Coefficient of Drag

At different speeds the best aerodynamic shape for any given speed may not be similar. Most pronounced between sub sonic and supersonic speeds. The Boxfish car that was referred to is an example, and dang! you beat me to it! Then there is designing for downforce...

Anyway, as regards aerodynamics of the Prius... We use RainX on our vehicles. It's kinda nice not to have to use the wipers at highway speeds when it's raining. So OK, we get the Prius and I fill the washer tank with RainX and run the washer. Take it out onto the highway and presto, nothing. It doesn't work, the rain sticks. Run the washer again, and the rain still sticks to the glass. RainX requires a certain amount of wind stream against the glass for it to work. At normal highway speeds the Prius lacks this. It appears the Prius windshield rides in a "bubble" of calm air created by the shape of the car, probably because of the bulbous nose. Kinda neat to see the aerodynamics at work, but we still like to use RainX rather than run the wipers and can't.

Too, has anyone else noticed that at normal highway speeds the rear glass stays dry? Not the little vertical window, but the long sloping glass. No suction pulling raindrops onto it near as I can figure. I've actually noticed the rear glass drying off as I drive in a downpour, at long as I maintain speed. This is the first car I've noticed that on.

 

<div class='quotetop'>QUOTE(Brian K @ Jun 10 2006, 12:27 AM) [snapback]269043[/snapback]</div>

YES.

At about 40MPH and above the rain does not hit the rear window.

Noticed that this past week during all of the rain. The first vehicle I have ever owned that did that.

But, I wasn't able to forgo the front wipers even traveling the NJ Turnpike in heavy rain.

 

Well... I used to drive a vehicle that has a Cd to put all those discussed here to shame. Lowest Cd of any production car ever put on the road, I'm told. 0.19. Accomplished by having NO air intake holes on the front (not radiators needed, as there is no engine), having a rear (non-literal) axel that was 11" narrower than the front, having a TOTALLY flat belly pan (no access needed under there!) and basically taking the tear-drop theme to the extreme. To most it looked funky. To those of us who got to live with them, they became very attractive indeed. Took a bit less than 2,000 Watts to keep it going at 65mph. And it would coast for freaking EVER.

 

<div class='quotetop'>QUOTE(TonyPSchaefer @ Sep 27 2004, 07:06 PM) [snapback]42058[/snapback]</div>

mayb this is not really related, but i feel driving the prius is alot "lighter" than other cars, mayb only b/c other cars ive driven are heavy, SUV,

and my friend say the prius look like a rabbit, somewat similar to your wife hahaha

 

<div class='quotetop'>QUOTE(Brian K @ Jun 9 2006, 11:27 PM) [snapback]269043[/snapback]</div>

If you apply RainX or similar using the old fashioned way, buffing it in by hand with the bottle, it works better. For best results, do it twice. I use the RainX washer fluid also, it makes it last longer on the window.

I agree it doesn't work as well as my last car, but still greatly improves visability.

 

<div class='quotetop'>QUOTE(Drift Motion @ Jun 10 2006, 02:21 PM) [snapback]269285[/snapback]</div>

My wife looks NOTHING like a Prius OR a rabbit. More like an AMC Pacer.

 

<div class='quotetop'>QUOTE(larkinmj @ Jun 8 2006, 09:08 AM) [snapback]267967[/snapback]</div>

Exactly! Actually its more of a "dolphin" shape which starts going towards the more efficient aero/hydro dynamic shapes.

 

<div class='quotetop'>QUOTE(darelldd @ Jun 10 2006, 03:35 PM) [snapback]269255[/snapback]</div>

Looking forward to seeing the movie!

 

<div class='quotetop'>QUOTE(larkinmj @ Jun 10 2006, 07:12 PM) [snapback]269368[/snapback]</div>

I'll be doing a Q&A session after the premier of it in my town. Should be fun. We'll probably have about 10 EVs parked in front of the theater and offer rides after as well.

So back to the topic... Is there a forumula or unit that defines frontal area/Cd that would allow us to compare any vehicle as far as air drag is concerned? Obviously Cd doesn't tell us all, so that can be misleading. But I have yet to hear of a "automobile air friction" unit. Anybody? Anybody even know what I'm asking?

 

<div class='quotetop'>QUOTE(darelldd @ Jun 11 2006, 06:26 PM) [snapback]269664[/snapback]</div>

This stuff is vaguely familiar- I used to do some work in hydrodynamics, and a lot of the equations are the same. My texts are in my office- I'll see if I can look this up tomorrow.
I do know the standard measure of drag is Ca (Cd times area); since Cd is just a number, Ca is in units of area. It's the same for underwater bodies (Cd times cross-section area)