Leon Posted October 18, 2009 Share Posted October 18, 2009 FWIW I was thinking of the shape in the picture. Uh... you... uh... you have a little typo in this sentence that makes it technically not make any sense. Ironic... Indeed, hahaha! Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted October 18, 2009 Share Posted October 18, 2009 Even the mythical "raindrop" does NOT look like an airfoil or a fuselage. The proportions are WAY OFF (i.e. it's chubby). You don't see fat wings and bubbly-like fuselages on aircraft. What you see are very lengthened shapes that would more be akin of "cigar" shapes or "needle" shapes. but never the short fat shaped of even the coloring-book inspired raindrop example. Proportions and ratios and loadings and power make planes fly more than an idealized symmetrical curve. Only time I've seen that shape in 35 years of flying is on a very few wing struts (and oval would have been just as good). Quote Link to comment Share on other sites More sharing options...
jt1 Posted October 18, 2009 Share Posted October 18, 2009 I will add this for the sake of discussion. When we were doing the wind tunnel tests, we had a good many conversations with Bob, the fellow who ran the tunnel. He is very knowledgable, loves to talk aero, and it would be quite interesting to buy him dinner and a few beers. Maybe we'll get to do that someday. One point he made very strongly was to not make the same assumptions for blunt object (car) aerodynamics, and aero for streamlined shapes (planes). Because of the vastly different shapes, coefficents, and characteristics of the airflow, what works with one may be totally wrong for the other. jt Quote Link to comment Share on other sites More sharing options...
bjhines Posted October 18, 2009 Share Posted October 18, 2009 A lot of this is counter-intuitive. There are a lot of considerations that are missed by many folks. I won't get into the specifics with this response, but I think I can shed some light on why things seem so weird when trying to get your head around fluid dynamics in general. Gasses, liquids, solids, and concretions can all be parted by objects moving through them. They all behave very differently from each other and under different conditions. When considering aerodynamics, you have to keep in mind the fact that air is highly compressable, The paths the particles follow are not what you might expect when considering a non-compressable fluid or something like a knife through a solid or powder. When considering air; The acceleration of the air around the object is key to determining what it will do. The air has mass, it also acts like a spring, energy can be locally stored in the air in a dramatic way because of it's springiness. This energy dissapates in all directions at the speed of sound. The faster you accelerate the air around the oject, the more energy builds up/is stored in the local area. I know you are thinking about supersonic aircraft being torn to ribbons by shockwaves and what does this have to do with cars.... but this is key to understanding even low speed aerodynamics becasue the same effect accounts for all sorts of weird non-intuitive aerodynamic principles. Under certain conditions liquids and gasses can mirror each other's performance, The Bernoulli principle is one effect that comes to mind. In it's most basic form it is simply conservation of energy. Though the air does have it's differences in the details. If you look closely, the air will have a much more complicated interation with the test device than water. As for the blunt nosed shape, In layman's terms, it is easier to part the air smoothly than it is to recombine the paths. The blunt nose reduces the surface area of the airfoil and therefore reduces friction. A long thin leading edge would work fine to part and recombine the air as long as the transition from leading to trailing was curved and smooth. Sharp angled transitions cause boundry separation and lead to turbulence. In reality, the compressability of air leads to a nuance called stagnation zones at the leading edge of the airfoil. The stagnation zone actually makes the leading edge seem pointier than it actually is. The stagnation zone is one of the most interesting realizations we had about the S30 cars in the tunnel tests. We found that increasing/fostering the stagnation zone lead to lower drag and less lift. Simply sealing the internal passages in the front clip made as dramatic an effect as changing the entire external nose of the car(G-nose). Quote Link to comment Share on other sites More sharing options...
Leon Posted October 18, 2009 Share Posted October 18, 2009 Even the mythical "raindrop" does NOT look like an airfoil or a fuselage. The proportions are WAY OFF (i.e. it's chubby). You don't see fat wings and bubbly-like fuselages on aircraft. What you see are very lengthened shapes that would more be akin of "cigar" shapes or "needle" shapes. but never the short fat shaped of even the coloring-book inspired raindrop example. Proportions and ratios and loadings and power make planes fly more than an idealized symmetrical curve. Only time I've seen that shape in 35 years of flying is on a very few wing struts (and oval would have been just as good). I don't know why you keep blabbering on about semantics. I've been trying to help the OP while you're doing nothing but arguing. Keep digging that hole buddy... Quote Link to comment Share on other sites More sharing options...
Challenger Posted October 18, 2009 Share Posted October 18, 2009 I don't know why you keep blabbering on about semantics. I've been trying to help the OP while you're doing nothing but arguing. Keep digging that hole buddy... Its the second time (that Ive seen) him do this. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted October 18, 2009 Share Posted October 18, 2009 while you're doing nothing but arguing. Keep digging that hole buddy... Sorry, I'm not trying to argue. I'm just trying to point out what I know and see. I'm leaving now to go fly my 4 seat airplane. I can say that it's composed of some curves but there is not one of those idealized "raindrop" shapes on the entire airframe, gear, or propeller. I've taken it apart myself for annual inspections and there are none there, inside or out. There is a Cub in the next hanger and it's tailplane are all ovals (tube structure) and it also flies very elegantly on just 65hp. All I'm saying is that where lo drag is concerned, a curved shape that's only 2.5 times as long as it is wide (your example) is not used where more needle-like structures (i.e J3 Cub tailplane) are used. Low drag = needle-like. chubby curvy have high drag. Bye, I'm going flying my slab-sided airplane right now. Too nice a day to waste here... Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted October 18, 2009 Share Posted October 18, 2009 Just got back. My slab-sided fuselage with flat-bottomed wing and triangular control surfaces and oval gear struts topped out at 160mph level cruise on 150hp. How fast should it go if it were designed "the right way"? Quote Link to comment Share on other sites More sharing options...
Leon Posted October 18, 2009 Share Posted October 18, 2009 Well isn't that special! You deserve a check plus and a sticker! Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted October 19, 2009 Share Posted October 19, 2009 I was looking for a discussion among adults, not coloring book outlines and 3rd grade attempts at sarcasm. I want to KNOW how things work and why. From 35 years of pilotage I just don't see such shapes as either practical nor ideal in the REAL WORLD. All I can say is that where it counts, the idealized raindrop shape is not used. My Grumman is damn fast and uses no teardrop shapes, just simple slab sides and rounded edges. Also, the faster you go, the more you want a thin needle shape. The F104 and the SR71 (all Kelly Johnson designs) come to mind. I'm raising my glass to Kelly. Quote Link to comment Share on other sites More sharing options...
Leon Posted October 19, 2009 Share Posted October 19, 2009 You are being the third grader, actually. You have digressed completely from the point of the original topic. Grim doesn't want to hear about your big fast plane, he was asking a question that was properly answered until you jumped in arguing semantics. Do you still not get this??? Obviously not. Quote Link to comment Share on other sites More sharing options...
kiwi303 Posted October 19, 2009 Share Posted October 19, 2009 I'm siding with RP here, Grim was asking about Aerodynamics, and the "Semantics" you are quibbling about is facts about aerodynamics. Big fast plane or small fat car, it's all aerodynamics, and subsonic, all has equal relevance in that it's air, and it's something moving through air. Quit grousing Leon... Quote Link to comment Share on other sites More sharing options...
Administrators RTz Posted October 19, 2009 Administrators Share Posted October 19, 2009 That'll be quite enough guy's. Play nice or don't play. Quote Link to comment Share on other sites More sharing options...
AkumaNoZeta Posted October 19, 2009 Author Share Posted October 19, 2009 I understand what I was asking about now. I'm sorry for using the "raindrop" shaped term now. Its just something I got in the habit of using while talking to other folks. Quote Link to comment Share on other sites More sharing options...
Administrators RTz Posted October 19, 2009 Administrators Share Posted October 19, 2009 For what its worth... The classic shape associated with a drop (with a pointy end in its upper side) is actually an optical effect due to light reflections and the drop's rapid movement. The shape of a drop falling through a gas is actually more or less spherical. Larger drops tend to be flatter on the bottom part due to the pressure of the gas they move through. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted October 19, 2009 Share Posted October 19, 2009 Maybe the original question probably caused the confusion? Aerodynamics is a science and if you ask the question "is a raindrop shape more efficient than a sewing needle shape" without defining the shapes then there will be honest disagreement. Simply "o> a better shape than <>" ignores too much in both aerodynamics and in the eventual application to come up with an answer. Leon had at least 9 different shapes that could be considered (and all had different drag coefficients). I had at least 3 different applications where "better" did not involve complex curvatures. It's hard to come up with an answer without a little more definition of precise shapes and precise applications. Quote Link to comment Share on other sites More sharing options...
Leon Posted October 19, 2009 Share Posted October 19, 2009 Yes, the original question was vague in defining the exact shapes we're talking about. Nobody mentioned the proportions of the shapes we were discussing until RP assumed some shape that was twice as long as it is wide, or concave, etc. What was given were proper pictures and links to the subject matter. I don't think RP's digression added to the discussion. It was a general question that was understood, and answered properly. Braap and others posted very good info on the effects of different shapes on drag. I understand that a raindrop does no physically look like that, but it seemed like everyone else understood what Grim was getting at. We're not talking about drag on a raindrop, so arguing the physical shape of one really is a moot point in this discussion. All it did was allow Grim to phrase his question concisely in a way that he and others understood. Quote Link to comment Share on other sites More sharing options...
AkumaNoZeta Posted October 19, 2009 Author Share Posted October 19, 2009 You've got to remember, not everybody here is en engineer or physicist. Some of us are just amateurs and beginners that need more information/help here and there. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted October 19, 2009 Share Posted October 19, 2009 ...It was a general question that was understood, and answered properly. Braap and others posted very good info on the effects of different shapes on drag....All it did was allow Grim to phrase his question concisely in a way that he and others understood. I did not feel that the generic answers addressed the question of speed that Grim had asked. Generic accepted answers on aerodynamics (like airflow over a wings gives an airplane lift to fly) are widely taught yet demonstratively wrong. I was guessing that Grim also wanted to know why the pictures in the book with the pretty airflows just were not matching up with what he was seeing people using for fast movers. Why were all these people using the "wrong" shape? The answer is actually as varried as the application (speed, dynamic stability, structural materials, rules, etc) where the best shape for the application is not the idealized art-deco curves of the 40's. There is a lot that goes into a fast moving machine. That's the "why" you'll see so many needle-shaped booms and fuselages and pointed land speed cars like you do. Quote Link to comment Share on other sites More sharing options...
Leon Posted October 19, 2009 Share Posted October 19, 2009 ...There is a lot that goes into a fast moving machine. That's the "why" you'll see so many needle-shaped booms and fuselages and pointed land speed cars like you do. Right, and it was mentioned by I believe JM that a supersonic aircraft/vehicle will have different geometric requirements than a subsonic one (such as a street legal vehicle) that does not enter the zone of compressible flow (M~0.4). Again, it was a general question and whether the flow was compressible or not was not explicitly established. This is a car forum however, so I made an assumption that Grim was talking about incompressible flow. In incompressible flow the "raindrop shape" (not raindrop proportion) i.e. blunt in front, slow taper towards the back, is very efficient. Quote Link to comment Share on other sites More sharing options...
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