Drag effects at low speeds?

[aerohead]

Originally Posted by examorph

wow thank you for the great paper!

Now that I am looking at cars such as the NUNA and Honda dream 2 I am starting to wonder, is the large surface area needed if the car is not using solar panel's, would reducing this area give an even lower CD? I ask because although the car I am designing is ran from an electric motor, solar panels or any additional power supply to the car battery is not allowed.

If you remove the photovoltaic arrays,these cars resemble Paul Jaray's 'pumpkin seed' of 1921 or wing sections.Jaray's car was resurrected by AeroVironment for the GM Sunraycer of 1987.With wheel fairings,Sunraycer produced Cd 0.089 as a wind tunnel scale model.With removal of the PV array,the wetted area of the car would be reduced,cutting skin friction drag for lower overall drag.

[examorph]

Hello again

I used the solar cars as a template and made a few little modifications so that the car still follows the rules and managed to get a drag coefficient of 0.38 which is a lot better than the 0.6 what I was getting from my previous designs but the only thing I am not sure about is that to get this drag coefficient I had to use more material for the car, I am not sure how much more weight is added yet because I have not yet finished the frame for the car but it is an estimated 1m x 1m sheet metal and about 2m of 30x30 box section, do you guys think its worth adding this material for a decrease of about 0.2 drag coefficient, I know I have not given you much to go off of as I yet do not have the weight values but could you please try estimate.

Thanks.

[drmiller100]

Lets not forget what the definition of drag coefficient is.

if you compare .6 to .4, the .4 is a 33 percent improvement. Or, the lower number will burn 33 percent less gasoline at speed than the bigger number.

it would take a LOT of weight at 60 mph to make that much difference.

On the order of thousands of pounds for most cars.

Let me try to state it another way. For a given frontal area, if you want 33 percent better mileage, you can drop drop the drag coefficient from .6 to .4 or you can cut the weight of the car by 2/3 or more.

I'd have to go use the calculator, but it is entirely possible you cannot raise your MPG by 1/3 at 60 mph by weight reduction.

[examorph]

Would the above post also hold true for a vehicle that travels at speeds of about 25-30mph and weights very little (can probably lift it up with one hand)?

[AndreLee]

I don't think so, its all about the engine efficiency and weight factor doesn't affect a lot. Anyway, the concept of aero-vironment seems to be very tactical, may lead to any evolution in this field.

[freebeard]

examorph wrote:

the size of the vehicle is only about 1.5m length x 0.8m width and 0.5m high.

~20" high? Is it a drone?

4 wheels required; and engine and not a motor; what class are you running in?

electrathonamerica

Plug 'Electrathon' into Google images (I'm not sure how to post a link to a search on that page, it wants to call out my web browser (Seamonkey) in the resulting URL). In the 2nd row 'electra3,GIF' @home.earthlink.net and 'electrathon.JPG' @hcps.org show a commercial fiberglass body and plexiglass canopy produced by Mark Murphy in Creswell, OR,USA.

To hit that 1.5m length you might need to make it a Kammback.

[freebeard]

Maybe something like this?

British Racing Green: Bentley Builds an Electric Car | Autopia | Wired.com

[aerohead]

Originally Posted by examorph

Would the above post also hold true for a vehicle that travels at speeds of about 25-30mph and weights very little (can probably lift it up with one hand)?

If you're driving on a track at a constant velocity you'll have your aero drag,rolling resistance,some curve resistance (which you can't do much about),and maybe some aero losses as a function of wind if the track is not sheltered.
*Of course your aero will be a function of frontal area and Cd.You want to minimize both as best you can
*Rolling resistance will revolve around your tire choice,inflation pressure (rule book?) and mass of vehicle and driver.
*Your battery range will be governed by the Road Load of aero and R-R at your given speed.
If you can cut weight do it.R-R will be a significant fraction of your overall load at low speeds.At higher velocity the aero would dominate (it;s 85% of drag with Honda's Dream 2 solar car at highway velocity).
*Your pit crew will be as important as the car if not more so.'Pre-race' the car all you can,and have no surprises come the day of the actual event.

[examorph]

Thank you for the very helpful information, the pit crew trials idea is great! and also to the other poster, thank you for the link.

So if weight is the main priority should I not worry about chasing a very low Cd? I have managed to achieve 0.38 but going lower is taking a lot of time designing and testing in the tunnel do you think its worth it?

Also, if anyone knows of any 4 wheel vehicles that have gone to Cd's of as low as 0.2 or even 0.1 could you please tell me the vehicle details.

Thanks

[aerohead]

Originally Posted by examorph

Thank you for the very helpful information, the pit crew trials idea is great! and also to the other poster, thank you for the link.

So if weight is the main priority should I not worry about chasing a very low Cd? I have managed to achieve 0.38 but going lower is taking a lot of time designing and testing in the tunnel do you think its worth it?

Also, if anyone knows of any 4 wheel vehicles that have gone to Cd's of as low as 0.2 or even 0.1 could you please tell me the vehicle details.

Thanks

The 1987 GM Sunraycer registered Cd 0.089 as a wind tunnel model when the wheel fairings were on.Without them,in full-scale,the actual race car registered Cd 0.12.
George Washington University reported that the wheel fairings on their Sunforce-1 car reduced drag by 34.48%.
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The 1996 HONDA Dream-2, 4-wheel conversion registered Cd 0.10 with CFD.
It has a frontal area of 12.269 square-feet (US),and a Drag Factor,CdA = 1.2269 square-feet.
The Dream is 5.788 times as long as it is tall,with a curb weight of 187.33 kg (413-pounds ).Top speed- 80.778 mph.
At 100 km/h (62.13 mph ) the Dream -2, 4-wheeler required 5.26 kW (7.062-hp) of power to overcome aero and R-R forces.
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The Dream,as a 4-wheeler would be a very good car to mimic.Just remove the PV array to reduce it down to a 'tadpole'/'pumpkin seed' and look for less than Cd 0.10.