Quote:
Originally Posted by theycallmeebryan
I understand that the NACA 6-series wings, and pretty much all foil shapes, produce the lowest drag while in flight, where air is less turbulent and there is no ground deflection.
Since these shapes are so susceptible to imperfections (bug guts, cracks in shape, roughness in shape, etc), what then would be the optimal shape for a road vehicle? Is it worth even attempting to use a low drag foil shape, like the NACA 6-series, on a road car at that rate? Are you suggesting that a simple shape following the template you have posted would have less drag and more laminar flow than a car designed around a NACA foil, considering every day road conditions?
Would it be safe to say that a shape which has the least average drag from -10 degrees to 10 degrees would be the optimal to select for a road vehicle?
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I don't have your wing section,but I spent some time with the NACA 67,1-215.It's drag coefficient(min) occurs between a lift coefficient of 0.175 and 0.375 which from the table,occurs for a range of angle-of-attack between zero and 2-degrees.Here's the kicker,by just changing the angle-of-attack 7-degrees(something quite common in crosswind driving),the Cd rises to 0.0162 a 490% increase.All the 6-series airfoils have a "sag" or "bucket" in their performance curves,and lift (drag) increases rapidly with very small wind direction change.----------------- The other thing to bear in mind is that laminar wings are laminar only up to the first minimum pressure location,and in this adverse pressure gradient,the wing will immediately transition to a turbulent boundary layer.----------- This doesn't mean that flow will separate,it only means that friction drag will increase,bad news in an airplane,but actually good news for automobiles because turbulent boundary layers can sustain attached flow just above the boundary layer,and attached flow is what we need for drag reduction in cars.-------------------------------------- The thing about the template,it is borrowed from a "wing guy"(Jaray"),who,while well acquainted with the tastiness of wings,found the "pumpkin seed" to be more delicious for low drag automobile forms.You will find numerous land speed record cars and solar cars which even today,rely of this form for top honors.-------------- For the template I chose the 2.5:1 fineness ratio as a "minimum" for a 3-dimensional form in 3-dimensional flow,in ground effect.This selection follows a logical linear progression from Jaray's streamline body of revolution completed in it's mirror-image ground reflection,and the German research which isolated the 2.5:1 ratio as having the lest drag.No real Einstein moments there.--------------------- The template respects streamline flow with an economy of structure and literally guarantees attached flow,at least if you don't cheat the minimums.----------- YOU CAN make the body with higher fineness ratio,I just want everyone to understand that there will be a small penalty to extra skin friction.And the good news is that since skin friction plays such a minor role in road vehicle aerodynamics,you won't really nshoot yourself in the foot if you do go longer.--------------------- Getting back to your question:the pumpkin seed will not exhibit drag increases anywhere near the same continent as wing sections in crosswind conditions,the drag will be lower,and while the boundary layer is turbulent,it will be impossible for the flow to separate up to the point you choose to "chop" the body,as Dr.Kamm did on his famous series of cars.