That is spectacular in it's own way...even bothering with an old truck like that deserves a "bow"...
2006 Jeep Liberty CRD...Founder of L.O.S.T.
OME 2.25" Lift w/ Toyo Open Country HTs 235/75/16s
ASFIR Alum Eng/Tranny/Transfercase/Fuel Skids
2002 Air Box Mod...Air Tabs (5) on Roof...(3)each behind rear windows
Partial Grill Block with Custom Air Scoop and 3" Open Catback Exhaust
Lambretta UNO150cc 4 Stroke Scooter
It from the book Race Car Aerodynamics by Joseph Katz with an excerpt on this website. Here is an excerpt if you don't want to read the whole thing:
As the slant angle is increased from zero, a positive lift will develop, which increases up to ? = 30°. At slant angles larger than 10° the rearward projection of this negative pressure causes quite a large increase in drag, as shown in this Figure. The most interesting feature of this data is that above a critical angle (close to ? = 30° the vortex structure breaks down and the drag and lift contribution of the slanted surface is much smaller. This fact has an effect on hatchback automobile design, where rear window inclination angle should be more than 35° or less than, say, 25°. Also, note that in this case, the basic body (with ? = 0° has negative lift due to ground effect, similar to the case with the ellipsoid, shown in Fig. 2.22.
Thanks for the Bow. Well as of right now barely into the first tank. Even though it is quitter and the wake is less turbulent (much less snow being kicked up). This project will come to a close. About an hour ago I bought a beater car. 1988 Buick Century 3.8L Auto has only 40K original miles. The guy I bought it from (my father) just bought it and did the run through, parts replacement. Best of all $1100, for 28mpg highway. It should be quite a change from the 15/17 I am used to.
Depending on how long it takes to process and insure the truck may see another tank or two. And when the truck needs some "exercise"
If you are making a fastback cover then you want to have it running from flush with the roof at an angle of 11 degrees to the back of the truck so you would drop about 18.5 inches (if my trig is right) across the 96 of the bed. If you look at the following diagram under the "A" column it shows the drag and lift produced my a fastback shape at different angles with the lowest drag at 11 degrees:
So there is a definite advantage to have sloping rather than horizontal because as long as the air stays attached (and it will at 11 degrees) you would be reducing the area of your wake which is what slows you down the most. It might also be good to have the last bit of the fastback curve back to the horizontal so that you aren't producing any lift. You could use plexyglass for a rear window in the cover if you are worried about viability.
Hey, so I have a question for you. I am planning on making a fastback for my pickup come spring, after reading this I took some measurements and found out if I make a fastback from the top of my cab to the back of my truck (top of the tailgate while up), it creates an 11.36 degree slope, so that sounds perfect.
But my original thought was to open the tailgate up and create a fastback from the top of the cab to the very end of the tailgate while down. As you can imagine this will increase the slope, it calculates out to exactly 18 degrees. Will flow stay attached at that angle?
Basically is it better to have an almost optimal slope (11.3 degrees) and THEN a 20" flat/drop off where the tailgate is while up, or would it be better to eliminate the flat surface of the tailgate by making an even bigger fastback but having it only be 18 degrees?
I would say that it would be better to have the tailgate up and the smaller angle as you would have less turbulence at that angle and smoother flow. A boat tail with the end chopped square is called a kammback, here is a quote from wikipedia:
Kamm showed that a better drag-reducing tail end design for a car is one that tapers and is then cut off abruptly.
The point at which this must happen, in order for the design to be a true Kammback, is controversial. A popular definition is that the cut-off should occur where the cross sectional area is approximately 50% of the car's maximum cross-section. Thus a minivan is not a Kammback.
Prior to Kamm's thesis, a teardrop shape that tapered smoothly to a point was considered optimal. Kamm showed that an abbreviated teardrop actually worked better; the air still flowed as if the entire teardrop were still there, but without the surface drag of the long point.
To minimise lift you would want the cover to start at horizontal at the roof of the truck, curve gently to 11 degrees and then back to horizontal at the tailgate so that the air flowing off the back doesn't produce force. To minimise total drag it will probably be better to let it continue at 10-11 degrees at the tailgate (but still make it gently at the roof). That way you would still have more room in the bed for carrying stuff as well so its win-win.
I am sceptical that the friction forces would be greater than the added turbulence that must occur at the cut off but I am pretty sure that the 10 degree angle will work better than the 18 degree one because there will be less turbulence where the sides and top meet. It would make a good simple experiment with some A-B-A testing, empty bed vs kammback vs higher angle boat-tail, all you would need is a large sheet of ply and something to secure it with and a loop of highway...
I was under the impression that a Kammback was a compromise. Where the Kammback cut-off is dictated by the utility of it. While a complete teardrop is the most aerodynamic it is also somewhat unpractical. Especially on a truck or any tall vehicle. Because a teardroped truck would have a very long tail, 14 foot. Assuming two 11 degree slopes with a 6' tall cab. This long tail would swing out in turns. I think that the Kammback was developed to get similar CD decreases( similar but not equal) while being much shorter -> more practical to drive with one equipped.
I know , I know one tanks means nothing. But for the past couple weeks during my lunch breaks from college. I have been running scrap metal loads. This whole tank has been 1/2 metal loads and 1/2 3 mile city trips (my dd needed repairs). The scrap runs were anywhere from 1000 - 3500 lbs of metal. Then empty on the way back.
This tank averaged 22mpg. Which is better than the previous best. 20 mpg I got last fall 100% highway.