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Aero drag comes from two sources 1) Disturbing smooth airflow... ,and 2) moving the air out of the way (displacing) as you penetrate it. (please note: Cd is covered in source 1 ... as is boundary layer disturbances ) |
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The more we play with aero mods...the slicker we become...even in our claims. I knew a guy (who was an auto body shop genius) who's last name was Kombach...boy, did he hear about his "square tail"! LOL |
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A question, and a theory for the discussion: 1. the rear angle of the EV1 (as for the Probe V) is rather close to the 30 degrees forbidden by theory. Why is that? 2. My gut feeling (not tested, sorry) is that the aerodynamic qualities of a car vary according to speed. The theory goes like this: air has mass; mass is slow (or how do you say this in English). The mass is pushed aside by the moving car, and once the car has passed, the mass comes back, attracted by the low pressure area behind the car. My point is this: because of the slowness of mass, the airflow picture will be different for a car depending on whether it's going 50 or 150. With 150, the wake will be longer, and the 'bow wave' will be stumper in shape. If you take one individual air molecule, and a car at 150, (as compared to car travelling 50) it goes like this: the molecule gets closer to the car before it starts being pushed to the side (by other molecules), it probably travels further outward since it hits a higher pressure area, and takes longer to travel back in the wake of the car - or would the lower pressure (stronger vacuum) cancel out this last effect? In other words, the air behind an Insight travelling at 50 would be back to normal pressure at a smaller distance behind it, than with an Insight travelling at 150. Or: does it make a difference for the Cd figure which wind speed is used in the wind tunnel that tests it? There might be a link between the answer to my two points: namely, that at low speeds (say up to 50 MpH), the rear third of the EV1/Probe is perfect, since air flow remains well attached to the car, from start to finish. But the rear third would not be perfect if the EV1/Probe were to travel at a speed of 130 MpH. At that speed, the air flow would not remain attached to the rear third since the car would be going to fast: and a smaller angle would be needed, like 10 or 15 degrees. This stuff is not necessarily based on existing (known) theory, but there may be a parallel (in the water, not in the air) that I know of: in windsurfing, the rear fin slices through the water, and prevents the tail from gliding sidewards. There is more pressure on one side of the fin than on the other. At low speeds, no problem. At high speeds however, you may encounter spinout: the pressure difference causes the angle of the fin (in comparison to the direction) to become too large, the water can no longer remain attached to the low pressure side (the wind side) and air bubbles start to form. The back of the board slides away to the leeside. Now of course the properties of water and air are much, much different, but I see no reason why a small similar effect does not exist for air. |
Quick point here - will write more later...
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So not quite the 30 degree angle of death. One reason both of these cars can use relatively steep rear angles is the shape of the rear is dramatically more conic than wedge like. There is a very generous curve from the sides to the backlight/decklid, and this would help avoid the formation of lift induced vortices formed at the rear of a more conventional wedge-like rear end shape with the same backlight angle but a sharper side-to-deck transition. The EV1 side-to-top rear radius is most obvious when seen from above: https://www.geencommentaar.nl/media/ev1.jpg |
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A pyramid is "boat tailed". But a cone is far more aerodynamically efficient owing to the elimination of the sharp angles where the pyramid's planes meet. I would say that most cars - even those which employ boat-tailing - retain distinct side & top "planes", like a pyramid. The EV1 did "conic" boat-tailing. Better. (EDIT: of course it's not really a cone, since the edges are curving in 2 dimensions, but the analogy still works.) |
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Not very applicable to highway speeds though, or even speeds attainable by street-legal cars. At speeds any of us are likely to travel in a car, the classic teardrop shape is supposed to be the most aerodynamic as mother nature has been telling us for years. |
re sharp vs blunt, all I can tell you is that on rc sailplanes they prefer NOT to round off the trailing edges, if you cant bring them to a point then leave them square. rounding will create control surface flutter and other noises. Granted the back of a car isn't exactly a trailing "edge". A boat too has a sharp transition to verticle at the back, if any verticle, don't know if it's for the same reasoning.
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Good link. Thanks. One day I will find an aerodynamicist to talk this over with.
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Get a block of foam and a big fan - shape the foam to the teardrop shape first and place in the fan air stream and measure the force on the shape . . . repeat as you cut the rear taper off.
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MetroMPG,
I haven't seen this referenced to anywhere on this forum yet. https://www.nasa.gov/centers/dryden/p...ain_H-2283.pdf Big file, 3.35 MB. It does give hard experimental evidence of whether radii help or not, at least for a van. It ought to rank at least "for kicks and giggles". |
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