aero cap idea.

[my first gmc]

OK, I decided that I want to make an aerocap for my truck. The only thing is I don't really like how the roof goes straight to the tail gate. I did a quick sketchup of something I would really like and think would be a little more practical. I wanted something with a rear window/hatch. Also, what would be the ideal slope of the roof? Any comments or suggestions are more than welcome.

[Frank Lee]

Shallower slope than possible compromises potential aero benefit although interior volume gains some utility. Pick your poison I guess.

[Memorytwo]

thats actually not that shallow.
but a kammback is the best
"According to the classic definition the tail should be cut off where it has tapered to approximately 50% of the car’s maximum cross section, which Kamm found represented a good compromise - by that point the turbulence typical of flat-back vehicles had been mostly eliminated at typical speeds."



although i like the military slant back hummer.

[sepp]

i suggest you compare your slope with some of the teardrop-shapes you can find around this forum, but judging by the eye, yours looks close to the ideal shape.
instead of making your rear plate bend forward, you could make it bend backwards, or at least make a top lip, that way you can win some precious height and guide down the airflow lower.
if you worry about sight loss to the rear, you can always turn away your mid rear view mirror and drive like that a couple of days, you will see that you really don't need it so badly.

[MechEngVT]

You could reduce your wake area a bit by progressively increasing the "tumblehome" toward the rear of the cap. Tumblehome is the angle which the side of the vehicle above the beltline departs from vertical. If you look at the rear of the new Fit the C/D pillars slant inwards more than the A/B pillars do. Keeping the progression of this increase subtle will reduce wake area, but too radical a shift will separate the flow from the sides of the cap.

A straight vertical cutoff at the rear plane would be better as shown on the A2 picture. This might permit a slight lowering of the vertical split height, reducing wake area, without increasing the downward slant at the rear enough to cause flow separation.

[mobilerik]

Shamelessly plugging my '03 Tacoma Prerunner thread...

Check out the exchanges between Aerohead, MetroMPG, and I about fitting my truck under the ideal curve and chopping the wake to deal with my short-bed. You could try the same with your truck.

The slope of the ideal curve averages about 10-11 degrees on the conservative side. Above the bed the curve is pretty straight -- it doesn't really get a lot steeper until it's past the tailgate. A rule-of-thumb you can follow is that you want it a tad shallower than 1-to-5.

That's not to say that you can't experiment with curving a wee bit more aggressively -- a couple guys on here have demonstrated attached flow (through tuft testing) with a curved "fastback" approach like in your sketch. But if you're sold on the rear-window idea (which I used as well), then you might simplify to a straight back. I personally like the way a straight back opens you up to curving in the side-to-side direction.

[my first gmc]

Well, the cap will have both a front to back curve and side to side curve, as well as the side being sloped. I just haven't figured out how to do all that in sketchup. My plan is to make a wooden frame that will have the basic shape and than cover it in insulating foam and refine it even more, then fiberglass it and do any final body work necessary.

Oh, and here's a side shot. If someone could put this against that airfoil, it would greatly be appreciated.

[my first gmc]

Ok, I changed a few things on the profile of the cap. Before with the rear height being half of the front the slope was 8*, now it's 10*. Also put the back straight up. So here's a quick photo chop for you guys. What do ya'll think?

Hopefully I'll be able to making a "cardboard prototype" frame on the 21st since I have a 5 day weekend then. Then hopefully I can start the real construction on spring break.

[flying_brick]

Hello,
I was inspired to build an 11 degree Kamm cap last year and built one this spring. I recently gave it a highway test and achieved 12.087km/L with a tonneau cover and 13.19km/L with the cap over the same 100km stretch of highway and identical windless conditions, for an efficiency increase of about 9% at a 100km/h speed.

Tonneau 6.718L/81.2km no wind, included climbing 500' Niagara escarpment on 401 in Ontario.
Kamm 8.512L/112.3km. Same as before + traversed two large river valleys.

In Ontario Canada we have "up to" 10% ethanol in fuel which worst case has 96.7% of the energy content of pure gasoline. I'm assuming my fuel was identical on both test runs. On the test run with the cap I filled up later and included two large river-valley hills. I think more fuel is lost uphill than is saved downhill. Anyhow, I think 9% is believable. I have a fisheye backup camera wired to the ignition.
For ice and snow I designed the closure to drain water out, and although it is not visible, I also have a flap at the cab end of the cap which drapes below the top of the box and ensures no water comes in (standard problem with all caps I have seen). The trailing overhang keeps the camera and caplock dry and extends the tail somewhat. The cap is solid but not beautiful: there are numerous design and construction improvements I would make on another one.
I look forward to the day when truck cap companies catch up and make these commercially.

[bondo]

Your rear visibility will be alot better than an aerocap that ends at the top of the tailgate. Best of luck on your project.

Bondo