View Single Post
Old 08-12-2020, 01:25 PM   #743 (permalink)
Master EcoModder
aerohead's Avatar
Join Date: Jan 2008
Location: Sanger,Texas,U.S.A.
Posts: 12,094
Thanks: 19,386
Thanked 6,135 Times in 3,771 Posts
misc. aero

Originally Posted by CeeforCitroen View Post
I've been thinking - fuel economy aside - these aerodynamic mods are going to make the car faster. Isn't it every boy racer's dream to own a fast Honda civic - far more than the drag inducing body kits and spoilers. Imagine this on a Type R. The Fast and The Furious would have nothing on it! Now, before I get banned from the forum for suggesting anything as fuel wasting as racing. I will add that as the drag coefficient goes down, the stock gear ratios become unnecessarily low now you don't need to be so far up the power band to fight Aeolus.

I have also noticed that a lot of cars (usually hatch back like my Citroen C2) just stop suddenly at the back. The air just kind of clashes together like a judge's hammer on a bench instead of "pinching" the car forward. The front of the car parts the air which needs power, but the air gains some potential energy, and you can reclaim some of it from the back end.

Could not even semi trucks have some sort of fold out or inflatible pyramid where the back trailer doors are? It would tuck back in when in town to not make the vehicle longer than it needs to be.

Another thing that is confusing is an article on Wikipedia that lists drag coefficients but a hemisphere is displayed as a lower drag coefficient than a sphere. This would seem to be wrong to me unless the hemisphere is side on - but there was an arrow pointing to the round side so I'm assuming that was faced into the wind.

Another weird quirk is whenever a car company puts out an "eco" car which has the aerodynamics of a shed. Which do you think has the most drag? A Smart Fortwo or a McLaren F1? Is there a speed at which the McLaren F1 would get better fuel economy?
1) Hucho wrote in his textbook that, a 30% drag reduction would essentially produce a 10% increase in terminal velocity.
2) And rather than engineer more powerful ( heavier) brakes, the automaker would limit top speed with over-driven gearing, which would not allow the engine to reach torque and power of it's engine map, capable of exceeding a safe target velocity, while allowing the better mpg.
3) The abrupt truncation of the car's body dates to the mid-1930s research of Fachsenfeld and Kamm at the FKFS, next door to Daimler-Benz. One simply takes the streamline body of the long-tail and chop off the tail wherever one likes, to satisfy ' practicality.'
4) For the distance allowed by the existing amount of tail, the aft-body airflow is provided a deceleration ramp, on which velocity is traded for increasing static pressure.
5) Wherever the tail is chopped, it remains at the highest possible pressure for that particular length.
6) This higher pressure imparts a higher base pressure, lower pressure drag, and lower overall drag. This IS by definition, 'STREAMLINING.'
7) The Fachsenfeld' inflatable semi-trailer tail dates to circa 1935. It uses exactly the mechanism mentioned above to deliver a tractor trailer with a drag coefficient potential below Cd 0.20. The efficacy of the technology has been demonstrated over and over again. I've used it myself.
8) Be careful of Wikipedia. None of there material is vetted, as one might imagine, as far as scientific rigor is concerned. Whatever drag table presented the drag coefficients you posted lacked caveats, with respect to Reynolds number effects. As a smooth sphere, a ballistic cannonball, will have a fraction of the drag of that of a ping-pong ball, all else being equal.
9) As to the Smart Fortwo vs McLaren F1, ultimate fuel economy would include many metrics beyond that of aerodynamics. I've never seen a drag coefficient published by McLaren for the F1. Nor it's frontal area. At Cd 0.38 for the Smart Fortwo, and it's frontal area, it may be possible that the two cars are aerodynamically identical in CdA. If at same weight, rolling resistance, power, and mechanical efficiency, they might be of equal mpg. Road velocity would not be a factor.
Photobucket album:
  Reply With Quote