Aerodynamic Formula

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momec
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Aerodynamic Formula

Post by momec »

Help
Does anyone know the site that gives the calculation formula for aero drag versus frontal area, HP required etc.
I thought i'd saved it but can't find the sucker.
Thanks Chris
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PJQ2
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Post by PJQ2 »

G'day Chris,

You've probably already found what you need, but here's a couple of tidbits anyway. The Most Reverend Headgash accumulated truckloads of stuff on the aero topic. You there Rev?

By Paul Haney, easy to follow page on drag calcs.
http://www.insideracingtechnology.com/tech102drag.htm

Wikipedia have the maths class version.
http://en.wikipedia.org/wiki/Drag_equation

Mid-ninties streamliner from Sweden, built for the Falkenburg Speed Record Attempt.
On the Aerodynamic Overview section there's a PDF of over-my-head academic stuff (no basic formula, but interesting read).
http://www.geocities.com/MotorCity/2078/index.html
Regs,
PJQ
Velocity Science Laboratories
Dr Goggles
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In drag

Post by Dr Goggles »

.......I'm suss on the ability to calculate drag from frontal area alone.....in my view no science is better than bad science , that said no doubt there are some pretty accurate estimates but frontal area is but a part of it....some of the worst drag is happening BEHIND the body.......
...few understand what I'm trying to do , but they vastly outnumber those who understand why..
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Reverend Hedgash
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Aero

Post by Reverend Hedgash »

Dr Go is quite correct. Try racing your ute with a cover on the back and then without one and although the frontal area hasn't changed I am sure you'll notice a difference in drag due to the trailing shape.

This is where the Cd coefficient of drag comes in and should be for whatever vehicle you're racing between 0 and 1. (1 is a bluff body rough no aero)

The drag effect is fixed but the shape is not and the Cd changes with every aero mod that you do to your car. The best way outside a wind tunnel to work out the Cd of your car is to find the Cd of a similar shaped car I'm afraid... Size doesn't matter. That is what the cross sectional area covers, the scale of the shape being judged.

Therefore:


Assuming standard air pressure, drag due to aero is:


F = - speed x speed x frontal area x Cd x 0.5

(note this is speed squared not speed times 2!)


(Measured in metres and metres per second. Cd has no units)


The power required to overcome this is F x speed

Therefore


Power = - speed x speed x speed x frontal area x Cd x 0.5


Speed cubed!!! Well there lies the importance of it all. The drag gets radically more significant the faster you go.

This doesn't take into account other friction such as rolling friction etc. but can be used for getting a rough figure.


Mechanical horsepower — 0.746 kW (or 33,000 ft lb per min)


Reverend H+
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momec
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Post by momec »

Why thanks guys.
The further I hunted on the net the more complex the maths got.
Thanks Rev, I can understand your maths.
Seeya Chris

Yes I do plan to play with various aspects of the utes body before I push the crank out through the sump but this research is for my 2010 project :wink:
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gazza414
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Post by gazza414 »

Chris , the maths is the easy part...its putting in the correct numbers which are relevant to the application cannot be guessed.
As has been mentioned the A part is easy ( relatively) as can be done with a digital camera. The Cd cannot and changes depending on velocity and geometry ( signature ) at speed.
1 FAST HAYABUSA 217.443mph so far
9 Official Timeslips over 200mph
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DirtyDave
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Post by DirtyDave »

What would the DC diffrence be between a Busa and a Naked bike be ???
and shit is not the answer im looking for cause i know this already.. :shock: :roll: :shock:
As my drag racing mentality is damb obvious and we come from the just make more grunt theory. looking at some of the sites posted above i'm realising what i'm up against, but didn't figure is was so much of a disadvantage.
still i set out to run open class and that is what i'm going to do.
well just make more power johnny...
Don't Worry, It only seems Kinky the first time..
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Like a Brick

Post by gazza414 »

Dave, the shape and position of the rider in how he/she interacts with the bike governs final Cd.
Needless to say the difference between a naked bike and a brick at 200mph...well there would be much in it!!!

actually the brick might be quicker!!!!!!!

You'll need to play with ballast also Dave.....its a challenge
1 FAST HAYABUSA 217.443mph so far
9 Official Timeslips over 200mph
Very much the apprentice
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Reverend Hedgash
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Hayabusa coefficient of drag

Post by Reverend Hedgash »

Davy Dirt,

Here is an excellent article discussing the CdA of a Hayabusa from wind tunnel tests, its comparison to other bikes, and discussions about the aero of these bikes in general.

http://www.sportrider.com/tech/146_0106_aero/index.html


The Hayabusa Cd is worked out to be 0.561

Honda 1996 Grand Prix bike RS125 Cd 0.644

Depending on your naked bike it would go from anywhere above these figures to about 1.0 depending on how much crap is on the front.


As Gazza points out, riding style makes an impact and this article claims up to 15% depends on the crouch...

Also simple things like wearing tight clothes to prevent the balloon effect etc.

A lot of aero is actually common sense and a few rules applied well.

You must read the hilarious postscript, in fact I'll post it here:

"Eight years ago, when Kawasaki engineers were designing the original replacement for the ZX-11, the prototype resulting from wind tunnel testing was almost a carbon copy of the Hayabusa--only the company figured it was too ugly to sell. And a Kawasaki engineer has confirmed the ZX-12R was in fact "neutered" to a lower top speed in its final stages of development."

He forgot to mention that Hayabusas sound like sewing machines too when compared to Ben's Duke. Fast but.

Rev H+
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Dr Goggles
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me head hurts

Post by Dr Goggles »

nerd city!!!!!!! :lol: ..........anyone for a game of chess? :twisted:
...few understand what I'm trying to do , but they vastly outnumber those who understand why..
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Lynchy
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Post by Lynchy »

Chris

Sitting at work - a bit bored, obviously.... I found a general article on fuel economy that might be useful for people building cars and wondering how Cd works and how to aerodynamically clean up their car.... I also found a decent summary type article regarding aerodynamics on the autospeed website if you'd like a read.

How Drag Affects Mileage
When you consider aerodynamics from a fuel economy standpoint, you're primarily looking at coefficient of drag (known in the business as "Cd"). Essentially, this is how easily a vehicle moves through the air, though drag isn't the only factor that is considered. "There's more to aerodynamics than just drag," "There's downforce and lift. And there is yawing moment, which is basically when you're in a crosswind, how much does the vehicle get steered by the wind that is pushing on it? And then there's noise. So we try to look for all of those factors”

Volvo's Frasher says the force acting against a car by the air it moves is a function of:
Cd x Frontal Area x Density of Air x Speed Squared

Speed clearly is an important part of the equation. At stop-and-go speeds, drag isn't a big deal, but the faster you go, the more it matters. At 70 mph, you've got four times the force working against your vehicle that you have at 35 mph.

To put Cd changes in perspective, Frasher put some numbers to a hypothetical sedan. Our imaginary car has a curb weight of 3,527 pounds, a Cd of 0.30, a frontal area of 23.7 square feet and 9 pounds of rolling resistance for every 1,000 pounds of weight.

According to Frasher, "If we put a gas-burning engine in this car, expect reasonable performance and drive it on a combined driving cycle, we can expect to get 23.8 mpg…. Add 10 percent to the drag coefficient, we'll now get 23.3 mpg…. Take 10 percent from the drag coefficient, we'll now get 24.3 mpg."

Comparing Vehicles' Drag Numbers
It's easy to get a feel for drag numbers by comparing Volvo sedans. According to Frasher, during Volvo's boxy-but-safe era, a Cd of 0.36 for the 960 model was typical for a sedan. Today's Volvos have come a long way, with the much sleeker S80 coming in at just 0.28.

"Not too long ago, anything below 0.3 was considered a sports carlike silhouette," says Bill Kwong, Toyota's product communications administrator. Now, Toyota has several vehicles in the sub-0.3 range, including the Avalon and Camry at 0.28 and the Solara at 0.29.

Cars typically have a much lower Cd than pickups and SUVs, which sit higher, are bigger and have greater cooling needs. Cooling is a big deal, aerodynamically speaking, since it requires airflow into the vehicle through the radiator, which increases drag.

Steve Wegryn, manager of Ford Motor Co.'s aerodynamics department, says, "For trucks, we're anywhere in the range of 0.40 to 0.43, 0.44…. For cars, on the order of 0.30 to 0.34. And SUVs are somewhere in between 0.36…to…0.41."

How Automakers Improve Aerodynamics
While some shapes are inherently more aerodynamic than others, aerodynamicists and designers subtly shape every vehicle to reduce drag. "We look at all areas of the car that come in contact with the air. Upper surface shape, under floor, wheels and even cooling and engine bay," says Ian Anderton, aerodynamicist at the Jaguar Design Studio in Gaydon, England. Automakers fine-tune the way the air attaches to the vehicle's surface, and the way it leaves the rear end.

To improve Cd, designers may make the following changes:
• Round the edges of the front end
• Tune the grille and fascia openings
• Tune the wheel openings
• Place spats (small spoilers) in front of the tires to reduce turbulence
• Tune the size and shape of the outside mirrors and their attachment arms
• Reshape the water channel on the A-pillars
• Adjust the front fascia and air dam to reduce drag under the vehicle
• Add side skirts
• Tune the deck height, length and edge radius
• Install a rear spoiler
• Adjust the angle of the rear window
• Tuck up the exhaust system
• Use a diffuser to tune air coming off the underside
• Install "belly pans," underbody panels that cover components and smooth airflow
How to Improve the Aerodynamics of Your Car
Automakers spend a lot of time optimizing vehicles' aerodynamics. But is there anything we can do to improve — or at least not degrade — the aerodynamics of our own vehicles?

It turns out the biggest gains are to be found on pickups — not by dropping the tailgate (a common misconception), but by installing a tonneau cover. "A tonneau cover improves the aerodynamics dramatically — on all pickup trucks," according to Ford's Wegryn. "In general, a tonneau cover can provide a drag reduction of 2 to 7 percent, depending on cab style, box length and overall vehicle Cd. Average fuel economy improvement ranges from 0.1 to 0.3 mpg." From an aero standpoint, it doesn't make a difference if you choose a soft or hard cover.

You'll also improve aerodynamics by:
• Reducing the use of roof racks
• Rolling up your windows and turning on the air conditioner at higher speeds, typically above 35 mph
• Replacing a broken or missing front air dam
• Lowering your vehicle
• Running narrower tires
• Choosing smoother wheels (ideally, flush discs like those on vehicles trying to set land speed records)
You can reduce your vehicle's aerodynamics by:
• Lifting it — "an inch of increased ride height degrades the coefficient of drag by about 10 drag counts [.01]," says Wegryn.
• Adding wider tires
• Choosing more "open" wheel designs (although, for many owners, this advantage will be offset by the fact that "open" wheels promote better brake cooling)
• Installing a bug shield
• Adding a rear spoiler, in some cases
As a rule, an increase in noise is a sign of increased drag that is reducing your aerodynamics.
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Lynchy
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Post by Lynchy »

Chris

Yeah - still bored....monthly reports done....no fires to put out.....

I found the calculation website you are likely looking for. It is...

http://www.bgsoflex.com/auto.html there are many calculation pages accessible from here. The one you would be after is:

Aerodynamic and Rolling HP Loss Calculation

Sumner (a frequent contributor to this forum and potential Lake Gairdner racer) has links to this and many other useful websites at:

http://purplesagetradingpost.com/sumner ... elinks.htm

He even has a link to our DLRA website.... Hopefully he will visit our little lake and we can put a face to the website
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Greg Watters
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Bored?

Post by Greg Watters »

momec
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Post by momec »

Thanks more guys.

The thing I'm considering building is a Diahatsu Mira with probable Toyota V8 quad cam and single turbo.
I can see there are going to be more bellytanks than bellybuttons on the Lake in the next few years so a competion coupe might be just the ticket.

A Mira with stock roof height has 1/3 the frontal area of the XP ute
With the same HP I currantly use the Mira should run about 70 to 90mph quicker than the ute.

Comp coupe allows you to streach the wheelbase which would be essential to get any sort of stability. The total width of The Mira is only 1420mm

I would build full purpose built heavy chassis and comp coupe is the most liberal of all the "auto body based" classes
The basis on such a narrow track worries me.

Do you guys have any thoughts on stability and safety of such a design.
Chris
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DirtyDave
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Post by DirtyDave »

Well i might not have been around this salt game for long but i have been involved in alot of motorsport,
And people say i might be brave or stupid for racing a bike but there is no way you will strap me into a short norrow light racecar..
sorry C Hanlon..
Maybe :roll: a Nitroburnin Altered :roll:
Don't Worry, It only seems Kinky the first time..
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