Project Number: P14221

Transcription

1 Multidisciplinary Senior Design Conference Kate Gleason College of Engineering Rochester Institute of Technology Rochester, New York Project Number: P14221 FSAE AERODYNAMIC DEVELOPMENT Shelby Acome Industrial Engineering Max Affleck Megan Kagenski Tim Moran Travis Newberry 1 ABSTRACT The FSAE Aerodynamic development project focuses on the implementation of a drag reduction system (DRS) on the aerodynamic package of RIT s 2014 Formula SAE competition vehicle. Aerodynamics allow the racecar to achieve maximum grip without extra mass, in return the racecar can maneuver around corners faster while still maintaining a light and fuel efficient car. Using simulation tools such as RIT s custom Lap-Time Simulator, preliminary analysis was done to analyze potential gain from DRS. Gains in endurance, autocross and fuel efficiency scores were significant compared to that of a car without DRS. As a baseline, the previous years racecar was retrofitted with a prototype DRS system on both the front and rear secondary elements, confirming the results of the Lap-Time Simulator. All elements of the DRS were packaged into the vehicle to meet Formula SAE rules and regulations and be aerodynamically efficient. 3D CAD, FEA and CFD software were used to design and develop the various unique components to withstand the extreme vibrations and forces the racecar endures. These components were then manufactured and installed on the Formula teams 2014 racecar for testing. 2 INTRODUCTION The Formula SAE collegiate design series is a competition where schools from across the world design, build and race a small open-wheeled racecar. The rules for these competitions are written to allow freedom of design while maintaining an emphasis on safety, performance, reliability, cost, and fuel efficiency. The competition includes both dynamic and static events that challenge both the racecar and the students who designed the car. Each event has its own scoring system and weight. Over the last several years a larger emphasis has been put on fuel economy, increasing it to 10% of the total competition score. The endurance event now not only depends on the total time but the total fuel used for the race. This change in scoring has lead teams to focus on improving performance while maintaining efficiency. The RIT Formula SAE team over the last 3 years has designed and developed a full aerodynamic package for their racecar. This system has allowed the racecar to become lighter while maintaining performance on the racetrack. A downfall of this system is increased drag during straightaway accelerations. The Formula team has expressed a need for a drag reduction system on their 2014 racecar, to decrease lap times and increase fuel efficiency. Currently, the use of drag reduction systems can most commonly be seen on Formula One cars. Introduced into the Formula One racing series in 2011, DRS has aided in allowing cars that are already very evenly matched in performance capabilities to overtake each other [1]. The series does, however, have a number of restrictions that influence when and where along a course the system can be activated. Drivers can only actuate the system in designated DRS zones, and only within those zones if they are within one second of the car in front of them. While

2 the Formula SAE design series permits the use of a drag reduction system, there are currently no limitations affecting when and where along a course it can be actuated. It is up to individual teams to make those decisions. Prior to any in-depth design of the DRS system, the team designed the aerodynamic package for which the DRS system would be incorporated into. CFD analysis was used to determine the variation of loads that the system would see under different driving conditions. This includes downforce and drag loads, as well as center of pressure locations and pressure distributions. Each of those parameters is used to analyze the individual DRS components. After comparing analyses with the elements at angles of attack for full downforce to zero angle of attack for minimum drag, it was determined that the DRS system could reduce the drag of the aerodynamics package by up to 50% at maximum course speeds. Figures 1 and 2 below are CFD simulations to demonstrate the effectiveness of a drag reduction system. Figure 1 shows the flow paths around the wing elements when they are angled for maximum downforce, while Figure 2 shows how the flow is disrupted less through open airfoils for minimum drag. Figure 1: Inactive DRS Figure 2: Active DRS The team has developed a lap time simulator that incorporates many vehicle parameters, all of which can be adjusted to determine different combinations of setups and determine what will yield the greatest scoring potential at competition. This tool was used to justify the need for implication of DRS on the 2014 racecar. In addition to the lap time simulation, the 2013 racecar was retrofitted with a prototype DRS system of the front and rear secondary wing elements. Dynamic testing data from the 2013 racecar also proved that lap time would decrease with DRS in place. The goals of the RIT Formula SAE team needed to be considered while designing the drag reduction system. These lead to critical design objectives for the project. The design objectives included keeping the weight of the entire system below 3 pounds, increasing the maximum power output, and keeping the design within Formula SAE regulations [2]. The RIT Formula SAE team also wanted the DRS to operate and actuate automatically without any driver input. The system also needed to draw as little current from the system as possible. All of these design criteria were taken into account when selecting a concept and designing the system. 3.1 DESIGN PROCESS When developing the drag reduction system, several design variations were considered. Initial brainstorming and research from what other FSAE teams had previously done showed a variety of systems, each with their own benefits and drawbacks. Some teams incorporated a DRS system into an aerodynamics package that had multiple secondary airfoils in the design, similar to the RIT Formula SAE team s current design. Most FSAE teams used a pneumatic system as a means for actuation [3]. One team in particular utilized a continuously actuated and optimized system with split wings. While the actuation of the system for this design is unknown, the team demonstrated the ability to actuate the left or right half of each set of airfoils independently, both front and rear. They also demonstrated the ability to automatically and continuously adjust the angle of attack for these elements. A system of this style was not desirable for this project. It becomes very difficult to study the aerodynamic forces at partial angles of attack during certain maneuvers. An active DRS system of this nature may make the car unbalanced during a turn, or be unpredictable for a driver who is controlling the car at the traction limit. Additionally, actuating only one side of the car adds complexity to a DRS system with minimal benefit. Actuation of one side of a split-system DRS would cause a vehicle to have less downforce on one side of the vehicle, plus a yawing moment about the vehicle s center of gravity. For the purposes of a DRS system, it was determined to be best designed as binary. The system is either on or off, meaning the secondary elements are at zero degrees angle of attack for minimum drag, or at full angle of attack for maximum downforce. Any partial actuation was determined to possibly upset the balance of the car when performing a maneuver, and make the vehicle less drivable. Since the vehicle already contained an electrical system for the ECU and a pneumatic system for shifting, actuation could be done with electric motors, electric linear actuators, or pneumatic actuators. Upon discovery that the electric windings from the electric motors and electric linear actuators would make the system over-weight and that it would heavily draw electrical current to maintain steady state conditions if using stepper-type motors, the decision to utilize pneumatic linear actuators was partially justified. Further testing would verify that it is a viable solution. After several iterations of concept generation for actuation geometry, the design shown in Figures 3 and 4 was decided upon. The benefit of this design is that there is only one pneumatic actuator for the set of elements,

3 Proceedings of the Multi-Disciplinary Senior Design Conference Page 3 including both the left and right halves. When the elements are actuated up, there is still a large enough slot gap between all of the elements to promote good airflow and minimize drag. The elements are rotated near their center of pressure to reduce any moments caused by aerodynamic forces on the elements. Figure 3: Inactive DRS Figure 4: Active DRS 3.2 MECHANICAL DESIGN The mechanical design for the DRS can be broken down into two components: mechanical systems and pneumatic actuation Mechanical Systems For this particular DRS system, the actuation had to be separated into a front system and a rear system, which can be seen in Figures 5 and 6. The front set of airfoil elements is similar to the rear in that there are 3 elements (2 secondary elements being actuated). Geometries needed to be modified slightly to accommodate airfoil dimensions, but the overall design between the two systems is very similar. Figure 5: Rear DRS Model Figure 6: Front DRS Model The rear system incorporated a vertical mid-plate between a split set of airfoils at the rear of the car. Within the mid plate housed the mechanical system shown in Figure 3 and Figure 4. Bellcranks attach to the wing elements with bolt hardware, and threaded mounts within the structure of the wing allow for a solid connection between the actuation system and the wing elements. While the rear elements are split, they are actuated together to avoid complexity, as discussed in the Design Process above. Special attention was focused on the location of the rotation point along the chord length of the secondary elements. Important considerations were made to pivot the elements close to their center of pressures to allow for the fastest possible actuation time between states. The center of pressure does not change significantly between the maximum angle of attack and the zero angle of attack of the wings with the DRS system activated. The center of pressure is located at approximately ¼ of the chord length from the leading edge. If actuation was to pivot about a point in front of the center of pressure, then the natural tendency of the system would be for the airfoils to produce a force that would rotate the elements into the low-drag state, which is undesirable when maximum downforce is required. To ensure that during a failure of the DRS system the wing elements would be in the maximum-downforce state, the pivot point is deliberately placed slightly behind the center of pressure for each element. This is also how the wings will return to their full-downforce positions when DRS is inactive. While the elements are actuated to their zero angle of attack position for low drag, they are not actuated back down when the system needs to switch off. Instead, as soon as the CO 2 pressure on the system is released, the inherent aerodynamic force on each element will be greater in front of the rotation point, causing them to rotate back to their maximum-downforce positions.

4 Figure 7: Rear Bellcrank Deflection Figure 8: Rear Bellcrank Stresses The bellcranks are to be made out of 0.125in 6061-T6 sheet aluminum, and will be cut to shape on a water jet. The FEA analysis was conducted with a 0.030inch mesh. The applied loads correspond to the maximum aerodynamic forces each wing element would see during driving conditions at maximum course speeds. The maximum defection under load on the bellcranks is minimal at just under 0.003inches with a maximum stress of 9500psi. This stress is well below the factor of safety that all components on the car are designed to. Figure 9: Rear Pin Deflection Figure 10: Rear Pin Stresses The rear DRS mechanical system lies in the center of the rear wing of the car. In order for the elements to respond together from central actuation, they must be tied together with a pin that spans through the mid-plate to both the left and right elements. This pin is made out of 7075-T6 aluminum and sees a cantilever load. Similar to the bellcrank analyses, the loads on each pin corresponded to the maximum aerodynamic forces that will occur while on course. Using a 0.030inch mesh, the analysis yielded a maximum stress of 38.8ksi and a maximum deflection of inches. Similar to the bellcranks, the stress is low enough that the factor of safety on this component is greater than the minimum FOS that the team designs to. Lastly, the pushrods that connect the two sets of bellcranks for the secondary and tertiary elements, both front and rear, were analyzed to ensure that they would not buckle under the loads they would see when the actuation force is applied. With their dimensions, given loads, and end conditions, each pushrod has a critical force that yields a buckling factor of safety greater than three. 3.3 ELECTRONIC DESIGN Control System The main hub for the controls of the DRS system is an Atmega32U4 microcontroller. With given inputs, the controller will determine if the car is accelerating in a straight line, and then be able to activate the solenoid valve to control the actuation of the airfoils. The microcontroller us programmed using an Arduino, which is based on C and C++ programming languages. The three inputs that the system will require are lateral acceleration, throttle position, and steering position. Each signal is read by 10 bit analog to digital converters on the Arduino and, based on the voltage input, is mapped to a number between 0 and The DRS output signal defaults to low, ensuring that if the system loses power the wings will default to their closed, maximum-downforce position. In order for the wings to open, the throttle needs to be at least 80% pressed, the steering wheel needs to be turned less than ten degrees, and the lateral acceleration needs to be less than 0.5G. These are all required to determine if the car is in a condition where less drag is desired, and less downforce is acceptable. This will occur as the car is exiting a corner and going through a straightaway, but before any braking occurs to enter the next corner. When exiting a corner, the lateral acceleration of the car is at a minimum, and throttle position increases towards its wide-open position. The steering angle will also aid in determining if the DRS system needs to be activated. It is important that a delay is also incorporated into the system. During a slalom, for instance, the car will momentarily have a zero steering angle, but it is critical that maximum downforce is being achieved. A delay will ensure that grip is not lost in the middle of a slalom, since the steering position and lateral acceleration will quickly increase. One of the main design objectives for this system is that it be driver-independent. While the system will have an override button to activate the system (mainly for demonstrative purposes), it will not require driver input.

5 Proceedings of the Multi-Disciplinary Senior Design Conference Page PNEUMATIC SYSTEM Pneumatic systems needed to be carefully designed to optimize response time of the DRS while using the least amount of CO 2 possible. Two sizes of clear pneumatic line were tested in order to estimate response time and CO 2 consumption per actuation. The test setup was run using a sample pneumatic cylinder, solenoid valve, timing lights, a CO 2 bottle, and regulator. A mass was attached to the active end of the cylinder to simulate the inertia of the mechanical DRS. Through experimentation it was shown that the ¼ inch hose showed a faster response time by about 40%, but utilized nearly 50% more CO 2 than the 1/8 th inch hose. Extrapolation of the amount of CO 2 consumed per actuation showed that there would be approximately 600 DRS actuations using the 1/8 th in pneumatic line and approximately 300 DRS actuations using the ¼ inch line. The decision was made to utilize the 1/8 th inch pneumatic line initially for testing, due to its lower CO 2 consumption rate Pneumatic Actuation The pneumatic cylinders used for actuation are Bimba brand pneumatic cylinders. Bore size is important for performance of the DRS. With a larger bore size, there is a larger holding power, and force applied to the system, but this comes at the expense of response time and CO 2 usage. Through bench testing, and weighing the benefits and tradeoffs of bore and stroke size, it was determined to use two cylinders that are 7/16inch bore and 3.4inch stroke in the front of the car, one on each side. In the rear there will only be one cylinder with at 7/8inch bore and a 3.5inch stroke. Since the front of the car would require significant mechanical complexity to actuate both sets of airfoils with one cylinder, it was determined that it would save weight and be simpler to run two cylinders for actuation in the front. The rear only utilizes one cylinder, however it needs to be slightly larger due to increased loads on the rear system Fittings and Hose Fittings and hose are directly threaded into the pneumatic components and are standard push-to-connect sizes. As mentioned previously, the outside diameter tubing sizes that have been considered for this project are 1/8 th inch and ¼ inch clear tubing. The pressure direction on the cylinder is controlled by a MAC 35A-AAA-DDBA-1BA solenoid valve. The valve, which draws 500mA, will not tax the electrical system significantly. This solenoidoperated, two-way valve provides pressure to either one side of the pneumatic cylinder or the other. By operating in this fashion, the system is either driven up by force, or down by force, with constant pressure applied to one side of the cylinder. A stiffer system is created by doing this and it is ensured that the elements remain fixed in their state. Due to potential CO 2 usage, it was decided to only actuate the airfoils in one direction. As discussed in the Mechanical Design above, the location for the rotation points will allow for the elements to return to their maximum-downforce positions without needing to be actuated Bottle The CO 2 reservoir being used is a 9oz bottle (originally intended for recreational paintball use). If necessary a 20oz bottle may be used, however, the smaller the bottle used, the more weight savings gained. This bottle will be independent of the shifting pneumatic circuit. Having two separate, independent systems ensures that if one system fails or runs out of pressure, the other system can stay online Regulator A Palmer PPSP010LP Regulator threads directly to the CO 2 bottle and steps the pressure down to 150psi. 4 RESULTS AND DISCUSSION The weight of the DRS system totals three pounds, meeting the target value specified in the engineering requirements. The overall reaction time for the system to actuate and move each of the elements from their maximum-downforce to minimum-drag position was found to be 0.1seconds, which is between the marginal and target values outlined in the engineering requirements. All DRS components comply with FSAE rules. Testing of the system proved that drag was minimized during on-course driving. Although there was a reduction in downforce when the system was activated, it was not needed for straight-line driving. There was a significant reduction in drag allowing the car to have faster lap times, but also a reduction in fuel consumption. Those two factors combined will ultimately result in scoring more competition points. Not only does data support these findings, but the driver can physically feel a difference when the system is activated during driving. 5 CONCLUSIONS AND RECOMMENDATIONS It was determined early in the design process that although the rear wing elements were split into left and right halves, they would be actuated together. Not only would this cut down on the overall weight of the system, but it also prevents added complexity, most of which is not understood. In the future, the RIT FSAE team could benefit from extensive analysis of the reactions of the system due to independent actuation of the rear elements. Actuating one side or the other will cause the aerodynamic balance of the car to shift even further from the car s centerline, especially while cornering when the balance is already shifted.

6 The design of this binary system resulted in the airfoils being in only one of two positions, either maximumdownforce or minimum-drag. While driving through a corner, maximum downforce is desirable. Going down a straightaway, minimum drag is desirable. However, there is an in-between stage where a different combination of downforce and drag might be the most desirable. Analysis of intermediate or continuously adaptable angles of attack could benefit the handling and overall lap times for the car. With the added complexity of a system like this, thorough analysis would be to the team s advantage. 6 REFERENCES [1] Overtaking and the DRS. Formula 1. Web. 08 May < _the_sport/5293.html>. [2] SAE International, Formula SAE Rules. [3] Monash Motorsport. Web. 08 May < >. 7 ACKNOWLEDGMENTS Team P14221 would like to acknowledge a handful of people for their support of this project: Doctor Alan Nye for his guidance for the entirety of the project. RIT Formula Racing for their support and sponsorship. Dave Hathaway, Rob Kraynik, and Jan Maneti in the Machine Shop for allowing us access to the facilities. Mark Smith and the Senior Design staff for their guidance and sponsorship of the project.