Aero drag reduction potential
 

I ran some numbers which can be attached to the Streamlining Template.
I've taken the Cd series from the fuselage data,ran the calcs starting with the Cd 0.04 L/D = 2.5 streamline body rather than the 12 % thickness fuselage of Cd 0.066.
On top of these values I've added 0.04 for the wheels,0.011 for a Korff/Morelli' cooling system,0.007 for 'Features' drag ( cut lines,door handles,wipers,mirrors,trim,etc.) based on the GM PNGV/Precept.
From this,I've 'subtracted' 0.015 for an under-car, full wheel fairing kit,as existed on the original GM Sunraycer or you'll see on the HONDA Dream-2 Solar racer.And I've subtracted an additional 0.008 for basjoos'-style full-floating front wheel skirts.
Since the Template is already broken down into ten segments,one can just pencil in the values below each 10 % segment line beginning at zero.
These values represent a conservative estimate of what we might achieve based on Template streamlining.Since the values are from full-scale wind tunnel testing with critical Reynolds number criteria applied,we should be able to use them with a high degree of confidence.
You can just choose a drag coefficient goal,find it on the Template,and build accordingly.You should be able to predict the new performance before you begin construction.
For values falling between 10 % stations you can just interpolate for a good approximation.

zero------ Cd 0.488
10 %----- Cd 0.375
20 %----- Cd 0.299
30 %----- Cd 0.240
40 %----- Cd 0.194
50 %----- Cd 0.162
60 %----- Cd 0.135
70 %----- Cd 0.125
80 %----- Cd 0.118
90 %----- Cd 0.118
100 %---- Cd 0.115

Note: Everyone will notice,that with proper ground clearance,the under-body of a vehicle projected rearward,would intersect the Template line around 80 %.You'll also notice that there is little drag reduction to be gained by going beyond 80 %.This is the region of 'phantom' tail.
For those contemplating a 100 %- length trailer,you can look at Solaraycer,1957 MG EX 181,Porsche 917 LH,or D-B C 111 III to see how they employed the Von Mises/ relev'e/ reflex-camber profile,simply bending the extra length up to clear the ground.
Final note: By taking the % Cd difference of where you are as opposed to where you want to go,half of that value will give your new HWY mpg multiplier @ 55 mph ( 88.6 Km/h ).
Happy modding!

Post-Script
 

I failed to mention,that for those,very daring and enterprising, who were to move their vehicles heat-exchangers to the rear of the vehicle,a la FORD Probe-IV,V,and configure your ducting likewise,you might drop another 0.011 from your Cd.
Truly scary efficiency!

Tail cutting and Cd variation
 

Basjoos was asked about mpg potentialities based on the length of his AEROCIVICs boat tail,so I thought I'd drag this thread back up as it is germane to the discussion.
Anyone familiar with the 'Aerodynamic Streamlining Template' can pencil in the values listed above in this thread onto their respective % position on the 'Template' for a quick reference as to drag potential vs boat tail length.

Thanks Phil.

A2 and ................
 

Thanks Vekke! For those of us without full-scale wind tunnels,these hard-science glimpses of real world testing really add to our portfolio and help substantiate or repute claims offered by others.
Really appreciate it.:D

Phil thank you for posting this (and Vekke for the PDF) but my left-brained art student logic is having trouble connecting the dots. Since the fronts of production cars are decidedly not template, and the boattails we build from them will not start at the 30% mark like the template does, how do we interpret the % points you posted? Also, is there a way to modify the baseline to fit our specific vehicles? I'd like to know how much I can get from 24" - 36" - 50" boat tails. (with 13* angles I can do a 100% tail and still lie within the length of a Ford F150 swb crew cab!) Thanks again for putting up with my relentless questions :)

for me, a real question is if I take an SUV, and add an 80 percent boattail to the end of it, what will my Cd be?

It's not that easy. If your SUV has huge side mirrors, roof rails/rack, mudflaps, wide tires, etc., then they will spoil the air going along the side/roof, so the Kamm extension will give you less gain. If the sides, roof and belly are smooth, then the same 80% Kamm can be used to its full potential and will give a larger Cd reduction.

it IS that easy.

the tail is by far the biggest effect. I wonder how much it is worth.

40 to 50 percent is the magic number.
for posterity, the BEST set of articles I have ever seen for those interested in this stuff.

http://wwwm.coventry.ac.uk/researchn...Challenge.aspx

tails
 

*First off,the boat tail begins at the zero-point of max roof camber for the elevation.
*For plan-view,the boat tail begins at the point of maximum body width.On EV-1,this would be halfway down the door,longitudinally.
*To determine your drag potential,you need to know how much aft-body you're starting with and the drag minimum possible for that much aft-body,then with any additional extension,what the Cd potential is at that length.
*If you've 'basjoos'd' your car,you'll have the greatest potential for drag reduction.
*At 70% you should be capable of Cd 0.13.Lower if you'll attack the wheels.
*The T-100 is at Cd 0.18 as it sits.
*the CRX was Cd 0.235 with 12-inches of tail and the rest of the 'kit.'
* The VW Transporter was Cd 0.285 with 18-inches.
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*For the Probe,it would be good to see a plan-view or measure to see where the body begins to converge at the rear.

what
 

I've got 8-new pictorial drag charts to post at the boat-tailed trailer thread if the scanner will work tomorrow.
Everyone should be able to deduce drag potentials from the comparative anatomy displayed.
Nearly all the numbers are directly out of the wind tunnels.I did a few extrapolations and interpolations but they should be of high confidence for prediction.
With respect to your SUV question,your results will be determined by the state of aerodynamic tuning on the base vehicle,as you will see in the railroad component data.
SUVs will react the same as any other vehicle to streamlining.
NASA took a late 70s Ford Econoline of Cd 0.48 and with softened nose,nice bellypan,and mediocre boat tail,achieved Cd 0.235.
If they had done the roof camber,longitudinal edge radii,cooling system streamlining,etc.,they would have come in at a lower number.
Gilkison's new 4RUNNER,at Cd 0.36 is nearly as clean as my CRX was new.It just has more frontal area.
With wheel covers,fender skirts,bellypan,cooling sys. mods,and tail,it can go to Cd 0.13 as well.But it would require a trailer-length tail to kill off all that frontal area.Or do part of it rigid,and the rest inflatable.I've done both.It all works.
With serious wheel streamlining as with so many of the solar cars,HPVs,and record cars,that Cd could also go lower.

50%
 

If someone doesn't beat me to it,I'll get Kamm's work posted.You'll see that his tail ended at 50% frontal area,as at this 'length',the lack of clearance for the SAE 10-degree departure would have knocked a longer tail off the car.
Fachsenfeld solved this challenge with the inflatable section to complete the rest of the tail,only on the open highway.

Phil, So I have seen the light.

I need to get the trike on the road, then I'm going to work on mileage for my suburban.

Thank you. I'm starting to understand this all, and the potential is HUGE.

That's true even for us dummies which makes it even better!!

Phil, what would be the results of a slotted box cavity mounted on the end of a shortened boattail, versus something like the back of an SUV ?

Suburban
 

When you get to the Suburban ( and I hope your projects go faster than mine),take a look back at the JEEP Cherokee mods on the Stunt-Busters thread.
You'll see a resemblance to the Cd 0.08 'Ideal' self-powered passenger rail car on the railroad components drag table at the boat-tailed trailer thread.
Any amount of rear extension will pay dividends at the pump.

This assumes a flat plate truncation?

Would ending in a box cavity create an inverted, 'virtual' (rather than inflated) extension to the boat-tail shape?

Edit: I notice Cd has the same question. Anyone have an answer?

assumes/box cavity
 

*yes,all truncations would be like a loaf of bread,with simple vertical slicing.
*As to the box cavity,I'd really need to compare published empirical data.
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*A proper boat tail will have zero separation until it terminates and this will allow the continuous pressure rise until separation which will yield the highest base pressure and lowest delta-P across the vehicle;ie. minimum pressure drag.
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*The box cavity would have immediate separation,although would be designed for reattachment and capturing locked-vortices all-around.
*In theory,the outer flow field would pass over the locked-vortices and the base pressure would be based on the pressure of the modified separation point of the cavity's rear termination point,minus the energy lost to the continuous 'feeding' of the vortices.
*The wake of both boat tail and box cavity would be the same cross-sectional area at equal length,but the boat tail would have a bit higher base pressure due to its lack of circulation along the pathway.
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I'm not sure if this answers the question.:o

Thanks for giving it a shot.

T'would be awesome to put one of Autospeed's Magnehelic gauges to work with a manifold of about 20 hoses you can valve on and off individually in a single run. :thumbup:

The reason I'm optimistic is that a half-tonneau on a pickup bed is essentially a box cavity facing up behind the obstruction of the cab, and it has a measurable effect.