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Old 01-28-2008, 09:28 PM   #1 (permalink)
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Aerodynamics Seminar #3 - 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 third in a series which I'm reproducing here with permission.

Go to: Aerodynamics Seminar Index


Aerodynamics Seminar # 3 - by Phil Knox

I've got about 50 minutes so I'll toss out some miscelania (sp?).

We've discussed that aero drag is defined by the size (frontal area) and shape (drag coefficient). At any speed considered,a change to either the size of a vehicle or its drag coefficient would alter its performance.

I'd like everyone to be acquainted with the significance speed (velocity) plays with drag, as it varies geometrically with speed, not arithmetically. If you double your speed, the power your powerplant must produce to overcome air drag does not double, it goes up eight times ( 8x !).

How can this be? Okay,consider that you hit the air twice as fast, twice as often,and your hitting twice as much of it. So your velocity affects the air in three ways,or VxVxV ( V cubed ).

In the formula for aerodynamic horsepower, which I'll post as a photograph ( because I don't know how to type it on this computer), you'll be considering AIR DENSITY,VELOCITY,FRONTAL AREA, Cd,to arrive at HP for a given speed.

The critical thing to remember is that speed costs money. A NASCAR race car can travel 50-mph with 15 horsepower. At race speeds,to go from 195 mph to 196 mph will take an additional horsepower just for a 1 mph increase in speed.

And to go back to Sean's question about those longitudinal fins on the tops of the race cars, They are "air fences".Their job comes to play if the driver gets the car sideways. At the speeds NASCAR races at,the cars become crude wings and will fly off the race track.The fences spoil the flow over the car,spoil the lift,and hopefully keep the racecar earthbound where the driver has some chance for control.

Also,at 200 mph,the NASCARS have much reserve power at 800-plus horsepower. The excess is used for passing acceleration. And at 195-200 mph, mileage for the MONTE CARLOs and TAURUS falls from 30-mpg to 4-mpg. A trip in a NASCAR today from Dallas to Austin,Texas at race speed would cost $187.00 in fuel.The same trip in the same cars at the posted speed limit would be $25.00. Again,speed costs money.

So what we're playing with is to fool the air into thinking our vehicle is either smaller or sleeker. Alex Tremulis said that the early aerodynamic pioneers had done all the "pick and shovel" work for us,and we're down to the details and refinement now. By looking back at their work,we can benefit from their lifes work.

John Gilkison mentioned KAMM and the ZEPPELIN WERKS,and in the next installment I hope to touch on some of these experts of their day and the enormous contribitions of which we are the beneficiary of there work.If we choose to do so.

Design obsolescence is a marketing construct, which has interfered with the natural technological evolution of ground transport. Some current offerings give us starting points from which to accentuate,modify,and approach the forms which might have existed,had certain influences not excercised power in the market.

'til next time, low drag to you all,and may we drill the sky for fuel. See ya, Phil.

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Old 02-14-2009, 05:45 PM   #2 (permalink)
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The picture showing "formula for aerodynamic horsepower" was lost when this got moved to a new thread.
Here's one from Wikipedia.

Horsepower required to overcome aerodynamic drag -

is the force of drag,
is the density of the fluid (Note that for the Earth's atmosphere, the density can be found using the barometric formula. It is 1.293 kg/m3 at 0 C and 1 atmosphere.),
is the speed of the object relative to the fluid,
is the reference area,
is the drag coefficient (a dimensionless parameter, e.g. 0.25 to 0.45 for a car), and
is the unit vector indicating the direction of the velocity (the negative sign indicating the drag is opposite to that of velocity).
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Old 02-14-2009, 09:47 PM   #3 (permalink)
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Don't you end up using 4x as much fuel at 2x the speed, because one of those 2x is cancelled out by getting to your destination in 1/2x the time?
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Old 02-15-2009, 02:55 AM   #4 (permalink)
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Remember he's talking about how much additional HP is required to overcome drag - not average fuel economy.

Less time burning fuel applies but, drag is only one factor. Parasitic losses in the engine, drive train and suspension go up as well - just not as fast as drag losses. So 2x speed (in same gear) means MORE than 4x fuel consumed.
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Old 02-16-2009, 08:15 PM   #5 (permalink)
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in 1/2x the time

Originally Posted by winkosmosis View Post
Don't you end up using 4x as much fuel at 2x the speed, because one of those 2x is cancelled out by getting to your destination in 1/2x the time?
We have to consider each vehicle on a case-specific basis, along with it's BSFC map under all operating regimes and changes to rolling resistance and any effects of road conditions,grades,temps,wind,etc.-------------------- For the aero portion only,doubling the velocity increases the road load power requirement by a factor of eight.The rolling resistance will not vary the same way.So you've got to calculate the total road load for any given speed.------------------- Using my truck for an example,at say 40 MPH you'd see low 40s mpg,and then pushing it to a steady 80 mph she falls off to around 30mpg.----------------- At the low speed the truck is mainly battling rolling resistance with only marginal air drag.At 80 mph,about 80% of available engine power is being absorbed by aero drag.--------------------- Every individual has to way out the implications of the higher velocity.

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