Small em,. .. .Gas turbine engine compressors are susceptible to two closely related, but distinctly different air flow problems. These are stall and surge. Because the two are so closely related, they are often confused with each other, or taken to be the same thing. If you were studying for your JAR ATPL exams, the distinction you would require is as follows:. .. .COMPRESSOR STALL is the partial breakdown of airflow caused by air approaching a blade or blades at an angle greater than the stalling angle. It may affect a single blade, or a small group of blades, but does not affect the entire compressor.. .. .COMPRESSOR SURGE is the total breakdown of airflow through the compressor. In extreme cases it can result in a reversal of airflow, such that exhaust gas is ejected from the front of the air inlet.. .. .The critical point in the above distinction is that stall is a partial breakdown of flow and surge is a total breakdown of flow. Unfortunately the above distinction often leads to the incorrect assumption that surge is simply a particularly bad case of stall. This assumption is potentially dangerous in that many surges do not originate in the compressor, but are in fact induced by the actions of the pilot. To understand why this is the case we need to take a closer look at stall and surge in isolation from each other.. .. .Virtually all modern turbojet and turbo fan engines employ axial flow compressors, so I will concentrate my discussion on how stall and surge affects these.. .. .AXIAL FLOW COMPRESSORS. .As Captain Squelch has stated, axial flow compressor blades are just like small wings. When air flows over the rotating blades at a suitable angle of attack, they generate lift. This lift tends to pull them out of the front of the engine, but this is not possible because they are fixed to discs attached to the compressor drive shaft. So instead of pulling the blades forward, the lift pushes the air backwards, accelerating it into the engine. But the spaces between the rotating blades are divergent, tending to decelerate the air. So some of the extra velocity is immediately converted into static pressure. The overall effect is that the rotors increase velocity, temperature and static pressure of the air. . .. .Each disc of rotating blades is followed a disc of stationary blades, which also form a ring of divergent ducts. As the air passing from the rotating blades moves through these divergent ducts, it slows down and its static pressure and temperature are again increased. The process is then repeated by further stages of rotors and stators. . .. .AXIAL FLOW COMPRESSOR STALL. .As stated above, the blades are just like small cambered wings. Axial flow compressor blades are generally of very high aspect ratio, and like high aspect ratio wings, they have a relatively low stalling angle. . .. .As angle of attack increases up to the stalling angle, the coefficient of lift increases. But at angles beyond the stalling angle, the airflow separates from the upper surfaces. This causes a rapid decrease in coefficient of lift, with the departing air forming a highly turbulent wake downstream of the blade. This turbulent flow then passes over the stator blades behind, and into the next row of rotors. . .. .The process from this point onwards can take a number of forms. In many cases the turbulence is simply flushed through the engine, such that only one or two blades stall. In this case there will be few if any outward indications that a stall has occurred. In the next level of severity, the initial stall might cascade rearwards, such that several rows of blades are affected, before the subsequent stages regain control of the air. In this case, the pilot might detect a sudden increase in internal temperature and increased vibration from the engine. The next level of severity, with the stall cascading all the way back through the compressor, will obviously increase the severity of the outward symptoms.. .. .The above processes do not necessarily mean that the airflow through the compressor has broken down completely. Some engine types can behave in quite bizarre manners. One of the US military turbofans introduced in the seventies or eighties for example, exhibited a locked-in rotating stall. In this case the stalling of a single blade cascaded not downstream, but onto the next rotor blade on that disc. This cased a small pocket of stall to rotate in the opposite direction to the engine, but at about half of the engine RPM. The engine would continue to run and produce some thrust, but in an extremely unhappy manner. Fortunately this particular phenomenon is not common to the majority of engine type. . .. .Stall occurs when a blade meets incoming air at an angle greater than its stalling angle. So any factor that increases angle of attack beyond the stalling angle is likely to cause compressor stall. To understand what these factor might be we need to look at the angle of attack of a rotating blade. . .. .To investigate this we can start with a blank sheet of paper. Draw a short (1 inch) line in the centre of the sheet, such that it is angled downwards from right to left at about 30 degrees. We will consider this to be the chord line of a compressor blade. The blade is rotating down the page and the airflow is coming in from left to right. Because the blade is rotating down the page, the airflow due to rotation is flowing up the page To represent this, draw a 2 inch arrow pointing vertically upwards to the trailing edge of the chord line. The remainder of the airflow is made up of axial flow into the engine. To represent this, draw a 1 inch horizontal arrow so that its head touches the tail of the rotational flow arrow. We can now draw the resultant of these flows, or the real airflow, by drawing a third arrow from the tail of the axial flow arrow to the head of the rotational flow arrow. The angle of attack is the angle between this third, resultant flow arrow and the chord line of the blade.. .. .To investigate increasing RPM, simply increase the length of the rotational flow arrow and move the axial flow arrow down to meet its tail, then draw a new resultant arrow. This will give a greater angle of attack. To investigate the effect of increasing the axial airflow velocity, simply increase the length of one of the axial flow arrows, then draw a new resultant. This will show that increasing axial flow decrease angle of attack, while decreasing axial flow increases it. . .. .By repeating this process using various combinations of rotational speed and axial flow speed we will find that angle of attack is increased by increasing RPM or by decreasing axial flow velocity. So any action which does either of these things in isolation, will increase the tendency to stall. We should however note that increasing RPM will usually increase axial airflow velocity, so stall will not necessarily be caused simply by increasing RPM. . .. .The important point here is that stall will occur when the ratio of RPM to axial airflow velocity reaches some critical value. So if for example we pull the aircraft into a very high angle of attack, or into a very high sideslip angle, the disruption of airflow into the intakes is likely to cause stall. Similarly if we accelerate the engine very rapidly, such that axial airflow cannot keep up with RPM, we will again cause stall. Common (pilot induced) causes of stall include:. .. .a. Excessive angles of attack or sideslip angles. This sometimes affects one engine position more than others.. .b. Operating with iced up intakes.. .c. Excessive engine acceleration or deceleration rates.. .d. Starting engines when parked in strong crosswinds or tailwinds (or with intake or exhaust blanks still fitted).. .e. Taxiing or flying too close behind other aircraft such that their exhausts or wakes flow into our intakes. . .. .But the stalling angle of a given blade is not entirely fixed. Structural damage caused by the ingestion of hard objects, erosion by sand or grit, icing, and surface contamination, can all reduce the stalling angle. So engines tend to stall more easily as they age, or if their compressors become iced or dirty. In the case or naval or SAR helicopters for example, this latter effect is particularly problematic. When hovering or operating close to the sea, the build up of salt crystals on the blades can quickly reduce stalling angle to dangerous levels. . .. .The degree to which the engine is able to control its airflow increases from front to rear. This is because the first row of rotor blades must accept whatever airflow they meet, whereas the subsequent rows receive air from the blades ahead of them. So the danger of stall in a serviceable engine is greatest at the front. The most common method of stall prevention is the use of variable angle inlet guide vanes or VIGVs. These are an additional row of non-rotating blades, immediately ahead of the first row of rotors. By changing the pitch angle of these vanes, the angle of the incoming airflow is varied to ensure that the angle of attack of the first row of rotors is always less than their stalling angle. This method is sometimes extended so that several rows of stator are of variable angle. I believe this has been applied to up to 10 stages in some engine.. .. .COMPRESSOR SURGE. .This phenomenon should perhaps be more correctly termed "compressor pressure surge". It occurs when the pressure downstream of the compressor, or in the downstream end of the compressor becomes too great. The airflow breaks down, the blades stall, and the pressure then surges forward, reversing the flow through the compressor. It is this that results in the ejection of exhaust gas from the front of the air intake. Like stall, the severity of compressor surge varies from minor to major. On an increasing scale of severity the effects can include minor vibration, increased internal temperatures, severe rumbling and loss of thrust, flame out, and (ultimately) immediate destruction of compressors and turbines.. .. .The most common pilot-induced cause of surge is excessive engine acceleration. If the throttle is opened too quickly, the pressure in the combustion chamber will immediately increase. If the compressor cannot accelerate quickly enough to match its outlet pressure to combustion chamber pressure, the airflow out of the back of the compressor will cease. This will increase the angle of attack of the blades at the rear of the compressor causing them to stall. Reduced flow through the stalled rear blades will reduce flow through the blades immediately upstream, causing them to stall. The stall will then cascade upstream, from rear to front. This stalling of progressively more of the compressor will reduce compressor outlet pressure still more, making the mismatch between combustion chamber pressure and compressor outlet pressure even greater. This allows the high pressure air in the combustion chamber to expand forward reversing the flow through the compressor.. .. .To understand another common cause of surge we need to look again at the compression process. The blades are little wings and like wings they are sensitive to airflow velocity. If the velocity is too low they will stall due to excessive angles of attack. If the velocity is too great they will produce shock waves, leading to shock-induced separation and shock stall. So the flow velocity through the compressor should ideally be kept constant. But the compression of the air as it flows through the compressor reduces its volume. If the cross-section of the air passage is constant, this reduced volume of air will slow down, or re-expand to fill the space, thereby undoing all of the work that has been done in compressing it. To prevent this, the cross-section of the air space gradually reduces from front to rear. . .. .This has the desired effect in maintaining constant velocity, but means that the engine is now committed to achieving a set rate of reduction in the volume of the air. This is no problem when running at optimum RPM, but at lower speeds, the compression process is less efficient. This means that the air is too big to pass through the reducing air space at the rear of the compressor. Left uncorrected this will cause the rear of the compressor to choke, with the airflow breaking down. This will stall the rear blades, reducing the compression efficiency still further. Once again the stalled rear stages will stall those in front of them so that a stall progresses from rear to font through the compressor. . .. .This problem is commonly prevented by the use of anti-surge bleed valves. These are controlled holes in the side of the compressor casing, usually about half or three-quarters of the way downstream. At low RPM they are open, allowing excess volumes of air to be dumped overboard. As RPM increases, the compressor efficiency improves, so the valves are gradually closed so that all of the airflow passes through the entire compressor.. .. .The subjects of compressor stall and surge are obviously very complex, and this post is already far too long. For readers with the required time (and interest) available, I would recommend "The Jet Engine" by Rolls Royce. ISBN 0 902121 04 9. (For the more cynical readers, "no I do not work for them, nor have I ever done so").