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Interview with Etienne Coetzee,
Landing Gear Systems Engineer
Airbus, Bristol
by Hinke Osinga,
University of Bristol, UK |
Etienne Coetzee at
Airbus, Bristol. |
Airbus is a global enterprise with industrial
facilities in France, Germany, Spain and the UK; additional design and
engineering satellite offices in the USA, Russia and China;
fully-owned subsidiaries in the United States, China, Japan and the
Middle East; spare parts centres in Hamburg, Frankfurt, Washington,
Beijing, and Singapore; training centres in Toulouse, Miami, Hamburg
and Beijing; and more than 130 field service offices around the
world.
Airbus' industrial sites in France, Germany, Spain
and the UK produce complete sections of aircraft, which are then
transported to one of the final assembly lines in France and Germany.
The company also relies on industrial co-operation and partnerships
with major companies all over the world, and a network of some 1,500
suppliers in 30 countries.
The A380 aircraft
Since the first Airbus
entered service in 1974, the product range has grown into four
families of aircraft comprehensively covering the market from 107 to
555 seats. To date Airbus has sold more than 6,500 aircraft, has 266
customers/operators worldwide and has delivered nearly 4,500 aircraft.
The newest member of the family, the A380, is the first true
double-deck passenger airliner for the long-range market. This most
spacious and efficient aircraft has seats for 555 passengers and is
designed to allow for projected passenger growth worldwide without
negatively impacting the environment.
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A380 in
flight.
(Courtesy of Airbus
UK) |
Etienne Coetzee is an employee of Airbus UK in
Bristol and has been working in the Landing Gear Systems (LGS) group
for six years. The LGS group is generally responsible for the
integration of all the landing gear systems, which includes the
braking, steering, and extension/retraction systems. It is also
responsible for ground maneuverability of the aircraft at low to
medium speeds, and complements the work of the Handling Qualities and
Flight Control groups in Toulouse. The LGS group is heavily involved
in the development of the A380. The A380 may look like it is just a
much bigger version of other Airbus aircraft, but it has the latest in
major advanced technologies incorporated in its design, which makes it
a very modern aircraft.
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The LGS group provides
part of the information needed to ensure that airports can accommodate
this large aircraft with little or no need to adapt their runways, so
that only alterations at the gates will be necessary in some
cases. Etienne explains how the A380 team conducted major
research. "They had to decide how many wheels the landing gear
of the A380 should have, and where they should be positioned. We
investigated the steering capabilities, how big the actuators should
be, and so on."
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Front view of the
A380.
(Courtesy of Airbus
UK) |
A modeling career at Airbus
So how did Etienne get involved with Airbus? He was
born in Pretoria, South Africa. As a child he lived in the US for
three years, while his father was an airforce attaché in
Washington. His dream was to become a pilot, but after a while he
realized that he had a greater passion for engineering. So, he went to
the University of
Pretoria to study mechanical engineering. He obtained a BEng
degree and left for Munich to work on light fighter aircraft at
Daimler Chrysler Aerospace (now EADS). His research there focused on the aeroelastic
tailoring of composite wings. In Munich he shared a house with
students from the University of Bath in the UK. After they had
returned to Bath he saw an opportunity to visit them and visit Airbus
in Bristol at the same time.
Etienne really liked Bristol and he managed to
organize a job within the Aeroelastics department of Airbus UK. "At
the time I thought that landing gears were really boring and that wing
design was the only real engineering challenge."
About six years ago,
the LGS Modeling and Simulation group consisted of only two people,
but with new projects, new people were required, and Etienne got
assigned to this group as part of the A380 project. "I found to my
surprise that landing gears are, in fact, amazingly difficult to
design! The environment that the gears operate in is very harsh, and
you also have highly nonlinear components on the gear such as tires,
that makes the analysis of this system very complex. Our group models
how all landing gears at Airbus operate on the aircraft, so we are in
a unique position to gain a deep understanding of how everything
works."
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Etienne in front of
the A380 design office building. |
Developing a good model for how the landing gear
behaves on aircraft has taken years of testing. The model runs in a
package called ADAMS, which is a completely integrated software
environment for running simulations and visualizing how the aircraft
behaves upon touchdown and during taxiing. This code is used to run
lots of scenarios based on experimental data to check, for example,
when control of the aircraft might be lost during certain maneuvers.
Such studies are designed to support certification, to demonstrate
that an aircraft satisfies all safety regulations. Etienne is working
on improved and diverse methods for this purpose. He wants to
demonstrate that the use of techniques from nonlinear dynamics is a
potential future improvement in aircraft design.
Continued professional development and chaos theory
The southwest of England is the aerospace
hub in the UK. Companies including Airbus and Rolls Royce are based
there and developed a scheme with the University of
Bristol and the University of the West of England (also based in
Bristol) to promote personal development of their employees. Employees
of Airbus are encouraged to attend the course Continued
Professional Development in Aerospace (CPDA), which leads to an
MSc degree. The CPDA course consists of four mandatory management
modules and an options part with specialization in either management
or technical skills. For Etienne, this opportunity meant that he was
exposed to dynamical systems theory for the first time, during the
class Nonlinear Flight Mechanics that was taught by Mark
Lowenberg (Aero, Univeristy of Bristol) and Mikhail Goman (Computing,
De Montfort
University). "I think that this may have been the first time they
taught that course, because their assignments were extremely
difficult. I mean, I was trying to learn about strange attractors and
Hopf bifurcations, and I had no idea what they were talking about."
Despite the difficulty of the course, Etienne did realize that the
landing gear on aircraft is inherently nonlinear and that he would be
able to apply these rather abstract concepts. "I knew about hysteresis
effects in landing gears and how difficult it is to design an
effective anti-skid system due to the properties of the tires and
brakes. I now know that the nonlinear effects are actually
dominant."
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Model of the A320
in the ADAMS package. |
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Even though Etienne
was not sure how to apply what he had learnt about nonlinearity, he
decided to choose his CPDA project on this topic. Supervised by Mark
Lowenberg and Bernd Krauskopf (Engineering Mathematics, University of
Bristol), he performed a nonlinear dynamics study of the landing
gear on the A320 aircraft. The A320 is one of the most popular
single-aisle jetliner aircraft offering around 150 seats. Etienne's
project Nonlinear Aircraft Ground Dynamics, on the A320
landing gear system, fits in well with a European research project
that looks at all aspects of aircraft on the ground, from the point of
touchdown until the arrival at the gate.
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A320 in
flight.
(Courtesy of Airbus
UK) |
The tires on aircraft landing gear operate over a
far larger envelope than that of cars. Aircraft tires can easily
operate at slip angles of 20° and higher, as opposed to only
5° for typical car tires. These high-slip angles mean that the
tires operate in the nonlinear area of the tire properties. "As you
steer an aircraft around a corner, you must adjust the speed such that
the loads on the tires are within the safety margins and the aircraft
does not veer off the runway." The main interest lies in what are
called high-speed runway exits. After landing, an aircraft must taxi
away as quickly as possible to maximize the capacity of
airports. Hence, it is an important question how fast an aircraft can
turn corners without compromising safety.
It would have been
possible to run a simulation in ADAMS using a specific thrust and
steering angle and check whether it is safe or not. However, Etienne
decided to try to use techniques from dynamical systems, especially
the numerical continuation of different types of solutions. He managed
to translate the basic ADAMS model into Matlab-SimMechanics
and link it with the continuation package
AUTO. With continuation he can extract, for example,
specific relations between thrust levels and steering angles. "I was
also able to check how the entire scenario depends on the steering
rate, which was thought to influence the dynamics as well. I now know
that, while it does have an effect, the steering rate is not always
important."
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Overall stability
diagram for the A320 with varying steering angle and fixed medium
thrust level. |
In his thesis Etienne considers the aircraft moving
at variable thrust and steering angle. This means that in his model
the aircraft is going around in a circular fashion. "If this circular
motion happens at constant speed, then I can think of this as an
equilibrium that depends on the two parameters thrust and steering
angle. With the use of
AUTO I could trace the equilibrium solution in
parameter space. In one parameter, you find saddle-node bifurcations,
meaning that for fixed thrust certain steering angles are physically
not possible, and Hopf bifurcations, where the equilibrium motion
becomes unstable. With the continuation software I can find the
stability boundaries directly." Enthusiastically, Etienne explains how
one can then run simulations starting from data obtained by the
continuation to investigate what exactly happens along the equilibrium
branches and how stability is lost. "I was even able to identify a
region where the aircraft moves around in a chaotic way!"
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One-parameter bifurcation diagram of the behavior of
the A320 as the steering angle varies using a fixed medium thrust
level. The diagram shows the region where chaotic dynamics
exist. |
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Nonlinear dynamics may guide flight tests
While Etienne has
not yet used his newly acquired nonlinear dynamics skills on the A380
to great extent, work is being done to expand and utilize these
methods. As an example, Etienne mentions the `0.5 g lateral
ground load regulation.' The regulations state that an aircraft must
be capable of taking the load generated by a lateral force of 0.5
g due to a circular manoeuver.
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The
Federal Aviation
Administration has demonstrated from an operational test campaign
that the 0.5 g load regulation is very conservative, and
depends on the size and mass of the aircraft. Etienne explains that
they are also coming to the same conclusions when they look at the
results from some of their simulations. "In order to generate a
0.5 g force in the computer simulations, we need to specify
unrealistic tire properties. It is physically not possible to get a
large aircraft into a condition where it has to withstand such a
force! We learnt from simulations that the bigger the aircraft, the
smaller the lateral loads seem to be. With the results of the
stability margins from my project I can work out the lateral loads,
and my predictions correspond very well with the empirical data." In
fact, Etienne believes that dynamical systems theory may well make it
possible to prove analytically that 0.5 g loads cannot occur
in a specified aircraft type under physically realistic
maneuvers.
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Etienne at the A380
landing gear test rig. |
Dynamical systems techniques are already used for
some work on the A380. "We recommended a specific flight test for the
A380 based on our simulations and proved some skeptics wrong when we
obtained a perfect match to our predictions!" Etienne is eager to tell
everybody at Airbus UK about the use of nonlinear dynamics and he is
slowly but surely making progress. "There are a lot of opportunities
for seminars due to our Airbus Professionals Network, where we hold
seminars that are open to all employees, and invite speakers who are
experts in specific technical areas. I gave a talk at Airbus UK
recently, and most of the attendees were certainly very
interested."
Awareness will come gradually, and good
computational tools are essential to ensure that others are confident
in working with nonlinear methods. "It is important that we are able
to use commercial tools for building our models and link this known
computation environment with the new tools." Etienne envisages a Matlab front end
linked with
AUTO in a framework where the nonlinear methods
automatically create the plots that make it easy to interpret the
results. His group is currently working with the
Mathworks to link
their modeling codes with Matlab. "I am lucky to be working in such a
talented environment. We do not take `no' for an answer. Everybody in
our group has their own specialties, and together we overcome very
difficult technical problems."