Aerodynamics Research Revolutionizes Truck Design. Originating Technology/NASA Contribution Dryden engineers modified a retired delivery van to test aerodynamic drag, first boxing the van with aluminum sheets at 9. The last 3. 5 years have seen a sea change in the design of trucks on America. Thanks to the ingenuity of a Dryden Flight Research Center researcher bicycling through the California desert and a team of engineers in Virginia, the shape of rigs and recreational vehicles (RVs) today owes as much to the skies as it does the open road. Bicyclists, motorcyclists, and even pedestrians feel a push and pull of air as large trucks pass. The larger a vehicle is and the faster it moves, the more air it pushes ahead. For a large truck, this can mean a particularly large surface moving a large quantity of air at a high velocity. The displaced air must go somewhere, spilling around the cab into swirling vortices. The air traveling along the side moves unevenly, adhering and breaking away, and sometimes dissipating into the surrounding air. At the end of the cab or trailer, the opposite effect of the high- pressure zone at the front develops; the airflow is confronted with an abrupt turn that it cannot negotiate, and a low- pressure zone develops. The high pressure up front, the turbid air alongside and under the vehicle, and the low pressure at the back all combine to generate considerable aerodynamic drag. ![]()
![]() 10 February 2004 16.00 Aerodynamics Lecture 4 Let’s discuss? Road Vehicle Aerodynamic Design Underbody influence. The major goal of aerodynamic design for racing cars is not to decrease drag like in. A study published in Automotive Engineering in August 1. In such cases, roughly half of the truck. Saltzman, Dryden aerospace engineer and bicyclist, noticed the push and pull of large trucks at highway speeds while riding to work. As a tractor trailer overtook him, he first felt the bow wave of air pushing him slightly away from the road and toward the sagebrush; as the truck swept past, its wake had the opposite effect, drawing him toward the road and even causing both rider and bicycle to lean toward the lane. Saltzman mused about ways to mitigate the bow wave and trailing partial vacuum, and resolved to help trucks glide through air instead of push through it, and, in the process, decrease drag and increase fuel efficiency. NASA colleagues at Dryden were working on the effects of drag and wind resistance on different kinds of aircraft and the early space shuttle designs, so they transferred their considerable knowledge to the design of large trucks. The first formal experiment involved a Ford van retired from delivery duties at Dryden. Mechanics attached an external frame which was then covered with sheet aluminum to give the van flat sides all around and 9. The vehicle looked like an aluminum shoebox on wheels, simulating the cruder motor homes of the period. The Dryden engineers measured the vehicle. Rounding all four front edges yielded a 5. The engineers estimated the potential gain in fuel economy to be between 1. During the following decade, Dryden researchers conducted numerous tests to determine which adjustments in the shape of trucks reduced aerodynamic drag and improved efficiency. The team leased and modified a cab over engine (COE) tractor trailer, the dominant cab design of the time, from a Southern California firm. Modifications included rounding the corners and edges of the box- shaped cab with sheet metal, placing a smooth fairing on the cab. Likewise, rounding the vertical and horizontal corners cut drag by 5. Closing the gap between the cab and the trailer realized a significant reduction in drag and 2. A second group of tests added a faired underbody and a boat tail, the latter feature resulting in drag reduction of about 1. Assuming annual mileage of 1. On the other coast from Saltzman and his Dryden team, Dr. Howard of Langley Research Center with Dr. Selby of Old Dominion University, Norfolk, Virginia, conducted a series of research projects in the late 1. One study conducted in 1. The study employed vortex generators, aerodynamic surfaces protruding from a body that draw faster moving air to the surface of the vehicle and disrupt the slower moving boundary layer air around a vehicle, the use of which can be traced back to research conducted by the National Advisory Committee for Aeronautics (NASA. The generated vortices . These studies quantified and characterized the behavior and performance of a variety of large- eddy breakup devices for turbulent flow separation control. Partnership. Answering the charge given by the U. S. Congress in the National Aeronautics and Space Act of 1. NASA makes the results of its research and expertise of its scientists and engineers available through a variety of means. Sponsored by the Innovative Partnerships Program, these include published studies, NASA outreach, the Small Business Innovation Research and Small Business Technology Transfer programs, technology transfer offices at each NASA field center, and the Space Alliance Technology Outreach Program (SATOP). The aerodynamics studies at Dryden have been made publicly available, and Aeroserve Technologies Ltd., of Ottawa, Canada, with its marketing arm, Airtab LLC, in Loveland, Colorado, applied these studies, the aerodynamic work at Langley, and the patented Wheeler vortex generator to the development of the Airtab vortex generator; designed to reduce drag and improve vehicle stability and fuel economy. Of the devices tested, the Wheeler showed the least parasitic drag, and Aeroserve optimized the Wheeler design for ease of installation and application to any vehicle. ![]() Product Outcome. The Surface Transportation Assistance Act of 1. As the previous regulation made the COE tractor a dominant choice, owing to its decreased length regardless of aerodynamic or fuel efficiency shortcomings, the new regulations opened the door for a renaissance of the . While COE designs place the cab directly above the engine, minimizing length and producing a cube- like tractor, conventional truck designs place the engine ahead of the cab. Though longer as a result, a protruding nose offers truck designers an inherently more aerodynamic shape from which to work. In 1. 98. 2, COE trucks constituted over 6. Peterbilt Motors Company, with similar numbers for other manufacturers; the cab- over design represented only 1 percent of sales for Peterbilt by 2. Streamlined cabs and fairings are now a common sight on our highways, and the once- prominent cab- over design has been abandoned in virtually all applications except small- capacity urban- oriented trucks where length remains a premium. ![]() The modifications tried by the engineers at Dryden were adopted by the truck manufacturers, as the same principles the NASA engineers demonstrated with COE trucks applied to conventionals. In addition, the cargo boxes of most delivery trucks today have rounded corners and edges, a direct application of the research conducted at Dryden on the . For livestock haulers, a key factor is that individual farmers have been the predominant owners of trailers, and these owners are difficult to convince about the costs of redesign versus the savings of superior aerodynamics. However, more and more livestock trailers are sporting boat- tail designs that ease the flow of air past the end of the trailer and minimize the low- pressure wake. Conventional trailer manufacturers have resisted change more so than others, in part because the aft end of such a trailer needs to be easy to manipulate at loading docks, where the optimal shape for superior aerodynamics. Two conventional means to address this issue are problematic: Adding side extenders (to decrease the exposed gap) is expensive and might impede maneuverability; moving the fifth wheel forward (to shorten the gap) places more weight on the steering axle. Airtab vortex generators create a controlled vortex to reduce truck and trailer wind resistance and aerodynamic drag. Each Airtab produces two counter- rotating vortices of air, each approximately four to five times the height of the Airtab and several feet in length, that smoothly bridge the gap between tractor and trailer or control airflow past the rear of the vehicle. Airtabs thus allow an operator to set the fifth wheel to the optimum position without incurring extra drag or steering gear wear penalties and gain some of the aerodynamic benefit of side extenders. At the back of a trailer, box van, or RV, Airtabs radically alter the airflow to reduce drag in two ways: Shifting the airflow pattern from vertical to horizontal to eliminate large eddies, and smoothing the airflow to artificially simulate a tapered rear of the vehicle. In fact, Airtabs have been shown effective on any vehicle with more than a 3. Smoothing the airflow results in markedly improved fuel economy without compromise to design utility, and additional benefits have been realized as well. The vortex generation reduces spray; users have reported improved rear and side view in wet or snowy weather, increasing safety and offering a clearer view of surrounding vehicles. Also, because Airtabs alter the airflow around the rear of a vehicle, the accumulation of road grime is reduced, keeping tail lights and reflectors clean and allowing less snow to build up, a significant safety benefit in foul weather. Less accumulation of road grime also means advertising and safety information on the back of a vehicle remains visible. Perhaps most importantly, drivers of vehicles fitted with Airtabs have reported improved stability and handling and dramatically reduced fishtailing of trailers. Increased stability also means that the trailer does not scrub on the sides of the road as much, increasing the life of tires. Drivers also report better handling when being passed in the same direction by other large vehicles. Cummins Rocky Mountain LLC, a diesel engine and generator wholesale and distribution company in Broomfield, Colorado, recognized these benefits and agreed to promote and sell Airtabs after internal testing and customer feedback indicated that Airtabs brought immediate safety and fuel economy benefits when running equipment at highway speeds. Car Aerodynamics Basics, How- To & Design Tips ~ FREE! Aerodynamics is the science of how air flows around and inside objects. Aerodynamic Principles. Drag. No matter how slowly a car is going, it takes some energy to move the car through the air. Drag, in vehicle aerodynamics, is comprised primarily of three forces: Frontal pressure, or the effect created by a vehicle body pushing air out of the way. Rear vacuum, or the effect created by air not being able to fill the hole left by the vehicle body. Boundary layer, or the effect of friction created by slow moving air at the surface of the vehicle body. Between these three forces, we can describe most of the interactions of the airflow with a vehicle body. Frontal Pressure. Frontal pressure is caused by the air attempting to flow around the front of the vehicle as shown in diagram D1 below. Frontal Pressure is a form of drag where the vehicle must push air molecules out of the way as it travels through the air. As millions of air molecules approach the front of the car, they begin to compress, and in doing so raise the air pressure in front of the car. At the same time, the air molecules travelling along the sides of the car are at atmospheric pressure, a lower pressure compared to the molecules at the front of the car. Just like an air tank, if the valve to the lower pressure atmosphere outside the tank is opened, the air molecules will naturally flow to the lower pressure area, eventually equalizing the pressure inside and outside the tank. The same rules apply to any vehicle. The compressed molecules of air naturally seek a way out of the high pressure zone in front of the vehicle, and they find it around the sides, top and bottom of the vehicle as demonstrated in diagram D1. Rear Vacuum. Rear vacuum is caused by the “hole” left in the air as a vehicle passes through it. To visualize this, let’s take a look at our demonstration car in diagram D2 below. These empty areas are the result of the air molecules not being able to fill the hole as quickly as the car can make it. The air molecules attempt to fill in to this area, but the car is always one step ahead, and as a result, a continuous vacuum sucks in the opposite direction of the car. Rear Vacuum (Also known as flow detachment) is another form of drag where the air the vehicle is passing through cannot fill the space of the hole left behind by the vehicle, leading to what amounts to a vacuum. This inability to fill the hole left by the car is technically called Flow detachment. Flow detachment applies only to the “rear vacuum” portion of the drag forces and has a greater and greater negative effect as vehicle speed increases. This extra bodywork allows the air molecules to converge back into the vacuum smoothly along the body into the hole left by the car’s cockpit, and front area, instead of having to suddenly fill a large empty space. The force created by the rear vacuum exceeds that created by frontal pressure, so there is very good reason to minimize the scale of the vacuum created at the rear of the vehicle. Diagram D3. The turbulence created by this detachment can then affect the air flow to parts of the car which lie behind the mirror. Intake ducts, for instance, function best when the air entering them flows smoothly. Wings generate far more downforce with smooth flows over them as well. Drag Coefficient. To enable the comparison of the drag produced by one vehicle versus another, a dimensionless value called the Coefficient of Drag or Cd was created. In truth though, to be ideal, a car body would be shaped like a tear drop, as even the best sports cars experience flow detachment. However, tear drop shapes are not conducive to the area where a car operates, and that is close to the ground. Airplanes don’t have this limitation, and therefore teardrop shapes work. The best road cars today manage a Cd of about 0. Formula 1 cars, with their wings and open wheels (a massive drag component) manage a minimum of about 0. If we consider that a flat plate has a Cd of about 1. F1 car really seems inefficient, but what an F1 car lacks in aerodynamic drag efficiency, it makes up for in downforce and horsepower. Aerodynamics How- To Tips (1/4)Cover Open wheels. Open wheels create a great deal of drag and air flow turbulence, similar to the diagram of the mirror in the “Turbulence” section above. Full covering bodywork is probably the best solution, if legal by regulations, but if partial bodywork is permitted, placing a converging fairing behind the wheel provides maximum benefit. Minimize Frontal Area. The smaller the hole your car punches through the air, the better it will accelerate, the higher the top speed, and the lower the fuel consumption it will have. Converge Bodywork Slowly. Bodywork which quickly converges or is simply truncated, forces the air flow into turbulence, and generates a great deal of drag. As mentioned above, it also can affect aerodynamic devices and bodywork further behind on the vehicle body.
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