Drafting and Cd reduction
 

A good (but not necessarily safe) way to measure your car's potential mpg improvement via Cd reduction mods is note how much of an improvement you get in mileage when drafting. When drafting, your engine rpm's are running at their usual levels, your rolling resistance is unchanged, and the only thing that has changed is your aero drag load. There have been some discussions in the past about how certain vehicle's gearing and engine loading don't allow them to to take full advantage of the reduction in engine loading resulting from a Cd reduction program, so this is one way you can test potential gains before going through the expense and effort of modifying your car to reduce its Cd. Ideally you should have an airspeed indicator in the nose of your car so you could graph the drop in airspeed as you get into a closer draft against the improvement in mileage. I'm not totally certain, but I'm guessing that the mileage achieved when the drafting airspeed has dropped by half would be the mileage achieved by your car when you have halved your Cd from its current value.

One thing I have noticed as I have dropped the Cd on my car is that drafting has less and less effect on my mileage and at a Cd of 0.17, it is not even worth bothering with even when a convenient draft presents itself. In some ways you could say I am drafting myself. Also the back of my car doesn't offer any mpg benefit to anybody trying to draft me, so my car is totally divorced from the drafting game.

The ability to benefit from drafting is a result of aerodynamic inefficiency both of the parts of the drafter and the drafted. An efficient aerodynamic design doesn't create the high and low pressure zones that can be somewhat canceled out (when you stick your high pressure nose in their low pressure wake) by drafting.

I have a question about your boat tail... what would happen to your .cd if you chopped it in half? (kammback style)... do you think it would go up dramatically or practically stay the same?

The Cd would go up because you'd have quadrupled the size of the recirculation eddy trailing the car.

When I first built my prototype boattail, it was cut off at the point where the back edge of my hatch glazing is now, then later I added a tail cone to complete the boattail (see pics in my Photobucket site). I got a noticeable increase in coasting performance when I went from the partial to the full boattail. There's a reason you don't see subsonic aircraft flying around with kammbacked boattails, its called increased drag.

half or full
 

basjoos,
I've been going over the old German stuff and it's in agreement with everything you say.And I was surprised that there is a certain degree of ledgerdermain with respect to the claims by the "experts" with respect to cropped posteriors on cars.---------------- With Jaray's and Lay's research,along with Dornier,Messerschmitt,etc.,full drag reduction does not occur until the full boattail is achieved.---------- Even when Korff eludes to Dornier's aircraft fuselage being truncated for the rear gunner's turret,we're only talking about 2 % of the tail length being compromised before extreme drag increase occurs!-------------------- Sure,there are practical limits to the length of a car,and Kamm openly admits that his 50-% tail- "Kammback" was to address "length" issues not"drag" ,but in purely aerodynamic terms,whether practical or not,its incorrect and untrue to say that a 50 % crop is as low drag as a full tail.The science simply does not support the claim!------------------------ Not to go off-topic,but if people expect to ever achieve high levels of fuel economy,they are seriously going to have to re-consider how they think about what cars should look like.---------------------- Thanks for all you do,and for remaining a constant source of inspiration for an old tin-knocker like me and so many others.It makes it easier for me when I know there are a few out there that "get it" and are willing to apply themselves,sometimes amidst an environment of zero encouragement and support.My hats off to you!

Actually you were an inspiration for me. At the time that I was getting more and more frustrated at the car manufacturers for their lack of inexpensive FE cars, I read your article "Free Fuel Riding on the Wind" about modifying your truck using materials from your local hardware store. That got me thinking about what I could do myself, so I researched automotive and aircraft drag reduction and got down to work modifying my own car.

EV WORLD: Free Fuel Riding on the Wind
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Nice article. I hadn't seen that one yet.