Some place I believe I once read that the most aerodynamic shape was roughly that of a raindrop. The ratio was, I believe, 7 times in length of the largest diameter of the drop. For example... 1" diameter = 7" in absolute length. But I can not locate any eveidence to support or refute my memory. Does anyone know where I might find this information? I am contemplating a project for a Physics class and/or a Technical drawing class I am taking.
depends a lot on what your talking about and for what? I am a mechanical engineering major at rutgers university where we build a race car (I know we have a couple more FSAE guys on here) and our team has a guy this year doing water/wind tunnel testing on an aero kit for the car to increase down force. If your talking for a boneville car at highspeeds, it's a lot different from what your looking into for an autocross style car. What exactly are you asking about
One more example of the brilliance in nature. Varies widely according to application as surfer said. Always been very interested in this. Some interesting techniques for controlling boundary layers, creating down force, reducing drag, etc. Keep this one going.
I had once read an article regarding aerodynamics, drag coefficient and vehicle design and it was stated that the lowest drag coefficient vehicle for many years was the VW Bug. I found it odd considering some of the other radical designs that came out during the Bugs production, which would seem to have been more streamlined.
One of the items I dream of finding in a dumpster is laminar flow film. As soon as I win the lotto I'll get one of those machines that laser drills holes the size of a human hair in steel. With those and a few louvers, an air pump and the right shape, you can make enough down force to pop the tires. A carefully designed belly pan, etc. Always wanted to try and build something with the drag of smoke.
Nope. The Bug had a high Cd ( 0.38 ) It wasnt even the lowest in 1939, because the '35 Tatra had a Cd of 0.212 The '53 Alfa Romeo BAT was better than that with a Cd of 0.19 And the '54 Fiat Turbina was even better with a Cd of 0.14 http://en.wikipedia.org/wiki/Drag_coefficient
Interesting article. Like I said it was some years ago so obviously my memory is failing me Some of those vehicles on the list were somewhat surprising though.
A little more info on the Tatra. http://www.tatraplan.co.uk/ Near the bottom of the page there are some other cars with bodywork based on the Teardrop. Including Jamais Contente, the first car to go faster than 100KPH ( about 60MPH ) it did 68.8 MPH in 1899.
Very cool info Alex. I dig the backbone fin and the whole shape/design from rear axle back. What kind of a effed up place is this? quote: (Ledwinka was released in 1951 and fully rehabilitated in 1992) Special thanx to all thos who stand in the way of independant thought: eff yourself mr. dictator. It's a very nice design on the tatra. I'm thinking I probably won't find one around here. Being a old porsche mechanic, I dig the flat 4 and the body reminds me of the early stuff.
Sorry for the HUGE pictures, but I found this on the JCB website and printed them a while back........Ron Ayers designed the streamliner for JCB and while at Bonneville he signed more his name more than Andy Green the driver and worlds fastest man. Also, here are a couple of "Favorite" saves that I had on my computer. Chris Nelson Kansas http://www.andrewgraves.biz/ssc_stuff/RonAyers.htm http://www.jcbdieselmax.com/html/team.php?team_id=5 http://en.wikipedia.org/wiki/Ron_Ayers http://www.jcbdieselmax.com/html/news_detail.php?id=49&month=0&year=2006
I would guess that it is the most aerodynamic shape that allows the smallest surface area of a given volume while travelling through the air of that density while acted upon by gravity only. I guess that is the equilibrium point shape. If you want to force it faster then I'd imagine you'd need a shape more like a bullet ( The X1 was designed after one essentially) I'm no physicist but I think that it would be different for a "passive" system like a rain drop and one where power is being applied to force the item through air. Stu Did ANY of that make sense??
I found something here. http://www.geocities.com/capecanaveral/lab/4515/geebeer2.html "It was a widely known engineering fact that the ideal shape for a body moving through a fluid was teardrop with a ratio between length and diameter of 3 to 3.5 to one " Some more from the same page. "The Fuselage For minimum profile drag coefficient, a streamline body should have a fineness ratio between 2 and 4. Also the maximum diameter should be located at about one-third of the length of the body. With these points in mind the Supersportster fuselage was laid out. The fineness ratio was set at 3.2 and the maximum body diameter was located at 34% of the fuselage length "
July 1993 Street Rodder. There is an article on Belly Tanks by Frank Oddo. In it he qoutes the Germans as saying 6:1 is the ideal ratio. Hope this helps. Wayno
"Aerodynamics are for people who cannot build engines." -Enzo Ferrari Really though, this is an interesting discussion, so BTTT...
Enzo also said ''The only people that brought an old Ferrari we're people that couldn't afford a new one'' Enzo was a jerk But not an a'hole like Bugatti
I am assuming that with all of the upgrades to the HAMB, our discussion on aerodynamics say circa 2001 is long gone?
Is a raindrop's shape detremined by the air flowing around it, or if it were to rain in a vacume ( I know !) would the drop take on a differnt shape? I'm just wondering if the drop is that shape because the air flowing around it is forcing it into the most efficent shape. If that is true I would find it hard to believe there could be a more efficent shape for aerodynamics. However, I'm not an engineer, or that bright so someone can probably prove me wrong.
A couple of problems to solve: As you double your airspeed you square the effects of resistance ie form drag and airflow turbulence drag. The undercarraige of the car either provides lift or drag, both effects are bad on traction a funny burst of wind could change one effect to sway twards the other.
As someone mentioned earlier, what application are you looking for? Any time you deal with the aerdynamics of a car you are comprimising the design from a purely aerodynamic design to a functional design in the application it is to be used. i.e. a drag car is different from an F1 car is different from a bonneville car... A raindrop is most likely very aerodynamic for it's function but I think the shape would promote lift at speed. You'd need to compromise that shape so you'd have traction. I helped a friend with windtunnel testing of his race kart a few years back and we noticed a correlation between aerodynamics and handling. Some of the imporvements aerodynamically adversely affected the handling and the engine cooling...
You're not wrong. Water drops in a vacuum will be spherical whether they are falling due to gravity or stationary. Air rushing past a liquid drop forces it into the most efficient shape possible, period. This applies to any air speed short of one that will simply disintegrate the drop. Lastly a water drop stationary in zero gee is spherical whether air surrounds it or not. In practice a drop will visibly wibble-wobble for quite a while in zero gee after it's formed but it's constantly "trying" to assume a spherical shape. As has been pointed out the high top speeds of record cars, and even ordinary passenger cars, require shapes that aren't very much like a classic raindrop. Additionally vehicle shapes have more complex requirements than simple drag reduction. They also have to control lift, they have to house equipment and people, and they have to allow for cooling airflow--all things that influence the shape away from perfect raindrop-ness. Aircraft wings have operational constraints as well. They need low drag of course but also adequate lift to support the vehicle in flight and an airfoil shape which suits the speed range and intended use of the particular airframe. All of which results in shapes that can be distinctly un drop-like. Speed ranges as severe as that required of an airliner require wings that have large movable surfaces such as multi-section flaps that lower takeoff/landing speeds to reasonable levels yet allow high cruising speeds when retracted. Airfoils such as used on ultra-lights and the original Wright flyer may not have a bottom surface at all but rather a single curved surface that will lift much weight but function properly only at very low speeds. Now that I'm so far OT let me air a pet peeve. What you may have learned in school to the effect that Bernoulli's Theorem, air flowing faster over the top of a curved wing, is what allows an airfoil to create lift is complete bunk. What allows it is Newton's first law of motion--you know, the one about equal and opposite reactions. Stick your hand out of the window at speed and you can "fly" it in the airstream. The air smacking the bottom of a wing as it is dragged along by a prop or turbine is what creates lift. This isn't just semantics. An airfoil can be as thin and flat as a sheet of cardboard and still create lift. The curved shape of an airfoil is not responsible for lift per se but it is critical for the reduction of drag and expanding the speed range over which it will operate which is why so much experimental work has been devoted to the problem. Most advanced aerobatic aircraft have wings with symmetrical airfoils because the flight characteristics need to be the same inverted as they are upright. According to Bernoulli's Theorem they shouldn't fly at all. The secret is that all wings, whatever their shape, need to be tilted up in front to generate lift. Obviously the faster the plane the less the wing will need to be tilted relative to the wind---a circumstance known as relative angle of attack. Even a jet fighter at Mach 2 needs a positive angle of attack, it's just really small. Position a plane's airfoil at zero degrees angle of attack and it's comin' down whatever Mr. Bernoulli has to say about it. Fortunately enough for us Bernoulli's Theorem does indeed account for the pressure drop needed to draw fuel out of a carburetor jet. Guess I should thank him for that. OT rant over.
I am by no means an expert on aero, but here is the little I know applied to cars. As it was said before, it is basically a big compromise. Downforce creates drag. Drag requires horsepower to overcome, especially at high speed. As it was also said, drag is an exponential function, not linear. Take for example a road racing car. It has a lot to do with the dynamics of tires. The more normal force you put on a tire, the more it "gives" back. Meaning the more vertical force between the tire and the road surface, the more lateral force (cornering force) the tire will give back, to a point. Downforce increases the normal force on a tire (obvious). Take for example a DSR (D sports racer). Without aero, the maximum lateral force the car can produce is roughly 1.3 g's. With aero (which they all are), the max lateral force is roughly 2-2.5 g's. The problem is that the car is displacement limited (by the rules), and therefore horsepower limited to roughly 240 BHP. So if the car has a lot of downforce it will corner very fast, but in the straights at high speed, it won't accelerate very fast and won't have a high top speed. So it's a compromise between high-end acceleration, top speed and cornering ability. In something like an F1 car, they create so much downforce because they have the horsepower to push it. At something like 100mph or just over they create a downforce equivalent to the weight of the car....aka they could drive upside down, bar any oiling issues. Now in something like a lakester, you want very little downforce, enough to keep it from flying, aso equally important you want a small frontal area. The drag force is equal to the coefficient of drag times the frontal area times the velocity squared. So the smaller the Cd, the smaller the frontal area, the less drag. As for specific shapes per aero, it is all about boundary layer seperation/ and laminar versus turbulent flow. Laminar flow is where in the boundary layer you have a consistant shear force, no mixing "tumbling" of the fluid. Turbulant flow has mixing of the fluid within the boundary layer. Turbulent flow creates more drag than laminar flow. Whether or not your flow is turbulent or laminar is a function of the fuild density, velocity, and surface roughness. There are two approaches you can take regarding the body shape. Either your design can try and stay within the laminar region or if it doesn't look like it is possible (velocity is too high, etc) then you can trip the boundary layer early. The point here is that you don't want to have a combination of laminar and turbulent flow because the trip point is a source of high drag. So you can trip the boundary layer early (with a lip, etc in the body) to create turbulent flow over the entire body. The other thing to worry about is boundary layer seperation. This occurs when you have a sudden change in shape (like the area right behind a cab on a pickup). What happens is you get a swirling of the fluid and a point of "zero" velocity somewhere else besides on the surface of the body. This is also a large point of drag. Hence the long shape of most streamliners, they are trying to avoid boundary layer seperation by extending the body and tapering it gradually. The other point where there is zero velocity is the molecules on the surface of the body due to the "no-slip condition". This simply means that on any surface with fluid flow over it, the molecules touching the surface don't move, and then the ones next move a little, all the way out to the full stream velocity. This is the boundary layer. Sorry if this seems long-winded or confusing, sometimes it sounds clearer in my head then when I write it down.
Seems clear enough to me. Some peculiar effects of this boundary layer can manifest themselves. In slow speed model planes for example a very slick surfaced airfoil sometimes doesn't work as well a a slightly rough one. In this case the boundary layer is too thick compared to the size of the wing so the roughness abreviates it, allows more flow closer to the layer, and greater lift results--a seemingly contrary result. Strings glued across the top front of the airfoil or sub-spars called "turbulators" that create a bump in same are also used for this purpose. Model airplanes have to deal with the fact that air molecules are always the same size and posessed of the same inertia so small airfoils are inherently less efficient because of this. This is the basis of the phenomenon called "scale effect". Big wings and props are more inherently efficient than smaller ones. A 50" wingspan model of a Cessna 170 that weighs one thousandth of a full sized one nevertheless requires about one hundredth the power to fly well. Boundary layer control is one of the main thrusts of hydrodynamic research these days.
This is really interesting guys, thanks. So, just for the sake of argument, let's say we were trying to make a smooth brick more aerodynamic (just thinking of the most un-aerodynamic shape that I could think of) and we couldn't do any radical shape changes to the brick (such as rounding the nose or needle pointing the rear). What aero tricks could we perform to help this brick be more aerodynamic?
It depends: At low speed nothing helps, at medium speeds you could wax the brick's pores, whereas you could drop a waxed and unwaxed brick from 50,000 and watch them fall on radar, and they would probably fall at the same speed.
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