Aerodynamic Coefficients Influence on the Performance of LMR1 Car
Car aerodynamics is governed by the same principles governing aircraft flights; the main focus being to produce down force instead of lift. In car aerodynamics, there is need to force more high speed-low pressure air to go under the car creating negative lift; down force (Rapid Racer, 2012). This results in higher grip levels for the tires and more traction, especially speeding around corners. For high performance cars, aerodynamics is achieved in several ways. The basic hypothesis is that faster driving increases down force thus pushing car’s tires down resulting in higher grips and tractions during races. The downward force also known as drag is the square of the car’s velocity; which is double the speed (Unlimited Performance Products, 2007). However, this increased down force reduces top speed resulting in more engine power to propel the car forward as shown below.
Nomenclature
In aerodynamics, there are several terms used unique to aerodynamics while some are used in unique ways when discussing aerodynamics. These include airfoil nomenclature, and glider axis among others.
The shape formed by the cross-section of a wing is known as airfoil and the round sides of the airfoil are leading edges (Rapid Racer, 2012). As the airfoil moves through the air, it experiences a relative wind and since the airfoil’s gliders have forward and downward movements, the relative wind comes from the front and below the airfoil.
There are three axes of rotation intersecting at the glider’s center of gravity with the glider’s mass evenly distributed around the center of gravity. Movement about the pitch axis makes the glider’s nose moves up or down while as it moves about the yaw axis, the nose moves from side to side. Additionally, whenever the glider rotates about the roll axis, one wing moves up, the other down.
Racing Car Aerodynamic
Drag is the force acting opposite to the path of the vehicle’s motion. This force results in increased fuel consumption as it hinders vehicles top speed. According to manufacturers, low drag vehicles have these characteristics; streamlined shape, low frontal area, and minimal openings in the bodywork for windows or cooling ducts.
The drag performance of vehicles is characterized by the drag coefficient (CD) equated as: CD = FD / (1/2 • •V2•AF). From this, FD is the drag force, ? is the air density, while V is the free stream velocity, and AF is the frontal area of the vehicle.
Lift is the force acting on vehicles as they move and may be manipulated to enhance the performance of a race car and decrease lap times as opposed to drag (Buresti, 2004). However, by manipulating the race car geometry it is possible to create negative lift, known as down force. Down force enhances vehicle performance by increasing the normal load on the tires. This increases the potential cornering force which results in the ability of the vehicle to corner faster and reduce lap times.
The lift of the vehicle is characterized by the lift coefficient (CL) and is calculated as; CL = FL / (1/2• •V2•AT) (2). In this equation, FL is the lift force; AT is the area of the top surface of the vehicle and the other. A negative lift coefficient shows a vehicle is experiencing down force.
Evaluation of Aerodynamic Forces
In aerodynamics, there are measures used to determine aerodynamic forces. The most common method is wind tunnels (Filip & Galetuse, 2011). This method is used in testing models of proposed aircraft and engine components. In this test, the model is situated in the test section of the tunnel and air made to flow past the model (National Aeronautics and Space Administration, 2012). Wind measurement encompasses four tests including measurement of aerodynamic forces directly, instrumenting the model with pressure taps and then calculating component performance from the pressure data (Society of Automotive Engineers, 2009). The other method is instrumenting the model to give diagnostic information regarding air flow around the model. The final method is using flow visualization techniques to provide diagnostic information.
Air velocity through the test section is determined using Bernoulli’s principle. The direction of airflow around a model is determined by tufts of yarn attached to the aerodynamic surfaces (Gal-Or, 1990). The other test is beam balances, connected to the test model with beams, strings, or cables (Goldstein, 2010). In the beams, pressure distributions are easily measured by the use of pressure sensitive paint; low fluorescence of the paint denotes high pressure. Besides, wake surveys are also used to determine pressure distributions on test models.
Above is the use of smoke trace for off-body flow visualization in the wind tunnel test section.
This denotes the use of beams to measure air pressure.
LMR Series
LMR1 cars are designed to be champions in street racing competitions. The vehicles feature Late Model Racecraft package with approximately 481 rear wheel horsepower and stock 20-inch rims and OEM clutch. The cars in addition, have Corsa exhaust systems complete with custom grind camshaft. The other features include MSD 8.5mm wires, under-drive pulley and NGK spark plugs (Katz, 2006). Besides, the vehicles have 160 degree thermostats and maximized factory mufflers. The LMR cars dominate the street races due to their powerful engines, reducing drag, and increased amount of air sucked in to their cylinders. LMR1 racing rules are initiated to ensure vehicles have bigger and larger tires of approximately 250mm to 300mm. The fuel tanks and chassis of the vehicles are supposed to the heavier and larger to increase down force while minimizing lift during racing and turning around sharp corners.
Lap Time Simulations
The LMR1 vehicle concept is developed after conducting an acceleration sensitivity analysis using simple constant acceleration equation lapsim seeking minimum lap time (Taleb, 2007). The lapsim result suggests the vehicle should be conceptually biased towards lateral acceleration over longitudinal acceleration (Haug, Choi, & Komkov, 1986). Whilst being a very simple lapsim, it is effective in quantifying the conceptual trade-offs. To bias the design towards lateral acceleration, a 50:50 weight distribution is chosen with four equal sized tires in an attempt to provide similar dynamic vertical loads on outside front and rear tires.
Conclusion
It can be concluded that aerodynamic coefficients greatly influence the performance of an LMRI car. In car aerodynamics, there is need to force more high speed-low pressure air to go under the car creating negative lifts. This would ensure cars have lower lift forces during acceleration to thus sucking the car into track with increasing speed. In designing racing cars, superior dynamic competiveness is required, affordability, reliability and maintainability (The University of New South Wales, 2010). The vehicle must require minimal maintenance and additionally have wider wheels to provide higher drag and traction.
References
Buresti, G. (2004). The Influence of Aerodynamics on the Design of High-Performance Road Vehicles. University of Pisa.
Filip, A., & Galetuse, S. (2011). Study of the Aerodynamic Forces Evaluation Methods. Incas Bulletin, 35-43.
Gal-Or, B. (1990). Vectored Propulsion, Supermaneuverability & Robot Aircraft. Springer Verlag.
Goldstein, E. (2010). Wind Tunnels, Don’t Count Them Out. Aerospace America, Vol. 48 N0. 4, 38-43.
Haug, E.J., Choi, K.K., & Komkov, V. (1986). Mathematics in Science and Engineering. Orlando, Florida: Academic Press, Inc.
Katz, J. (2006). Race Car Aerodynamics: Designing for Speed. Cambridge, Massachusetts: Bentley Publishers.
National Aeronautics and Space Administration. (2012, September 13). Wind Tunnel Testing. Retrieved October 25, 2012, from www.nasa.gov: http://www.grc.nasa.gov/WWW/k-12/airplane/tuntest.html
Rapid Racer. (2012). Aerodynamics. Retrieved October 25, 2012, from www.rapid-racer.com: http://www.rapid-racer.com/aerodynamics.php
Society of Automotive Engineers. (2009). Going with the Flow. Aerospace Engineering & Manufacturing, 27-28.
Taleb, N.N. (2007). The Black Swan: The Impact of the Highly Improbable. Random House.
The University of New South Wales. (2010). ACME Racing Design Report 2010. Retrieved October 25, 2012, from www.unsw.edu.au: http://seit.unsw.adfa.edu.au/studentactivities/saevehicle/car.php
Unlimited Performance Products. (2007). Aerodynamics. Retrieved October 25, 2012, from www.up22.com: http://www.up22.com/Aerodynamics.htm
Appendixes
Appendix 1 — Views of the CAD model
Figure A1 — Front view of the modelled LMR1 car
Figure A2 — Side and Top views of the modelled LMR1 car
Appendix 3 — Lapsim input data and sample output
