Compressor stall
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A compressor stall is a situation of abnormal airflow found in dynamic compressors, as used in jet engines and turbocharged reciprocating engines, which results from a stall of the aerofoils within the compressor rotor. When a compressor stalls, then starts compressing again, then stalls again in a repeating cycle, the condition is known as compressor surge.
Compressor stalls result in a loss of engine power, which can vary in severity from a momentary drop (occurring so quickly it is barely registered on engine instruments) to a complete shut down of the engine, as with a jet flameout. This was a common problem on early jet engines with manual or mechanical fuel control units, but has been virtually eliminated by the use of hydromechanical and electronic (FADEC) control systems.
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[edit] Types
There are two general types of compressor stall.
"Axis-symmetric stall" is a complete breakdown in compression resulting in a violent expulsion of compressed air out the intake, due to the compressor's inability to maintain pressure. In an axis-symmetric stall, the compressor has reached the limit of its pressure rise capabilities: local airfoil stalls quickly propagate to the entire compressor. The result of this is the loss of the compressor's ability to maintain a pressure ratio and the subsequent backflow of all the air from the high pressure section.
"Rotational stall" is a local disruption of airflow within the compressor which continues to provide compressed air but with reduced effectiveness. "Rotational stall" arises when a region of the compressor experiences local airfoil stall. The resulting pockets of stagnant air (referred to as "stall cells") rotate within the compressor, passing to the next airfoil around the rotor without moving in the flow direction. A rotational stall may be momentary or may be steady, however, often the stall cells will grow to a complete compressor axis-symmetric stall.
[edit] Causes
Compressor stalls are aerodynamic stalls in which the airfoils in the compressor lose their lifting capability. This often results in a sudden change in the pressure differential between the intake and combustion chamber. While modern engines with advanced control units can avoid many causes of stall - jet aircraft pilots must take this into account when dropping airspeed or increasing throttle.
The following factors can induce compressor stall:
- Aircraft operation outside of design envelope - e.g. Extreme flight manoeuvre, or engine thrust too high for the operating altitude.
- Engine operation outside specified design parameters - e.g. Abrupt increases in engine thrust ("slam acceleration").
- Turbulent or disrupted airflow to the engine intake - e.g. Use of reverse thrust at low forward speed, resulting in re-ingestion of hot turbulent air, or for military aircraft, ingestion of hot wake gases from fired missile.
- Damaged, worn or contaminated engine components - e.g. such as damaged rotor blades or wrongly positioned guide vanes)
One of the most common causes of compressor stalls in commercial aviation aircraft is a bird strike. On take-off, while manoeuvring on the ground or while on approach to landing, planes operate in proximity to birds. It is not uncommon for birds to be sucked into the intake of the engine and the disruption to the airflow and damage to the blades often causes compressor stall.
[edit] Effects
Compressor axis-symmetric stalls, or compressor surges, are immediately identifiable producing one or more extremely loud bangs from the engine and reports of fire or flames "shooting" out of the engine are common during this type of compressor stall. These stalls may be accompanied by an increased exhaust gas temperature, an increase in rotor speed due to the large reduction in work done by the stalled compressor and yawing of the aircraft in the direction of the affected engine. Severe stresses occur within the engine and aircraft particularly from the intense aerodynamic buffeting within the compressor.
Compressors are carefully designed to avoid stall and as a result the effects of a stall can vary depending on the design and operating conditions. A minor stall may create an alarming noise but have little other effect. On the other hand, a violent compressor surge might completely destroy the engine and set it on fire. Locked-in stall, where the compressor stalls repeatedly and continuously, is particularly dangerous, with very high levels of vibration causing accelerated engine wear and possible damage.
The appropriate response to compressor stalls varies according the engine type and situation—but usually consists of immediately and steadily decreasing thrust on the affected engine.
[edit] Notable stall occurrences
The Lockheed SR-71 Blackbird, a supersonic reconnaissance aircraft developed in the United States, employed special jet engines that were known for their tendency to “hard unstart,” that is, their tendency to produce spectacular compressor stalls, often violent enough to throw the pilot's head against the canopy of the aircraft. The stalls occurred when shockwaves over the jet intakes moved out of their proper location during high-speed supersonic flight. The stalls produced a very dramatic loss of thrust and a violent yaw moment, and required quick action by the crew to avoid compromise of the mission or airframe. Unstarts were the bane of SR-71 pilots until computer controls on the engines later in the SR-71 program significantly reduced their incidence and simplified recovery.
The Rolls-Royce Avon was affected by compressor surge early on in its development. Such was the perceived importance and urgency of the engine that Rolls-Royce licensed the Sapphire compressor design from Armstrong Siddeley to speed development.
A compressor stall, caused by inadvisably trying to sideslip to lose altitude, contributed to the death of Lt. Kara Hultgreen.
