Thoughts on gain from boat tail on 94 eclipse

[Rango318]

Originally Posted by Rango318

So, i did some testing today. Did a tuft test, which i will upload pics of tonight. I did one test with the hatch closed, then one with the hatch open, and all gaps sealed. I made it as close to the correct angle as possible. I then ran one tuft test. I decided to fill up, and drive to where i work and back to get a real life style test.

My tank last week, monday thru friday was a new high of 43.84.

To run my test, i did everything i would usually do when going to work. Stopped at the store, stopped in the parking lot at work.

The only things that i did not test for : usually the return trip is at night, so it is colder, engine is cold, and the headlights are on. All of which i did not include in this test.

All I can say about the results is WOW. I drove 58 miles on 1.091 gallons (53mpg). I am going to leave it like this for this week of work and see if i see this big of a change again. One can only hope.

So i must have somehow, even with being extremely careful, messed up filling on my second fill. The 53mpg is not accurate.

[aerohead]

Originally Posted by Xist

That much? What is the most that anyone has achieved so far?

What could I expect for my Forester?

Without digging,do we have a Cd for the Forrester? Is it based on the Legacy station wagon,or is that the Outback?

[aerohead]

Originally Posted by Rango318

That is a great idea just to test the idea. I had thought about using it as part of the tail, but not to use it to see if it would help. Thank. I think I will try it today.





Can i ask how you got those numbers? I love to do math and such and would really like to know how to calculate such a thing. I can only hope i get these numbers.

GM's Styling Dept. had a couple of guys who published an SAE paper back in the early 1960s.
They reported that a 10% drag reduction would mean a 5% mpg increase at a steady 55-mph.
The same 10% drag reduction would yield a 6% mpg improvement at 70-mph.
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After running the CRX at Bonneville in 1990,I called the GM Aero Lab and spoke with Glen Scharf (sp?),who I believe was the head aero guru at the time.Mr. Scharf told me that the 10%/5%,10%/6% relationship was still valid and could be used with high confidence.
We discussed the numbers I came up with for the salt and mpg figures and he felt that a reasonable guestimate for the modified CRX would be Cd 0.235.
Short of a trip to the A2 Wind Tunnel it's one of the few tools we have at or disposal for attaching numerical; values to our mods.

[Xist]

Originally Posted by aerohead

Without digging,do we have a Cd for the Forrester? Is it based on the Legacy station wagon,or is that the Outback?

Aerohead, I had looked it up for http://ecomodder.com/forum/showthrea...dia-25681.html

Originally Posted by aerohead

A full boat tail would drop the Subie's drag from Cd 0.405,to 0.12.At 55 mph you'd be looking at a 35% mpg increase.

I currently average around 28 MPG, so that would push me to 38.7, but it would be a full ten feet long!

[aerohead]

Originally Posted by Xist

Aerohead, I had looked it up for http://ecomodder.com/forum/showthrea...dia-25681.html



I currently average around 28 MPG, so that would push me to 38.7, but it would be a full ten feet long!

I'm no good at links yet,but at the full-boat-tail trailer thread there are some pictorial drag tables for limited-length tails as I used on my VW bus,NASA used on their Ford Econoline,and Kamm/Fachsenfeld and other researchers came up with buses and 18-wheelers.
If memory serves me the tail will have:
*hard 20-degree downslope.
*hard 10-degree in-slope (plan-taper) on the two sides.
*hard 10-degree taper for diffuser area.
*top-to-side edges are rounded over,increasing in radius as the tail progresses rearward.
This will be very much like the General Motors 'Optimum' boat tail that researchers at Texas Tech were investigating in 1995.
If I were to do a limited-length tail I would build exactly as GM did.
You won't have to sweat the great length and there's no guesswork.It's off-the-shelf technology.
Just bear in mind that with such a tail,you're 'done.' There's no economical drag reduction potential by extending it further.

[Xist]

Originally Posted by aerohead

This will be very much like the General Motors 'Optimum' boat tail that researchers at Texas Tech were investigating in 1995.

I tried searching for that, but did not come up with anything.

[aerohead]

Originally Posted by Xist

I tried searching for that, but did not come up with anything.

*The GM Optimum tail made the cover of the Winter 1995 magazine which Texas Tech publishes.It's in their article about the Aero Lab's Project Tailwind (in which they tape over GM,leaving only 'Optimum Tail.')
The GM tail is only shown in side elevation.It's 20-degrees on top,10-degrees on bottom,and is as long as the trailer is tall.
*My boat tail was also 20-degrees in 1980,with a length equal to 75% of body height,bending gently into the slope.
*NASA's 1981 boat tail was also 20-degrees,bending gradually into the slope.
*Walter Korff (Lockheed Aeronautical) also used 20-degrees,with gentle transition into the slope,with length equal to 70% of body height.
*W.A.Mair,who specialized in boat tail research recommended no more than 22-degrees.He used 1.8X body height to reach the 22-degree slope.
*Ahmed et. al. used 25-degrees at top with hard corner,10-degrees on bottom,10-degrees on sides.It was 62.5% of body height.It lowered the models Cd from 0.25,to 0.214,a 14.4% drag reduction.
*An ideal boat tail,on that same model would have returned a 52% drag reduction.

[Xist]

Should we move this conversation to http://ecomodder.com/forum/showthrea...tml#post368673? This is not necessarily relevant to Rango318's thread.

From: http://www.nasa.gov/centers/dryden/p...ain_163113.pdf

The full boattail provided an average 32-percent reduction in drag at "highway speeds" whereas the truncated boattail provdied [sic] an average 31-percent reduction in drag as compared to the configuration having the blunt base.

[Sven7]

Jeez your rear window flow looks great. Just copy my boat tail.

http://ecomodder.com/forum/showthrea...tml#post326923

[Cd]

Rango 318 :

Thanks for the testing.

I'm curious about how that the tufts reacted with the hatch down.

( I'm expecting separation around half way down the glass ) Did you manage to get a picture of the hatch down test at speed ?

And the reason for the error with the earlier fill was the quantity of fuel. You need several gallons to get a good result.
The smaller the amount of fuel, the more room for errors.