This post was originally written by Phil Knox (aerohead), and it first appeared on the MaxMPG group. Phil has done a lot of work educating the masses about the critical role aerodynamics play in efficiency, and has spurred many in the DIY crowd to take matters into their own hands.
This is the first in a series which I'm reproducing here with permission.
Go to: Aerodynamics Seminar Index
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Aerodynamics Seminar #1 - by Phil Knox
Since we all can't sit down together,its hard to assess everyones background and get on the same page, aerodynamically speaking. Had public education not dropped the ball,we wouldn't be on an obscure website trying to fill in the blanks,and self-educate ourselves on a topic which quite literally effects all people,everywhere.
The crux of the matter is that we all live in the troposhere at the bottom of our planets ocean of air. Air as measured at a arbitrary pressure and temperature standard,weighs about 0.075 pounds per cubic foot.A 22-cubic foot refrigerator (empty) holds about a pound of air.
Since we are three dimensional,we occupy space,and anytime we move,we displace atmosphere.It takes work to do this. The power required to do work is denoted in the U.S. by HORSEPOWER. If you lift 550-pounds,one foot,in one minute,you have used one horsepower. If your electric car expends 746-Watts to do a task,it has used one horsepower.
When moving through the air,we are displacing a volume of air which has mass,in some slice of time.Our cars,when they travel,must tunnel through a volume of air which is a function of their FRONTAL AREA. When you stand at a distance in front of your car and think about the area of its height and width,you are viewing its frontal area.
A simple example would be a van,6-feet tall,and 6-feet wide.No curves,all sharp corners,lets ignore the wheels for simplicity.Your vans frontal area is 6-feet times 6-feet,or,36-square feet.For every mile our van goes,it punches a hole through the air that is 36-feet by 5,280 feet.Thats 190,080 cubic feet of air.If we multiply 190,080 by 0.075 we get the mass of the air we tunneled through,which is 14,256 pounds of air.Dividing by 2,000 pounds/ton,we get 7.128 tons of air moved for every mile driven.
How quickly we move these tons and how efficiently we move them,determines how much fuel,battery,fuel cell etc. to do it.For the current state of the art for auto design,we stand to gain up to 35% better range from our power source just by changing the shape of our cars,trucks,and SUVs.If we're happy with the size (frontal area)of our vehicle,then its shape will be the primary area of attention for improving its highway mileage.
Going back to the
EVWORLD article,we consider that a hen egg and sugar cube have identical air resistance,even though the hen egg is twenty times larger than the cube, its shape is our clue as to how this virtual leviathan can move through the air with no more resistance than the tiny cube.The secrets in the shape.
Considering the bottlenose dolphin,the sailfish, canoes, kayaks, aircraft fuselage,submarine hulls, land speed record cars, all high-performers,we begin to gain insight into how nature and man work with and not against our tropospheric environment.
Since I'm running down the clock,I'll close with something to think about.If I can insulate my home against the effects of hot and cold, thereby lowering my monthly energy bill, is it possible to "insulate" my car against the effects of the troposphere in which it is emmersed? Thermal insulation has a numerical performance rating system from which one can calculate potential savings. Next time we'll see that aerodynamics offers a very similar rating system for which to anticipate change.
See ya down the road, Phil.
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