Interesting Slide Show about Aerodynamics and Car Design

I hate when websites split article text across so many pages. I would have liked photos that showed the side views better.

 

I agree with that on the highway but it is not true for low speed or stop & go traffic.

EPA's combined MPG counts 55% city and 45% highway figures.

 

I wonder at what speed Prius' aerodynamic was optimized for. I think 55 mph for Gen2 and 65 mph for Gen3.

 

This is not a valid question. Assuming we avoid transonic and supersonic speeds, which is a safe assumption for the Prius, aerodynamic optimization is the same across all speeds.

The speed issue is that aerodynamic drag goes up by the square of the speed, and power required to move against this drag goes up by the cube of the speed. This is why aerodynamic drag is important at highway speeds, but inconsequential around town.

Tom

 

Tom, if you are going to drive like an old lady....

 

How about 40 mph vs. 112 mph (Prius top speed)? Would the drag coefficient of 0.25 Cd hold the same at both of those speeds?

 

unless you have some active features the Cd is mainly a constant.

Remember energy (think gallons of gas to drive a distance under drag) is proportional to CdA*v*v*d. Where A is the effective Frontal Area and v is the velocity, d is distance.

Power (think horsepower to over come drag) is is proportional to CdA*v*v*v. So speeds are important for tuning the engine for efficiency, power.

If you were routinely going over 100, you may even increase drag to give more control. You would also lower the car which reduces effective frontal area.

 

I wonder why the article didn't mention the cab forward design in the intrepid, the camaro and all the recent minivans.

 

Does this hold true for factors other than drag, such as lift? Or are the two related closely enough that for all intents and purposes at Prius speeds, it doesn't matter?

 

Cd isn't a constant, although for the 40 to 112mph range in a car it is close enough to being constant.

Rolling resistance isn't constant or linear with speed either. It goes further from theoretical than Cd does.

You can find a lot of information on Cd vs speed using Google, unfortunately for the average person it's usually plotted as Cd vs Reynolds number. The interesting stuff happens above normal automotive speeds.

 

It would also be interesting to see some of the many 'slippery' cars that were designed without benefit of wind tunnel. Like a 1961 Jaguar E-Type, a 1933 Dymaxion car, and a 1930s Tatra.

 

There are problems with the article, too many generalities and not enough numbers.

The Chrysler Airflow, actually had a worse CD than a more ordinary looking model of Lincoln, but at about 0.56 it was still a lot better than most cars of the era

The Ferrari F1 car isn't aerodynamic in the conventional sense. It has about the same Cd as a parachute and worse than a pick-up truck.. What it does do well is generate a lot of down force.

 

The wings used to push down for better traction are not needed to counteract lift on the prius. It is front wheel drive and the front of the car is pushed down by aerodynamics, the bottom is smoothed to reduce lift. The standard wheels and tires would likely be changed out for better traction at higher speeds (energy is only proportional to speed, not speed squared) before the aerodynamics needed changing. More tire area though does impact city, that is slower speed efficiency.

I do get surprising amount of wind noise for such a low cd car. They can probably improve aerodynamics to reduce the sources of this noise, or maybe add a little more sound deadening material.

 

Tatra T77A was 0.21, Dymaxion 0.25 according to Automobile drag coefficient - Wikipedia, the free encyclopedia.

The E type wasn't all that good, pretty poor by modern sedan standards. It'sCd should be easy to find

 

Yes, for all practical purposes.

Yes over normal speeds. At very low speeds the viscosity of the air becomes a bigger factor. At transonic speeds the compressibility of air changes the way it behaves, and shock waves start to form. At normal speeds air behaves like an incompressible fluid.

Tom

 

R

And for low power vehicles, such as bicycles, air resistance is even more of a factor, and aerodynamics yet more significant. Speed records need to be adjusted for altitude, and are only considered to be official under 'legal wind' conditions. (Effectively, no wind at all.)

 

I appreciate the author's attempt to focus on aerodynamics, but it fascinates me how little he analyzes the aerodynamics. Take the Prius for example. When the Gen III first came out, we heard all about all the little things Toyota did to the shape to channel the airflow around the wheels and the explanation of the high-hatch rear end.

It would be nice if he has touched bases on some of these topics, such as the very slanted windshield, short hood, very slanted rear window, short trunk or no trunk, channeling the air around the wheels and placing panels on the bottom of the vehicle (he slightly touched bases on the last two, but barely). Here are some discussions on this subject:
http://priuschat.com/forums/prius-h...onda-civic-hybrid-powertrain.html#post1344773

http://priuschat.com/forums/prius-c...car-in-southern-california-2.html#post1345128

 

Those fake scoops at the lower corners of the front end, where the foglights are positioned, are real scoops on higher speed cars. Apparently, air captured by the scoops is forced into the front wheel wells then out along the length of the car. The air is shaped into a layer that hugs the car the way to the back and, somehow, increases the aerodynamic efficiency of the car.

Has anyone punched holes in under the fender behind those scoops so air could be forced into them? If this trick works on higher-speed cars, why not on a Prius at freeway speeds?

 

#1 - this thread is roughly 1½ decades old. Several of the contributions are from no longer active members. In fact the Gen III hadn't been released yet.
#2 - chopping up an access through the inner fender well in an attempt to 'improve' dynamics would more likely be deleterious than helpful, & that's why the build on the various models are done the way they are. It's best to give the design engineers Credit due and not presume owners could do a better job.