Land speed racing is, to a degree, about power. The faster you go, the more power you need to overcome air resistance. Thus, the more power you have, the faster you can go with the appropriate gearing. Alternatively, you can cut air resistance and go faster with the same power. Most racers put effort into doing both.
Matt Brown is an automotive engineer, occasional Autopian contributor, and the YouTuber behind the channel SuperfastMatt. As you might guess by the name, he likes going fast, and he’s working to make his homebrew land speed racer faster than ever. To that end, he’s decided to design an aerodynamic body for his formerly bare-framed build.
Matt starts by outlining the basic factors that will influence his design. The car needs room for him inside, along with the wheels, a windscreen, an exhaust exit, and so on. He needs to allow for the basic functioning of the vehicle while creating a streamlined body that is as slippery and aerodynamic as possible.
The video explains the typical way to design a streamlined vehicle, wherein the nose is rounded off and the body tapered to a point at the rear. Indeed, allowing the air to come back together smoothly at the back of the vehicle is just as, if not more important than very gently punching a hole in it at the front of the vehicle.
In contrast, Matt’s land-speed car is nothing like that. It’s currently a bare frame, with plenty of protrusions from various components sticking out here and there, all contributing to a pretty poor drag figure.
To design his streamliner body, Matt decided to use a tool called AirShaper, with the company sponsoring the build. AirShaper is a tool for Computational Fluid Dynamics analysis, or CFD for short. Basically, it allows you to do digital simulations of airflow or water flow over an object, which can often be more convenient than building a scale model and using a wind tunnel. Performing meaningful CFD analysis is a highly technical skill that typically requires a great deal of advanced knowledge and experience to do well. Do it poorly, and your computational results will have little correlation with what happens in the real world.
The point of AirShaper is to make CFD more accessible than traditional packages which can have a very steep learning curve. As someone who took an introductory CFD course at university, I can easily understand the value there. Matt was also particularly lucky, with AirShaper CEO Wouter Remmerie also sitting in to provide his own insight on design.
Oh, and before we go further – here’s your crash course in aerodynamics. There are two kinds of drag of prime concern. There’s pressure drag, also known as form drag, which is caused by the shape of the object. It’s all about the pressure variation across the vehicle. Imagine a sphere traveling through the air. You’d see the air compressed as it impacts the front of the sphere. Meanwhile, as the air flows around the sphere, it creates a boundary layer, which then separates towards the back. This leaves a turbulent “wake” behind the sphere where there’s a low-pressure zone. This pressure differential creates drag, pushing the sphere back. Then, there’s friction drag, which is much simpler. It’s just down to the friction of air molecules on the surface of a vehicle.
Matt ran his car at the Bonneville salt flats previously with no body, and used his results there as a verification for the figures he was getting out of AirShaper. The CFD analysis suggested his bare-framed design had a coefficient of drag (Cd) worse than an 18-wheeler, at 0.825. Total drag of the vehicle is much less, as the land speed car has a much lower frontal area, but it basically indicates that the vehicle is less aerodynamically efficient than a semi-truck.
Matt also noted the car was kind of unstable from his experience out on the salt flats. AirShaper similarly noted there wasn’t any aerodynamic force that would help keep it in a straight line—undesirable for a land speed car. Indeed, the car was so sketchy, he topped out at 157 mph according to his dash before backing off, though he suspected the car would actually be capable of closer to 180 mph with no body. AirShaper only accounts for aerodynamic resistance, and suggests the car would be capable of 180 mph with just 130 horsepower. Matt had the car dyno tested at 195 horsepower, but between things like rolling resistance on gooey salt and the loss of horsepower to altitude at Bonneville, he reckons the analysis was pretty much in the ballpark.
When designing the body, Matt started by keeping things simple where possible. He started by roughing out a design that’s round at the front and tapers towards the rear, leaving room for a driver, the engine, and the wheels. He noted the car was 22 inches wide to accommodate the front and rear wheels, and that there was little to gain from slimming the area between the two. That let him form the main lower body section from three flat panels, easing the job of constructing the body. He’s contemplating either doing that, or bending a large plastic piece into a single U-shaped panel that sits underneath the car.
Matt then got into the meat of things, optimizing the nose to minimize the coefficient of drag. He set up AirShaper to simulate a vehicle moving along the ground. This configures the CFD analysis with the correct parameters for interactions between the ground and the vehicle itself. The AirShaper analysis kept finding that a long, lower nose offered lower drag. However, making it too long or too pointy didn’t really help. He now has to contemplate how to carve out a gap between the front wheels to allow better visibility without compromising drag.
At the back, Matt experimented by changing the angle at which the tail tapered together. He found that for his basic design, a longer, sleeker tail was worse. This was because while the subtler taper angle cut pressure drag by blending the air more gently, it added more friction drag from the greater surface area of this design. He ended up shortening the basic design until he found the point where the drag figures stopped improving.
As for stability, Matt wanted the body to help keep the car on the straight and narrow. This meant ensuring that the center of pressure was behind the center of gravity of the car. Basically, imagine the center of gravity is the balancing point where we can say the mass of an object acts. The center of pressure is the point where the sum of the pressure field on the car acts.
Finding the center of pressure is complicated, but basically, adding more drag at the rear of the car pulls it rearward. Matt achieved this by adding a fin to the back of the car. It’s the same principle behind the fletches on the back of an arrow, or the flights on the back of a dart. They shift the center of pressure rearward, behind the center of gravity, providing aerodynamic stability in motion.
Adding the fin does add some drag, as it increases frontal area. However, by making it fairly small and giving it a streamlined teardrop shape, it keeps the additional drag to a minimum, both in friction drag and pressure drag respectively.
Matt then put together a rough top speed estimate for the car, based on similar rolling resistance and altitude losses as his prior bare-framed run at Bonneville. He reckons, even running the car on two cylinders rather than four, that it will be much faster than his previous run with the streamlined body installed. He estimates a top speed of 265 mph in that configuration. Running full power, just like his previous run, his calculations expect a top speed of 325 mph.
These figures are all very rough. They’re based on a great deal of assumptions and back-of-the-envelope extrapolations. However, it provides a vague idea of just how much impact the streamliner body could have. Yes, it’s a hugely significant gain in top speed, but that’s what you get when you slash your Cd from 0.825 down to 0.12.
Construction will pose some challenges. Matt has to figure out how to build the majority of the design in fiberglass. Plus, real-world considerations like fasteners and cutouts for wheels and the like will hurt efficiency. But regardless, Matt should expect to go much faster when he returns to the salt with a new streamlined body of his own design. We can’t wait to see how the real world results measure up against the simulations.
Image credits: Superfast Matt via YouTube Screenshot, Drag diagram credit to BoH, CC BY-SA 3.0