Quote:
Originally Posted by CFECO
How does the Pillbug do so well, as it is considerably "under the Template"? My theory is that the air coming around the sides of the greenhouse add to the air coming over the roof and is able to more easily follow the sharper angle, same with the EV-1. Here is my unfinished X-Car, "unscientifically" compared to the Pillbug, but it's a close representation. The Elevation lines, also done spur of the moment, show a similar pattern to the Pillbug. I will do that better in the future!
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The Schl'o'rwagen is Cd 0.186,same as the 2014 VW XL1,both very low drag by contemporary standards.
The GM Impact/EV1 is rated by GM (as of 1999) at Cd 0.197.Also a good number.
Hucho gave us half a dozen ways to get to Cd 0.15 in his second edition book.
He said that we could go below 0.15 with extra elongation of the body,and with further integration of the wheels into the body,that we could approach Cd 0.07,to 0.09.
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The reason the Pillbug and EV1 don't have lower drag is because of their small fineness ratios.
By adding 16-inches of boat tail and lowering 1.5-inches,GM was able to get the Impact down to around Cd 0.153 for their world speed record at Ft.Stockton.
Had the Pillbug had more aft-body length,and a more gentle roofline,it would also have had lower drag like the SolarWorld GT and Cambridge University CUER eco racer.
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According to Hucho's elliptical drag table,your car,as drawn,will have the potential for Cd 0.21,due to its effective fineness ratio of 1.3:1.
*If you basjoos both the front and rear wheels as on AeroCivic,you should come in at Cd 0.185 if the Beetle's cooling system adds no more than 0.01,and all your cut lines,wipers,
mirrors' are no worse than the 1999 GM Precept PNGV car.
Cd 0.185 is nothing to sneeze at.Certainly.
What Kamm might have had you think about is, that if you'd vault the roofline up to the 'Template',you could truncate the length for in-town 'practicality',while reserving the potential to add an extensible tail,only for the highway,which could get you down around Cd 0.13-0.12.
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As your design sits,I predict that you'll lose your boundary layer on the roof,even with the point,25% the length aft of the face of your rear tire.
The Pseudo-Jaray fastback contour will have too much pressure rise to sustain attachment,and the flow will roll up into vigorous,power-robbing, attached longitudinal vortices.
"...the shape of a body in front of the largest cross-section has only minor influence on the total drag.The main contributions to the drag force originate from the rear part of the body.It is not important to find a proper shape to divide the oncoming flow but it is very important to design a rear body surface which brings the divided streamlines smoothly together.Optimum shapes are 'streamlined' bodies having a very slender rear part." Hucho,1987