Aerodynamics Seminar # 9 - by Phil Knox

[MetroMPG]

This post was originally written by Phil Knox (aerohead), and it first appeared on the MaxMPG group. Phil has done a lot of work educating the masses about the critical role aerodynamics play in efficiency, and has spurred many in the DIY crowd to take matters into their own hands.

This is the ninth in a series which I'm reproducing here with permission.

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I believe ellipse is what I meant to use. 6-to-1 ratio is also what I meant. The ellipse is so close to the ideal streamline shape its not a bad model to follow. And we are talking about only the back half of a 6-to - 1 ellipse. In ground-effect,the 6-to-1 form has the aero characteristics of the ideal 3-to-1 streamline form up in free air. Also,the half of the ellipse is cut in half longitudinally.

Measuring the height of the vehicle where it begins its contraction,you would measure back three heights behind the car,take an enormous French curve,and scribe an imaginary elliptical curve from the top of the car to the ground. Its just a rule of thumb. Its ridiculously long! The whole point is that you can cut this boattail off anywhere you want and you will have attached flow and drag reduction up to your cuttoff point.

Since the air under your car is moving slower than anywhere else, vortex generators cannot provide the benefit to a car that they can to an aircraft outside of ground effect. And while suction slot boundary layer control HAS been shown to reduce drag, the energy required to provide the energy for suction EXCEEDS the enrgy saved in the drag reduction.

Virtually all late model vehicles are "clean" up to there backs. Any major drag reduction will come from the back of the vehicle. And every auto aerodynamicist knows that increasing the "fineness" ratio of a vehicle will provide the only path to truly low drag.

And yes, below 250-mph, anything more than a hemispherical shaped nose will offer no additional drag reduction.

With the T-100, I had over eight feet of body length for cross-sectional area reduction behind the cab so it made it a perfect candidate for the tapered shell. In its photos, you can see the elliptical curvature of the shell both in plan and elevation. By the time the air makes it to where the tailgate used to be, the air has recovered atmospheric pressure which shortly before had been velocity pressure.

At 75-mph my ridiculous elliptical boattailing adds 8.5 mpg, a 27% improvement in economy, 27% reduction in CO2 emissions, 27% in additional range, and 27% actual cost in highway travel. You wouldn't want to use it.