less lift at the tip than the root. in extreme cases, this can lead to the wingtip entering stall long before the wing as a whole. In this case the average lift of the wing as a whole moves forward; the inboard sections are continuing to generate lift and are generally in front of the center of gravity (CoG), while the tips are no longer contributing and are behind the CoG. This produces a strong nose-up pitch in the aircraft, which can lead to more of the wing stalling, the lift moving further forward, and so forth. This chain reaction is considered very dangerous and was known as the pitch-up.

Tip stall can be prevented in a number of ways, at least one of which is found on almost all modern aircraft. An early solution was the addition of wing fences to re-direct sideways moving air back towards the rear of the wing. A similar solution is the dog-tooth notch seen on some aircraft, like the Avro Arrow. A more common modern solution is to use some degree of washout.

References:
Low Speed Handling with Special Reference to the Super Stall” Trubshaw, Appendix III in “Trubshaw Test Pilot” Trubshaw and Edmondson, Sutton Publishing 1998, ISBN 0 7509 1838 1, p.166

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