You'd want to blunt the corners on the nose as much as practical and then put a spoiler on the top of the back of the brick. To a degree that tricks the air into thinking the shape is longer than it really is. Slicing a slab off the top of the brick works as well (chopped top). But let's face it an early car body that isn't very highly modified is pretty much an aerodynamic disaster area.
So blunting edges would make the air "stick" to the surface instead of being kicked out away from the surface the way that sharp corners would? Assuming that we we're looking for top speed or fuel economy (these two area's would entail similar aerodynamics, yes?) the spoiler on the back of the brick would be designed how? Just straight off the end of the brick flush with the top causing no lift or downforce or would it need a certain shape that would trick the air somehow? I've seen those plastic strips of vortex generators glued to the sides of vans toward the back. From reading what has been written here, it sounds like these things are disrupting the air in such a way that it can't create a vacuum on the backside of the van. Is this correct? Provided that it is shaped and sized correctly, does the previously mentioned spoiler have a greater affect than these little vortex generators or is it just a different way of going about overcoming the same problem?
If the drop shape is the most efficient, why are race cars such as the Cobra Daytona coupes or GT40's more aerodynamic with flat backs instead of tapered ones?
That is called a Kammback, after Wunibald Kamm. http://faculty.concord.edu/chrisz/hobby/80-AMXitems/Information/production/KammbackStory.html And http://www.alfaclub.cc/feature/f_kamm.htm ( in German, but you can get a pretty good idea of how it works by the illustration and pics... ) Edit. Keeping the Kamm Effect in mind, I wonder if a 2Dr Sedan Duece or Model A, could be faster than its Coupe version for LSR Racing...
That chopped off rear end is known a Kammback for reasons that I've forgotten. It was found that chopping off the back in a certain way could simulate the much longer tail needed for the best drag reduction. It seemed to work but further aero developments superseded the design. It also tended to reduce lift and handling problems and was combined with spoilers to actually provide downforce. Racing body design these days is as much or more concerned with down force since power to drive draggier bodies through the air is abundant and road racing track straightaways are shorter. This is less important on big ovals but a trade-off between drag and downforce must still be made. Open wheel cars, and to a lesser degree sports-racing cars are festooned with wings, spoilers, diffusers and sundry other air management devices to glue them to the track. F1 cars take this to such an extreme that they routinely corner at 5 Gs. Sanctioning bodies have wisely outlawed movable surfaces. If they were to fail at the wrong spot a car would fly off a turn like it was punched in the side. It is a severe challenge to design suspensions that can handle the extreme down force loadings and still provide a degree of suppleness. The dominance of downforce is such that bizarre situations can occur such as the fact that a car might be able to go around a turn at 100mph but not be able to at 70. Lift (downforce) varies as the square of the airspeed. Double the speed and quadruple the downforce (and drag). Aircraft manage this by simply trimming their angles of attack for any given airspeed. Can't do dat on a race car.
Good info here guys. This looks like a good start for a Tech article. Can anyone recommend good books or articles on this subject for the beginner/apprentice, the journeyman, the master? Some comments, or more like thinking out loud: VirgilHilts writes: "I would guess that it (raindrop) is the most aerodynamic shape that allows the smallest surface area of a given volume while traveling through the air of that density while acted upon by gravity only." And moving at that velocity. If it was moving faster, it would probably distort into a smaller diameter, longer shape due to increased drag. A fixed raindrop shape moving at higher speed would see increased drag due to onset of turbulent flow or boundary layer separation (see discussion by Blair). The comment from Elmo Rodge about the article by Frank Oddo saying 6:1 is the ideal ratio seems to back this up. Belly tanks on WWII aircraft moved faster than raindrops. Roothawg mentions aerodynamics discussions circa 2001. Is this still around? Can anyone re-post it? Scratch Built mentions raindrops in vacuum. A brick is as aerodynamic as a drop in a vacuum. Shape doesn't matter there. Take a look at the Apollo mission lunar landers, a box with legs sticking out. Worked fine getting on and off the moon.
The old aerodynamics post had to have went 5-6 pages and was really in depth covering everything including louvers for LSR cars.
more please. Bump....bump bump that's the sound of the fiftys when we're rolling down the street. Speaking of boundary layers. There's an F-16 at Edwards with millions of laser drilled micro sized holes in the upper surfaces (and leading edge) of the wing. Bleed air is pumped out in an attempt to control the boundary layer. Last I heard, high and low speed flight was dramatically improved. Can somebody who knows discuss this? Are the results similiar to the dimples on a golf ball keeping the boundary layer tight to the surface? Please explain.
Are we talking about dropping a brick? Or Racing a brick on land? If dropping it, you would add fins like a dart or a bomb. These would keep it straight up and down so there is less surface area, and it would not flop around. Sealing the pores might help a little too, but a bullet shaped nose cone would probably be best.
cant say much else about this.. but I have heard about the same thing.. additionally I have heard that experiments with "rough surfaces" to control Eddy currents have provided similar results.. obviously I havent been a part of said tests.. but have learned of them from various sources.. (where theres smoke, there's fire)
Here is an old thread about Golf Ball surfaces and Aerodynamics. http://www.jalopyjournal.com/forum/showthread.php?t=3688&highlight=Aerodynamics
So if the ideal shape is a Teardrop with a certain ratio ( say 5:1 ), I assume this is the ideal shape to push a given volume of something through the Air. But what if the thing you want to build has a ratio of 10:1 ( like a LSR Racer...) What would give less drag? Sticking to the Ideal shape or stretching it out? ( which reduces the frontal area...)
The typical raindrop shape is only good at speeds that real ones fall through the air. Faster you go the more elongated the "ideal" shape becomes. Raindrops don't fall at 2-300mph so that 6-1 ratio won't apply at much higher speeds. The experimental F16 performs better because the air flowing through the multitude of holes not only helps keep the boundary layer attached but also helps keep it attached over a much greater area of the wing at high angles of attack which is when air "wants" to separate from a wing's surface and reduce lift. In humid air you can actually see little attached clouds forming at areas of particularly low barometric pressure. It takes a high angle of attack and a huge pressure differential to keep a heavy fighter aloft at low airspeeds or high gee turns. The hole drilling thing presumably does an even better job but isn't usually used on production fighters because of cost and maintenance issues. Golf balls, like model airplanes, operate at very different "Reynolds Numbers" which is a mathematical formulation that integrates both wingloading and aircraft speed. A small model airplane moving at very low speeds has a low Reynolds number and will respond to techniques develop to aid lift in that particular regime like surface roughness and airfoil concavity. A golf ball is of course even smaller yet but it moves through the air much more quickly so its inherent Reynold's Number is not too different from the model's so that's why the dimples (roughness) work as well as they do. A small model that flies very fast (scale jet models can approach 300mph) has a higher Reynolds number and other techiques are called for. Airliners are huge and move fairly fast so their Reynolds numbers are very large. Real jet fighters are smaller than airliners but they move much faster so the Reynold's regime changes yet again. It is basically a mathematical explanation of "scale effect" which is just a way of saying that air molecules only come in one size but aircraft come in many. A 30 foot long LSR streamliner operates in a Reynolds Number regime similar to a twin turboprop aircraft and so would not likely benefit from roughened surfaces or other low speed tricks. Plus you want to keep the thing in contact with the ground which complicates matters significantly. In this case you're trying to avoid lift at all costs but this imperative can interfere with drag reduction. Round and round you go endlessly balancing one critical need with another contradictory one. Suffice to say that raindrops don't set records and all these contradictory needs explain why such vast sums of money have been spent on hydrodynamic computer modeling and wind tunnel testing.
Cool, thanks for the explination. I have read about the Reynolds Number before, but I can't say I totally understand it... I thought it was a number to compensate for the aerodynamic effects of scaling a body down. But that number is used on full scale bodies too?
Actually, water falling down through the air does NOT assume the "teardrop" shape - it looks like a hamburger (flat on the bottom, round on top). The teardrop shape only occurs when a drop of water is sliding down a surface that grabs and pulls the tail out.
Yeah any and all objects moving through a fluid medium have a Reynold's number regime. The number for golf ball is I think in the 100,000s range and jet fighters are in the millions. Things like water pipes and submarines have them too--any hydrodynamic flow (air is merely a thin fluid) situation has a corresponding Reynolds Number regime. Atomickcustom is quite right about the real shape of a raindrop. Real world hydrodynamic drag is all about fluid flow across a surface not the shape of a liquid in free fall. A liquid's surface tension comes into play there. Different equations for that presumably.
What a cool post. Just speaking of drag, whatever has the smallest frontal area has the smallest drag. That's easy enough, but you can't drive a needle. Realistically speaking, the rest of the car's design is going to determine the minimum frontal area. By the way, frontal area doesn't mean only the "front" of the car, think of the shadow cast from a light shining at the front of the car. Stretching the car doesn't reduce the frontal area. Longer is not necessarily better either. That discussion gets back to laminar vs. turbulent flow.
WOW! A simple question I lost track of after I asked it turned into this amazing thread I don't have time to read right now. I must bookmark and come back when I have more time. And BTW, I was thinking/theorizing about shape for a Bonneville type race vehicle.
Turbulent flow is caused by the air molecules not being able to "follow" the curvature of a body due to their inherent inertia. At some given speed they just won't follow the curvature because the ambient air pressure is insufficient to keep air molecules plastered to the surface and proceeding along in an orderly fashion. These turbulently flowing molecules bounce around on the surface and extract needed energy from the moving body. So keeping the flow attached to a surface as long as possible reduces drag. As a first approximation it is easy to get smooth (laminar) flow at low speeds and hard at higher speeds. This is the whole science of hydrodynamics in a nutshell. Total frontal area is important in calculating drag but total surface area is as well. Obviously a big ol' long streamliner has thousands of square inches of skin. Even if flow isn't turbulent this "surface" or "skin" drag is not inconsequential especially on something with as much surface area as a streamliner. Frontal area, turbulent flow, and surface drag all take their toll on a vehicle trying to go fast. To that you have to add something called induced drag regarding airfoils which represents the "work" being done by the air to hold up an aircraft. This isn't much of an issue with land bound vehicles. Add all these things up and you still have issues like stability to consider which is why you sometimes see aircraft like tails on streamliners to keep them going straight. A complex frustrating business at times and frequently it seems far more art than science.
Metalshapes OK... So as the Speed increases the body needs to taper in at a shallower angle ( still talking Teardrop shapes...), because otherwise it can't keep the Laminair airflow? And by how much is defined by the Reynolds number?[/quote] I believe you are correct, as a hawk is far more "tapered" than a duck, when you consider their form at speed. Reynold's equation works at predicting when a fluid would go turbulent in a length of pipe. In the business of gliding through "static" air molecules at higher speeds it should be a useful tool for predicting how far you can maintain laminar flow along the streamline paths on the body of the car. Recall that there is a boundary layer of air that attaches to a surface. Our tech school competed in the Tour-de-Sol electric car races held in the Northeast. So we were trying to "cheat" the drag penalty at far lower speeds. I found this book by Tamai...who I believe was involved at MIT, quite helpful. The Leading Edge: Aerodynamic Design of Ultra-Streamlined Land Vehicles (Engineering and Performance) by Goro Tamai
Thats boundry layer control I belive. They did this with the X-21. Any moving surface has a layer of turbulant air "stuck" to it. If you've seen Dodge's T/A Challenger, thats why they rased the intake scoop. If you suck away this crappy air, you reduce drag, and increase lift. Good things. But the X-21 was a maintanance bitch. The thousands of tiny slots in the wing were very small, like 1/32nd and any clog would foul up the wing. The plumbing for this was a nightmare. It worked, but it wasn't practicle.
I'm sorry but Bernouilli is right! What Bernouilli's law says is that, in a fluid flow, the sum of potential, kinetic and pressure energy is constant. Since our air particle will see a minimal difference in height when it approaches the wing, potential energy is constant, meaning that kinetic energy, speed, will be converted in pressure. What causes a flat plate, or your hand, to create lift is the fact that you give it a positive angle of attack, therefore reducing speed of air under the surface and increasing the speed of air over it. This has the effect of reducing the pressure on top of the surface and increasing it under it, creating lift, all in accordance with Bernouilli! Of course, increasing the angle of attack will increase the lift, but only to the stall point! At this point, you can add angle as much as you want, the plane is going down! Just look at the picture I added. It is the typical pressure distribution on an aircraft wing (the vertical scale decreases as you get higher in the graph). You will notice that the pressure on top is smaller and it's effect is greater than the pressure on the bottom. the grey zone represents the net normal force, the difference between pressure on the two sides. Lots of wing profiles create lift at 0 degree of angle of attack by the way. I have NACA data that shows that some profiles still create lift even with a negative angle of attack! that means that I could choose a profile and set it upside down with a positive angle of attack and it would still go down, all because of the curvature.
Hey kids, buy this month's issue of Motor Trend Classic, the best magazine out there in my opinion. There's a great article in there that debunks the story about '59 Chevy's becoming airborn at speed. It's a terrific read. The '59 Chevy has a Cd of 0.445. The Saab 92 is a remarkable 0.35. Amazing Harley Earl was influenced by airplanes, Saab actually built 'em.
These guys (The Granvilles, out of Springfield, MA) knew a little about aero shapes back in the early thirties. As you can see, the teardrop shape was prevalent in their designs. I believe this one was good for over 325+ in the air. Looked cool as hell too.
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