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Old 01-29-2008, 11:43 PM   #1 (permalink)
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Aerodynamics Seminar # 6 - by Phil Knox

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 sixth in a series which I'm reproducing here with permission.

Go to: Aerodynamics Seminar index


Aerodynamics Seminar # 6 - by Phil Knox

(Note: Phil wrote this preface to #6 in a separate message; I think it's worth posting along with the #6 info. - Darin)


Hello all. As my Tuesday departure deadline approaches much remains to do. I have been collating materials for the "math" section of the seminars. It looks like it will be after I return from New Mexico before I can post the formulas I wanted to share.

There is much to complete on the T-100 and after I leave here tonight,its off to HOME DEPOT for more supplies and work into the night. Hurricane Rita could drop 4-5 inches on us and I have no idea what kind of "work" conditions I will have for fabricating. It was 102-degrees today,so again I'll cook well into the night.

The formulas,once posted,will allow everyone to calculate aerodynamic loads for any vehicle,knowing a few perameters. You'll be able to estimate road load horsepower for a vehicle at various conditions.With your own mileage numbers you'll be able to calculate your brake specific fuel consumption,estimate top speeds,fuel economy,range etc.

A pocket calculator will be handy.There is no trigonometry or calculus.Its all pretty much multiplication,division,and working with percentages.We'll walk through some examples together.No tests!!!!!!!!!!!!

If you do not presently have a "baseline" for your vehicle,you'll want to get a simple notepad for your vehicle and something to write with.I record: date, odometer reading, miles driven, fuel price, fuel cost, gallons, mpg, station,and any notes about modifications to vehicle, weather,etc.

By tracking mileage I can establish what the vehicle does with no modifications.When I make a change and see different results,I can use the formulas to quantify what if anything I've accomplished. If the vehicle performance changes for no reason,I can consider a tuneup or whatever. The main thing is "write stuff down!" If you know where you are and want to investigate if a modification might make sense, you can do it on paper and see if you want to proceed.

Long ago I wrote a simple program in BASIC and witha few keystrokes,could race through some calculations. Technology asside,for the day to day world we are basically looking at odometers and fuel pump readings. If you have an electric,you'll be working with kilowatts, power transfer efficiency, motor efficiency.Then,once your at the drive wheels, your down to rolling resistance and aerodynamic drag like everything else. If anyone has any specific questions, I'll try and get something for you.

When your looking at your car,think of a sailfish or perigrine falcon. Think of bottlenose dolphin and penguins. I was in a category-5 hurricane today and thought little of it. At 150-mph the Piper Cherokee I was flying in was right at home with this kind of wind loading. All the ample radii and gently sloping surfaces of the Cherokees air frame gave the air nothing to grab hold of.

As John G. mentioned in his blog,If I encounter headwinds on my trek next week, I'll easily be driving in a Class-1 hurricane. The aero mods to the T-100 hopefully will allow me to pass without incident.

I won't worry about my home as it is also aerodynamic, a ferro-cement dome which a tornado would shy away from and tidal surge,while wet,would eventually abate and leave me relatively unscathed.

Wish me well and I'll catch up in a couple of weeks, Phil.

Seminar # 6
(Note: Where Phil refers to photos/images in his text, I intend to come back and post them once all the Seminars are online. - Darin)

Hellow all. I had hoped to post large,neat,formulas in the photo section of the site. I think everyone needs these sooner than later so here goes.

The horsepower it takes at the drivewheel of your vehicle to overcome aerodynamic drag can be estimated by the formula
HP =V/375 [ 0.00256 X Cd X A X (V squared)]
where V= speed in miles per hour, Cd is your drag coefficient,A= frontal area of your vehicle,and (V squared) is your speed times itself.

Frontal area can be approximated by multiplying your vehicles width times its height,times 0.84. Frontal area is in square feet, so as dimensions are usually given in inches,divide the result by 144 to get square feet.

After you get the result,you can obtain flywheel horsepower by dividing your HP by 0.95 which yields Brake horsepower(Bhp).

These numbers are for standard barometric pressure, no wind, dry road, no more than 0.5% grade, 60-degrees F.

If you don't have frontal area,estimate it. Plug in your drag coefficient,and speed,multiply everything inside the parenthesis together,then multiply all that by your speed again and finally,divide by 375.

Project MPGiata! Mods for getting 50+ MPG from a 1990 Miata
Honda mods: Ecomodding my $800 Honda Fit 5-speed beater
Mitsu mods: 70 MPG in my ecomodded, dirt cheap, 3-cylinder Mirage.
Ecodriving test: Manual vs. automatic transmission MPG showdown

has launched a forum for the efficient new Mitsubishi Mirage - fuel efficiency info for Geo Metro owners - electric car conversion on a beer budget
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