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NO STRAIGHTFORWARD SOLUTION
FORMULA SAE/STUDENT 50
THE AERO PACKAGE CONUNDRUM The average speed of the Formula
Student UK and Germany events has significantly increased in the
last few years, making the need for an aero package an essential to
win. UH Racing's Karl Mackle talks about the challenges behind
designing the perfect aero package for FSAE
FORMULA SAE and Student, its UK and German equivalent is, at its
core, a design engineering competit ion for student teams across
the world rather than a race. This means that the regulations
governing the car are few, to help encourage innovation, leading to
a field of cars that can look very different. This is particularly
true when it comes to aerodynamics wi th primarily positional
constraints. You also have concepts like powered ground effect
(like the Brabham BT46b fan car) and skirts being illegal, but
purely for safety. Being a design engineering competit ion brings a
whole new set on restrictions on what can be designed though, as
the car is not just scored on its speed in dynamic events but on
static events as well .
The static events consist of a design, business and cost event,
which historically have not scored aero packages that well as they
do not meet the requirements. This restricted the number of teams
that actually ran aero packages for a number of years as the gain
that they gave in dynamic events were lower than the loss they
bought to the static events. In recent years, though, the average
speed has increased rapidly making aero packages almost an
essential item for any team that wants to w in . The biggest
restraint on their design is how to minimise the losses in the
dynamic events.
The cost event is the biggest restraint on aero design. The car
will always be cheaper wi thout an aero package, so the design will
always be constrained heavily by performance vs cost analysis. This
tends to keep the aerodynamic packages simple as it
is the most cost-effective way to implement any package, but it
does not deter some teams from implementing 'expensive' systems.
Even 'DRS' style systems are implemented and tested but the
advantage they give to straightline speed versus their cost means
that their use in competit ion is still very l imited.
The design event is less of a l imiting factor on the challenge
as it will reward engineering effort in the design and build of the
car. How it does limit the design choices is that the aero package
has to meet the original design intent, to design a formula style
car for the weekend racer. An argument could stand that a weekend
racer would not be able to understand how aero maps work, therefore
they should not belong in Formula Student. This encourages the
implementation of a simple aero package again as arguments can be
made that a simple package could be understood by the weekend
racer.
With most teams now fol lowing this philosophy, the loss will be
incurred by all the top teams though, meaning aero packages will
have less of an effect this year than it has previously.
So from the static events it seems that although the teams have
the freedom to do almost anything, the design must be kept simple.
It's only when the dynamic events are studied that the problem
becomes more complex. The first day of dynamic events alone shows
that the wings cannot be straightforward as acceleration and skid
pan events require opposite set ups aerodynamicalfy.
The acceleration event is a 75m sprint that takes less than four
seconds and sees speeds exceed 70 km/h amongst the top teams. This
leaves the emphasis on keeping drag to a minimum with a secondary
thought given to downforce for traction.
The Formula Student event held in the UK in particular requires
thought to be given to downforce for traction as the event is
typically wet and it did see many cars struggling in 2012 with the
high power-to-weight ratios.
Contrastingly, the skidpan event measures the steady-state
turning ability of the car around a 15.25m diameter circle. Here
the emphasis is purely on downforce to maximise the speed. With no
acceleration taking place, there is no need to consider drag at any
point, and maximum speed a car can turn at a 15.25m diameter is
relatively low ensuring drag will never affect the speed that can
be achieved.
To gain the maximum downforce possible for this event,
Australian team Monash Motorsport has tested out running the rear
wing at a high rake angle, showing just how little emphasis needs
to be given to drag or efficiency for this event. This need for
ultimate downforce values is even more critical at the German
Formula Student event as the skidpan is artificially wetted, making
traction even more important.
These two events show that if designing a fixed aero package
then the best compromise would be to create a very efficient one to
maximise performance for both events. The general solution used by
most teams that use wings is to make them adjustable instead. This
allows the maximum angle of attack to be used for the skidpan event
and a lower angle of attack or open 'DRS' style set up for the
acceleration event.
With mainly double or triple element wings being used, the
design remains relatively simple and can still be validated f c the
static events, while also keeping the true manufacture cost down.
This is an impor tar : consideration wi th most teams, wi th budge
t being small to design and build a full race car f rom scratch
every year. This adds an unusual problem, wi th the student
designer also having to track down sponsorship to have their parts
manufactured.
The second day of dynamic events sees the autocross event take
place, a 0.8km point to point sprint around a twisting circuit with
a supposed average speed no greater than 48km/h /30mph. The
average
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51
RIGHT & BELOW The Monash Motorsport team has been at the
forefront of running a rear wing at a high rake angle (right). The
author has opted for a split wing on the UH Racing car (below) for
maximum downforce in front of the wheels while giving cleaner air
flow to the radiator
speed stated in the rules is slightly off as the teams will
probably be challenging closer to 60km/h/37mph, allowing over 4 0 %
more downforce to be created than the rules suggest. The top speed
achieved will not be much higher than 100km/h/62mph either, which
means that drag isn't too much of a concern amongst the teams. In
fact, with the tracks consisting of corners wi th a diameter
A QUESTION OF BALANCE
The third day of dynamic events sees the finale, the endurance.
This is also the highest scoring event and integrates the
efficiency event as well. The efficiency event adds an interesting
component to the event, as the usual high downforce setup will
bring an economy penalty. For electric vehicles
between 23-45m and the longest straight being no more than 60m,
there is a demand to have as much downforce as possible at all
times to allow a higher average speed. The only t ime the
aerodynamics will not be effective is in the hairpins that can have
a diameter as small as 9m and see the speed briefly drop below
30km/h/18mph. This means that setups similar to the skidpan are
usually seen, although high rake angles on rear wings are put to a
more modest angle, taking into account efficiency for the higher
speed encountered. Simulation work suggests that small adjustments
may be needed between skidpan and autocross though to regain
aerodynamic balance for the different style of track.
this is a problem that definitely needs to be considered as the
drop-off in performance due to power drain is visibly noticeable by
the end of the 22km event T^e two solutions used to achieve the
best event time rather than ultimate lap t ime are to reduce the
maximum pov.er amiab le f rom the start of the event or to run a
lower drag set up aero package. The : : c-omise for the endurance
event isn't just limited to the electric cars.
The petrol cars must also balance ultimate downforce for
ultimate : e : drag for higher efficiency score. This event also
holds a unique challenge for the four-cylindered engines, as this
is the first event where heat in the e rg --E : 2::-;
become a major concern. This problem will also exist in the twin
and single cylinder engine cars but to a much smaller degree. This
leads to a compromise between lap times, efficiency and cooling,
leading to a more complex design solution for most teams running
wings.
The main concern always seems to tend towards ultimate lap t ime
wi th all teams wanting the honours of being the fastest car. To
get around the cooling and efficiency requirements has seen a few
teams use a unique solution this year, include myself at UH Racing.
The implementation of a split wing allows the maximum downforce to
be achieved in front of the wheels while giving a cleaner air f low
to the radiator to achieve the required cooling.
The second solution is to have only a multi element wing in
front of the radiator, which may sound like a large compromise to
ultimate downforce but will allow a much easier implementation of a
diffuser. The front wings that are generally used in Formula
Student tend to starve the inlet f low for diffusers, causing them
to create more static downforce than aerodynamic downforce. This
problem causes many teams to choose between a wing package and a
diffuser rather than have a full aero package, although some have
successfully managed to implement full aero packages..
The weight of having a full aero package must also be
questioned. Formula Student is a sport w i thou t a m in imum
weight requirement. This means that cars weighing as small as 1
30kg or less are expected this year, and having a full aero package
wil l have a noticeable difference to the weight and performance.
The w ing package at UH Racing is expected to weight just under
10kg, and a diffuser is likely to weigh the same due to its size
and support requirements.
As far as what the correct solution to go for is, this is still
an unknown. Simulations are predicting a 2.5 second improvement in
lap time from the implementation of the wings alone for UH Racing,
a massive improvement when the starting time was just 55 seconds.
Gauging how this increased performance will compare to other teams
is difficult, with all teams quoting massively different downforce
values, ones that do not correlate to on-track performance last
year. Also, the different ways in which the teams validate the aero
packages will have a large effect on the static event scores,
meaning the winner of the Formula Student event for this year is
anyone's guess. Q
CO LJ
< to
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Issue 12 - 2013
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FORMULA SAE/STUDENT FRONT WINGS ANALYSIS
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WINGS are typically fitted to race cars to generate downforce
which increases the load reacted at the tyre contact patch. Greater
load increases acceleration under braking and cornering reducing
laptimes. However, it does reduce the tractive acceleration and
therefore the top speed achieved on the straights. Downforce and
the efficiency by which it is produced is therefore an area of
significant development in motorsport.
For fluids to exert a force on a wing a pressure difference
between the two surfaces must be established. Wings commonly
utilised in motorsport applications reduce the fluid pressure
about the lower surface and increase the pressure about the upper
surface. This results in a net force proportional to the pressure
difference between the upper and lower surfaces over the wings plan
area which is reacted at the tyres contact patch.
If we refer to aerodynamic texts we will also f ind that drag
force is generated through both form and induced components, the
magnitude of which can be significant for finite wings.
The use of aerodynamic devices has
expense of drag. The team took the decision to design devices,
wi th the author taking on the front wing project.
Upon taking on the front wing design a number of questions
quickly cropped up, the most predominant of which were: where do
you start when trying to design a front wing; what wing sections do
you use; and how many wing elements should be employed?
To answer these questions a simple model assuming two
dimensional f low was produced to predict the downforce of a mult i
tude of different wing sections, elements and orientations.
DOWNFORCE FORCE MODELLING
Initially the wing section of the flap and vane elements was
selected based upon its steady stall characteristics. The lift
coefficient of the profile at a suitable Reynolds numbers given the
maximum permissible chord length and freestream velocity was
obtained from a low speed aerofoil database (Selig, 1995). Assuming
attached flow, the downforce generated by the flap and vane
elements at varying angles of attack can be modelled utilising
Equation 1 and Equation 2.
CO
< Z < CO
Z
o
U J
< GO
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Nathan Barrett takes time out of his studies at Oxford Brookes
University to analyse the effectiveness of a front wing on a
Formula Student/SAE car
NOMENCLATURE
5 h L P
P
Re
V
Plan Area ( r n z ) Angle of Attack (radians) Lift Coefficient
Lift Coefficient when a= 0-Boundary Layer Thickness Ground
Clearance ( m ) Lift Force (N) Fluid Pressure (Pa)
M Fluid Density
1 (m3)
: Reynolds Number
: Fluid Velocity ( )
Chord Length ( m )
become an area of divided opinion between the teams competing at
Formula Student competitions. With average vehide speeds
approaching 30mph it would seem easy to condemn the use of
aerodynamic devices due to the reduced loads a n c Reynolds number
achievable. In recent years, though, a number of teams have been
very outspoken in the performance gains they have achieved through
f i tment of wings and underbodies to their cars. It was therefore
felt that clarity on the matter was required prior to the design
cycle of the 2013 Oxford Brookes Racing Formula Stuoe-* car.
Last year an evaluate- : - e - : t parameters including a : : -2
: : : e s was compiled by fellow 0*-:.-t: 5-oo