Hitched Detachable Boat Tail finished. With tests!

[brucey]

I'll try testing it tonight, but I dont think this idea works practically. At least not the way I've built it. It's too heavy, too awkward to use, a ***** to put on and off, and way too big to store anywhere. Nice concept.

I think I'm going to start from scratch again and have a kammback permanently on the car to get the right angles going and go from there.

[BamZipPow]

Found something interesting...at least from an inflateable point of view...

Balloons.net makes those inflateables you see around...they even have some clear ones.

Probably be kinda expensive though...but it would be a cool approach.

[brucey]

OFFICIAL RESULTS:

Weather Conditions:

45°F | °C
Current: Partly Cloudy
Wind: N at 3 mph
Humidity: 48%

Method:

Cruise was set at 55 and run on my 1 mile test track. A - B 3 times each, cancelled with cruise and resuming with resume. I put the boat tail on and duct taped a few areas where I felt it could be catching. Then did another A-B 3 times, and went back, took it off, and did ANOTHER A-B 3 times.

Data:

NO TAIL:

A: 33.2
B: 33.4
A: 33.2
B: 33.2
A: 33.4
B: 33.3

TAIL:

A: 33.6
B: 33.4
A: 33.5
B: 33.1
A: 33.4
B: 33.0

NO TAIL:

A: 34.7
B: 33.3
A: 32.9
B: 33.3
A: 33.5
B: 33.4

Results:

GORRAMIT

Pretty much a wash.

Its interesting that the best AND the worse results came from the same run set, in the same direction.

Other than that, it seems I've made a 4 foot rolling blind spot. And I now have enough data for myself to start over pretty much from scratch.

*sigh*

It would have been really useful for the trip to Cape Canaveral this weekend for the shuttle launch. Hopefully I can rig up a kamm quickly and drag it down.

[Cd]

That was certainly unexpected !
Sorry to hear about those results.
It would be interesting to see some tuft testing to see what is happening back there.

[dluck]

Hi,
I'm not surprised too much by the 55 mph data. If you tried it at 45 I suspect it would have gotten worse as the boattail would just extra weight. Since at 45 the wind resistance becomes insignificant compared to the other losses. However, if you tried it at 65 or 70, we might see that although the mpg goes down in both cases, it would go down less in the case of the boattail.

Could you rerun the tests at 65 or even 70?

[Cd]

Originally Posted by dluck

Hi,
Since at 45 the wind resistance becomes insignificant compared to the other losses.

Um..........."insignificant" ??

[Sorrow Six Star]

lesbaru! I like it!

the boat tail makes the OB look like a Chinook.

[BamZipPow]

Some data I'd like to see is the weight of the boat tail along with the new angle/rake of the car with the boat tail installed.

[dluck]

Originally Posted by Cd

Um..........."insignificant" ??

Yes, insignificant for a car, not insignificant if you are peddling a bicycle. The boattail effectiveness is proportional to square of the speed. At low speeds where there is little if any energy being spent to push air out of the way, or sucking it back in behind you cannot get any benefit. Also the boattail length/curvature will create a sweet spot for a particular speed. If its not long enough there were still be drag as a vacuum is created behind the boattail. However being longer than necessary will start to add drag back and unnecessary weight.

[MetroMPG]

dluck: even at 45 mph, probably half or more of the power required to go down the road is lost to aero drag.

Have a look at this tool: Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com

Also the boattail length/curvature will create a sweet spot for a particular speed.

Cd is effectively constant for our purposes - the shape either works, or it doesn't, regardless of speed.