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 second in a series which I'm reproducing here with permission.
Go to: Aerodynamics Seminar Index
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Aerodynamics Seminar: installment # 2 - by Phil Knox
Okay,so we were talking about insulation, which you can see next time your at the home improvement store.Thermal insulation is rated by R- Factor,which denotes its resistance to tranfering heat ( there is no such thing as cold,only an absence of heat).The larger the R- Factor,the greater resistance to transfer.
In thermodynamics,the study of heat flow,the silly engineers take the inverse of the R- Factor,to come up with a heat transfer COEFFICIENT.By taking R and dividing by one,you get
U, the heat transfer.
With a pocket calculator,you can play around with R-Factors and realise that it follows that the higher the R,the lower the U, and the less heat is transfered.
In aerodynamics,engineers skip the air resistance factor ,and go straight to a coefficient. Since we're messin' with aerodynamic drag,we call our coefficient,the coefficient of aerodynamic drag, or, Cd for short. Simple!
The lowest Cd would occur for any shape in space,where there is no air. And to briefly answer John Gilkisons question about how our car would do in a vacuum? At 50 mph,about half the resistance your engine or motor works against is air drag. On the Moon,at 50 mph,your car would have 50% less resistance,and see a 25% increase in mileage. The faster you went the more the savings as we'll discuss later.
So we've got this rating system for car shapes, or anything else that moves through air or water, as air behaves exactly like water up to about 250 mph. The least drag would be found for a jet aircraft wing in free air (away from the ground up in the sky) with a Cd of 0.02. The highest drag shape we can create may come at no surprise,its the parachute!,and it has a Cd of 1.35. A sheet of plywood flat into the air is Cd 1.11.
For us on the ground, things get a little weird.If we take that jet aircraft wing,put four wheels on it ,and drive it on the ground,its drag coefficient leaps to Cd 0.11. This is considered the lowest drag possible for a ground vehicle in "ground effect", a phenomenon which occurs for all vehicles in closde proximity to the ground. What it does is to make the air think your twice as tall as you actually are, a mirror image of yourself or your car,progected below the ground.
The lesson here,is that without active, powered,boundary layer control, it is impossible to go below Cd0.11 on the road.
What is boundary layer? It is the strata of air which is directly adjacent to the skin of your vehicle,at the boundary of flow. Up to a point,this layer is affected by surface roughness. For small objects like golfballs,roughness or dimpling can create a turbulent boundary layer which can stay attached longer on a body of decreasing cross-section.
Size is important here and is governed by REYNOLD'S NUMBERS effects which do not effect things as large as automobiles.
All our cars have turbulent boundary layers,and separated flow becomes the major component of aerodynamic drag,called PROFILE DRAG. No additional smoothing or polishing of a cars finish can reduce its FRICTION DRAG which is a function of surface area, and constitutes about 7% of total drag.
Presently,typical U.S.passenger cars have Cds in the range of Cd0.30. Pickup trucks have drag coefficients on the order of Cd 0.45. SUVs have coefficients on the order of Cd0.7 to 0.34. Vans run as low as 0.30.
The aerodynamic "SIGNITURE" of your car is its DRAG INDEX,which is a product of its Cd and its frontal area.
For example,a Dodge Caravan with Cd 0.3 and aera of 36square feet has an index of 10.8 square feet. A CRX witha Cd of 0.3 and frontal area of 18 square feet has an index of 5.4 square feet. If the CRX was shaped like a HUMMER H-2,its index would be 10.8 square feet , just like the Caravan. This goes back to the egg and sugar cube. Consequently,if the H-2 shape was changed to that of a CRX, its drag would fall precipitously,and performance skyrocket!
So again,if size is not negotiable,we still have shape to play with. At 55 mph, a 10% drag reduction translates to a 5% increase in fuel economy. At 70mph,a 10% drag reduction translates into a 4% increase. With the Cds, you can actually predict how any given vehicle will respond to drag reduction. And with carefull mileage records,fuel gain numbers can be reverse-engineered to provide drag data. Its what the big dogs do,and its a tool we can also use.
I'm outta time,more next week,happy trails! Phil.