Formula Body Aerodynamics
The UNR Formula SAE Club’s aim is to produce aerodynamic components to improve the performance of the University of Nevada, Reno formula SAE car. Formula SAE is a competition among students in which cars are designed, built, and raced. Currently the club’s formula car has a wrapped composite designed to improve racing performance, however, improvements can be made. The main points of aerodynamic design in a racecar are to reduce drag as well as provide downward force. Drag is the opposing aerodynamic force exerted on the car as it moves through the air, slowing it down. Downward force is used to counter lift which can be created when air passes above and below the car. Too much lift and the tires lose traction; traction that is essential when going around a turn at high speeds. To help improve performance we are creating a streamline design to help reduce drag as well as adding spoilers to create a downforce. These modifications will help the UNR Formula SAE Club’s racer perform better.
Fig. 1: Proof of Concept Design
The latest concept for the aerodynamic body is shown above (a) side view b) front view c) isometric view). It features a nose cone designed to redirect air above the body. The steeper slope on the top accelerates the moving air to ensure that the least amount of flow goes beneath the car, to help reduce lift. The front fairing has the most streamline slope possible given the parameters of the frame. It is also easier to manufacture because it doesn’t have many intricate curves. It has the minimum cross-sectional area while maintaining enough material to be able to handle the shear stresses of the air flowing through it. The top roll bars will be uncovered to minimize the area that the air needs to flow around, making it more streamline. The rear of the body has a rounded end to minimize the formation of wakes as the car moves. This reduces the vacuum pressure produced by vortices.
Proof of Concept
To test the body, the team scaled it down to 0.058 of the original size. A blunt box and a perfect streamline tear drop shape with the same cross-sectional area and surface area was created as the worst and best extreme respectively. To not was material and budget space, the box, streamline, and body was run through a flow simulation in SolidWorks. The airspeed was chosen at 40 mph as it is the average speed of the cars. After the concept was optimized via iterative simulations, it was then 3D printed. All 3 models were sanded, and a light resin was applied to ensure equal smoothness across the surfaces. They were then tested in a wind tunnel and the friction force was measured for the three cases. Shown below are the 3 simulated friction forces and it is evident that the body is closer to the streamline extreme.
Fig. 2: Aerodynamic simulations in SolidWorks of the box, streamline, and design.
Detailed Design Overview
The body will be smooth, easily removable, and cost efficient. Fiberglass was chosen as over other composites such as carbon fiber because it is cheap to use and strong enough according to the research. The resin used for the fiberglass will ideally be the cheapest among the recommended resins that coincide with the fiberglass sheets chosen. A specific brand for small craft fiberglass has been chosen based on cost, specs, and shipping time and price. Fiberglass panels were designed to be removable and durable. The exterior will be split into several panels and attached using DZUS quarter turn fasteners which will be mounted to the frame. This method of attaching the panels was chosen based on function over cost and will have little to no impact on the car’s drag. Based on the previous Formula SAE car, 1/8 inch thick panels are cost effective and durable. If the panels are thinner they tend to crack during transportation of the vehicle. If the panels are any thicker, the costs of the fiberglass sheets and resin increase, and the time to cure increases as well.
Reduction from drag will come from the smoothness of the exterior and the geometry of the body. The geometry of the body is dictated by the frame size and shape but is not entirely controlled by it. An expected drag coefficient of .34 was found by rounding the edges of the body and curving the surfaces as much as possible combined with polishing the surface of the panels when finished. Between the geometry, material, and attachment choices, all design specification were reached as seen in Table 1. The final design is shown in fig. 3.
|Geometrically Predicted Drag||0.38||0.3 – 0.4||Based on similar geometry,a drag coefficient range was found|
|Ease of Removal||24 seconds per panel||30 seconds per panel||Repairs and inspection during the race|
|Weight||9.5lbs||10lbs||SAE Formula Club specified a weight cap|
|Shatter Resistance||Tears||Doesn’t shatter||Safety hazards during race|
Table 1: Engineering Analysis Table
Fig. 3: Design of prototype to be built.
The Nevada Formula body panels will be constructed with fiberglass. The fiberglass that will be used is 70 gauge, the same material used for light watercrafts. To create the profile that the panels will take, plexi-glass sheets were used as male molds of the body. Sheets were cut to size allowing for some tolerance, this proved handy since the panels needed to be trimmed to fit. The plexi-glass was heated then bent to achieve the correct curvature for an aerodynamic profile.
Fig. 4: The mold is being created using plexi-glass sheets.
After the mold was cut and shaped, the fiberglass was cut to the size of the mold. A layer of wax paper was used between the fiberglass and the mold to act as a release agent. Only one sheet was used for each panel. The nose cone proved to be the most complex part. Since it has different curves on it, foam was cut then formed together to achieve the proper shape. The fiberglass had to be lain over a few days since the resin could keep running and give the resin inconsistent thickness. The fiberglass was coated with a specific mixture of resin and hardener. The ratio dictates how rigid the panels will be, with that in mind the panels that was fabricated have both rigidity and flexibility. Three coats of the resin and hardener mixture ensures that the panels achieve a one-sixteenth inch thickness. A mock frame was constructed using PVC pipes and fittings to ensure that the panels are the proper dimensions.
Fig. 5: The resin being laid on the fiberglass sheets and the nose cone being made with smaller strips. Preliminary fitting on the frame.
The panels were then measured against the frame to have a perfect fit. The permanent panels were then mounted with bolts. The removable panels are mounted using quarter-turn fasteners for the inspections. Thee fabrication process provided the team with smooth panels that are rigid enough to take an impact yet flexible enough to take the curve of the frame. A gel coat was used to give the surface a shiny finish. The finished panels were painted navy blue and mounted all together. Tires were also added to perform a roll test.
Fig. 6: The finished panels are painted then mounted onto the frame as well as the tires.
Billy Mann is a senior at UNR majoring in mechanical engineering. He is from Truckee, CA and his hobbies include skiing, hiking, biking, comedy, and acting.
Sean Wells is a senior mechanical engineer after graduation his goal is to design cars. He currently works for a company called Scooterbug doing repairs and maintenance on electric and manual wheelchairs. Sean spends his free time riding dirt bikes and working on cars.
Phumi Hlape was born in Torrence, California. Moved to Reno fifteen years ago. Currently a Senior pursing a degree in Mechanical Engineering. Phumi provides knowledge and expertise in SolidWorks modeling and other CAD software. Currently an engineering intern at Micromanipulator Company to gain experience in the engineering field.
Tim Wirshing is a student working on his undergraduate in Mechanical Engineering at UNR. He graduated from Green Valley High School in Las Vegas, NV. While attending High school, Tim earned the Eagle Scout Award and participated in several volunteer programs including Title 1 Hope and St. Thomas More Outreach Projects. Frequently he travels to visit family members across the US. He engages in activities ranging surfing to hiking to enjoying the arts and attempting to master games such as chess and go while on his travels.
John Victorino is a mechanical engineering major at the University of Nevada, Reno. Originally from the Philippines, John moved to Reno at the age of 12. He now spends his free time skiing, climbing, and biking when he isn’t in the library or computer labs.