Aero drag reduction potential

[drmiller100]

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

[aerohead]

Originally Posted by Sven7

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

*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.
__________________________________________________ ______________
*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.

[aerohead]

Originally Posted by drmiller100

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?

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.

[aerohead]

Originally Posted by drmiller100

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.

Coventry University - The Aerodynamics Challenge

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.

[drmiller100]

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.

[slowmover]

Originally Posted by drmiller100

Phil, So I have seen the light. 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!!

[Cd]

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 ?

[aerohead]

Originally Posted by drmiller100

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.

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.

[betasniper]

I have just made this table showing the percent benefit of a boat tail as compared to a full boat tail. Enjoy

LENGTH	CD	BENEFIT
0%	.488	0%
10%	.375	30.3%
20%	.299	50.7%
30%	.240	66.5%
40%	.194	78.8%
50%	.162	87.4%
60%	.135	94.6%
70%	.125	97.3%
80%	.118	99.2%
90%	.118	99.2%
100%	.115	100%

[freebeard]

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?

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 ?