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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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

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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.

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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

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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