This document contains a research report on axial compressor surge in gas turbine engines. It defines compressor surge as a breakdown in airflow through the compressor blades caused by stalling. When surge occurs, compressed air reverses direction and exits the front or back of the engine explosively. The report describes two methods engine manufacturers use to prevent surge: variable inlet guide vanes and blow-off valves. It also outlines the process for testing surge characteristics and calculating an engine's surge line on a performance map. The report provides examples to illustrate the concepts and concludes with references in the CUHarvard citation style.

1. Module 212SE – Aircraft maintenance engineering –
Research activity 2.
Michael Etienne Page 1 1/19/2015
Task name: Surge in an Axial
Compressor
Date: 31/10/2013
Student name: Michael Etienne Group: N/A
i. Undertake research to answer the following questions. A good starting place would
be the book, “The Jet Engine” produced by Rolls Royce Ltd. However, you will need
to expand your research beyond this in order to answer the questions fully. Please
feel free to add annotated diagrams to aid explanation. Text within diagrams will not
count towards word count.
ii. You should use this template for your answer and insert it into your portfolio of work
(CW1).
iii. Ideally your answer should be no more than 1000 to 1500 words.
iv. Additional sheets can be added as required.

2. Module 212SE – Aircraft maintenance engineering –
Research activity 2.
Michael Etienne Page 2 1/19/2015
1. Define what constitutes an axial-flow compressor surge and write a technical
description of how this phenomenon can occur?
An axial-flow compressor surge is constituted when the airflow through the compressor breaks down.
When this happens the compressor blades stall and it stops the air being forced to flow from front to
back. This enables the high-pressure air in the centre of the engine to escape from the front or back in
an explosive manner. This phenomenon when looked at in more detail has been found to occur due to
the stalling of the blades. The blades are small aerofoils, which behave in the same manner as a wing
when airflow passes over them. When the “angle of attack of the blades exceeds the critical angle of
attack” (Learnings 2010) or there is a disturbance in the airflow, e.g. FOD, bird strikes and intake
distortion, the blades stall causing “a pocket of stagnant air, also known as stall cells, rotate around the
circumference of the compressor” (Ward 2010:348). With this pocket of air flowing around the
compressor it leads to an increase of structural load on the blades and reduced efficiency of the system.
This stalling effect can propagate causing all the blades to stall throughout the whole compressor, also
known as an “axisymmetric oscillation”(Ward 2010:348), resulting in the system not being able to
maintain a steady airflow against the pressure gradient of the air. The rotator and stator blades create
the pressure gradient as the convergent stator blades increase the pressure of the air as it flows from
the rotating blades. As a result the velocity of the air decreases and becomes more stable for the
compression cycle to be efficient. In this particular situation the compressor begins to act as a turbine,
converting the air’s energy into work, forcing it out the front of the engine. This leads to a reverse of the
pressure and velocity of the air as it decreases in pressure but increases in the velocity. Some surges
may only occur momentarily but a deep surge may occur which is more severe and damaging. This
damage can become a deep surge that “involves a large mass flow fluctuation with airflow reversal
(expulsion of previously compressed air out of the intake) accompanied by a highly disturbing acoustic
effects (large bang)” (Ward 2010:349). With this effect it has known to be accompanied by a flame out
which is visible flames seen at both ends of the engine. The stall of the blades of the compressor and
the surge are often coupled but can independently occur in flight. When the two phenomenons occur
simultaneously this can lead to a surge cycle, this a recurrence of the stall over the blades of the
compressor due to the air’s pressure and velocity unchanged from the previous stall.

3. Module 212SE – Aircraft maintenance engineering –
Research activity 2.
Michael Etienne Page 3 1/19/2015
2. Describe and evaluate 2 methods utilised by engine manufacturers to alleviate
the effects of a compressor surge, including at least 3 examples of actual
implementation of these devices to illustrate historical development?
One of the methods utilised by engine manufacturer is using airflow control devices like the, VIGV (Variable
Stagger Intake Guide Valves) and VSV (Variable Stagger Stator Vanes). The VIGV is used to prevent the
problem of front-end stall at low or variable rotational speeds, stopping the build up of stagnant air in the forward
parts of the compressor. The way in which the vanes prevent this is by directing the airflow at the front of the
compressor onto the first row of blades at the correct angle of incidence. During flight, the axial velocity of the air
increases and “the vanes must be repositioned to maintain the correct angle of incidence” (Ward 2010:356)
otherwise this can lead to the blades stalling and creating a surge. With this type of method of airflow control, the
characteristic of the compressor changes and this results in the surge line moving but doesn’t alter the working
line. The same method can also be applied to the VSV (figure 1), which are found throughout the compressor as
the pressure increases along each stage. The effectiveness of having this type of component within an engine is
it a preventative measures so that the stages leading up to a surge do not occur. However one of the problems of
having this component integrated within the engine is when it comes to maintenance of the aircraft, which can
include inspection or removal of VIGV, this could prove difficult and tricky. The reason for this is that with this
integration, if one VIGV or VSV becomes damaged then it requires the entire removal of the axial compressor to
fix it. This can be time consuming for engineers during maintenance tasks as well as being costly if one broken
part is ingested through the whole working system of the engine and damages other parts. This type of method
has been implemented in a wide variety of modern aircraft such as the Boeing 737, most commonly used short-
haul aircraft in the world, and military aircraft like the Boeing Globe Master which is the largest transport aircraft
the RAF has flown in its 95 year history.
Another method used by engine manufacturers to alleviate the effects of a surge is a blow-off valve. The purpose
of this type of component is to not only prevent surge in the compressor but also reduce the damage on the
turbocharger and engine. The way which it does this is “relieve the damaging effects of compressor "surge
loading" by allowing the compressed air to vent to the atmosphere, making a distinct hissing sound, or recirculate
into the intake upstream of the compressor inlet” (Allard 1982). Overall the effectiveness of this type of system is
good as it offers an additional safety net for the engine in the scenario where the VIGVS fail at the front of the
engine and this system detects the surge (through the ECU controlled by the FADEC). Another effect measure
by this system is that it improves the surge margin; this is the “distance between the surge line and the operating
point on a vertical line for the constant corrected mass flow value”(GSP 2013). In one scenario the blow-off valve
can prove dangerous depending on the position of other components round it. If a MAF, a mass airflow sensor
which finds out the mass of the flowing air entering the engine, is located “upstream from the blow-off valve, the
engine control unit (ECU) will inject excess fuel because the atmospherically vented air is not subtracted from the
intake charge measurements. This leads with a fuel-rich mixture after each actuation which leads to a rich
mixing that could possible cause stalling of the engine when the throttle is closed” (Allard 1982). What this is
saying is if the detector is behind the blow off valve where it is venting the compressed air for the atmosphere,
the computer isn’t then able to calculate the correct fuel to air mixture causing the mixture to become too
enriched, as there is less compressed air than it had calculated. This type of scenario could damage other
components of the engine it produces “inefficiently combusted fuel, known as soot, which can foul spark plugs
and destroy catalyst converters” (Ward 2010). An example of this type of system has been implemented on the
C-130J Hercules which is currently used by the RAF to transport of troops/vehicles, air to air refueling or tactical
assault sorties. The blow-off valves on the turbo-prop engines on the Hercules are monitored by FADEC (Full
Authority Digital Engine Control). This system monitors the engine for its efficiency in temperature and pressure
so it will be able to detect a surge and act accordingly.
Figure 1: Variable Stator Vanes in an Axial Compressor (Rolls-Royce 1996:29)

4. Module 212SE – Aircraft maintenance engineering –
Research activity 2.
Michael Etienne Page 4 1/19/2015
3. Describe a typical surge characteristics testing process, including a method for
calculating a particular gas turbine engine’s “surge line”?
With every engine there is an operating map, this aids manufacturers in predicting the performance of it.
The map looks in particular at the pressure and temperature rises and efficiency of the engine against the
airflow through the engine. The performance characteristics of the engine can be divided into five different
parameters:
 Correct mass flow
 Correct rational speed
 Pressure ratio
 Component efficiency
 Reynolds index
(GSP 2013)
The most convenient and easiest way for engineers to read these characteristics is to represent them in
component characteristic performance graphs. With a compressor you have a operating map which is
plotted with an x-axis of compressor entry flow and the y-axis showing the pressure ratio index. The
compressor entry flow is a flow that has been corrected into a non-dimensional flow instead of the actual
flow of air. The difference between the two is the corrected flow is the flow of air if the engine was at sea
level pressure with the ambient temperature. If this was taken from an ISA table, that would be P=101.325
KPa with the temperature at 288.15K. The standardisation of these units is used as a good estimate for
performance to near design conditions as well as allowing engineers to be analytical of the data. The
formula used to calculate the corrected entry flow:
𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 𝑒𝑛𝑡𝑟𝑦 𝑓𝑙𝑜𝑤 =
𝑚√ 𝐶 𝑝 𝑇02
̇
𝐴 𝑃02
 𝐶 𝑝 = 𝑆𝑝𝑒𝑐𝑖𝑓𝑖𝑐 ℎ𝑒𝑎𝑡 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑎𝑡 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
 𝑃02 = 𝑆𝑡𝑎𝑡𝑖𝑐 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒 (𝑒𝑛𝑔𝑖𝑛𝑒 𝑖𝑛𝑙𝑒𝑡)
 𝑇02 = 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 (𝑒𝑛𝑔𝑖𝑛𝑒 𝑖𝑛𝑙𝑒𝑡)
 𝐴 = 𝐴𝑟𝑒𝑎
 M = Mass
(Cumpsty 2003:127)
With this type of flow, the engine manufacturer is represented by plotting it over a period of time as kg/s or
lb/min. The y-axis is plotted with the pressure ratio; this is the difference between the inlet pressure and
outlet pressure. On the graph there are two axes, the working line is a line that joins the points of the
equilibrium of the engine at different airflows and pressures. Alongside this there is another line known as
a surge line, which represents an area above it, the region of unstable airflow, which is what engineers
want to avoid, as these are areas where engine surge may occur. This line is plotted after calculating the
surge margin; this incorporates both the surge line, and the surge margin. The surge margin is the
“distance between the surge line and the operating point on a vertical line for the constant corrected mass
flow value”(GSP 2013). The formula to calculate this is:
𝑆𝑢𝑟𝑔𝑒 𝑀𝑎𝑟𝑔𝑖𝑛 = 100 ×
𝑃𝑅𝑠𝑢𝑟𝑔𝑒 − 𝑃𝑅𝑜𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑙𝑖𝑛𝑒
𝑃𝑅𝑜𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑙𝑖𝑛𝑒
= 100 × (
𝑃𝑅𝑠𝑢𝑟𝑔𝑒
𝑃𝑅𝑜𝑝𝑒𝑟𝑎𝑡𝑖𝑛𝑔 𝑙𝑖𝑛𝑒
− 1)
(GSP 2013)

5. Module 212SE – Aircraft maintenance engineering –
Research activity 2.
Michael Etienne Page 5 1/19/2015
4. Reference and Bibliography: (Must conform to CUHarvard format)
 A.Ward, T. (2010). Aerospace Propulsion Systems (1st ed.). Sinapore: John Wiley & Sons.
 Allard, A. (1982). Turbocharging and Supercharging. Cambridge, England: Patrick Stevens
Limited.
 General Electric Aviation (2013) General Electric Aviation J85 [Online] Available at
http://Http://www.geaviation.com/engines/v2 military/j85/ [Accessed 2 November
2013]
 Cumpsty, N. (2003). Jet Propulsion (2nd Edition ed.). Cambridge: University of Cambridge
Press.
 Flight Learnings (2010) Turbine Engine Operational Conditions [Online] Available at
http://Http://www.flightlearnings.com/2010/03/11/turbine-engine-operational-
considerations-part-two-compressor-stalls/ [Accessed 1 November 2013]
 Rolls-Royce. (1996). The Jet Engine (5th Edition ed.). Derby, UK: Renault Printing.
 G.D Team (2013) Surge Margin [Online] Available at
http://Http://www.gspteam.com/GSPsupport/OnlineHelp/index.html?surge_margin.ht
m [Accessed 3 November 2013]