Drag effects at low speeds?
 

Me and a group of people are making a small car which is powered by an electric motor for a race, I am having a hard time deciding on a design for the bodywork, after researching I thought the best design would be a tear drop since many say it has the lowest drag coefficient of any natural shape, what are your thoughts on this?

Some extra information: The cars maximum speed will probably not go over 30mph, the front and rear wheels have to be in-line.

An additional question, we are using bike tyres for the wheels on the vehicle, is it worth covering these up or at such low speeds will the drag not be noticeable?

Think of riding a bicycle at 20mph; let alone at 30mph. Drag is significant, and the churning wheels also add drag. Look at the performance of a velomobile vs a bicycle. The only time when the weight of the body is a greater penalty than the lower drag is up long steep hills.

You can also run the numbers yourself: aero & rolling drag calculator

Look up "the template" thread here on EM. I would imagine a really good and not too difficult design would be a version of the 1930s Schlorwagen's shape, which resembles a "tear-drop" a little, adapted with a trailing edge that is a bit more like the template, which also resembles the tear drop a little. Get a copy of Hucho's book on the "Aerodynamics of Road Vehicles" through your library's inter library loan program, or buy it ($$$). For fun and design ideas, spend some time on the thread called "interesting aerodynamic vehicles." It features pics of classic and little known vehicles almost entirely. The Schorwagen is on it too. Good luck!

Instead of the Schlorwagen I would imagine something like this:
http://landspeedrecordrocketcar.com/.../Goldenrod.jpg

But following the template where possible. Full boat tail. Full under body covering. Enclosed wheels (unless you'll be making tight turns)

Definitely.
The spokes add a lot of drag.
On many Velomobiles, the spokes get covered with a circular cloth so the shape is smooth.

Keep weight low, and use narrow bike race wheels and tyres pumped up to their advertised limit for low rolling resistance.

I would look to the HPV crowd for the best examples of things to copy.

Yeah, having to power something yourself tends to make people more inventive at reducing their workload ;)

I have two running HPV's
This one takes about 100 watts of power at 30 mph
http://www.veloliner.com/nissan07/_dsc9052_sm.jpg

This one is a tricycle.
http://www.veloliner.com/nissan07/_dsc8769_sm.jpg

There are lots of photos of my liners at: veloliner.com

Even a simlpe touring fairing on my city bike improved my cruise speed from 11 mph to 14 mph. Every bit helps.

best
 

The lowest drag forms are 'streamlined bodies of revolution.'Or 'teardrops.'
'Half-bodies',derived from streamlined bodies of revolution produce drag coefficients on the order of Cd 0.08 in ground proximity.When wheels are added the drag goes up to Cd 0.12-.13.With the narrow bicycle tires you might see on the order of Cd 0.10 if wheel fairings were employed.
If your team is attempting to stretch the range of the battery pack the 'teardrop' form could help with that.
In the Sunrayce '95,a 'torpedo'-shaped 'TNE-3',3-wheeler from Northern Essex Community College was achieving 150 mile range at 55-mph (US statute miles) on a single charge,equivalent to over 350 mpg.
This car was about 3.84 times as long as it's body diameter with Cd 0.10.It would be worth checking out.

what does the race track look like?

Will you accelrate to 30 mph, then maintain forever, or is there start, slow down, speed up?

Below 30 mph, weight often has more drag then wind. So, minimum weight is obviously a BIG goal.

From there, whether to add the weight of the body or not depends on the course. If you are usually below 15 mph, it might be tough to justify a body.

Bodyless, below 30 mph, they will LOSE.

miller: I really doubt if the weight is that important on track bike. I am not trying to argue but to point out my perspective as a 60+ year old cyclist.

Aero is everything except for power available. On my upright bike, a 17 pound carbon Trek road bike, I can push it to about 28 mph for a short burst. but am comfortable at about 18-20 mph on a smooth road with little wind. On a simple, mostly faired recumbent (weight about 36pounds), I am comfortable at 22-25 and can go about 35-37 in a sprint.
http://veloliner.com/waterford06/DSCN0547_sm.JPG

If you have the option, put pedals in your electric cart. You will win.
I remember a solar contest in Japan in 1994. A VARNA was sent over with a small solar panel on the top and it kicked every other solar bikes butt. The rider was Myhee. He had just set the 24 hour record of 633 miles and was hot. I rode with Myhee in Michigan 25 hour in 2004 and spent a lot of the ride talking and hearing about his exploits. At Michigan he carried a bugle with him and every time I saw him during the night I was rewarded with a tune.

I recommend these two books:
High-Tech Cycling by Edmond Burke
The Leading Edge by Goro Tamai
I make my bike fairings from either 4 mm Coroplast or 1" hex mesh bonded between two layers of 5 mil plastic supported by angle aluminum and hard nylon tubing.

Wow, so many replies and some very good information! Thank you, now, I have a new question, length vs height vs width?
I have changed some things around on the car to reduce the length of it by almost half! but it has added a bit to the width but the material used to compensate for the width is no where as near as much as the material saved on length, also I have took the advice the majority of you have given and covered the wheels up.
so in conclusion, is it worth increasing the height and width to compensate for the reduction in length or should I really not worry as much about length and try keep the car as slim and low down in height as possible?

Also, one last question, is there any disadvantages of allowing the airflow under the vehicle?

new question
 

*For an 'enclosed-wheel' car,the air 'see's the car and it's reflected mirror image below the ground.
*For this 'ground-reflection' situation the air likes a car which is 5 times as long as it's height,or 2.5:1 for the body and reflection.This is demonstrated with the Aerodynamic Streamlining Template.
*If you're doing an exposed wheel design,you might want to treat the body as an aerodynamic section instead of a body of revolution,where the majority of air goes around the body.With sections,the lowest drag is reached when the length of the body is about 3.8 times the body width,just as with the TNE-3 car mentioned earlier.
*With faired-in bicycle wheels you're looking at Cd 0.10.
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*you're trying to minimize frontal area,reduce unnecessary body surface area (and it's friction),and slip in with the lowest coefficient of aerodynamic drag by shrink-wrapping the body around the driver and mechanicals while respecting aerodynamic minimums for attached flow.
*length of the body will be dictated by whether you're going to do a streamline body of revolution or streamline section.
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As to the under body flow,it appears that some of the 'special' shapes fair no better than bodies with 'normal' ground clearance (as the TNE-3 car).
*You've probably seen NUON's NUNA series of solar racers.They have exceedingly low drag coefficients.For a battery-only EV this shape might not have the interior packaging capabilities you'd need.You should check these cars out if you haven't already done so.
Sounds like a really fun project!:)

Hello, I am back, this project has not been started yet and I have been very busy lately so have had no time to do much extra work on it but now I finally have a bit more time to spend thinking about how I should design the car.
I have modelled the car and have ran some wind tunnel tests on it and got some results but dont really know how to read them....the wind tunnel test results show me the drag coeficient (what I wanted) but this is on a graph over time, as time pass's the drag coeficient changes, the velocity of the air going over the vehicle also changes, this is strange because I set the value for the air speed and it should be constant, so I dont understand why the drag coeficient and air speeds are changing, any ideas?

The graph I see is the exact same graph as the one shown at the bottom of this picture:

http://gfxspeak.com/wp-content/uploa...6541711650.jpg

and here is the 2d slice air flow image I see constantly changing with time:

http://gfxspeak.com/wp-content/uploa...6541170698.jpg

Please note, these images were from google and are not of the car I have designed, I will get some images of that soon.

Need 5 posts

1 more post

edit: sorry about that, please continue from 2 posts up.

Thanks

Cd vs Velocity
 

For vehicles the size of an automobile,their length as associated with velocity renders a transition to turbulent boundary layer at around 20-mph (32 km/h).The air around a road vehicle is considered turbulent itself and Reynolds number acts accordingly.
For this reason,coefficients of aerodynamic drag are constant above 20 mph and do not vary.So I'm as confused as you are about some of the results you are getting.
CFD modelling for 3-Dimensional flow requires the complete Navier-Stokes solution which may not be included in your software package.Not long ago,it would require supercomputers to solve these 3-D 'problems.'

Thank you for the reply, I spent some more time playing around with the software and found that I was wrong, the speed the vehicle is going is not constant, it is accelerating or slowing down from its previous speed in real time meaning it takes time for the speed to become the one you tell it.

Now that I have a drag coefficient I am again confused, I am getting low numbers for the car I have modelled and similar ones I modelled in the past, the drag coefficients are ranging from 0.5-0.7, from comparing them to real size vehicles these are very very small numbers but it's understandable because the vehicle size is also a lot smaller, the size of the vehicle is only about 1.5m length x 0.8m width and 0.5m high.

The two questions I have are, do you think these drag coefficients are correct for the vehicle size given and if they are correct wouldn't that mean that the drag coefficient of this vehicle is so low that the motor running it will not be able to get to the speeds required for air resistance to slow the vehicle down allot. If this is also true then wouldn't that mean that I shouldn't really have to worry about aerodynamics and my main goal should be making the vehicle as light as possible?

Coefficients by definition don't consider size.

I am confused now, because I thought that the larger the vehicle the larger the drag therefore the larger the coefficient?

Drag increases with size because the frontal area increases. Total drag = Cd * A (projected / frontal area).

The coefficient of drag is the same for two identical shapes regardless of whether one is larger than the other.

So is a change of about 0.1-0.3 in the coefficient of drag a big change?

Define "big". :)

(Yes, that probably qualifies as big.)

You can view the impact of changes by comparing results from the aero & rolling drag calculator

thank you for the calculator, very helpful :)

New question :) , I asked this before but after researching more I ask again, what shape should I make the car? I ask this again because I made a tear drop shape for a body around the car and tested it in the wind tunnel, the drag coefficient was about 0.5, I read on other websites that it is possible to achieve CD's of as low as 0.07! so I think I could probably do a little better than 0.5 if I change the shape, I saw images of some of the lowest CD vehicles, here is one that looked interesting:

http://www.actiflow.com/images/nuna5.jpg
http://www.techniche-europe.com/files/Nuna5_Car.jpg

What do you guys think of this? do you think I should spend my time modelling it and trying it in the wind tunnel or do you think there are better designs?

Also the car has to have 4 wheels which may cause problems with this design.

better?
 

I believe that the NUNA cars are at the low end of the drag spectrum.I think they are all 3-wheelers though.
In 1996,the Honda Dream-2 solar car team published a paper for the Japanese Society of Automotive Engineers about some research they did.It was all CFD as they ran out of time before they could put their 'car' in the tunnel.
Anyway,their Cd 0.10, 3-wheel Dream-2 was modified to 4-wheels and I think they were able to maintain around Cd 0.10.
You might check out this paper.Cd 0.10 is pretty good.They show front and side elevation views as well as the plan view.

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.

PVs
 

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.

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.

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.

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)?

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.

examorph wrote:
~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. :)

Maybe something like this?

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

speeds
 

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.

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

as low as
 

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.