Describing my Beetle wing (VW New Beetle)

[Ernie Rogers]

I started working on the wing within a few months after buying the car in 2003. I spent a few weeks reading all the journal articles I could find on the connection between rear shape and drag. From those, I concluded that I should have a "shelf" about equal in path length to the distance down from the top. I got a photo of the assembly into this email. I will have to come back with photos of parts after I figure out how to do it.

Ernie

[lectruck]

Wow Ernie, Looks really good! I used to have a 2002 turbo beetle, traded it in for my Scion xB. It only got 20mpg. The diesel bug gets really good mileage.

Don

[Ernie Rogers]

Originally Posted by Ernie Rogers

I started working on the wing within a few months after buying the car in 2003. I spent a few weeks reading all the journal articles I could find on the connection between rear shape and drag. From those, I concluded that I should have a "shelf" about equal in path length to the distance down from the top. I got a photo of the assembly into this email. I will have to come back with photos of parts after I figure out how to do it.
Ernie

The first picture shows the plan of the center shelf. It's about 20 inches wide x 42 inches long, closely follows the rear shape.

Second picture shows shelf profile at one end.

Third picture is shape and position of end "winglet." About 24 inches front to back x 20 inches high. Bolt holes indicate how the wing center slopes slightly to the rear.

Fourth picture is inside face of a winglet. You can see the bracket that fits over the edge of the rear hatch.

[Ernie Rogers]

Originally Posted by Ernie Rogers

I started working on the wing within a few months after buying the car in 2003. I spent a few weeks reading all the journal articles I could find on the connection between rear shape and drag. From those, I concluded that I should have a "shelf" about equal in path length to the distance down from the top. I got a photo of the assembly into this email. I will have to come back with photos of parts after I figure out how to do it.
Ernie

First picture is of bottom edge of a winglet. The winglet is supported by a backet toward the front, with two screws holding it from the bottom. The winglets also rest on the fenders for additional support. The winglet has a felt pad at the contact area, and the fender has a patch of clear plastic sheet to prevent scuffing of the paint.

Second picture shows the way the bracket is secured from beneath by screws.

Third picture shows a small gap between the leading edge of the wing and the car surface. The intended purpose of the gap is to suck out a separation bubble that could form there. The gap also serves to protect the car paint.

The middle part of the wing is made from class cloth and epoxy resin over a foam core, with some paper and glue between. Each end of it includes one inch thick plywood with threaded inserts. Winglets have matching through holes for countersunk flat-head allen screws. The winglets were fashioned from pattern-grade birch plywood.

Assembly involves screwing the winglets in place, then inserting the center section between. The complete assembly captures the edges of the hatch so the attachment is very secure. Time to put on or take off is about five to ten minutes. Nothing shows on the exterior when the wing is off except the clear plastic patches that protect the paint.

[alohaspirit]

thanks for the info & pics

how much mpg gain did you say you got vs stock?

[Ernie Rogers]

Originally Posted by alohaspirit

thanks for the info & pics

how much mpg gain did you say you got vs stock?

This is a hard question for me. The reason is I'm not 100% sure.

Shortly after building the wing, I tested it very carefully and got a 3 mpg improvement no matter what my speed was. (At 60 mph, I got 58 mpg without it and 61 with it, as I recall.) This actually makes some sense because at higher speeds, the 3 mpg represents a bigger gain.

Later, I measured my Cd and Crr (rolling resistance coefficient). Armed with this and some other car parameters, I can calculated miles per gallon a-priori.

I will check my figures on this and write a little later, I just got a call for dinner.

Ernie Rogers

[trebuchet03]

Ever since I saw your beetle... I've always wondered what the inspiration behind the winglet shape was.

Beautiful build though, I'm always a fan of quality

[MetroMPG]

This is really good detail - thanks for posting it, Ernie.

I'm with Trebuchet: looks like a quality build.

[Ernie Rogers]

Now, the rest of the story--

Last summer, I measured my fuel economy very carefully. My "test track" was I-80 from the Toole truck stop near Salt Lake City to the Wendover, Nevada, exit and return. Distance, very close to 200 miles, almost perfectly level. Mileage was very close to 65.5 mpg at 60 mph. Speed and distance were checked with a GPS.

I just finished calculating the mpgs, assuming no wind, 4500 ft elevation and 70 degrees. Here are the input numbers--

Frontal area...............23 (Uhh, I guessed that number)
Drag coefficient.........0.31 (my measured value)
Air density................1.01 kg/cu.m.
Weight....................3050 lb (measured, with me + full tank)
Rolling resistance......0.0065 (my measured value)
Fuel density.............3200 gm /gallon
Engine efficiency.......Empirical fit to published data for my engine
Transmission eff........0.85

The program gets 65.56, which is too close to be believed.

That all looks very nice. But, if I plug in the factory Cd = 0.38, I get about a 6 mpg drop, which doesn't match my measurement.

/Ernie

Originally Posted by Ernie Rogers

This is a hard question for me. The reason is I'm not 100% sure. [When asked about the benefit of the wing.]

Shortly after building the wing, I tested it very carefully and got a 3 mpg improvement no matter what my speed was. (At 60 mph, I got 58 mpg without it and 61 with it, as I recall.) This actually makes some sense because at higher speeds, the 3 mpg represents a bigger gain.

Later, I measured my Cd and Crr (rolling resistance coefficient). Armed with this and some other car parameters, I can calculated miles per gallon a-priori.

I will check my figures on this and write a little later, I just got a call for dinner.

Ernie Rogers

[cfg83]

MetroMPG -

Originally Posted by MetroMPG

This is really good detail - thanks for posting it, Ernie.

I'm with Trebuchet: looks like a quality build.

Me three. The diablo is in the details. The profile looks like an upside down wing. Is that intentional?

CarloSW2