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SAE TECHNICALPAPER SERIES 2007-01-0897

Aerodynamics for Formula SAE:On-Track Performance Evaluation

Scott Wordley, Jessie Pettigrew and Jeff SaundersMonash University

Reprinted From: Vehicle Aerodynamics 2007(SP-2066)

2007 World CongressDetroit, MichiganApril 16-19, 2007

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ABSTRACT

The measured on-track performance of a Formula SAE car with a high downforce aerodynamics package is presented. Data logged from variety of different driving tests is used to determine how the addition of ‘wings’ affects the car’s acceleration, cornering, braking and slaloming abilities. These results are then compared with analytical predictions for the same car, presented in earlier papers [1,2].

INTRODUCTION

Race cars commonly make use of aerodynamic devices to generate increased normal load on the tires, which can improve the car’s acceleration (in all directions) in segments of the track where it is grip-limited. The ‘downforce’ produced by wings and other aerodynamic devices is usually accompanied by an increase in drag, but with careful design the net result is often a faster car and reduced lap times.

It is well known that aerodynamic forces increase as a function of velocity squared, so the higher the speed the more important aerodynamics become. The specifications for Formula SAE tracks generally limit speeds to below 100 km/h, which makes it difficult (but not impossible) to generate effective levels of downforce. Other negative factors must also be considered when using wings for FSAE, including their additional drag, weight, and their effect on the car’s centre of gravity height and polar moment of inertia.

Because of these conflicting interactions, the only reliable way to determine if wings can improve the performance of a FSAE car is to make back-to-back measurements on-track, using the same car, with and without wings. This is the object of this paper.

This work is the third in a series of papers which document the design, development and validation of a high downforce aerodynamics package for the 2003 Monash University Formula SAE car. The first paper [1] explains FSAE rule considerations for the use of aerodynamic devices and the process used in their initial

specification. In this paper, an aerodynamically balanced wing package was designed to produce maximum downforce within the stated acceptable limits of increased drag and reduced top speed. The net effect of these wings on the car’s performance in the FSAE dynamic events was then predicted. The addition of the wing package described showed the potential for significantly improved cornering, braking and slaloming with only slightly diminished straight line acceleration.

The second paper [2] documented the detail design and testing process for this wing package. A range of CFD, wind tunnel and on-track testing was presented to quantify and validate the aerodynamic performance of these wings.

This paper will examine how the addition of this wing package to the Monash Formula SAE car affects its on-track performance in the four dynamic events; Acceleration, Skid Pan, Autocross and Endurance.

VEHICLE PARAMETERS

The 2003 Monash Formula SAE vehicle was utilised for the following tests (see Figure 1). The full specifications of this car including its engine power, gearing, weight, CG height, polar moment of inertia and aerodynamic coefficients, both with and without wings, can be found in [1]. These specifications describe the race ready condition of the car for 2003. Since that time the car has been modified to increase its durability and robustness for long term testing. The major changes include a slightly detuned engine map and improved cooling system to extend engine life, and stronger one-piece cast wheels to enable machine changing of tires.

These changes, along with the addition of sensors, wiring, break-out box and a dedicated data logging power supply have resulted in reduced engine power, and increased weight, CG height and polar moment of inertia compared to the values quoted in [1]. These changes have not been quantified, but the increased weight is estimated at 50 kg.

2007-01-0897

Aerodynamics for Formula SAE: On-Track Performance Evaluation

Scott Wordley, Jessie Pettigrew and Jeff Saunders Monash University

Copyright © 2007 SAE International

Figure 1: 2003 Monash FSAE car, with and without wings.

DATA AQUISTITION

A MoTec Advanced Dash Logger (ADL) was used to record a range of data from the car during on-track testing. The data logger used a dedicated power supply to eliminate alternator charging noise from the reference signal. The various sensors used for these tests are detailed below.

WHEEL SPEED

Wheel speed was measured at the left front wheel only using a 0.9mm thick laser cut steel trigger wheel with 15 teeth attached at the hub. A Honeywell Hall-effect sensor was then attached to the upright and adjusted to give the desired 1.0mm gap between the trigger wheel and sensor face. Logged at 200 Hz, this set-up gave a resolution of 1% at a velocity of 36 km/h which was considered acceptable. Due to the fact that only one wheel speed sensor was used, right hand turns (where the left hand side of the car tracks a larger turn radius) read slightly higher speeds than left hand turns.

ACCELEROMETER

Vehicle acceleration was measured using a Crossbow LP 3-axis accelerometer which was mounted near the front axle line of the car and logged at 100 Hz. The polar alignment of this sensor was set by performing straight line runs with the car and adjusting the alignment until the recorded lateral accelerations averaged zero. The non-linearity of each axis as quoted by the manufacturer was ± 0.2%.

STEERING ANGLE

Steering angle was recorded at 100 Hz using a Longfellow II LPT linear potentiometer which measured

steering rack displacement. The accuracy of this device is quoted as ±1%.

TEST PROCEDURE

The different on-track tests were designed with two aims in mind. Firstly, the tests had to accurately replicate the four different driving events (Acceleration, Skid Pan, Autocross and Endurance) defined in the FSAE competition rules [3]. This allowed the overall change in the car’s performance to be gauged. In addition to this, an understanding of how the wings affect acceleration, braking, steady state cornering and transient response was also desirable. For this reason a braking zone was added to the end of the acceleration run, and a series slaloms were tested (within the size range allowed by FSAE rules) to gauge differences in transient response.

The four different tests are described in more detail below.

ACCELERATION AND BRAKING TEST

The acceleration and braking test involved the car accelerating as fast as possible from a standing start for a set distance (110m) at which point the driver braked as hard as possible (see Figure 2 below). The acceleration distance of 110m was chosen as it corresponded to a point at which the engine reached its maximum RPM in 4th gear. Using data from these tests it was possible to extract times for the Acceleration Event (0-75m) and also to measure braking distances and decelerations.

Figure 2: Explanation of acceleration and braking test

SKID PAN TEST

The skid pan test (defined in FSAE rules [3]) is a figure-8 track with two circles, each 15.25 m in diameter. The car enters the track at the centre of the ‘8’ and completes 2 consecutive laps on the right circle before crossing over and completing 2 consecutive laps of the left circle and exiting out the centre of the ‘8’ (see Figure 3 below). In competition, the second lap on each side is timed as a test of the vehicle’s maximum steady state cornering speed.

Figure 3: Explanation of skid pan test

SLALOM TESTS

A series of slaloms were laid out in a straight line using cones. The driver was given space to accelerate to a comfortable speed before beginning the slaloms. Data from the first two slaloms was disregarded to allow the car and driver to find their maximum speed and establish a smooth rhythm. Three different slalom spacings were separately tested, 8m, 12m and 16m as shown in Figure 4 below.

Figure 4: Explanation of slalom test

AUTOCROSS/ENDURANCE TRACK TEST

A course design, typical of those seen in the Autocross and Endurance events was also tested. A detailed track map of this course can be found in the appendix. The course includes two straights, eight corners and three slalom sections.

TESTING VENUES

The acceleration, braking, skid pan and slalom tests were conducted on the infield of the Calder Park Thunderdome Raceway in Melbourne, Australia. The autocross/endurance track tests were conducted at Gippsland Park, in Morwell, Australia. The Morwell circuit was quite hilly (unlike the traditionally flat competition courses), and the map provided has been annotated to show this. The track surface at both locations was reasonably old and worn asphalt with rock aggregate.

For each test, back-to-back runs where conducted on the vehicle, with and without wings. Each test was repeated a minimum of 6 times to allow the tires to reach a stable operating temperature and to allow for driver errors or inconsistencies.

Tire compound, driver and vehicle set-up where held constant throughout all tests, to ensure that changes in performance where affected by the addition of wings only.

RESULTS

ACCELERATION AND BRAKING RESULTS

Figure 6 plots vehicle speed versus distance for the acceleration and braking test. Three different aerodynamic configurations were tested; no wings; wings in low drag configuration and wings in standard (high downforce) configuration. Due to the close similarity in the ‘no wings’ and ‘low drag wings’ results, a detail view of this graph is provided in Figure 7.

For the low drag configuration, the angle of the three flaps is adjusted to decouple their interaction with the main plane (see Figure 5). These angles were determined from wind tunnel tests which aimed to minimize the drag of the rear wing, which usually constitutes around 40% of the car’s overall drag. As the front wing only contributes 10% of the overall drag, it was not changed for the low drag configuration.

Figure 5: Rear wing in standard configuration (left), rear wing in low drag configuration (right).

Figure 6: Acceleration and braking, vehicle speed versus distance.

These results show that when the wings are set in their low drag configuration, the car can accelerate at the same rate as the car without wings. The difference in performance is barely measurable. Whilst wings can not be completely removed nor re-added to a car for different events, adjustments (such as changes in angle of attack) are allowed. This means that the low drag rear wing setting can be used in the Acceleration Event for negligible losses compared to the bare car.

In their standard, high downforce configuration, the addition of wings to the car results in a slightly slower rate of acceleration. Comparing the times taken to cover 75m from a stationary start (Table 1 below), the standard configuration winged car is 0.23 seconds, or 4.4% slower. This difference can be used as an indication of how wings affect the car’s acceleration in the autocross and endurance events where such a high downforce wing setting would be used.

Table 1: Acceleration times.

Figure 7: Previous graph, detail view.

The comparison of braking performance again shows little difference between the bare car and the low drag wing configuration, with the graphs very closely overlaying each other. Conversely, the car with wings in standard configuration has significantly improved braking performance. Figure 5 shows that this gain occurs in the initial stage of braking where the aerodynamic drag is highest. This additional drag allows the car to reach its maximum deceleration rate as soon as the brakes are applied.

Due to the fact that braking performance is important in the endurance and autocross events where speeds range between 40 km/h and 80 km/h, the most appropriate metric is the average deceleration rate between these speeds. From this data, braking distances can also be calculated. These results are given in Table 2 below.

Table 2: Braking decelerations and distances.

The wing car in standard configuration is seen to generate an average of 0.33 g more braking deceleration in this speed range, allowing it to brake approximately 8m later than the non-winged car.

SKID PAN RESULTS

Figure 8 below plots front left wheel speed versus distance for the skid pan test, while Figure 9 plots measured lateral accelerations versus distance. The car was tested with wings (in the standard configuration) and without wings.

Figure 8: Skid pan front left wheel speed versus distance.

As expected, these results show that the FL wheel speed for the right hand loops (0-100m) reads faster than the left hand loops (100-200m) due to the difference in the turn radius that the left wheel tracks. It will be assumed that the car is actually maintaining the same speed for each side, and this speed can be found by averaging the results of both sides (see Table 3).

Figure 9: Skid pan lateral acceleration versus distance.

Table 3: Skid pan average speeds, lateral accelerations and times.

The winged car gave an average skid pan speed of 32.5 km/h, versus 29.2 km/h for the same car without wings; a 10.2% improvement. This gain was also reflected in the measured lateral accelerations, with the car averaging 1.09 g and 0.82 g, with and without wings respectively, a gain of 0.27 g due to the wings.

Using the assumed driven skid pad radius of 8.5m (as in the predictions made in [1]) with the measured average vehicle speeds equates to average times of 5.92 seconds (wings) and 6.28 seconds (no wings) for a single lap. Timing lights are required to make more accurate recordings of the actual lap times, but were not available for this test.

SLALOM RESULTS

The left front wheel speeds and lateral accelerations for slalom spacings of 8, 12 and 16m are plotted in Figures 10 to 15. As in the skid pan results, the maximum peaks in the speed graphs indicate that the car is passing the left hand side of the slalom cone, resulting in a higher left front wheel speed.

Figure 10: 8m slaloms, front left wheel speed versus distance.

Figure 11: 8m slaloms, lateral acceleration versus distance.

Figure 12: 12m slaloms, front left wheel speed versus distance.

Figure 13: 12m slaloms, lateral acceleration versus distance.

Figure 14: 16m slaloms, front left wheel speed versus distance.

Figure 15: 16m slaloms, lateral acceleration versus distance.

In all cases, the average values of front left wheel speed are higher for the winged car, demonstrating that it is capable of traversing the slaloms faster.

A similar difference is evident in the lateral acceleration plots, but it is less visually pronounced due to the necessary scale of the graphs.

A clearer means of comparison is to take the average front left wheel speeds and plot them versus slalom spacing as shown in Figure 16.

Figure 16: Average vehicle speed versus slalom spacing.

Figure 16 shows that average speed increases approximately linearly with increasing slalom spacing.

The winged car is always faster than the bare car, by an average of 9% for the three different slalom spacings tested.

AUTOCROSS / ENDURANCE TRACK RESULTS

The Gippsland Park circuit was used for autocross/endurance testing. Cones were used on track to add slalom sections to some of the more open straights. The resulting course was logged at 750 meters in length, and was considered typical of the type of track seen in the Australasian FSAE competition (see Figure 17 below). A larger, annotated track map can be found in the appendix.

Figure 17: Track-map coloured by wheel speed, car without wings.

Logged data comparing the performance of the car with and without wings for one lap of this course is provided in the appendix. These results plot the time variance, vehicle speed, lateral acceleration and throttle position versus distance traveled. The two data sets (wings and no wings) are overlaid to make comparison easier.

The start-finish line (0m) is located near the braking point for the first series of 9m slaloms. The speed trace shows that the winged car is able to maintain a higher speed through these slaloms and reach wide open throttle sooner resulting in a higher exit speed into the first straight (30m). Once on the straight, the non-winged car is able to accelerate faster than the winged car and at the braking point (85m) both cars are traveling at the same speed. The variance graph shows that the winged car has already gained 0.5 of a second on the non-winged car by this point.

The winged car brakes slightly harder into the first wide left hand turn and is able to generate an average of 0.2g more lateral acceleration and hold a higher speed through this series of three open corners. At the 300 meter mark the winged car is 1.3 seconds ahead.

Performance through the following 9 meter wide slaloms is similar for both cars, while the next 12 meter wide slaloms see the wing car gain a little time only to lose it again entering the long left hand corner beginning at 400m. Once it reaches its maximum steady state corning potential the winged car is again able to maintain a higher lateral acceleration and feed in throttle much earlier on corner exit. Through the open, sweeping section of track which follows, the winged car is able to maintain speeds which are between 10 and 20 km/h faster than the bare car, which increases its lead to 3.5 seconds by the 625m mark.

In the final series of tight low speed corners the performance of both cars is very similar, and by the end of the lap the winged car is a total of 3.3 seconds ahead.

DISCUSSION

The experimental data will now be compared with the initial performance predictions made in the earlier paper.

ACCELERATION COMPARISON

Figure 18 shows a comparison between the predicted acceleration results [1], and the results measured through on-track testing. The considerable difference between the theoretical predictions and the experimental data can be attributed to a number of factors. It is believed that the tire coefficient of friction used in the predictions (1.6) was unrealistically high for this event, given that the tires are generally cold at the start of the test. Also, the predictions do not account for the loss of acceleration due to shifting times (~0.25 sec per shift for 3 shifts), which are quite pronounced in the experimental data. The test vehicle is also heavier and producing less engine power than is assumed by the initial model.

Despite these discrepancies, both sets of results show a very similar loss in speed due to the use of wings, when viewed as a function of vehicle speed (rather than distance). Further testing, model development and correlation is needed to improve the predictions to a point where they can accurately simulate this event.

Figure 18: Comparison of acceleration predictions and measurements.

SKID PAN COMPARISON

Predicted skid pan speeds from [1] are compared with the measured skid pan speeds below in Table 4. Once again, the absolute values of the predicted speeds are significantly higher than those measured (~20%). Again, this difference is attributed to the theoretical coefficient of friction used in the simulation and the increased weight of the test vehicle.

The relative differences seen for the wings-on versus wings-off cases are more similar, with theory predicting a 5% increase in speed compared to the 11% increase measured on-track.

Table 4: Comparison of skid pan predictions and measurements.

SLALOM COMPARISON

In the first paper [1], the improvement in relative slalom performance due to the addition of wings was gauged by comparing both the cornering potential and the yaw acceleration potential of the car, with and without wings, as these are the main factors that influence slalom performance. This analysis predicted that the winged car should always be faster than the non-winged car, but without a complicated dynamic simulation, it was not possible estimate the magnitude of the expected improvement.

The experimental results presented in Figure 16 show that the winged car is in fact always faster than the non-winged car, by an average of 9%.

If we examine Figures 19 and 20 (reproduced from [1]) which plot relative cornering and yaw acceleration potential versus speed, and consider the range of slalom speeds measured on-track (30 km/h – 60 km/h) we find that the average predicted margin of improvement is quite similar at 10%.

Figure 19: Predicted relative cornering potential versus velocity, with and without wings.

Figure 20: Predicted relative yaw acceleration potential versus velocity, with and without wings.

AUTOCROSS / ENDURANCE COMPARISON

The track testing results given in the appendix showed an overall improvement of 3.3 seconds per lap (for a 52 second lap) or a 6.3% reduction in lap times due to the addition of wings. Over the course of a 22 km endurance event such an improvement would result in the winged car lapping the non-winged car not once, but twice.

Analysis of the data showed that the winged car gained most of its advantage from the higher corner exit speeds it was able to generate and carry onto the following straights. It was also able to brake harder and later than the non-winged car. Although it was not able to accelerate quite as fast in a straight line, the winged car still achieved the same top speed by virtue of its higher corner exit speeds.

These experimental observations compare reasonably well with the qualitative predictions made in [1].

SUMMARY OF RESULTS

In summary, it was found that the addition of the wings to the 2003 Monash FSAE vehicle resulted in the following performance changes:

Table 5: Summary of results.

It should also be remembered that the test vehicle is around 50 kg heavier than its race weight due to the addition of the data logging system and more robust components designed for long term durability.

The improvement in performance due to the addition of wings will be a function of the ratio of aerodynamic downforce to the total static weight of the car and driver. If the downforce is kept the same and the total weight of the car and driver is reduced from its current 350 kg to the race weight of 300 kg the performance improvement due to the addition of wings will be increased. For this reason the summary results tabled above should be considered conservative.

CONCLUSIONS

Data measured from on-track testing has shown that wings can be used to significantly improve the performance of a Formula SAE car. These gains are most evident in braking, large radius corners and corner exit speeds, with the trade off being a small reduction in straight line acceleration.

ACKNOWLEDGMENTS

The authors would like to thank Scott Younnes; Philip Juric, Bob Wright; Roan Lyddy-Meaney; Jarrod Hammond; Nick Trevorrow; Shaun Johnston; Annika Harvey; Borzou Shahsavand; Ryan Gordon; Rob Harbig; Andrew Brandt; and Jayce Moore for their continued support and many years of hard work on this project.

REFERENCES

1. Wordley, S.J., and Saunders, J.W., Aerodynamics for Formula SAE: Initial Design and Performance Prediction, SAE Paper 2006-01-0806, 2006.

2. Wordley, S.J., and Saunders, J.W., Aerodynamics for Formula SAE: A CFD, Wind Tunnel and On-Track Study, SAE Paper 2006-01-0808, 2006.

3. SAE, 2006 Formula SAE Rules, US Comp Edition, Society of Automotive Engineers, USA, 2004.

4. Case, D., Formula SAE: Competition History 1981-2004, Society of Automotive Engineers, USA, 2005.

5. McBeath, S., Competition Car Downforce, Haynes Publishers, Somerset, 1998

6. Katz, J., Race Car Aerodynamics, Bentley Publishers, USA, 1995.

7. Hucho, W., The aerodynamics of road vehicles, Butterworths Publishers, London, 1965.

8. Coiro, D.P., et al, Experiments and Numerical Investigation on a Multi-Component Airfoil Employed in a Racing Car Wing, SAE paper 970411, Topics in Vehicle Aerodynamics, pp. 221-231, 1997.

9. McKay, N.J. and Gopalarathnam, A., The Effects of Wing Aerodynamics on Race Vehicle Performance, SAE Paper 2002-01-3294, 2002.

10. Milliken, W.F., and Milliken, D.L., Race Car Vehicle Dynamics, SAE International, 1995.

CONTACT

Scott Wordley: scott.wordley@eng.monash.edu.au

Jessie Pettigrew: jpet7@student.monash.edu.au

Website: http://users.monash.edu.au/%7Efsae/

APPENDIX

LOGGED AUTOCROSS / ENDURANCE DATA:

Black traces: no wings; Grey traces: with wings